Survival motor neuron protein facilitates assembly of stress granules

Tudor-Containing Methyl-Lysine and Methyl-Arginine Reader Proteins: Disease Implications and Chemical Tool Development

Tudor domains are histone readers that can recognize various methylation marks on lysine and arginine. This recognition event plays a key role in the recruitment of other epigenetic effectors and the control of gene accessibility. The Tudor-containing protein family contains 42 members, many of which are involved in the development and progression of various diseases, especially cancer. The development of chemical tools for this family will not only lead to a deeper understanding of the biological functions of Tudor domains but also lay the foundation for therapeutic discoveries. In this review, we discuss the role of several Tudor domain-containing proteins in a range of relevant diseases and progress toward the development of chemical tools such as peptides, peptidomimetics, or small-molecules that bind Tudor domains. Overall, we highlight how Tudor domains are promising targets for therapeutic development and would benefit from the development of novel chemical tools.

Tudor-dimethylarginine interactions: the condensed version

Biomolecular condensates (BMCs) can facilitate or inhibit diverse cellular functions. BMC formation is driven by noncovalent protein-protein, protein-RNA, and RNA-RNA interactions. Here, we focus on Tudor domain-containing proteins - such as survival motor neuron protein (SMN) - that contribute to BMC formation by binding to dimethylarginine (DMA) modifications on protein ligands. SMN is present in RNA-rich BMCs, and its absence causes spinal muscular atrophy (SMA). SMN's Tudor domain forms cytoplasmic and nuclear BMCs, but its DMA ligands are largely unknown, highlighting open questions about the function of SMN. Moreover, DMA modification can alter intramolecular interactions and affect protein localization. Despite these emerging functions, the lack of direct methods of DMA detection remains an obstacle to understanding Tudor-DMA interactions in cells.

Stress granules dynamics and promising functions in pancreatic cancer

Stress granules (SGs), non-membrane subcellular organelles made up of non-translational messenger ribonucleoproteins (mRNPs), assemble in response to various environmental stimuli in cancer cells, including pancreatic cancer, particularly pancreatic ductal adenocarcinoma (PDAC) which has a low 5-year survival rate of 10%. The pertinent research on SGs and pancreatic cancer has not, however, been compiled. In this review, we talk about the dynamics of SGs and their positive effects on pancreatic cancer such as SGs promote PDAC viability and repress apoptosis, meanwhile emphasizing the connection between SGs in pancreatic cancer and signature mutations such KRAS, P53, and SMAD4 as well as the functions of SGs in antitumor drug resistance. This novel stress management technique may open the door to better treatment options in the future.

Stress granules in the spinal muscular atrophy and amyotrophic lateral sclerosis: The correlation and promising therapy

Increasing genetic and biochemical evidence has broadened our view of the pathomechanisms that lead to Spinal muscular atrophy (SMA) and Amyotrophic lateral sclerosis (ALS), two fatal neurodegenerative diseases with similar symptoms and causes. Stress granules are dynamic cytosolic storage hubs for mRNAs in response to stress exposures, that are evolutionarily conserved cytoplasmic RNA granules in somatic cells. A lot of previous studies have shown that the impaired stress granules are crucial events in SMA/ALS pathogenesis. In this review, we described the key stress granules related RNA binding proteins (SMN, TDP-43, and FUS) involved in SMA/ALS, summarized the reported mutations in these RNA binding proteins involved in SMA/ALS pathogenesis, and discussed the mechanisms through which stress granules dynamics participate in the diseases. Meanwhile, we described the applications and limitation of current therapies targeting SMA/ALS. We futher proposed the promising targets on stress granules in the future therapeutic interventions of SMA/ALS.

The Role of Ubiquitin in Regulating Stress Granule Dynamics

Stress granules (SGs) are dynamic, reversible biomolecular condensates, which assemble in the cytoplasm of eukaryotic cells under various stress conditions. Formation of SGs typically occurs upon stress-induced translational arrest and polysome disassembly. The increase in cytoplasmic mRNAs triggers the formation of a protein-RNA network that undergoes liquid-liquid phase separation when a critical interaction threshold has been reached. This adaptive stress response allows a transient shutdown of several cellular processes until the stress is removed. During the recovery from stress, SGs disassemble to re-establish cellular activities. Persistent stress and disease-related mutations in SG components favor the formation of aberrant SGs that are impaired in disassembly and prone to aggregation. Recently, posttranslational modifications of SG components have been identified as major regulators of SG dynamics. Here, we summarize new insights into the role of ubiquitination in affecting SG dynamics and clearance and discuss implications for neurodegenerative diseases linked to aberrant SG formation.

Proteostatic imbalance and protein spreading in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder whose exact causative mechanisms are still under intense investigation. Several lines of evidence suggest that the anatomical and temporal propagation of pathological protein species along the neural axis could be among the main driving mechanisms for the fast and irreversible progression of ALS pathology. Many ALS-associated proteins form intracellular aggregates as a result of their intrinsic prion-like properties and/or following impairment of the protein quality control systems. During the disease course, these mutated proteins and aberrant peptides are released in the extracellular milieu as soluble or aggregated forms through a variety of mechanisms. Internalization by recipient cells may seed further aggregation and amplify existing proteostatic imbalances, thus triggering a vicious cycle that propagates pathology in vulnerable cells, such as motor neurons and other susceptible neuronal subtypes. Here, we provide an in-depth review of ALS pathology with a particular focus on the disease mechanisms of seeding and transmission of the most common ALS-associated proteins, including SOD1, FUS, TDP-43, and C9orf72-linked dipeptide repeats. For each of these proteins, we report historical, biochemical, and pathological evidence of their behaviors in ALS. We further discuss the possibility to harness pathological proteins as biomarkers and reflect on the implications of these findings for future research.

Connecting the "dots": RNP granule network in health and disease

All cells contain ribonucleoprotein (RNP) granules - large membraneless structures composed of RNA and proteins. Recent breakthroughs in RNP granule research have brought a new appreciation of their crucial role in organising virtually all cellular processes. Cells widely exploit the flexible, dynamic nature of RNP granules to adapt to a variety of functional states and the ever-changing environment. Constant exchange of molecules between the different RNP granules connects them into a network. This network controls basal cellular activities and is remodelled to enable efficient stress response. Alterations in RNP granule structure and regulation have been found to lead to fatal human diseases. The interconnectedness of RNP granules suggests that the RNP granule network as a whole becomes affected in disease states such as a representative neurodegenerative disease amyotrophic lateral sclerosis (ALS). In this review, we summarize available evidence on the communication between different RNP granules and on the RNP granule network disruption as a primary ALS pathomechanism.

