F5  CATHEPSIN L IS IMPORTANT FOR CATAGEN PROGRESSION, ENTRY INTO ANAGEN AND THE FORMATION OF THE INNER ROOT SHEATH AND TELOGEN CLUB HAIR.

¹Tobin DJ, ²Foitzik K, ²Mecklenburg ³L, ³Reinheckel T, ⁴Botchkarev VA, ³Peters C, ²Paus R. ¹Dept. of Biomedical Sciences, Univ. of Bradford, England, ²Depts. of Dermatology, Univ. Hosp. Eppendorf, ³Univ. of Hamburg, Germany. ⁴Boston Univ., Boston, USA, ³Inst.of Mol. Medicine, ⁴Univ. of Freiburg, Germany.

Cathepsin L (CTSL) is a ubiquitous lysosomal proteinase involved in skin and hair follicle (HF) homeostasis. Knockout mice exhibit massive shedding of their first hair coat, followed by regrowth and renewed partial hair loss during subsequent hair cycles. Here we examined cell proliferation and apoptosis by Ki67/TUNEL double immunofluorescence in wildtype and knockout mice during HF regression (catagen) and regrowth (anagen). In addition, the expression of NT-3/TrkC and p75 was investigated. The catagen-telogen-anagen transitions were analysed by high resolution light microscopy and transmission electron microscopy.

Apoptosis-associated HF regression was well advanced in 19 day old wildtype mice. By contrast, CTSL-L deficient mice exhibited significantly lower levels of HF apoptosis in the outer root sheath while very high levels of Ki67-IR were present in the epidermis and distal HFs. Indeed, proliferating cells were still readily detectable even in the outer root sheath of the lower HF in CTSL-/- mice. Overall NT-3 expression was increased in HFs of CTSL-deficient mice. By p25 massive proliferation, resulting in a significant advance in anagen progression (anagen IV-VI), was observed in CTSL -/- mice, at a time when wildtype HFs were just entering early anagen. Ultrastructurally, HFs in CTSL-/- mice exhibited abnormal, excessively bulbous club hairs during telogen. The keratinized rootlets that anchor club hairs to surrounding germ cells were incompletely formed, compromising hair shaft mooring in the HF. These traits were associated with severe defects in inner root sheath (IRS) differentiation and desquamation which resulted in part from incomplete IRS cell hardening. Abnormal IRS desquamation was also associated with marked dilatation of the hair canal and disrupted sebum secretion. The first genuine hair cycle exhibited much less HF pathology, yet anagen was remarkably advanced in 28 day-old CTSL -/- mice. Transient defects in hair bulb melanocytes and differentiating cortical keratinocytes of CTSL -/- mice included vacuolation and the increased formation of lysosomes respectively. These observations imply that CSTL plays previously unrecognised functionally important roles during normal HF development and cycling.