The splicing regulators RBM5 and RBM10 are subunits of the U2 snRNP engaged with intron branch sites on chromatin

Understanding the mechanisms of pre-mRNA splicing is limited by the technical challenges to examining spliceosomes in vivo. Here, we report the isolation of RNP complexes derived from precatalytic A or B-like spliceosomes solubilized from the chromatin pellet of mammalian cell nuclei. We found that these complexes contain U2 snRNP proteins and a portion of the U2 snRNA bound with protected RNA fragments that precisely map to intronic branch sites across the transcriptome. These U2 complexes also contained the splicing regulators RBM5 and RBM10. We found RBM5 and RBM10 bound to nearly all branch site complexes and not simply those at regulated exons. The deletion of a conserved RBM5/RBM10 peptide sequence, including a zinc finger motif, disrupted U2 interaction and rendered the proteins inactive for the repression of many alternative exons. We propose a model where RBM5 and RBM10 regulate splicing as components of the U2 snRNP complex following branch site base pairing.

PEI, a new transfection method, augments the inhibitory effect of RBM5 on prostate cancer

PEI is a cationic polymer, serving as a non-viral transfection carrier grounded in nanotechnology that enhances transfection efficiency via the proton sponge effect. RBM5 is an RNA-binding protein that can inhibit tumor development. This study involved the transfection of RBM5 in prostate cancer cells with PEI, Lipo2000, and their combination. Transwell and wound healing assays were used to observe invasion and migration of prostate cancer cells and flow cytometry was used to observe the apoptosis. Detect the expression of invasion and migration-related protein MMP9 through western blotting experiment. An activity detection kit was used to detect the activity of apoptotic protein caspase-3. We found that there was no significant difference in transfection efficiency when PEI and Lipo2000 were used alone but it significantly improved when they are combined. RBM5 reduced invasion, migration, and proliferation of prostate cancer and enhanced apoptosis. MMP9 expression was reduced, and the activity of caspase-3 was increased. PEI transfection could improve the inhibition of RBM5 on tumors more than Lipo2000. The inhibitory effect is more obvious when the two are used together. RBM5 transfected with PEI can amplify its inhibitory effect on prostate cancer, and this effect is more evident when combined with Lipo2000.

Gene knockout of RNA binding motif 5 in the brain alters RIMS2 protein homeostasis in the cerebellum and Hippocampus and exacerbates behavioral deficits after a TBI in mice

RNA binding motif 5 (RBM5) is a tumor suppressor in cancer but its role in the brain is unclear. We used conditional gene knockout (KO) mice to test if RBM5 inhibition in the brain affects chronic cortical brain tissue survival or function after a controlled cortical impact (CCI) traumatic brain injury (TBI). RBM5 KO decreased baseline contralateral hemispheric volume (p < 0.0001) and exacerbated ipsilateral tissue loss at 21 d after CCI in male mice vs. wild type (WT) (p = 0.0019). CCI injury, but not RBM5 KO, impaired beam balance performance (0-5d post-injury) and swim speed on the Morris Water Maze (MWM) (19-20d) (p < 0.0001). RBM5 KO was associated with mild learning impairment in female mice (p = 0.0426), reflected as a modest increase in escape latency early in training (14-18d post-injury). However, KO did not affect spatial memory at 19d post-injury in male or in female mice but it was impaired by CCI in females (p = 0.0061). RBM5 KO was associated with impaired visual function in male mice on the visible platform test at 20d post-injury (p = 0.0256). To explore signaling disturbances in KOs related to behavior, we first cross-referenced known brain-specific RBM5-regulated gene targets with genes in the curated RetNet database that impact vision. We then performed a secondary literature search on RBM5-regulated genes with a putative role in hippocampal function. Regulating synaptic membrane exocytosis 2 (RIMS) 2 was identified as a gene of interest because it regulates both vision and hippocampal function. Immunoprecipitation and western blot confirmed protein expression of a novel ~170 kDa RIMS2 variant in the cerebellum, and in the hippocampus, it was significantly increased in KO vs WT (p < 0.0001), and in a sex-dependent manner (p = 0.0390). Furthermore, male KOs had decreased total canonical RIMS2 levels in the cerebellum (p = 0.0027) and hippocampus (p < 0.0001), whereas female KOs had increased total RIMS1 levels in the cerebellum (p = 0.0389). In summary, RBM5 modulates brain function in mammals. Future work is needed to test if RBM5 dependent regulation of RIMS2 splicing effects vision and cognition, and to verify potential sex differences on behavior in a larger cohort of mice.

