Translational control in germ cell development: A role for the RNA-binding proteins Musashi-1 and Musashi-2

Knockdown of MSI2 inhibits metastasis by interacting with caveolin-1 and inhibiting its ubiquitylation in human NF1-MPNST cells

Malignant peripheral nerve sheath tumours (MPNSTs) are highly aggressive Schwann cell-derived sarcomas, and they are either associated with neurofibromatosis type 1 (NF1) or sporadic. Our previous study found that high mobility group protein A2 (HMGA2) regulates NF1-MPNST growth through Musashi-2 (MSI2); however, whether MSI2 regulates MPNST metastasis and what the mechanism is remain unclear. Here, we demonstrated that the protein caveolin-1 (CAV1) directly interacts with MSI2 in human NF1-MPNST cells. Moreover, we discovered that knockdown of MSI2 induces CAV1 protein expression by inhibiting its ubiquitylation level in NF1-MPNSTs. In addition, CAV1 mediates the suppressive function of MSI2 in epithelial-mesenchymal transition, migration and invasion in vitro and metastasis in vivo. These results help to reveal the potential mechanisms of MSI2 as a target of antimetastatic treatment for human NF1-MPNST.

Human oncoprotein Musashi-2 N-terminal RNA recognition motif backbone assignment and identification of RNA-binding pocket

RNA-binding protein Musashi-2 (MSI2) is a key regulator in stem cells, it is over-expressed in a variety of cancers and its higher expression is associated with poor prognosis. Like Musashi-1, it contains two N-terminal RRMs (RNA-recognition Motifs, also called RBDs (RNA-binding Domains)), RRM1 and RRM2, which mediate the binding to their target mRNAs. Previous studies have obtained the three-dimensional structures of the RBDs of Musashi-1 and the RBD1:RNA complex. Here we show the binding of MSI2-RRM1 to a 15nt Numb RNA in Fluorescence Polarization assay and time resolved Fluorescence Resonance Energy Transfer assay. Using nuclear magnetic resonance (NMR) spectroscopy we assigned the backbone resonances of MSI2-RRM1, and characterized the direct interaction of RRM1 to Numb RNA r(GUAGU). Our NMR titration and structure modeling studies showed that MSI2-RRM1 and MSI1-RBD1 have similar RNA binding events and binding pockets. This work adds significant information to MSI2-RRM1 structure and RNA binding pocket, and contributes to the development of MSI2 specific and MSI1/MSI2 dual inhibitors.

Musashi RNA-Binding Proteins as Cancer Drivers and Novel Therapeutic Targets

Aberrant gene expression that drives human cancer can arise from epigenetic dysregulation. Although much attention has focused on altered activity of transcription factors and chromatin-modulating proteins, proteins that act posttranscriptionally can potently affect expression of oncogenic signaling proteins. The RNA-binding proteins (RBP) Musashi-1 (MSI1) and Musashi-2 (MSI2) are emerging as regulators of multiple critical biological processes relevant to cancer initiation, progression, and drug resistance. Following identification of Musashi as a regulator of progenitor cell identity in Drosophila, the human Musashi proteins were initially linked to control of maintenance of hematopoietic stem cells, then stem cell compartments for additional cell types. More recently, the Musashi proteins were found to be overexpressed and prognostic of outcome in numerous cancer types, including colorectal, lung, and pancreatic cancers; glioblastoma; and several leukemias. MSI1 and MSI2 bind and regulate the mRNA stability and translation of proteins operating in essential oncogenic signaling pathways, including NUMB/Notch, PTEN/mTOR, TGFβ/SMAD3, MYC, cMET, and others. On the basis of these activities, MSI proteins maintain cancer stem cell populations and regulate cancer invasion, metastasis, and development of more aggressive cancer phenotypes, including drug resistance. Although RBPs are viewed as difficult therapeutic targets, initial efforts to develop MSI-specific inhibitors are promising, and RNA interference-based approaches to inhibiting these proteins have had promising outcomes in preclinical studies. In the interim, understanding the function of these translational regulators may yield insight into the relationship between mRNA expression and protein expression in tumors, guiding tumor-profiling analysis. This review provides a current overview of Musashi as a cancer driver and novel therapeutic target. Clin Cancer Res; 23(9); 2143-53. ©2017 AACR.

