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

Abstract

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.

INTRODUCTION

The retina is part of the central nervous system and is essential for the visual cycle (1). The neural retina lies in the back of the eye, and the retinal-pigmented epithelium (RPE) forms the blood-retina barrier (2, 3). Ion channel function within the retina’s complex neural circuitry is crucial to maintaining the visual cycle in the central nervous system’s light and dark and neurologic vision components (4). Inwardly rectifying potassium (Kir) channels are a subfamily of K⁺ channels expressed within the developing and mature retina and RPE (5). Seven distinct Kir channels (Kir 1–7) comprise the Kir family members in the mammalian system. Kir channels are regulated in multiple ways, such as membrane potential, ligand gating, and adenosine triphosphate (ATP) dependence (6). Within the retina, Kir channels mediate neural signaling, K⁺ homeostasis, membrane potentials, and the tight blood-retina barrier in the RPE (7, 8). Kir channel mutations within the retina reportedly cause early-onset blindness, including mutations in KCNJ13, which encode for Kir7.1, leading to Leber's congenital amaurosis (LCA16) (8). Kir channels also play a role in the neuronal response in Müller glial cells (Kir4.1) (9). The review provides detailed pathophysiology of Kir2.x, Kir4.x, and Kir7.1 channels in retinal disease.

MOLECULAR EXPRESSION OF Kir CHANNELS IN THE NEURAL RETINA AND RPE

The neural retina expresses most of the Kir channel subunits, with a higher level seen for Kir2.1, Kir3.1, and Kir4.1 than other subunits (5). A cell-type-specific subunit expression exists for these transcripts, yet Kir channels are not used as biomarkers. Transcripts for all subunits, except Kir2.3, Kir2.4, Kir3.2, Kir3.3, Kir4.1, Kir5.1, and Kir6.2, have been identified in the RPE. Kir channel proteins are present in retinal ganglion cells, amacrine cells, Müller cells, bipolar cells, and possibly photoreceptors within the neural retina (5). In the same paper, Yang et al. used immunoblot analysis to demonstrate the relatively low Kir1.1 and Kir3.1 expression in the RPE cells compared with other channel subunits (5). Immunostaining indicated that Kir2.1 protein is expressed in mature bipolar cells and horizontal cells in rodents (9, 10). The labeling for Kir2.1 was below the photoreceptor terminal and, more specifically, within the rod spherule proximity (9). Moreover, Kir2.1 does not colocalize with the Müller cell marker but is present in the rod bipolar cell termini (9). Kir2.1 staining was also noticed in ganglion cells, but it was mainly localized to the nuclei without an established significance (11). Kir3.2 protein is localized to the cytoplasm of rat retinal ganglion cell somata, proximal dendrites, and axons. When present in the membrane, Kir3.2 is associated with the G protein-coupled receptor (GPCR), the γ-aminobutyric acid B (GABAB) (11). Kir3.2 protein has also been identified within the inner plexiform layer of the chick retina during development based on neuronal activity (12).

Immunohistochemistry identified Kir4.1 localization in the Müller glial cells of the rat retina and interfacing with the apical RPE processes (13). Kir4.1 is colocalized with the glial fibrillary acidic protein in the Müller glial cells within the rat retina (9). Further confirmation of polarized Kir4.1 expression was due to its interactions with dystrophin-glycoprotein complex (DP71) and aquaporin 4 (AQP4) in both mouse and rat retina. The specific Kir4.1 localization was confined to the endfeet by the vitreal border and perivascular processes in the outer retinal layers (14–17). The Kir4.1/Kir5.1 heterotetramer is also present in the perisynaptic processes in the inner plexiform layer in addition to the Kir4.1 homotetramer (18). Kir6.1 protein is expressed in the RPE and may alter blood glucose homeostasis and retinal vasculature. Moreover, it maintains insulin sensitivity and reduces inflammation (19). Kir7.1 is present in the specialized apical RPE processes that establish communications with the adjacent photoreceptor cells (5, 7, 20, 21).

