Retinoschisin and Cardiac Glycoside Crosstalk at the Retinal Na/K-ATPase

Advances in understanding the molecular structure of retinoschisin while questions remain of biological function

Retinoschisin (RS1) is a secreted protein that is essential for maintaining integrity of the retina. Numerous mutations in RS1 cause X-linked retinoschisis (XLRS), a progressive degeneration of the retina that leads to vision loss in young males. A key manifestation of XLRS is the formation of cavities (cysts) in the retina and separation of the layers (schisis), disrupting synaptic transmission. There are currently no approved treatments for patients with XLRS. Strategies using adeno-associated viral (AAV) vectors to deliver functional copies of RS1 as a form of gene augmentation therapy, are under clinical evaluation. To improve therapeutic strategies for treating XLRS, it is critical to better understand the secretion of RS1 and its molecular function. Immunofluorescence and immunoelectron microscopy show that RS1 is located on the surfaces of the photoreceptor inner segments and bipolar cells. Sequence homology indicates a discoidin domain fold, similar to many other proteins with demonstrated adhesion functions. Recent structural studies revealed the tertiary structure of RS1 as two back-to-back octameric rings, each cross-linked by disulfides. The observation of higher order structures in vitro suggests the formation of an adhesive matrix spanning the distance between cells (∼100 nm). Several studies indicated that RS1 readily binds to other proteins such as the sodium-potassium ATPase (NaK-ATPase) and extracellular matrix proteins. Alternatively, RS1 may influence fluid regulation via interaction with membrane proteins such as the NaK-ATPase, largely inferred from the use of carbonic anhydrase inhibitors to shrink the typical intra-retinal cysts in XLRS. We discuss these models in light of RS1 structure and address the difficulty in understanding the function of RS1.

Retinoschisin and novel Na/K-ATPase interaction partners Kv2.1 and Kv8.2 define a growing protein complex at the inner segments of mammalian photoreceptors

The RS1 gene on Xp 22.13 encodes retinoschisin which is known to directly interact with the retinal Na/K-ATPase at the photoreceptor inner segments. Pathologic mutations in RS1 cause X-linked juvenile retinoschisis (XLRS), a hereditary retinal dystrophy in young males. To further delineate the retinoschisin-Na/K-ATPase complex, co-immunoprecipitation was performed with porcine and murine retinal lysates targeting the ATP1A3 subunit. This identified the voltage-gated potassium (Kv) channel subunits Kv2.1 and Kv8.2 as direct interaction partners of the retinal Na/K-ATPase. Colocalization of the individual components of the complex was demonstrated at the membrane of photoreceptor inner segments. We further show that retinoschisin-deficiency, a frequent consequence of molecular pathology in XLRS, causes mislocalization of the macromolecular complex during postnatal retinal development with a simultaneous reduction of Kv2.1 and Kv8.2 protein expression, while the level of retinal Na/K-ATPase expression remains unaffected. Patch-clamp analysis revealed no effect of retinoschisin-deficiency on Kv channel mediated potassium ion currents in vitro. Together, our data suggest that Kv2.1 and Kv8.2 together with retinoschisin and the retinal Na/K-ATPase are integral parts of a macromolecular complex at the photoreceptor inner segments. Defective compartmentalization of this complex due to retinoschisin-deficiency may be a crucial step in initial XLRS pathogenesis.

Retinal Proteomic Alterations and Combined Transcriptomic-Proteomic Analysis in the Early Stages of Progression of a Mouse Model of X-Linked Retinoschisis

