Sc65-Null Mice Provide Evidence for a Novel Endoplasmic Reticulum Complex Regulating Collagen Lysyl Hydroxylation

Collagen is a major component of the extracellular matrix and its integrity is essential for connective tissue and organ function. The importance of proteins involved in intracellular collagen post-translational modification, folding and transport was recently highlighted from studies on recessive forms of osteogenesis imperfecta (OI). Here we describe the critical role of SC65 (Synaptonemal Complex 65, P3H4), a leprecan-family member, as part of an endoplasmic reticulum (ER) complex with prolyl 3-hydroxylase 3. This complex affects the activity of lysyl-hydroxylase 1 potentially through interactions with the enzyme and/or cyclophilin B. Loss of Sc65 in the mouse results in instability of this complex, altered collagen lysine hydroxylation and cross-linking leading to connective tissue defects that include low bone mass and skin fragility. This is the first indication of a prolyl-hydroxylase complex in the ER controlling lysyl-hydroxylase activity during collagen synthesis.
Author Summary Fibrillar collagens are major components of connective tissue extracellular matrix (ECM). Among them, type I collagen is the most abundant protein in the human body and a large constituent of bone, dermis, tendon and ligament ECMs; type I collagen is also present in the stroma of other organs including heart, lung and kidney where, when dysregulated, it significantly contributes to pathological fibrosis. Type I and other collagen molecules have triple-helical folding requirements and undergo numerous intracellular post-translational modifications in the endoplasmic reticulum (ER) and Golgi apparatus. We and others have shown that alterations/loss of specific collagen modifications can lead to severe congenital disease such as osteogenesis imperfecta (OI). Here, using a multidisciplinary approach, we describe functional studies of the SC65 protein (Synaptonemal Complex 65 or P3H4), a poorly characterized member of the Leprecan gene family of proteins. We provide evidence that SC65 is a critical component of an ER complex with prolyl 3-hydroxylase 3 (P3H3), lysyl-hydroxylase 1 (LH1), and potentially cyclophilin B (CYPB). Loss of Sc65 in the mouse results in instability of this complex, site-specific reduction in collagen lysine hydroxylation and connective tissue defects including osteopenia and skin fragility.