Synonymous variants in HTRA1 implicated in AMD susceptibility impair its capacity to regulate TGF-β signaling

High-temperature requirement A1 (HTRA1) is a secreted serine protease reported to play a role in the development of several cancers and neurodegenerative diseases. Still, the mechanism underlying the disease processes largely remains undetermined. In age-related macular degeneration (AMD), a common cause of vision impairment and blindness in industrialized societies, two synonymous polymorphisms (rs1049331:C>T, and rs2293870:G>T) in exon 1 of the HTRA1 genewere associated with a high risk to develop disease. Here, we show that the two polymorphisms result in a protein with altered thermophoretic properties upon heat-induced unfolding, trypsin accessibility and secretion behavior, suggesting unique structural features of the AMD-risk-associated HTRA1 protein. Applying MicroScale Thermophoresis and protease digestion analysis, we demonstrate direct binding and proteolysis of transforming growth factor beta 1 (TGF-beta 1) by normal HTRA1 but not the AMD-risk-associated isoform. As a consequence, both HTRA1 isoforms strongly differed in their ability to control TGF-beta mediated signaling, as revealed by reporter assays targeting the TGF-beta 1-induced serpin peptidase inhibitor (SERPINE1, alias PAI-1) promoter. In addition, structurally altered HTRA1 led to an impaired autocrine TGF-beta signaling in microglia, as measured by a strong down-regulation of downstream effectors of the TGF-beta cascade such as phosphorylated SMAD2 and PAI-1 expression. Taken together, our findings demonstrate the effects of two synonymous HTRA1 variants on protein structure and protein interaction with TGF-beta 1. As a consequence, this leads to an impairment of TGF-beta signaling and microglial regulation. Functional implications of the altered properties on AMD pathogenesis remain to be clarified.