WHAT ROLES DO Kir CHANNELS PLAY IN THE RETINAL DISEASE STATES?

The existing model for Kir channels in the retina and RPE is based on the expression studies above and consists of a strong inward rectifier Kir2.1 expressed in bipolar cells (9), a weak inward rectifier Kir4.1 in Müller glial cells as a homotetramer, and a heterotetramer with Kir5.1, and a weak inward rectifier Kir7.1 in the apical RPE processes (Fig. 1). In contrast, Fig. 2 shows the expression of Kir channels in the diseased retina.

Figure 1.

The role of Kir channels in K⁺ spatial buffering within the retina and RPE. Kir7.1 releases a small outward current from the apical processes of the RPE into the subretinal space. Kir4.1 is expressed as a homotetramer in the Müller glial cell endfeet and releases K⁺ into the subretinal space. Kir4.1 is also expressed as a heterotetramer in the inner plexiform layer and exhibits a bidirectional K⁺ current. Müller glial cells, bipolar cells, and retinal glial cells all exhibit a strong inward K⁺ current from Kir2.1. Amacrine cells also express Kir3.2. Arrows indicate the direction of K⁺ flux through the respective channels. Rectification properties of individual retinal Kir channel subtypes is also color coded. AC, amacrine cells; BC, bipolar cells; GC, ganglion cells; HC, horizontal cells; IPL, inner plexiform layer; MGC, Müller glial cells; OPL, outer plexiform layer; PR, photoreceptors.

Figure 2.

Kir channels in the diseased retina. Kir2.1 is present in the apical processes of the RPE, in addition to the cone photoreceptor, and is believed to regulate VEGF secretion from the RPE and control retinal vasculature. Pathophysiology associated with Kir4.1 and Kir7.1 contributes to blindness. Shown are also blood vessels in the retina that benefit from expressing strong inward rectifier channels in the pericytes. RPE, retinal pigmented epithelium.

Kir2.1 Physiology and Pathophysiology

cKir2.1 is activated by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) when the retina is illuminated (22). Previous evidence indicated Kir2.1 expression in Drosophila (22) and mouse (10) photoreceptors. These studies may have been undertaken at various retinal development stages, or the antibody was nonspecific for the Kir2.1 channel. In addition, researchers previously identified Kir2.1 in mouse photoreceptors colocalized with channel-associated protein synapse-110 and indicated its involvement in synaptic cell signaling (10). However, functional studies suggest that Kir2.1 exists only in bipolar cells (9, 23).

Research has recently identified Kir2.1 in the apical processes of rat RPE and the bipolar cells within the inner plexiform layer and Müller glial cells (Fig. 2) (22). However, one limitation of the study was that the cultured RPE cells were not mature monolayers, and the authors did not evaluate markers, such as RPE65, Best1, and TRP1, to determine monolayer maturity (23). The tight monolayers and polarity of the cultured RPE cells were also not evaluated by transepithelial electrical resistance (24). The only evidence provided that the Kir2.1 channel was trafficked to the apical process was the polarized distribution with more significant staining in the apical region of the cell compared with the basolateral region (22). However, this finding does not necessarily mean that the channel was trafficked to the cell membrane. Thus, a previously characterized polarized RPE model, such as human induced pluripotent stem cells (iPSC)-RPE monolayers, could address these limitations. It is also possible that Kir2.1 remain in the endosomal pathway and does not reach the cell membrane. This model aligns with what is observed in yeast, where the required endosomal sorting complex for transport pathways regulates the number of Kir2.1 channels that reach the cell membrane depending on the extracellular K⁺ concentration (25). We predict that Kir2.1 would only be trafficked to the membrane in disease conditions, such as diabetic retinopathy.

