Emerging role for RNA binding motif protein 4 in the development of brown adipocytes

Activation of TrkB signaling mitigates cerebellar anomalies caused by Rbm4-Bdnf deficiency

A molecular and functional link between neurotrophin signaling and cerebellar foliation is lacking. Here we show that constitutive knockout of two homologous genes encoding the RNA binding protein RBM4 results in foliation defects at cerebellar lobules VI-VII and delayed motor learning in mice. Moreover, the features of Rbm4 double knockout (dKO), including impaired differentiation of cerebellar granule cells and dendritic arborization of Purkinje cells, are reminiscent of neurotrophin deficiency. Loss of RBM4 indeed reduced brain-derived neurotrophic factor (BDNF). RBM4 promoted the expression of BDNF and full-length TrkB, implicating RBM4 in efficient BDNF-TrkB signaling. Finally, prenatal supplementation with 7,8-dihydroxyflavone, a TrkB agonist, restored granule cell differentiation, Purkinje cell dendritic complexity and foliation-the intercrural fissure in particular-in the neonatal cerebellum of Rbm4dKO mice, which also showed improved motor learning in adulthood. This study provides evidence that prenatal activation of TrkB signaling ameliorates cerebellar malformation caused by BDNF deficiency.

The Role of Splicing Factors in Adipogenesis and Thermogenesis

Obesity is a significant global health risk that can cause a range of serious metabolic problems, such as type 2 diabetes and cardiovascular diseases. Adipose tissue plays a pivotal role in regulating energy and lipid storage. New research has underlined the crucial role of splicing factors in the physiological and functional regulation of adipose tissue. By generating multiple transcripts from a single gene, alternative splicing allows for a greater diversity of the proteome and transcriptome, which subsequently influence adipocyte development and metabolism. In this review, we provide an outlook on the part of splicing factors in adipogenesis and thermogenesis, and investigate how the different spliced isoforms can affect the development and function of adipose tissue.

RNA-Binding Proteins in the Regulation of Adipogenesis and Adipose Function

The obesity epidemic represents a critical public health issue worldwide, as it is a vital risk factor for many diseases, including type 2 diabetes (T2D) and cardiovascular disease. Obesity is a complex disease involving excessive fat accumulation. Proper adipose tissue accumulation and function are highly transcriptional and regulated by many genes. Recent studies have discovered that post-transcriptional regulation, mainly mediated by RNA-binding proteins (RBPs), also plays a crucial role. In the lifetime of RNA, it is bound by various RBPs that determine every step of RNA metabolism, from RNA processing to alternative splicing, nucleus export, rate of translation, and finally decay. In humans, it is predicted that RBPs account for more than 10% of proteins based on the presence of RNA-binding domains. However, only very few RBPs have been studied in adipose tissue. The primary aim of this paper is to provide an overview of RBPs in adipogenesis and adipose function. Specifically, the following best-characterized RBPs will be discussed, including HuR, PSPC1, Sam68, RBM4, Ybx1, Ybx2, IGF2BP2, and KSRP. Characterization of these proteins will increase our understanding of the regulatory mechanisms of RBPs in adipogenesis and provide clues for the etiology and pathology of adipose-tissue-related diseases.

miR-504 Promoted Gastric Cancer Cell Proliferation and Inhibited Cell Apoptosis by Targeting RBM4

Background

The purpose of this study was to explore the role and underlying mechanism of miR-504 and RBM4 in gastric cancer.

Methods

The qRT-PCR or Western blot was performed to determine the expressions of miR-504 and RBM4 in the gastric cancer tissues and normal tissues. Human SGC-7901 cells were transfected with miR-504 mimic/inhibitor or pcDNA-RBM4. Cell proliferation and cell apoptosis were assessed by colony formation assay and flow cytometry, respectively. Luciferase reporter gene assays were used to investigate interactions between miR-504 and RBM4 in SGC-7901 cells.

Results

The relative expression of miR-504 was significantly upregulated in the gastric cancer group (n = 25) than in the paired normal group (n = 25), but the relative RBM4 expression was remarkably downregulated in the gastric tumor group, compared with the normal group. Additionally, miR-504 overexpression increased the viability of gastric cancer cells. Moreover, RBM4 is a functional target of miR-504 in gastric cancer cells. miR-504 was further confirmed to promote SGC-7901 cell proliferation and inhibit cell apoptosis by downregulation RBM4 in vitro.

