The Lysosomal Protease Cathepsin L Is an Important Regulator of Keratinocyte and Melanocyte Differentiation During Hair Follicle Morphogenesis and Cycling

Abstract

We have previously shown that the ubiquitously expressed lysosomal cysteine protease, cathepsin L (CTSL), is essential for skin and hair follicle homeostasis. Here we examine the effect of CTSL deficiency on hair follicle development and cycling in ctsl ؊/؊ mice by light and electron microscopy, Ki67/ terminal dUTP nick-end labeling, and trichohyalin immunofluorescence. Hair follicle morphogenesis in ctsl ؊/؊ mice was associated with several abnormalities. Defective terminal differentiation of keratinocytes occurred during the formation of the hair canal, resulting in disruption of hair shaft outgrowth. Both proliferation and apoptosis levels in keratinocytes and melanocytes were higher in ctsl ؊/؊ than in ctsl ؉/؉ hair follicles. The development of the hair follicle pigmentary unit was disrupted by vacuolation of differentiating melanocytes. Hair cycling was also abnormal in ctsl ؊/؊ mice. Final stages of hair follicle morphogenesis and the induction of hair follicle cycling were retarded. Thereafter, these follicles exhibited a truncated resting phase (telogen) and a premature entry into the first growth phase. Further abnormalities of telogen development included the defective anchoring of club hairs in the skin, which resulted in their abnormal shedding. Melanocyte vacuolation was again apparent during the hair cycleassociated reconstruction of the hair pigmentary unit. A hallmark of these ctsl ؊/؊ mice was the severe disruption in the exiting of hair shafts to the skin surface. This was mostly because of a failure of the inner root sheath (keratinocyte layer next to the hair shaft) to fully desquamate. These changes resulted in a massive dilation of the hair canal and the abnormal routing of sebaceous gland products to the skin surface. In summary, this study suggests novel roles for cathepsin proteases in skin, hair, and pigment biology. Principal target tissues that may contain protein substrate(s) for this cysteine protease include the developing hair cone, inner root sheath, anchoring apparatus of the telogen club, and organelles of lysosomal origin (eg, melanosomes). The hair follicle (HF) is a unique neuroectodermal-mesodermal interactive organoid that results in the elaboration of at least 15 distinct interacting cell subpopulations, organized into five or six concentric cylinders.

Out-of-frame start codons prevent translation of truncated nucleo-cytosolic cathepsin L in vivo

“…Similar vesicular accumulations have already been found in MEFs, keratinocytes and cardiomyocytes of Ctsl À / À mice 11,15,45,46 . Most likely these vesicular accumulations represent a variation of classical storage disorders triggered by impaired lysosomal degradation 47 .…”

“…Cathepsin L-deficient (Ctsl À / À ) mice reproduce normally, although pups showed a slightly increased mortality during the weaning period 10 . The phenotype of Ctsl À / À mice is characterized by periodical loss of fur hair 10,11 , epidermal hyperplasia 10,12 , reduced numbers of CD4 þ -T cells 13,14 and dilated cardiomyopathy in aged animals 15,16 . Several studies revealed that cathepsin L affects tumour progression in cancer mouse models 17,18 .…”

Selective suppression of cathepsin L by antisense cDNA impairs human brain tumor cell invasion in vitro and promotes apoptosis

“…Tobin et al 37 found that apoptosis levels in keratinocytes and melanocytes were higher in cathepsin L( À /À ) than in cathepsin L( +/ +) hair follicles.…”

Cathepsin-L Influences the Expression of Extracellular Matrix in Lymphoid Organs and Plays a Role in the Regulation of Thymic Output and of Peripheral T Cell Number

“…There is accumulating evidence for specific functions of CTSL and other lysosomal proteases in health and disease (16 -22). The use of "knockout" (KO) or mutant mice deficient in CTSL has provided a valuable tool for gaining new insights into the in vivo functions of this protease (21)(22)(23)(24)(25)(26)(27).…”

“…In mice, CTSL has been shown to be critical for epidermal homeostasis, the regulation of the hair cycle (18,23), and the maintenance of the heart structure and function (24). A role for CTSL in trophoblast invasion (17), tumor metastasis (16), thyroid function (25), and mammary gland involution (19) has also been demonstrated.…”