Niemann-Pick type C (NPC) is a devastating autosomal recessive neurodegenerative disorder characterized by the accumulation of cholesterol and other lipids in viscera and the central nervous system. The clinical presentation typically includes progressive ataxia, dystonia, and dementia, first presenting in early childhood and ultimately leading to death in the early teens [1
]. NPC is one of over 40 known lysosomal storage disorders (reviewed in [2
]), which collectively have an incidence of one in 8,000 live births [4
]. The specific molecules that accumulate in each disease vary, but most of the disorders share the feature of prominent neurological symptoms. Though great progress has been made in characterizing the biochemical and genetic defects in these diseases, the pathways that lead from these defects to cell and tissue dysfunction are inadequately understood.
The decline in neurological function in NPC patients is caused by mutations in either the npc1
]. Npc1 is a multiple-membrane-spanning late endosomal/lysosomal protein that can bind cholesterol [8
] and acts as a transmembrane pump when expressed in bacteria [9
]. The Npc1 protein has a sequence related to the “sterol sensing domain” of known regulators of cholesterol metabolism (SCAP and HMG-CoA reductase) and Patched, a receptor for the secreted developmental signaling protein Hedgehog. The exact role of Npc1 in regulating lipid homeostasis and in maintaining neurological function is far from clear. Even less is known about Npc2, the small cholesterol-binding protein [10
] responsible for a minority (<5%) of NPC cases.
Phenotypes similar to human NPC are seen in two mouse strains, C57BLKS/J spm
] and BALB/c npc1nih
], both of which harbor spontaneous mutations in npc1
]. The most striking and well-documented histological change in npc1
mice is the progressive loss of cerebellar Purkinje cells (PCs) [15
]. Stereotactic cell counting confirmed that the PC is the type that exhibits the greatest percentage loss in npc1
mice (<10% remaining at 10 wk) but surprisingly revealed that glia in the corpus callosum have a greater early reduction in number (48% for glia versus 13% for PCs, at 3 wk) [17
]. This early decline in glia and the detection of Npc1 protein in astrocytic processes [18
] led to the idea that the primary function of the Npc1 protein may be in glia and that loss of PCs and other neurons may be a secondary consequence of glial dysfunction [17
]. However, Npc1 mRNA is abundantly present in neurons as well as glia throughout the brain [20
]. Thus, it is unclear which cells in the brain require Npc1 protein and why PCs degenerate in mice lacking Npc1.
Four broad categories of possible explanations for the PC degeneration have emerged (). (1) Npc1 is required within PCs and loss of the protein results in degeneration as a consequence of the toxic accumulation of lipids. High concentrations of unesterified cholesterol can cause cell death by altering membrane rigidity, forming intracellular crystals that may interfere with organelle function, or triggering apoptotic signaling events [21
]. (2) Npc1 is required within PCs and loss of the protein causes a deficiency at specific subcellular locations. In this model, the accumulation of cholesterol and other lipids is not harmful per se, but the diminished capacity to transport these or other molecules to the correct places within the cell leads to degeneration. (3) Npc1 is required within glia or another cell type within the cerebellum, or in a distant organ such as a secretory gland, and loss of the protein results in degeneration of PCs through a deficiency in a secreted molecule. This could be a trophic factor or possibly even cholesterol itself, as studies have shown that glia-derived cholesterol is required for synaptogenesis [22
]. (4) Loss of Npc1 function results in the release of toxic/pro-inflammatory compounds into the local environment or systemic circulation, causing PC degeneration.
Models of PC Degeneration in npc1 Mice
In this study, we employed chimeric mice to help resolve the fundamental mystery of why neurons degenerate in NPC disease. This powerful approach can distinguish intrinsic and extrinsic causes of cell death but has only rarely been used to study human neurodegenerative disease [23
]. The results indicate that PC loss in npc1−/−
mutants is a cell-autonomous process, i.e., Npc1 function is critical within PCs. Ultrastructural and biochemical analyses support the idea that this cell-autonomous loss is due to the activation of a particular genetic program, autophagy, that leads to cell death.