Conclusions

miR-504 promotes gastric cancer cell proliferation and inhibits cell apoptosis by targeting RBM4, and this provides a potential diagnostic biomarker and treatment for patients with gastric cancer.

LncRNA Ctcflos orchestrates transcription and alternative splicing in thermogenic adipogenesis

The recruitment of thermogenic brite adipocytes within white adipose tissue attenuates obesity and metabolic comorbidities, arousing interest in understanding the underlying regulatory mechanisms. The molecular network of brite adipogenesis, however, remains largely unresolved. In this light, long noncoding RNAs (lncRNAs) emerged as a versatile class of modulators that control many steps within the differentiation machinery. Leveraging the naturally varying propensities of different inbred mouse strains for white adipose tissue browning, we identify the nuclear lncRNA Ctcflos as a pivotal orchestrator of thermogenic gene expression during brite adipocyte differentiation. Mechanistically, Ctcflos acts as a pleiotropic regulator, being essential for the transcriptional recruitment of the early core thermogenic regulatory program and the modulation of alternative splicing to drive brite adipogenesis. This is showcased by Ctcflos‐regulated gene transcription and splicing of the key browning factor Prdm16 toward the isoform that is specific for the thermogenic gene program. Conclusively, our findings emphasize the mechanistic versatility of lncRNAs acting at several independent levels of gene expression for effective regulation of key differentiation factors to direct cell fate and function.

The distinct roles of zinc finger CCHC-type (ZCCHC) superfamily proteins in the regulation of RNA metabolism

The zinc finger CCHC-type (ZCCHC) superfamily proteins, characterized with the consensus sequence C-X₂-C-X₄-H-X₄-C, are accepted to have high-affinity binding to single-stranded nucleic acids, especially single-stranded RNAs. In human beings 25 ZCCHC proteins have been annotated in the HGNC database. Of interest is that among the family, most members are involved in the multiple steps of RNA metabolism. In this review, we focus on the diverged roles of human ZCCHC proteins on RNA transcription, biogenesis, splicing, as well as translation and degradation.

Regulatory roles and mechanisms of alternative RNA splicing in adipogenesis and human metabolic health

Alternative splicing (AS) regulates gene expression patterns at the post-transcriptional level and generates a striking expansion of coding capacities of genomes and cellular protein diversity. RNA splicing could undergo modulation and close interaction with genetic and epigenetic machinery. Notably, during the adipogenesis processes of white, brown and beige adipocytes, AS tightly interplays with the differentiation gene program networks. Here, we integrate the available findings on specific splicing events and distinct functions of different splicing regulators as examples to highlight the directive biological contribution of AS mechanism in adipogenesis and adipocyte biology. Furthermore, accumulating evidence has suggested that mutations and/or altered expression in splicing regulators and aberrant splicing alterations in the obesity-associated genes are often linked to humans’ diet-induced obesity and metabolic dysregulation phenotypes. Therefore, significant attempts have been finally made to overview novel detailed discussion on the prospects of splicing machinery with obesity and metabolic disorders to supply featured potential management mechanisms in clinical applicability for obesity treatment strategies.

Altered expressions and splicing profiles of Acin1 transcripts differentially modulate brown adipogenesis through an alternative splicing mechanism

Apoptotic chromatin condensation inducer in the nucleus (also referred as Acin1) was first characterized as an RNA-binding protein involved in apoptosis. In later reports, Acin1 was identified as an auxiliary component of the exon junction complex (EJC) which is assembled throughout pre-messenger RNA splicing. In this study, results of whole-transcriptome analyses revealed reduced expressions and reprogrammed splicing profiles of Acin1 transcripts throughout development of brown adipose tissues (BATs) that execute non-shivering thermogenesis in small rodents and infants by consuming lipids. Depletion of endogenous Acin1 isoforms led to activation of brown adipogenic signatures in mouse C3H10T1/2 fibroblasts. Nevertheless, overexpressions of the Acin1-L or Acin1-S isoform exerted discriminative influences on brown adipogenesis and reprogramming of the expression of serine/arginine-rich splicing factor 3 (SRSF3) through an alternative splicing-coupled nonsense-mediated decay mechanism in a sequence-specific manner. Moreover, the Acin1-SRSF3 axis constitutes a regulatory pathway that participates in the brown adipocyte-related splicing network. Taken together, the interplay between accessory EJC components and splicing regulators constitutes an emerging mechanism for differentially manipulating the activity of brown adipogenesis via alternative splicing network.

