Overexpression of nuclear FUS induces neuronal cell death

hnRNPs: roles in neurodevelopment and implication for brain disorders

Heterogeneous nuclear ribonucleoproteins (hnRNPs) constitute a family of multifunctional RNA-binding proteins able to process nuclear pre-mRNAs into mature mRNAs and regulate gene expression in multiple ways. They comprise at least 20 different members in mammals, named from A (HNRNP A1) to U (HNRNP U). Many of these proteins are components of the spliceosome complex and can modulate alternative splicing in a tissue-specific manner. Notably, while genes encoding hnRNPs exhibit ubiquitous expression, increasing evidence associate these proteins to various neurodevelopmental and neurodegenerative disorders, such as intellectual disability, epilepsy, microcephaly, amyotrophic lateral sclerosis, or dementias, highlighting their crucial role in the central nervous system. This review explores the evolution of the hnRNPs family, highlighting the emergence of numerous new members within this family, and sheds light on their implications for brain development.

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

circ-Gucy1a2 Protects Mice from Cerebral Ischemia-Reperfusion Injury by Attenuating Neuronal Apoptosis and Mitochondrial Membrane Potential Loss

Cerebral ischemia-reperfusion (I/R) injury (CI/RI) is a severe problem in patients with cerebral ischemia. The current study explored the influences of circular (circ)-Gucy1a2 on neuronal apoptosis and mitochondrial membrane potential (MMP) in the brain tissue of CI/RI mice. Forty-eight mice were randomized into the sham group, transient middle cerebral artery occlusion (tMCAO) group, lentivirus negative control (LV-NC) group, and LV-Gucy1a2 group. Mice were first injected with lentivirus loaded with LV-Gucy1a2 or LV-NC via lateral ventricle, followed by the establishment of CI/RI models 2 weeks later. Twenty-four hours after CI/RI, the neurological impairment of mice was assessed using a 6-point scoring system. The cerebral infarct volume and brain histopathological changes were determined in CI/RI mice through histological staining. In vitro, pcDNA3.1-NC and pcDNA3.1-Gucy1a2 were transfected into mouse primary cortical neurons for 48 hours, followed by the establishment of oxygen-glucose deprivation/reoxygenation (OGD/R) models. The levels of circ-Gucy1a2 in mouse brain tissues and neurons were examined using RT-qPCR. Neuronal proliferation and apoptosis, MMP loss, and oxidative stress (OS)-related indexes in neurons were detected using CCK-8 assay, flow cytometry, JC-1 staining, and H2DFFDA staining. CI/RI mouse models and OGD/R cell models were successfully established. After CI/RI, neurons in mice were impaired and the cerebral infarction volume was increased. circ-Gucy1a2 was poorly expressed in CI/RI mouse brain tissues. Overexpression of circ-Gucy1a2 increased OGD/R-induced neuronal proliferation and mitigated apoptosis, MMP loss, and OS. Overall, circ-Gucy1a2 was down-regulated in brain tissues of CI/RI mice, and overexpression of circ-Gucy1a2 can protect mice from CI/RI.

Apoptosis and long non-coding RNAs: Focus on their roles in Heart diseases

Heart disease is one of the principal death reasons around the world and there is a growing requirement to discover novel healing targets that have the potential to avert or manage these illnesses. On the other hand, apoptosis is a strongly controlled, cell removal procedure that has a crucial part in numerous cardiac problems, such as reperfusion injury, MI (myocardial infarction), consecutive heart failure, and inflammation of myocardium. Completely comprehending the managing procedures of cell death signaling is critical as it is the primary factor that influences patient mortality and morbidity, owing to cardiomyocyte damage. Indeed, the prevention of heart cell death appears to be a viable treatment approach for heart illnesses. According to current researches, a number of long non-coding RNAs cause the heart cells death via different methods that are embroiled in controlling the activity of transcription elements, the pathways that signals transmission within cells, small miRNAs, and the constancy of proteins. When there is too much cell death in the heart, it can cause problems like reduced blood flow, heart damage after restoring blood flow, heart disease in diabetics, and changes in the heart after reduced blood flow. Therefore, studying how lncRNAs control apoptosis could help us find new treatments for heart diseases. In this review, we present recent discoveries about how lncRNAs are involved in causing cell death in different cardiovascular diseases.

