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Conditional N-Myc deletion limits the proliferation of granule neuron progenitors (GNPs), perturbs foliation, and leads to reduced cerebellar mass. We show that c-Myc mRNA levels increase in N-Myc-null GNPs and that simultaneous deletion of both c- and N-Myc exacerbates defective cerebellar development. Moreover, N-Myc loss has been shown to trigger the precocious expression of two cyclin-dependent kinase inhibitors, Kip1 and Ink4c, in the cerebellar primordium. We now further demonstrate that the engineered disruption of the Kip1 and Ink4c genes in N-Myc-null cerebella partially rescues GNP cell proliferation and cerebellar foliation. These results provide definitive genetic evidence that expression of N-Myc and concomitant down-regulation of Ink4c and Kip1 contribute to the proper development of the cerebellum. Myc | N-Myc deficiency | cerebellum | granule neuron progenitor | cell cycle inhibitor

Polymerase delta interacting protein 38 (PDIP38) was identified as a human DNA polymerase (pol) delta interacting protein through a direct interaction with p50, the small subunit of human pol delta. PDIP38 was also found to interact with proliferating cell nuclear antigen, which suggested that it might play a role in vivo in the processes of DNA replication and DNA repair in the nucleus. In order to characterize further this novel protein, we have examined its subcellular localization by the use of immunochemical and cellular fractionation techniques. These studies show that PDIP38 is a novel mitochondrial protein and is localized mainly to the mitochondria. PDIP38 was shown to possess a functional mitochondrial targeting sequence that is located within the first 35 N-terminal amino acid residues. The mature PDIP38 protein is about 50 amino acid residues smaller than the full-length precursor PDIP38 protein, consistent with it being processed by cleavage of the mitochondrial targeting sequence during entry into the mitochondria. His-tagged mature PDIP38 inhibited pol delta activity in vitro and interacted with human papillomavirus 16 E7 oncoprotein, suggesting that PDIP38 might play a role in the pol delta-mediated viral DNA replication. Although the localization of PDIP38 to the mitochondria suggests that it serves functions within the mitochondria, we cannot eliminate the possibility that it may be involved in pol delta-mediated DNA replication or DNA repair under certain conditions such as viral infection.

Background: Hypersecretion of the precursor of the lysosomal protease cathepsin D (procathepsin D) has been implicated in the invasive phenotype of human breast cancer. However, the mechanism of the abnormal secretion of procathepsin D remains unclear. Since the mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) plays a central role in the intracellular transport and endocytosis of M6P-containing lysosomal enzymes, a deficiency in functional M6P/IGF2R may underlie the hypersecretion of procathepsin D in invasive tumors., Material/methods: In the present study, we compared the profiles of cathepsin D and the M6P/IGF2R between the highly invasive MDA-MB-231 and the non-invasive MCF-7 cell lines., Results: MDA-MB-231cells were confirmed to secrete a much larger proportion of procathepsin D into the medium than MCF-7 cells. Addition of M6P to the culture medium significantly altered the secretion of procathepsin D by MCF-7 cells, but had little effect on cathepsin D distribution in MDA-MB-231cells. Both the M6P-binding capacity and the endocytosis of exogenous M6P-bearing proteins in MDA-MB-231 cells were far less than those in MCF-7 cells. mRNA analysis indicated that the levels of the M6P/IGF2R mRNA in MDA-MB-231 cells were not lower but were even higher than that in MCF-7 cells. Sequence analysis indicated a difference in the 3'-untranslated region of M6P/IGF2R between the two cell lines, but no mutation in the M6P-binding domain of the receptor., Conclusions: The results suggest that a potential defect in a post-transcriptional process (e.g., translation) may exist during synthesis of the M6P/IGF2R in MDA-MB-231cells, leading to failure to express sufficient functional M6P/IGF2R and thereby resulting in the hypersecretion of procathepsin D.