G protein-coupled receptors differentially regulate glycosylation and activity of the inwardly rectifying potassium channel Kir7.1

Endocytic activation and exosomal secretion of matriptase stimulate the second wave of EGF signaling to promote skin and breast cancer invasion

The dysfunction of matriptase, a membrane-anchored protease, is highly related to the progression of skin and breast cancers. Epidermal growth factor (EGF)-induced matriptase activation and cancer invasion are known but with obscure mechanisms. Here, we demonstrate a vesicular-trafficking-mediated interplay between matriptase and EGF signaling in cancer promotion. We found that EGF induces matriptase to undergo endocytosis together with the EGF receptor, followed by acid-induced activation in endosomes. Activated matriptase is then secreted extracellularly on exosomes to catalyze hepatocyte growth factor precursor (pro-HGF) cleavage, resulting in autocrine HGF/c-Met signaling. Matriptase-induced HGF/c-Met signaling represents the second signal wave of EGF, which promotes cancer cell scattering, migration, and invasion. These findings demonstrate a role of vesicular trafficking in efficient activation and secretion of membrane matriptase and a reciprocal regulation of matriptase and EGF signaling in cancer promotion, providing insights into the physiological functions of vesicular trafficking and the molecular pathological mechanisms of skin and breast cancers.

Hypothalamic POMC neuron-specific knockout of MC4R affects insulin sensitivity by regulating Kir2.1

BACKGROUND

Imbalance in energy regulation is a major cause of insulin resistance and diabetes. Melanocortin-4 receptor (MC4R) signaling at specific sites in the central nervous system has synergistic but non-overlapping functions. However, the mechanism by which MC4R in the arcuate nucleus (ARC) region regulates energy balance and insulin resistance remains unclear.

METHODS

The MC4Rflox/flox mice with proopiomelanocortin (POMC) -Cre mice were crossed to generate the POMC-MC4Rflox/+ mice. Then POMC-MC4Rflox/+ mice were further mated with MC4Rflox/flox mice to generate the POMC-MC4Rflox/flox mice in which MC4R is selectively deleted in POMC neurons. Bilateral injections of 200 nl of AAV-sh-Kir2.1 (AAV-sh-NC was used as control) were made into the ARC of the hypothalamus. Oxygen consumption, carbon dioxide production, respiratory exchange ratio and energy expenditure were measured by using the CLAMS; Total, visceral and subcutaneous fat was analyzed using micro-CT. Co-immunoprecipitation assays (Co-IP) were used to analyze the interaction between MC4R and Kir2.1 in GT1-7 cells.

RESULTS

POMC neuron-specific ablation of MC4R in the ARC region promoted food intake, impaired energy expenditure, leading to increased weight gain and impaired systemic glucose homeostasis. Additionally, MC4R ablation reduced the activation of POMC neuron, and is not tissue-specific for peripheral regulation, suggesting the importance of its central regulation. Mechanistically, sequencing analysis and Co-IP assay demonstrated a direct interaction of MC4R with Kir2.1. Knockdown of Kir2.1 in POMC neuron-specific ablation of MC4R restored the effect of MC4R ablation on energy expenditure and systemic glucose homeostasis, indicating by reduced body weight and ameliorated insulin resistance.

CONCLUSION

Hypothalamic POMC neuron-specific knockout of MC4R affects energy balance and insulin sensitivity by regulating Kir2.1. Kir2.1 represents a new target and pathway that could be targeted in obesity.

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.

MC4R in Central and Peripheral Systems

Obesity has emerged as a critical and urgent health burden during the current global pandemic. Among multiple genetic causes, melanocortin receptor-4 (MC4R), involved in food intake and energy metabolism regulation through various signaling pathways, has been reported to be the lead genetic factor in severe and early onset obesity and hyperphagia disorders. Most previous studies have illustrated the roles of MC4R signaling in energy intake versus expenditure in the central system, while some evidence indicates that MC4R is also expressed in peripheral systems, such as the gut and endocrine organs. However, its physiopathological function remains poorly defined. This review aims to depict the central and peripheral roles of MC4R in energy metabolism and endocrine hormone homeostasis, the diversity of phenotypes, biased downstream signaling caused by distinct MC4R mutations, and current drug development targeting the receptor.