Identification of key candidate genes and molecular pathways in white fat browning: an anti-obesity drug discovery based on computational biology

Background

Obesity—with its increased risk of obesity-associated metabolic diseases—has become one of the greatest public health epidemics of the twenty-first century in affluent countries. To date, there are no ideal drugs for treating obesity. Studies have shown that activation of brown adipose tissue (BAT) can promote energy consumption and inhibit obesity, which makes browning of white adipose tissue (WAT) a potential therapeutic target for obesity. Our objective was to identify genes and molecular pathways associated with WAT and the activation of BAT to WAT browning, by using publicly available data and computational tools; this knowledge might help in targeting relevant signaling pathways for treating obesity and other related metabolic diseases.

Results

In this study, we used text mining to find out genes related to brown fat and white fat browning. Combined with biological process and pathway analysis in GeneCodis and protein-protein interaction analysis by using STRING and Cytoscape, a list of high priority target genes was developed. The Human Protein Atlas was used to analyze protein expression. Candidate drugs were derived on the basis of the drug-gene interaction analysis of the final genes. Our study identified 18 genes representing 6 different pathways, targetable by a total of 33 drugs as possible drug treatments. The final list included 18 peroxisome proliferator-activated receptor gamma (PPAR-γ) agonists, 4 beta 3 adrenoceptor (β3-AR) agonists, 1 insulin sensitizer, 3 insulins, 6 lipase clearing factor stimulants and other drugs.

Conclusions

Drug discovery using in silico text mining, pathway, and protein-protein interaction analysis tools may be a method of exploring drugs targeting the activation of brown fat or white fat browning, which provides a basis for the development of novel targeted therapies as potential treatments for obesity and related metabolic diseases.

RNA-Binding Motif 4 (RBM4) Suppresses Tumor Growth and Metastasis in Human Gastric Cancer

BACKGROUND Dysregulation of the splicing activator, RNA-binding motif 4 (RBM4), has recently been reported to be involved in the progression of several cancers. However, the mechanisms that underpin the activity of RBM4 in gastric cancer (GC) remain unknown. The purpose of our study was to explore how RBM4 affects the biological behavior of GC through in vivo and in vitro experiments. MATERIAL AND METHODS Western blot and flow cytometry analyses were used to investigate the RBM4 protein levels in normal gastric epithelial cells and 5 types of GC cells. Cell Counting Kit-8 assay, flow cytometry analysis, wound-healing, and migration and invasion assays were evaluated in vitro in BGC823 and MGC803 GC cells. A xenograft tumor model was used to assess whether RBM4 inhibits GC growth in vivo. Mitogen-activated protein kinase (MAPK) protein levels were determined using western blot analyses. RESULTS Our study revealed that RBM4 protein was downregulated in GC cells. Re-expression of RBM4 inhibited the proliferation, migration, and invasion of GC cells, while promoting apoptosis. Thus, the overexpression of RBM4 can inhibit tumor growth in GC mouse models. We also report that RBM4 was involved in the activation of MAPK-dependent signaling pathways in human GC. CONCLUSIONS It is hoped that these findings will improve our understanding of GC pathogenesis while also helping us to explore the feasibility of RBM4-targeted therapy for GC treatment.

RBM4a-SRSF3-MAP4K4 Splicing Cascade Constitutes a Molecular Mechanism for Regulating Brown Adipogenesis

An increase in mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) reportedly attenuates insulin-mediated signaling which participates in the development of brown adipose tissues (BATs). Nevertheless, the effect of MAP4K4 on brown adipogenesis remains largely uncharacterized. In this study, results of a transcriptome analysis (also referred as RNA-sequencing) showed differential expressions of MAP4K4 or SRSF3 transcripts isolated from distinct stages of embryonic BATs. The discriminative splicing profiles of MAP4K4 or SRSF3 were noted as well in brown adipocytes (BAs) with RNA-binding motif protein 4-knockout (RBM4-/-) compared to the wild-type counterparts. Moreover, the relatively high expressions of authentic SRSF3 transcripts encoding the splicing factor functioned as a novel regulator toward MAP4K4 splicing during brown adipogenesis. The presence of alternatively spliced MAP4K4 variants exerted differential effects on the phosphorylation of c-Jun N-terminal protein kinase (JNK) which was correlated with the differentiation or metabolic signature of BAs. Collectively, the RBM4-SRSF3-MAP4K4 splicing cascade constitutes a novel molecular mechanism in manipulating the development of BAs through related signaling pathways.

