https://orcid.org/0000-0003-2599-1034
Abstract
Ribosome-associated quality control (RQC), a ubiquitous process essential for maintaining protein homeostasis in eukaryotes, acts as a critical surveillance system for protein translation. By identifying and eliminating stalled ribosomes, RQC prevents aberrant translation and the production of potentially toxic misfolded proteins. The review focuses on the role of RQC in mammals, where its complete functionality remains to be elucidated. This study delves into the mechanisms through which dysfunction in RQC plays a role in the development of neurological disorders, focusing on neurodegenerative and neurodevelopmental diseases. We explore the underlying mechanisms by which RQC dysfunction contributes to the pathogenesis of neurological disorders, particularly neurodegenerative and neurodevelopmental diseases. Further research is crucial to unravel the intricate mechanisms governing RQC’s influence on neurological function. This knowledge will pave the way for exploring therapeutic avenues targeting RQC factors as potential interventions for these debilitating diseases. By shedding light on RQC’s contribution to neurological disorders, this review opens doors for developing targeted therapies and interventions.
Ribosome-associated quality control (RQC) is a highly conserved mechanism in eukaryotes that is activated by ribosomes becoming abnormally stalled. This process targets nascent proteins and/or messenger ribonucleic acids (mRNAs) for degradation (1). RQC acts as a fundamental proofreading system, addressing the relatively infrequent but diverse causes of failed translation events that can arise during stress, development, or conditions with increased protein synthesis (2). During the elongation cycle, ribosome stalling can occur under various pathological conditions, including structural defects in mRNA, the presence of uncharged transfer RNA (tRNA) molecules, and hereditary variations (1,3–5). RQC is an essential component to ensure the fidelity of protein synthesis. However, insufficient RQC activity causes aggregation of proteins, which lose their intended function or gain new, potentially harmful properties (1). Additionally, stalled ribosomes that are not cleared by RQC can impede subsequent rounds of translation (6).
Neurological disorders encompass primary injury or dysfunction of the central or peripheral nervous system, manifesting as neurodegenerative and neurodevelopmental conditions. Emerging studies suggest that RQC dysfunction is linked to multiple neurological diseases, such as Alzheimer’s disease (AD), Huntington’s disease (HD), and peripheral neuropathy (7–11). These diseases highlight the critical role for the RQC pathway in proper neuronal function. The key mechanism underlying this link could be that RQC failure results in protein misfolding and aggregation, ultimately leading to neurodegeneration (Figure 1).
The relationship between ribosome-associated quality control (RQC) and neurological disorders. Under various pathological conditions, the RQC mechanism is activated to degrade aberrant peptides, thereby preserving normal nervous system function. However, the failure of the RQC system, caused by factors such as mutations in RQC-associated complexes, persistent ribosome stalling, aberrant peptide aggregation, or reduced protein synthesis, can lead to nervous system damage and the development of neurological disorders.
Given the critical role that RQC dysfunction may play in understanding the pathogenic mechanisms and exploring treatments for neurological disorders, a deeper exploration of the relationship between RQC and these conditions is crucial. This review will explore the current research landscape of neurological disease perspectives and the available literature on the molecular mechanisms of RQC in mammals. By providing a comprehensive and inclusive description of the current understanding of this topic, we aim to guide future research efforts.
RQC System
It is now understood that ribosomes stalled due to malfunction or collisions are recognized by specific quality control factors, triggering the RQC surveillance pathway. RQC comprises a multi-step process involving multiple enzymes and protein factors, including Zinc Finger Protein 598 (ZNF598), Pelota, HBS1-like translational GTPase (Hbs1L), Listerin, nuclear export mediator factor (NEMF), Ankyrin Repeat and Zinc Finger Peptidyl tRNA Hydrolase 1 (ANKZF1), p97, and other cofactors (12,13). During this process, translation is halted, incomplete nascent peptides are released and degraded, defective mRNAs are eliminated, followed by the occupied ribosomes recycling (14,15). RQC acts as a self-protective system, degrading aberrant proteins and rescuing stalled ribosomes as early as during translation itself.
Under exposure to harmful internal or external stimuli, RQC regulates processes such as ribosome biogenesis, transport, and degradation. These processes can be divided into 3 consecutive steps (14). The first step, known as ribosome rescue to primarily rescue stalled ribosomes. The subsequent step targets stalled ribosomes and mediates their dissociation into ribosomal subunits. The final step involves recognition of the aberrant complex, the 60S subunit with the nascent peptidyl-tRNA, ultimately triggering the ubiquitination degradation pathway of the aberrant complex. This process not only releases the free, translationally competent 60S ribosomal subunit but also triggers the proteolysis of nascent chains (Figure 2). While some studies propose that RQC starts after ribosomal subunit splitting (1), we posit that RQC encompasses the entire process, beginning with ribosome stalling recognition. However, ubiquitination of the aberrant complex containing the 60S subunit and the nascent peptidyl-tRNA undeniably represents a critical and decisive step within RQCs.
Stalled ribosome rescue and RQC pathways. Ribosomal stalling, whether occurring internally within an mRNA or at its 3 termini, is sensed by distinct mechanisms. However, both sensing systems converge to trigger ribosome splitting, ultimately leaving 60S subunits obstructed with peptidyl-tRNA. RQC = ribosome-associated quality control; tRNA = transfer RNA.
RQC encompasses 2 major pathways: canonical RQC (cRQC) and initiation RQC (iRQC). In eukaryotes, the cRQC pathway entails NEMF binding to the 60S subunit, which then recruits and stabilizes Listerin for the ubiquitination of stalled nascent polypeptide chains on ribosomes (16,17). Nevertheless, the iRQC pathway is triggered by the activation of the integrated stress response or translation initiation inhibitors. This pathway contains a unique E3 ligase, ring finger protein 10 (RNF10), to specifically ubiquitinate a specific site of ribosomal protein and subsequently degrade the 40S subunit (18). Notably, the cRQC pathway targets ribosomes stalled during elongation due to collisions (19), while the iRQC pathway acts near start codons when translation initiation struggles to progress to elongation (18). In the next content, we focus on the cRQC.
