Increased Fibronectin Serotonylation in Stretched Optic Nerve Head Astrocytes

Purpose: Elevated intraocular pressure contributes to glaucomatous optic nerve degeneration by inducing biomechanical stress at the optic nerve head (ONH), especially in the lamina cribrosa (LC). ONH astrocytes (ONHA) in the LC respond to biomechanical signals through extracellular matrix (ECM) remodeling activities, promoting tissue fibrosis and damage to retinal ganglion cell axons. The enzyme transglutaminase 2 (TG2) plays a role in ECM remodeling, in part, due to its ability to post-translationally modify and cross-link ECM proteins. A unique post-translational modification mediated by TG2 is "serotonylation” - the transamidation of the monoamine serotonin (5-hydroxytryptamine, 5HT) to glutamine residues on proteins. It is speculated that serotonylation contributes to fibrotic tissue remodeling, but this process has not been studied in ocular tissues or in primary glial cells. In this study, we examined changes in the serotonylation of fibronectin (FN; a major ECM glycoprotein) by ONHA after exposure to cyclic stretch.

Methods: Primary human ONHA strains (n=3) were exposed to 0-12% cyclic stretch for 24h using a FlexCell FX-6000 system. Cell lysates and conditioned medium samples were collected from stretched and control cells. Serotonylation was assessed by probing for serotonin in samples of FN immunoprecipitated out of conditioned media. Protein levels for potential extra- and intra-cellular mediators of serotonylation were examined using western blotting of concentrated conditioned medium samples and cell lysates, respectively.

Results: Serotonylated fibronectin was detected in ONHA. Exposure to stretch increased the amount of fibronectin that was serotonylated by 2.49-fold (p=0.0080). After stretching, extracellular FN levels were not changed. Extracellular TG2 levels were increased by 3.76-fold (p=0.0004). In cell lysates, post-stretch levels of both FN and TG2 were decreased by 5.56-fold (p=0.0181) and 2.51-fold (p=0.0441), respectively. Additionally, serotonin 2A and 2C (5HT2A, 5HT2C) receptor levels were unchanged, and serotonin transporter (SERT) levels decreased by 2.94-fold (p=0.0297).

Conclusions: Increased FN serotonylation by TG2 is observed in ONHA after exposure to 24h of 0-12% cyclic stretch. Serotonylation may promote increased FN-crosslinking and fibrotic ECM remodeling, an important feature of glaucomatous pathology. The secreted TG2 – which was elevated in response to stretch – is likely to be the primary mediator of this increased serotonylation. The observed decrease in SERT may lead to increased extracellular 5HT levels, which increases the substrate availability for TG2-mediated serotonylation. Though unchanged, activity at the 5HT2A/C G-coupled protein receptors could increase the availability of intracellular calcium required for TG2 activity. Future experiments will be focused on furthering our understanding of how these proteins may interact to promote serotoynlation.