Mechanisms and Regulation of RNA Condensation in RNP Granule Formation

Ribonucleoprotein (RNP) granules are RNA-protein assemblies that are involved in multiple aspects of RNA metabolism and are linked to memory, development, and disease. Some RNP granules form, in part, through the formation of intermolecular RNA-RNA interactions. In vitro, such trans RNA condensation occurs readily, suggesting that cells require mechanisms to modulate RNA-based condensation. We assess the mechanisms of RNA condensation and how cells modulate this phenomenon. We propose that cells control RNA condensation through ATP-dependent processes, static RNA buffering, and dynamic post-translational mechanisms. Moreover, perturbations in these mechanisms can be involved in disease. This reveals multiple cellular mechanisms of kinetic and thermodynamic control that maintain the proper distribution of RNA molecules between dispersed and condensed forms.

Cytoplasmic localization of amyotrophic lateral sclerosis-related TDP-43 proteins modulates stress granule formation

TDP-43 is an RNA/DNA-binding protein associated with amyotrophic lateral sclerosis (ALS). Under pathological conditions, TDP-43 exported from the nucleus accumulates in the cytoplasm, forming inclusion bodies. However, the molecular mechanisms that contribute to such aggregation are unclear. The pathogenic processes that lead to aggregation in ALS were investigated by analysing the effects of wildtype human TDP-43 or with mutations in the nuclear localization sequence (NLS) or those associated with ALS in stress granule formation. TDP-43 (WT, ∆NLS or G348C), with or without a GFP-tag, was expressed in SH-SY5Y neuroblastoma or HeLa cells and stress granules induced by oxidative stress or heat shock. Stress granule formation was altered in cells strongly expressing GFP-TDP-∆NLS, or untagged TDP-43-∆NLS in the cytoplasm but not the negative controls, GFP or GFP-UtrCH. In contrast, there was no reduction in stress granule formation by cells that expressed untagged TDP-43 (WT or G348C) in the nucleus upon stress induction. GFP labelling of TDP-43 (WT or G348C) promotes high cytoplasmic expression and nuclear aggregation. Stress granule formation was impaired in cells expressing GFP-TDP-43 (WT or G348C) in the cytoplasm. Overall, these results suggest that stress granule formation may be inhibited by high levels of TDP-43 protein in the cytoplasm. As stress granules serve a protective function, their deregulation may promote neurodegeneration due to an aberrant stress response.

Disruption of RNA Metabolism in Neurological Diseases and Emerging Therapeutic Interventions

RNA binding proteins are critical to the maintenance of the transcriptome via controlled regulation of RNA processing and transport. Alterations of these proteins impact multiple steps of the RNA life cycle resulting in various molecular phenotypes such as aberrant RNA splicing, transport, and stability. Disruption of RNA binding proteins and widespread RNA processing defects are increasingly recognized as critical determinants of neurological diseases. Here, we describe distinct mechanisms by which the homeostasis of RNA binding proteins is compromised in neurological disorders through their reduced expression level, increased propensity to aggregate or sequestration by abnormal RNAs. These mechanisms all converge toward altered neuronal function highlighting the susceptibility of neurons to deleterious changes in RNA expression and the central role of RNA binding proteins in preserving neuronal integrity. Emerging therapeutic approaches to mitigate or reverse alterations of RNA binding proteins in neurological diseases are discussed.

Physiological, Pathological, and Targetable Membraneless Organelles in Neurons

Neurons require unique subcellular compartmentalization to function efficiently. Formed from proteins and RNAs through liquid-liquid phase separation, membraneless organelles (MLOs) have emerged as one way in which cells form distinct, specialized compartments in the absence of lipid membranes. We first discuss MLOs that are common to many cell types as well as those that are specific to neurons. Interestingly, many proteins associated with neurodegenerative disease are found in MLOs, particularly in stress and transport granules. We next review possible links between neurodegeneration and MLOs, and the hypothesis that the protein and RNA inclusions formed in disease are related to the functional complexes occurring inside these MLOs. Finally, we discuss the hypothesis that protein post-translational modifications (PTMs), which can alter phase separation, can modulate MLO formation and provide potential new therapeutic strategies for currently untreatable neurodegenerative diseases.

Apolipoprotein L9 interacts with LC3/GABARAP and is a microtubule-associated protein with a widespread subcellular distribution

Mouse Apolipoprotein L9 is a 34-kDa phosphatidylethanolamine (PE)-binding protein. The gene is present only in mouse and rat genomes; hence it is restricted to two species. To understand why, it is essential to uncover details about its functions in cellular processes. Here we show that ApoL9 interacts with the proteins of the LC3 and GABARAP subfamilies, which are key players in macroautophagy. In vitro binding studies show a strong association with GABARAP, and in amino acid-starved cells it preferentially interacts with lipidated LC3B, likely by binding to its PE moiety through its lipid-binding domain. On treatment with autophagy inhibitors bafilomycin A1 and chloroquine, ApoL9 is found near swollen mitochondria and on lysosomes/LAMP1-positive compartments. However, ApoL9 itself does not seem to be degraded as a result of autophagy, suggesting that it is not an autophagy cargo receptor. Deletions in a putative transmembrane region between amino acids 110 and 145 abolish binding to PE. In addition, ApoL9 can redistribute to stress granules, can homo-oligomerize, and is a microtubule-associated protein. In short, its distribution in the cell is quite widespread, suggesting that it could have functions at the intersection of membrane binding and reorganization, autophagy, cellular stress and intracellular lipid transport.This article has an associated First Person interview with the first author of the paper.

ALS and FTD: Where RNA metabolism meets protein quality control

Recent genetic and biochemical evidence has improved our understanding of the pathomechanisms that lead to amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), two devastating neurodegenerative diseases with overlapping symptoms and causes. Impaired RNA metabolism, enhanced aggregation of protein-RNA complexes, aberrant formation of ribonucleoprotein (RNP) granules and dysfunctional protein clearance via autophagy are emerging as crucial events in ALS/FTD pathogenesis. Importantly, these processes interact at the molecular level, converging on a common pathogenic cascade. In this review, we summarize key principles underlying ALS and FTD, and we discuss how mutations in genes involved in RNA metabolism, protein quality control and protein degradation meet mechanistically to impair the functionality and dynamics of RNP granules, and how this leads to cellular toxicity and death. Finally, we describe recent advances in understanding signaling pathways that become dysfunctional in ALS/FTD, partly due to altered RNP granule dynamics, but also with stress granule-independent mechanisms and, thus could be promising targets for future therapeutic intervention.