Sensing Stalled Ribosome and Splitting Ribosomal Subunits
Ribosome stalling is the crucial trigger for the RQC pathway. Various physical, chemical, and biological factors can lead to this problem, with the most common cause being abnormalities in mRNA structure, including pseudoknots, stem-loops, as well as GC-rich regions (20). Slowed and stalled ribosomes cause collisions with trailing ribosomes, forming disomes or trisomes. A subset of these collisions is recognized by the sensor ZNF598. As the first recruited RQC factor, ZNF598 ubiquitinates the specific 40S ribosomal proteins, initiating the recruitment of downstream RQC factors (14,21) and promoting the disassembly of the stalled ribosome. This ZNF598-mediated ribosomal ubiquitination and dissociation is the specific cRQC characteristics. The other ZNF598-independent collision sensor is endothelial differentiation-related factor 1 (EDF1). EDF1 interacts with collided ribosomes, retaining for a long time and thereby preventing new ribosomes binding (22). Persistent stalling increases the recruitment of EDF1-dependent ZNF598 to disomes, triggering the cRQC pathway (23). Therefore, ZNF598 plays a critical role by initiating the cRQC pathway (24), and recruiting downstream factors like the RNA helicase ASCC3, which further promotes subunit dissociation (25). Importantly, ZNF598’s role in ubiquitination and disassembly is specific to collided ribosomes and contributes solely to the cRQC pathway.
Another mechanism for ribosomal splitting involves Pelota, Hbs1L, and ATP binding cassette subfamily E member 1 (ABCE1) and functions independently of collisions (26). This pathway addresses ribosomal stalling at mRNA 3 end, which occur due to stop codon read through or lack of a stop codon without involving collisions with other ribosomes (27). In these scenarios, the Hbs1L-Pelota complex is recognized as identifying stalled ribosomes (28,29). Subsequently, it recruits ABCE1 to facilitate ribosomal splitting into 40S and 60S subunits. Notably, ABCE1 is a canonical recycling factor to dissociate ribosomes (28,29). This mode of resolution for stalled ribosomes, mediated by Hbs1L, is distinct from the ZNF598-dependent pathway triggered by collided ribosomes.
When ribosomes stall at either terminal or internal mRNA, dissociation of the ribosomal subunits occurs. This results in several key events: degradation of the mRNA, recycling small ribosomal subunit, and, importantly, the attaching to the trapped nascent chain within the 60S subunit (30). The faulty mRNA is ultimately degraded by endonucleolytic cleavage (20,31). Meanwhile, the ribosomal subunits are efficiently recycled (19). Subsequently, classic RQC mechanisms are activated to degrade the aberrant peptidyl-tRNA-60S complex (14,21).
Degrading Aberrant Translation Intermediates
Sequential recruitment of RQC factors leads to the partial degradation of incomplete peptides through ubiquitination. After dissociation of the ribosomal subunits, tRNA and folded polypeptide regions are found at different ends of the ribosomal exit tunnel, creating the abnormal peptidyl-tRNA-60S complex (5,14,21). Listerin, an E3 ubiquitin ligase, recognizes this complex for ubiquitin tagging that targets it for degradation (32,33). NEMF, a different element, attaches to the blocked large ribosomal subunit, enlists Listerin, and hinders the 40S subunit from reattaching (5,14,21). If no suitable ubiquitination site exists on the exposed nascent chains, NEMF recruits additional tRNA to label them with carboxy-terminal alanine and threonine tails (CAT tails) (16,33). These CAT tails function as a pushrod, extending the nascent chain and exposing a suitable ubiquitination site through the ribosomal exit channel, ultimately leading to degradation (16). Notably, NEMF is essential for efficient Listerin recruitment and subsequent ubiquitination, regardless of CAT tail presence. The primary function of CAT tails is to facilitate the exposure of lysine residues on the nascent chain, which are critical recognition sites for Listerin-mediated ubiquitination (Figure 3).
The mechanism for the degradation of abnormal nascent chain. The RQC marks the 60S-associated nascent chain for proteasomal degradation through 2 possible mechanisms: Listerin-mediated ubiquitination or NEMF-mediated CAT tails. CAT = C-terminal alanine and threonine; RQC = ribosome-associated quality control.
Eventually, the ubiquitinated chain of newly synthesized proteins is released from the 60S subunit through the action of the AAA + ATPase p97/valosin-containing protein (VCP) and subsequently broken down by the proteasome (34,35). VCP recognizes the ubiquitin tag on the nascent chain, facilitating its extraction for proteasomal degradation. ANKZF1 cleaves the tRNA, separating it from the 60S subunit and inhibiting CAT tails function while releasing the nascent polypeptide. ANKZF1 interacts with VCP, enhancing the effective extraction and transportation of polyubiquitinated nascent strands to the proteasome pathway (36) (Figure 4). After the disassociation of the RQC complex from the 60S subunit, E3 ubiquitin ligases (such as Tom1/HUWE1 E3) additionally aid in the full breakdown of any remaining peptide chains (35).
RQC pathways of degrading aberrant translation intermediates. Ribosomes stalled on the 60S subunit with peptidyl-tRNA bound are recognized by NEMF. NEMF then recruits and stabilizes the binding of the E3 ubiquitin ligase Listerin. Listerin, in turn, mediates the ubiquitylation of the nascent polypeptide chain. Subsequently, ubiquitylated polypeptides are recognized by the VCP (p97) complex, which includes its cofactors. The VCP complex then extracts the nascent polypeptides from the 60S ribosomal subunit after they have been released from the conjugated tRNA by ANKZF1. Finally, these released polypeptides can be degraded by the proteasome. NEMF = nuclear export mediator factor; RQC = ribosome-associated quality control; tRNA = transfer RNA; VCP = valosin-containing protein.
Subtypes of RQC Pathways
The RQC pathway plays a crucial role in resolving ribosome stalling during the process of cotranslational import in both the endoplasmic reticulum (ER) and mitochondria (14). This pathway helps ensure that protein synthesis proceeds smoothly and efficiently in these organelles. By understanding the mechanisms by which the RQC pathway operates in both the ER and mitochondria, researchers can gain valuable insights into how cells maintain protein homeostasis and prevent the accumulation of defective proteins.