CHIP suppresses the proliferation and migration of A549 cells by mediating the ubiquitination of eIF2α and upregulation of tumor suppressor RBM5

The protein kinase RNA-like endoplasmic reticulum kinase (PERK)-eukaryotic translation initiation factor 2 subunit α (eIF2α) pathway plays an essential role in endoplasmic reticulum (ER) stress. When the PERK-eIF2α pathway is activated, PERK phosphorylates eIF2α (p-eIF2α) at Ser51 and quenches global protein synthesis. In this study, we verified eIF2α as a bona fide substrate of the E3 ubiquitin ligase carboxyl terminus of the HSC70-interaction protein (CHIP) both in vitro and in cells. CHIP mediated the ubiquitination and degradation of nonphosphorylated eIF2α in a chaperone-independent manner and promoted the upregulation of the cyclic AMP-dependent transcription factor under endoplasmic reticulum stress conditions. Cyclic AMP-dependent transcription factor induced the transcriptional enhancement of the tumor suppressor genes PTEN and RBM5. Although transcription was enhanced, the PTEN protein was subsequently degraded by CHIP, but the expression of the RBM5 protein was upregulated, thereby suppressing the proliferation and migration of A549 cells. Overall, our study established a new mechanism that deepened the understanding of the PERK-eIF2α pathway through the ubiquitination and degradation of eIF2α. The crosstalk between the phosphorylation and ubiquitination of eIF2α shed light on a new perspective for tumor progression.

RNA-binding protein RBM5 plays an essential role in acute myeloid leukemia by activating the oncogenic protein HOXA9

BACKGROUND

The oncogenic protein HOXA9 plays a critical role in leukemia transformation and maintenance, and its aberrant expression is a hallmark of most aggressive acute leukemia. Although inhibiting the upstream regulators of HOXA9 has been proven as a significant therapeutic intervention, the comprehensive regulation network controlling HOXA9 expression in leukemia has not been systematically investigated.

RESULTS

Here, we perform genome-wide CRISPR/Cas9 screening in the HOXA9-driven reporter acute leukemia cells. We identify a poorly characterized RNA-binding protein, RBM5, as the top candidate gene required to maintain leukemia cell fitness. RBM5 is highly overexpressed in acute myeloid leukemia (AML) patients compared to healthy individuals. RBM5 loss triggered by CRISPR knockout and shRNA knockdown significantly impairs leukemia maintenance in vitro and in vivo. Through domain CRISPR screening, we reveal that RBM5 functions through a noncanonical transcriptional regulation circuitry rather than RNA splicing, such an effect depending on DNA-binding domains. By integrative analysis and functional assays, we identify HOXA9 as the downstream target of RBM5. Ectopic expression of HOXA9 rescues impaired leukemia cell proliferation upon RBM5 loss. Importantly, acute protein degradation of RBM5 through auxin-inducible degron system immediately reduces HOXA9 transcription.

CONCLUSIONS

We identify RBM5 as a new upstream regulator of HOXA9 and reveal its essential role in controlling the survival of AML. These functional and molecular mechanisms further support RBM5 as a promising therapeutic target for myeloid leukemia treatment.

Downregulated RBM5 Enhances CARM1 Expression and Activates the PRKACA/GSK3β Signaling Pathway through Alternative Splicing-Coupled Nonsense-Mediated Decay

Downregulated RNA-binding motif protein 5 (RBM5) promotes the development and progression of various tumors, including bladder cancer (BC). Alternative splicing (AS) plays a crucial role in the progression of cancer by producing protein isomers with different functions or by promoting nonsense-mediated mRNA decay (NMD). However, whether RBM5 modulates the progression of BC through AS-NMD remains unexplored. In this study, we revealed that the downregulation of RBM5 expression promoted the expression of coactivator-associated arginine methyltransferase 1 (CARM1) in BC cells and tissues. Increased expression of CARM1 facilitated the activation of the Wnt/β-catenin axis and cell proliferation, which then contributed to the poor prognosis of patients with BC. Interestingly, RBM5 bound directly to CARM1 mRNA and participated in AS-NMD, downregulating the expression of CARM1. In addition, we revealed that protein kinase catalytic subunit alpha (PRKACA) functioned as a phosphorylated kinase of GSK3β, was regulated by CARM1 at the transcription level, and promoted the growth and progression of BC cells. Furthermore, in this study, we demonstrated a regulatory mechanism of Wnt/β-catenin activation through the RBM5/CARM1/PRKACA axis and identified a novel potential target for treating BC.