Nucleolin reorganization and nucleolar stress in Purkinje cells of mutant PCD mice

The Purkinje cell (PC) degeneration (pcd) mouse harbors a mutation in Agtpbp1 gene that encodes for the cytosolic carboxypeptidase, CCP1. The mutation causes degeneration and death of PCs during the postnatal life, resulting in clinical and pathological manifestation of cerebellar ataxia. Monogenic biallelic damaging variants in the Agtpbp1 gene cause infantile-onset neurodegeneration and cerebellar atrophy, linking loss of functional CCP1 with human neurodegeneration. Although CCP1 plays a key role in the regulation of tubulin stabilization, its loss of function in PCs leads to a severe nuclear phenotype with heterochromatinization and accumulation of DNA damage. Therefore, the pcd mice provides a useful neuronal model to investigate nuclear mechanisms involved in neurodegeneration, particularly the nucleolar stress. In this study, we demonstrated that the Agtpbp1 gene mutation induces a p53-dependent nucleolar stress response in PCs, which is characterized by nucleolar fragmentation, nucleoplasmic and cytoplasmic mislocalization of nucleolin, and dysfunction of both pre-rRNA processing and mRNA translation. RT-qPCR analysis revealed reduction of mature 18S rRNA, with a parallel increase of its intermediate 18S-5'-ETS precursor, that correlates with a reduced expression of Fbl mRNA, which encodes an essential factor for rRNA processing. Moreover, nucleolar alterations were accompanied by a reduction of PTEN mRNA and protein levels, which appears to be related to the chromosome instability and accumulation of DNA damage in degenerating PCs. Our results highlight the essential contribution of nucleolar stress to PC degeneration and also underscore the nucleoplasmic mislocalization of nucleolin as a potential indicator of neurodegenerative processes.

A severe mouse model of spinal muscular atrophy develops early systemic inflammation

Spinal muscular atrophy (SMA) is a fatal genetic disease, mainly affecting children. A number of recent studies show, aside from lower motor neuron degeneration and atrophy of skeletal muscles, widespread defects present in the central nervous system (CNS) and peripheral non-neuronal cell types of SMA patients and mouse models, particularly of severe forms. However, molecular mechanisms underlying the multi-organ manifestations of SMA were hardly understood. Here, using histology, flow cytometry and gene expression analysis in both messenger RNA and protein levels in various tissues, we found that a severe SMA mouse model develops systemic inflammation in early symptomatic stages. SMA mice had an enhanced intestinal permeability, resulting in microbial invasion into the circulatory system. Expression of proinflammatory cytokines was increased in all tissues and the acute phase response in the liver was activated. Systemic inflammation further mobilized glucocorticoid signaling and in turn led to dysregulation of a large set of genes, including robust upregulation of FAM107A in the spinal cord, increased expression of which has been implicated in neurodegeneration. Moreover, we show that lipopolysaccharide challenge markedly suppressed survival of motor neuron 2 exon 7 splicing in all examined peripheral and CNS tissues, resulting in global survival of motor neuron level reduction. Therefore, we identified a novel pathological mechanism in a severe SMA mouse model, which affects phenotypic severity through multiple paths and should contribute to progression of broad neuronal and non-neuronal defects.

mRNP assembly, axonal transport, and local translation in neurodegenerative diseases

The development, maturation, and maintenance of the mammalian nervous system rely on complex spatiotemporal patterns of gene expression. In neurons, this is achieved by the expression of differentially localized isoforms and specific sets of mRNA-binding proteins (mRBPs) that regulate RNA processing, mRNA trafficking, and local protein synthesis at remote sites within dendrites and axons. There is growing evidence that axons contain a specialized transcriptome and are endowed with the machinery that allows them to rapidly alter their local proteome via local translation and protein degradation. This enables axons to quickly respond to changes in their environment during development, and to facilitate axon regeneration and maintenance in adult organisms. Aside from providing autonomy to neuronal processes, local translation allows axons to send retrograde injury signals to the cell soma. In this review, we discuss evidence that disturbances in mRNP transport, granule assembly, axonal localization, and local translation contribute to pathology in various neurodegenerative diseases, including spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease (AD).

R-Loops in Motor Neuron Diseases

R loops are transient three-stranded nucleic acid structures that form physiologically during transcription when a nascent RNA transcript hybridizes with the DNA template strand, leaving a single strand of displaced nontemplate DNA. However, aberrant persistence of R-loops can cause DNA damage by inducing genomic instability. Indeed, evidence has emerged that R-loops might represent a key element in the pathogenesis of human diseases, including cancer, neurodegeneration, and motor neuron disorders. Mutations in genes directly involved in R-loop biology, such as SETX (senataxin), or unstable DNA expansion eliciting R-loop generation, such as C9ORF72 HRE, can cause DNA damage and ultimately result in motor neuron cell death. In this review, we discuss current advancements in this field with a specific focus on motor neuron diseases associated with deregulation of R-loop structures. These mechanisms can represent novel therapeutic targets for these devastating, incurable diseases.

A complex of C9ORF72 and p62 uses arginine methylation to eliminate stress granules by autophagy

Mutations in proteins like FUS which cause Amyotrophic Lateral Sclerosis (ALS) result in the aberrant formation of stress granules while ALS-linked mutations in other proteins impede elimination of stress granules. Repeat expansions in C9ORF72, the major cause of ALS, reduce C9ORF72 levels but how this impacts stress granules is uncertain. Here, we demonstrate that C9ORF72 associates with the autophagy receptor p62 and controls elimination of stress granules by autophagy. This requires p62 to associate via the Tudor protein SMN with proteins, including FUS, that are symmetrically methylated on arginines. Mice lacking p62 accumulate arginine-methylated proteins and alterations in FUS-dependent splicing. Patients with C9ORF72 repeat expansions accumulate symmetric arginine dimethylated proteins which co-localize with p62. This suggests that C9ORF72 initiates a cascade of ALS-linked proteins (C9ORF72, p62, SMN, FUS) to recognize stress granules for degradation by autophagy and hallmarks of a defect in this process are observable in ALS patients.