Endoplasmic reticulum RQC
Active mammalian RQC may be linked to ER membranes, and a halt in translation on the ER site enhances the attraction of RQC elements to these specific membranes (37). In cell culture models, ER proteins with translational stalling are successfully targeted for degradation by a specialized RQC machinery termed endoplasmic reticulum RQC (ER-RQC) (37,38). Importantly, NEMF is crucial in the polyubiquitination that is ribosome-dependent and the ensuing degradation of these ER proteins (37). While ZNF598, VCP, and the proteasome are also involved in ER-RQC substrate degradation (39), the precise roles of other RQC factors in this pathway remain to be elucidated. Considering the intricate process of protein biogenesis at the interface of the cytoplasm and ER, along with the distinct mechanisms utilized by ER-RQC in contrast to cytosolic RQC, additional investigation into this pathway is necessary.
Mitoribosome-RQC
Mitochondria, with their own genome and ribosomes (mitoribosomes), play essential roles in cellular metabolism and ATP generation. In yeast, RQC mechanisms target heat-shock-induced misfolded proteins for import and degradation within the mitochondria (40). Surprisingly, dysfunctional mitochondria can release reactive oxygen species that impact cytoplasmic translation, emphasizing a possible connection between mitochondrial well-being and cytosolic pathways for RQC (41). When limited translation factors or abnormal mRNA are present in mitochondria, mitoribosomes may stall, triggering the activation of rescue mechanisms. Notably, during mitochondrial protein cotranslational translocation, ribosomal stalling triggers NEMF-dependent addition of CAT tails by the mitoribosome-RQC (mtRQC) machinery (42,43). VCP, a crucial participant, plays a key role in the regulation of mtRQC to ultimately avert the buildup of potentially harmful protein aggregations in the mitochondria (42). Nevertheless, a more thorough comprehension of the precise steps and functions of the mtRQC mechanism is still lacking.
The Link Between RQC Dysfunction and Neurodegenerative Diseases
RQC complexes are essential for resolving stalled ribosomes within cells, including collisions, suggesting that dysfunction of these factors can have severe consequences. Indeed, mounting evidence links RQC dysfunction to neurophysiological impairment and the development of neurological diseases (44) (Table 1). A deeper understanding of how RQC factors coordinate and regulate each other may be key to unraveling the mechanisms underlying these diseases. The toxic effects of ribosome stalling are implicated in neurodegenerative diseases. For example, the protein TDP-43, a hallmark of some neurodegenerative diseases, can interact with the activated ribosomal protein receptor, causing ribosome stalling and subsequent the formation of cytoplasmic vacuoles (45). Similarly, mutant huntingtin protein can cause ribosome stalling, hindering protein synthesis and resulting in a loss of function (46). This interference with normal cellular processes can have serious consequences for cell health and function. Additionally, mutations in other factors involved in the RQC pathway could potentially lead to the accumulation of toxic protein aggregates. These aggregates can have a “gain-of-function” characteristic, which is commonly associated with degenerative diseases. The build-up of these aggregates can overwhelm the cell’s natural clearance mechanisms, leading to further cellular dysfunction and potentially contributing to disease progression. Therefore, understanding the impact of mutations in RQC factors and how they contribute to protein aggregation could provide important insights into the development and progression of degenerative diseases.
Studies of RQC involved in neurodegenerative diseases
| Diseases | Evidences from patients | Evidences from model organisms | Evidences from cell lines |
|---|---|---|---|
| AD | ZNF598, NEMF, and ANKZF1 deposited surrounding an amyloid plaque core. | The mice with Listerin allele mutation show accumulation of hyperphosphorylated tau protein. | The continuous collision of ribosomes caused by stalled APP/APP.C99 translation. |
| HD | LISTERIN is differentially expressed in Huntington’s disease. | Deficiency of LISTERIN leads to cells being unable to properly tag full-length mHTT with ubiquitin. | |
| ALS | Pelota protein expression is lower in C9ORF72-ALS/FTD patient-derived iPSN. Missense mutation of ZNF598 caused ALS. | ZNF598 deletion can also enhance the progression of C9ORF72-ALS/FTD in Drosophila. | Depletion of NEMF, LISTERIN, or ANKZF1 increases the accumulation of RAN translation products and aggravates toxicity. |
| PD | ABCE1 and HBS1L were significantly downregulated in PD brain tissues. | In Drosophila, the effectiveness of rescuing PINK1 was observed through increasing the activity of ABCE1 and other RQC factors, or by inhibiting the activity of NEMF. |
| Diseases | Evidences from patients | Evidences from model organisms | Evidences from cell lines |
|---|---|---|---|
| AD | ZNF598, NEMF, and ANKZF1 deposited surrounding an amyloid plaque core. | The mice with Listerin allele mutation show accumulation of hyperphosphorylated tau protein. | The continuous collision of ribosomes caused by stalled APP/APP.C99 translation. |
| HD | LISTERIN is differentially expressed in Huntington’s disease. | Deficiency of LISTERIN leads to cells being unable to properly tag full-length mHTT with ubiquitin. | |
| ALS | Pelota protein expression is lower in C9ORF72-ALS/FTD patient-derived iPSN. Missense mutation of ZNF598 caused ALS. | ZNF598 deletion can also enhance the progression of C9ORF72-ALS/FTD in Drosophila. | Depletion of NEMF, LISTERIN, or ANKZF1 increases the accumulation of RAN translation products and aggravates toxicity. |
| PD | ABCE1 and HBS1L were significantly downregulated in PD brain tissues. | In Drosophila, the effectiveness of rescuing PINK1 was observed through increasing the activity of ABCE1 and other RQC factors, or by inhibiting the activity of NEMF. |
Notes: AD = Alzheimer’s disease; ALS = amyotrophic lateral sclerosis; C9ORF72-ALS/FTD = C9ORF72-amyotrophic lateral sclerosis with frontotemporal lobar degeneration; HD = Huntington’s disease; iPSN = induced pluripotent stem cell differentiated neurons; NEMF = nuclear export mediator factor; PD = Parkinson’s disease; RQC = ribosome-associated quality control.
