Mammalian hemicentin 1 is assembled into tracks in the extracellular matrix of multiple tissues

Background Hemicentins (HMCNs) are a family of extracellular matrix proteins first identified in Caenorhabditis elegans, with two orthologs (HMCN1 and 2) in vertebrates. In worms, HMCN is deposited at specific sites where it forms long, fine tracks that link two tissues by connecting adjacent basement membranes (BMs). By generating CRISPR/Cas9‐mediated Hmcn1 and Hmcn2 knockout mice, we tested the hypothesis that HMCNs perform similar functions in mammals. Results Hmcn1 −/− mice were viable and fertile. Using new, knockout mouse‐validated HMCN1 antibodies, HMCN1 was detected in wild‐type mice as fine tracks along the BM of hair and whisker follicles, in the sclera of the eyes, and in the lumen of some lymphoid conduits. It was also observed in the mesangial matrix of the kidney glomerulus. However, HMCN1 deficiency did not affect the functions of these tissues, including adherence of coat hairs and whiskers, the sieving function of lymphoid conduits, or the immune response to injected antigens. HMCN2 deficiency did not lead to any discernible phenotypes on its own or when combined with HMCN1 deficiency. Conclusion That Hmcn1 −/−, Hmcn2 −/−, and Hmcn1/2 double knockout mice did not display any overt phenotypes implicates compensation by other members of the fibulin family. Key Findings New, authenticated antibodies to hemicentin‐1 demonstrate that it assembles into the extracellular matrix in track patterns, similar to the pattern in C. elegans. Mice are viable without hemicentin‐1, hemicentin‐2, or both. Hemicentin‐1 is identified as a new component of some lymphoid conduits. Hemicentin‐1 is detected in eye, hair, whisker pads, and kidney mesangium.