Retinal Mechanical Sensation

Abstract

Retinal neurons process light signals and respond to mechanical signals. mechanosensitive channels (MSCs) have been revealed in all retinal layers in humans, monkeys, mice, rats, porcine, salamanders, goldfish, etc. Some MSCs open in physiological conditions to regulate membrane potential, light responses, and neurotransmitter release, and some MSCs can mediate neurodegenerative effects. Alterations in the intraocular and external pressure critically involve the pathogenesis of glaucoma, traumatic retinal injury (TRI), and other retinal disorders. Our team revealed several MSCs in the outer and inner retinal neurons and first reported the pressure-evoked current and voltage response in salamander photoreceptors and primate bipolar cells. It is still unclear how retinal light pathways deal with endogenous and exogenous mechanical stimulation, and the physiological and pathological significance for retinal neurons to express multiple types of MSCs is not fully understood. This chapter will focus on the variety and functions of MSCs permeable to K+, Na+, and Ca2+, primarily including the big potassium channel (BK), two-pore domain potassium channel TRAAK and TREK, Piezo, epithelial sodium channel (ENaC), transient receptor potential channel vanilloid (TRPV) TRPV1, TRPV2, TRPV4, etc., in retinal photoreceptors, bipolar cells, horizontal cells, amacrine cells, and ganglion cells.

1. Introduction

Pressure stresses are associated with multiple retinal disorders [1], such as acute and chronic glaucoma, traumatic retinal injury (TRI), and retinal dysfunctions that occur during air travel, diving, and mountain hiking. Glaucoma is a blinding disease characterized by the elevation of the intraocular pressure (IOP) and histological retinal damage primarily observed in the optic disk and retinal ganglion cells (RGCs). The worldwide prevalence of glaucoma in the population above 40 years is ∼3·5% [2, 3]. Glaucoma patients typically develop tunnel vision, a severe loss of the peripheral and paracentral visual fields. The higher vulnerability of RGCs and the peripheral vision to mechanical stresses are generally similar between glaucoma and the pressurized air wave-caused TRI [3, 4, 5, 6, 7].

In mammals like humans [8, 9, 10] and mice [11, 12], cone photoreceptors have a lower light sensitivity and a distribution peak at the central retina/fovea, which provides the best visual resolution for the central retina during daytime (except for S-cone that has a gradually higher density in the ventral retina [13, 14, 15]). Rods, on the other hand, dominantly occupy vast regions of the peripheral and paracentral retina [16], offering higher light sensitivity. Rod signals go through three pathways to reach RGCs, (1) the primary rod pathway formed by Rods➔Rod bipolar cells (RBCs)➔AII amacrine cells (AII ACs)---Cone-driven depolarizing BCs (cDBCs)➔RGCs, (2) the secondary rod pathway formed by Rods---Cones ➔ cBCs➔RGCs, and (3) the tertiary rod pathway composed by Rods➔ Hyperpolarizing BCs (HBCs)➔RGCs, where “➔” indicates glutamatergic synapses and “---” indicates electric synapses. The central vision is dominated by cones, and cone signals may reach RGCs via Cones➔cDBCs➔ON RGCs and ON-OFF RGCs and Cones➔cHBCs➔OFF RGCs and ON-OFF RGCs. It is uncertain whether mechanical signals can differentially affect retinal neurons in the rod and cone pathways.

Do retinal neurons express mechanosensitive ion channels (MSCs)? The answer is yes. All eukaryotes express MSCs to cope with physical stimuli [17]. In multiple mammalian species, MSCs have been observed in neurons and processes in the outer segment layer (OSL), inner segment layer (ISL), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL, including somas of BCs and ACs), inner plexiform layer (IPL), ganglion cell layer (GCL, including somas of RGCs and displaced ACs), and nerve fiber layer (NFL), such as the big potassium channel (BK), two-pore domain potassium channel TRAAK and TREK, Piezo, epithelial sodium channel (ENaC), and transient receptor potential channel vanilloid TRPV1, TRPV2, and TRPV4. These MSCs have been confirmed as mechano-gated channels in sensory neurons, Xenopus oocytes, or cultured cell lines [18, 19], as they open directly by membrane tension and transduce the mechanical signals into electrical currents. Retinal neurons show variable levels and distribution patterns of MSCs, implying cell-type specific mechanical-dealing strategies and pressure effects.

MSCs may be activated by membrane stretch, mechanical and osmotic pressures, changes in osmolarity and temperature, membrane stretches, pH, and other modulators. The physical factors are often present in the ocular environment in physiological conditions, and some channels are known to be constitutionally open (see below). The retina lines the inner surface of the posterior eyeball, and a positive IOP is homogeneously maintained inside the eyeball by the volume of the aqueous humor against the limited space formed by the relatively rigid eyeball shell. Although the circulation of the aqueous humor critically regulates the IOP level, IOP still fluctuates. The eyeball shell is primarily formed by the sclera and cornea, and with a bulk modulus of ∼2.2 × 10⁹ Pa, water is incompressible under normal conditions. Thus, any increase in aqueous humor volume would expand the eyeball space to stretch the retina. The elevated IOP in glaucoma can significantly enlarge eyeballs in children, known as buphthalmos, and it also focally expands the retina at the cupping region of the optic disk in chronic glaucoma patients. Besides, aging may directly induce structural remodeling and retinal deformation [12]. Therefore, MSCs in retinal neurons may be activated in physiological and pathological conditions. This chapter will briefly review the literature on several representative MSCs in retinal photoreceptors, BCs, horizontal cells (HCs), ACs, and RGCs for a better understanding of the significance of MSCs in retinal light pathways.

2. Ocular mechanical hemostasis

Stress and strain, like IOP and retinal stretch, are a pair of inseparable physical parameters that can activate MSCs. The eyeball structure resembles a hollow spherical shell with an enclosed space and a positive inner pressure. Normal IOP in human eyeballs is 10–21 mmHg (1.3–2.8 kPa) higher than the atmospheric pressure, and the pressure source is aqueous humor. Aqueous humor flows from the ciliary body to the posterior and anterior chambers and drains into the canal of Schlemm and uveoscleral vein. Aqueous humor contains 99.9% of water [20] and molecules like amino acids, proteins, and others like those found in blood plasma [21, 22]. Its formation involves three mechanisms [21, 23, 24]: (a) the ultrafiltration, that is, water and water-soluble substances pass the fenestrated ciliary capillary endothelium and enter into the ciliary interstitium along the hydrostatic and osmotic pressure gradients, (b) diffusion of water- and lipid-soluble substances from ciliary processes to the posterior chamber, and (c) active transportation/secretion of anions, cations, and other molecules by the non-pigmented epithelial cells at ciliary processes. A majority (80–90%) of aqueous humor is produced by active transportation and is not pressure-dependent.