Many Amyotrophic Lateral Sclerosis (ALS)-linked mutations cause accumulation of stress granules, and most ALS cases are caused by repeat expansions in C9ORF72. Here the authors show that C9ORF72 and the autophagy receptor p62 interact to associate with proteins symmetrically dimethylated on arginines such as FUS, to eliminate stress granules by autophagy.

Unexpected similarities between C9ORF72 and sporadic forms of ALS/FTD suggest a common disease mechanism

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) represent two ends of a disease spectrum with shared clinical, genetic and pathological features. These include near ubiquitous pathological inclusions of the RNA-binding protein (RBP) TDP-43, and often the presence of a GGGGCC expansion in the C9ORF72 (C9) gene. Previously, we reported that the sequestration of hnRNP H altered the splicing of target transcripts in C9ALS patients (Conlon et al., 2016). Here, we show that this signature also occurs in half of 50 postmortem sporadic, non-C9 ALS/FTD brains. Furthermore, and equally surprisingly, these ‘like-C9’ brains also contained correspondingly high amounts of insoluble TDP-43, as well as several other disease-related RBPs, and this correlates with widespread global splicing defects. Finally, we show that the like-C9 sporadic patients, like actual C9ALS patients, were much more likely to have developed FTD. We propose that these unexpected links between C9 and sporadic ALS/FTD define a common mechanism in this disease spectrum.

Accumulation of poly(A) RNA in nuclear granules enriched in Sam68 in motor neurons from the SMNΔ7 mouse model of SMA

Spinal muscular atrophy (SMA) is a severe motor neuron (MN) disease caused by the deletion or mutation of the survival motor neuron 1 (SMN1) gene, which results in reduced levels of the SMN protein and the selective degeneration of lower MNs. The best-known function of SMN is the biogenesis of spliceosomal snRNPs, the major components of the pre-mRNA splicing machinery. Therefore, SMN deficiency in SMA leads to widespread splicing abnormalities. We used the SMN∆7 mouse model of SMA to investigate the cellular reorganization of polyadenylated mRNAs associated with the splicing dysfunction in MNs. We demonstrate that SMN deficiency induced the abnormal nuclear accumulation in euchromatin domains of poly(A) RNA granules (PARGs) enriched in the splicing regulator Sam68. However, these granules lacked other RNA-binding proteins, such as TDP43, PABPN1, hnRNPA12B, REF and Y14, which are essential for mRNA processing and nuclear export. These effects were accompanied by changes in the alternative splicing of the Sam68-dependent Bcl-x and Nrnx1 genes, as well as changes in the relative accumulation of the intron-containing Chat, Chodl, Myh9 and Myh14 mRNAs, which are all important for MN functions. PARG-containing MNs were observed at presymptomatic SMA stage, increasing their number during the symptomatic stage. Moreover, the massive accumulations of poly(A) RNA granules in MNs was accompanied by the cytoplasmic depletion of polyadenylated mRNAs for their translation. We suggest that the SMN-dependent abnormal accumulation of polyadenylated mRNAs and Sam68 in PARGs reflects a severe dysfunction of both mRNA processing and translation, which could contribute to SMA pathogenesis.

Blocking p62-dependent SMN degradation ameliorates spinal muscular atrophy disease phenotypes

Spinal muscular atrophy (SMA), a degenerative motor neuron (MN) disease, caused by loss of functional survival of motor neuron (SMN) protein due to SMN1 gene mutations, is a leading cause of infant mortality. Increasing SMN levels ameliorates the disease phenotype and is unanimously accepted as a therapeutic approach for patients with SMA. The ubiquitin/proteasome system is known to regulate SMN protein levels; however, whether autophagy controls SMN levels remains poorly explored. Here, we show that SMN protein is degraded by autophagy. Pharmacological and genetic inhibition of autophagy increases SMN levels, while induction of autophagy decreases these levels. SMN degradation occurs via its interaction with the autophagy adapter p62 (also known as SQSTM1). We also show that SMA neurons display reduced autophagosome clearance, increased p62 and ubiquitinated proteins levels, and hyperactivated mTORC1 signaling. Importantly, reducing p62 levels markedly increases SMN and its binding partner gemin2, promotes MN survival, and extends lifespan in fly and mouse SMA models, revealing p62 as a potential new therapeutic target for the treatment of SMA.

Evolution of DNMT2 in drosophilids: Evidence for positive and purifying selection and insights into new protein (pathways) interactions

The DNA methyltransferase 2 (DNMT2) protein is the most conserved member of the DNA methyltransferase family. Nevertheless, its substrate specificity is still controversial and elusive. The genomic role and determinants of DNA methylation are poorly understood in invertebrates, and several mechanisms and associations are suggested. In Drosophila, the only known DNMT gene is Dnmt2. Here we present our findings from a wide search for Dnmt2 homologs in 68 species of Drosophilidae. We investigated its molecular evolution, and in our phylogenetic analyses the main clades of Drosophilidae species were recovered. We tested whether the Dnmt2 has evolved neutrally or under positive selection along the subgenera Drosophila and Sophophora and investigated positive selection in relation to several physicochemical properties. Despite of a major selective constraint on Dnmt2, we detected six sites under positive selection. Regarding the DNMT2 protein, 12 sites under positive-destabilizing selection were found, which suggests a selection that favors structural and functional shifts in the protein. The search for new potential protein partners with DNMT2 revealed 15 proteins with high evolutionary rate covariation (ERC), indicating a plurality of DNMT2 functions in different pathways. These events might represent signs of molecular adaptation, with molecular peculiarities arising from the diversity of evolutionary histories experienced by drosophilids.

Neurochondrin interacts with the SMN protein suggesting a novel mechanism for spinal muscular atrophy pathology

Spinal muscular atrophy (SMA) is an inherited neurodegenerative condition caused by a reduction in the amount of functional survival motor neuron (SMN) protein. SMN has been implicated in transport of mRNA in neural cells for local translation. We previously identified microtubule-dependent mobile vesicles rich in SMN and SNRPB, a member of the Sm family of small nuclear ribonucleoprotein (snRNP)-associated proteins, in neural cells. By comparing the interactomes of SNRPB and SNRPN, a neural-specific Sm protein, we now show that the essential neural protein neurochondrin (NCDN) interacts with Sm proteins and SMN in the context of mobile vesicles in neurites. NCDN has roles in protein localisation in neural cells and in maintenance of cell polarity. NCDN is required for the correct localisation of SMN, suggesting they may both be required for formation and transport of trafficking vesicles. NCDN may have potential as a therapeutic target for SMA together with, or in place of the targeting of SMN expression.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The essential neural protein neurochondrin interacts with the spinal muscular atrophy (SMA) protein SMN in cell lines and in mice. This might be relevant to the molecular pathology of SMA.

