Claudin-1 and-3 maintain proper hair follicle structure and regulate telogen effluvium

Claudins (CLDNs) are major transmembrane proteins of tight junctions (TJs), which seal the intercellular space of epithelial cells. Patients with a nonsense mutation in CLDN1 gene develop ichthyosis and scalp hair loss. The importance of TJs in epidermal barrier formation is well-known; however, little is known about how TJs affect the homeostasis and structure of hair follicles (HFs). To tackle this issue, we investigated the impact of TJs on retaining hairs.In this study, murine back skin samples were immunohistochemically stained for CLDN1 and CLDN3 to detect their localization in HFs. Additionally, we conducted daily skin observation and histological analysis in wild-type (WT), Cldn1 knockdown (1KD), Cldn3 knockout (3KO) and double mutant with 1KD and 3KO (1KD3KO) mice. Both CLDN1 and CLDN3 were localized in inner bulge cell layer, isthmus, infundibulm and sebaceous gland. Although 1KD or 3KO single mutant mice showed normal hair coat, 1KD3KO double mutant mice lost almost all hairs in their first telogen between postnatal day 20 (P20) and P25. As for the structure of HFs, the bulge region, whose inner layer cells sustain telogen hairs, in WT mice was composed of well-organized two cell layers and was localized below sebaceous glands. On the other hand, 1KD3KO bulge region was enlarged with disorganized three or four cell layers and was localized closer to the skin surface, adjacent to sebaceous glands. Furthermore, hair canals of isthmus and infundibulm in 1KD3KO mice were widened. Taken together, our findings indicate that CLDN1 and CLDN3 are critical for proper HF structure, especially cell alignment and localization of bulge region, thus preventing telogen hair loss. Induction or maintenance of CLDN1 and CLDN3 may become a new treatment target for patients with alopecia.