Lost in Translation: Evidence for Protein Synthesis Deficits in ALS/FTD and Related Neurodegenerative Diseases

pTDP-43 levels correlate with cell type specific molecular alterations in the prefrontal cortex of C9orf72 ALS/FTD patients

Repeat expansions in the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis and familial frontotemporal dementia (ALS/FTD). To identify molecular defects that take place in the dorsolateral frontal cortex of patients with C9orf72 ALS/FTD, we compared healthy controls with C9orf72 ALS/FTD donor samples staged based on the levels of cortical phosphorylated TAR DNA binding protein (pTDP-43), a neuropathological hallmark of disease progression. We identified distinct molecular changes in different cell types that take place during FTD development. Loss of neurosurveillance microglia and activation of the complement cascade take place early, when pTDP-43 aggregates are absent or very low, and become more pronounced in late stages, suggesting an initial involvement of microglia in disease progression. Reduction of layer 2-3 cortical projection neurons with high expression of CUX2/LAMP5 also occurs early, and the reduction becomes more pronounced as pTDP-43 accumulates. Several unique features were observed only in samples with high levels of pTDP-43, including global alteration of chromatin accessibility in oligodendrocytes, microglia, and astrocytes; higher ratios of premature oligodendrocytes; increased levels of the noncoding RNA NEAT1 in astrocytes and neurons, and higher amount of phosphorylated ribosomal protein S6. Our findings reveal previously unknown progressive functional changes in major cell types found in the frontal cortex of C9orf72 ALS/FTD patients that shed light on the mechanisms underlying the pathology of this disease.

C9ORF72 patient-derived endothelial cells drive blood-brain barrier disruption and contribute to neurotoxicity

The blood-brain barrier (BBB) serves as a highly intricate and dynamic interface connecting the brain and the bloodstream, playing a vital role in maintaining brain homeostasis. BBB dysfunction has been associated with multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS); however, the role of the BBB in neurodegeneration is understudied. We developed an ALS patient-derived model of the BBB by using cells derived from 5 patient donors carrying C9ORF72 mutations. Brain microvascular endothelial-like cells (BMEC-like cells) derived from C9ORF72-ALS patients showed altered gene expression, compromised barrier integrity, and increased P-glycoprotein transporter activity. In addition, mitochondrial metabolic tests demonstrated that C9ORF72-ALS BMECs display a significant decrease in basal glycolysis accompanied by increased basal and ATP-linked respiration. Moreover, our study reveals that C9-ALS derived astrocytes can further affect BMECs function and affect the expression of the glucose transporter Glut-1. Finally, C9ORF72 patient-derived BMECs form leaky barriers through a cell-autonomous mechanism and have neurotoxic properties towards motor neurons.

Titin is a nucleolar protein in neurons

Titin is the largest protein produced by living cells and its function as a molecular spring in striated muscle is well characterized (1, 2). Here we demonstrate that titin isoforms in the same size range as found in muscle are prominent neuronal proteins in both the central and peripheral nervous systems, including motor neurons in the spinal cord and brain. Within these neurons, titin localizes to the dense fibrillar component of the nucleolus, the site of ribosomal RNA biogenesis and modification, and a critical site of dysfunction in neurodegenerative disease (3-5). Additionally, we show that the levels of both titin mRNA and protein are altered in the spinal cord of SOD1G93A mice, a commonly used model of amyotrophic lateral sclerosis, indicating that titin mediated nucleolar events may in fact contribute to the pathobiology of disease.

TDP-43-stratified single-cell proteomics of postmortem human spinal motor neurons reveals protein dynamics in amyotrophic lateral sclerosis

A limitation of conventional bulk-tissue proteome studies in amyotrophic lateral sclerosis (ALS) is the confounding of motor neuron (MN) signals by admixed non-MN proteins. Here, we leverage laser capture microdissection and nanoPOTS single-cell mass spectrometry-based proteomics to query changes in protein expression in single MNs from postmortem ALS and control tissues. In a follow-up analysis, we examine the impact of stratification of MNs based on cytoplasmic transactive response DNA-binding protein 43 (TDP-43)+ inclusion pathology on the profiles of 2,238 proteins. We report extensive overlap in differentially abundant proteins identified in ALS MNs with or without overt TDP-43 pathology, suggesting early and sustained dysregulation of cellular respiration, mRNA splicing, translation, and vesicular transport in ALS. Together, these data provide insights into proteome-level changes associated with TDP-43 proteinopathy and begin to demonstrate the utility of pathology-stratified trace sample proteomics for understanding single-cell protein dynamics in human neurologic diseases.

Screens in aging-relevant human ALS-motor neurons identify MAP4Ks as therapeutic targets for the disease

Effective therapeutics is much needed for amyotrophic lateral sclerosis (ALS), an adult-onset neurodegenerative disease mainly affecting motor neurons. By screening chemical compounds in human patient-derived and aging-relevant motor neurons, we identify a neuroprotective compound and show that MAP4Ks may serve as therapeutic targets for treating ALS. The lead compound broadly improves survival and function of motor neurons directly converted from human ALS patients. Mechanistically, it works as an inhibitor of MAP4Ks, regulates the MAP4Ks-HDAC6-TUBA4A-RANGAP1 pathway, and normalizes subcellular distribution of RANGAP1 and TDP-43. Finally, in an ALS mouse model we show that inhibiting MAP4Ks preserves motor neurons and significantly extends animal lifespan.