Accumulation of RNA-dependent protein kinase (PKR) in the nuclei of lung cancer cells mediates radiation resistance

We have previously demonstrated that radiation induced cell death in PKR (−/−) deficient mouse embryo fibroblasts (MEFs) but not in PKR (+/+) wild type MEFs. Our study indicated that PKR can also be involved in survival pathways following radiation therapy through activation of the AKT survival pathways in these MEFs is mediated in part through PKR. The role of PKR on radiation sensitivity in cancer cells has not been evaluated. In this study, we demonstrated that radiation treatment causes nuclear translocation of PKR in human lung cancer cells. The transduction of lung cancer cells with a dominant negative adenoviral PKR vector blocks nuclear translocation of PKR and leads to the reversal of radiation resistance. Plasmid transduction of lung cancer cells with nuclear targeted wild type PKR vectors also increased radiation resistance. This effect is selectively abrogated by plasmid transduction of dominant negative PKR vectors which restore radiation sensitivity. These findings suggest a novel role for PKR in lung cancer cells as a mediator of radiation resistance possibly through translocation of the protein product to the nucleus.

The emerging role of minor intron splicing in neurological disorders

Pre-mRNA splicing is an essential step in eukaryotic gene expression. Mutations in cis-acting sequence elements within pre-mRNA molecules or trans-acting factors involved in pre-mRNA processing have both been linked to splicing dysfunction that give rise to a large number of human diseases. These mutations typically affect the major splicing pathway, which excises more than 99% of all introns in humans. However, approximately 700-800 human introns feature divergent intron consensus sequences at their 5' and 3' ends and are recognized by a separate pre-mRNA processing machinery denoted as the minor spliceosome. This spliceosome has been studied less than its major counterpart, but has received increasing attention during the last few years as a novel pathomechanistic player on the stage in neurodevelopmental and neurodegenerative diseases. Here, we review the current knowledge on minor spliceosome function and discuss its potential pathomechanistic role and impact in neurodegeneration.

RNP Assembly Defects in Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a motor neuron disease caused by mutations/deletions within the survival of motor neuron 1 (SMN1) gene that lead to a pathological reduction of SMN protein levels. SMN is part of a multiprotein complex, functioning as a molecular chaperone that facilitates the assembly of spliceosomal small nuclear ribonucleoproteins (snRNP). In addition to its role in spliceosome formation, SMN has also been found to interact with mRNA-binding proteins (mRBPs), and facilitate their assembly into mRNP transport granules. The association of protein and RNA in RNP complexes plays an important role in an extensive and diverse set of cellular processes that regulate neuronal growth, differentiation, and the maturation and plasticity of synapses. This review discusses the role of SMN in RNP assembly and localization, focusing on molecular defects that affect mRNA processing and may contribute to SMA pathology.

Stress Granules and ALS: A Case of Causation or Correlation?

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease characterized by cytoplasmic protein aggregates within motor neurons. These aggregates are linked to ALS pathogenesis. Recent evidence has suggested that stress granules may aid the formation of ALS protein aggregates. Here, we summarize current understanding of stress granules, focusing on assembly and clearance. We also assess the evidence linking alterations in stress granule formation and dynamics to ALS protein aggregates and disease pathology.

Motor Neuron Gene Therapy: Lessons from Spinal Muscular Atrophy for Amyotrophic Lateral Sclerosis

Spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS) are severe nervous system diseases characterized by the degeneration of lower motor neurons. They share a number of additional pathological, cellular, and genetic parallels suggesting that mechanistic and clinical insights into one disorder may have value for the other. While there are currently no clinical ALS gene therapies, the splice-switching antisense oligonucleotide, nusinersen, was recently approved for SMA. This milestone was achieved through extensive pre-clinical research and patient trials, which together have spawned fundamental insights into motor neuron gene therapy. We have thus tried to distil key information garnered from SMA research, in the hope that it may stimulate a more directed approach to ALS gene therapy. Not only must the type of therapeutic (e.g., antisense oligonucleotide vs. viral vector) be sensibly selected, but considerable thought must be applied to the where, which, what, and when in order to enhance treatment benefit: to where (cell types and tissues) must the drug be delivered and how can this be best achieved? Which perturbed pathways must be corrected and can they be concurrently targeted? What dosing regime and concentration should be used? When should medication be administered? These questions are intuitive, but central to identifying and optimizing a successful gene therapy. Providing definitive solutions to these quandaries will be difficult, but clear thinking about therapeutic testing is necessary if we are to have the best chance of developing viable ALS gene therapies and improving upon early generation SMA treatments.

SMN regulation in SMA and in response to stress: new paradigms and therapeutic possibilities

Low levels of the survival of motor neuron (SMN) protein cause the neurodegenerative disease spinal muscular atrophy (SMA). SMA is a pediatric disease characterized by spinal motor neuron degeneration. SMA exhibits several levels of severity ranging from early antenatal fatality to only mild muscular weakness, and disease prognosis is related directly to the amount of functional SMN protein that a patient is able to express. Current therapies are being developed to increase the production of functional SMN protein; however, understanding the effect that natural stresses have on the production and function of SMN is of critical importance to ensuring that these therapies will have the greatest possible effect for patients. Research has shown that SMN, both on the mRNA and protein level, is highly affected by cellular stress. In this review we will summarize the research that highlights the roles of SMN in the disease process and the response of SMN to various environmental stresses.

Impaired spliceosomal UsnRNP assembly leads to Sm mRNA down-regulation and Sm protein degradation

Cellular spliceosomal UsnRNP assembly is assisted by the PRMT5 and SMN complexes. Prusty et al. demonstrate that perturbations in the assembly machinery of UsnRNPs trigger complex cellular responses, using ribosomes, exosome-mediated RNA degradation, and autophagy to prevent Sm protein aggregation.