RPE cells can be inferred to regulate the subretinal space osmolarity. In vitro, RPE cells regulate cell culture media osmolarity and alter the angiogenic vascular endothelial growth factor (VEGF) secretion (26). The angiogenic effects of extracellular VEGF in the human fetal retina are required for retinal vasculature development (27). Based on scientific facts, the current model suggests that hypoxic astrocytes secrete VEGF in the developing human retina, thereby facilitating angiogenesis from the optic disk (28). Excess VEGF is the primary cause of the blood-retina barrier breakdown (29). An increased extracellular VEGF is associated with Kir2.1 expression in the developing fetal retina (Fig. 2). Prenatal hypoxia by intrauterine growth restriction induces retinal dysfunction in the RPE caused by the excess release of VEGF (30). In addition, the increased Kir2.1 mRNA expression is correlated with increased VEGF mRNA (22). Previous studies have shown that Kir2.1 expression in the RPE, and potentially in the apical processes, primarily responds to hypoxia followed by sudden hyperoxia and RPE monolayer disruption.

Kir2.1 dysfunction causes cardiovascular (31), metabolic (32), and retinal diseases (22). Kir2.1 is trafficked to the cell membrane through the endosomal pathways by dynamin and Rac1, which regulates ion flux (33). The Andersen–Tawil syndrome due to nonfunctional Kir2.1 has also been linked to neurologic symptoms and paralysis (34). A single case report showed that a KCNJ5 mutation, which encodes Kir3.4, led to Andersen–Tawil syndrome through the indirect Kir2.1 channel function inhibition (35). Table 1 lists the known genetic mutations in KCNJ2 that lead to cardiovascular and neurologic diseases.

Table 1.

Disease-causing mutations in Kir2.1

Channel	Disease	Mutation	Reference
Kir2.1	Andersen-Tawil syndrome	R40X	(36)
		G52V	(37)
		R67Q	(38)
		R67W	(39)
		Y68D	(34)
		D71N	(40)
		D71V	(41)
		D71Y	(42)
		T74A	(43)
		T75R	(40)
		T75M	(34)
		D78G	(34)
		D78Y	(44)
		R82Q	(34)
		V123G	(34)
		S136F	(41)
		G138K	(37)
		G144S	(41)
		G144D	(45)
		G144A	(43)
		Y145C	(46)
		G146S	(38)
		G146R	(37)
		G146D	(40)
		G146A	(47)
		C154F	(48)
		P186T	(49)
		P186L	(50)
		K187R	(51)
		R189S	(49)
		R189I	(40)
		T192I	(52)
		T192A	(53)
		G215D	(54)
		N216H	(50)
		L217P	(34)
		R218Q	(41)
		R218W	(41)
		L222S	(55)
		R260P	(56)
		L298R	(51)
		G300D	(40)
		G300A	(57)
		G300V	(41)
		V302M	(50)
		E303K	(41)
		T305P	(51)
		M307I	(58)
		M307V	(59)
		T309I	(48)
		R312H	(51)
		R312C	(40)
		N318S	(60)
		W322C	(60)
		S369X	(61)
	Long QT syndrome	T75A	(62)
		G206S	(63)
		P351S	(38)
		T400M	(64)
		N410S	(65)
	Short QT syndrome	D172N	(66)
		E299V	(67)
		M301K	(68)
		K346T	(69)
	Ventricular tachycardia	R82W	(70)
		C101R	(71)
		V227F	(70)
	Arrythmia	T305A	(72)
	Atrial fibrillation	V93I	(73)

Two primary retinal vasculature diseases are caused by Kir2.1 channelopathy, including retinopathy of prematurity (ROP) and diabetic retinopathy. Diabetic retinopathy results in an increased inward current in the retina pericytes (74). The responsible Kir subunits for this current are not yet determined. From 1997 to 2005, the incidence of ROP was 0.17% of the overall live births in the United States, which increased to 15.58% in premature births (75). In addition, 28.5% of diabetic adults in the United States develop diabetic retinopathy with significant vision loss (76). These retinal diseases are mediated by VEGF (77, 78). Anti-VEGF therapies have been routinely used to treat these conditions (79). These anti-VEGF therapies may indirectly reduce the Kir2.1 expression in the RPE. Moreover, increased Kir2.1 expression could be a biomarker for abnormal retinal vasculature.