Elucidating the role of RBM5 in osteoclastogenesis: a novel potential therapeutic target for osteoporosis

Osteoporosis is a prevalent bone disease with multigene involved, and the molecular mechanisms of its pathogenesis are not entirely understood. This study aims to identify novel key genes involved in osteoporosis to discover potential pharmacological targets. We analyzed three microarray datasets and identified four differentially expressed genes. The LASSO model indicated that RNA-binding motif protein 5 (RBM5) is associated with osteoporosis and is a potential drug target. We conducted the Spearman correlation analysis and found 52 genes that were significantly related to RBM5. Enrichment analysis showed that these genes were primarily involved in RNA splicing and osteoclast differentiation pathways. By using lentivirus-based shRNA, we successfully knocked down RBM5 expression in RAW264.7 cell line, which showed that RBM5 knockdown significantly impaired their differentiation potential to mature osteoclasts and significantly inhibited bone-resorbing activity. RT-qPCR analyses revealed the expression of osteoclastogenesis marker genes was downregulated along with RBM5 expression. These findings suggest that RBM5 plays a crucial role in the pathogenesis of osteoporosis and provides a new potential pharmacological target.

The role of molecular diagnostics in aneurysmal and simple bone cysts - a prospective analysis of 19 lesions

Aneurysmal (ABC) and simple bone cysts (SBC) have been traditionally distinguished by radiological and histopathological features. However, there is some radiological and histopathological overlap between ABC and SBC. ABC is characterised by USP6 fusions while, recently, NFATC2 fusions have been found in a large proportion of SBC. Identifying these fusions may assist in confirming the diagnosis of either ABC or SBC. To elaborate the potential benefit of molecular testing, we report a prospective series of 19 consecutive bone cysts with comprehensive radiological, histopathological and molecular diagnostics. Integrating radiological, histopathological and molecular findings, 11 cysts were diagnosed as SBC and 8 as ABC. Radiologically, 6 of 11 SBC and 6 of 8 ABC were diagnosed as ABC. Fibrin-like collagen deposits were identified in 8 of 11 (73%) SBC and 3 of 8 (38%) ABC. Nodular fasciitis-like areas were identified in 6 of 8 (75%) ABC and in 7 of 11 (64%) SBC. A USP6 fusion was identified in all 8 ABC, including a novel RBM5-USP6 fusion. An NFATC2 fusion was found in 7 of 11 SBC (FUS-NFATC2 fusion in 5 and EWSR1-NFATC2 in 2 cases). There is radiological and histopathological overlap between SBC and ABC in a significant proportion of cases. A diagnosis of ABC is frequently suggested radiologically in SBC, and fibrin-like deposits, thought to be specific for SBC, may be found in some ABC. Molecular testing may significantly improve diagnostic accuracy in bone cysts.

RBM10: Structure, functions, and associated diseases

RBM10 is a nuclear RNA-binding protein (RBP) that regulates the alternative splicing of primary transcripts. Recently, research on RBM10 has become increasingly active owing to its clinical importance, as indicated by studies on RBM0 mutations that cause TARP syndrome, an X-linked congenital pleiotropic developmental anomaly, and various cancers such as lung adenocarcinoma in adults. Herein, the molecular biology of RBM10 and its significance in medicine are reviewed, focusing on the gene and protein structures of RBM10, its cell biology, molecular functions and regulation, relationship with the paralogous protein RBM5, and the mutations of RBM10 and their associated diseases. Finally, the challenges in future studies of RBM10 are discussed in the concluding remarks.

RNA Binding Motif 5 (RBM5) in the CNS-Moving Beyond Cancer to Harness RNA Splicing to Mitigate the Consequences of Brain Injury

Gene splicing modulates the potency of cell death effectors, alters neuropathological disease processes, influences neuronal recovery, but may also direct distinct mechanisms of secondary brain injury. Therapeutic targeting of RNA splicing is a promising avenue for next-generation CNS treatments. RNA-binding proteins (RBPs) regulate a variety of RNA species and are prime candidates in the hunt for druggable targets to manipulate and tailor gene-splicing responses in the brain. RBPs preferentially recognize unique consensus sequences in targeted mRNAs. Also, RBPs often contain multiple RNA-binding domains (RBDs)—each having a unique consensus sequence—suggesting the possibility that drugs could be developed to block individual functional domains, increasing the precision of RBP-targeting therapies. Empirical characterization of most RBPs is lacking and represents a major barrier to advance this emerging therapeutic area. There is a paucity of data on the role of RBPs in the brain including, identification of their unique mRNA targets, defining how CNS insults affect their levels and elucidating which RBPs (and individual domains within) to target to improve neurological outcomes. This review focuses on the state-of-the-art of the RBP tumor suppressor RNA binding motif 5 (RBM5) in the CNS. We discuss its potent pro-death roles in cancer, which motivated our interest to study it in the brain. We review recent studies showing that RBM5 levels are increased after CNS trauma and that it promotes neuronal death in vitro. Finally, we conclude with recent reports on the first set of RBM5 regulated genes identified in the intact brain, and discuss how those findings provide new clues germane to its potential function(s) in the CNS, and pose new questions on its therapeutic utility to mitigate CNS injury.