Blockage of VEGF function by bevacizumab alleviates early-stage cerebrovascular dysfunction and improves cognitive function in a mouse model of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder and the predominant type of dementia worldwide. It is characterized by the progressive and irreversible decline of cognitive functions. In addition to the pathological beta-amyloid (Aβ) deposition, glial activation, and neuronal injury in the postmortem brains of AD patients, increasing evidence suggests that the often overlooked vascular dysfunction is an important early event in AD pathophysiology. Vascular endothelial growth factor (VEGF) plays a critical role in regulating physiological functions and pathological changes in blood vessels, but whether VEGF is involved in the early stage of vascular pathology in AD remains unclear.

METHODS

We used an antiangiogenic agent for clinical cancer treatment, the humanized monoclonal anti-VEGF antibody bevacizumab, to block VEGF binding to its receptors in the 5×FAD mouse model at an early age. After treatment, memory performance was evaluated by a novel object recognition test, and cerebral vascular permeability and perfusion were examined by an Evans blue assay and blood flow scanning imaging analysis. Immunofluorescence staining was used to measure glial activation and Aβ deposits. VEGF and its receptors were analyzed by enzyme-linked immunosorbent assay and immunoblotting. RNA sequencing was performed to elucidate bevacizumab-associated transcriptional signatures in the hippocampus of 5×FAD mice.

RESULTS

Bevacizumab treatment administered from 4 months of age dramatically improved cerebrovascular functions, reduced glial activation, and restored long-term memory in both sexes of 5×FAD mice. Notably, a sex-specific change in different VEGF receptors was identified in the cortex and hippocampus of 5×FAD mice. Soluble VEGFR1 was decreased in female mice, while full-length VEGFR2 was increased in male mice. Bevacizumab treatment reversed the altered expression of receptors to be comparable to the level in the wild-type mice. Gene Set Enrichment Analysis of transcriptomic changes revealed that bevacizumab effectively reversed the changes in the gene sets associated with blood-brain barrier integrity and vascular smooth muscle contraction in 5×FAD mice.

CONCLUSIONS

Our study demonstrated the mechanistic roles of VEGF at the early stage of amyloidopathy and the protective effects of bevacizumab on cerebrovascular function and memory performance in 5×FAD mice. These findings also suggest the therapeutic potential of bevacizumab for the early intervention of AD.

Navigating the ALS Genetic Labyrinth: The Role of MAPT Haplotypes

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by wide clinical and biological heterogeneity, with a large proportion of ALS patients also exhibiting frontotemporal dementia (FTD) spectrum symptoms. This project aimed to characterize risk subtypes of the H1 haplotype within the MAPT (microtubule-associated protein tau) gene, according to their possible effect as a risk factor and as a modifying factor in relation to the age of disease onset. One hundred patients from Bulgaria with sporadic ALS were genotyped for the variants rs1467967, rs242557, rs1800547, rs3785883, rs2471738, and rs7521. Haploview 4.2 and SHEsisPlus were used to reconstruct haplotype frequencies using genotyping data from the 1000 Genomes project as controls. Genotype-phenotype correlation was investigated in the context of age of disease onset and risk of disease development. While the individual variants of the subtypes do not influence the age of onset of the disease, a correlation was found between the specific haplotype GGAGCA (H1b) and the risk of developing sALS, with results showing that individuals harboring this haplotype have a nearly two-fold increased risk of developing sALS compared to other H1 subtypes. The results from this study suggest that fine transcriptional regulation at the MAPT locus can influence the risk of ALS.

Transcriptome driven discovery of novel candidate genes for human neurological disorders in the telomer-to-telomer genome assembly era

BACKGROUND

With the first complete draft of a human genome, the Telomere-to-Telomere Consortium unlocked previously concealed genomic regions for genetic analyses. These regions harbour nearly 2000 potential novel genes with unknown function. In order to uncover candidate genes associated with human neurological pathologies, a comparative transcriptome study using the T2T-CHM13 and the GRCh38 genome assemblies was conducted on previously published datasets for eight distinct human neurological disorders.

RESULTS

The analysis of differential expression in RNA sequencing data led to the identification of 336 novel candidate genes linked to human neurological disorders. Additionally, it was revealed that, on average, 3.6% of the differentially expressed genes detected with the GRCh38 assembly may represent potential false positives. Among the noteworthy findings, two novel genes were discovered, one encoding a pore-structured protein and the other a highly ordered β-strand-rich protein. These genes exhibited upregulation in multiple epilepsy datasets and hold promise as candidate genes potentially modulating the progression of the disease. Furthermore, an analysis of RNA derived from white matter lesions in multiple sclerosis patients indicated significant upregulation of 26 rRNA encoding genes. Additionally, putative pathology related genes were identified for Alzheimer's disease, amyotrophic lateral sclerosis, glioblastoma, glioma, and conditions resulting from the m.3242 A > G mtDNA mutation.

