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

Salidroside modulates repolarization through stimulating Kv2.1 in rats

BACKGROUND

Voltage-gated potassium (Kv) channel growth is strongly associated with the development of arrhythmia. Salidroside (Sal), an active component from Rhodiola crenulata, has been shown to exert protective effects against heart disease. The present study was conducted to investigate the effects of Sal on Kv2.1 channel, and to explore the ionic mechanism of anti-arrhythmic.

METHODS

In this study, we utilized cisapride (Cis., A stimulant that prolongs the QT interval and causes cardiac arrhythmias) by intravenous injection to establish an arrhythmia model, and detected the effects of Sal on electrocardiography (ECG) and pressure volume loop (P-V loop) in SD rats. The effect of Sal on ECG of citalopram (Cit., a Kv2 channel inhibition)-evoked arrhythmia rat models was further evaluated by monitoring the dynamic changes of multiple indicators of ECG. Then, we detected the effect of Sal on the viability of hypoxic H9c2 cells using CCK-8 assay. After that, the effect of Sal on Kv channel currents (IKv) and Kv2.1 channel currents (IKv2.1) in H9c2 cells under normal and hypoxic conditions was examined using whole-cell patch clamp technique. In addition, the effect of Sal on IKv and IKv2.1 in H9c2 cells was determined under the inhibition of Kv and Kv2.1 channels. HEK293 cells stably transfected with Kv2.1 plasmids were also used to investigate the IKv2.1 changes under Sal pre-treated and co-incubated conditions. In addition, potential interactions of Sal with Kv2.1 protein were predicted and tested by molecular docking, molecular dynamics simulation (MDS), localized surface plasmon resonance (LSPR), and cellular thermal shift assay (CETSA) techniques, respectively. Furthermore, gene and protein levels of Kv2.1 in Sal-treated H9c2 cell were estimated by qRT-PCR, Western blot (WB) and immunofluorescence (IF) analysis.

RESULTS

Sal shortened the prolongated QT interval and ameliorated the cardiac impairment associated with arrhythmia in SD rats caused by Cis., as reflected in the ECG and P-V loop data. And Sal was also protective against arrhythmia in rats caused by Kv2 channel inhibition. At the cellular level, Sal increased cell viability after CoCl2-induced hypoxic injury in H9c2 cells. Whole-cell patch clamp assay confirmed that Sal inhibited both IKv and IKv2.1 in normal H9c2 cells, while enhanced IKv and IKv2.1 in cardiomyocytes after hypoxic injury. And Sal enhanced IKv inhibited by 1.5 mM 4-AP and upregulated all inhibition of Kv2 channels induced by 20 mM 4-AP administration, antagonized the IKv2.1 inhibitory effect of Cit. Moreover, Sal pre-administration for 24 h and immediate administration increased IKv2.1 in HEK293 cells stably transfected with Kv2.1 plasmids. Molecular docking demonstrated the potential binding of Sal to the Kv2.1 protein, with calculated binding energy of -5.4 kcal/mol. MDS test illustrated that the average hydrogen bonding of the Sal-Kv2.1 complexes was 30.89%. LSPR results verified the potential binding of Sal to Kv2.1 protein with an affinity value of 9.95 × 10-4 M. CETSA assay confirmed Sal can enhance the expression of Kv2.1 protein in H9c2 cells treated with heat, which suggests that Sal may bind to Kv2.1 protein. The results of WB, qRT-PCR, and IF further argued that Sal pre-administration for 24 h enhanced the levels of the Kv2.1 gene and protein (with no effects on the Kv2.1 gene and protein for H9c2 cells co-incubated with Sal for 6 h and 12 h).

CONCLUSION

Overall, our findings indicate that Sal can resist drug-induced arrhythmias in SD rats, partially by modulating repolarization through stimulating Kv2.1.

Retinoschisin Is Required for Pineal Gland Calcification and Cellular Communication in Pinealocytes of Rats and Mice

Retinoschisin (RS1) is a secretory protein specifically localized to the extracellular domains in both the lateral retina and the pineal gland (PG). However, the functions of RS1 in the pineal body are poorly understood. To address this knowledge gap, in this study, we undertook histochemical, ultrastructural, and Western blotting analyses of the PG in rats and RS1-knock-in transgenic. We found that RS1 plays a key role in pineal gland calcification (PGC) in mice through both extracellular and intracellular pathways. RS1 was clustered around the cell membrane or intracellularly in pinealocytes, actively participating in the exchange of calcium and thereby mediating PGC. Additionally, RS1 deposition is essential for maintaining PGC architecture in the intercellular space of the adult PG. In RS1-knock-in mice with a nonsense mutation (p.Y65X) in the Rs1-domain of RS1, the Rs1-domain is chaotically dispersed in pinealocytes and the intercellular region of the PG. This prevents RS1 from binding calcified spots and forming calcified nodules, ultimately leading to the accumulation of calcareous lamellae in microvesicles. Additionally, RS1 was observed to colocalize with connexin-36, thereby modulating intercellular communication in the PG of both rats and mice. Our study revealed for the first time that RS1 is essential for maintaining PGC architecture and that it colocalizes with connexin 36 to modulate intercellular communication in the PG. These findings provide novel insights into the function of the RS1 gene in the PG.

Fundus autofluorescence, optical coherence tomography and electroretinography abnormalities in a patient with digoxin retinopathy that resemble those in KCNV2-associated retinopathy

BACKGROUND

Digoxin related retinal toxicity causes blurred vision, photophobia, central scotoma, color vision abnormality, and electroretinography (ERG) abnormalities. Here, we report a case with transient abnormalities in vison, in which fundus autofluorescence (FAF), optical coherence tomography (OCT), and ERG findings resembled those in KCNV2 (potassium voltage-gated channel modifier subfamily V member 2)-associated retinopathy.

