Using Small Molecules to Reprogram RPE Cells in Regenerative Medicine for Degenerative Eye Disease

Abstract

The main purpose of regenerative medicine for degenerative eye diseases is to create cells to replace lost or damaged ones. Due to their anatomical, genetic, and epigenetic features, characteristics of origin, evolutionary inheritance, capacity for dedifferentiation, proliferation, and plasticity, mammalian and human RPE cells are of great interest as endogenous sources of new photoreceptors and other neurons for the degrading retina. Promising methods for the reprogramming of RPE cells into retinal cells include genetic methods and chemical methods under the influence of certain low-molecular-weight compounds, so-called small molecules. Depending on the goal, which can be the preservation or the replacement of lost RPE cells and cellular structures, various small molecules are used to influence certain biological processes at different levels of cellular regulation. This review discusses the potential of the chemical reprogramming of RPE cells in comparison with other somatic cells and induced pluripotent stem cells (iPSCs) into neural cells of the brain and retina. Possible mechanisms of the chemically induced reprogramming of somatic cells under the influence of small molecules are explored and compared. This review also considers other possibilities in using them in the treatment of retinal degenerative diseases based on the protection, preservation, and support of survived RPE and retinal cells.

Keywords: RPE; chemically directed reprogramming; eye degeneration disease; inherited retinal degeneration; regenerative and rehabilitation medicine; reprogramming; retinal pigment epithelium; small molecules.

Figure 1

Eye injury induces different RPE responses in lower vertebrates and mammals, including humans. The initial stages of cellular reprogramming are the same in all studied organisms: cells enter the cell cycle, begin to proliferate, lose pigment, and dedifferentiate. However, subsequently, the cells develop completely differently: in some species, RPE cells are transformed into neuronal retinal cells, thereby restoring the retina, while in humans, dedifferentiated RPE cells differentiate into myofibroblasts, which leads to serious pathologies.

Figure 2

Some small molecules and signaling pathways promoting the development of the nervous system (based on the poster https://www.stemcell.com/media/files/wallchart/WA10014-Small_Molecules_Big_Impact.pdf (accessed on 16 November 2024)). Notes: The transforming growth factor β (TGFβ) is involved in a whole range of biological functions, from cell growth to cell differentiation and apoptosis. SMAD1, 2, 3, 5, and 8 are receptor-regulated SMADs. They bind to membrane-bound serine/threonine receptors and are activated by the kinase activity of the receptors. SMAD4 acts as a cofactor that binds to activated R-SMADS (SMADs) forming a complex that translocates into the nucleus [63]. Pathway inhibitors: SB431542, LY364947, RepSox, Dorsomorphin, LDN193189. The Notch signaling pathway regulates cell proliferation, cell fate, differentiation, and cell death in all metazoans. The Notch pathway is activated when Delta or Jagged ligands on neighboring cells activate cleavage of the receptor releasing the Notch intracellular domain (NICD). The Notch pathway plays a role in specifying neural subtypes [64]. Pathway inhibitors: DAPT, LY411575. Fibroblast growth factor (FGF) signaling regulates several developmental processes, including cellular proliferation, differentiation, migration, morphogenesis, and patterning. FGF signaling via MEK/ERK is critical for self-renewal and proliferation of human PSCs [65]. The WNT signaling pathway is an ancient and evolutionarily conserved pathway that regulates crucial aspects of cell fate determination, cell migration, cell polarity, neural patterning, and organogenesis during embryonic development [66]. Pathway activators: CHIR99021, SB216763; pathway inhibitors: IWR-1-endo. The Hedgehog (Shh) pathway is important in post-embryonic tissue regeneration and repair processes. Specifically, Shh signaling is implicated in the induction of multifarious neuronal populations in central nervous system [67]. Pathway activators: Purmorphamine, SAg. The RHO/ROCK pathway regulates cytoskeletal dynamics and plays an important role in cell adhesion, proliferation, motility, contraction, and apoptosis. Loss of cadherin or integrin binding activates the Rho pathway in human PSCs, leading to anoikis [68]. Pathway inhibitors: Y-27632, thiazovivin. The 3′,5′-cyclic adenosine monophosphate (cAMP) is a second messenger important in reprogramming and differentiation for many cell subtypes [69]. Pathway activator: forskolin. The protein kinase C (PKC) family of kinases is commonly activated by diacylglycerol (DAG) and calcium and is involved in several signaling pathways that can regulate differentiation [70]. Pathway activators: prostaglandin E2, (−)-Indolactam V; pathway inhibitors: HA-100, GO6983. Retinoic acid (RA) is a potent morphogen required for embryonic development. RA acts in a paracrine fashion to shape the developing eye and is essential for normal optic vesicle and anterior segment formation [71]. Activators: 9-cis retinoic acid, all-trans retinoic acid, CD437, TTNPB. RAR, RXR -RA receptors. Epigenetic marks such as acetylation (Ac) of histones and methylation (Me) of histones or DNA serve to induce or inhibit gene expression in a heritable manner. Global changes in epigenetic marks are critical for reprogramming [35]. DNA Methyltransferase inhibitors: RG108; histone methyltransferase inhibitors: BIX01294; histone demethylase inhibitors: tranylcypromine; histone acetyltransferase inhibitors: garcinol; histone deacetylase inhibitors: sodium butyrate, trichostatin A, valproic acid.