Sirtuins regulate proteomic responses near thermal tolerance limits in the blue mussels Mytilus galloprovincialis and Mytilus trossulus

The blue mussels Mytilus galloprovincialis and M. trossulus are competing species with biogeographical ranges set in part by environmental exposure to heat and hyposalinity. The underlying cellular mechanisms influencing interspecific differences in stress tolerance are unknown, but are believed to be under regulation by sirtuins, NAD-dependent deacylases that play a critical role in the cellular stress response. A comparison of the proteomic responses of M. galloprovincialis and M. trossulus to an acute heat shock in the presence and absence of the sirtuin inhibitor suramin (SIRT1, 2 and 5), showed that sirtuins affected molecular chaperones, oxidative stress proteins, metabolic enzymes, cytoskeletal and signaling proteins more in the heat-sensitive M. trossulus than in the heat-tolerant M. galloprovincialis. Interactions between sirtuin inhibition and changes in the abundance of proteins of β-oxidation and oxidative stress in M. trossulus suggest a greater role of sirtuins in shifting metabolism to reduce the production of reactive oxygen species near thermal limits. Furthermore, RNA-binding proteins initiating and inhibiting translation were affected by suramin in M. galloprovincialis and in M. trossulus, respectively. Western blot analysis showed that the levels of mitochondrial sirtuin 5 (SIRT5) were generally three times higher and increased with acute heat stress in response to sirtuin inhibition in M. trossulus but not in M. galloprovincialis, suggesting a possible feedback response in the former species and a greater reliance on SIRT5 for its stress response. Our findings suggest that SIRT5 plays an important role in setting interspecific differences in stress tolerance in Mytilus by affecting the stress proteome.

RNA binding proteins in spermatogenesis: an in depth focus on the Musashi family

Controlled gene regulation during gamete development is vital for maintaining reproductive potential. During the complex process of mammalian spermatogenesis, male germ cells experience extended periods of the inactive transcription despite heavy translational requirements for continued growth and differentiation. Hence, spermatogenesis is highly reliant on mechanisms of posttranscriptional regulation of gene expression, facilitated by RNA binding proteins (RBPs), which remain abundantly expressed throughout this process. One such group of proteins is the Musashi family, previously identified as critical regulators of testis germ cell development and meiosis in Drosophila, and also shown to be vital to sperm development and reproductive potential in the mouse. This review describes the role and function of RBPs within the scope of male germ cell development, focusing on our recent knowledge of the Musashi proteins in spermatogenesis. The functional mechanisms utilized by RBPs within the cell are outlined in depth, and the significance of sub-cellular localization and stage-specific expression in relation to the mode and impact of posttranscriptional regulation is also highlighted. We emphasize the historical role of the Musashi family of RBPs in stem cell function and cell fate determination, as originally characterized in Drosophila and Xenopus, and conclude with our current understanding of the differential roles and functions of the mammalian Musashi proteins, Musashi-1 and Musashi-2, with a primary focus on our findings in spermatogenesis. This review highlights both the essential contribution of RBPs to posttranscriptional regulation and the importance of the Musashi family as master regulators of male gamete development.

DDX3Y gene rescue of a Y chromosome AZFa deletion restores germ cell formation and transcriptional programs