The Surfactant Positive Airway Pressure and Pulse Oximetry Randomized Trial (SUPPORT) (80) in premature infants administered supplemental oxygen shortly after birth, in whom oxygen saturation was 91%–95% and revealed a significantly increased severe ROP frequency compared with infants whose oxygen saturations remained between 85% and 89%. This finding indicates that oxygen therapy affects pathogenic retinal neovascularization development. This finding is corroborated by a mouse model of oxygen-induced ROP, where mice from postnatal day 0 to eye-opening postnatal day 15 were exposed to 5 days of hyperoxia. Hyperoxia-induced neovascularization was identified as the increased presence of neovascular nuclei in the retinal vasculature (81). This altered neovascularization is an ROP characteristic. Considering VEGF as a critical angiogenic factor within the developing retina (82), we infer that hyperoxia induces VEGF expression and thus Kir2.1 expression in the RPE, resulting in retinal disease. However, we did not find any literature on the Kir2.1 expression in ROP.

Some evidence has suggested that normal Kir2.1 function is required for healthy retinal vascularization. One example is fetal alcohol syndrome, in which gestational alcohol exposure has multiple negative neurologic and cardiovascular effects on infant development. These include retinal dysplasia, abnormal vessel tortuosity, and altered ERG in nonhuman primates and humans (83, 84). Specifically, 49% of fundus images indicated a retinal vascular anomaly (85). In addition, Kir2.1 is inhibited by alcohol, and loss of function mutations mirror all the characteristic features of fetal alcohol syndrome (86). The effects of alcohol on Kir7.1 have not been evaluated; thus, whether inadequate K⁺ inward current in RPE is due to the Kir2.1 or Kir7.1 channels remains unclear. Inhibiting Kir2.1 would prevent the channel from counteracting the VEGF effects.

Missense mutations in Kir2.1 cause long-QT cardiac arrhythmias in cardiomyocytes, as shown in Table 1 (72). As demonstrated using stem cells, long-QT syndromes have also been associated with visual defects, including retinal vascular anomalies (87).

Kir4.1 PHYSIOLOGY AND PATHOPHYSIOLOGY

Glaucoma is associated with the Kir channel expression changes in the inner retina. Specifically, Kir4.1 expression (88) decreases under increasing pressures, whereas Kir2.1 increases (88). Elevated pressure induces retinal ganglion cell death in vitro and in vivo (89, 90). Kir4.1 is a weakly rectifying channel in the retinal Müller glial cells perisynaptic processes, which forms a heterotetramer with Kir5.1. Kir4.1 is also present as a homotetramer at the endfeet facing the subretinal space (6). In addition, the Kir4.1 homotetramer is only 85%–90% blocked by spermines compared with other Kir channels (91). VU0134992 is a Kir4.1-specific inhibitor that interacts with amino acid side chains E158 and I159 in the inner pore of the channel (115).

Previous researchers have observed that the Kir4.1/Kir5.1 heterotetramer increases inward and outward conductance compared with the Kir4.1 homotetramer, yielding a more bidirectional current flux (92). The gating mechanism for Kir4.1/Kir5.1 was recently described, and inward conductance is inversely dependent on exogenous PIP2 concentration and is sensitive to pH (93). The Kir4.1/Kir5.1 heterotetramer in primary renal cells modulates ENaC and regulates electrolyte homeostasis (94). This finding suggests that the Kir4.1/Kir5.1 heterotetramer may play a similar role in electrolyte homeostasis within the retina.