Figure 3

Schematic representation of chemically induced reprogramming of fibroblasts into neural stem cells and into neurons in the brain and retina [12,36,37,39,41,42,43,85,86]. This approach utilized small molecules that acted on the cellular epigenome (Epi) and on various signaling pathways that control cellular identity (TGFβ, GSK3β, PKC, BMP, SHH, JNR, ROCK, and others). FG—growth factor; m—mouse; h—human; CiNs—chemically induced neurons; CiNSCs—chemically induced neural stem cells; CiPCs—chemically induced photoreceptor-like cells; m rd1—mouse model of retinal degeneration; m NaIO₃—mouse model of sodium iodate (NaIO₃)-induced retinal degeneration; SRT—subretinal transplantation, LVT—lateral ventricle transplantation. Pathway activators: green color; pathway inhibitors: red color.

Figure 4

Schematic representation of chemically induced reprogramming of astrocytes into neural stem cells and into neurons in the brain and retina [34,38,50,87,88,89,90,91,92]. Chemically induced reprogramming utilized small molecules that acted on the cellular epigenome (Epi) and on various signaling pathways that control cellular identity (TGFβ, GSK3β, PKC, SHH, Notch, RAR, ROCK, and others). FG—growth factor; m—mouse; nm—neonatal mouse; h—human; r—rat; CiNs—chemically induced neurons; CiNSCs—chemically induced neural stem cells; CRI—microinjection into the cortices; STI—microinjection into the striatum; LVT—lateral ventricle transplantation. Pathway activators: green color; pathway inhibitors: red color.

Figure 5

Schematic representation of chemically induced reprogramming of ESCs and iPSCs into neural stem cells and into neurons of the brain and retina [2,47,52,54,97,98,99]. Chemically induced reprogramming utilized small molecules that acted on the cellular epigenome (Epi) and on various signaling pathways that control cellular identity (TGFβ, GSK3β, WNT, BMP, NOTCH, ROCK, and others). GF—growth factor; iPSC—induced pluripotent stem cell; ESCs—embryonic stem cells; RSCs—retinal stem cell; CiRGCs—chemically induced retinal ganglion cells; CiR—chemically induced retina; CiROD—chemically induced rods; CiRPE—chemically induced retinal pigment epithelium; CiPCs—chemically induced photoreceptor-like cells; m NOD-SCID—the nonobese diabetic/severe combined immunodeficient mouse; m Crx^tvrm65/IL2rγ^−/−—model of immunosuppressive mouse/retinal degeneration; m NOD.SCID-rd1—the nonobese diabetic/severe combined immunodeficient mouse model of retinal degeneration; RCS rat—rat model of retinal degeneration from Royal College of Surgeons; IVT—intravitreal injection; SRT—subretinal injection; mMNU—mouse model of N-Nitroso-N-methylurea (MNU)-induced retinal degeneration, m—mouse, h—human. Pathway activators: green color; pathway inhibitors: red color. 2.3.4. Reprogramming of the RPE into CiNSCs and CiNs.

Figure 6

Schematic representation of chemically induced reprogramming of RPE into neural stem cells and into neurons in the brain and retina [12,27,46,93]. Chemically induced reprogramming utilized small molecules that acted on the cellular epigenome (Epi) and on various signaling pathways that control cellular identity (TGFβ, GSK3β, WNT, BMP, NOTCH, PKC, and others). Fh—fetal human; Mn—cynomolgus monkeys (Macaca fascicularis); MPTP hydrochloride—induced Parkinson’s disease model; PPI—implantation into posterior putamen; sphere—free floating conditions; for other abbreviations, refer to Figure 3 and Figure 5. Pathway activators: green color; pathway inhibitors: red color.