Glaucoma is generally classified as high-tension glaucoma and normal-tension glaucoma, and both are diagnosable by similar pathological changes at the optic disc and the loss of visual field. High-tension glaucoma is characterized by the accumulation of aqueous humor due to the blockage of the iris or trabecular meshwork on the drainage of the aqueous humor at the area of the anterior chamber angle, including angle closure glaucoma and open-angle glaucoma.

IOP level is primarily determined by the volume of aqueous humor and has been expressed by $\mathit{Pi}\left(\mathit{IOP}\right)=\mathit{Pe}+\left(\mathit{Fin}-\mathit{Fu}\right)/\mathit{\text{Ctrab}}$ [23, 24], where P_e is the episcleral venous pressure, F_in is the inflow/formation rate, F_u is the outflow rate via uveoscleral pathway, and C_trab is the facility of the outflow through the trabecular pathway. The equation, however, does not include the ocular elasticity and volumes as variables affecting IOP.

The flow rate of the aqueous humor and IOP both fluctuate. The circulation of the aqueous humor acts to stabilize IOP, but IOP still changes with the heartbeat, breath, and exercise in a range of 2–10 mmHg (reviewed by Ref. [1]). Young’s modulus (E, tensile elasticity) was measured as 1.5 × 10⁵ to 8.3 × 10⁵ Pa for the sclera and 2.5 × 10⁴ to 2.4 × 10⁵ Pa in the cornea, and water has a bulk modulus (K) of 2.2 × 10⁹ Pa and is incompressible under normal conditions. Recently, we reported the first mathematical equation with the modified bulk (K) modulus and Young’s modulus (E) for the relationship between ocular elasticity, aqueous humor volume, eyeball volume, the thickness of the eyeball shell, and IOP [25]. The equation (eq) is presented in the following six forms:

$$\mathit{\text{ΔIOP}}=K_s\ast \frac{∆V_W}{V_s}$$E1

$$\mathit{\text{ΔIOP}}=1.33\ast \left(\frac{H}{R}\right)\ast E_s\ast \left(\frac{∆V_W}{V_s}\right)$$E2

$$V_s=K_s\ast \frac{∆V_W}{\mathit{\text{ΔIOP}}}$$E3

$$V_s=1.33\ast \left(\frac{H}{R}\right)\ast E_s\ast \left(\frac{∆V_W}{\mathit{\text{ΔIOP}}}\right)$$E4

$$\frac{∆V_W}{V_s}=\frac{\mathit{\text{ΔIOP}}}{K_s}$$E5

$$\frac{∆V_W}{V_s}=\frac{\mathit{\text{ΔIOP}}}{1.33\ast E_s}\ast \left(\frac{R}{H}\right)$$E6

where V_S is the eyeball volume, V_W is the volume of aqueous humor, IOP is the intraocular pressure, Δ indicates changes in the parameters, K_S is the bulk modulus of the eyeball, E_S is Young’s modulus of the eyeball shell, H is the thickness of the eyeball shell, and R is the inner radius of the eyeball. Assuming the aqueous humor is fully regulated, Eqs. (1) and (2) state that IOP alteration is positively correlated with the elasticity and the volume fluctuation of the aqueous humor relative to the eyeball volume. Eqs. (3) and (4) state that the eyeball volume is positively correlated with the elasticity and the volume fluctuation of the aqueous humor relative to the IOP change. A larger ΔV_W/ΔIOP (severe accumulation of aqueous humor with less IOP elevation) predicts more intensive eyeball and retinal expansion. Eqs. (5) and (6) state that the eyeball expansion rate is negatively correlated with the eyeball elasticity and positively correlated with the IOP elevation.

3. Physiological properties of MSCs and MSCs expressed in photoreceptors

3.1 MSCs permeable to K⁺

BK channel is also known as the calcium- and voltage-gated large conductance potassium channel, Maxi-K, KCNMA1, Slo1, Kca1.1, and stretch-activated potassium channel. It is a homotetrameric channel of four identical, pore-forming, and Ca²⁺ and voltage-sensing units with or without associating with the regulatory β and γ subunits. The single-channel conductance is between 200 and 300 pS (reviewed by Ref. [26]).

BKs were reported in rods in goldfish [27] and salamander retinas [28, 29, 30]. They were expressed in the axon terminal [27], soma, and inner segment (IS) [28, 31] of rods and associated with membrane hyperpolarization and glutamate release [29]. BK at the presynaptic terminals of goldfish bipolar cells [27] mediates calcium-activated potassium current (I(KCa)). BK also influences calcium-mediated glutamate release in salamander rods [29] under normal conditions and provides an effective “clamp” of the dark membrane potential [31] to its normal resting level. The I(KCa) was largely inhibited by 100 nM iberiotoxin, a BK blocker, and sensitive to tetraethylammonium (TEA) [29]. BK blocker iberiotoxin increases the light response of some rods [30]. BK may be activated by flupirtine [32]. Cones from the rabbit and salamander retina also express BK [30, 33]. Studies from other tissues have revealed the mechanical sensitivity of BKs [34, 35, 36].

Mechanosensitive P2K channels (TRAAK and TREK) [37, 38, 39, 40, 41, 42] are gated primarily by membrane tension, and their opening is facilitated by convex membrane deformation [43, 44, 45]. Activation of the channel by mechanical stimulus is graded and reversible. The negative pressure that can activate half-maximally TREK-1 and TRAAK is −36 and −46 mmHg, respectively [46]. The channel activities are also affected by other factors, such as pH, temperature, membrane potential, arachidonic acid, and G protein-mediated regulation [43, 44, 47, 48, 49]. TRAAK and TREK give rise to leak (also called background) K⁺ currents. A well-known role of background K⁺ currents is to stabilize the negative resting membrane potential and counterbalance depolarization. TRAAK was recently reported to open upon ultrasound, and the single-channel conductance was ∼73pS [50]. Without the stimulation, TRAAK had an average open probability of 1.9%, while ultrasound and pressure increased channel open probability to 6.3 and 26%, respectively.

