Corneal cell therapy: with iPSCs, it is no more a far-sight

Abstract

Human-induced pluripotent stem cells (hiPSCs) provide a personalized approach to study conditions and diseases including those of the eye that lack appropriate animal models to facilitate the development of novel therapeutics. Corneal disease is one of the most common causes of blindness. Hence, significant efforts are made to develop novel therapeutic approaches including stem cell-derived strategies to replace the diseased or damaged corneal tissues, thus restoring the vision. The use of adult limbal stem cells in the management of corneal conditions has been clinically successful. However, its limited availability and phenotypic plasticity necessitate the need for alternative stem cell sources to manage corneal conditions. Mesenchymal and embryonic stem cell-based approaches are being explored; nevertheless, their limited differentiation potential and ethical concerns have posed a significant hurdle in its clinical use. hiPSCs have emerged to fill these technical and ethical gaps to render clinical utility. In this review, we discuss and summarize protocols that have been devised so far to direct differentiation of human pluripotent stem cells (hPSCs) to different corneal cell phenotypes. With the summarization, our review intends to facilitate an understanding which would allow developing efficient and robust protocols to obtain specific corneal cell phenotype from hPSCs for corneal disease modeling and for the clinics to treat corneal diseases and injury.

Keywords: Cell replacement therapy; Cornea; Differentiation; Disease modeling; Induced pluripotent stem cells.

Fig. 1

Schema of layers in the cornea and its development. The cornea constitutes of three cellular layers: the CEC, CS, and CEn and two acellular membranes. The Bm separating the CEP and CS. Dm sandwiched between CS and CEnC. The CEP is derived from the PEP originating from the OSEs. Both CS and CEnC derive from NCC which rise from the MSC

Fig. 2

Schema of deriving corneal cell phenotype from iPSCs. Human iPSCs treated with competitors of activin, and nodal pathways result in the inhibition of SMAD signaling inducing neuroectodermal progenitor (NEP) fate by activation of Zic and Fox gene family. Subsequent directed differentiation of NEPs to corneal epithelial cells (CEPs) having expression of Pax6, ABCG2, p63, and cytokeratin 12 and 13 is done by inhibiting TGFβ and WNT signaling pathways. To obtain CSKs, iPSCs are at first directed towards NCC phenotype by inhibiting TGFβ and BMP4 signaling using SB431542 and Noggin respectively. NCCs can be differentiated to keratocan and ABCB5-positive CSKs by following a co-culture system involving PA6 stromal cells for SDIA or by following a more defined culture method utilizing the bFGF and ascorbic acid (ascorpate-2-phosphate, A-2-P) signaling pathway. ZO-1 and Na,K-ATPase-positive CEnCs (see references [68, 78] for hCEnC markers) can be differentiated from NCC following a sequential differentiation procedure where the NCCs are first treated with a GSK3b inhibitor to activate the WNT/β-catenin pathway followed by treatment with SB431542 to inhibit TGFβ-mediated SMAD signaling. RA promotes terminal CEnC differentiation inhibiting while ROCK inhibitor promotes survival and enhances functional properties of the CEnCS [83, 84]