Studies of RQC involved in neurodegenerative diseases
| Diseases | Evidences from patients | Evidences from model organisms | Evidences from cell lines |
|---|---|---|---|
| AD | ZNF598, NEMF, and ANKZF1 deposited surrounding an amyloid plaque core. | The mice with Listerin allele mutation show accumulation of hyperphosphorylated tau protein. | The continuous collision of ribosomes caused by stalled APP/APP.C99 translation. |
| HD | LISTERIN is differentially expressed in Huntington’s disease. | Deficiency of LISTERIN leads to cells being unable to properly tag full-length mHTT with ubiquitin. | |
| ALS | Pelota protein expression is lower in C9ORF72-ALS/FTD patient-derived iPSN. Missense mutation of ZNF598 caused ALS. | ZNF598 deletion can also enhance the progression of C9ORF72-ALS/FTD in Drosophila. | Depletion of NEMF, LISTERIN, or ANKZF1 increases the accumulation of RAN translation products and aggravates toxicity. |
| PD | ABCE1 and HBS1L were significantly downregulated in PD brain tissues. | In Drosophila, the effectiveness of rescuing PINK1 was observed through increasing the activity of ABCE1 and other RQC factors, or by inhibiting the activity of NEMF. |
| Diseases | Evidences from patients | Evidences from model organisms | Evidences from cell lines |
|---|---|---|---|
| AD | ZNF598, NEMF, and ANKZF1 deposited surrounding an amyloid plaque core. | The mice with Listerin allele mutation show accumulation of hyperphosphorylated tau protein. | The continuous collision of ribosomes caused by stalled APP/APP.C99 translation. |
| HD | LISTERIN is differentially expressed in Huntington’s disease. | Deficiency of LISTERIN leads to cells being unable to properly tag full-length mHTT with ubiquitin. | |
| ALS | Pelota protein expression is lower in C9ORF72-ALS/FTD patient-derived iPSN. Missense mutation of ZNF598 caused ALS. | ZNF598 deletion can also enhance the progression of C9ORF72-ALS/FTD in Drosophila. | Depletion of NEMF, LISTERIN, or ANKZF1 increases the accumulation of RAN translation products and aggravates toxicity. |
| PD | ABCE1 and HBS1L were significantly downregulated in PD brain tissues. | In Drosophila, the effectiveness of rescuing PINK1 was observed through increasing the activity of ABCE1 and other RQC factors, or by inhibiting the activity of NEMF. |
Notes: AD = Alzheimer’s disease; ALS = amyotrophic lateral sclerosis; C9ORF72-ALS/FTD = C9ORF72-amyotrophic lateral sclerosis with frontotemporal lobar degeneration; HD = Huntington’s disease; iPSN = induced pluripotent stem cell differentiated neurons; NEMF = nuclear export mediator factor; PD = Parkinson’s disease; RQC = ribosome-associated quality control.
Alzheimer’s Disease
AD is a chronic, progressive neurodegenerative condition marked by deteriorating memory and cognitive function. It is the prevalent type of dementia in elderly individuals. A 2022 study shows around 6.5 million Americans over the age of 65 have been diagnosed with AD, and estimates predict a surge to 13.8 million by 2060 (47). AD is neuropathologically characterized by the build-up of plaques made of amyloid-beta (Aβ) and anomalous clumps of tau protein that are hyperphosphorylated. Mounting evidence suggests a link between AD and RQC pathways. Defects in RQC can lead to the aggregation of amyloid protein (48), the source of Aβ. Although Aβ has typically been viewed as the primary neurotoxic factor, new research indicates that the C-terminal segment of APP (APP.C99) could be of greater significance (49). The mechanism behind this involves the stalling of ER ribosomes during the translocation of APP.C99 at the membrane, leading to the activation of the cGAS-STING pathway, a response to DNA damage. This exaggerated activation, consequently, results in neuroinflammation and ultimately promotes AD development (50).
RQC dysfunction and stalled translation are emerging as possible contributors to AD pathogenesis. A study identified RQC factors, including ZNF598, NEMF, ANKZF1, and Rack1, deposited in the center of amyloid plaques within brain tissue from AD patients (10). Furthermore, mouse models harboring a Listerin allele mutation exhibit an accumulation of hyperphosphorylated tau protein (51), a hallmark specifically associated with AD. One potential mechanism involves the persistence of collided ribosomes, leading to the accumulation of nascent peptides tagged with CAT tails without subsequent degradation. This accumulation may induce cellular cytotoxicity (52), including in brain cells and eventually lead to AD (10). Another potential mechanism by which colliding ribosomes might contribute to AD involves the activation of stress response pathways (53,54). Further studies are needed to elucidate the precise role of targeted collision ribosomes in AD pathogenesis.
Huntington’s Disease
HD presents with psychological, cognitive, and movement impairments. This degenerative condition is linked to variations and expansion of CAG trinucleotide repeats in exon 1 of the Huntingtin (HTT) gene (55). These genetic alterations lead to an abnormal extension of the polyglutamine (polyQ) region in the mutated N-terminal HTT protein, causing it to aggregate (56). An essential pathological feature of HD is the presence of intracellular aggregates composed of mutated Huntingtin (mHTT) protein (57).
HTT is a multifunctional protein implicated in vital cellular processes, including vesicular transport, synaptic transmission, and autophagy (58). However, the mutated form of HTT disrupts its normal function and inhibits protein synthesis by slowing ribosomal translocation. Notably, mHTT binds to ribosomes and specifically affects the translocation of numerous target messenger RNAs (mRNAs) (46). This abnormal interaction hinders protein synthesis and interferes with ribosomal translocation, resulting in ribosome stalling during translation elongation. As a result, mHTT serves as a strong inhibitor of translation elongation rates, leading to persistent ribosome stalling that causes harmful collisions and disrupted protein synthesis. Interestingly, studies indicate that LISTERIN expression is changed in individuals with HD (59). Additionally, LISTERIN deficiency impairs the proper tagging of full-length mHTT with ubiquitin by cellular machinery (9). LISTERIN plays a crucial role in the formation of mHTT inclusion bodies and its subsequent ubiquitination, which leads to detoxification through a Tae2/Hsf1-dependent sequestration pathway (9). The exact contribution of mHTT to HD pathogenesis is not yet fully understood, highlighting the need for additional research to explore a possible connection between HD and RQC pathway.
