Deficiency in Retinal TGFß Signaling Aggravates Neurodegeneration by Modulating Pro-Apoptotic and MAP Kinase Pathways.

Transforming growth factor ß (TGFß) signaling has manifold functions such as regulation of cell growth, differentiation, migration, and apoptosis. Moreover, there is increasing evidence that it also acts in a neuroprotective manner. We recently showed that TGFß receptor type 2 (Tgfbr2) is upregulated in retinal neurons and Müller cells during retinal degeneration. In this study we investigated if this upregulation of TGFß signaling would have functional consequences in protecting retinal neurons. To this end, we analyzed the impact of TGFß signaling on photoreceptor viability using mice with cell type-specific deletion of Tgfbr2 in retinal neurons and Müller cells (Tgfbr2ΔOC) in combination with a genetic model of photoreceptor degeneration (VPP). We examined retinal morphology and the degree of photoreceptor degeneration, as well as alterations of the retinal transcriptome. In summary, retinal morphology was not altered due to TGFß signaling deficiency. In contrast, VPP-induced photoreceptor degeneration was drastically exacerbated in double mutant mice (Tgfbr2ΔOC; VPP) by induction of pro-apoptotic genes and dysregulation of the MAP kinase pathway. Therefore, TGFß signaling in retinal neurons and Müller cells exhibits a neuroprotective effect and might pose promising therapeutic options to attenuate photoreceptor degeneration in humans.

Transcriptional Profiling Identifies Upregulation of Neuroprotective Pathways in Retinitis Pigmentosa.

Hereditary retinal degenerations like retinitis pigmentosa (RP) are among the leading causes of blindness in younger patients. To enable in vivo investigation of cellular and molecular mechanisms responsible for photoreceptor cell death and to allow testing of therapeutic strategies that could prevent retinal degeneration, animal models have been created. In this study, we deeply characterized the transcriptional profile of mice carrying the transgene rhodopsin V20G/P23H/P27L (VPP), which is a model for autosomal dominant RP. We examined the degree of photoreceptor degeneration and studied the impact of the VPP transgene-induced retinal degeneration on the transcriptome level of the retina using next generation RNA sequencing (RNASeq) analyses followed by weighted correlation network analysis (WGCNA). We furthermore identified cellular subpopulations responsible for some of the observed dysregulations using in situ hybridizations, immunofluorescence staining, and 3D reconstruction. Using RNASeq analysis, we identified 9256 dysregulated genes and six significantly associated gene modules in the subsequently performed WGCNA. Gene ontology enrichment showed, among others, dysregulation of genes involved in TGF-ß regulated extracellular matrix organization, the (ocular) immune system/response, and cellular homeostasis. Moreover, heatmaps confirmed clustering of significantly dysregulated genes coding for components of the TGF-ß, G-protein activated, and VEGF signaling pathway. 3D reconstructions of immunostained/in situ hybridized sections revealed retinal neurons and Müller cells as the major cellular population expressing representative components of these signaling pathways. The predominant effect of VPP-induced photoreceptor degeneration pointed towards induction of neuroinflammation and the upregulation of neuroprotective pathways like TGF-ß, G-protein activated, and VEGF signaling. Thus, modulation of these processes and signaling pathways might represent new therapeutic options to delay the degeneration of photoreceptors in diseases like RP.

Light Intensity-Dependent Dysregulation of Retinal Reference Genes.

The degeneration of photoreceptors is a common hallmark of ocular diseases like retinitis pigmentosa (RP) or age-related macular degeneration (AMD). To experimentally induce photoreceptor degeneration, the light damage paradigm is frequently used. In this study we show that the exposure to high amounts of cool white light (10,000 lux, 1 h) resulted in a more than 11-fold higher apoptotic rate in the retina compared to light exposure with 5000 lux for 30 min. Consequently, exposure to intense light resulted in a significant downregulation of retinal mRNA expression levels of the reference genes Gapdh, Gnb2l, Rpl32, Rps9, Actb, Ubc or Tbp compared to untreated controls. Investigators performing light-induced photoreceptor degeneration should be aware of the fact that higher light intensities will result in a dysregulation of reference genes.

New Insights into Endothelin Signaling and Its Diverse Roles in the Retina.