TRAAK has been reported in the mouse [47, 51] and salamander photoreceptors [30]. Mouse photoreceptors also express TREK1 [52]. Our team applied pressure on the rod IS and evoked sustained currents of three components in rods [30]. The pressure-induced outward current at membrane potentials ≥ −80 mV (termed I_po) was sensitive to the blockage of intracellular Cs⁺, in line with the expression of MSCs permeable to K⁺. Using tetraethylammonium (TEA) to block most potassium channels largely depolarized salamander rods (33 mV, 82.5%) and reduced the light response by 24% [30].

Fink and colleagues cloned and expressed TRAAK in Xenopus oocytes and COS cells (fibroblast-like cell lines derived from monkey kidney tissue) and first reported the instantaneous and non-inactivating currents of TRAAK. The currents were not gated by voltage, only partially inhibited by Ba²⁺ at high concentrations, and were insensitive to the other classical K⁺ channel blockers TEA, 4-aminopyridine, and Cs⁺. TRAAK could be stimulated by the neuroprotective drug riluzole [47, 51], aprepitant [53], arachidonic acid (AA), and other unsaturated fatty acids but not by saturated fatty acids [47]. Wang and colleagues [51] explored the expression of TRAAK and the apoptosis of the ONL with immunostaining, Western blotting, and real-time polymerase chain reaction (RT-PCR). The channel agonist riluzole activated TRAAK and delayed the apoptosis of photoreceptors in the ONL of rd1 mice of retinal degeneration (Pde6brd1). They pointed out that activation of TRAAK in rd1 mice protects photoreceptors from apoptosis.

TRAAK, TREK1, and BK are permeable to K⁺, and a typical reversal potential (Erev) for K currents is around −75 to −90 mV [50, 54]. TREK is sensitive to the blockage of Cs⁺ [55], and TREK1 is inhibited by fluoxetine, norfluoxetine, and L-methionine [48, 56]. TREK2 is a 78% homology of TREK1. TREK2 produces rapidly activating and non-inactivating outward rectifier K⁺ currents. The single-channel conductance is 100 pS. The currents can be strongly stimulated by lysophosphatidylcholine, riluzole, polyunsaturated fatty acids such as arachidonic, docosahexaenoic, and linoleic acids, as well as volatile general anesthetics like chloroform, halothane, and isoflurane [41].

3.2 TRPs permeable to Na⁺ and Ca²⁺

TRPs are involved in various medical conditions (reviewed by Refs. [57, 58]) related to the bone [59], lung [60], kidney [61, 62, 63], digestive system [64, 65], pain [66, 67, 68, 69, 70], tooth, scapuloperoneal spinal muscular atrophy, hereditary motor and sensory neuropathy type IIC (HMSN IIC, also known as Charcot-Marie-Tooth disease type 2C (CMT2C)) [71, 72], Bardet-Biedl syndrome [73], hyponatremia [74], etc. TRPs include seven subfamilies, namely TRPC (canonical), TRPV, TRPM (melastatin), TRPN (NOMPC), TRPA (ANKTM1), TRPP (polycystin), and TRPML (mucolipin) [75, 76]. TRPs share the common feature of six transmembrane domains and various degrees of sequence similarity and permeability to cations.

The retina expresses multiple types of TRPs, but not all of them are known as mechano-gated channels. TRPs are variably modulated by membrane tension, osmolality, temperature, phorbol esters, and G-protein-mediated regulation. TRPA1, TRPC1/3/5/6; TRPM3/4/7, TRPP1/2, and TRPV2/4 are activated by stretch, osmolality, and/or pressure and identified as mechano-gated channels (reviewed by Refs. [77, 78, 79, 80]).

TRPVs include TRPV1-6. Their permeability to calcium (PCa) is higher than that to sodium (PNa). PCa/PNa is 9.6 (vanilloids) /3.8 (heat) for TRPV1, 3 for TRPV2, 2.8 for TRPV3, 6 for TRPV4, and > 100 for TRPV5 and 6 [77]. TRPV2 is activated by membrane tension, heat ≥52°C, cannabidiols, and 2-aminoethoxydiphenyl borate (2-APB) [81, 82, 83, 84, 85, 86], and piperlongumine is recently identified as a selective, reversible, and allosteric antagonist of human TRPV2 (hTRPV2) [87]. TRPV4 opens by mechanical pressure [88], membrane stretch [89], osmotic pressure [61, 90, 91, 92], warm temperature (27–34°C) [77, 93, 94, 95, 96], specific pharmacological agonists like GSK1016790A (GSK101, 1-2uM) and 4α-phorbol 12,13-didecanoate (4aPDD, 1-2uM), and other molecules [97], and it is inhibited by RN-1734 (∼20uM) [76, 98, 99, 100]. TRPV1 involves pressure-induced retinal pathology [101] and hypertonicity sensing in the primary osmosensory neurons in the mouse organum vasculosum lamina terminalis [102]; however, TRPV1 unlike TRPV2 is restricted to a subset of mechanically insensitive cutaneous nociceptors responding to heat [103]. TRPV1 is activated by noxious heat of 40–50°C [81, 104, 105], capsaicin, and resiniferatoxin [106, 107, 108], and it is selectively inhibited by capsazepine and 5′-iodoresiniferatoxin [107] and variably blocked by divalent heavy metals [109].

TRPs have been found in all retinal layers [110]. Photoreceptors, ONL, and OPL express TRPV1, TRPV2, TRPV4, TRPM7, TRPP2, TRPC1, and TRPML1 in the vertebrate and mammalian retina [30, 110, 111, 112]. TRPV1/VR1 immunoreactivity in goldfish and zebrafish retinas was present in photoreceptor synaptic ribbons [113]. TRPV2 was present in axons of photoreceptors and outer plexiform layer (OPL) in the rat, cat, primate [111], mouse [110], and salamander [30] retinas, and TRPV4 was observed in the OPL in the mouse [101, 114, 115] and primate retinas [54], and axons of salamander photoreceptors [30].

There are still disputes on whether the outer retinal neurons express TRPV4. In TRPV4 knockout mice, Yarishkin and colleagues [116] did not find significant changes in ERG a-wave and b-wave evoked by whole-field lights (presumably white) of 0.00025–79 cd.s/m², and they concluded that TRPV4 did not regulate the distal retinal light responses. Several studies reported TRPV4 in retinal Müller cells, microglia cells, or astrocytes to critically involve volume regulation and swollen-related pathologies [117, 118, 119], indicating the importance of TRPV4 in glial cells. On the other hand, TRPV4 signals show a horizontal distribution pattern in the OPL in the mice, porcine, primate, and salamander retinas [30, 54, 110, 115], which are not well consistent with the vertical orientation of Müller cells and dispersed distribution of microglial cells. In mice of acute retinal detachment, the number of apoptotic photoreceptors was reduced by approximately 50% in TRPV4 knockout mice relative to wild-type mice [120].