RBM4a modulates the impact of PRDM16 on development of brown adipocytes through an alternative splicing mechanism

Brown adipocytes (BAs) exhibit an energy-expending signature that is important in balancing metabolic homeostasis. In this study, results of transcriptome analyses revealed the reprogrammed splicing profile of the PR domain containing 16 (PRDM16) gene, a key transcription factor involved in brown adipogenesis, throughout development of wild-type brown adipose tissues (BATs). Moreover, discriminative splicing patterns of PRDM16 transcripts were noted in embryonic and postnatal RBM4a^-/- BATs. Overexpression of RBM4a enhanced the relative levels of PRDM16^{-ex 16} transcripts by simultaneously interacting with exonic and intronic CU elements, which encoded the PRDM16_S isoform containing a distinct C-terminus. The presence of the overexpressed PRDM16_S isoform showed a stronger effect than the overexpressed PRDM16_L isoform on enhancing transcriptional activity of the RBM4a and the PGC-1α promoter. Overexpression of the PRDM16_S isoform exerted more-prominent effects on enhancing the BAT-related gene program and energy expenditure compared to those of PRDM16_L-overexpressing cells. Our studies demonstrated that RBM4a-regulated alternative splicing constituted another regulatory mechanism for strengthening the influence of PRDM16 on the development of brown adipocytes.

RBM4 Modulates Radial Migration via Alternative Splicing of Dab1 during Cortex Development

The RNA-binding motif 4 (RBM4) protein participates in cell differentiation via its role in regulating the expression of tissue-specific or developmentally regulated mRNA splice isoforms. RBM4 is expressed in embryonic brain during development; it is initially enriched in the ventricular zone/subventricular zone and subsequently distributed throughout the cerebral cortex. Rbm4a knockout brain exhibited delayed migration of late-born neurons. Using in utero electroporation, we confirmed that knockdown of RBM4 impaired cortical neuronal migration. RNA immunoprecipitation with high-throughput sequencing identified Disabled-1 (Dab1), which encodes a critical reelin signaling adaptor, as a potential target of RBM4. Rbm4a knockout embryonic brain showed altered Dab1 isoform ratios. Overexpression of RBM4 promoted the inclusion of Dab1 exons 7 and 8 (7/8), whereas its antagonist polypyrimidine tract-binding protein 1 (PTBP1) acted in an opposite manner. RBM4 directly counteracted the effect of PTBP1 on exon 7/8 selection. Finally, we showed that the full-length Dab1, but not exon 7/8-truncated Dab1, rescued neuronal migration defects in RBM4-depleted neurons, indicating that RBM4 plays a role in neuronal migration via modulating the expression of Dab1 splice isoforms. Our findings imply that RBM4 is necessary during brain development and that its deficiency may lead to developmental brain abnormality.

Multi-posttranscriptional regulations lessen the repressive effect of SRPK1 on brown adipogenesis

Alternative splicing has been widely demonstrated to function as pivotal regulation in specifying cellular fates and biological functions. The relative expression or cellular localization of a splicing factor constitutes an important mechanism in spatiotemporal programming of cell- and stage-specific splicing profiles. In this study, results of deep RNA-sequencing (RNA-Seq) analyses first revealed the reprogrammed splicing profile and reduced expression of serine/arginine-rich splicing factor protein kinase 1 (SRPK1) throughout the development of brown adipose tissue (BAT). A gradual increase in the exon 10-skipped SRPK1 transcript, a potential target of a nonsense-mediated decay (NMD) mechanism, was noted during brown adipogenesis. Elevated RBM4a constituted the regulatory mechanism that led to skipping of SRPK1 exon 10. Moreover, brown adipogenesis-induced upregulation of microRNA (miR)-485 interfered with SRPK1 expression by targeting its 3'-untranslated region (UTR). Depletion of endogenous SRPK1 enhanced the development of C3H10T1/2 cells toward brown adipocytes. Taking our results together, multiple post-transcriptional regulations reduced SRPK1 expression, which subsequently affected brown adipogenesis.

