EXOSOME TREATMENT OF RGCs

 

Mesenchymal stem cells (MSC) are self-replicating multipotent stromal cells isolated from mesenchymal tissues such as bone marrow (BMSC), adipose tissues, dental pulp and umbilical cord blood as well as other tissues.

MSCs promote the protection and regeneration of central nervous system (CNS) neurons, which lack the capacity to regenerate, or be replaced following their loss. The retina is an outgrowth of the brain and is thus part of the CNS and subject to the same regenerative limitations.

MSCs secrete exosomes, which are endocytic-derived structures composed of proteins, lipids, and mRNA surrounded by a phospholipid bi-layer that are secreted into the extracellular space. Their size ranges from 30 to 100 nm. Exosomes contain mRNA and miRNA, which are both functional and, when delivered to another cell via fusion with the cell membrane, lead to the translation of new proteins.

They can be easily stored and do not proliferate, making the application of specific doses easier. Due to their smaller size, they are also capable of migrating into the ganglion cell layer (GCL) from the vitreous (unlike transplanted cells) and delivering their content to the retinal ganglion cells (RGC). The surrounding phospholipid bilayer of exosomes protects the contents against degradation and makes them immunologically inert, qualities important for a therapeutic delivery system.

Treatment of primary retinal cultures with BMSC-exosomes demonstrated significant neuroprotective and neuritogenic effects. BMSC-derived exosomes promoted statistically significant survival of RGCs and regeneration of their axons while partially preventing RGC axonal loss and RGC dysfunction.

After optic nerve crush injury, MSCs transplanted into the vitreous are able to promote significant neuroprotection of RGCs and moderate regeneration of their axons. In animal models of glaucoma, MSCs promote the survival of RGCs and their axons and preserve their function.

The mechanism of exosome-derived neuroprotection is apparently through a paracrine-mediated effect with secreted factors being necessary.

In culture, MSC are efficacious when cocultured (yet physically separated) from the injured retinal cells. The assumption that neurotrophic growth factors (NTF) are important is corroborated both by the expansive NTF rich secretome of MSC and by the attenuated neuroprotective and neuritogenic effects when particular NTF receptors are inhibited. Secreted NTF such as platelet-derived growth factor and brain-derived neurotrophic factor have been shown to be important to the neuroprotection of RGCs whereas MSC mediated-neuritogenesis depended more on nerve growth factor. Other secreted factors, such as Wnt3a have been implicated in the neuroprotective effect of MSC on CNS neurons.

Transplantation into the vitreous of healthy and diseased eyes yields no evidence of differentiation or migration/integration into retinal tissue, strongly implicating paracrine over cell replacement as the dominant mechanism.

Exosomes offer a cell-free alternative to BMSC therapy, which can be easily isolated, purified and stored. They lack the risk of complications associated with transplanting live cells into the vitreous (immune rejection, unwanted proliferation/differentiation).

A limitation of exosome-related treatment is that regeneration is only significant at short distances from the lesion site (<1 mm) limiting its potential at promoting functional reconnection of the visual pathway. It is currently unknown what the ideal timeframe for treatment is, whether a single injection of exosomes is sufficient or weekly/bi-weekly/monthly injections are required.