TRPVs carry cation currents that reverse at ∼ −10 to 20 mV [30, 54, 85, 121], and the channel opening can cause membrane depolarization, regulate neuronal excitability, and mediate excitotoxicity [122, 123, 124]. Our recent data revealed the expression of TRPV4 and TRPV2 in the axon terminal of rods and cones and pressure-evoked currents and potentials in salamander photoreceptors [30]. We applied positive pressures onto the rod IS and elicited three components of currents in salamander rods, and one of them was a cation current that reversed at ∼ − 10 mV (termed I_pc). Meanwhile, hypotonicity induced a slow cation current of a similar reversal potential [30]. The data aligns with the immunocytochemical data and is consistent with the expression of TRPVs in photoreceptors. Also, pressure applied to the outer segment (OS) of rods and cones closed a Ca²⁺-dependent cation conductance reversed at ∼0 mV, in line with the closing of TRPV2 in the OS [30]. The wild-type rat TRPV2 was reported to be constitutively fully open [125], and the spatial structure of the agonist-free full-length TRPV2 molecular [126] showed larger upper and lower gates than the agonist-opened TRPV1. The pressure response in photoreceptors saturates at 25.9 mmHg. The agonists and antagonists of TRPV2 and TRPV4 variably affect the light response of salamander rods [30]. The light-evoked potentials recorded at various light intensities were larger at 23°C than at 31°C and severely disrupted at 43°C. The results indicate that mechanical stimuli may affect light signals in outer retinal neurons via TRPVs.

Besides, retinal pigment epithelium (RPE) is vital for the integrity of photoreceptors through its phagocytic function, and the RPE of the porcine, mouse, and human expresses TRPV2 [127, 128, 129]. The expression is enhanced by cannabidiol and inhibited by SKF96365 [128], a nonselective blocker of TRPs and inhibitor of the store-operated Ca²⁺ entry (SOCE). TRPV2 in RPE mediates both the heat-dependent and insulin-like growth factor-1-induced secretion of vascular endothelial growth factor (VEGF) [129]. RPE also expresses TRPV1-4, TRPMs, and TRPCs in the human retina [129, 130], TRPM3 in the mouse retina [110], and ENaCα in the rat and human retinas [131]. In freshly isolated RPE cells and ARPE-19 cells, Cordeiro and colleagues found that raising the temperature increased the secretion rate of VEGF-A. This effect was sensitive to ruthenium red (20uM). Heat also induced a Ca²⁺ signal, which was diminished by the TRPV blockers La³⁺ and ruthenium red and enhanced by 2-APB (3 mM) but insensitive to the TRPV1 agonist capsaicin and TRPV3 activator camphor.

3.3 ENaC and piezo permeable to Na⁺ or Na⁺/Ca²⁺

ENaC is a heteromultimeric channel usually composed of three homologous subunits (α, β, and γ) with a 30–40% identity at the level of their amino acid sequence [132, 133]. The immunoreactivity and mRNA of the pore-forming α-subunit have been observed in rods and cones of rat and human retinas [131, 134]. The specific blocker amiloride [135] enhanced ERG a-wave in the rat and rabbit retinas. Unlike other family members, ENaC is constitutively active [133, 136], which, in line with the observation in photoreceptors [135], suggests that ENaCα may regulate visual signals in photoreceptors in physiological conditions.

ENaC belongs to the ENaC/degenerin (ENaC/DEG) family and involves the functions of sensing and responding to mechanical and chemical stimuli [133, 137, 138]. It is regulated by the intracellular Na⁺ and H⁺, peptidases, and other small molecules [139, 140, 141, 142, 143, 144]. It is highly Na⁺-selective and amiloride-sensitive (EC₅₀ 150 nM). The permeability (P) ratio of PNa⁺: PK⁺ = 100: 1. ENaC/DEG is characterized by a relatively long extracellular loop bounded by two transmembrane pore-forming helices (TM1 and TM2). The single-channel conduction is 7.4pS in low-Na⁺ solution and 7.5pS in high-Na⁺ solution (reviewed by Refs. [133, 144]). AP 301 is an agonist of ENaCα [136, 145, 146, 147] that has been trialed for treating pulmonary permeability edema [148]. These results indicate that MSCs may open in physiological conditions to affect visual signals.

About one decade ago, Coste and colleagues [149, 150] revealed a novel family of mechanically activated cation channels in eukaryotes, consisting of Piezo1 and Piezo2 channels [149, 150, 151, 152]. Piezo senses touch and stretch in cells of fruit flies and mammals [79, 152, 153, 154, 155]. Piezo is the nonselective cationic channel that is permeable to alkali ions (K⁺, Na⁺, and Cs⁺), divalent cations (Ba²⁺, Ca²⁺, Mg²⁺, and Mn²⁺), and several organic cations (tetramethyl ammonium (TMA), TEA, etc.). The chord conductance of human Piezo 1 [156] at the membrane potential of -100 mV is 35-55pS for Cs⁺, Na⁺, and K⁺ and ∼ 23pS for Li⁺, and the conductance for divalent at 90 mM extracellular concentration is 25pS for Ba²⁺ and 15pS for Ca²⁺ at -80 mV and 10pS for Mg²⁺ at -50 mV measured at the cell-attached mode. Like TRPVs, Piezo carries cation currents, which reverse around −10 to 20 mV [18, 157]. The channel opening is anticipated to depolarize the membrane in physiological conditions and involve excitotoxicity in pathological conditions [158].

Piezo1 mRNA was detected in the mouse ONL [159]. Immunocytochemical and electrophysiological data from Bocchero and colleagues identified Piezo 1 and TRPC1 in rods of Xenopus retinas [160]. Mechanical stimulation in the order of 10pN applied to OS or IS evoked a “calcium transient,” and the channel blocker reduced the duration of the photo response to bright flashes. More interestingly, bright flashes of light caused a rapid shorting of OSs. The authors proposed that MSCs, including TRPC1 and Piezo, play an integral role in rod phototransduction in the vertebrate retina.