RBM5 Acts as Tumor Suppressor in Medulloblastoma through Regulating Wnt/β-Catenin Signaling

INTRODUCTION

RBM5 acts as a tumor suppressor gene in lung and breast cancers; however, its role in the pathogenesis of medulloblastoma (MB) remains unclear. We previously identified 4 RBM5 mutations in whole exome sequencing analysis of 40 MB patients. This study examined the role of RBM5 in MB progression.

METHODS

The expression patterns of RBM5 in tissues of 40 MB patients were analyzed using immunohistochemistry. Associations between RBM5 expression and overall survival (OS) were evaluated using Kaplan-Meier analysis. The RBM5 role in Daoy cells' proliferation, migration, and Wnt/β-catenin signaling was analyzed after RBM5 knockdown and overexpression.

RESULTS

The expression level of RBM5 mRNA and protein was significantly lower in MB than that in adjacent normal control tissues, and low RBM5 expression was significantly associated with reduced OS (p = 0.034). RBM5 knockdown induced Daoy and ONS-76 cells proliferation, while RBM5 overexpression repressed cell proliferation and migration in vitro (all p < 0.05). β-Catenin, LEF1, and cyclin D1 mRNA levels were upregulated, while DKK1 expression was downregulated in Daoy cells following RBM5 knockdown.

CONCLUSION

RBM5 may function as a tumor suppressor in MB by regulating Wnt/β-catenin signaling, and its reduced expression is associated with lower OS.

Conformational Dynamics from Ambiguous Zinc Coordination in the RanBP2-Type Zinc Finger of RBM5

The multi-domain RNA binding protein RBM5 is a molecular signature of metastasis. RBM5 regulates alternative splicing of apoptotic genes including the cell death receptor Fas and the initiator Caspase-2. The RBM5 RanBP2-type zinc finger (Zf1) is known to specifically recognize single-stranded RNAs with high affinity. Here, we study the structure and conformational dynamics of the Zf1 zinc finger of human RBM5 using NMR. We show that the presence of a non-canonical cysteine in Zf1 kinetically destabilizes the protein. Metal-exchange kinetics show that mutation of the cysteine establishes high-affinity coordination of the zinc. Our data indicate that selection of such a structurally destabilizing mutation during the course of evolution could present an opportunity for functional adaptation of the protein.

Hepatitis delta virus interacts with splicing factor SF3B155 and alters pre-mRNA splicing of cell cycle control genes

Hepatitis delta virus (HDV) is the agent responsible for the most severe form of human viral hepatitis. The HDV genome consists of a single-stranded circular RNA molecule that encodes for one single protein, the delta antigen. Given its simplicity, HDV must make use of several host cellular proteins to accomplish its life cycle processes, including transcription, replication, post-transcriptional, and post-translational modifications. Consequently, identification of the interactions established between HDV components and host proteins assumes a pivotal interest in the search of novel therapeutic targets. Here, we used the yeast three-hybrid system to screen a human liver cDNA library to identify host proteins that interact with the HDV genomic RNA. One of the identified proteins corresponded to the splicing factor SF3B155, a component of the U2snRNP complex that is essential for the early recognition of 3' splice sites in the pre-mRNAs of human genes. We show that the interaction between the HDV genomic RNA and SF3B155 occurs in vivo and that the expression of HDV promotes changes in splicing of human genes whose alternative splicing is SF3B155-dependent. We further show that expression of HDV triggers alterations in several constitutive and alternative splicing events in the tumor suppressor RBM5 transcript, with consequent reduction of its protein levels. This is the first description that HDV expression promotes changes in the splicing of human genes, and we suggest that the HDV-induced alternative splicing changes, through SF3B155 sequester, may contribute for the early progression to hepatocellular carcinoma characteristic of HDV-infected patients.