The role of Kir4.1 in maintaining K⁺ homeostasis changes throughout retinal development in mice between postnatal day 1 and eye-opening at postnatal day 15. The Kir4.1 and AQP4 channels regulate the osmolarity of the horizontal cells in the developing neural retina (95). Interestingly, immature horizontal cells are responsible for maintaining K⁺ buffering during retinogenesis before the postnatal differentiation of the Müller glial cells. The Kir4.1 channel and AQP4 had traveled from the horizontal cells and reached the endfeet of the Müller glial cells by day 15 (96). The Kir4.1 channel and AQP4 trafficking to the endfeet in Müller glial cells in mice is mediated by β-1 syntrophin (97) and DP71. Kir4.1 and AQP4 lose their polarization and localize throughout the Müller cell membrane in DP71-knockout mice (98), resulting in retinal edema. The absence of DP71 leads to retinal ischemia, ocular inflammation, retinal detachment, and blood-retina barrier degeneration. Given these, it is hypothesized that DP7.1 mutations would also affect Kir4.1 and AQP4 trafficking. Adeno-associated virus-mediated DP71 gene therapy restores the normal Kir4.1 and AQP4 distribution, the blood-retina barrier integrity, and retinal osmolarity (98).

Kir4.1 antibody inhibition in the rat retina leads to a marked decrease in a posthyperpolarizing negative response in the electroretinogram (ERG) potential, suggesting Kir4.1 responsibility in the Müller glial cells (9). Mice with Kir4.1 mutations lack a slow PIII response by the Müller glial cells from light-evoked ERG, indicating a role for Kir4.1 in the light response. The PIII response is the dominant slowly developing potential as a light stimuli response. It has a direct effect on the ERG c-wave amplitude, which indicates the physiological interdependence between the Müller glial cells and the RPE (99). The inadequate, slow PIII response in these mice suggests that Kir4.1 is the primary K⁺ channel in Müller glial cells (100).

Selective serotonin reuptake inhibitors (SSRIs) inhibit Kir4.1 function in astroglia cells in the brain (101). However, the effects of SSRIs on the retina and retinal vasculature have not been evaluated. We predict that inhibition of Kir4.1 by SSRIs in the retina will result in ROP and diabetic retinopathy phenotype through the alterations in the retinal vasculature. Kir4.1 is a therapeutic target for diabetic retinopathy (102), which is consistent with other studies that defined the role of Kir4.1 in maintaining retinal vasculature. Metformin is a clinical drug used in diabetes therapy that regulates insulin sensitivity (103). Metformin targets Müller glial cells in mice and has been shown to correct altered circadian rhythm, mitigate diabetic retinopathy-associated neovascularization, and restore the normal Kir4.1 function (104).

Mutations in KCNJ10, which encodes Kir4.1, cause epilepsy, ataxia, sensorineural deafness, and tubulopathy (EAST) syndrome (105). Table 2 lists these specific missense mutations. Seizure disorder, developmental delays, and altered renal biochemical homeostasis have been observed in young children with Kir4.1 mutations (105). This is unsurprising because Kir4.1 is highly expressed in the brain (115) and the renal tubule (91). Kir4.1 is believed to coordinate with NaK–ATPase to maintain K⁺ homeostasis in the renal tubule (116), the proposed mechanism for altered urine chemistries in patients with Kir4.1 mutations (105). Visual defects are not considered a hallmark of EAST syndrome, but they occur in some patients. These include nystagmus in a few patients (106) and abnormal ERG (117). Abnormal ERG has been associated with the Müller cell component; thus, mutations may lead to loss of function. These findings indicate that vision assessment is warranted in individuals with EAST syndrome.

Table 2.

Disease-causing mutations in Kir4.1

Channel	Disease	Mutation	Reference
Kir4.1	EAST syndrome	T57I	(106)
		R65P	(107)
		R65C	(108)
		L68P	(109)
		F75C	(110)
		F75L	(108)
		G77R	(107)
		I129V	(109)
		C140R	(111)
		T164I	(111)
		A167V	(111)
		R175Q	(112)
		R199X	(111)
		R297C	(111)
	Hearing loss	P194H	(113)
		R348C	(113)
	Seizures	R204H	(114)

EAST, epilepsy, ataxia, sensorineural deafness, and tubulopathy syndrome.