4. The expression and activity of MSCs in HCs

The OPL consists of the horizontally ramified axon terminals of photoreceptors, dendrites of BCs, and processes of HCs, and the IPL is composed of the axon terminals of BCs, processes of ACs, and dendrites of RGCs. The somas of BCs, ACs, and Müller cells reside in INL, BCs in the upper half of the INL, ACs in the lower half, and Müller cells in the middle [161]. Some earlier works revealed moderate loss of neurons in INL in glaucoma and pressure-caused retinal pathology in the outer retina. It has been unclear how MSCs are expressed in retinal interneuron BCs, HCs, and ACs.

In dissociated HCs from the retinas of the rat and mouse [162], Sun and colleagues first identified BK-mediated outward current in HCs. The single-channel conductance measured in symmetrical 150 mM K⁺ in mouse HCs was ∼250 pS (202–279 pS). BK-mediated membrane current was identified by the blockage of BK antagonists iberiotoxin (100 nM) and paxillin (2.5 mM), as well as Ca²⁺-free solutions, divalent cation, and voltage-gated calcium channel blockers. The potassium current was outwardly rectified and reversed around -75 mV. Blocking BK with paxillin depolarized the membrane and produced oscillation of increasing frequency, and the synthetic BK agonist NS1619 inhibited these oscillations. The authors concluded that the activation of BKa channels put a ceiling on membrane depolarization and regulated the temporal responsivity of HCs. The finding is consistent with the notion that BK hyperpolarizes HCs, counterbalancing the membrane depolarization and excitatory visual noises. In salamander retinas [30], the TRPV2 antibody brightly labeled the OPL, and half of the calretinin/GABA (gamma-aminobutyric acid)-positive processes of horizontal cells (HCs) were labeled for TRPV2. Pressure applied on the IS of salamander rods could evoke a Co²⁺-sensitive inward current component in rods at membrane potentials <−50 mV (termed I_pi). The data indicates the presence of TRPs in a presynaptic site, likely the processes of HCs.

Besides, D2 mice with congenital glaucoma showed thinning of OPL and loss of processes of BCs and HCs [163, 164].

5. The expression and activity of MSCs in BCs

BK was first observed in isolated ON BCs from the goldfish retina [27, 165] to mediate resonance (60–70 Hz or 5–10 Hz) K⁺ current sensitive to 100 nM charybdotoxin [165]. The potassium current was coupled with L-type voltage-gated calcium channels (L-CaV). BCs express TRAAK [47] and BK [166] in the mouse and rat retinas, TRPV4 in primates [54], and ENaC in rats [134]. Mammalian rod BCs (RBCs) express TRPV4 [54] and BKs [33, 166]. Nagai and colleagues used situ hybridization and immunohistochemistry and observed BK in rabbit RBCs and cone ON BCs [33]. In BKa mutant mice, ERG b-wave at the mesopic range (high scotopic range) is reduced, while others were not affected, indicating that BK involves the function of ON BCs [166, 167].

Dendrites of BCs ramify in the OPL, and OPL expresses BK [27, 29, 31], TRAAK [47], TREK1 [52], ENaC [168], TRPC1/4 [169], TRPV4 [114, 115], TRPV1 [113], and TRPV2 [30, 110, 111]. In the mouse retina, in situ hybridization and immunofluorescence revealed widespread expression throughout multiple retinal layers for TRPC1, TRPC3, TRPC4, TRPML1, and TRPP2 [110]. The somas of BCs reside in the INL, where TREK1, TREK2 [52], TRAAK [47, 52], ENaC [170], TRPV1 [106], and TRPV2 [111] have also been observed in mammals. The IPL was positive for TRAAK [30, 171], TRPV2 [111], TRPV4 [54, 114, 115], TRPC1/4 [169], and ENaCa [131, 168] in mammals and vertebrate retinas.

The dendrites and somas of BCs in the primate retina are positive for TRPV4 [54]. Pressure evoked transient cation currents in primate BCs, which reversed at ∼ −10 mV and were enhanced upon heating from 24° to 34° in line with TRPV4. The pressure for the half-maximal effect in the primate retina was ∼20 mmHg [54], comparable to the borderline of harmful IOP level in glaucoma. The data indicate that BCs may respond to the fluctuation of IOP levels in physiological and pathological conditions. Besides, TRPV1 was observed in the excitatory synapses from BCs in the dendrites of RGCs [172].

The mRNA and immunoreactivity of ENaCα have been observed in BCs in human and rat retinas [131, 134]. The function of BCs is accessible in vivo with ERG. In the presence of ENaC blocker amiloride, Brockway and colleagues [135] found that ERG a-, b- and d-waves were all enhanced in the rat retina, while the slow PIII-Müller cell response [135] was reduced. The data indicate that higher activities of ENaC can reduce the function of ON BCs in normal conditions. Besides, Piezo1 mRNA was detected in the mouse INL [159].

D2 mice of congenital glaucoma have shown thinning of OPL, loss of processes of BCs and HCs [163, 164], and reduction of ERG a- and b-waves. High IOP can result in the disturbance to the IPL [173] and reduction of BC synapses in RGC dendrites before RGC death [174, 175, 176] and RBC dendrites [173]. Pressurized airwaves can reduce ERG b-wave at post-blast 3–7 days [177, 178] and damage dendrites of rod BCs (RBCs) [179]. Interestingly, in studies of TRI, the data on ERG b-wave observed within days [177, 178] are inconsistent with those obtained in months [180, 181], resembling glaucoma [182, 183, 184]. ERG b-wave measures a field potential from DBC populations, which hardly measure activities of HBCs due to the very short duration of flash stimuli (∼5 ms for animals and 0.1–0.2 sec for patients [185, 186]). ERG a-wave [184, 187, 188] may reduce in glaucoma, and b-wave may reduce [163, 188, 189] or briefly increase in earlier stages of IOP elevation [184]. Besides potential experimental errors, the discrepancy in b-wave could indicate two progressive pathophysiological stages; the early one likely involves more closely to the responses of MSCs in BCs to initial IOP elevation, and the later one may be more attributable to damage to the structural integrity of BCs.

In a mouse model, the expression of PKCα was decreased progressively in cell bodies and dendrites of RBCs by the acute ocular hypertension (IOP ∼100 mmHg) induced by injection of physiological saline [190]. In the retinas injured by the intravitreal injection of 10 mM NMDA (N-methyl-D-aspartate), a patch-clamping study showed that RBCs were more vulnerable to excitotoxicity than cone BCs and that PKCα critically involved the function of mGluR6. The study pointed out that in the three mice models (ocular hypertension, excitotoxic neurodegeneration, or optic nerve crash), RBC dysfunction all occurs before RGC loss. Similarly, in a glaucoma model rat, the sodium hyaluronate-induced ocular hypertension caused damage to the OPL and IPL and was associated with morphologic and morphometric changes in BCs [173].

