Sensory nerve supports epithelial stem cell function in healing of corneal epithelium in mice: the role of trigeminal nerve transient receptor potential vanilloid 4

Abstract

In order to understand the pathobiology of neurotrophic keratopathy, we established a mouse model by coagulating the first branch of the trigeminal nerve (V1 nerve). In our model, the sensory nerve in the central cornea disappeared and remaining fibers were sparse in the peripheral limbal region. Impaired corneal epithelial healing in the mouse model was associated with suppression of both cell proliferation and expression of stem cell markers in peripheral/limbal epithelium as well as a reduction of transient receptor potential vanilloid 4 (TRPV4) expression in tissue. TRPV4 gene knockout also suppressed epithelial repair in mouse cornea, although it did not seem to directly modulate migration of epithelium. In a co-culture experiment, TRPV4-introduced KO trigeminal ganglion upregulated nerve growth factor (NGF) in cultured corneal epithelial cells, but ganglion with a control vector did not. TRPV4 gene introduction into a damaged V1 nerve rescues the impairment of epithelial healing in association with partial recovery of the stem/progenitor cell markers and upregulation of cell proliferation and of NGF expression in the peripheral/limbal epithelium. Gene transfer of TRPV4 did not accelerate the regeneration of nerve fibers. Sensory nerve TRPV4 is critical to maintain stemness of peripheral/limbal basal cells, and is one of the major mechanisms of homeostasis maintenance of corneal epithelium.

<p>Self-Retained Cryopreserved Amniotic Membrane for the Management of Corneal Ulcers</p>

“…One such condition, neurotrophic keratitis (NK), involves damage to the corneal nerves that can diminish corneal epithelial viability and dampen the blinking and tearing reflexes that maintain ocular surface health. 3,4 This is especially devastating as corneal nerves and epithelial cells mutually support one another via neuropeptides and growth factors to ensure corneal wound healing [5][6][7][8] and nerve repair and survival. 9 Thus, early diagnosis and treatment of underlying disease are essential in restoring the ocular surface and preventing progressive ulceration, imminent perforation, and irreversible vision loss.…”

Polymodal Sensory Transduction in Mouse Corneal Epithelial Cells

“…Consistent with prominent transcript expression, immunohistochemistry showed robust TRPV4 expression in central and limbal regions and a well-defined basal-to-squamous gradient of TRPV4-ir and TRPV4 eGFP , a pattern that resembles the basal-apical TRPV4 expression patterns reported in urothelial, skin, and mammary epithelia, 17,18,60 but differs from the rabbit cornea in which TRPV4 was localized to apical cells. 61 Basal expression implicates TRPV4 in differentiation/stemness and/or barrier permeability functions, 15,21,33 whereas plasma membrane TRPV4-ir reflects the well-known roles of the channel in ion transport, Ca 2+ homeostasis, and volume regulation. 45,62,63 Although the function of cytosolic TRPV4-ir puncta is less clear, total internal reflection microscopy studies have linked such puncta to stimulus-and development-dependent trafficking of endosomes, membrane translocation, insertion, and/or internalization.…”

“…It is possible that faster and larger responses correspond to basal cells, which express channels at high densities and may utilize them to propel regeneration. 15 Because TRPV4-mediated currents and [Ca 2+ ] i elevations took several minutes to peak, we hypothesize that the channel is gated by intermediary mechanisms, such as phospholipase A2, phosphatidyl inositol phosphate, and/or eicosanoid metabolites that require minutes lag time for activation. 24,40,[74][75][76]77 Dry eye treatments rely on hypotonic artificial tears to counter evaporative water loss and epithelial cell damage, 8,78 and are therefore likely to trigger mechanisms that sense CEC swelling.…”

“… 45 , 62 , 63 Although the function of cytosolic TRPV4-ir puncta is less clear, total internal reflection microscopy studies have linked such puncta to stimulus- and development-dependent trafficking of endosomes, membrane translocation, insertion, and/or internalization. 64 – 68 Given that TRPV4 is expressed in the CE, 12 , 14 afferents, 15 fibroblasts, 69 stromal keratocytes, 70 and the endothelium ( Fig. 1 ) it probably functions as a pan-corneal regulator of sensory inputs.…”

“…9 The mammalian CE expresses several TRPV isoforms, with TRPV1 implicated in regulation of wound healing, 10 inflammatory signaling, 11 and osmotransduction 12 ; TRPV3 linked to thermosensing and cell proliferation; 13 and TRPV4 associated with transduction of moderate temperature, regulatory volume decrease (RVD), 14 and regulation of stemness. 15 However, previous studies tended to be conducted in immortalized epithelial cell lines 16 in which cells lack native contacts and may have altered their sensory transduction mechanisms. It is therefore important to determine which TRPV channels are expressed and functional in the intact epithelium, how native cells respond to stretch, temperature, and swelling, and whether TRPV activation by extrinsic stressors regulates release of substances that can modulate the firing of trigeminal afferents.…”