RNA-binding protein PSPC1 promotes the differentiation-dependent nuclear export of adipocyte RNAs

A highly orchestrated gene expression program establishes the properties that define mature adipocytes, but the contribution of posttranscriptional factors to the adipocyte phenotype is poorly understood. Here we have shown that the RNA-binding protein PSPC1, a component of the paraspeckle complex, promotes adipogenesis in vitro and is important for mature adipocyte function in vivo. Cross-linking and immunoprecipitation followed by RNA sequencing revealed that PSPC1 binds to intronic and 3'-untranslated regions of a number of adipocyte RNAs, including the RNA encoding the transcriptional regulator EBF1. Purification of the paraspeckle complex from adipocytes further showed that PSPC1 associates with the RNA export factor DDX3X in a differentiation-dependent manner. Remarkably, PSPC1 relocates from the nucleus to the cytoplasm during differentiation, coinciding with enhanced export of adipogenic RNAs. Mice lacking PSPC1 in fat displayed reduced lipid storage and adipose tissue mass and were resistant to diet-induced obesity and insulin resistance due to a compensatory increase in energy expenditure. These findings highlight a role for PSPC1-dependent RNA maturation in the posttranscriptional control of adipose development and function.

Differential Impacts of Alternative Splicing Networks on Apoptosis

Apoptosis functions as a common mechanism to eliminate unnecessary or damaged cells during cell renewal and tissue development in multicellular organisms. More than 200 proteins constitute complex networks involved in apoptotic regulation. Imbalanced expressions of apoptosis-related factors frequently lead to malignant diseases. The biological functions of several apoptotic factors are manipulated through alternative splicing mechanisms which expand gene diversity by generating discrete variants from one messenger RNA precursor. It is widely observed that alternatively-spliced variants encoded from apoptosis-related genes exhibit differential effects on apoptotic regulation. Alternative splicing events are meticulously regulated by the interplay between trans-splicing factors and cis-responsive elements surrounding the regulated exons. The major focus of this review is to highlight recent studies that illustrate the influences of alternative splicing networks on apoptotic regulation which participates in diverse cellular processes and diseases.

The impact of RNA binding motif protein 4-regulated splicing cascade on the progression and metabolism of colorectal cancer cells

Dysregulated splicing of pre-messenger (m)RNA is considered a molecular occasion of carcinogenesis. However, the underlying mechanism is complex and remains to be investigated. Herein, we report that the upregulated miR-92a reduced the RNA-binding motif 4 (RBM4) protein expression, leading to the imbalanced expression of the neuronal polypyrimidine tract-binding (nPTB) protein through alternative splicing-coupled nonsense mediated decay (NMD) mechanism. Increase in nPTB protein enhances the relative level of fibroblast growth factor receptor 2 IIIc (FGFR2) and pyruvate kinase M2 (PKM2) transcripts which contribute to the progression and metabolic signature of CRC cells. Expression profiles of RBM4 and downstream alternative splicing events are consistently observed in cancerous tissues compared to adjacent normal tissues. These results constitute a mechanistic understanding of RBM4 on repressing the carcinogenesis of colorectal cells.

RBM4-Nova1-SRSF6 splicing cascade modulates the development of brown adipocytes

Alternative splicing (AS) is a pivotal mechanism for the expansion of gene diversity, which determines the cellular fate or specification. However, the effect of AS networks on brown adipogenesis has not been comprehensively investigated. In this study, we identified the discriminative splicing profiles of RNA-binding motif protein 4a-knockout (RBM4a^-/-) brown adipocytes (BAs) and compared them with those of their wild-type counterparts through deep RNA-sequencing. Among these candidates, RBM4a ablation enhanced the relative level of exon 4-excluded neuro-oncological ventral antigen 1 (Nova1^-4) transcripts, which were predominantly generated in embryonic BAs. By contrast, most of the Nova1 transcripts were exon 4-included (Nova1⁺⁴) in mature BAs. The Nova1 isoforms exhibited differential effects on repressing the development of BAs. Moreover, overexpression of Nova1 proteins reduced the serine/arginine splicing factor 6 (SRSF6) level by enhancing the generation of intron 2-included (SRSF6^{+intron 2}) transcripts, which are a putative candidate of the AS-coupled nonsense-mediated decay mechanism. Furthermore, we observed the positive effect of SRSF6 on BA development. These results highlight the hierarchical role of RBM4a in an AS cascade that manipulates brown adipogenesis.