Amyotrophic Lateral Sclerosis
ALS is a heterogeneous neurodegenerative disease that primarily affects motor neurons with progressive muscle weakness, respiratory failure, and most death within 5 years. The incidence of ALS ranges from 2 to 3/100 000 and its prevalence reaches approximately 5.2 per 100 000 population (60). Although the basic pathophysiological mechanism of ALS is currently unknown, the neuropathology is characterized with the deposition of ubiquitination protein inclusions in motor neurons (61).
The expansion of intronic G4C2 hexanucleotide repeats within C9ORF72 is the most frequent genetic mutation observed in familial cases of amyotrophic lateral sclerosis with frontotemporal lobar degeneration (ALS/FTD) (62). The G4C2 repeats have the propensity to form complex secondary structures and encode repeat amino acids, resulting in the aggregation of dipeptide repeat (DPR) proteins. More importantly, the G4C2 repeat may potentially induce ribosome collisions by inhibiting ribosomal elongation, which eventually initiates the RQC pathway (63). Dysfunction of RQC pathways may increase DPR expression and aggregation, which eventually stimulate cellular stress and toxicity (64). Despite initiating the RQC pathway, these DPRs, which lack lysine residues, are unable to be efficiently ubiquitinated by the RQC complexes, LISTERIN and NEMF (64). Interestingly, the Pelota and ZNF598 RQC factors can modulate translation dynamics through repeat RNA sequences (65). In vitro, it was found that Pelota protein expression is diminished in C9ORF72-ALS/FTD patient-derived induced pluripotent stem cell (iPSC)-differentiated neurons, potentially contributing to elevated DPR expression and disease exacerbation (65). Moreover, ZNF598 deletion can also enhance the progression of C9ORF72-ALS/FTD in Drosophila (66); meanwhile, depletion of NEMF, LISTERIN, or ANKZF1 exacerbates the abnormal products aggregation and aggravates toxicity (44). Overexpression of RQC factors restrains the expression of DPR, especially poly (GR), whereas knockdown of these factors exacerbates abnormal CAT tails amplification (67). These results support that the RQC factors are potent modifiers that restrain DPR expression in C9ORF72-associated ALS/FTD, whereas insufficient RQC contributes to its occurrence and development.
Sequencing of an ALS cohort confirmed a missense variant in ZNF598 is a genetic cause for ALS (68). Additionally, some ALS cases may harbor genetic variants in RNA-binding proteins (RBPs) (69,70). One possible explanation for this association is that RBPs play a critical role in regulating translation by recruiting essential regulatory components (71). While the precise contribution of RQC dysfunction to ALS pathogenesis remains unclear, it is possible that such dysfunction increases the risk of developing ALS.
Parkinson’s Disease
Parkinson’s disease (PD) is a prevalent and devastating neurodegenerative movement disorder with a worldwide prevalence ranging from 4971 to 7325 per 100 000 individuals (72). It shares some clinical features with other neurodegenerative conditions. The main manifestation of PD is steadily progressive movement dysfunction, accompanied by numerous nonmotor symptoms such as affective disorders, psychosis, and cognitive impairment. The classical biological features of PD are the abnormal depositions of α-synuclein, along with degeneration of dopaminergic neurons in the substantia nigra area, a region of the midbrain critical for motor control. Hence, it is not surprising that PD is also classified as a synucleinopathy.
Biallelic mutations in Pten-induced kinase 1 (PINK1) or Parkin (PRKN) are known to cause familial PD. Notably, PINK1 and PRKN are considered core components of the mtRQC pathway (73), whose main role is to promote mitochondrial complex-I subunit expression during the cotranslational translocation process (74). This mitochondrial complex-I subunit undergoes a CAT tail-like modification which hint an active function of mtRQC pathway. Further experiments suggest that PINK1 can effectively rescue the neuromuscular degeneration observed in PINK1-deficient Drosophila by enhancing the activity of RQC factors such as ABCE1 or inhibiting the activity of NEMF (75). The mRNA levels with ABCE1 and HBS1L were significantly down-regulated in brain tissue from patients with PD (73). This selective downregulation of RQC pathway-related genes highlights the importance of RQC failure in contributing to PD pathogenesis. Overall, failure of the RQC pathway could mediate CAT tail-like modifications of mitochondrial proteins, impairing proteostasis and ultimately leading to the development of PD.
Schizophrenia
Schizophrenia (SCZ) is a complex syndrome characterized with a myriad of clinical manifestations. These manifestations can be broadly categorized into positive and negative symptoms. Positive symptoms include delusions and hallucinations, while negative symptoms encompass apathy and diminished emotional expression. The etiology of SCZ is complex, impacting by both genetic and environmental factors. The precise mechanisms underlying its pathogenesis remain unclear. However, recent research suggests that dysregulation of translational control may contribute to the development of SCZ (76). A study identified a variant in the LISTERIN increased the risk of SCZ (77). Furthermore, LISTERIN expression levels were reduced in SCZ patients. These observations support the theory that LISTERIN involved in SCZ pathogenesis, potentially by impacting translation and thereby affecting neural activity (78).
Translational Regulation and RQC in Neurodevelopmental Disorders
Neurons exhibit high sensitivity to aberrant translation products due to their limited repertoire of translational RQC mechanisms. This sensitivity arises from the potential for accumulation of these abnormal products when the machinery responsible for rescuing stalled ribosomes or processing nascent peptide chains is compromised. Inappropriate RQC can result in the aggregation of misfolded protein deposits, which can be highly toxic to neurons. The following section highlights specific genes encoding proteins critical for RQC function. Mutations of these genes is proven to link with diverse neurodevelopmental disorders (NDDs, as detailed in Table 2).