Potassium Channels, Glucose Metabolism and Glycosylation in Cancer Cells

Potassium channels emerge as one of the crucial groups of proteins that shape the biology of cancer cells. Their involvement in processes like cell growth, migration, or electric signaling, seems obvious. However, the relationship between the function of K+ channels, glucose metabolism, and cancer glycome appears much more intriguing. Among the typical hallmarks of cancer, one can mention the switch to aerobic glycolysis as the most favorable mechanism for glucose metabolism and glycome alterations. This review outlines the interconnections between the expression and activity of potassium channels, carbohydrate metabolism, and altered glycosylation in cancer cells, which have not been broadly discussed in the literature hitherto. Moreover, we propose the potential mediators for the described relations (e.g., enzymes, microRNAs) and the novel promising directions (e.g., glycans-orinented drugs) for further research.

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.

The Multifaceted Melanocortin Receptors

The 5 known melanocortin receptors (MCs) have established physiological roles. With the exception of MC2, these receptors can behave unpredictably, and since they are more widely expressed than their established roles would suggest, it is likely that they have other poorly characterized functions. The aim of this review is to discuss some of the less well-explored aspects of the 4 enigmatic members of this receptor family (MC1,3-5) and describe how these are multifaceted G protein-coupled receptors (GPCRs). These receptors appear to be promiscuous in that they bind several endogenous agonists (products of the proopiomelanocortin [POMC] gene) and antagonists but with inconsistent relative affinities and effects. We propose that this is a result of posttranslational modifications that determine receptor localization within nanodomains. Within each nanodomain there will be a variety of proteins, including ion channels, modifying proteins, and other GPCRs, that can interact with the MCs to alter the availability of receptor at the cell surface as well as the intracellular signaling resulting from receptor activation. Different combinations of interacting proteins and MCs may therefore give rise to the complex and inconsistent functional profiles reported for the MCs. For further progress in understanding this family, improved characterization of tissue-specific functions is required. Current evidence for interactions of these receptors with a range of partners, resulting in modulation of cell signaling, suggests that each should be studied within the full context of their interacting partners. The role of physiological status in determining this context also remains to be characterized.

Envisioning the role of inwardly rectifying potassium (Kir) channel in epilepsy

Epilepsy is a devastating neurological disorder characterized by recurrent seizures attributed to the disruption of the dynamic excitatory and inhibitory balance in the brain. Epilepsy has emerged as a global health concern affecting about 70 million people worldwide. Despite recent advances in pre-clinical and clinical research, its etiopathogenesis remains obscure, and there are still no treatment strategies modifying disease progression. Although the precise molecular mechanisms underlying epileptogenesis have not been clarified yet, the role of ion channels as regulators of cellular excitability has increasingly gained attention. In this regard, emerging evidence highlights the potential implication of inwardly rectifying potassium (Kir) channels in epileptogenesis. Kir channels consist of seven different subfamilies (Kir1-Kir7), and they are highly expressed in both neuronal and glial cells in the central nervous system. These channels control the cell volume and excitability. In this review, we discuss preclinical and clinical evidence on the role of the several subfamilies of Kir channels in epileptogenesis, aiming to shed more light on the pathogenesis of this disorder and pave the way for future novel therapeutic approaches.