MSCs permeable to Na⁺ and Ca²⁺ respond to pressure with excitatory currents, and their role in pressure-induced glutamate release from photoreceptors and BCs requires more attention and further exploration.

6. The expression and activity of MSCs in ACs

BK is in the open state and mediates small outward miniature currents at −60 to −40 mV in salamander ACs, which were sensitive to iberiotoxin [191]. A17 ACs from rat retinas provide reciprocal inhibitory synapses to RBCs that mediate the primary rod pathway. BK reduces GABA release from A17 ACs and regulates the flow of excitatory signals through the primary rod pathway [192]. TRPC5 is present in mouse ACs and RGCs, serving as a negative regulator of RGCs for axon outgrowth [193].

Starburst ACs (SACs) are cholinergic GABAergic ACs. They critically modulate the activities of the direction-selective RGCs and mediate the retinal wave during development. Fort and colleagues [194] observed a strong expression of the KCNK2 gene that encodes TREK1 in isolated SACs from the mouse retina. At the postnatal day P1-6, TREK1−/− mice exhibited an altered frequency of the retinal wave, which is known to be set by the slow afterhyperpolarization (sAHP) of SACs. The sAHP conductance was found to be calcium-dependent, reversed at potassium reversal potential (E_K), blocked by barium, insensitive to apamin and TEA, and activated by arachidonic acid.

Mammalian ACs express the TRAAK and TRPV2 transcripts [47, 111]. The INL shows the immunoreactivity of TRAAK, TREK1, and TREK2 [52]. A low level of TRAAK is present in the IPL of the salamander retina [30]. The mRNA and immunoreactivity of ENaCα were increased in the IPL and INL in D2 mice [168], and the INL of human retinas expressed the mRNA of β- and γ-subunits of ENaC [170]. These studies suggest that individual interneurons require several types of MSCs to perform normal functions.

It has been poorly understood how ACs express MSCs and respond to the mechanical activation of MSCs. ACs mediate spontaneous postsynaptic currents (sIPSCs) in RGCs and BCs. In rat RGCs, TRPV4 antagonist HC-067047 reduces sIPSCs, and TRPV4 agonist GSK 1016790A enhances sIPSCs in ON and OFF RGCs. TRPV4 activation has different effects on the release of glycine and GABA and the function of the neurotransmitter receptors [195]. TRPV1 was found in processes and perikarya of putative ACs in the INL, which was in the vicinity of nNOS-positive ACs but did not colocalize nNOS, and retinal explants exposed to capsaicin presented high protein nitration and cell death in the INL and GCL [106].

ACs have shown some changes in glaucoma retinas. Pressure insult led to Type III neuron death restricted to the INL [196] in an early report. Later studies in glaucoma models further identified significantly [25, 197] or minor loss (∼15%) of ACs [198, 199], low light sensitivity of AII ACs [184, 200], thinning of OPL, loss of processes of BCs and HCs [163, 164], disturbance to the IPL [173], and reduction of BC synapses in RGC dendrites [174, 175, 176, 201] before RGC death. AII ACs are glycinergic ACs, making ∼11% of all ACs [202] in the rabbit retina. GABAergic ACs, including cholinergic ACs [203], were also reduced in glaucoma model D2 mice [198] and rats with elevated IOP [204]. It was concurrent with the RGC death and appeared to be selective for ACs coupled to RGCs [204]. Different changes were observed in nitroxidergic (NO) ACs [198] in glaucoma models. IOP elevation in rats reduced the glutamate- and K⁺-induced GABA release and increased GABA uptake [205]. In a glaucoma model rat, the sodium hyaluronate-induced ocular hypertension [173] damaged the direction-selective circuit, IPL, and OPL besides RGC populations. The treated eyes exhibited morphologic and morphometric changes in BCs, ON-OFF direction-selective RGCs, ON and OFF SACs, and the IPL. The data demonstrated the variation of pressure responsiveness among ACs.

7. The expression and activity of MSCs in RGCs

7.1 MSCs permeable to K⁺ in RGCs

BK has been observed in RGCs from mammalian and other vertebrate retinas [191, 206, 207, 208]. It is in open states and involves the development and normal function of RGCs in vertebrate retinas. In isolated trout retinal RGCs [208], Iberiotoxin-sensitive, calcium-activated potassium currents mediated by BK were minimal before hatching but increased significantly then.

In the isolated and intact RGCs [207] from ferret retinas, charybdotoxin, a blocker of BK, increased RGC spiking. Charybdotoxin and apamin (SKCa channel blocker) reduced the time to the threshold and the hyperpolarization after the spike in isolated RGCs and 80% of a- and b-RGCs. In mouse RGCs, blocking BK with charybdotoxin increased the spontaneous excitatory post-synaptic currents (EPSCs) and light-evoked ON-EPSCs but decreased the light-evoked OFF-IPSCs [206]. These results align with the notion that BK regulates the membrane potential, excitability, and visual signals of mammalian RGCs.

The GCL of the mouse retina exhibits immunoreactivities of TREK-1, TREK2 [52, 171], and TRAAK [47, 171]. Hughes and colleagues also observed other K2P channels in RGCs, such as TASK-1, TWIK-1, TWIK-2, and TWIK-3 [52]. In rat RGCs, TREK2 at the sites postsynaptic to GABAergic ACs may be activated by GABAB receptors to affect the activities of RGCs [209]. Using immunocytochemistry, patch-clamp, PCR, and western blot, Zhang and colleagues [210] found that rd1 mice express higher levels of the mRNA and protein of TREK1 and TRAAK, and the arachidonic acid-evoked current was bigger in RGCs. TASK-3 also showed some regulation on the excitability of mouse RGCs [211]. The data suggest that K2Ps have physiological and pathological roles in RGCs.