Deletions of the AZFa region (AZoospermia Factor-a) region of the human Y chromosome cause irreversible spermatogenic failure that presents clinically in men as Sertoli-cell only (SCO) pathology of the testis. Deletions of the AZFa region typically encompass two genes: DDX3Y and USP9Y. However, human genetic evidence indicates that SCO is most tightly linked to deletion of DDX3Y and that deletions/mutations of USP9Y can be transmitted from one generation to the next. Here, we generated stable iPSC lines with AZFa deletions, tested complementation via introduction of DDX3Y, and assessed ability to form germ cells in vivo in a xenotransplantation model. We observed a quantifiable improvement in formation of germ cell like cells (GCLCs) from complemented donor iPSCs. Moreover, expression of UTF1, a prospermatogonial protein, was restored in cells complemented by introduction of DDX3Y on the AZFa background. Whole-genome RNA sequencing of purified GCLCs revealed an enrichment of genes involved in translational suppression and transcriptional control in DDX3Y-rescued GCLCs over mutant GCLCs, which maintained a molecular phenotype more similar to undifferentiated iPSCs. This study demonstrates the ability to probe fundamental genetics of human germ cell formation by complementation and indicates that DDX3Y functions in the earliest stages of human germ cell development.

Knockout of RNA Binding Protein MSI2 Impairs Follicle Development in the Mouse Ovary: Characterization of MSI1 and MSI2 during Folliculogenesis

Characterizing the mechanisms underlying follicle development in the ovary is crucial to understanding female fertility and is an area of increasing research interest. The RNA binding protein Musashi is essential for post-transcriptional regulation of oocyte maturation in Xenopus and is expressed during ovarian development in Drosophila. In mammals Musashi is important for spermatogenesis and male fertility, but its role in the ovary has yet to be characterized. In this study we determined the expression of mammalian Musashi proteins MSI1 and MSI2 during mouse folliculogenesis, and through the use of a MSI2-specific knockout mouse model we identified that MSI2 is essential for normal follicle development. Time-course characterization of MSI1 and MSI2 revealed distinct differences in steady-state mRNA levels and protein expression/localization at important developmental time-points during folliculogenesis. Using a gene-trap mouse model that inactivates Msi2, we observed a significant decrease in ovarian mass, and change in follicle-stage composition due to developmental blocking of antral stage follicles and pre-antral follicle loss through atresia. We also confirmed that hormonally stimulated Msi2-deficient mice produce significantly fewer MII oocytes (60.9% less than controls, p < 0.05). Furthermore, the majority of these oocytes are of poor viability (62.2% non-viable/apoptotic, p < 0.05), which causes a reduction in female fertility evidenced by decreased litter size in Msi2-deficient animals (33.1% reduction to controls, p < 0.05). Our findings indicate that MSI1 and MSI2 display distinct expression profiles during mammalian folliculogenesis and that MSI2 is required for pre-antral follicle development.

Zebrafish stem/progenitor factor msi2b exhibits two phases of activity mediated by different splice variants

The Musashi (Msi) family of RNA-binding proteins is important in stem and differentiating cells in many species. Here, we present a zebrafish gene/protein trap line gt(msi2b-citrine)(ct) (57) (a) that expresses a Citrine fusion protein with endogenous Msi2b. Our results reveal two phases of Msi2b expression: ubiquitous expression in progenitor cells in the early embryo and later, tissue-specific expression in differentiating cells in the olfactory organ, pineal gland, and subpopulations of neurons in the central nervous system (CNS). Interestingly, this division between early and late phases is paralleled by differential expression of msi2b alternative splicing products. Whereas the full-length and long variant v3 Msi2b predominate at early stages, the later expression of variants in differentiating tissues appears to be tissue specific. Using the gt(msi2b-citrine)(ct) (57) (a), we characterized tissue-specific expression of Msi2b with cellular resolution in subsets of differentiating cells in the olfactory organ, pineal gland, CNS, and ventral neural tube. By performing transcription activator-like effectors nuclease-mediated biallelic genome editing or morpholino knockdown of Msi2b in zebrafish, our results show that early inactivation of Msi2b results in severe embryonic defects including hypertrophy of the ventricles and shortening of the body, consistent with an important role in cell proliferation and survival. Moreover, specific inactivation of Msi2b full-length indicates that this species is essential for the early role of Msi2b. This line provides a valuable tool both for live imaging of the endogenous Msi2b at subcellular resolution and manipulation of Msi2b-expressing cells.

