Contribution of extracellular vesicles for the pathogenesis of retinal diseases: shedding light on blood-retinal barrier dysfunction

Abstract

Retinal degenerative diseases, including diabetic retinopathy (DR) and age-related macular degeneration (AMD), loom as threats to vision, causing detrimental effects on the structure and function of the retina. Central to understanding these diseases, is the compromised state of the blood-retinal barrier (BRB), an effective barrier that regulates the influx of immune and inflammatory components. Whether BRB breakdown initiates retinal distress, or is a consequence of disease progression, remains enigmatic. Nevertheless, it is an indication of retinal dysfunction and potential vision loss.The intricate intercellular dialogues among retinal cell populations remain unintelligible in the complex retinal milieu, under conditions of inflammation and oxidative stress. The retina, a specialized neural tissue, sustains a ceaseless demand for oxygen and nutrients from two vascular networks. The BRB orchestrates the exchange of molecules and fluids within this specialized region, comprising the inner BRB (iBRB) and the outer BRB (oBRB). Extracellular vesicles (EVs) are small membranous structures, and act as messengers facilitating intercellular communication in this milieu.EVs, both from retinal and peripheral immune cells, increase complexity to BRB dysfunction in DR and AMD. Laden with bioactive cargoes, these EVs can modulate the retinal microenvironment, influencing disease progression. Our review delves into the multifaceted role of EVs in retinal degenerative diseases, elucidating the molecular crosstalk they orchestrate, and their microRNA (miRNA) content. By shedding light on these nanoscale messengers, from their biogenesis, release, to interaction and uptake by target cells, we aim to deepen the comprehension of BRB dysfunction and explore their therapeutic potential, therefore increasing our understanding of DR and AMD pathophysiology.

Keywords: Age-related macular degeneration; Blood-retinal barrier; Diabetic retinopathy; Extracellular vesicles; Retinal degenerative diseases; miRNA.

Fig. 1

Overview of different subtypes of extracellular vesicles. The various subtypes of extracellular vesicles (EVs) differ from each other in their biogenesis’ pathways and functions. The exosomes are the smallest vesicles, are formed within multivesicular bodies (MVBs) and then released upon MVB fusion with the plasma membrane. Exosomes are enriched with markers such ESCRT-related proteins, tetraspanins or flotillins. Microvesicles bud directly from the plasma membrane and often carry markers like Annexin A1, integrins, and selectins. Protrusion-derived vesicles generate at the tips of membrane protrusions, such as filopodia and microvilli, and often carry CD133 in their content. Migrasomes are a specialized subtype of EVs associated with cell migration that are formed from the disassembly of actin-based membrane protrusions and can be identified by the presence of TSPAN4. Finally, secreted amphisomes and secreted autophagosomes are vesicles originated by the fusion of autophagosomes with MVBs that are released to the extracellular compartment or by the release of autophagosomes, respectively. On both cases, LC3 is a protein commonly associated with these subtypes of EVs. This comprehensive overview of EVs subtypes, illustrated in this schematic representation of a general eukaryotic cell, emphasizes their remarkable diversity and underscores their crucial roles in intercellular communication, contributing to a wide array of physiological and pathological processes. Created with BioRender.com

Fig. 2

Contribution of extracellular vesicles to age-related macular degeneration pathogenesis. The extracellular vesicles (EVs) have a multifaceted role in the pathogenesis of age-related macular degeneration (AMD), being central players in several critical processes contributing to AMD progression. It is described the presence of EVs co-localized with fibulin-3 within drusen deposits, a classical hallmark of AMD, highlighting their involvement in the disease’s early stages. Once submersed within an oxidative stress and pro-inflammatory environment, retinal pigment epithelial (RPE) cells release EVs that can contribute to the progression of the disease. These RPE-derived EVs are involved in the activation of the immune system, since EVs carrying damaged mitochondrial DNA (mtDNA) have the potential to activate microglia. Their involvement in the activation of the immune system is also related to their ability to participate in the recruitment of macrophages to the site of AMD lesions. Along with these, RPE-derived EVs that carry complement 3 (C3) also activate the inflammasome and the complement system, leading to the release of pro-inflammatory cytokines which consequently contributes to the inflammatory milieu of the AMD microenvironment. Moreover, these EVs can also act in an autocrine manner, being directly involved not only in the disruption of tight junction (TJ) complexes and in the breakdown of the outer blood-retinal barrier (oBRB), carrying histone deacetylase 6 (HDAC6), but also in the dysfunction of other RPE cells and in the induction of the endothelial-mesenchymal transition (EMT) that often occurs in the RPE monolayer, leading to subretinal fibrosis and the formation of hyperreflective foci present in AMD patients. Ultimately, RPE-derived EVs are also implicated in angiogenesis of choroidal vessels, a pivotal event in wet AMD pathogenesis, since they carry pro-angiogenic cargo such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase 9 (MMP-9). All of these events highlight the pivotal role of EVs in driving various AMD-related processes, making them attractive targets for therapeutic interventions in the management of AMD. Created with BioRender.com

Fig. 3

Contribution of extracellular vesicles to the pathogenesis of diabetic retinopathy. Extracellular Vesicles (EVs), both circulating EVs and retinal cells-derived EVs, play critical roles in several key processes associated with the pathogenesis of diabetic retinopathy (DR). Particularly, circulating EVs derived from mesenchymal stem cells (MSCs) that have matrix metalloproteinase 2 (MMP-2) in their cargo, are central players in the detachment and loss of pericytes, an early hallmark event in DR, which leads to microvascular dysfunction. These MMP-2-containing EVs are also known to be involved in the breakdown of the inner blood-retinal barrier (iBRB) and in the angiogenic process, both important hallmarks of DR. Peroxisome proliferator-activated receptor gamma (PPARγ)-containing EVs have also an important role in the endothelial cell proliferation. Additionally, circulating EVs containing IgG as well as preadipocyte-derived EVs positive for LINC00968 (long intergenic non-protein coding RNA 968) and EVs containing C-X-C Motif Chemokine Ligand 10 (CXCL10) contribute to endothelial cell dysfunction. Moreover, retinal cells-derived EVs, such as EVs secreted by pericytes containing cPWW2P2A and reactive astrocytes-derived EVs also contribute to the progression of DR, since they exacerbate endothelial cell dysfunction and angiogenesis thereby contributing to the disruption of retinal microvascular integrity. Created with BioRender.com