7.2 Piezo and ENaC in RGCs

Piezo proteins are pore-forming mechano-gated channels [150, 154]. Optic never head expresses multiple MSCs [212]. The mRNA of Piezo1 and Piezo2 are present in the myelinated region of the mouse optic nerve, and the expression level of Piezo2 is high in the optic never head. Immunostaining revealed Piezo1 and Piezo2 in the GCL [213] and non-neuronal ocular tissues [213, 214], as well as astrocytes [215]. Zhu and colleagues used single-molecule fluorescence in situ hybridization (smFISH) and transgenic reporter mice expressing Piezo fusion proteins [159] to explore the distribution of Piezo in the mouse retina. Piezo1 and Piezo2 were found in the GCL, trabecular meshwork, and ciliary body, and Piezo1 mRNA was more abundant. In genetically encoded Ca²⁺ indicator mice and an ex vivo pressurized retina preparation, Harraz and colleagues reported Ca²⁺-permeable Piezo1 in the endothelial cells in retinal and cortical capillaries [214].

Piezo channel plays a part in RGC damage in the mouse retina. The expression of retinal Piezo2 increases in the mouse model of high IOP. Piezo1 agonist Yoda 1 suppressed neurite outgrowth in RGCs. On the other hand, Piezo antagonist GsMTx4 promoted neurite outgrowth in RGCs [213, 216]. A gain of function variant of Piezo1 (e756del) was observed in 30% of African Americans and associated with higher IOPs, thinner NFL, and lower optic nerve head capillary densities, but these did not reach statistical significance [158]. Piezo1 regulates the cell cycle progression of astrocytes, and loss of Piezo1 can lead to the cell cycle of astrocytes at the optic nerve head arrest at the G0/G1 phase [215].

ENaC is a novel therapeutic target for serval human diseases [133]. It is activated by protease and blocked by amiloride [136, 144]. ENaCa immunoreactivity and mRNA expression are present in the GCL and IPL in the mouse retina [168] and RGCs in the rat and human retina [131, 134]. GCL expresses the mRNA and protein of β- and γ-subunits of ENaC in the human retina [170]. In D2 mice, Dyka and colleagues found an upregulation of ENaCα gene expression in the IPL and GCL, but they did not find β- and γ-subunits [168]. A recent study reveals that mobile Zn²⁺ from interneurons triggers RGC death in optic nerve injury [217], while Zn²⁺ may inhibit TRPM1 and activate TREK2 [218, 219].

7.3 TRPVs in RGCs

The retina expresses multiple types of TRPs of variable functional significance (reviewed by Refs. [57, 58]). Gilliam and colleagues identified TRPs in the mouse retina with RT-PCR and immunohistology [110]. The strongest signals were reported for TRPC1, TRPC3, TRPM1, TRPM3, and TRPML1, and clear signals were obtained for TRPC4, TRPM7, TRPP2, TRPV2, and TRPV4. Their results revealed widespread expression throughout multiple retinal layers for TRPC1, TRPC3, TRPC4, and TRPML1. Other early studies have described the distribution patterns of multiple TRPs in a variety of species [1, 101, 114, 115, 220]. RGCs and their axons have been widely identified as the most vulnerable to pressure in both TRI [5, 221, 222] and glaucoma [3, 4], and recent data from glaucoma models have revealed RGC dysfunction [200] and synaptic damage [174, 223, 224] before axon loss. Yet, the involvement of MSCs has been uncertain.

GCL and IPL express TRPV2 in rat, cat, primate, and salamander retinas [30, 54, 106, 111, 112], TRPV1 in rat and primate retina [101, 106], and TRPV4 in the mouse [101, 114], porcine [115], and primate retina [54]. The optic nerve head was revealed mRNAs of TRPV2 and TRPV4 [212]. Lakk and colleagues observed mRNAs of TRPVs in isolated RGCs of 7–15 um diameter from the mouse retina, whose levels were TRPV4 > TRPV2 > TRPV3 and TRPV1 [225].

TRPV4 single-channel conductance was measured as an inward conductance of 60pS and outward conductance of 102pS. TRPV3 showed a higher single-channel conductance, 172 pS at +60 mV, compared with ∼100pS for TRPV1/2/4 [121]. Liedtke and colleagues first cloned cDNAs encoding the vanilloid receptor-related osmotically activated channel (VR-OAC, i.e., TRPV4) from the rat, mouse, human, and chicken. They showed that TRPV4 was a cation channel gated by exposure to hypotonicity within the physiological range [89]. Later, the researchers confirmed the pressure responsiveness of TRPV4 in transfected cell lines and TRPV4 mutant mice [61, 92]. Another group of scientists [88] separately reported the impaired osmotic sensation in mice lacking TRPV4.

The large peripheral RGCs of the primate retina express more heavily TRPV4 [54]. RGCs in the mouse [114] and primate retina [54] can be activated by micromolar TRPV4 agonists GSK1016790A and 4aPDD, exhibiting membrane depolarization and higher firing rate. In cultured RGCs, TRPV4 agonists evoked calcium influxes and were associated with apoptosis of the neurons [114]. TRPV4 antagonist RN-1734 has been tested in retinal slices in culture and revealed a neuroprotective role in the porcine retina [115]. These observations have confirmed the expression of TRPV4 in RGCs. TRPV4 agonist enhances the frequency of excitatory postsynaptic currents consistent with the expression of TRPV4 in presynaptic BCs [54]. Notably, activation of TRPV4 can increase the excitability of RGCs via shortening the delay [54] and augmenting the amplitude [226] of voltage-gated sodium current sensitive to tetrodotoxin. Li and colleagues [118] reported an increased expression of TRPV4 under high IOP. Intravitreal injection of TRPV4 agonist induces Müller cell gliosis, and activation of TRPV4 induces the release of tumor necrosis factor-α (TNF-α) from cultured Müller cells. Inhibition of TNF-a could reduce TRPV4-mediated RGC apoptosis [118]. The results together support that the normal functions and apoptosis of RGCs involve TRPV4 located in RGCs, BCs, and Müller cells. In Müller cells, TRPV4 serves as a volume regulator [117, 120, 226, 227, 228, 229].

TRPVs also involve diabetic retinopathy. TRPV4 knockout or inhibition could prevent the increased water diffusion and blood-retina barrier breakdown in the retina of streptozotocin-induced diabetic mice [230]. In rat models, TRPV2 contributes to stretch-activated cation currents and myogenic constriction in retinal arterioles, and genetic deletion of TRPV2 impaired the myogenic reaction of retinal arterioles, resembling that observed in diabetic animals [85, 231]. Systemic activation of TRPV4 could cause endothelial failure and circulatory collapse [60], and TRPV4 promotes vascular permeability in retinal vascular disease [232]. Calcium influx through TRPV4 modulates the adherens contacts between retinal microvascular endothelial cells [233], and pharmacologic inhibition of TRPV1 or TRPV4 suppresses retinal angiogenesis in vitro [234].