Specialized assembly factors facilitate the formation of many macromolecular complexes in vivo. The formation of Sm core structures of spliceosomal U-rich small nuclear ribonucleoprotein particles (UsnRNPs) requires assembly factors united in protein arginine methyltransferase 5 (PRMT5) and survival motor neuron (SMN) complexes. We demonstrate that perturbations of this assembly machinery trigger complex cellular responses that prevent aggregation of unassembled Sm proteins. Inactivation of the SMN complex results in the initial tailback of Sm proteins on the PRMT5 complex, followed by down-regulation of their encoding mRNAs. In contrast, reduction of pICln, a PRMT5 complex subunit, leads to the retention of newly synthesized Sm proteins on ribosomes and their subsequent lysosomal degradation. Overexpression of Sm proteins under these conditions results in a surplus of Sm proteins over pICln, promoting their aggregation. Our studies identify an elaborate safeguarding system that prevents individual Sm proteins from aggregating, contributing to cellular UsnRNP homeostasis.

SMN - A chaperone for nuclear RNP social occasions?

Survival Motor Neuron (SMN) protein localizes to both the nucleus and the cytoplasm. Cytoplasmic SMN is diffusely localized in large oligomeric complexes with core member proteins, called Gemins. Biochemical and cell biological studies have demonstrated that the SMN complex is required for the cytoplasmic assembly and nuclear transport of Sm-class ribonucleoproteins (RNPs). Nuclear SMN accumulates with spliceosomal small nuclear (sn)RNPs in Cajal bodies, sub-domains involved in multiple facets of snRNP maturation. Thus, the SMN complex forms stable associations with both nuclear and cytoplasmic snRNPs, and plays a critical role in their biogenesis. In this review, we focus on potential functions of the nuclear SMN complex, with particular emphasis on its role within the Cajal body.

SMN deficiency in severe models of spinal muscular atrophy causes widespread intron retention and DNA damage

Spinal muscular atrophy (SMA), an autosomal recessive neuromuscular disease, is the leading monogenic cause of infant mortality. Homozygous loss of the gene survival of motor neuron 1 (SMN1) causes the selective degeneration of lower motor neurons and subsequent atrophy of proximal skeletal muscles. The SMN1 protein product, survival of motor neuron (SMN), is ubiquitously expressed and is a key factor in the assembly of the core splicing machinery. The molecular mechanisms by which disruption of the broad functions of SMN leads to neurodegeneration remain unclear. We used an antisense oligonucleotide (ASO)-based inducible mouse model of SMA to investigate the SMN-specific transcriptome changes associated with neurodegeneration. We found evidence of widespread intron retention, particularly of minor U12 introns, in the spinal cord of mice 30 d after SMA induction, which was then rescued by a therapeutic ASO. Intron retention was concomitant with a strong induction of the p53 pathway and DNA damage response, manifesting as γ-H2A.X positivity in neurons of the spinal cord and brain. Widespread intron retention and markers of the DNA damage response were also observed with SMN depletion in human SH-SY5Y neuroblastoma cells and human induced pluripotent stem cell-derived motor neurons. We also found that retained introns, high in GC content, served as substrates for the formation of transcriptional R-loops. We propose that defects in intron removal in SMA promote DNA damage in part through the formation of RNA:DNA hybrid structures, leading to motor neuron death.

Diverse role of survival motor neuron protein

The multifunctional Survival Motor Neuron (SMN) protein is required for the survival of all organisms of the animal kingdom. SMN impacts various aspects of RNA metabolism through the formation and/or interaction with ribonucleoprotein (RNP) complexes. SMN regulates biogenesis of small nuclear RNPs, small nucleolar RNPs, small Cajal body-associated RNPs, signal recognition particles and telomerase. SMN also plays an important role in DNA repair, transcription, pre-mRNA splicing, histone mRNA processing, translation, selenoprotein synthesis, macromolecular trafficking, stress granule formation, cell signaling and cytoskeleton maintenance. The tissue-specific requirement of SMN is dictated by the variety and the abundance of its interacting partners. Reduced expression of SMN causes spinal muscular atrophy (SMA), a leading genetic cause of infant mortality. SMA displays a broad spectrum ranging from embryonic lethality to an adult onset. Aberrant expression and/or localization of SMN has also been associated with male infertility, inclusion body myositis, amyotrophic lateral sclerosis and osteoarthritis. This review provides a summary of various SMN functions with implications to a better understanding of SMA and other pathological conditions.

Relationships between Stress Granules, Oxidative Stress, and Neurodegenerative Diseases

Cytoplasmic stress granules (SGs) are critical for facilitating stress responses and for preventing the accumulation of misfolded proteins. SGs, however, have been linked to the pathogenesis of neurodegenerative diseases, in part because SGs share many components with neuronal granules. Oxidative stress is one of the conditions that induce SG formation. SGs regulate redox levels, and SG formation in turn is differently regulated by various types of oxidative stress. These associations and other evidences suggest that SG formation contributes to the development of neurodegenerative diseases. In this paper, we review the regulation of SG formation/assembly and discuss the interactions between oxidative stress and SG formation. We then discuss the links between SGs and neurodegenerative diseases and the current therapeutic approaches for neurodegenerative diseases that target SGs.

Cytoplasmic stress granules: Dynamic modulators of cell signaling and disease

Stress granule (SG) assembly is a conserved cellular strategy to minimize stress-related damage and promote cell survival. Beyond their fundamental role in the stress response, SGs have emerged as key players for human health. As such, SG assembly is associated with cancer, neurodegenerative disorders, ischemia, and virus infections. SGs and granule-related signaling circuits are therefore promising targets to improve therapeutic intervention for several diseases. This is clinically relevant, because pharmacological drugs can affect treatment outcome by modulating SG formation. As membraneless and highly dynamic compartments, SGs regulate translation, ribostasis and proteostasis. Moreover, they serve as signaling hubs that determine cell viability and stress recovery. Various compounds can modulate SG formation and dynamics. Rewiring cell signaling through SG manipulation thus represents a new strategy to control cell fate under various physiological and pathological conditions.

A novel human-specific splice isoform alters the critical C-terminus of Survival Motor Neuron protein

Spinal muscular atrophy (SMA), a leading genetic disease of children and infants, is caused by mutations or deletions of Survival Motor Neuron 1 (SMN1) gene. SMN2, a nearly identical copy of SMN1, fails to compensate for the loss of SMN1 due to skipping of exon 7. SMN2 predominantly produces SMNΔ7, an unstable protein. Here we report exon 6B, a novel exon, generated by exonization of an intronic Alu-like sequence of SMN. We validate the expression of exon 6B-containing transcripts SMN6B and SMN6BΔ7 in human tissues and cell lines. We confirm generation of SMN6B transcripts from both SMN1 and SMN2. We detect expression of SMN6B protein using antibodies raised against a unique polypeptide encoded by exon 6B. We analyze RNA-Seq data to show that hnRNP C is a potential regulator of SMN6B expression and demonstrate that SMN6B is a substrate of nonsense-mediated decay. We show interaction of SMN6B with Gemin2, a critical SMN-interacting protein. We demonstrate that SMN6B is more stable than SMNΔ7 and localizes to both the nucleus and the cytoplasm. Our finding expands the diversity of transcripts generated from human SMN genes and reveals a novel protein isoform predicted to be stably expressed during conditions of stress.