RQC factors in neurological disorders
| Protein | Function | Relevance to neurological disorders |
|---|---|---|
| LISTERIN | E3 ubiquitin ligase, tags nascent-chain with ubiquitin to degrade by the proteasome | Listerin variant mice exhibit severe signs of peripheral neuropathies and motor neuron diseases. Listerin knockout mice display phenotypes associated with cognitive disorders and pathology characterizes by the overaccumulation of TTC3. LISTERIN is differentially expressed in HD. LISTERIN expression was reduced in the peripheral blood of patients with schizophrenia. |
| NEMF | Recruits Listerin and adds CAT tails. | Pathogenic NEMF variants associated with intellectual disability or/and axonal polyneuropathy. Variants in Nemf result in neurological defects in muscle and neuromuscular junctions, as well as a shortened life span in mice. The impairment of CAT tails has been shown to correlate with the severity of the phenotype. CAT tails are also associated with AD and AP-C99. Furthermore, NEMF has been identified in amyloid plaque in AD. |
| GTPBP2 | Ribosome recycling factor | GTPBP2 variants caused a complex syndromic NDDs associated with profound neurodevelopmental impairment, intellectual disability, epilepsy, choreiform movement disorder, and ectodermal abnormalities, which are accompanied by pathognomonic craniofacial features. |
| ABCE1 | ATPase that splits 80S into 40S and 60S | Low levels of ABCE1 mRNA were observed in PD brain tissue. Overexpression of ABCE1 in a Drosophila PD model of Pink1 led to the rescue of mitochondrial aggregation and neuronal loss. |
| HBS1L | Recognizes stalled ribosomes at the 3-end of mRNA and splits the subunits together with Pelota | Low expression levels of HBS1L mRNA are observed in PD brain tissue. Pathogenic HBS1L variants associated with severe intrauterine growth restriction, global developmental delay, hypotonia, retinal pigmentary deposits and multiple deformity. Deletion of Hbs1L in mice resulted in embryonic development failure, while mice with the Hbs1Ltm1a/tm1a genotype displayed various NDDs phenotypes such as growth restriction, facial dysmorphism, and retinal pigmentation deposits. |
| Protein | Function | Relevance to neurological disorders |
|---|---|---|
| LISTERIN | E3 ubiquitin ligase, tags nascent-chain with ubiquitin to degrade by the proteasome | Listerin variant mice exhibit severe signs of peripheral neuropathies and motor neuron diseases. Listerin knockout mice display phenotypes associated with cognitive disorders and pathology characterizes by the overaccumulation of TTC3. LISTERIN is differentially expressed in HD. LISTERIN expression was reduced in the peripheral blood of patients with schizophrenia. |
| NEMF | Recruits Listerin and adds CAT tails. | Pathogenic NEMF variants associated with intellectual disability or/and axonal polyneuropathy. Variants in Nemf result in neurological defects in muscle and neuromuscular junctions, as well as a shortened life span in mice. The impairment of CAT tails has been shown to correlate with the severity of the phenotype. CAT tails are also associated with AD and AP-C99. Furthermore, NEMF has been identified in amyloid plaque in AD. |
| GTPBP2 | Ribosome recycling factor | GTPBP2 variants caused a complex syndromic NDDs associated with profound neurodevelopmental impairment, intellectual disability, epilepsy, choreiform movement disorder, and ectodermal abnormalities, which are accompanied by pathognomonic craniofacial features. |
| ABCE1 | ATPase that splits 80S into 40S and 60S | Low levels of ABCE1 mRNA were observed in PD brain tissue. Overexpression of ABCE1 in a Drosophila PD model of Pink1 led to the rescue of mitochondrial aggregation and neuronal loss. |
| HBS1L | Recognizes stalled ribosomes at the 3-end of mRNA and splits the subunits together with Pelota | Low expression levels of HBS1L mRNA are observed in PD brain tissue. Pathogenic HBS1L variants associated with severe intrauterine growth restriction, global developmental delay, hypotonia, retinal pigmentary deposits and multiple deformity. Deletion of Hbs1L in mice resulted in embryonic development failure, while mice with the Hbs1Ltm1a/tm1a genotype displayed various NDDs phenotypes such as growth restriction, facial dysmorphism, and retinal pigmentation deposits. |
Notes: AD = Alzheimer’s disease; CAT = C-terminal alanine and threonine; HD = Huntington’ disease; NDDs = neurodevelopmental disorders; NEMF = nuclear export mediator factor; PD = Parkinson’s disease; RQC = ribosome-associated quality control.