CONCLUSION

The results presented here underline the potential of the T2T-CHM13 assembly in facilitating the discovery of candidate genes from transcriptome data in the context of human disorders. Moreover, the results demonstrate the value of remapping sequencing data to a superior genome assembly. Numerous potential pathology related genes, either as causative factors or related elements, have been unveiled, warranting further experimental validation.

Chemical screens in aging-relevant human motor neurons identify MAP4Ks as therapeutic targets for amyotrophic lateral sclerosis

Effective therapeutics is much needed for amyotrophic lateral sclerosis (ALS), an adult-onset neurodegenerative disease mainly affecting motor neurons. By screening chemical compounds in human patient-derived and aging-relevant motor neurons, we identify a neuroprotective compound and show that MAP4Ks may serve as therapeutic targets for treating ALS. The lead compound broadly improves survival and function of motor neurons directly converted from human ALS patients. Mechanistically, it works as an inhibitor of MAP4Ks, regulates the MAP4Ks-HDAC6-TUBA4A-RANGAP1 pathway, and normalizes subcellular distribution of RANGAP1 and TDP-43. Finally, in an ALS mouse model we show that inhibiting MAP4Ks preserves motor neurons and significantly extends animal lifespan.

Cytoplasmic aggregation of mutant FUS causes multistep RNA splicing perturbations in the course of motor neuron pathology

Dysfunction of the RNA-binding protein (RBP) FUS implicated in RNA metabolism can cause amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. Mutations affecting FUS nuclear localization can drive RNA splicing defects and stimulate the formation of non-amyloid inclusions in affected neurons. However, the mechanism by which FUS mutations contribute to the development of ALS remains uncertain. Here we describe a pattern of RNA splicing changes in the dynamics of the continuous proteinopathy induced by mislocalized FUS. We show that the decrease in intron retention of FUS-associated transcripts represents the hallmark of the pathogenesis of ALS and is the earliest molecular event in the course of progression of the disease. As FUS aggregation increases, the pattern of RNA splicing changes, becoming more complex, including a decrease in the inclusion of neuron-specific microexons and induction of cryptic exon splicing due to the sequestration of additional RBPs into FUS aggregates. Crucially, the identified features of the pathological splicing pattern are also observed in ALS patients in both sporadic and familial cases. Our data provide evidence that both a loss of nuclear FUS function due to mislocalization and the subsequent cytoplasmic aggregation of mutant protein lead to the disruption of RNA splicing in a multistep fashion during FUS aggregation.

TDP-43-stratified single-cell proteomic profiling of postmortem human spinal motor neurons reveals protein dynamics in amyotrophic lateral sclerosis

Unbiased proteomics has been employed to interrogate central nervous system (CNS) tissues (brain, spinal cord) and fluid matrices (CSF, plasma) from amyotrophic lateral sclerosis (ALS) patients; yet, a limitation of conventional bulk tissue studies is that motor neuron (MN) proteome signals may be confounded by admixed non-MN proteins. Recent advances in trace sample proteomics have enabled quantitative protein abundance datasets from single human MNs (Cong et al., 2020b). In this study, we leveraged laser capture microdissection (LCM) and nanoPOTS (Zhu et al., 2018c) single-cell mass spectrometry (MS)-based proteomics to query changes in protein expression in single MNs from postmortem ALS and control donor spinal cord tissues, leading to the identification of 2515 proteins across MNs samples (>900 per single MN) and quantitative comparison of 1870 proteins between disease groups. Furthermore, we studied the impact of enriching/stratifying MN proteome samples based on the presence and extent of immunoreactive, cytoplasmic TDP-43 inclusions, allowing identification of 3368 proteins across MNs samples and profiling of 2238 proteins across TDP-43 strata. We found extensive overlap in differential protein abundance profiles between MNs with or without obvious TDP-43 cytoplasmic inclusions that together point to early and sustained dysregulation of oxidative phosphorylation, mRNA splicing and translation, and retromer-mediated vesicular transport in ALS. Our data are the first unbiased quantification of single MN protein abundance changes associated with TDP-43 proteinopathy and begin to demonstrate the utility of pathology-stratified trace sample proteomics for understanding single-cell protein abundance changes in human neurologic diseases.

Amyotrophic Lateral Sclerosis Risk Genes and Suppressor

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that leads to death by progressive paralysis and respiratory failure within 2-4 years of onset. About 90-95% of ALS cases are sporadic (sALS), and 5-10% are inherited through family (fALS). Though the mechanisms of the disease are still poorly understood, so far, approximately 40 genes have been reported as ALS causative genes. The mutations in some crucial genes, like SOD1, C9ORF72, FUS, and TDP-43, are majorly associated with ALS, resulting in ROS-associated oxidative stress, excitotoxicity, protein aggregation, altered RNA processing, axonal and vesicular trafficking dysregulation, and mitochondrial dysfunction. Recent studies show that dysfunctional cellular pathways get restored as a result of the repair of a single pathway in ALS. In this review article, our aim is to identify putative targets for therapeutic development and the importance of a single suppressor to reduce multiple symptoms by focusing on important mutations and the phenotypic suppressors of dysfunctional cellular pathways in crucial genes as reported by other studies.