Catching a killer: Mechanisms of programmed cell death and immune activation in Amyotrophic Lateral Sclerosis

In the central nervous system (CNS), execution of programmed cell death (PCD) is crucial for proper neurodevelopment. However, aberrant activation of these pathways in adult CNS leads to neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). How a cell dies is critical, as it can drive local immune activation and tissue damage. Classical apoptosis engages several mechanisms to evoke "immunologically silent" responses, whereas other forms of programmed death such as pyroptosis, necroptosis, and ferroptosis release molecules that can potentiate immune responses and inflammation. In ALS, a fatal neuromuscular disorder marked by progressive death of lower and upper motor neurons, several cell types in the CNS express machinery for multiple PCD pathways. The specific cell types engaging PCD, and ultimate mechanisms by which neuronal death occurs in ALS are not well defined. Here, we provide an overview of different PCD pathways implicated in ALS. We also examine immune activation in ALS and differentiate apoptosis from necrotic mechanisms based on downstream immunological consequences. Lastly, we highlight therapeutic strategies that target cell death pathways in the treatment of neurodegeneration and inflammation in ALS.

The Complex Mechanisms by Which Neurons Die Following DNA Damage in Neurodegenerative Diseases

Human cells are exposed to numerous exogenous and endogenous insults every day. Unlike other molecules, DNA cannot be replaced by resynthesis, hence damage to DNA can have major consequences for the cell. The DNA damage response contains overlapping signalling networks that repair DNA and hence maintain genomic integrity, and aberrant DNA damage responses are increasingly described in neurodegenerative diseases. Furthermore, DNA repair declines during aging, which is the biggest risk factor for these conditions. If unrepaired, the accumulation of DNA damage results in death to eliminate cells with defective genomes. This is particularly important for postmitotic neurons because they have a limited capacity to proliferate, thus they must be maintained for life. Neuronal death is thus an important process in neurodegenerative disorders. In addition, the inability of neurons to divide renders them susceptible to senescence or re-entry to the cell cycle. The field of cell death has expanded significantly in recent years, and many new mechanisms have been described in various cell types, including neurons. Several of these mechanisms are linked to DNA damage. In this review, we provide an overview of the cell death pathways induced by DNA damage that are relevant to neurons and discuss the possible involvement of these mechanisms in neurodegenerative conditions.

Wild-Type and Mutant FUS Expression Reduce Proliferation and Neuronal Differentiation Properties of Neural Stem Progenitor Cells

Nervous system development involves proliferation and cell specification of progenitor cells into neurons and glial cells. Unveiling how this complex process is orchestrated under physiological conditions and deciphering the molecular and cellular changes leading to neurological diseases is mandatory. To date, great efforts have been aimed at identifying gene mutations associated with many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Mutations in the RNA/DNA binding protein Fused in Sarcoma/Translocated in Liposarcoma (FUS/TLS) have been associated with motor neuron degeneration in rodents and humans. Furthermore, increased levels of the wild-type protein can promote neuronal cell death. Despite the well-established causal link between FUS mutations and ALS, its role in neural cells remains elusive. In order to shed new light on FUS functions we studied its role in the control of neural stem progenitor cell (NSPC) properties. Here, we report that human wild-type Fused in Sarcoma (WT FUS), exogenously expressed in mouse embryonic spinal cord-derived NSPCs, was localized in the nucleus, caused cell cycle arrest in G1 phase by affecting cell cycle regulator expression, and strongly reduced neuronal differentiation. Furthermore, the expression of the human mutant form of FUS (P525L-FUS), associated with early-onset ALS, drives the cells preferentially towards a glial lineage, strongly reducing the number of developing neurons. These results provide insight into the involvement of FUS in NSPC proliferation and differentiation into neurons and glia.