X-linked retinoschisis (XLRS) is among the most commonly inherited degenerative retinopathies. XLRS is caused by functional impairment of RS1. However, the molecular mechanisms underlying RS1 malfunction remain largely uncharacterized. Here, we performed a data-independent acquisition-mass spectrometry-based proteomic analysis in RS1-null mouse retina with different postal days (Ps), including the onset (P15) and early progression stage (P56). Gene set enrichment analysis showed that type I interferon-mediated signaling was upregulated and photoreceptor proteins responsible for detection of light stimuli were downregulated at P15. Positive regulation of Tor signaling was downregulated and nuclear transcribed mRNA catabolic process nonsense-mediated decay was upregulated at P56. Moreover, the differentially expressed proteins at P15 were enriched in metabolism of RNA and RNA destabilization. A broader subcellular localization distribution and enriched proteins in visual perception and phototransduction were evident at P56. Combined transcriptomic-proteomic analysis revealed that functional impairments, including detection of visible light, visual perception, and visual phototransduction, occurred at P21 and continued until P56. Our work provides insights into the molecular mechanisms underlying the onset and progression of an XLRS mouse model during the early stages, thus enhancing the understanding of the mechanism of XLRS.

RS1 gene is a novel prognostic biomarker for lung adenocarcinoma

Background

Although it has a poor prognosis, patients with lung adenocarcinoma (LUAD) have a relatively higher 5‐year survival period. Thus, it is necessary to identify effective prognostic markers to evaluate the effect of early treatment. RS1 gene encodes retinoschisin, a key protein in congenital retinoschisis, while few studies have been reported on the association between RS1 and cancer prognosis.

Methods

We performed bioinformatic analyses based on the data obtained from The Cancer Genome Atlas and Gene Expression Omnibus databases to demonstrate the expression level of RS1 was related to the LUAD prognosis and our findings were verified in‐vitro and clinical samples. Then, we explored the potential mechanism of how RS1 expression influenced the prognosis of LUAD.

Results

Compared with normal tissues, the RS1 expression was significantly lower in tumor tissues. The Multivariate Cox regression model showed that RS1 could be used as an independent prognostic indicator. Furthermore, we found significant differences in immune cell infiltration between RS1 high and low expression groups, and the proteasome pathway was found enriched in RS1 low expression samples.

Conclusion

In conclusion, our study suggests that RS1 is a novel prognostic biomarker for LUAD. Differences in immune cell infiltration and signaling pathways may contribute to the poor prognosis of LUAD caused by low RS1 expression.

Altered Protein Function Caused by AMD-associated Variant rs704 Links Vitronectin to Disease Pathology

Purpose

Vitronectin, a cell adhesion and spreading factor, is suspected to play a role in the pathogenesis of age-related macular degeneration (AMD), as it is a major component of AMD-specific extracellular deposits (e.g., soft drusen, subretinal drusenoid deposits). The present study addressed the impact of AMD-associated non-synonymous variant rs704 in the vitronectin-encoding gene VTN on vitronectin functionality.

Methods

Effects of rs704 on vitronectin expression and processing were analyzed by semi-quantitative sequencing of VTN transcripts from retinal pigment epithelium (RPE) cells generated from human induced pluripotent stem cells (hiPSCs) and from human neural retina, as well as by western blot analyses on heterologously expressed vitronectin isoforms. Binding of vitronectin isoforms to retinal and endothelial cells was analyzed by western blot. Immunofluorescence staining followed extracellular matrix (ECM) deposition in cultured RPE cells heterologously expressing the vitronectin isoforms. Adhesion of fluorescently labeled RPE or endothelial cells in dependence of recombinant vitronectin or vitronectin-containing ECM was investigated fluorometrically or microscopically. Tube formation and migration assays addressed effects of vitronectin on angiogenesis-related processes.

Results

Variant rs704 affected expression, secretion, and processing but not oligomerization of vitronectin. Cell binding and influence on RPE-mediated ECM deposition differed between AMD-risk-associated and non-AMD-risk-associated protein isoforms. Finally, vitronectin affected adhesion and endothelial tube formation.

Conclusions

The AMD-risk-associated vitronectin isoform exhibits increased expression and altered functionality in cellular processes related to the sub-RPE aspects of AMD pathology. Although further research is required to address the subretinal disease aspects, this initial study supports an involvement of vitronectin in AMD pathogenesis.