Brown adipose tissue: Updates in cellular and molecular biology

Brown adipose tissue (BAT) is mainly composed of adipocytes, it is highly vascularized and innervated, and can be activated in adult humans. Brown adipocytes are responsible for performing non-shivering thermogenesis, which is exclusively mediated by uncoupling protein (UCP) -1 (a protein found in the inner mitochondrial membrane), the hallmark of BAT, responsible for the uncoupling of the proton leakage from the ATP production, therefore, generating heat (i.e. thermogenesis). Besides UCP1, other compounds are essential not only to thermogenesis, but also to the proliferation and differentiation of BAT, including peroxisome proliferator-activated receptor (PPAR) family, PPARgamma coactivator 1 (PGC1)-alpha, and PRD1-BF-1-RIZ1 homologous domain protein containing protein (PRDM) -16. The sympathetic nervous system centrally regulates thermogenesis through norepinephrine, which acts on the adrenergic receptors of BAT. This bound leads to the initialization of the many pathways that may activate thermogenesis in acute and/or chronic ways. In summary, this mini-review aims to demonstrate the latest advances in the knowledge of BAT.

Prognostic value of decreased expression of RBM4 in human gastric cancer

RNA-binding motif 4 (RBM4) is a multifunctional protein that participates in regulating alternative splicing and mRNA translation. Its reduced expression has been associated with poor overall survival in lung cancer, breast cancer and ovarian cancer. We assessed RBM4 protein expression levels with immunohistochemistry in tissue microarrays containing malignant gastric cancer tissues and benign tissues from 813 patients. We also examined the expression levels of RBM4 mRNA in twenty-five paired gastric cancer samples and adjacent noncancerous tissues. Both RBM4 protein and mRNA expression levels were significantly lower in gastric cancer tissues compared with the adjacent noncancerous tissues. There was a significant association between reduced RBM4 protein expression and differentiation (P < 0.001), lymph node metastasis (P = 0.026), TNM state (P = 0.014) and distant metastasis (P = 0.036). Patients with reduced RBM4 expression (P < 0.001, CI = 0.315–0.710) and TNM stage III and IV (P < 0.001, CI = 4.757–11.166) had a poor overall survival. These findings suggest that RBM4 is a new biomarker in gastric cancer, as the reduced expression of this protein is correlated with poor differentiation, lymph node status and distant metastasis. Further, lower RBM4 expression is an independent prognostic marker for gastric cancer.

RBM4 promotes neuronal differentiation and neurite outgrowth by modulating Numb isoform expression

RBM4 modulates alternative exon selection of Numb and up-regulates proneural Mash1 gene expression, possibly via specific Numb isoforms. RBM4 overexpression promotes neuronal cell differentiation. Moreover, RBM4 is essential for neurite outgrowth in primary cortical neurons by modulating specific Numb isoform expression.

RBM4 participates in cell differentiation by regulating tissue-specific alternative pre-mRNA splicing. RBM4 also has been implicated in neurogenesis in the mouse embryonic brain. Using mouse embryonal carcinoma P19 cells as a neural differentiation model, we observed a temporal correlation between RBM4 expression and a change in splicing isoforms of Numb, a cell-fate determination gene. Knockdown of RBM4 affected the inclusion/exclusion of exons 3 and 9 of Numb in P19 cells. RBM4-deficient embryonic mouse brain also exhibited aberrant splicing of Numb pre-mRNA. Using a splicing reporter minigene assay, we demonstrated that RBM4 promoted exon 3 inclusion and exon 9 exclusion. Moreover, we found that RBM4 depletion reduced the expression of the proneural gene Mash1, and such reduction was reversed by an RBM4-induced Numb isoform containing exon 3 but lacking exon 9. Accordingly, induction of ectopic RBM4 expression in neuronal progenitor cells increased Mash1 expression and promoted cell differentiation. Finally, we found that RBM4 was also essential for neurite outgrowth from cortical neurons in vitro. Neurite outgrowth defects of RBM4-depleted neurons were rescued by RBM4-induced exon 9–lacking Numb isoforms. Therefore our findings indicate that RBM4 modulates exon selection of Numb to generate isoforms that promote neuronal cell differentiation and neurite outgrowth.