Tagged actin mRNA dysregulation in IGF2BP1[Formula: see text] mice

Gene expression is tightly regulated by RNA-binding proteins (RBPs) to facilitate cell survival, differentiation, and migration. Previous reports have shown the importance of the Insulin-like Growth Factor II mRNA-Binding Protein (IGF2BP1/IMP1/ZBP1) in regulating RNA fate, including localization, transport, and translation. Here, we generated and characterized a knockout mouse to study RBP regulation. We report that IGF2BP1 is essential for proper brain development and neonatal survival. Specifically, these mice display disorganization in the developing neocortex, and further investigation revealed a loss of cortical marginal cell density at E17.5. We also investigated migratory cell populations in the IGF2BP1[Formula: see text] mice, using BrdU labeling, and detected fewer mitotically active cells in the cortical plate. Since RNA localization is important for cellular migration and directionality, we investigated the regulation of β-actin messenger RNA (mRNA), a well-characterized target with established roles in cell motility and development. To aid in our understanding of RBP and target mRNA regulation, we generated mice with endogenously labeled β-actin mRNA (IGF2BP1[Formula: see text]; β-actin-MS2[Formula: see text]). Using endogenously labeled β-actin transcripts, we report IGF2BP1[Formula: see text] neurons have increased transcription rates and total β-actin protein content. In addition, we found decreased transport and anchoring in knockout neurons. Overall, we present an important model for understanding RBP regulation of target mRNA.

Downstream Effects of Mutations in SOD1 and TARDBP Converge on Gene Expression Impairment in Patient-Derived Motor Neurons

Amyotrophic Lateral Sclerosis (ALS) is a progressive and fatal neurodegenerative disease marked by death of motor neurons (MNs) present in the spinal cord, brain stem and motor cortex. Despite extensive research, the reason for neurodegeneration is still not understood. To generate novel hypotheses of putative underlying molecular mechanisms, we used human induced pluripotent stem cell (hiPSCs)-derived motor neurons (MNs) from SOD1- and TARDBP (TDP-43 protein)-mutant-ALS patients and healthy controls to perform high-throughput RNA-sequencing (RNA-Seq). An integrated bioinformatics approach was employed to identify differentially expressed genes (DEGs) and key pathways underlying these familial forms of the disease (fALS). In TDP43-ALS, we found dysregulation of transcripts encoding components of the transcriptional machinery and transcripts involved in splicing regulation were particularly affected. In contrast, less is known about the role of SOD1 in RNA metabolism in motor neurons. Here, we found that many transcripts relevant for mitochondrial function were specifically altered in SOD1-ALS, indicating that transcriptional signatures and expression patterns can vary significantly depending on the causal gene that is mutated. Surprisingly, however, we identified a clear downregulation of genes involved in protein translation in SOD1-ALS suggesting that ALS-causing SOD1 mutations shift cellular RNA abundance profiles to cause neural dysfunction. Altogether, we provided here an extensive profiling of mRNA expression in two ALS models at the cellular level, corroborating the major role of RNA metabolism and gene expression as a common pathomechanism in ALS.

SMN-deficient cells exhibit increased ribosomal DNA damage

Spinal muscular atrophy, the leading genetic cause of infant mortality, is a motor neuron disease caused by low levels of survival motor neuron (SMN) protein. SMN is a multifunctional protein that is implicated in numerous cytoplasmic and nuclear processes. Recently, increasing attention is being paid to the role of SMN in the maintenance of DNA integrity. DNA damage and genome instability have been linked to a range of neurodegenerative diseases. The ribosomal DNA (rDNA) represents a particularly unstable locus undergoing frequent breakage. Instability in rDNA has been associated with cancer, premature ageing syndromes, and a number of neurodegenerative disorders. Here, we report that SMN-deficient cells exhibit increased rDNA damage leading to impaired ribosomal RNA synthesis and translation. We also unravel an interaction between SMN and RNA polymerase I. Moreover, we uncover an spinal muscular atrophy motor neuron-specific deficiency of DDX21 protein, which is required for resolving R-loops in the nucleolus. Taken together, our findings suggest a new role of SMN in rDNA integrity.

Manganese-Induced Neurotoxicity through Impairment of Cross-Talk Pathways in Human Neuroblastoma Cell Line SH-SY5Y Differentiated with Retinoic Acid

Manganese (Mn) is an important element; yet acute and/or chronic exposure to this metal has been linked to neurotoxicity and neurodegenerative illnesses such as Parkinson’s disease and others via an unknown mechanism. To better understand it, we exposed a human neuroblastoma cell model (SH-SY5Y) to two Mn chemical species, MnCl₂ and Citrate of Mn(II) (0–2000 µM), followed by a cell viability assay, transcriptomics, and bioinformatics. Even though these cells have been chemically and genetically modified, which may limit the significance of our findings, we discovered that by using RA-differentiated cells instead of undifferentiated SH-SY5Y cell line, both chemical species induce a similar toxicity, potentially governed by disruption of protein metabolism, with some differences. The MnCl₂ altered amino acid metabolism, which affects RNA metabolism and protein synthesis. Citrate of Mn(II), however, inhibited the E3 ubiquitin ligases–target protein degradation pathway, which can lead to the buildup of damaged/unfolded proteins, consistent with histone modification. Finally, we discovered that Mn(II)-induced cytotoxicity in RA-SH-SY5Y cells shared 84 percent of the pathways involved in neurodegenerative diseases.

