Sensing through Non-Sensing Ocular Ion Channels

Nonviral base editing of KCNJ13 mutation preserves vision in a model of inherited retinal channelopathy

Clinical genome editing is emerging for rare disease treatment, but one of the major limitations is the targeting of CRISPR editors' delivery. We delivered base editors to the retinal pigmented epithelium (RPE) in the mouse eye using silica nanocapsules (SNCs) as a treatment for retinal degeneration. Leber congenital amaurosis type 16 (LCA16) is a rare pediatric blindness caused by point mutations in the KCNJ13 gene, a loss of function inwardly rectifying potassium channel (Kir7.1) in the RPE. SNCs carrying adenine base editor 8e (ABE8e) mRNA and sgRNA precisely and efficiently corrected the KCNJ13W53X/W53X mutation. Editing in both patient fibroblasts (47%) and human induced pluripotent stem cell-derived RPE (LCA16-iPSC-RPE) (17%) showed minimal off-target editing. We detected functional Kir7.1 channels in the edited LCA16-iPSC-RPE. In the LCA16 mouse model (Kcnj13W53X/+ΔR), RPE cells targeted SNC delivery of ABE8e mRNA preserved normal vision, measured by full-field electroretinogram (ERG). Moreover, multifocal ERG confirmed the topographic measure of electrical activity primarily originating from the edited retinal area at the injection site. Preserved retina structure after treatment was established by optical coherence tomography (OCT). This preclinical validation of targeted ion channel functional rescue, a challenge for pharmacological and genomic interventions, reinforced the effectiveness of nonviral genome-editing therapy for rare inherited disorders.

V-ATPase Regulates Retinal Progenitor Cell Proliferation During Eye Regrowth in Xenopus

Purpose: The induction of retinal progenitor cell (RPC) proliferation is a strategy that holds promise for alleviating retinal degeneration. However, the mechanisms that can stimulate RPC proliferation during repair remain unclear. Xenopus tailbud embryos successfully regrow functional eyes within 5 days after ablation, and this process requires increased RPC proliferation. This model facilitates identification of mechanisms that can drive in vivo reparative RPC proliferation. This study assesses the role of the essential H+ pump, V-ATPase, in promoting stem cell proliferation. Methods: Pharmacological and molecular loss of function studies were performed to determine the requirement for V-ATPase during embryonic eye regrowth. The resultant eye phenotypes were examined using histology and antibody markers. Misexpression of a yeast H+ pump was used to test whether the requirement for V-ATPase in regrowth is dependent on its H+ pump function. Results: V-ATPase inhibition blocked eye regrowth. Regrowth-incompetent eyes resulting from V-ATPase inhibition contained the normal complement of tissues but were much smaller. V-ATPase inhibition caused a significant reduction in reparative RPC proliferation but did not alter differentiation and patterning. Modulation of V-ATPase activity did not affect apoptosis, a process known to be required for eye regrowth. Finally, increasing H+ pump activity was sufficient to induce regrowth. Conclusions: V-ATPase is required for eye regrowth. These results reveal a key role for V-ATPase in activating regenerative RPC proliferation and expansion during successful eye regrowth.

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

Inward rectifier potassium (Kir) channels in the retina: living our vision

Channel proteins are vital for conducting ions throughout the body and are especially relevant to retina physiology. Inward rectifier potassium (Kir) channels are a class of K+ channels responsible for maintaining membrane potential and extracellular K+ concentrations. Studies of the KCNJ gene (that encodes Kir protein) expression identified the presence of all of the subclasses (Kir 1-7) of Kir channels in the retina or retinal-pigmented epithelium (RPE). However, functional studies have established the involvement of the Kir4.1 homotetramer and Kir4.1/5.1 heterotetramer in Müller glial cells, Kir2.1 in bipolar cells, and Kir7.1 in the RPE cell physiology. Here, we propose the potential roles of Kir channels in the retina based on the physiological contributions to the brain, pancreatic, and cardiac tissue functions. There are several open questions regarding the expressed KCNJ genes in the retina and RPE. For example, why does not the Kir channel subtype gene expression correspond with protein expression? Catching up with multiomics or functional "omics" approaches might shed light on posttranscriptional changes that might influence Kir subunit mRNA translation within the retina that guides our vision.

Ocular Surface Ion-Channels Are Closely Related to Dry Eye: Key Research Focus on Innovative Drugs for Dry Eye

Abundant ion-channels, including various perceptual receptors, chloride channels, purinergic receptor channels, and water channels that exist on the ocular surface, play an important role in the pathogenesis of dry eye. Channel-targeting activators or inhibitor compounds, which have shown positive effects in in vivo and in vitro experiments, have become the focus of the dry eye drug research and development, and individual compounds have been applied in clinical experimental treatment. This review summarized various types of ion-channels on the ocular surface related to dry eye, their basic functions, and spatial distribution, and discussed basic and clinical research results of various channel receptor regulatory compounds. Therefore, further elucidating the relationship between ion-channels and dry eye will warrant research of dry eye targeted drug therapy.

Molecular Approaches Fighting Nonsense

Nonsense mutations are the result of single nucleotide substitutions in the DNA that change a sense codon (coding for an amino acid) to a nonsense or premature termination codon (PTC) within the coding region of the mRNA [...].

New Omics-Derived Perspectives on Retinal Dystrophies: Could Ion Channels-Encoding or Related Genes Act as Modifier of Pathological Phenotype?

Ion channels are membrane-spanning integral proteins expressed in multiple organs, including the eye. Here, ion channels play a role in several physiological processes, like signal transmission and visual processing. A wide range of mutations have been reported in the corresponding genes and their interacting subunit coding genes, which contribute significantly to a wide spectrum of ocular diseases collectively called channelopathies, a subgroup of inherited retinal dystrophies. Such mutations result in either a loss or gain-of channel functions affecting the structure, assembly, trafficking and localization of channel proteins. We investigated the probands of seven Italian and Egyptian families affected by not completely defined forms of inherited retinal dystrophies, by whole exome sequencing (WES) experiments, and found interesting variants in already known causative genes probably able to impair retinal functionalities. However, because such variants did not completely explain the phenotype manifested by each patient, we proceed to further investigate possible related genes carrying mutations that might complement previously found data, based on the common aspect linked to neurotransmission impairments. We found 10 mutated genes whose variants might alter important ligand binding sites differently distributed through all considered patients. Such genes encode for ion channels, or their regulatory proteins, and strictly interact with known causative genes, also sharing with them synaptic-related pathways. Taking into account several limitations that will be resolved by further experiments, we believe that our exploratory investigation will help scientists to provide a new promising paradigm for precise diagnosis of retinal dystrophies to facilitate the development of rational treatments.