RGCs express TRPV1 [101, 108, 172, 235, 236, 237, 238]. Sappington and colleagues reported mRNA of TRPV1 in the cell body and axon of RGCs, and the TRPV1 level increased by IOP elevation in D2 mice. They applied hydrostatic pressure (70 mmHg) to RGCs in culture and reported that TRPV1 antagonism could reduce the pressure-induced RGC apoptosis. Similar results were obtained from RGCs of the monkey, human, and rat retina, and TRPV1 knockout and pharmacological antagonism of TRPV1 were found to prevent the pressure-induced RGC apoptosis [101]. TRPV1 and TRPV4 were shown to form a protein complex in RGCs [101], but the interaction between TRPV1 and TRPV4 appears to be insignificant [225]. In isolated mouse RGCs from wild-type and TRPV1−/− mice, Lakk and colleagues examined calcium influxes via TRPV4 by calcium imaging, and TRPV1 mutation showed no effect on TRPV4 activities evoked by the channel agonist GSK1016790 [225]. However, TRPV1 mutation and antagonist were reported to be neuroprotective for isolated RGCs [101] and retinal explants [101] but neurodegenerative in vivo [101, 108, 238], indicating the differential roles of TRPV1 in RGCs and presynaptic neurons.

In transgenic mice where a few subtypes of RGCs were genetically labeled, the RGCs ramified in the sublamina a showed the greatest change in the dendritic morphology after 1 week of IOP elevation in the microbead occlusion model of glaucoma [176]. On the other hand, both ONaGCs and OFFaGC exhibited reduced light sensitivity after a few weeks of IOP elevation, as shown in our results [200]. Rountree and colleagues applied pulsatile injection (pulse-width > 50 ms at 0.69 kPa pressure) of Ames mediums into retinal tissue and evoked neuronal responses comparable to light responses. The response of RGCs was reduced by a TRPV blocker [239]. Genetic deletion of TRPV1 can differently affect the excitability of RGCs firing continuously to light onset (ON-sustained) vs. light offset (OFF-sustained) [236]. These results together indicate that TRPVs may play physiological and excitotoxic roles in RGCs.

8. Other mechanosensitive TRPs in retinal neurons

TRPA1, TRPC1/3/5/6, TRPM3/4/7, and TRPP1 are mechano-gated channels (reviewed by Refs. [79, 80]). TRPA1, TRPM1, and TRPCs mediate light response in sensory neurons. Planarians possess extraocular photoreception and display an unconventional TRPA1-mediated photophobic response to near-UV light [240]. Glutamate released from photoreceptors hyperpolarizes ON BCs by stimulating mGluR6, which was mediated by the inactivation of TRPM1 after binding with the G-protein a or bg subunits [241, 242] (reviewed in Refs. [57, 243]) to mediate light signals. TRPC6 and TRPC7 are expressed in the intrinsic photosensitive RGCs (ipRGCs) in the mammalian retina to involve melanopsin phototransduction. The melanopsin photocurrent in ipRGCs was abolished by blocking or eliminating TRPC3/6/7 channels in M1-type ipRGCs and TRPC3/6 in M2-type ipRGCs, suggesting that the target of melanopsin phototransduction varies in these ipRGCs (reviewed in Ref. [244]). TRPC1/4/5/6 are expressed in endothelial and Müller cells, and the TRPC1/4/5/6(−/−) compound knockout mice showed resistance to diabetic retinopathy [245].

TRPC5 is present in mouse RGCs and ACs to serve as a negative regulator of RGCs for axon outgrowth [193]. Optic never head expresses multiple types of MSCs. The mRNA of TRPP1 and TRPP2 were observed in the myelinated region of the mouse optic nerve [212]. The optic nerve head and the glial lamina were revealed TRPC1-6, TRPV2, TRPV4, TRPM1-4/6/7, TRPP1, and TRPP2. About 43–87% of individual astrocytes express the mRNA of TRPC1, TRPM7, TRPP1, and TRPP2. The expression level of TRPP2 was high in the optic nerve head. TRPM3 immunofluorescence was present in a subset of RGCs in mice during postnatal 7–14 days and in adult mice. Activation of TRPM3 with the synthetic TRPM3 agonist CIM0216 (CIM) induced prolonged calcium transients in RGCs, which were mostly absent in TRPM3 mutant mice. The prolonged calcium transient was not associated with strong membrane depolarizations but induced c-Fos expression [246, 247].

Ischemia elicited a decrease in the ERG-recorded retinal responsiveness to light along with reactive gliosis and a significant increase in the expression of TRPM7 in Müller cells [248]. In another report [249], blue light triggered apoptosis of retinal pigment epithelial (RPE) cells, and its deleterious effects were partially attenuated by the synergistic action of TRPM7 and the pigment epithelium-derived factor (PEDF) via the PKC/ERK signaling pathway. The reverse transcription PCR analysis demonstrated the mRNA expression of TRPC1, TRPM7, TRPV1/2/4, and TRPP1, but not TRPC6 or TRPM4 in retinal vascular smooth muscle cells (VSMCs). TRPV2 inhibitor tranilast and specific TRPV2 pore-blocking antibodies reserved the hypoosmotic stretch-induced Ca²⁺ influx in VSMCs of isolated retinal arterioles, but inhibitors of TRPC1, TRPM7, TRPV1, and TRPV4 had no effect. The authors concluded that retinal VSMCs expressed a range of mechanosensitive TRP channels, but only TRPV2 contributed to myogenic signaling in this vascular bed [85].

9. Conclusions

Fluctuating mechanical stresses and strains are present in the ocular environment and are associated with multiple retinal disorders. Individual retinal neurons often co-express several types of MSCs, which may mediate either depolarizing or hyperpolarizing membrane currents upon mechanical and chemical activation. Each MSC contributes more-or-less to the membrane currents or potentials, regulating neuronal excitability and visual signals or resulting in neuronal excitotoxicity. A few TRPs directly mediate phototransduction in ipRGCs and synaptic transmission in ON BCs in mammalian retinas, and some of them are also mechano-gated channels. MSCs are still to be fully characterized for their significance in maintaining cellular mechanical homeostasis, aging change, and cooperation in physiological and pathological conditions, facilitating the development of effective therapeutic strategies for pressure-related visual disorders.

Acknowledgments

I thank the funding support from US Army Medical Research Acquisition Activities VR210010 and the NIH core grant EY02520.

Conflict of interest

The author declares no conflict of interest.