Axonal TDP-43 condensates drive neuromuscular junction disruption through inhibition of local synthesis of nuclear encoded mitochondrial proteins

Mislocalization of the predominantly nuclear RNA/DNA binding protein, TDP-43, occurs in motor neurons of ~95% of amyotrophic lateral sclerosis (ALS) patients, but the contribution of axonal TDP-43 to this neurodegenerative disease is unclear. Here, we show TDP-43 accumulation in intra-muscular nerves from ALS patients and in axons of human iPSC-derived motor neurons of ALS patient, as well as in motor neurons and neuromuscular junctions (NMJs) of a TDP-43 mislocalization mouse model. In axons, TDP-43 is hyper-phosphorylated and promotes G3BP1-positive ribonucleoprotein (RNP) condensate assembly, consequently inhibiting local protein synthesis in distal axons and NMJs. Specifically, the axonal and synaptic levels of nuclear-encoded mitochondrial proteins are reduced. Clearance of axonal TDP-43 or dissociation of G3BP1 condensates restored local translation and resolved TDP-43-derived toxicity in both axons and NMJs. These findings support an axonal gain of function of TDP-43 in ALS, which can be targeted for therapeutic development.

Dysfunction of RNA/RNA-Binding Proteins in ALS Astrocytes and Microglia

Amyotrophic Lateral Sclerosis is a neurological disease that primarily affects motor neurons in the cortex, brainstem, and spinal cord. The process that leads to motor neuron degeneration is strongly influenced by non-motor neuronal events that occur in a variety of cell types. Among these, neuroinflammatory processes mediated by activated astrocytes and microglia play a relevant role. In recent years, it has become clear that dysregulation of essential steps of RNA metabolism, as a consequence of alterations in RNA-binding proteins (RBPs), is a central event in the degeneration of motor neurons. Yet, a causal link between dysfunctional RNA metabolism and the neuroinflammatory processes mediated by astrocytes and microglia in ALS has been poorly defined. In this review, we will discuss the available evidence showing that RBPs and associated RNA processing are affected in ALS astrocytes and microglia, and the possible mechanisms involved in these events.

A system-wide mislocalization of RNA-binding proteins in motor neurons is a new feature of ALS

Amyotrophic lateral sclerosis (ALS) is a motor neuron disease characterized by progressive degeneration of motor neurons. Mislocalization of TAR DNA-binding protein 43 (TDP-43) is an early event in the formation of cytoplasmic TDP-43-positive inclusions in motor neurons and a hallmark of ALS. However, the underlying mechanism and the pathogenic impact of this mislocalization are relatively unexplored. We previously reported that abnormal AMPK activation mediates TDP-43 mislocalization in motor neurons of humans and mice with ALS. In the present study, we hypothesized that other nuclear proteins are mislocalized in the cytoplasm of motor neurons due to the AMPK-mediated phosphorylation of importin-α1 and subsequently contribute to neuronal degeneration in ALS. To test this hypothesis, we analyzed motor neurons of sporadic ALS patients and found that when AMPK is activated, importin-α1 is abnormally located in the nucleus. Multiple integrative molecular and cellular approaches (including proteomics, immunoprecipitation/western blot analysis, immunohistological evaluations and gradient analysis of preribosomal complexes) were employed to demonstrate that numerous RNA binding proteins are mislocalized in a rodent motor neuron cell line (NSC34) and human motor neurons derived from iPSCs during AMPK activation. We used comparative proteomic analysis of importin-α1 complexes that were immunoprecipitated with a phosphorylation-deficient mutant of importin-α1 (importin-α1-S105A) and a phosphomimetic mutant of importin-α1 (importin-α1-S105D) to identify 194 proteins that have stronger affinity for the unphosphorylated form than the phosphorylated form of importin-α1. Furthermore, GO and STRING analyses suggested that RNA processing and protein translation is the major machinery affected by abnormalities in the AMPK-importin-α1 axis. Consistently, the expression of importin-α1-S105D alters the assembly of preribosomal complexes and increases cell apoptosis. Collectively, we propose that by impairing importin-α1-mediated nuclear import, abnormal AMPK activation in motor neurons alters the cellular distribution of many RNA-binding proteins, which pathogenically affect multiple cellular machineries in motor neurons and contribute to ALS pathogenesis.