Identification of Novel Targets of RBM5 in the Healthy and Injured Brain

The tumor suppressor RNA-binding motif 5 (RBM5) regulates the expression levels and cassette exon-definition (i.e. splicing) of a select set of mRNAs in a tissue-specific manner. Most RBM5-regulated targets were identified in oncological investigations and frequently involve genes which mediate apoptotic cell death. Little is known about the role of RBM5 in the brain. Also, it is unclear if a brain injury may be required to detect RBM5 mediated effects on pro-apoptotic genes due to their low expression levels in the healthy adult CNS at baseline. Conditional/floxed (brain-specific) gene deleter mice were generated to elucidate CNS-specific RBM5 mRNA targets. Male/female mice were subjected to a severe controlled cortical impact (CCI) traumatic brain injury (TBI) in order to increase the background expression of pro-death mRNAs and facilitate testing of the hypothesis that RBM5 inhibition decreases post-injury upregulation of caspases/FAS in the CNS. As expected, a CCI increased caspases/FAS mRNA in the injured cortex. RBM5 KO did not affect their levels or splicing. Surprisingly, KO increased the mRNA levels of novel targets including casein kinase 2 alpha prime interacting protein (Csnka2ip/CKT2) - a gene not thought to be expressed in the brain, contrary to findings here. Twenty-two unique splicing events were also detected in KOs including increased block-inclusion of cassette exons 20-22 in regulating synaptic membrane exocytosis 2 (Rims2). In conclusion, here we used genome-wide transcriptomic analysis on healthy and injured RBM5 KO mouse brain tissue to elucidate the first known gene targets of this enigmatic RBP in this CNS.

miR-938 promotes cell proliferation by regulating RBM5 in lung adenocarcinoma cells

A growing body of research suggests that microRNAs (miRNAs) may play a key part in the progression of various cancers, including lung adenocarcinoma (LUAD). However, the expression and mechanism of miR-938 (microRNA-938) in LUAD have not been defined. Compared with adjacent tissues, the level of miR-938 was up-regulated in LUAD tissues. miR-938 expression was significantly associated with tumor size. In vitro assays indicated that miR-938 expression was also increased in the LUAD cell lines. Overexpression of miR-938 promoted LUAD cell proliferation, whereas down-regulation of miR-938 had the opposite effect. We identified RNA-binding protein 5 (RBM5) as a potential target gene of miR-938 in LUAD. Expression of RBM5 was down-regulated in LUAD tumor tissues and negatively correlated with expression of miR-938. Up-regulation of RBM5 reversed cell proliferation by inhibition of miR-938 expression in LUAD cells. These results showed that miR-938 may act as an oncogenic miRNA by targeting RBM5 in LUAD, indicating that miR-938 could be used as a potential therapeutic target for LUAD patients.

Proteins as the Molecular Markers of Male Fertility

Proteins play a key role in many functions such as metabolic activity, differentiation, as cargos, and cell fate regulators. It is necessary to know about the proteins involved in male fertility to develop remedies for the treatment of male infertility. However, the role of the proteins is not limited to particular aspect in the biological systems. Some of the proteins act as ion channels such as catsper, and protein such as Nanos is a translational repressor in germ cells and expressed in prenatal period whose role in male fertility is not clearly understood. Rbm5 is a pre-mRNA splicing factor necessary for sperm differentiation whose loss results in deficit in sperm production. DEFB114 is a beta-defensin family protein necessary for sperm motility in lipopolysaccharide-challenged mice. TEX101 is a plasma membrane specific germ cell protein whose function is not clearly identified. Gpr56 is an another adhesion protein whose null mutation leads to arrest of production of pupps. Amyloid precursor protein in Alzheimer's disease plays a role in male fertility whose function is uncertain which has to be considered while targeting them. The study on amyloid precursor protein in male fertility is a novel thing, but requires further study in correlation to Alzheimer's disease.

RBM-5 modulates U2AF large subunit-dependent alternative splicing in C. elegans

A key step in pre-mRNA splicing is the recognition of 3' splicing sites by the U2AF large and small subunits, a process regulated by numerous trans-acting splicing factors. How these trans-acting factors interact with U2AF in vivo is unclear. From a screen for suppressors of the temperature-sensitive (ts) lethality of the C. elegans U2AF large subunit gene uaf-1(n4588) mutants, we identified mutations in the RNA binding motif gene rbm-5, a homolog of the tumor suppressor gene RBM5. rbm-5 mutations can suppress uaf-1(n4588) ts-lethality by loss of function and neuronal expression of rbm-5 was sufficient to rescue the suppression. Transcriptome analyses indicate that uaf-1(n4588) affected the expression of numerous genes and rbm-5 mutations can partially reverse the abnormal gene expression to levels similar to that of wild type. Though rbm-5 mutations did not obviously affect alternative splicing per se, they can suppress or enhance, in a gene-specific manner, the altered splicing of genes in uaf-1(n4588) mutants. Specifically, the recognition of a weak 3' splice site was more susceptible to the effect of rbm-5. Our findings provide novel in vivo evidence that RBM-5 can modulate UAF-1-dependent RNA splicing and suggest that RBM5 might interact with U2AF large subunit to affect tumor formation.