LncRNA KCNQ1OT1 contributes to cardiomyocyte apoptosis by targeting FUS in heart failure

Long noncoding RNAs (lncRNAs) have recently been recognized as the important regulators in cardiac diseases. This study was aimed to investigate the role and molecular mechanism of lncRNA KCNQ1OT1 in regulating cardiomyocyte apoptosis in heart failure (HF). The mouse model of HF was induced by doxorubicin (ADR). Cell apoptosis was detected by Hoechst and TUNEL staining. Molecule expressions were determined by qRT-PCR and western blot. The interaction between KCNQ1OT1 and Fused in sarcoma (FUS) was assessed by RNA immunoprecipitation (RIP) and RNA pull-down assays. KCNQ1OT1 was up-regulated in the myocardial tissues of HF mice and the ADR-stimulated mouse myocardial cell line (HL-1). KCNQ1OT1 overexpression promoted apoptosis of ADR-stimulated HL-1 cells, while KCNQ1OT1 knockdown caused the opposite effect. The RIP and RNA pull-down results showed that KCNQ1OT1 - bound to FUS and negatively regulated its protein level. Knockdown of FUS inhibited apoptosis of ADR-stimulated HL-1 cells and reversed the effect of KCNQ1OT1 overexpression on cardiomyocyte apoptosis. In vivo experiment showed that KCNQ1OT1 ovexpression improved myocardial histopathological changes, reduced myocardial fibrosis areas, down-regulated FUS expression, and inhibited cell apoptosis of HF mice. In conclusion, KCNQ1OT1 facilitates cardiomyocyte apoptosis by - targeting FUS in ADR-induced HF.

Circular RNA CircFndc3b modulates cardiac repair after myocardial infarction via FUS/VEGF-A axis

Circular RNAs are generated from many protein-coding genes, but their role in cardiovascular health and disease states remains unknown. Here we report identification of circRNA transcripts that are differentially expressed in post myocardial infarction (MI) mouse hearts including circFndc3b which is significantly down-regulated in the post-MI hearts. Notably, the human circFndc3b ortholog is also significantly down-regulated in cardiac tissues of ischemic cardiomyopathy patients. Overexpression of circFndc3b in cardiac endothelial cells increases vascular endothelial growth factor-A expression and enhances their angiogenic activity and reduces cardiomyocytes and endothelial cell apoptosis. Adeno-associated virus 9 -mediated cardiac overexpression of circFndc3b in post-MI hearts reduces cardiomyocyte apoptosis, enhances neovascularization and improves left ventricular functions. Mechanistically, circFndc3b interacts with the RNA binding protein Fused in Sarcoma to regulate VEGF expression and signaling. These findings highlight a physiological role for circRNAs in cardiac repair and indicate that modulation of circFndc3b expression may represent a potential strategy to promote cardiac function and remodeling after MI.

Circular RNAs (circRNAs) are non-coding RNAs generated from pre-mRNAs of coding genes by the splicing machinery whose function in the heart is poorly understood. Here the authors show that AAV-mediated delivery of the circRNA circFndc3b prevents cardiomyocyte apoptosis, enhances angiogenesis, and attenuates LV dysfunction post-MI in mice by regulating FUS-VEGF-A signalling.

FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Unraveling the Pathways to Neuronal Homeostasis and Disease: Mechanistic Insights into the Role of RNA-Binding Proteins and Associated Factors

The timing, dosage and location of gene expression are fundamental determinants of brain architectural complexity. In neurons, this is, primarily, achieved by specific sets of trans-acting RNA-binding proteins (RBPs) and their associated factors that bind to specific cis elements throughout the RNA sequence to regulate splicing, polyadenylation, stability, transport and localized translation at both axons and dendrites. Not surprisingly, misregulation of RBP expression or disruption of its function due to mutations or sequestration into nuclear or cytoplasmic inclusions have been linked to the pathogenesis of several neuropsychiatric and neurodegenerative disorders such as fragile-X syndrome, autism spectrum disorders, spinal muscular atrophy, amyotrophic lateral sclerosis and frontotemporal dementia. This review discusses the roles of Pumilio, Staufen, IGF2BP, FMRP, Sam68, CPEB, NOVA, ELAVL, SMN, TDP43, FUS, TAF15, and TIA1/TIAR in RNA metabolism by analyzing their specific molecular and cellular function, the neurological symptoms associated with their perturbation, and their axodendritic transport/localization along with their target mRNAs as part of larger macromolecular complexes termed ribonucleoprotein (RNP) granules.

NPM-hMLF1 fusion protein suppresses defects of a Drosophila FTLD model expressing the human FUS gene

Fused in sarcoma (FUS) was identified as a component of typical inclusions in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). In FTLD, both nuclear and cytoplasmic inclusions with wild-type FUS exist, while cytoplasmic inclusions with a mutant-form of FUS occur in many ALS cases. These observations imply that FUS plays a role across these two diseases. In this study, we examined the effect of several proteins including molecular chaperons on the aberrant eye morphology phenotype induced by overexpression of wild-type human FUS (hFUS) in Drosophila eye imaginal discs. By screening, we found that the co-expression of nucleophosmin–human myeloid leukemia factor 1 (NPM-hMLF1) fusion protein could suppress the aberrant eye morphology phenotype induced by hFUS. The driving of hFUS expression at 28 °C down-regulated levels of hFUS and endogenous cabeza, a Drosophila homolog of hFUS. The down-regulation was mediated by proteasome dependent degradation. Co-expression of NPM-hMLF1 suppressed this down-regulation. In addition, co-expression of NPM-hMLF1 partially rescued pharate adult lethal phenotype induced by hFUS in motor neurons. These findings with a Drosophila model that mimics FTLD provide clues for the development of novel FTLD therapies.

Lymphoblastoid cell lines as a model to understand amyotrophic lateral sclerosis disease mechanisms

In the past, amyotrophic lateral sclerosis (ALS) has been considered a 'neurocentric' disease; however, new evidence suggests that it should instead be looked at from a 'multisystemic' or 'non-neurocentric' point of view. From 2006, we focused on the study of non-neural cells: ALS patients' peripheral blood mononuclear cells (PMBCs) and lymphoblastoid cell lines (LCLs). Here, we characterize LCLs of sporadic ALS (sALS) and patients carrying SOD1, TARDBP and FUS mutations to identify an ALS biologically relevant molecular signature, and determine whether and how mutations differentially affect ALS-linked pathways. Although LCLs are different from motor neurons (MNs), in LCLs we found some features typical of degenerating MNs in ALS, i.e. protein aggregation and mitochondrial dysfunction. Moreover, different gene mutations have different effects on ALS cellular mechanisms. TARDBP and FUS mutations imbalance mitochondrial dynamism toward increased fusion, whereas sALS and SOD1 mutations mainly affect fission. With regards to protein aggregation and/or mislocalization, TARDBP and SOD1 mutations show the presence of aggregates, whereas FUS mutation does not induce protein aggregation and/or mislocalization. Finally, all LCLs, independently from mutation, are not able to work in a condition of excessive energy request, suggesting that mitochondria from ALS patients are characterized by a significant metabolic defect. Taken together, these data indicate that LCLs could be a valid cellular model in ALS research in the identification and study of specific pathological pathways.