RBM4a-regulated splicing cascade modulates the differentiation and metabolic activities of brown adipocytes

RNA-binding motif protein 4a (RBM4a) reportedly reprograms splicing profiles of the insulin receptor (IR) and myocyte enhancer factor 2C (MEF2C) genes, facilitating the differentiation of brown adipocytes. Using an RNA-sequencing analysis, we first compared the gene expressing profiles between wild-type and RBM4a^−/− brown adipocytes. The ablation of RBM4a led to increases in the PTBP1, PTBP2 (nPTB), and Nova1 proteins, whereas elevated RBM4a reduced the expression of PTBP1 and PTBP2 proteins in brown adipocytes through an alternative splicing-coupled nonsense-mediated decay mechanism. Subsequently, RBM4a indirectly shortened the half-life of the Nova1 transcript which was comparatively stable in the presence of PTBP2. RBM4a diminished the influence of PTBP2 in adipogenic development by reprogramming the splicing profiles of the FGFR2 and PKM genes. These results constitute a mechanistic understanding of the RBM4a-modulated splicing cascade during the brown adipogenesis.

RBM4-MEF2C network constitutes a feed-forward circuit that facilitates the differentiation of brown adipocytes

Myocyte enhancer factor 2c (MEF2C) is the MADS-box type transcription factor involved in the differentiation of cardiac and skeletal muscle and synaptic formation. Alternatively spliced transcripts of the MEF2C gene were proven to encode isoforms which exert distinct functions in transcriptional regulation. During the differentiation of brown adipocytes, upregulated RBM4 enhanced skipping of the MEF2Cγ region which functions as a transcriptional repressor. The presence of an overexpressed MEF2Cγ- isoform in turn induced transcriptional activity of the RBM4 promoter, constituting a positive feedback circuit in differentiating brown adipocytes. The RBM4-MEF2Cγ- network induced the expression of "myogenic" miR-1 to a greater extent than did PRDM17, BMP7 C/EBPβ, or UCP1 transcripts in C3H10T1/2 cells. Overexpression of miR-1 independently exerted the same activity as RBM4 and the MEF2Cγ- isoform of upregulating brown adipocyte-specific factors in C3H10T1/2 cells, which suggests a potential effect of miR-1 on brown adipocytes. These results indicated that the RBM4-MEF2C-miR-1 network constitutes a novel mechanism which programs the gene expression profile toward the development of brown adipocytes.

Impacts of Alternative Splicing Events on the Differentiation of Adipocytes

Alternative splicing was found to be a common phenomenon after the advent of whole transcriptome analyses or next generation sequencing. Over 90% of human genes were demonstrated to undergo at least one alternative splicing event. Alternative splicing is an effective mechanism to spatiotemporally expand protein diversity, which influences the cell fate and tissue development. The first focus of this review is to highlight recent studies, which demonstrated effects of alternative splicing on the differentiation of adipocytes. Moreover, use of evolving high-throughput approaches, such as transcriptome analyses (RNA sequencing), to profile adipogenic transcriptomes, is also addressed.

Elevated SRPK1 lessens apoptosis in breast cancer cells through RBM4-regulated splicing events

Imbalanced splicing of premessenger RNA is typical of tumorous malignancies, and the regulatory mechanisms involved in several tumorigenesis-associated splicing events are identified. Elevated expression of serine-arginine protein kinase 1 (SRPK1) may participate in the pathway responsible for the dysregulation of splicing events in malignant tumor cells. In this study, we observed a correlation between the cytoplasmic accumulation of RNA-binding motif protein 4 (RBM4) and up-regulated SRPK1 in breast cancer cells. The production of the IR-B and MCL-1S transcripts was induced separately by the overexpression of RBM4 and SRPK1 gene silencing. Overexpressed RBM4 simultaneously bound to the CU-rich elements within the MCL-1 exon2 and the downstream intron, which subsequently facilitated the exclusion of the regulated exon. Breast cancer cells are deprived of apoptotic resistance through the RBM4-mediated up-regulation of the IR-B and MCL-1S transcripts. These findings suggest that the splicing events regulated by the SRPK1-RMB4 network may contribute to tumorigenesis through altered sensitivity to apoptotic signals in breast cancer cells.