Effect of miR-483-5p on apoptosis of lung cancer cells through targeting of RBM5

RBM5 has been reported to be a candidate tumor suppressor gene which plays an important role in the induction of apoptosis. In this study, we investigated the effect of miR-483-5p on apoptosis of lung cancer cells and the underlying mechanism. We found that the expression of miR-483-5p mRNA was significantly up-regulated in lung cancer compared with adjacent para-cancerous tissues by using real-time PCR. Silencing miR-483-5p promoted A549 cell apoptosis and enhanced caspase-3 activity by flow cytometry with annexin V-FITC/PI staining and caspase-3 activity report kit. Western blotting demonstrated that miR-483-5p mimicked down-regulated RBM5 protein expression and miR-483-5p inhibitor up-regulated RBM5 protein expression. With additional bioinformatics analysis, we confirmed that RBM5 is a target gene of miR-483-5p and is favored for treating NSCLC. The immunohistochemical pattern of RBM5 could be used to predictoutcome for NSCLC. In conclusion, our results support that RBM5 expression can be regulated by miR-483-5p which is a prognostic marker for NSCLC patients. miR-483-5p inhibitor plays a role in lung cancer through targeting RBM5 to induce apoptosis.

RBM10: Harmful or helpful-many factors to consider

RBM10 is an RNA binding motif (RBM) protein expressed in most, if not all, human and animal cells. Interest in RBM10 is rapidly increasing and its clinical importance is highlighted by its identification as the causative agent of TARP syndrome, a developmental condition that significantly impacts affected children. RBM10's cellular functions are beginning to be explored, with initial studies demonstrating a tumor suppressor role. Very recently, however, contradictory results have emerged, suggesting a tumor promoter role for RBM10. In this review, we describe the current state of knowledge on RBM10, and address this dichotomy in RBM10 function. Furthermore, we discuss what may be regulating RBM10 function, particularly the importance of RBM10 alternative splicing, and the relationship between RBM10 and its paralogue, RBM5. As RBM10‐related work is gaining momentum, it is critical that the various aspects of RBM10 molecular biology revealed by recent studies be considered moving forward. It is only if these recent advances in RBM10 structure and function are considered that a clearer insight into RBM10 function, and the disease states with which RBM10 mutation is associated, will be gained.

BrainPhys® increases neurofilament levels in CNS cultures, and facilitates investigation of axonal damage after a mechanical stretch-injury in vitro

Neurobasal®/B27 is a gold standard culture media used to study primary neurons in vitro. An alternative media (BrainPhys®/SM1) was recently developed which robustly enhances neuronal activity vs. Neurobasal® or DMEM. To the best of our knowledge BrainPhys® has not been explored in the setting of neuronal injury. Here we characterized the utility of BrainPhys® in a model of in vitro mechanical-stretch injury.

METHODS/RESULTS

Primary rat cortical neurons were maintained in classic Neurobasal®, or sequentially maintained in Neurocult® followed by BrainPhys® (hereafter simply referred to as "BrainPhys® maintained neurons"). The levels of axonal markers and proteins involved in neurotransmission were compared on day in vitro 10 (DIV10). BrainPhys® maintained neurons had higher levels of GluN2B, GluR1, Neurofilament light/heavy chain (NF-L & NF-H), and protein phosphatase 2 A (PP2A) vs. neurons in Neurobasal®. Mechanical stretch-injury (50ms/54% biaxial stretch) to BrainPhys® maintained neurons modestly (albeit significantly) increased 24h lactate dehydrogenase (LDH) levels but markedly decreased axonal NF-L levels post-injury vs. uninjured controls or neurons given a milder 38% stretch-injury. Furthermore, two 54% stretch-injuries (in tandem) exacerbated 24h LDH release, increased α-spectrin breakdown products (SBDPs), and decreased Tau levels. Also, BrainPhys® maintained cultures had decreased markers of cell damage 24h after a single 54% stretch-injury vs. neurons in Neurobasal®. Finally, we tested the hypothesis that lentivirus mediated overexpression of the pro-death protein RBM5 exacerbates neuronal and/or axonal injury in primary CNS cultures. RBM5 overexpression vs. empty-vector controls increased 24h LDH release, and SBDP levels, after a single 54% stretch-injury but did not affect NF-L levels or Tau.

CONCLUSION

BrainPhys® is a promising new reagent which facilities the investigation of molecular targets involved in axonal and/or neuronal injury in vitro.

Whole-transcriptome microarray analysis reveals regulation of Rab4 by RBM5 in neurons

RNA binding motif 5 (RBM5) is a nuclear protein that modulates gene transcription and mRNA splicing in cancer cells. The brain is among the highest RBM5-expressing organ in the body but its mRNA target(s) or functions in the CNS have not been elucidated. Here we knocked down (KO) RBM5 in primary rat cortical neurons and analyzed total RNA extracts by gene microarray vs. neurons transduced with lentivirus to deliver control (non-targeting) shRNA. The mRNA levels of Sec23A (involved in ER-Golgi transport) and the small GTPase Rab4a (involved in endocytosis/protein trafficking) were increased in RBM5 KO neurons relative to controls. At the protein level, only Rab4a was significantly increased in RBM5 KO extracts. Also, elevated Rab4a levels in KO neurons were associated with decreased membrane levels of oligomeric serotonin transporters (SERT). Finally, RBM5 KO was associated with increased uptake of membrane-derived monomeric SERT.

