# Structure and Function of Ocular Proteoglycans: Essential Proteins for Vision

## TL;DR

This review explains how proteoglycans in the eye are crucial for vision by maintaining tissue structure and communication.

## Contribution

The paper provides a comprehensive overview of the structural and functional roles of various ocular proteoglycans in visual processing.

## Key findings

- Ocular proteoglycans like SPACRCAN and hyaluronan protect photoreceptors from oxidative stress.
- Pikachurin and other heparan sulfate proteoglycans stabilize synapses and support visual signal transmission.
- Small leucine-rich repeat proteoglycans maintain corneal transparency and tissue stability.

## Abstract

This narrative review outlines the structure and essential functions of ocular proteoglycans (PGs) in visual processing as documented in the extensive literature on this subject matter. The eye, as one of the most complex sensory organs, relies on the coordinated activity of various tissues and cell types, with PGs playing a central role in facilitating communication and maintaining tissue function. These molecules stabilise ocular tissues; for example, SPACRCAN (IMPG2) and hyaluronan aggregates in the interphotoreceptor matrix protect photoreceptors from oxidative stress. Specialised heparan sulfate PGs, such as pikachurin, eyes-shut, and the neurexin family, stabilise synapses and ensure synaptic specificity and plasticity. Pikachurin is particularly important for the rapid transmission of visual signals at the bipolar ribbon synapse. A diverse array of chondroitin sulfate (aggrecan, versican, neurocan, brevican, phosphacan, NG2), keratan sulfate (SV2), and heparan sulfate (perlecan, agrin, collagen XVIII) PGs are differentially expressed in ocular tissues, contributing to tissue stability and homeostasis. In the cornea, sclera, and choroid, small leucine-rich repeat PGs (SLRPs) maintain three-dimensional structure, corneal transparency, and tissue function through interactions with cytokines and growth factors. The vitreous humour contains opticin and nyctalopin, which support the nutrition of avascular regions and facilitate bipolar ribbon synapse signalling. Ultimately, the effectiveness of the eye as a visual organ depends significantly on the functional roles of its constituent PGs.