CASE REPORT

An 89-year-old woman presented with complaints of acute blurred vision, nyctalopia, photophobia, and color vision abnormality. She received digoxin for tachycardia induced by atrial fibrillation for a month. The fundi showed a faint white ring at the fovea, which showed hyperfluorescence in FAF. OCT showed a thickened EZ in the macula. A dark-adapted (DA)-30 ERG showed a reduced and "squaring (trough-flattened)" a-wave, and a delayed, supernormal b-wave, resulting in a high b/a-wave amplitude ratio. The digoxin dose was reduced following an elevation in serum levels. Five weeks later, her visual acuities improved, and abnormal hyperfluorescence on FAF disappeared. After 6 months, no visual symptoms were reported. The ellipsoid-zone thickening in OCT improved; however, the b/a-wave amplitude ratio on DA-30 ERG remained high. The b-wave in LA-long-flash ERG was initially reduced, which improved after correction of serum level of digoxin.

CONCLUSIONS

The patient's clinical findings resembled those of patients with KCNV2-associated retinopathy or temporal hyperkalemia. These disorders appear to have a common pathogenesis, which may be related to abnormal extracellular potassium levels in the retina. The on-bipolar cells seemed to be more affected than the off-bipolar cells in digoxin related retinal toxicity.

Long-term functional and structural outcomes in X-linked retinoschisis: implications for clinical trials

Introduction

X-linked retinoschisis (XLRS) is an inherited retinal disease (IRD) caused by pathogenic mutations in the retinoschisin gene, RS1. Affected individuals develop retinal layer separation, leading to loss of visual acuity (VA). Several XLRS gene therapy trials have been attempted but none have met their primary endpoints. An improved understanding of XLRS natural history and clinical outcomes may better inform future trials. Here, we report the long-term functional and structural outcomes of XLRS and the relevance of RS1 genotypes to the visual prognosis of affected individuals.

Methods

A retrospective chart review of patients with molecularly confirmed X-linked retinoschisis was performed. Functional and structural outcomes, and RS1 genotype data, were included for analysis.

Results

Fifty-two patients with XLRS from 33 families were included in the study. Median age at symptom onset was 5 years (range 0-49) and median follow-up was 5.7 years (range 0.1-56.8). Macular retinoschisis occurred in 103 of 104 eyes (99.0%), while peripheral retinoschisis occurred in 48 of 104 eyes (46.2%), most often in the inferotemporal quadrant (40.4%). Initial and final VA were similar (logMAR 0.498 vs. 0.521; p = 0.203). Fifty of 54 eyes (92.6%) developed detectable outer retinal loss by age 20, and 29 of 66 eyes (43.9%) had focal or diffuse outer retinal atrophy (ORA) by age 40. ORA but not central subfield thickness (CST) was associated with reduced VA. Inter-eye correlation was modest for VA (r-squared = 0.03; p = 0.08) and CST (r-squared = 0.15; p = 0.001). Carbonic anhydrase inhibitors (CAIs) improved CST (p = 0.026), but not VA (p = 0.380). Eight of 104 eyes (7.7%) had XLRS-related retinal detachment (RD), which was associated with poorer outcomes compared to eyes without RD (median final VA 0.875 vs. 0.487; p <0.0001). RS1 null genotypes had greater odds of at least moderate visual impairment at final follow-up (OR 7.81; 95% CI 2.17, 28.10; p = 0.002) which was independent of age at onset, initial CST, initial ORA, or previous RD.

Discussion

Overall, long-term follow-up of XLRS patients demonstrated relatively stable VA, with presenting CST, development of ORA, and null RS1 mutations associated with poorer long-term visual outcomes, indicating a clinically relevant genotype-phenotype correlation in XLRS.

Targeted Expression of Retinoschisin by Retinal Bipolar Cells in XLRS Promotes Resolution of Retinoschisis Cysts Sans RS1 From Photoreceptors

Purpose

Loss of retinoschisin (RS1) function underlies X-linked retinoschisis (XLRS) pathology. In the retina, both photoreceptor inner segments and bipolar cells express RS1. However, the loss of RS1 function causes schisis primarily in the inner retina. To understand these cell type-specific phenotypes, we decoupled RS1 effects in bipolar cells from that in photoreceptors.

Methods

Bipolar cell transgene RS1 expression was achieved using two inner retina-specific promoters: (1) a minimal promoter engineered from glutamate receptor, metabotropic glutamate receptor 6 gene (mini-mGluR6/ Grm6) and (2) MiniPromoter (Ple155). Adeno-associated virus vectors encoding RS1 gene under either the mini-mGluR6 or Ple-155 promoter were delivered to the XLRS mouse retina through intravitreal or subretinal injection on postnatal day 14. Retinal structure and function were assessed 5 weeks later: immunohistochemistry for morphological characterization, optical coherence tomography and electroretinography (ERG) for structural and functional evaluation.

Results

Immunohistochemical analysis of RS1expression showed that expression with the MiniPromoter (Ple155) was heavily enriched in bipolar cells. Despite variations in vector penetrance and gene transfer efficiency across the injected retinas, those retinal areas with robust bipolar cell RS1 expression showed tightly packed bipolar cells with fewer cavities and marked improvement in inner retinal structure and synaptic function as judged by optical coherence tomography and electroretinography, respectively.

Conclusions

These results demonstrate that RS1 gene expression primarily in bipolar cells of the XLRS mouse retina, independent of photoreceptor expression, can ameliorate retinoschisis structural pathology and provide further evidence of RS1 role in cell adhesion.