RQC factors in neurological disorders
| Protein | Function | Relevance to neurological disorders |
|---|---|---|
| LISTERIN | E3 ubiquitin ligase, tags nascent-chain with ubiquitin to degrade by the proteasome | Listerin variant mice exhibit severe signs of peripheral neuropathies and motor neuron diseases. Listerin knockout mice display phenotypes associated with cognitive disorders and pathology characterizes by the overaccumulation of TTC3. LISTERIN is differentially expressed in HD. LISTERIN expression was reduced in the peripheral blood of patients with schizophrenia. |
| NEMF | Recruits Listerin and adds CAT tails. | Pathogenic NEMF variants associated with intellectual disability or/and axonal polyneuropathy. Variants in Nemf result in neurological defects in muscle and neuromuscular junctions, as well as a shortened life span in mice. The impairment of CAT tails has been shown to correlate with the severity of the phenotype. CAT tails are also associated with AD and AP-C99. Furthermore, NEMF has been identified in amyloid plaque in AD. |
| GTPBP2 | Ribosome recycling factor | GTPBP2 variants caused a complex syndromic NDDs associated with profound neurodevelopmental impairment, intellectual disability, epilepsy, choreiform movement disorder, and ectodermal abnormalities, which are accompanied by pathognomonic craniofacial features. |
| ABCE1 | ATPase that splits 80S into 40S and 60S | Low levels of ABCE1 mRNA were observed in PD brain tissue. Overexpression of ABCE1 in a Drosophila PD model of Pink1 led to the rescue of mitochondrial aggregation and neuronal loss. |
| HBS1L | Recognizes stalled ribosomes at the 3-end of mRNA and splits the subunits together with Pelota | Low expression levels of HBS1L mRNA are observed in PD brain tissue. Pathogenic HBS1L variants associated with severe intrauterine growth restriction, global developmental delay, hypotonia, retinal pigmentary deposits and multiple deformity. Deletion of Hbs1L in mice resulted in embryonic development failure, while mice with the Hbs1Ltm1a/tm1a genotype displayed various NDDs phenotypes such as growth restriction, facial dysmorphism, and retinal pigmentation deposits. |
| Protein | Function | Relevance to neurological disorders |
|---|---|---|
| LISTERIN | E3 ubiquitin ligase, tags nascent-chain with ubiquitin to degrade by the proteasome | Listerin variant mice exhibit severe signs of peripheral neuropathies and motor neuron diseases. Listerin knockout mice display phenotypes associated with cognitive disorders and pathology characterizes by the overaccumulation of TTC3. LISTERIN is differentially expressed in HD. LISTERIN expression was reduced in the peripheral blood of patients with schizophrenia. |
| NEMF | Recruits Listerin and adds CAT tails. | Pathogenic NEMF variants associated with intellectual disability or/and axonal polyneuropathy. Variants in Nemf result in neurological defects in muscle and neuromuscular junctions, as well as a shortened life span in mice. The impairment of CAT tails has been shown to correlate with the severity of the phenotype. CAT tails are also associated with AD and AP-C99. Furthermore, NEMF has been identified in amyloid plaque in AD. |
| GTPBP2 | Ribosome recycling factor | GTPBP2 variants caused a complex syndromic NDDs associated with profound neurodevelopmental impairment, intellectual disability, epilepsy, choreiform movement disorder, and ectodermal abnormalities, which are accompanied by pathognomonic craniofacial features. |
| ABCE1 | ATPase that splits 80S into 40S and 60S | Low levels of ABCE1 mRNA were observed in PD brain tissue. Overexpression of ABCE1 in a Drosophila PD model of Pink1 led to the rescue of mitochondrial aggregation and neuronal loss. |
| HBS1L | Recognizes stalled ribosomes at the 3-end of mRNA and splits the subunits together with Pelota | Low expression levels of HBS1L mRNA are observed in PD brain tissue. Pathogenic HBS1L variants associated with severe intrauterine growth restriction, global developmental delay, hypotonia, retinal pigmentary deposits and multiple deformity. Deletion of Hbs1L in mice resulted in embryonic development failure, while mice with the Hbs1Ltm1a/tm1a genotype displayed various NDDs phenotypes such as growth restriction, facial dysmorphism, and retinal pigmentation deposits. |
Notes: AD = Alzheimer’s disease; CAT = C-terminal alanine and threonine; HD = Huntington’ disease; NDDs = neurodevelopmental disorders; NEMF = nuclear export mediator factor; PD = Parkinson’s disease; RQC = ribosome-associated quality control.
HBS1L Variants
HBS1L, belonging to the GTP-binding elongation factor family, encodes a specialized ribosomal rescue factor. It interacts with Pelota to dissociate ribosomes into subunits (79). HBS1L is essential for embryonic development, as its deletion results in early embryonic lethality (80). The pathogenic mechanism underlying this lethality involves a tissue-specific reduction in Pelota and EDF1 due to HBS1L deletion. This reduction disrupts the cellular response to ribosome collisions, alters transcription regulation with an elevation of 80S ribosomes, and leads to continuous ribosome stalling during mRNA translation (80). Recent studies have identified pathogenic variants in HBS1L as an ultra-rare cause of NDDs. Hbs1Ltm1a/tm1a mice, a model for HBS1L deficiency, exhibit several NDD phenotypes, including growth restriction, dysmorphic facial features, and retinal abnormalities (81). In humans, only one case with compound heterozygous HBS1L mutations has been reported. This 3-year-old girl presented with severe developmental delay, retinal pigmentary deposits, and multiple congenital malformations (including microcephaly, lax joints, spinal deformity, and bladder diverticula) (82). The complete function of HBS1L has not been fully elucidated. Substantial investigations are necessary to explore the precise physiological function and the downstream consequences of its dysfunction.
GTP Binding Protein 1 and GTP Binding Protein 2 Variants
GTP binding protein 1 (GTPBP1) and GTP binding protein 2 (GTPBP2) encode guanosine triphosphate (GTP)-binding proteins that belong to the GTPase superfamily. These proteins share a high degree of homology, exhibiting 68% amino acid sequence similarity. They contribute to various types of ribosomal homeostasis, while their deficiency leads to codon-specific ribosome pausing (83).
Homozygous GTPBP1 or GTPBP2 mutations cause NDDs characterized by profound neurodevelopmental impairment, intellectual disability, epilepsy, movement disorders, and facial dysmorphisms (84–86). This distinct syndrome has been designated GTPBP1/2-related ectodermal neurodevelopmental (GREND) syndrome (84). However, the precise molecular mechanisms underlying these pathologies remain to be elucidated. Functional studies have revealed that GTPBP1 is essential for alleviating ribosome stalling and maintaining neuronal homeostasis (87). Meanwhile, GTPBP2 interacts with a critical partner, Pelota, and plays a vital role in regulating neuronal function (88). Interestingly, rare GTPBP2 variants cause neurogenetic conditions with brain iron accumulation (86,89,90). A surprising discovery emerged from a mouse model study. Inhibition of GTPBP2 activity exacerbated a heterozygous variant in a tRNA synthase gene known to cause Charcot-Marie-Tooth peripheral neuropathy (91). This finding suggests that GTPBP2 inactivation may contribute to the progression of peripheral nerve dysfunction. Therefore, the mechanisms of abnormal or insufficient levels of GTPBP1 and GTPBP2 contribute to brain developmental disorders need further study. Additionally, investigations into the impact of aberrant translational regulation on central nervous system function are warranted.