Overview of Beneficial Effects of (Poly)phenol Metabolites in the Context of Neurodegenerative Diseases on Model Organisms

The rise of neurodegenerative diseases in an aging population is an increasing problem of health, social and economic consequences. Epidemiological and intervention studies have demonstrated that diets rich in (poly)phenols can have potent health benefits on cognitive decline and neurodegenerative diseases. Meanwhile, the role of gut microbiota is ever more evident in modulating the catabolism of (poly)phenols to dozens of low molecular weight (poly)phenol metabolites that have been identified in plasma and urine. These metabolites can reach circulation in higher concentrations than parent (poly)phenols and persist for longer periods of time. However, studies addressing their potential brain effects are still lacking. In this review, we will discuss different model organisms that have been used to study how low molecular weight (poly)phenol metabolites affect neuronal related mechanisms gathering critical insight on their potential to tackle the major hallmarks of neurodegeneration.

Contribution of Energy Dysfunction to Impaired Protein Translation in Neurodegenerative Diseases

Impaired energy homeostasis and aberrant translational control have independently been implicated in the pathogenesis of neurodegenerative diseases. AMP kinase (AMPK), regulated by the ratio of cellular AMP and ATP, is a major gatekeeper for cellular energy homeostasis. Abnormal regulation of AMPK has been reported in several neurodegenerative diseases, including Alzheimer’s disease (AD) and amyotrophic lateral sclerosis (ALS). Most importantly, AMPK activation is known to suppress the translational machinery by inhibiting the mechanistic target of rapamycin complex 1 (mTORC1), activating translational regulators, and phosphorylating nuclear transporter factors. In this review, we describe recent findings on the emerging role of protein translation impairment caused by energy dysregulation in neurodegenerative diseases.

Nucleolin Rescues TDP-43 Toxicity in Yeast and Human Cell Models

TDP-43 is a nuclear protein involved in pivotal processes, extensively studied for its implication in neurodegenerative disorders. TDP-43 cytosolic inclusions are a common neuropathologic hallmark in amyotrophic lateral sclerosis (ALS) and related diseases, and it is now established that TDP-43 misfolding and aggregation play a key role in their etiopathology. TDP-43 neurotoxic mechanisms are not yet clarified, but the identification of proteins able to modulate TDP-43-mediated damage may be promising therapeutic targets for TDP-43 proteinopathies. Here we show by the use of refined yeast models that the nucleolar protein nucleolin (NCL) acts as a potent suppressor of TDP-43 toxicity, restoring cell viability. We provide evidence that NCL co-expression is able to alleviate TDP-43-induced damage also in human cells, further supporting its beneficial effects in a more consistent pathophysiological context. Presented data suggest that NCL could promote TDP-43 nuclear retention, reducing the formation of toxic cytosolic TDP-43 inclusions.

Germline SAMD9L truncation variants trigger global translational repression

SAMD9L is an interferon-induced tumor suppressor implicated in a spectrum of multisystem disorders, including risk for myeloid malignancies and immune deficiency. We identified a heterozygous de novo frameshift variant in SAMD9L in an infant with B cell aplasia and clinical autoinflammatory features who died from respiratory failure with chronic rhinovirus infection. Autopsy demonstrated absent bone marrow and peripheral B cells as well as selective loss of Langerhans and Purkinje cells. The frameshift variant led to expression of a truncated protein with interferon treatment. This protein exhibited a gain-of-function phenotype, resulting in interference in global protein synthesis via inhibition of translational elongation. Using a mutational scan, we identified a region within SAMD9L where stop-gain variants trigger a similar translational arrest. SAMD9L variants that globally suppress translation had no effect or increased mRNA transcription. The complex-reported phenotype likely reflects lineage-dominant sensitivities to this translation block. Taken together, our findings indicate that interferon-triggered SAMD9L gain-of-function variants globally suppress translation.

Neurodegeneration and axonal mRNA transportation

The prevalence of neurodegenerative diseases is accelerating in rapidly aging global population. Novel and effective diagnostic and therapeutic methods are required to tackle the global issue of neurodegeneration in the future. A better understanding of the potential molecular mechanism causing neurodegeneration can shed light on dysfunctional processes in diseased neurons, which can pave the way to design and synthesize novel targets for early diagnosis during the asymptomatic phase of the disease. Abnormal protein aggregation is a hallmark of neurodegenerative diseases which can hamper transportation of cargoes into axons. Recent evidence suggests that disruption of local protein synthesis has been observed in neurodegenerative diseases. Because of their highly asymmetric structure, highly polarized neurons require trafficking of cargoes from the cell body to different subcellular regions to meet the extensive demands of cellular physiology. Localization of mRNAs and subsequent local translation to corresponding proteins in axons is a mechanism which allows neurons to rapidly respond to external stimuli as well as establishing neuronal networks by synthesizing proteins on demand. Axonal protein synthesis is required for axon guidance, synapse formation and plasticity, axon maintenance and regeneration in response to injury. Different types of excitatory and inhibitory neurons in the central and peripheral nervous systems have been shown to localize mRNA. Rising evidence suggests that the repertoire of localizing mRNA in axons can change during aging, indicating a connection between axonal mRNA trafficking and aging diseases such as neurodegeneration. Here, I briefly review the latest findings on the importance of mRNA localization and local translation in neurons and the consequences of their disruption in neurodegenerative diseases. In addition, I discuss recent evidence that dysregulation of mRNA localization and local protein translation can contribute to the formation of neurodegenerative diseases such as Alzheimer's disease, Amyotrophic Lateral Sclerosis, and Spinal Muscular Atrophy. In addition, I discuss recent findings on mRNAs localizing to mitochondria in neurodegeneration.

