The Musashi proteins direct post-transcriptional control of protein expression and alternate exon splicing in vertebrate photoreceptors

Abstract

The Musashi proteins, MSI1 and MSI2, are conserved RNA binding proteins with a role in the maintenance and renewal of stem cells. Contrasting with this role, terminally differentiated photoreceptor cells express high levels of MSI1 and MSI2, pointing to a role for the two proteins in vision. Combined knockout of Msi1 and Msi2 in mature photoreceptor cells abrogated the retinal response to light and caused photoreceptor cell death. In photoreceptor cells the Musashi proteins perform distinct nuclear and cytoplasmic functions. In the nucleus, the Musashi proteins promote splicing of photoreceptor-specific alternative exons. Surprisingly, conserved photoreceptor-specific alternative exons in genes critical for vision proved to be dispensable, raising questions about the selective pressures that lead to their conservation. In the cytoplasm MSI1 and MSI2 activate protein expression. Loss of Msi1 and Msi2 lead to reduction in the levels of multiple proteins including proteins required for vision and photoreceptor survival. The requirement for MSI1 and MSI2 in terminally differentiated photoreceptors alongside their role in stem cells shows that, depending on cellular context, these two proteins can control processes ranging from cell proliferation to sensory perception.

Unique transcriptomes of sensory and non-sensory neurons: insights from Splicing Regulatory States

“…Consistent with these functional enrichments, a zebrafish mutant for srrm3 , showing a large mis-regulation of RetMICs, exhibited severe OS alteration, photoreceptor death and blindness (Ciampi et al, 2022 ). However, despite the strong phenotype of a global microexon mis-regulation, it should be noted that depletion of individual RetMICs in mice have not shown any evident visual phenotypes (Matalkah et al, 2022 ), likely as a result of compensatory effects and the high robustness of living systems to relatively subtle perturbations as those exerted by individual microexon deletions.…”

“…The patterns of AS in retina and photoreceptors are known to be in part driven by the direct action of an RNA-binding protein called Musashi-1 (MSI1) (Murphy et al, 2016 ; Ling et al, 2020 ; Matalkah et al, 2022 ). The Musashi family comprises two members, MSI1 and MSI2 , and it has been involved in stem cell renewal and negative regulation of cell differentiation.…”

“…Moreover, MSI1 overexpression in neuroblastoma cells was shown to promote the inclusion of some of these photoreceptor-specific exons by binding to the downstream proximal introns. Msi1 and Msi2 have a distinct pattern of expression across retinal development, with Msi1 being highly expressed at birth until postnatal day 16, while both proteins show equal expression in adult photoreceptors (Matalkah et al, 2022 ). Indeed, both genes are functionally redundant in mature retina, as inducible double knockout mice have a strong visual impairment and mis-splicing of photoreceptor exons, while single-knockout mice have no alterations in photoreceptors (Matalkah et al, 2022 ).…”

Proteomic profiling of retina and retinal pigment epithelium combined embryonic tissue to facilitate ocular disease gene discovery

“…Contributing to this is regulation at the post-transcriptional level -that involves non-coding RNA or RNA-binding protein (RBP)-mediated control over mRNA splicing, transport, stability and translation -all of which determines the cellular proteome (Brinegar and Cooper 2016;Hentze et al 2018;Gebauer et al 2021). Indeed, recent findings have demonstrated that RBPs play a conserved role in mediating post-transcriptional control in eye and lens development Lachke and Maas 2011;Dash et al 2015Dash et al , 2016Dash et al , 2020Siddam et al 2018;Shao et al 2020;Barnum et al 2020;Nakazawa et al 2020;Sundar et al 2020;Aryal et al 2020b;Lachke 2022;Matalkah et al 2022). Together, these findings suggest that along with the transcriptome, characterization of the proteome is important to determine the factors that are important in a specific cell/tissue of interest.…”

Establishing the contribution of active histone methylation marks to the aging transcriptional landscape of Drosophila photoreceptors

“…H3K4me3 and H3K36me3 have been linked to the regulation of transcription initiation and elongation, respectively. Since photoreceptors experience age-associated changes in transcription factor activity 24 , 25 , as well as splicing 26 , 27 , we sought to study how histone methylation contributed to the aging transcriptome. Further, we wondered if decreasing levels of one of these active marks would directly contribute to any of the age-associated changes observed in the transcriptome of aging photoreceptors.…”