Post-transcriptional regulation of FUS and EWS protein expression by miR-141 during neural differentiation

Brain development involves proliferation, migration and specification of neural progenitor cells, culminating in neuronal circuit formation. Mounting evidence indicates that improper regulation of RNA binding proteins (RBPs), including members of the FET (FUS, EWS, TAF15) family, results in defective cortical development and/or neurodegenerative disorders. However, in spite of their physiological relevance, the precise pattern of FET protein expression in developing neurons is largely unknown. Herein, we found that FUS, EWS and TAF15 expression is differentially regulated during brain development, both in time and in space. In particular, our study identifies a fine-tuned regulation of FUS and EWS during neuronal differentiation, whereas TAF15 appears to be more constitutively expressed. Mechanistically FUS and EWS protein expression is regulated at the post-transcriptional level during neuron differentiation and brain development. Moreover, we identified miR-141 as a key regulator of these FET proteins that modulate their expression levels in differentiating neuronal cells. Thus, our studies uncover a novel link between post-transcriptional regulation of FET proteins expression and neurogenesis.

hnRNPA1 autoregulates its own mRNA expression to remain non-cytotoxic

Heterogeneous nuclear ribonucleoprotein (hnRNP)A1, a member of the hnRNP family, is involved in a variety of RNA metabolisms. The hnRNPA1 expression is altered in some human diseases and mutations of the hnRNPA1 gene cause amyotrophic lateral sclerosis and multisystem proteinopathy. It has been therefore assumed that the dysregulation of hnRNPA1 is linked to the pathogenesis of the diseases. However, the mechanism underlying the regulation of the hnRNPA1 expression remains unknown. In this study, using cell-based models, we have found that hnRNPA1 negatively regulates its own mRNA expression by inhibiting the intron10 splicing of hnRNPA1 pre-mRNA. This mechanism likely serves as an autoregulation of the hnRNPA1 expression. We have also found that a low-grade excess of hnRNPA1 expression causes cytotoxicity by activating the mitochondrial apoptosis pathway. Collectively, these data suggest that the level of hnRNPA1 is strictly controlled to be within a certain range by the mRNA autoregulation in the physiological condition so that the cytotoxicity-causative alteration of hnRNPA1 expression does not take place.

Linking mitochondrial dysfunction, metabolic syndrome and stress signaling in Neurodegeneration

Mounting evidence suggests a link between metabolic syndrome (MetS) such as diabetes, obesity, non-alcoholic fatty liver disease in the progression of Alzheimer's disease (AD), Parkinson's disease (PD) and other neurodegenerative diseases (NDDs). For instance, accumulated Aβ oligomer is enhancing neuronal Ca²⁺ release and neural NO where increased NO level in the brain through post translational modification is modulating the level of insulin production. It has been further confirmed that irrespective of origin; brain insulin resistance triggers a cascade of the neurodegeneration phenomenon which can be aggravated by free reactive oxygen species burden, ER stress, metabolic dysfunction, neuorinflammation, reduced cell survival and altered lipid metabolism. Moreover, several studies confirmed that MetS and diabetic sharing common mechanisms in the progression of AD and NDDs where mitochondrial dynamics playing a critical role. Any mutation in mitochondrial DNA, exposure of environmental toxin, high-calorie intake, homeostasis imbalance, glucolipotoxicity is causative factors for mitochondrial dysfunction. These cumulative pleiotropic burdens in mitochondria leads to insulin resistance, increased ROS production; enhanced stress-related enzymes that is directly linked MetS and diabetes in neurodegeneration. Since, the linkup mechanism between mitochondrial dysfunction and disease phenomenon of both MetS and NDDs is quite intriguing, therefore, it is pertinent for the researchers to identify and implement the therapeutic interventions for targeting MetS and NDDs. Herein, we elucidated the pertinent role of MetS induced mitochondrial dysfunction in neurons and their consequences in NDDs. Further, therapeutic potential of well-known biomolecules and chaperones to target altered mitochondria has been comprehensively documented. This article is part of a Special Issue entitled: Oxidative Stress and Mitochondrial Quality in Diabetes/Obesity and Critical Illness Spectrum of Diseases - edited by P. Hemachandra Reddy.