Bone morphogenetic protein 4 inhibits TGF-β2 stimulation of extracellular matrix proteins in optic nerve head cells: Role of gremlin in ECM modulation

The characteristic cupping of the optic nerve head (ONH) in glaucoma is associated with elevated TGF-β2 and increased synthesis and deposition of extracellular matrix (ECM) proteins. In addition to TGF-β2, the human ONH also expresses bone morphogenetic proteins (BMPs) and BMP receptors, which are members of the TGF-β superfamily. We examined the potential effects of BMP4 and the BMP antagonist gremlin on TGF-β2 induction of ECM proteins in ONH cells. BMP-4 dose dependently inhibited TGF-β2-induced fibronectin (FN) and PAI-1 expression in ONH astrocytes and lamina cribrosa (LC) cells and also reduced TGF-β2 stimulation of collagen I, collagen VI, and elastin. Addition of gremlin blocked this BMP-4 response, increasing cellular and secreted FN as well as PAI-1 levels in both cell types. Gremlin was expressed in ONH tissues and ONH cells, and gremlin protein levels were significantly increased in the LC region of human glaucomatous ONH tissues. Interestingly, recombinant gremlin dose dependently increased ECM protein expression in cultured ONH astrocytes and LC cells. Gremlin stimulation of ECM required activation of TGF-β receptor and R-Smad3. TGF-β2 increased gremlin mRNA expression and protein levels in ONH cells. Inhibition of either the type I TGF-β receptor or Smad3 phosphorylation blocked TGF-β2-induced gremlin expression. In conclusion, BMP4 blocked the TGF-β2 induction of ECM proteins in ONH cells. The BMP antagonist gremlin reversed this inhibition, allowing TGF-β2 stimulation of ECM synthesis. Increased expression of gremlin in the glaucomatous ONH may further exacerbate TGF-β2 effects on ONH ECM metabolism by inhibiting BMP-4 antagonism of TGF-β2 signaling. Modulation of the ECM via gremlin provides a novel therapeutic target for glaucoma.