Targeting IRES-dependent translation as a novel approach for treating Duchenne muscular dystrophy

Internal-ribosomal entry sites (IRES) are translational elements that allow the initiation machinery to start protein synthesis via internal initiation. IRESs promote tissue-specific translation in stress conditions when conventional cap-dependent translation is inhibited. Since many IRES-containing mRNAs are relevant to diseases, this cellular mechanism is emerging as an attractive therapeutic target for pharmacological and genetic modulations. Indeed, there has been growing interest over the past years in determining the therapeutic potential of IRESs for several disease conditions such as cancer, neurodegeneration and neuromuscular diseases including Duchenne muscular dystrophy (DMD). IRESs relevant for DMD have been identified in several transcripts whose protein product results in functional improvements in dystrophic muscles. Together, these converging lines of evidence indicate that activation of IRES-mediated translation of relevant transcripts in DMD muscle represents a novel and appropriate therapeutic strategy for DMD that warrants further investigation, particularly to identify agents that can modulate their activity.

Localization of CDR2L and CDR2 in paraneoplastic cerebellar degeneration

OBJECTIVE

Identify the subcellular location and potential binding partners of two cerebellar degeneration-related proteins, CDR2L and CDR2, associated with anti-Yo-mediated paraneoplastic cerebellar degeneration.

METHODS

Cancer cells, rat Purkinje neuron cultures, and human cerebellar sections were exposed to cerebrospinal fluid and serum from patients with paraneoplastic cerebellar degeneration with Yo antibodies and with several antibodies against CDR2L and CDR2. We used mass spectrometry-based proteomics, super-resolution microscopy, proximity ligation assay, and co-immunoprecipitation to verify the antibodies and to identify potential binding partners.

RESULTS

We confirmed the CDR2L specificity of Yo antibodies by mass spectrometry-based proteomics and found that CDR2L localized to the cytoplasm and CDR2 to the nucleus. CDR2L co-localized with the 40S ribosomal protein S6, while CDR2 co-localized with the nuclear speckle proteins SON, eukaryotic initiation factor 4A-III, and serine/arginine-rich splicing factor 2.

INTERPRETATION

We showed that Yo antibodies specifically bind to CDR2L in Purkinje neurons of PCD patients where they potentially interfere with the function of the ribosomal machinery resulting in disrupted mRNA translation and/or protein synthesis. Our findings demonstrating that CDR2L interacts with ribosomal proteins and CDR2 with nuclear speckle proteins is an important step toward understanding PCD pathogenesis.

Loss of Tdp-43 disrupts the axonal transcriptome of motoneurons accompanied by impaired axonal translation and mitochondria function

Protein inclusions containing the RNA-binding protein TDP-43 are a pathological hallmark of amyotrophic lateral sclerosis and other neurodegenerative disorders. The loss of TDP-43 function that is associated with these inclusions affects post-transcriptional processing of RNAs in multiple ways including pre-mRNA splicing, nucleocytoplasmic transport, modulation of mRNA stability and translation. In contrast, less is known about the role of TDP-43 in axonal RNA metabolism in motoneurons. Here we show that depletion of Tdp-43 in primary motoneurons affects axon growth. This defect is accompanied by subcellular transcriptome alterations in the axonal and somatodendritic compartment. The axonal localization of transcripts encoding components of the cytoskeleton, the translational machinery and transcripts involved in mitochondrial energy metabolism were particularly affected by loss of Tdp-43. Accordingly, we observed reduced protein synthesis and disturbed mitochondrial functions in axons of Tdp-43-depleted motoneurons. Treatment with nicotinamide rescued the axon growth defect associated with loss of Tdp-43. These results show that Tdp-43 depletion in motoneurons affects several pathways integral to axon health indicating that loss of TDP-43 function could thus make a major contribution to axonal pathomechanisms in ALS.

Phenotypic Suppression of ALS/FTD-Associated Neurodegeneration Highlights Mechanisms of Dysfunction

A fundamental question regarding the etiology of amyotrophic lateral sclerosis (ALS) is whether the various gene mutations associated with the disease converge on a single molecular pathway or act through multiple pathways to trigger neurodegeneration. Notably, several of the genes and cellular processes implicated in ALS have also been linked to frontotemporal dementia (FTD), suggesting these two diseases share common origins with varied clinical presentations. Scientists are rapidly identifying ALS/FTD suppressors that act on conserved pathways from invertebrates to vertebrates to alleviate degeneration. The elucidation of such genetic modifiers provides insight into the molecular pathways underlying this rapidly progressing neurodegenerative disease, while also revealing new targets for therapeutic development.

