Exosomes and their miRNA/protein profile in keratoconus-derived corneal stromal cells

Keratoconus (KC) is a corneal thinning disorder and a leading cause of corneal transplantation worldwide. Exosomes are small, secreted extracellular vesicles (30-150 nm) that mediate cellular communication via their protein, lipid, and nucleic acid content. We aimed to characterize the exosomes secreted by primary corneal fibroblasts from subjects with or without KC. Using human keratoconus stromal fibroblast cells (HKC, n = 4) and healthy stromal fibroblasts (HCF, n = 4), we collected and isolated exosomes using serial ultracentrifugation. Using nanoparticle tracking analysis (NTA) with ZetaView®, we compared the size and concentration of isolated exosomes. Different exosomal markers were identified and quantified using a transmission electron microscope (TEM) (CD81) and Western blot (CD9 and CD63). Exosomal miRNA profiles were determined by qRT-PCR using Exiqon Human panel I miRNA assays of 368 pre-selected miRNAs. Proteomic profiles were determined using a label-free spectral counting method with mass spectrometry. Differential expression analysis for miRNAs and proteins was done using student's t-test with a significance cutoff of p-value ≤0.05. We successfully characterized exosomes isolated from HCFs using several complementary techniques. We found no significant differences in the size, quantity, or morphology between exosomes secreted by HCFs with or without KC. Expression of CD81 was confirmed by immuno-EM, and expression of CD63 and CD9 with western blots in all exosome samples. We detected the expression of 72-144 miRNAs (threshold cycle Ct < 36) in all exosome samples. In HKC-derived exosome samples, miR-328-3p, miR-532-5p, miR-345-5p, and miR-424-5p showed unique expression, while let-7c-5p and miR-665 have increased expression. Protein profiling identified 157 proteins in at least half of the exosome samples, with 38 known exosomal proteins. We identified 12 up- and 2 down-regulated proteins in HKC-derived exosomes. The proteins are involved in membrane-bounded vesicles, cytoskeletal, calcium binding, and nucleotide binding. These proteins are predicted to be regulated by NRF2, miR-205, and TGF-β1, which are involved in KC pathogenesis. We successfully characterized the HKC-derived exosomes and profiled their miRNA and protein contents, suggesting their potential role in KC development. Further studies are necessary to determine if and how these exosomes with differential protein/miRNA profiles contribute to the pathogenesis of KC.