Cyclin-dependent kinase 5 (Cdk5) and its regulatory subunit p35 are integral players in the proper development of the mammalian central nervous system. Proteolytic cleavage of p35 generates p25, leading to aberrant Cdk5 activation. The accumulation of p25 is implicated in several neurodegenerative diseases. In primary neurons, p25 causes apoptosis and tau hyperphosphorylation. Current mouse models expressing p25, however, fail to rigorously recapitulate these phenotypes in vivo. Here, we generated inducible transgenic mouse lines overexpressing p25 in the postnatal forebrain. Induction of p25 preferentially directed Cdk5 to pathological substrates. These animals exhibited neuronal loss in the cortex and hippocampus, accompanied by forebrain atrophy, astrogliosis, and caspase-3 activation. Endogenous tau was hyperphosphorylated at many epitopes, aggregated tau accumulated, and neurofibrillary pathology developed progressively in these animals. Our cumulative findings provide compelling evidence that in vivo deregulation of Cdk5 by p25 plays a causative role in neurodegeneration and the development of neurofibrillary pathology.

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Niemann-Pick type C (NPC) is a disease that affects intracellular cholesterol-trafficking pathways. By cloning the hamster ortholog of NPC1, we identified the molecular lesions in two independently isolated Chinese hamster ovary cell mutants, CT60 and CT43. Both mutants lead to premature translational terminations of the NPC1 protein. Transfecting hamster NPC1 cDNA complemented the defects of the mutants. Investigation of the CT mutants, their parental cells, and an NPC1-stable transfectant allow us to present evidence that NPC1 is involved in a post-plasma membrane cholesterol-trafficking pathway. We found that the initial movement of low density lipoprotein (LDL)-derived cholesterol to the plasma membrane (PM) did not require NPC1. After reaching the PM and subsequent internalization, however, cholesterol trafficking back to the PM did involve NPC1. Both LDL-derived cholesterol and cholesterol originating from the PM accumulated in a dense, intracellular compartment in the CT mutants. Cholesterol movement from this compartment to the PM or endoplasmic reticulum was defective in the CT mutants. Our results functionally distinguish the dense, intracellular compartment from the early endocytic hydrolytic organelle and imply that NPC1 is involved in sorting cholesterol from the intracellular compartment back to the PM or to the endoplasmic reticulum.

Aberrant processing of the amyloid precursor protein (APP) and the subsequent accumulation of amyloid ␤ (A␤) peptide has been widely established as a central event in Alzheimer's disease (AD) pathogenesis. The sequential cleavage steps required for the generation of A␤ are well outlined; however, there is a relative dearth of knowledge pertaining to signaling pathways and molecular mechanisms that can modulate this process. Here, we demonstrate a novel role for p25/cyclin-dependent kinase 5 (Cdk5) in regulating APP processing, A␤ peptide generation, and intraneuronal A␤ accumulation in inducible p25 transgenic and compound PD-APP transgenic mouse models that demonstrate deregulated Cdk5 activity and a neurodegenerative phenotype. Induction of p25 resulted in enhanced forebrain A␤ levels before any evidence of neuropathology in these mice. Intracellular A␤ accumulated in perinuclear regions and distended axons within the forebrains of these mice. Evidence for modulations in axonal transport or ␤-site APP cleaving enzyme 1 protein levels and activity are presented as mechanisms that may account for the A␤ accumulation caused by p25/Cdk5 deregulation. Collectively, these findings delineate a novel pathological mechanism involving aberrant APP processing by p25/Cdk5 and have important implications in AD pathogenesis.