SIGNIFICANCE

Rab4a is involved in the regulation of endocytosis and protein trafficking in cells. In the CNS it regulates diverse neurobiological functions including (but not limited to) trafficking of transmembrane proteins involved in neurotransmission (e.g. SERT), maintaining dendritic spine size, promoting axonal growth, and modulating cognition. Our findings suggest that RBM5 regulates Rab4a in rat neurons.

RBM5 inhibits tumorigenesis of gliomas through inhibition of Wnt/β-catenin signaling and induction of apoptosis

BACKGROUND

Gliomas are one of the most common malignant brain tumors and bring a big threat to human life as traditional therapy is unsatisfactory. RBM5 was a RNA-binding motif protein and was reported as a tumor suppressor. But the role of RBM5 in gliomas was unknown.

METHODS

The mRNA level of RBM5 was determined in gliomas tissues and cell lines by real-time quantitative PCR (qRT-PCR) assay while the association of RBM5 expression with prognosis was analyzed by Kaplan-Meier method and compared by log-rank test. Lentivirus was used to overexpress RBM5 in gliomas cells. MTT and BrdU incorporation assay were used to determine cell proliferation and DNA synthesis when the ability of cell migration and invasion was analyzed by transwell assay with/without Matrigel. Cell apoptosis rate was determined with fluorescence-activated cell sorting (FACS) method. Then, expression of apoptosis molecules and critical members in Wnt/β-catenin pathway were detected by western blot analysis.

RESULTS

RBM5 was shown to be downregulated in gliomas tissues and gliomas cell lines. And decreased RBM5 expression was clinically correlated with tumor stage, patient age, and poor prognosis of gliomas patients. The proliferation and DNA synthesis was dramatically inhibited when RBM5 was overexpressed in SHG44 or U251 cells. Also, the ability of cell migration and invasion was disrupted. Then, the level of β-catenin and Cyclin D1 significantly decreased when DKK1 and P-GSK-3β increased reversely in SHG44 cells, which suggested that RBM5 inhibited canonical Wnt/β-catenin signaling. Meanwhile, we demonstrated that caspase3-mediated apoptotic pathway was activated by RBM5 as Bax, TNF-α, and cleaved caspase3 were greatly upregulated while antiapoptotic molecule Bcl-2 was downregulated. Additionally, that apoptotic rate increased significantly from less than 1 to 32% in RBM5-overexpressed SHG44 cells further supported the pro-apoptosis role of RBM5 in gliomas cells.

CONCLUSIONS

RBM5 plays a suppressor role in human gliomas by inhibiting Wnt/β-catenin signaling and inducing cell apoptosis. This study improves our knowledge about the carcinogenesis and progression of human gliomas, which would greatly contribute to the therapy for gliomas patients.

Co- and post-transcriptional regulation of Rbm5 and Rbm10 in mouse cells as evidenced by tissue-specific, developmental and disease-associated variation of splice variant and protein expression levels

BACKGROUND

Expression and function of the two RNA binding proteins and regulators of alternative splicing, RBM5 and RBM10, have largely been studied in human tissue and cell lines. The objective of the study described herein was to examine their expression in mouse tissue, in order to lay the framework for comprehensive functional studies using mouse models.

METHODS

All RNA variants of Rbm5 and Rbm10 were examined in a range of normal primary mouse tissues. RNA and protein were examined in differentiating C2C12 myoblasts and in denervated and dystonin-deficient mouse skeletal muscle.

RESULTS

All Rbm5 and Rbm10 variants examined were expressed in all mouse tissues and cell lines. In general, Rbm5 and Rbm10 RNA expression was higher in brain than in skin. RNA expression levels were more varied between cardiac and skeletal muscle, depending on the splice variant: for instance, Rbm10v1 RNA was higher in skeletal than cardiac muscle, whereas Rbm10v3 RNA was higher in cardiac than skeletal muscle. In mouse brain, cardiac and skeletal muscle, RNA encoding an approximately 17kDa potential paralogue of a small human RBM10 isoform was detected, and the protein observed in myoblasts and myotubes. Expression of Rbm5 and Rbm10 RNA remained constant during C2C12 myogenesis, but protein levels significantly decreased. In two muscle disease models, neither Rbm10 nor Rbm5 showed significant transcriptional changes, although significant specific alternative splicing changes of Rbm5 pre-mRNA were observed. Increased RBM10 protein levels were observed following denervation.