Decreased function of survival motor neuron protein impairs endocytic pathways

Spinal muscular atrophy (SMA) is caused by depletion of the ubiquitously expressed survival motor neuron (SMN) protein, with 1 in 40 Caucasians being heterozygous for a disease allele. SMN is critical for the assembly of numerous ribonucleoprotein complexes, yet it is still unclear how reduced SMN levels affect motor neuron function. Here, we examined the impact of SMN depletion in Caenorhabditis elegans and found that decreased function of the SMN ortholog SMN-1 perturbed endocytic pathways at motor neuron synapses and in other tissues. Diminished SMN-1 levels caused defects in C. elegans neuromuscular function, and smn-1 genetic interactions were consistent with an endocytic defect. Changes were observed in synaptic endocytic proteins when SMN-1 levels decreased. At the ultrastructural level, defects were observed in endosomal compartments, including significantly fewer docked synaptic vesicles. Finally, endocytosis-dependent infection by JC polyomavirus (JCPyV) was reduced in human cells with decreased SMN levels. Collectively, these results demonstrate for the first time, to our knowledge, that SMN depletion causes defects in endosomal trafficking that impair synaptic function, even in the absence of motor neuron cell death.

A role for the survival of motor neuron protein in mRNP assembly and transport

Localization and local translation of mRNA plays a key role in neuronal development and function. While studies in various systems have provided insights into molecular mechanisms of mRNA transport and local protein synthesis, the factors that control the assembly of mRNAs and mRNA binding proteins into messenger ribonucleoprotein (mRNP) transport granules remain largely unknown. In this review we will discuss how insights on a motor neuron disease, spinal muscular atrophy (SMA), is advancing our understanding of regulated assembly of transport competent mRNPs and how defects in their assembly and delivery may contribute to the degeneration of motor neurons observed in SMA and other neurological disorders.

Alterations in stress granule dynamics driven by TDP-43 and FUS: a link to pathological inclusions in ALS?

Stress granules (SGs) are RNA-containing cytoplasmic foci formed in response to stress exposure. Since their discovery in 1999, over 120 proteins have been described to be localized to these structures (in 154 publications). Most of these components are RNA binding proteins (RBPs) or are involved in RNA metabolism and translation. SGs have been linked to several pathologies including inflammatory diseases, cancer, viral infection, and neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). In ALS and FTD, the majority of cases have no known etiology and exposure to external stress is frequently proposed as a contributor to either disease initiation or the rate of disease progression. Of note, both ALS and FTD are characterized by pathological inclusions, where some well-known SG markers localize with the ALS related proteins TDP-43 and FUS. We propose that TDP-43 and FUS serve as an interface between genetic susceptibility and environmental stress exposure in disease pathogenesis. Here, we will discuss the role of TDP-43 and FUS in SG dynamics and how disease-linked mutations affect this process.

Intracellular Bacterial Pathogens Trigger the Formation of U Small Nuclear RNA Bodies (U Bodies) through Metabolic Stress Induction

Invasive bacterial pathogens induce an amino acid starvation (AAS) response in infected host cells that controls host defense in part by promoting autophagy. However, whether AAS has additional significant effects on the host response to intracellular bacteria remains poorly characterized. Here we showed that Shigella, Salmonella, and Listeria interfere with spliceosomal U snRNA maturation in the cytosol. Bacterial infection resulted in the rerouting of U snRNAs and their cytoplasmic escort, the survival motor neuron (SMN) complex, to processing bodies, thus forming U snRNA bodies (U bodies). This process likely contributes to the decline in the cytosolic levels of U snRNAs and of the SMN complex proteins SMN and DDX20 that we observed in infected cells. U body formation was triggered by membrane damage in infected cells and was associated with the induction of metabolic stresses, such as AAS or endoplasmic reticulum stress. Mechanistically, targeting of U snRNAs to U bodies was regulated by translation initiation inhibition and the ATF4/ATF3 pathway, and U bodies rapidly disappeared upon removal of the stress, suggesting that their accumulation represented an adaptive response to metabolic stress. Importantly, this process likely contributed to shape the host response to invasive bacteria because down-regulation of DDX20 expression using short hairpin RNA (shRNA) amplified ATF3- and NF-κB-dependent signaling. Together, these results identify a critical role for metabolic stress and invasive bacterial pathogens in U body formation and suggest that this process contributes to host defense.

The function of RNA-binding proteins at the synapse: implications for neurodegeneration

The loss of synapses is a central event in neurodegenerative diseases. Synaptic proteins are often associated with disease neuropathology, but their role in synaptic loss is not fully understood. Of the many processes involved in sustaining the integrity of synapses, local protein translation can directly impact synaptic formation, communication, and maintenance. RNA-binding proteins and their association with RNA granules serve to regulate mRNA transportation and translation at synapses and in turn regulate the synapse. Genetic mutations in RNA-binding proteins FUS and TDP-43 have been linked with causing neurodegenerative diseases: amyotrophic lateral sclerosis and frontotemporal dementia. The observation that mutations in FUS and TDP-43 coincide with changes in RNA granules provides evidence that dysfunction of RNA metabolism may underlie the mechanism of synaptic loss in these diseases. However, we do not know how mutations in RNA-binding proteins would affect RNA granule dynamics and local translation, or if these alterations would cause neurodegeneration. Further investigation into this area will lead to important insights into how disruption of RNA metabolism and local translation at synapses can cause neurodegenerative diseases.

