Cathepsin D (Ctsd) is a ubiquitously expressed aspartic protease functioning primarily in the acidic endosomal/lysosomal cell compartment. At postnatal day 26, mice with constitutive Ctsd-deficiency die from neurodegeneration equivalent to the congenital neuronal ceroid lipofuscinosis (NCL) type 10 in humans. This lethal phenotype hindered addressing the role of Ctsd in important physiological or pathological processes in adult animals, such as the long-suspected role for Ctsd in promotion of breast cancer. To overcome these limitations we developed a conditional knock-out allele for Ctsd using the Cre/loxp technology, which allows for selective spatial and/or temporal inactivation of the gene in vivo.
First, we compared a ubiquitous Ctsd deletion with a deletion in cells of neuroectodermal origin, e.g. in neurons and astroglia. The absence of Ctsd in the respective cell- and tissue types was confirmed by Western Blot and immunohistochemistry. 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. These results are in support of a critical NCL-causing role for Ctsd deletion in neurons. Remaining Ctsd in other cell types of the brain has only a modifying impact on the disease process.
In a second - ongoing - approach, we crossed conditional Ctsd knock-out mice to the transgenic MMTV-PyMT breast cancer mice. We used transgenic mice with specific Cre-recombinase expression pattern to delete Ctsd from mammary cancer cells or from myeloid cells/macrophages. These mouse lines are not affected by neurodegeneration and are currently investigated for cancer development and metastasis up to an age of 6 months.
In summary, the new conditional Ctsd knock-out mouse not only demonstrates that type 10 NCL is initiated by cells of neuroectodermal origin, but will also help to further study tissue-specific functions of Ctsd in vivo.