The epithelial potassium channel Kir7.1 is stimulated by progesterone

The choroid plexus (CP) epithelium secretes cerebrospinal fluid and plays an important role in healthy homeostasis of the brain. CP function can be influenced by sex steroid hormones; however, the precise molecular mechanism of such regulation is not well understood. Here, using whole-cell patch-clamp recordings from male and female murine CP cells, we show that application of progesterone resulted in specific and strong potentiation of the inwardly rectifying potassium channel Kir7.1, an essential protein that is expressed in CP and is required for survival. The potentiation was progesterone specific and independent of other known progesterone receptors expressed in CP. This effect was recapitulated with recombinant Kir7.1, as well as with endogenous Kir7.1 expressed in the retinal pigment epithelium. Current-clamp studies further showed a progesterone-induced hyperpolarization of CP cells. Our results provide evidence of a progesterone-driven control of tissues in which Kir7.1 is present.

Altered phosphatidylinositol regulation of mutant inwardly rectifying K+ Kir7.1 channels associated with inherited retinal degeneration disease

KEY POINTS

Kir7.1 K⁺ channel expressed in retinal pigment epithelium is mutated in inherited retinal degeneration diseases. We study Kir7.1 in heterologous expression to test the hypothesis that pathological R162 mutation to neutral amino acids results in loss of a crucial site that binds PI(4,5)P₂ . Although R162W mutation inactivates Kir7.1, changes to smaller volume (e.g. Gln) amino acids are tolerated or even enhance function (Ala or Cys). Chemical modification of Kir7.1-R162C confirms that large residues of the size of Trp are incompatible with normal channel function even if positively charged. In addition to R162, K164 (and possibly K159) forms a binding site for the phosphoinositide and is essential for channel activity. R162 substitution with a large, neutral side chain like Trp exerts a dominant negative effect on Kir7.1 activity such that less than one fifth of the full activity is expected in a cell expressing the same amount of mutant and wild-type channels.

ABSTRACT

Mutations in the Kir7.1 K⁺ channel, highly expressed in retinal pigment epithelium, have been linked to inherited retinal degeneration diseases. Examples are mutations changing Arg 162 to Trp in snowflake vitreoretinal degeneration (SVD) and Gln in retinitis pigmentosa. R162 is believed to be part of a site that binds PI(4,5)P₂ and stabilises the open state. We have tested the hypothesis that R162 mutation to neutral amino acids will result in the loss of this crucial interaction to the detriment of channel function. Our findings indicate that although R612W mutation inactivates Kir7.1, changes to smaller volume (e.g. Gln) amino acids are tolerated or even enhance function (Ala or Cys). Cys chemical modification of Kir7.1-R162C confirms that large residues of the size of Trp are incompatible with normal channel function even if positively charged. Experiments titrating the levels of plasma membrane PI(4,5)P₂ with voltage-dependent phosphatase DrVSP reveal that, in addition to R162, K164 (and possibly K159) forms a binding site for the phosphoinositide and ensures channel activity. Finally, the use of a concatemeric approach shows that substitution of R162 with a large, neutral side chain mimicking a Trp residue exerts a dominant negative effect on Kir7.1 activity such that less than one fifth of the full activity is expected in heterozygous cells carrying the SVD mutation. Our results suggest that if mutations in the human KCNJ13 gene resulting in the neutralisation of R162 and Kir7.1 malfunction led to retinal degeneration diseases, their severity might depend on the nature of the side chain of the replacing amino acid.

Sensing through Non-Sensing Ocular Ion Channels

Ion channels are membrane-spanning integral proteins expressed in multiple organs, including the eye. In the eye, ion channels are involved in various 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 an array of blindness, termed ocular channelopathies. These mutations result in either a loss- or gain-of channel functions affecting the structure, assembly, trafficking, and localization of channel proteins. A dominant-negative effect is caused in a few channels formed by the assembly of several subunits that exist as homo- or heteromeric proteins. Here, we review the role of different mutations in switching a “sensing” ion channel to “non-sensing,” leading to ocular channelopathies like Leber’s congenital amaurosis 16 (LCA16), cone dystrophy, congenital stationary night blindness (CSNB), achromatopsia, bestrophinopathies, retinitis pigmentosa, etc. We also discuss the various in vitro and in vivo disease models available to investigate the impact of mutations on channel properties, to dissect the disease mechanism, and understand the pathophysiology. Innovating the potential pharmacological and therapeutic approaches and their efficient delivery to the eye for reversing a “non-sensing” channel to “sensing” would be life-changing.