The vasoactive peptide endothelin is an effective regulator of blood pressure and vascular homeostasis. In addition, the dysregulation of the endothelin signaling pathway is discussed to contribute to ocular diseases like glaucoma or diabetic retinopathy. Furthermore, our workgroup and others showed a protective effect of endothelin 2 for the survival of photoreceptors. In this study, we analyzed mRNA expression levels of the endothelin signaling family in wild-type mice after a puncture of the eye, intravitreal PBS injections, or light-induced photoreceptor degeneration. We observed elevated endothelin receptor a (Eta), endothelin receptor b (Etb), endothelin 1(Et1), and endothelin 2 (Et2) levels, while endothelin 3 (Et3) mRNA levels were not significantly altered. Our findings indicate an important role of the endothelin signaling pathway in response to ocular trauma or disease. These findings make endothelin signaling a promising target to attenuate retinal degeneration.

H2A.Z.2.2 is an alternatively spliced histone H2A.Z variant that causes severe nucleosome destabilization.

The histone variant H2A.Z has been implicated in many biological processes, such as gene regulation and genome stability. Here, we present the identification of H2A.Z.2.2 (Z.2.2), a novel alternatively spliced variant of histone H2A.Z and provide a comprehensive characterization of its expression and chromatin incorporation properties. Z.2.2 mRNA is found in all human cell lines and tissues with highest levels in brain. We show the proper splicing and in vivo existence of this variant protein in humans. Furthermore, we demonstrate the binding of Z.2.2 to H2A.Z-specific TIP60 and SRCAP chaperone complexes and its active replication-independent deposition into chromatin. Strikingly, various independent in vivo and in vitro analyses, such as biochemical fractionation, comparative FRAP studies of GFP-tagged H2A variants, size exclusion chromatography and single molecule FRET, in combination with in silico molecular dynamics simulations, consistently demonstrate that Z.2.2 causes major structural changes and significantly destabilizes nucleosomes. Analyses of deletion mutants and chimeric proteins pinpoint this property to its unique C-terminus. Our findings enrich the list of known human variants by an unusual protein belonging to the H2A.Z family that leads to the least stable nucleosome known to date.

Calcium spikes, waves and oscillations in a large, patterned epithelial tissue.

While calcium signaling in excitable cells, such as muscle or neurons, is extensively characterized, calcium signaling in epithelial tissues is little understood. Specifically, the range of intercellular calcium signaling patterns elicited by tightly coupled epithelial cells and their function in the regulation of epithelial characteristics are little explored. We found that in Drosophila imaginal discs, a widely studied epithelial model organ, complex spatiotemporal calcium dynamics occur. We describe patterns that include intercellular waves traversing large tissue domains in striking oscillatory patterns as well as spikes confined to local domains of neighboring cells. The spatiotemporal characteristics of intercellular waves and oscillations arise as emergent properties of calcium mobilization within a sheet of gap-junction coupled cells and are influenced by cell size and environmental history. While the in vivo function of spikes, waves and oscillations requires further characterization, our genetic experiments suggest that core calcium signaling components guide actomyosin organization. Our study thus suggests a possible role for calcium signaling in epithelia but importantly, introduces a model epithelium enabling the dissection of cellular mechanisms supporting the initiation, transmission and regeneration of long-range intercellular calcium waves and the emergence of oscillations in a highly coupled multicellular sheet.

Interface Contractility between Differently Fated Cells Drives Cell Elimination and Cyst Formation.

Although cellular tumor-suppression mechanisms are widely studied, little is known about mechanisms that act at the level of tissues to suppress the occurrence of aberrant cells in epithelia. We find that ectopic expression of transcription factors that specify cell fates causes abnormal epithelial cysts in Drosophila imaginal discs. Cysts do not form cell autonomously but result from the juxtaposition of two cell populations with divergent fates. Juxtaposition of wild-type and aberrantly specified cells induces enrichment of actomyosin at their entire shared interface, both at adherens junctions as well as along basolateral interfaces. Experimental validation of 3D vertex model simulations demonstrates that enhanced interface contractility is sufficient to explain many morphogenetic behaviors, which depend on cell cluster size. These range from cyst formation by intermediate-sized clusters to segregation of large cell populations by formation of smooth boundaries or apical constriction in small groups of cells. In addition, we find that single cells experiencing lateral interface contractility are eliminated from tissues by apoptosis. Cysts, which disrupt epithelial continuity, form when elimination of single, aberrantly specified cells fails and cells proliferate to intermediate cell cluster sizes. Thus, increased interface contractility functions as error correction mechanism eliminating single aberrant cells from tissues, but failure leads to the formation of large, potentially disease-promoting cysts. Our results provide a novel perspective on morphogenetic mechanisms, which arise from cell-fate heterogeneities within tissues and maintain or disrupt epithelial homeostasis.