Effects of Oxidative Stress on Protein Translation: Implications for Cardiovascular Diseases

Cardiovascular diseases (CVDs) are a group of disorders that affect the heart and blood vessels. Due to their multifactorial nature and wide variation, CVDs are the leading cause of death worldwide. Understanding the molecular alterations leading to the development of heart and vessel pathologies is crucial for successfully treating and preventing CVDs. One of the causative factors of CVD etiology and progression is acute oxidative stress, a toxic condition characterized by elevated intracellular levels of reactive oxygen species (ROS). Left unabated, ROS can damage virtually any cellular component and affect essential biological processes, including protein synthesis. Defective or insufficient protein translation results in production of faulty protein products and disturbances of protein homeostasis, thus promoting pathologies. The relationships between translational dysregulation, ROS, and cardiovascular disorders will be examined in this review.

To Be or Not To Be…Toxic-Is RNA Association With TDP-43 Complexes Deleterious or Protective in Neurodegeneration?

TAR DNA binding protein (TDP-43) is a nucleic acid binding protein associated with insoluble cytoplasmic aggregates in several neurodegenerative disorders, including 97% of the ALS cases. In healthy individuals, TDP-43 is primarily localized to the nucleus; it can shuttle between the nucleus and the cytoplasm, and is involved in several aspects of RNA processing including transcription, splicing, RNA stability, transport, localization, stress granule (SG) formation, and translation. Upon stress, TDP-43 aggregates in the cytoplasm and associates with several types of RNA and protein assemblies, resulting in nuclear depletion of TDP-43. Under conditions of prolonged stress, cytoplasmic TDP-43 undergoes liquid-liquid phase separation (LLPS) and becomes less mobile. Evidence exists to support a scenario in which insoluble TDP-43 complexes sequester RNA and/or proteins causing disturbances in both ribostasis and proteostasis, which in turn contribute to neurodegeneration. However, the relationship between RNA binding and TDP-43 toxicity remains unclear. Recent studies provide conflicting views on the role of RNA in TDP-43 toxicity, with some finding RNA as a toxic factor whereby RNA binding contributes to TDP-43 toxicity, while others find RNA to be a protective factor that inhibits TDP-43 aggregation. Here we review and discuss these recent reports, which ultimately highlight the importance of understanding the heterogeneity of TDP-43 assemblies and collectively point to solubilizing TDP-43 as a potential therapeutic strategy.

Loss of the ribosomal RNA methyltransferase NSUN5 impairs global protein synthesis and normal growth

Modifications of ribosomal RNA expand the nucleotide repertoire and thereby contribute to ribosome heterogeneity and translational regulation of gene expression. One particular m5C modification of 25S ribosomal RNA, which is introduced by Rcm1p, was previously shown to modulate stress responses and lifespan in yeast and other small organisms. Here, we report that NSUN5 is the functional orthologue of Rcm1p, introducing m5C3782 into human and m5C3438 into mouse 28S ribosomal RNA. Haploinsufficiency of the NSUN5 gene in fibroblasts from William Beuren syndrome patients causes partial loss of this modification. The N-terminal domain of NSUN5 is required for targeting to nucleoli, while two evolutionary highly conserved cysteines mediate catalysis. Phenotypic consequences of NSUN5 deficiency in mammalian cells include decreased proliferation and size, which can be attributed to a reduction in total protein synthesis by altered ribosomes. Strikingly, Nsun5 knockout in mice causes decreased body weight and lean mass without alterations in food intake, as well as a trend towards reduced protein synthesis in several tissues. Together, our findings emphasize the importance of single RNA modifications for ribosome function and normal cellular and organismal physiology.

Tau drives translational selectivity by interacting with ribosomal proteins

There is a fundamental gap in understanding the consequences of tau-ribosome interactions. Tau oligomers and filaments hinder protein synthesis in vitro, and they associate strongly with ribosomes in vivo. Here, we investigated the consequences of tau interactions with ribosomes in transgenic mice, in cells, and in human brain tissues to identify tau as a direct modulator of ribosomal selectivity. First, we performed microarrays and nascent proteomics to measure changes in protein synthesis. Using regulatable rTg4510 tau transgenic mice, we determined that tau expression differentially shifts both the transcriptome and the nascent proteome, and that the synthesis of ribosomal proteins is reversibly dependent on tau levels. We further extended these results to human brains and found that tau pathologically interacts with ribosomal protein S6 (rpS6 or S6), a crucial regulator of translation. Consequently, protein synthesis under translational control of rpS6 was reduced under tauopathic conditions in Alzheimer's disease brains. Our data establish tau as a driver of RNA translation selectivity. Moreover, since regulation of protein synthesis is critical for learning and memory, aberrant tau-ribosome interactions in disease could explain the linkage between tauopathies and cognitive impairment.

rRNA and tRNA Bridges to Neuronal Homeostasis in Health and Disease

Dysregulation of protein translation is emerging as a unifying mechanism in the pathogenesis of many neuronal disorders. Ribosomal RNA (rRNA) and transfer RNA (tRNA) are structural molecules that have complementary and coordinated functions in protein synthesis. Defects in both rRNAs and tRNAs have been described in mammalian brain development, neurological syndromes, and neurodegeneration. In this review, we present the molecular mechanisms that link aberrant rRNA and tRNA transcription, processing and modifications to translation deficits, and neuropathogenesis. We also discuss the interdependence of rRNA and tRNA biosynthesis and how their metabolism brings together proteotoxic stress and impaired neuronal homeostasis.