NEMF Variants
NEMF, another critical factor within the RQC pathway, functions in concert with its partner, LISTERIN. The primary role of NEMF is to recruit LISTERIN to nascent peptidyl-tRNA-60S complexes. This ensures that the stalled nascent polypeptide is properly positioned for ubiquitination by LISTERIN. In the absence of functional LISTERIN, NEMF itself promotes the degradation of stalled polypeptides through a mechanism dependent on its C-terminal alanine tail sequences (17).
NEMF plays an instructive role in nervous system development and neuron differentiation in Drosophila (92). Unfortunately, its precise biological function in mammals remains poorly understood. While limited research exists, some studies have begun to demonstrate its potential role in human health. NEMF is considered a risk gene associated with cognitive abnormalities (93). Additionally, other separate investigations have reported associations between variants in human NEMF and cognitive abnormalities combined with axonal polyneuropathy (8,11). Further research using primary cultured mouse cortical neurons revealed that NEMF deficiency restricts axonal elongation and synaptic development (11). Furthermore, studies in mice harboring Nemf variants have shown neurological defects in muscles and neuromuscular junctions, along with a shortened lifespan (8). It points avenues for future explorations to explore the detailed changes that occur within the central nervous system due to NEMF deficiency.
LISTERIN Variants
LISTERIN, an E3 ubiquitin ligase, occupied a core position in the RQC pathway by ubiquitinating nascent polypeptides, thereby targeting them for degradation and attenuating the accumulation of protein aggregates. Deficiency of LISTERIN impairs the efficient ubiquitination of nascent chains, rendering them prone to aggregation (94).
ENU-induced Listerin mutation in mice results in a rapid onset of severe peripheral neuropathies and motor neuron diseases within 3 weeks of age (51). Listerin knockout mice exhibit cognitive impairment and accumulate tetratricopeptide repeat domain 3 (TTC3). This accumulation is reversed by TTC3 knockdown (95). On the other hand, it is suggested that abnormal LISTERIN can lead to proteotoxic stress. These suggest that pathogenic variants in LISTERIN could be associated with a group of diseases affecting both the peripheral and central nervous systems. However, LISTERIN has not been implicated in human disease.
Advances in the Treatment of Neurological Disorders
A recurring theme in the above research is that abnormalities in the RQC pathway result in ribosome stalling, which cannot be cleared in time. This causes translation disruption, protein misfolding, misassembly, and aggregation, ultimately dysregulating proteostasis and contributing to neurological disorders. Interestingly, there is growing interest in targeting the RQC complex as a potential therapeutic avenue to alleviate these deficiencies.
Repression of transcription or translation of toxic polypeptides represents an attractive option. Abnormal protein deposits and inclusion body formation are hallmarks of many neurodegenerative diseases, including Aβ plaques, tau protein, and TDP-43. Significantly, research has confirmed that RQC complex is very important for maintaining protein homeostasis (43). Dysfunction of RQC pathway provides a potential link between inclusion bodies and the development of neurological disorders. Consequently, targeting the removal of inclusion bodies has emerged as a promising therapeutic strategy. Under persistent proteotoxic stress conditions, the re-ubiquitination of nascent protein products stalled on cytosolic ribosomes becomes inefficient, ultimately leading to their intracellular accumulation. Interestingly, research suggests that targeting cytoplasmic deubiquitinases may offer a therapeutic approach to promote the degradation of nascent peptide chains, thereby preventing their aggregation and reducing their toxicity.
Another potential therapeutic target lies in eukaryotic translation initiation factor 2 subunit alpha (eIF2α). This factor becomes activated in various RQC-associated dysfunctions (96,97). Targeting the phosphorylation of eIF2α, a key translation initiation factor, offers a potential 2-fold benefit: relieving unwanted inhibition of translation and minimizing the production of harmful RAN translation products (98). This strategy was effectively demonstrated in a cellular model of C9ORF72 hexanucleotide expansion using GSK260641 and ISRIB, drugs that inhibit downstream signaling events triggered by protein kinase RNA-like endoplasmic reticulum kinase (PERK) and phosphorylated-eIF2α (99). However, further research is necessary to determine whether these compounds can ultimately exert a protective effect at the cellular level and demonstrate therapeutic efficacy in vivo against diseases characterized by abnormal eIF2α activation.
In addition to the aforementioned strategies targeting RQC dysfunction in neurological diseases, progress has also been made in utilizing novel technologies. Researchers are exploring the application of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) technology to reduce repeat sequences or downregulate the levels of expansion transcripts (100).
Conclusion
This review examines the intricate interactions between RQC and neurological disorders. As a critical cellular quality control mechanism, RQC is involved in both rescuing stalled ribosomes and degrading aberrant proteins. However, the molecular connections between RQC dysfunction and neurodegeneration remain complex. Despite emerging evidence suggesting that the RQC pathway plays a crucial role in regulating neurological functions, particularly in neurogenetic disorders, a comprehensive understanding of these processes remains elusive.
Further research is necessary to elucidate the specific mechanisms through which RQC dysfunction impacts the nervous system. A deeper understanding of RQC and its downstream signaling pathways may provide valuable insights into the pathogenesis of neurological disorders. Despite existing knowledge gaps, the therapeutic potential of the RQC pathway is significant. Future efforts should focus on clarifying its role in the pathogenesis of neurological diseases to develop new therapeutic strategies and interventions.
Funding
This review was supported by the grants from the National Natural Science Foundation of China (82001222 to X.C.); the Innovation Research Foundation of China International Medical Exchange Foundation (Z-201-20-1801 to X.C.); and the Shanxi Science and Technology Department (202103021223437 to J.W., 202203021212037 to R.Z.).
Conflict of Interest
None.
Acknowledgments
We thank Home for Researchers editorial team (www.home-for-researchers.com) for language editing service.
Author Contributions
X.C., W.Z., R.Z., and J.G. designed the study; J.W. and J.W. wrote the original draft; H.C., Y.X., Z.W., and J.M. drew illustration; X.C. wrote the final manuscript. All authors approved the final manuscript and the submission to this journal.
Consent for Publication
All authors agree with the content of the manuscript, and consent to the publication.