Kir7.1 PHYSIOLOGY AND PATHOPHYSIOLOGY

NaK–ATPase pumps K⁺ ions into the RPE cells on light exposure, and Kir7.1 releases a small outward K⁺ current back into the subretinal space from the apical processes (118). Kir7.1 has a relatively small inward unitary conductance compared with other Kir channels, with ∼50 fS (119). The small Kir7.1 conductance is consistent with the notion that it primarily mediates small outward currents. The permeability characteristics of Kir7.1 could result from methionine at amino acid position 125, which is unique to Kir7.1 (120). The extracellular selectivity loop and narrow inner pore regulate ion permeability (120). In addition, VU590, a Kir7.1-specific blocker, has been identified to bind in the inner pore region of the Kir7.1 channel (121, 122). Thus, Kir7.1 has unique inner pore properties compared with other Kir channels, which warrants further investigation.

RPE K⁺ released into the subretinal space is essential in buffering K⁺ concentration changes produced by light-evoked photoreceptor activity (123). The PIP2 hydrolysis inhibits Kir7.1, which might be regulated by dark-light transitions (124). Using a patient iPSC-RPE model, the loss of Kir7.1 function altered phagocytosis of photoreceptor outer segments (125). Inward K⁺ conductance does not appear in the absence of Kir7.1 in the apical processes, and ERG c-wave amplitude is not maintained in mice with suppressed Kcnj13 expression in RPE (7).

Kir7.1 is regulated by GPCRs both directly by N-linked glycosylation (126) and indirectly by the RPE oxytocin receptor (127, 128) and progesterone brain receptor (129). Nonsense and missense mutations within the N-terminal cytoplasmic domain, selectivity loop, TM2 domain, and C-terminal cytoplasmic domain lead to early-onset blindness development, including snowflake vitreoretinal degeneration (SVD) and LCA16 (130–132). These effects are due to RPE, photoreceptor degeneration, retinal detachment, and nystagmus (130).

Table 3 lists the known KCNJ13 mutations that lead to blindness. Our laboratory has studied an autosomal recessive nonsense mutation at tryptophan 53, which yields a premature stop codon in the N-terminal cytoplasmic domain. This truncated protein does not reach the cell membrane and lacks K⁺ inward current (132). In addition, the W53X mutation yields a reduced ERG c-wave in mouse models compared with wild-type mice (7) but could be restored with gene therapy (138). Another study of a human iPSC cell disease-in-a-dish model revealed that read-through drug NB84 and gene augmentation via lentivirus fixed channel trafficking to the cell membrane restored K⁺ inward currents and demonstrated Rb⁺ enhanced current (125).

Table 3.

Disease-causing mutations in Kir7.1

Channel	Disease	Mutation	Paper
Kir7.1	LCA16	W53X	(132)
		S105I	(133)
		T153I	(134)
		Q117R	(135)
		R162Q	(135)
		R166X	(135)
		Q219X	(133)
		L241P	(135)
		E276A	(135)
	SVD	I120T	(136)
		R162W	(137)

Moreover, researchers currently evaluate the therapeutic potential of gene editing, base editing, and engineered tRNA for KCNJ13 point mutations (139). The glutamine to arginine mutation in the selectivity loop has been identified at amino acid position 117 (Q117R) and yields a nonfunctional channel that develops LCA16 (135). A missense mutation alters threonine to isoleucine at amino acid position 153 (T153I) (134). The T153I mutation within the inner pore region yields a dysfunctional channel that reaches the cell membrane. The T153I mutation highlights the importance of the narrow inner pore in channel conductance (140). Another allelic mutation in which arginine is changed to tryptophan at amino acid position 162 (R162W) leads to SVD because the channel is nonfunctional and does not reach the cell membrane (131). Notably, the R162Q mutation is not located in the signal sequence, amino acids 323–360 (141); thus, its mistrafficking likely comes from another mechanism, such as the endosomal pathways. An additional C-terminal cytoplasmic domain mutation included an arginine converted to a premature stop codon (R166X), which yields a truncated protein and leads to LCA16 development. Evidence suggests that mice’s proteosome degrades the R166X mutated channel (142). Together, the harmful effects of these point mutations on vision, including the RPE degeneration and loss of photoreceptors, highlight the importance of the Kir7.1 channel in retinal physiology.