Gonad RNA-specific qRT-PCR analyses identify genes with potential functions in schistosome reproduction such as SmFz1 and SmFGFRs

In the search for new strategies to fight schistosomiasis, the unique reproductive biology of Schistosoma mansoni has come into the focus of research. The development of the gonads and the ability of egg production are fundamental not only for continuing the life cycle but also for pathogenicity. Previous studies of schistosome biology demonstrated an influence of pairing on gonad development of the female and on gene expression profiles in both genders. Due to the limited access to specific tissues, however, most of these studies were done at the level of whole worms neglecting individual tissues that may be targets of pairing-dependent processes. Recently, we established a protocol allowing the isolation of testes and ovaries from adult S. mansoni. Here, we describe tissue-specific qRT-PCR analyses comparing transcript levels of selected genes on the basis of RNA from gonads and whole worms. Gene expression in ovary and testes was in some cases found to be significantly influenced by pairing, which was not traceable in whole worms. Among the candidate genes identified as regulated by pairing in gonads were the frizzled homolog SmFz1 and the two fibroblast growth factor receptor homologs SmFGFR-A and SmFGFR-B. First functional characterizations were done, including comparative qRT-PCR analyses, in situ-localization experiments, heterologous expression in Xenopus oocytes (SmFGFR-A/B), and inhibitor studies using the Fz/Dvl-pathway inhibitor 3289-8625, or BIBF1120 blocking FGFR-signaling. Besides confirming gonad localization and receptor functions, inhibitor-induced phenotypes were observed in vitro such as decreased egg production as well as drastic effects on gonad differentiation, morphology, embryogenesis, and survival of adult worms. In summary, these results emphasise the usefulness of tissue-specific qRT-PCRs for selection of candidate genes with important roles in reproduction, allowing subsequent studies to determine their suitability as drug targets.

Specificity factors in cytoplasmic polyadenylation

Poly(A) tail elongation after export of an messenger RNA (mRNA) to the cytoplasm is called cytoplasmic polyadenylation. It was first discovered in oocytes and embryos, where it has roles in meiosis and development. In recent years, however, has been implicated in many other processes, including synaptic plasticity and mitosis. This review aims to introduce cytoplasmic polyadenylation with an emphasis on the factors and elements mediating this process for different mRNAs and in different animal species. We will discuss the RNA sequence elements mediating cytoplasmic polyadenylation in the 3' untranslated regions of mRNAs, including the CPE, MBE, TCS, eCPE, and C-CPE. In addition to describing the role of general polyadenylation factors, we discuss the specific RNA binding protein families associated with cytoplasmic polyadenylation elements, including CPEB (CPEB1, CPEB2, CPEB3, and CPEB4), Pumilio (PUM2), Musashi (MSI1, MSI2), zygote arrest (ZAR2), ELAV like proteins (ELAVL1, HuR), poly(C) binding proteins (PCBP2, αCP2, hnRNP-E2), and Bicaudal C (BICC1). Some emerging themes in cytoplasmic polyadenylation will be highlighted. To facilitate understanding for those working in different organisms and fields, particularly those who are analyzing high throughput data, HUGO gene nomenclature for the human orthologs is used throughout. Where human orthologs have not been clearly identified, reference is made to protein families identified in man.

The RNA-Binding Protein Musashi1: A Major Player in Intestinal Epithelium Renewal and Colon Cancer Development

Aberrant gene expression is the cause and the consequence of tumorigenesis. A major component of gene expression is translation regulation; a process whose main players are RNA-binding-proteins (RBPs). More than 800 RBPs have been identified in the human genome and several of them have been shown to control gene networks associated with relevant cancer processes. A more systematic characterization of RBPs starts to reveal that similar to transcription factors, they can function as tumor suppressors or oncogenes. A relevant example is Musashi1 (Msi1), which is emerging as a critical regulator of tumorigenesis in multiple cancer types, including colon cancer. Msi1 is a stem marker in several tissues and is critical in maintaining the balance between self-renewal and differentiation. However, a boost in Msi1 expression can most likely lead cells towards an oncogenic pathway. In this article, we discuss the parallels between Msi1 function in normal renewal of intestinal epithelium and in colon cancer.