CONCLUSIONS

The varied co-transcriptional and post-transcriptional regulation aspects of Rbm5 and Rbm10 expression associated with mouse tissues, myogenesis and muscle disease states suggest that a mouse model would be an interesting and useful model in which to study comprehensive functional aspects of RBM5 and RBM10.

RBM5 and p53 expression after rat spinal cord injury: implications for neuronal apoptosis

RBM5 (RNA-binding motif protein 5), a nuclear RNA binding protein, is known to trigger apoptosis and induce cell cycle arrest by regulating the activity of the tumor suppressor protein p53. However, its expression and function in spinal cord injury (SCI) are still unknown. To investigate whether RBM5 is involved in central nervous system injury and repair, we performed an acute SCI model in adult rats in this study. Our results showed RBM5 was unregulated significantly after SCI, which was accompanied with an increase in the levels of apoptotic proteins such as p53, Bax, and active caspase-3. Immunofluorescent labeling also showed that traumatic SCI induced RBM5 location changes and co-localization with active caspase-3 in neurons. To further probe the role of RBM5, a neuronal cell line PC12 was employed to establish an apoptotic model. Knockdown of RBM5 apparently decreased the level of p53 as well as active caspase-3, demonstrating its pro-apoptotic role in neurons by regulating expressions of p53 and caspase-3. Taken together, our findings indicate that RBM5 promotes neuronal apoptosis through modulating p53 signaling pathway following SCI.

RBM10 regulates alternative splicing

RBM10, originally called S1-1, is a nuclear RNA-binding protein with domains characteristic of RNA processing proteins. It has been reported that RBM10 constitutes spliceosome complexes and that RBM5, a close homologue of RBM10, regulates alternative splicing of apoptosis-related genes, Fas and cFLIP. In this study, we examined whether RBM10 has a regulatory function in splicing similar to RBM5, and determined that it indeed regulates alternative splicing of Fas and Bcl-x genes. RBM10 promotes exon skipping of Fas pre-mRNA as well as selection of an internal 5'-splice site in Bcl-x pre-mRNA. We propose a consensus RBM10-binding sequence at 5'-splice sites of target exons and a mechanistic model of RBM10 action in the alternative splicing.

The RNA recognition motif domains of RBM5 are required for RNA binding and cancer cell proliferation inhibition

RBM5 is a known putative tumor suppressor gene that has been shown to function in cell growth inhibition by modulating apoptosis. RBM5 also plays a critical role in alternative splicing as an RNA binding protein. However, it is still unclear which domains of RBM5 are required for RNA binding and related functional activities. We hypothesized the two putative RNA recognition motif (RRM) domains of RBM5 spanning from amino acids 98-178 and 231-315 are essential for RBM5-mediated cell growth inhibition, apoptosis regulation, and RNA binding. To investigate this hypothesis, we evaluated the activities of the wide-type and mutant RBM5 gene transfer in low-RBM5 expressing A549 cells. We found that, unlike wild-type RBM5 (RBM5-wt), a RBM5 mutant lacking the two RRM domains (RBM5-ΔRRM), is unable to bind RNA, has compromised caspase-2 alternative splicing activity, lacks cell proliferation inhibition and apoptosis induction function in A549 cells. These data provide direct evidence that the two RRM domains of RBM5 are required for RNA binding and the RNA binding activity of RBM5 contributes to its function on apoptosis induction and cell growth inhibition.

RBM10 Modulates Apoptosis and Influences TNF-α Gene Expression

Recent evidence suggests that protein encoded by the RNA Binding Motif 10 (RBM10) gene has the ability to modulate apoptosis. The objective of this study was to test this hypothesis by manipulating RBM10 expression levels and examining the downstream consequences. The results showed that transient overexpression of RBM10 correlated with significantly elevated levels of tumour necrosis factor alpha (TNF-α) mRNA and soluble TNF-α (sTNF-α) protein, and increased apoptosis (phosphatidyl serine exposure on the outer cell membrane and nuclear condensation). Stable RNA interference-mediated RBM10 knockdown clones were less susceptible to TNF-α-mediated apoptosis, and had decreased sTNF-α protein levels. Elevated levels of TNF-α associated with RBM10 overexpression resulted from increased TNF-α transcription, not TNF-α mRNA stabilization. These results suggest that RBM10 has the ability to modulate apoptosis, and that it does so via a mechanism involving alterations to TNFR super family-mediated signaling. These data provide the first direct evidence that human RBM10 can function as an apoptosis modulator and cytokine expression regulator.