Dysregulated expression of lipid storage and membrane dynamics factors in Tia1 knockout mouse nervous tissue

During cell stress, the transcription and translation of immediate early genes are prioritized, while most other messenger RNAs (mRNAs) are stored away in stress granules or degraded in processing bodies (P-bodies). TIA-1 is an mRNA-binding protein that needs to translocate from the nucleus to seed the formation of stress granules in the cytoplasm. Because other stress granule components such as TDP-43, FUS, ATXN2, SMN, MAPT, HNRNPA2B1, and HNRNPA1 are crucial for the motor neuron diseases amyotrophic lateral sclerosis (ALS)/spinal muscular atrophy (SMA) and for the frontotemporal dementia (FTD), here we studied mouse nervous tissue to identify mRNAs with selective dependence on Tia1 deletion. Transcriptome profiling with oligonucleotide microarrays in comparison of spinal cord and cerebellum, together with independent validation in quantitative reverse transcriptase PCR and immunoblots demonstrated several strong and consistent dysregulations. In agreement with previously reported TIA1 knock down effects, cell cycle and apoptosis regulators were affected markedly with expression changes up to +2-fold, exhibiting increased levels for Cdkn1a, Ccnf, and Tprkb vs. decreased levels for Bid and Inca1 transcripts. Novel and surprisingly strong expression alterations were detected for fat storage and membrane trafficking factors, with prominent +3-fold upregulations of Plin4, Wdfy1, Tbc1d24, and Pnpla2 vs. a −2.4-fold downregulation of Cntn4 transcript, encoding an axonal membrane adhesion factor with established haploinsufficiency. In comparison, subtle effects on the RNA processing machinery included up to 1.2-fold upregulations of Dcp1b and Tial1. The effect on lipid dynamics factors is noteworthy, since also the gene deletion of Tardbp (encoding TDP-43) and Atxn2 led to fat metabolism phenotypes in mouse. In conclusion, genetic ablation of the stress granule nucleator TIA-1 has a novel major effect on mRNAs encoding lipid homeostasis factors in the brain, similar to the fasting effect.

Assays for the identification and prioritization of drug candidates for spinal muscular atrophy

Spinal muscular atrophy (SMA) is an autosomal recessive genetic disorder resulting in degeneration of α-motor neurons of the anterior horn and proximal muscle weakness. It is the leading cause of genetic mortality in children younger than 2 years. It affects ∼1 in 11,000 live births. In 95% of cases, SMA is caused by homozygous deletion of the SMN1 gene. In addition, all patients possess at least one copy of an almost identical gene called SMN2. A single point mutation in exon 7 of the SMN2 gene results in the production of low levels of full-length survival of motor neuron (SMN) protein at amounts insufficient to compensate for the loss of the SMN1 gene. Although no drug treatments are available for SMA, a number of drug discovery and development programs are ongoing, with several currently in clinical trials. This review describes the assays used to identify candidate drugs for SMA that modulate SMN2 gene expression by various means. Specifically, it discusses the use of high-throughput screening to identify candidate molecules from primary screens, as well as the technical aspects of a number of widely used secondary assays to assess SMN messenger ribonucleic acid (mRNA) and protein expression, localization, and function. Finally, it describes the process of iterative drug optimization utilized during preclinical SMA drug development to identify clinical candidates for testing in human clinical trials.

Methylation levels of SLC23A2 and NCOR2 genes correlate with spinal muscular atrophy severity

Spinal muscular atrophy (SMA) is a monogenic neurodegenerative disorder subdivided into four different types. Whole genome methylation analysis revealed 40 CpG sites associated with genes that are significantly differentially methylated between SMA patients and healthy individuals of the same age. To investigate the contribution of methylation changes to SMA severity, we compared the methylation level of found CpG sites, designed as "targets", as well as the nearest CpG sites in regulatory regions of ARHGAP22, CDK2AP1, CHML, NCOR2, SLC23A2 and RPL9 in three groups of SMA patients. Of notable interest, compared to type I SMA male patients, the methylation level of a target CpG site and one nearby CpG site belonging to the 5'UTR of SLC23A2 were significantly hypomethylated 19-22% in type III-IV patients. In contrast to type I SMA male patients, type III-IV patients demonstrated a 16% decrease in the methylation levels of a target CpG site, belonging to the 5'UTR of NCOR2. To conclude, this study validates the data of our previous study and confirms significant methylation changes in the SLC23A2 and NCOR2 regulatory regions correlates with SMA severity.

Antiviral innate immunity and stress granule responses

Viral infection triggers the activation of antiviral innate immune responses in mammalian cells. Viral RNA in the cytoplasm activates signaling pathways that result in the production of interferons (IFNs) and IFN-stimulated genes. Some viral infections have been shown to induce cytoplasmic granular aggregates similar to the dynamic ribonucleoprotein aggregates termed stress granules (SGs), suggesting that these viruses may utilize this stress response for their own benefit. By contrast, some viruses actively inhibit SG formation, suggesting an antiviral function for these structures. We review here the relationship between different viral infections and SG formation. We examine the evidence for antiviral functions for SGs and highlight important areas of inquiry towards understanding cellular stress responses to viral infection.

Pathological stress granules in Alzheimer's disease

A feature of neurodegenerative disease is the accumulation of insoluble protein aggregates in the brain. In some conditions, including Amyotrophic Lateral Sclerosis and Frontotemporal lobar degeneration, the primary aggregating entities are RNA binding proteins. Through regulated prion-like assembly, RNA binding proteins serve many functions in RNA metabolism that are essential for the healthy maintenance of cells of the central nervous system. Those RNA binding proteins that are the core nucleating factors of stress granules (SGs), including TIA-1, TIAR, TTP and G3BP1, are also found in the pathological lesions of other neurological conditions, such as Alzheimer's disease, where the hallmark aggregating protein is not an RNA binding protein. This discovery suggests that the regulated cellular pathway, which utilizes assembly of RNA binding proteins to package and silence mRNAs during stress, may be integral in the aberrant pathological protein aggregation that occurs in numerous neurodegenerative conditions.

RNA binding proteins: a common denominator of neuronal function and dysfunction

In eukaryotic cells, gene activity is not directly reflected by protein levels because mRNA processing, transport, stability, and translation are co- and post-transcriptionally regulated. These processes, collectively known as the ribonome, are tightly controlled and carried out by a plethora of trans-acting RNA-binding proteins (RBPs) that bind to specific cis elements throughout the RNA sequence. Within the nervous system, the role of RBPs in brain function turns out to be essential due to the architectural complexity of neurons exemplified by a relatively small somal size and an extensive network of projections and connections. Thus far, RBPs have been shown to be indispensable for several aspects of neurogenesis, neurite outgrowth, synapse formation, and plasticity. Consequently, perturbation of their function is central in the etiology of an ever-growing spectrum of neurological diseases, including fragile X syndrome and the neurodegenerative disorders frontotemporal lobar degeneration and amyotrophic lateral sclerosis.