Neuroectoderm-specific deletion of cathepsin D in mice models human inherited neuronal ceroid lipofuscinosis type 10.

Cathepsin D (Ctsd) is a ubiquitously expressed aspartic protease functioning primarily in the acidic endosomal/lysosomal cell compartment. At an age of 26 ± 1 days, mice with constitutive Ctsd deficiency (Ctsd(-/-)) die from a neurodegenerative lysosomal storage disease equivalent to the congenital neuronal ceroid lipofuscinosis (NCL) type 10 in humans. In addition to neurodegeneration, Ctsd(-/-) mice exhibit a loss of CD4(+)/CD8(+)-double-positive thymocytes and an atrophy of the intestinal mucosa. To date, it is not understood if and how these phenotypes are triggering each other. In addition, the cell type causing initiation of NCL in Ctsd(-/-) mice has not been identified yet. To investigate the tissue- and cell type-specific functions of Ctsd, we generated a novel conditional Ctsd allele by flanking the second exon with loxP sites. We compared a ubiquitous Ctsd deletion with a deletion of the protease by a Nestin-promoter controlled Cre-recombinase expression in cells of neuroectodermal origin, e.g. in neurons and astroglia, but not in microglia. First, we confirmed absence of Ctsd in the respective cell- and tissue types. The neuroectoderm specific knock-out mice survived about 5.5 days longer than the mice with ubiquitous Ctsd deletion, which was in line with the progress in brain histopathology. Atrophies of thymus and small intestine were delayed to similar extend. The conditional Ctsd knock-out mouse model established in this study not only demonstrates that this type of NCL is initiated by cells of neuroectodermal origin, but will also help to further study tissue-specific functions of Ctsd in vivo.

The lysosomal cysteine protease cathepsin L regulates keratinocyte proliferation by control of growth factor recycling.

Mice deficient for cathepsin L (CTSL) show epidermal hyperplasia due to a hyperproliferation of basal keratinocytes. Here we show that the critical function of CTSL in the skin is keratinocyte specific. This is revealed by transgenic re-expression of CTSL in the keratinocytes of ctsl-/- mice, resulting in a rescue of the ctsl-/- skin phenotype. Cultivation of primary mouse keratinocytes with fibroblast- and keratinocyte-conditioned media, as well as heterologous organotypic co-cultures of mouse fibroblasts and human keratinocytes, showed that the altered keratinocyte proliferation is caused primarily by CTSL-deficiency in keratinocytes. In the absence of EGF, wild type and CTSL-knockout keratinocytes proliferate with the same rates, while in presence of EGF, ctsl-/- keratinocytes showed enhanced proliferation compared with controls. Internalization and degradation of radioactively labeled EGF was identical in both ctsl-/- and ctsl+/+ keratinocytes. However, ctsl-/- keratinocytes recycled more EGF to the cell surface, where it is bound to the EGF-receptor, which is also more abundant in ctsl-/- cells. We conclude that the hyperproliferation of keratinocytes in CTSL-knockout mice is caused by an enhanced recycling of growth factors and growth factor receptors from the endosomes to the keratinocyte plasma membrane, which result in sustained growth stimulation.