Extracellular Vesicles in Corneal Fibrosis/Scarring

Abstract

Communication between cells and the microenvironment is a complex, yet crucial, element in the development and progression of varied physiological and pathological processes. Accumulating evidence in different disease models highlights roles of extracellular vesicles (EVs), either in modulating cell signaling paracrine mechanism(s) or harnessing their therapeutic moiety. Of interest, the human cornea functions as a refractive and transparent barrier that protects the intraocular elements from the external environment. Corneal trauma at the ocular surface may lead to diminished corneal clarity and detrimental effects on visual acuity. The aberrant activation of corneal stromal cells, which leads to myofibroblast differentiation and a disorganized extracellular matrix is a central biological process that may result in corneal fibrosis/scarring. In recent years, understanding the pathological and therapeutic EV mechanism(s) of action in the context of corneal biology has been a topic of increasing interest. In this review, we describe the clinical relevance of corneal fibrosis/scarring and how corneal stromal cells contribute to wound repair and their generation of the stromal haze. Furthermore, we will delve into EV characterization, their subtypes, and the pathological and therapeutic roles they play in corneal scarring/fibrosis.

Keywords: cell-cell communication; cornea; exosomes; extracellular vesicles (EV); fibrosis; microvesicles; scarring; therapeutic; wound healing.

Figure 1

A schematic overview to corneal scarring/fibrosis: associated clinical factors; the corneal wound healing and angiogenesis response with the four major steps indicative of the regeneration and restoration mechanisms; and current management strategies.

Figure 2

A schematic overview of different extracellular vesicles (EVs) types that include microvesicles, exosomes, exomeres, and supermeres. Microvesicles are typically formed by the outward budding of the plasma membrane. Exosomes are derived from multivesicular bodies (referred to as intraluminal vesicles) during formation and can be derived from the cell surface. The biogenesis of exomeres and supermeres remain unknown and are complexes of proteins and nucleic acids that are not membrane enclosed. Collectively, each of these EVs types are enriched in distinct markers that currently defines their composition.

Figure 3

Typical structure and molecular composition of exosomes. Exosomes are surrounded by a phospholipid bilayer enriched in lipids such as ceramide, cholesterol, phosphatidylserine, and sphingomyelin. They are enriched in glycoproteins such as β-galactosidase, and N-linked or O-linked Glycans. Exosomes are enriched in proteins associated with biogenesis, such as programmed cell death 6-interacting protein (ALIX), syntenin-1 (SDCBP), tumor susceptibility gene 101 (TSG101), and vacuolar protein sorting 4 and -32 (VPS4 and VPS32). Upon trafficking of multi-vesicular bodies encompassing exosomes, they express small GTPase Ras-related proteins 5A, -7, -11, -27A/B and -35 (RAB5A, RAB7, RAB11, RAB27A/B, and RAB35). When MVBs fuse with the plasma membrane to release exosomes into the extracellular space, they also express soluble N-ethylmaleimide-sensitive fusion attachment protein receptor (SNARE) proteins such as synaptosome associated protein 23 (SNAP23), syntaxin1a (SYX1A), vesicle associated membrane protein 7 (VAMP7), and YKT6 V-SNARE Homolog (YKT6). Exosomes are enriched in; tetraspanin proteins, such as CD9, CD37, CD53, CD63, CD81, and CD82; flotillin (FLOT) molecules, such as FLOT1 and FLOT2; major histocompatibility complex-I and -II (MHC-I and -II); adhesion molecules for example epithelial cellular adhesion molecule (EpCAM), intercellular adhesion molecule-1 (ICAM-1), integrin subunit α1-6 (ITGA1-6), integrin subunit αV (ITGAV), integrin subunit β1-4 (ITGB1-4), and lactadherin (MFGE8); heparan sulfate proteoglycans (HSPGs) that include agrin (AGRN), glypican 1-6 (GPC1-6), perlecan (HSPG2), and syndecan1-4 (SDC1-4). Exosomes can also contain cytosolic proteins that include actin (ACTB), cofilin1 (CFL1), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), heat shock protein 70 and -90 (HSP70 and HSP90), myosin, and tubulin. The exosomal surface molecular and internal cargo serves to mediate intracellular communication between different cell types within the body, thus functioning differently in either normal homeostasis or pathological conditions.