A review of neuropsychiatric adverse events from topical ophthalmic brimonidine

Brimonidine is a first-line topical medication for increased intraocular pressure and glaucoma which may be used alone or in conjunction with other topical therapies. Its structural and pharmacological comparabilities to clonidine give way to the hypothesis that it may cause neuropsychiatric side effects. The majority of case reports citing brimonidine toxicity, either for topical or peripheral exposure, include pediatric age groups but especially infants. Among the latter, a dose-response phenomenon is evident. Dose-response correlates have also been shown among adults. Case series and prospective double-blind treatment studies also give evidence for the occurrence of several central nervous system adverse reactions. Topical ophthalmic brimonidine use should be followed for the occurrence of neuropsychiatric disturbances generally, and enhanced vigilance should be maintained for at-risk populations.

Glial and neuronal expression of the Inward Rectifying Potassium Channel Kir7.1 in the adult mouse brain

Inward Rectifying Potassium channels (Kir) are a large family of ion channels that play key roles in ion homeostasis and neuronal excitability. The most recently described Kir subtype is Kir7.1, which is known as a K⁺ transporting subtype. Earlier studies localised Kir7.1 to subpopulations of neurones in the brain. However, the pattern of Kir7.1 expression across the brain has not previously been examined. Here, we have determined neuronal and glial expression of Kir7.1 in the adult mouse brain, using immunohistochemistry and transgenic mouse lines expressing reporters specific for astrocytes [glial fibrillary acidic protein‐enhanced green fluorescent protein (GFAP‐EGFP], myelinating oligodendrocytes (PLP‐DsRed), oligodendrocyte progenitor cells (OPC, Pdgfra‐creER^T2/Rosa26‐YFP double‐transgenic mice) and all oligodendrocyte lineage cells (SOX10‐EGFP). The results demonstrate significant neuronal Kir7.1 immunostaining in the cortex, hippocampus, cerebellum and pons, as well as the striatum and hypothalamus. In addition, astrocytes are shown to be immunopositive for Kir7.1 throughout grey and white matter, with dense immunostaining on cell somata, primary processes and perivascular end‐feet. Immunostaining for Kir7.1 was observed in oligodendrocytes, myelin and OPCs throughout the brain, although immunostaining was heterogeneous. Neuronal and glial expression of Kir7.1 is confirmed using neurone‐glial cortical cultures and optic nerve glial cultures. Notably, Kir7.1 have been shown to regulate the excitability of thalamic neurones and our results indicate this may be a widespread function of Kir7.1 in neurones throughout the brain. Moreover, based on the function of Kir7.1 in multiple transporting epithelia, Kir7.1 are likely to play an equivalent role in the primary glial function of K⁺ homeostasis. Our results indicate Kir7.1 are far more pervasive in the brain than previously recognised and have potential importance in regulating neuronal and glial function.

Melanocortin-4 Receptor Signalling: Importance for Weight Regulation and Obesity Treatment

The melanocortin-4 receptor (MC4R) - embedded in the leptin-melanocortin pathway - is activated by proopiomelanocortin (POMC)-derived neuropeptides such as α- and β-melanocyte-stimulating hormone (MSH) and plays an important role in hypothalamic body-weight regulation. Accordingly, MC4R is a potential drug target to combat obesity. Previous attempts to develop MC4R agonists failed due to ineffectiveness or severe adverse events. Recently, a new generation of MC4R ligands was developed. Specifically, setmelanotide was found to be effective by inducing biased signalling of the MC4R and thereby reducing feelings of hunger and leading to substantial weight loss in patients with POMC or leptin receptor deficiency. This new potential pharmacological treatment option could be beneficial for further groups of obese patients with defects in the leptin-melanocortin signalling pathway.