CONCLUSIONS AND FUTURE IMPACT

Kir channels play a critical role in maintaining extracellular K⁺ concentrations within the retina and RPE in both rodents and humans. Gene expression studies have identified all subfamilies of Kir channels in the RPE and retina. This review mainly focused on functional Kir channels, including Kir2.1, Kir4.1, and Kir7.1. Recently, a role for Kir2.1 has been suggested in both normal and diseased RPE in retinal vasculature development and is closely associated with VEGF release from the RPE and retinal vasculature. This review evaluated how this novel finding alters the Kir channel function model within the RPE and retina in health and disease. We can identify several future research directions that would lead to our understanding of the role of Kir channel biology in the retina. We expect that a combination of blockers and in vivo and ex vivo electrophysiological measurements will prove channel subtype-specific role that contributes to retina physiology with the Kir channel subtype-specific blocker advancement. Future therapeutics development will rely on Kir channel tetramers’ specific structure (143). Another critical area would be the interactions between Kir channels and molecules that make up complex plasma membranes, such as cholesterol or other lipids/proteins. Therefore, studies should determine if the Kir channel function in the retina is similar to their role in other organs.

Defining the Kir channel expression levels in the retina cells will be critical in using cell transplantation as a treatment for several degenerative blindness. Hence, augmented expression of specific Kir channels in the heterologous cells would have a functional advantage therapeutically. The study of Kir channel biology will also increase our understanding of the correlation between gene expression and protein levels. As we have understood from this review, going by the several Kir channel subunits expression in the retina, it will be an overinterpretation that protein expression is proportional. Future multiomics or functional “omics” approaches might uncover mechanisms such as microRNA inhibition of mRNA that does not yield functional protein under physiological conditions but could have a role in disease pathophysiology.

Kir biology in the retina is an emerging field that benefited from gene mutation discovery that leads to blindness. Future work should focus on identifying mutations at intronic sites and their effects on gene expression/function. We reviewed the disease-causing mutations in Kir2.1, Kir4.1, and Kir7.1, as well as any associated visual defects. Kir2.1, Kir4.1, and Kir7.1 mainly play a role in maintaining subretinal K⁺ concentration and the RPE monolayer integrity. Specifically, Kir4.1 mutations cause neurologic diseases with and without visual defects. Neurologic disorders are generally associated with retinal defects, and our review indicates that the vision of individuals with EAST syndrome should be evaluated. Some of these mutations have been directly linked to visual defects, but others have not yet been considered. This review is our vision that identifies areas for further research to understand the biology of Kir channels and the roles of these channels in the RPE and retina.

GRANTS

This work was supported by Endocrinology and Reproductive Physiology NICHDT32 (to K.M.B.). The work was also supported by NIH R01 EY024995, NIH R24 EY032434, and Retina Research Foundation M.D. Matthews Research Professorship to B.R.P.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

K.M.B. and B.R.P. conceived and designed research; K.M.B. and B.R.P. performed experiments; K.M.B. and B.R.P. analyzed data; K.M.B. and B.R.P. interpreted results of experiments; K.M.B. and B.R.P. prepared figures; K.M.B. and B.R.P. drafted manuscript; K.M.B. and B.R.P. edited and revised manuscript; K.M.B. and B.R.P. approved final version of manuscript.