BBA - Molecular and Cell Biology of Lipids (v.1685, #1-3)

Introduction by L. Liscum; S.L. Sturley (1).

Niemann–Pick C research from mouse to gene by Peter G. Pentchev (3-7).
Understanding the molecular basis of Niemann–Pick C (NP-C) disease took decades of struggle. Here I describe our early efforts to unravel the complex lipid storage found in NP-C tissues, and how the mouse model for NP-C pointed us in the right direction. Our success in cloning the NP-C1 gene in 1997 can be attributed to collaboration between an international body of scientists and families coping with NP-C disease. The next challenge is to delineate the biological function of the NP-C1 protein.
Keywords: Cholesterol; Cholesteol transport; Niemann–Pick C; Positional cloning; Sphingomyelinase;

The NPC1 protein: structure implies function by Catherine Scott; Y.A. Ioannou (8-13).
Niemann–Pick type C (NPC) is a lysosomal storage disorder, characterized by intracellular accumulation of low-density lipoprotein (LDL)-derived cholesterol and neurodegeneration leading to premature death. The most common form of the disease, NPC1, results from mutations in the NPC1 gene. Thus, the NPC1 protein is the focus of intense investigation to elucidate the function of this protein and its role in the disease pathogenesis. Recent studies have revealed the NPC1 subcellular location, topology and potential functions of the NPC1 protein. In lieu of direct experimental evidence, certain hypotheses about the function of NPC1 can be inferred by analyzing disease-causing mutations, NPC1 protein sequence homology to other related proteins, and the potential tertiary structure similarity between NPC1 and its prokaryotic ancestors, such as the E. coli RND permease AcrB. This review will discuss recent work on the characterization and function of the NPC1 protein and highlight structural features that may be important in assisting in the elucidation of NPC1 function and role in subcellular lipid transport and homeostasis.
Keywords: NPC1 protein; Structure; Function;

Structure and function of the NPC2 protein by Marie T. Vanier; Gilles Millat (14-21).
Somatic cell hydridization and linkage studies indicated the implication of a second gene as a cause of Niemann–Pick C disease in a minority (5%) of patients. A study of the lysosomal proteome led to the identification of a previously known gene, HE1, as the NPC2 gene. The mature NPC2/HE1 protein is a ubiquitous soluble small 132-amino-acid glycoprotein, first characterized as a major secretory protein in the human epididymis, but also detected in most tissues. Seventeen families with mutations in the NPC2 gene are known. Good genotype–phenotype correlations were observed. No distinction can be made between the biochemical phenotypes of NPC1 or NPC2 mutants. The NPC2 protein binds cholesterol with submicromolar affinity at neutral and acidic pH. The bovine protein has been crystallized, and the cholesterol-binding site assigned to a hydrophobic loosely packed region. There is strong evidence that the NPC1 and NPC2 proteins must function in a closely related fashion. Current data have led to the hypothesis that NPC2 would bind cholesterol from internal lysosomal membranes, enabling a physical interaction with NPC1 (or another protein) and allowing postlysosomal export of cholesterol. In this model, the activity of NPC1 would depend on that of NPC2. The precise function of the NPC2 protein has, however, not been fully elucidated.
Keywords: Niemann–Pick C disease; NPC2; HE1; Cholesterol; Glycolipid;

Niemann–Pick C 1 (NPC1) is a large integral membrane glycoprotein that resides in late endosomes, whereas NPC2 is a small soluble protein found in the lumen of lysosomes. Mutations in either NPC1 or NPC2 result in aberrant lipid transport from endocytic compartments, which results in lysosomal storage of a complex mixture of lipids, primarily cholesterol and glycosphingolipids. What are the biological functions of the NPC1 and NPC2 proteins? Here we review what is known about the intracellular itinerary of these two proteins as they facilitate lipid transport. We propose that the intracellular trafficking patterns of these proteins will provide clues about their function.
Keywords: Trafficking; Niemann–Pick C protein; Lipid;

Lipid and cholesterol trafficking in NPC by Sushmita Mukherjee; Frederick R. Maxfield (28-37).
Niemann–Pick type C, or NPC for short, is an early childhood disease exhibiting progressive neurological degeneration, associated with hepatosplenomegaly in some cases. The disease, at the cellular level, is a result of improper trafficking of lipids such as cholesterol and glycosphingolipids (GSLs) to lysosome-like storage organelles (LSOs), which become engorged with these lipids. It is believed that the initial defect in trafficking, whether of cholesterol or a GSL, results in an eventual traffic jam in these LSOs. This leads to the retention of not only other lipids, but also of transmembrane proteins that transiently associate with the late endosomes (LE) in normal cells, on their way to other cellular destinations such as the trans-Golgi network (TGN). In this review, we discuss the biophysical properties of lipids and cholesterol that might determine their intracellular itineraries, and how these itineraries are altered in NPC cells, which have defects in the proteins NPC1 or NPC2. We also discuss some potential therapeutic directions being suggested by recent research.
Keywords: Endocytosis; Membrane domain; Niemann–Pick C; NPC1; Sphingolipid storage disorder;

Metazoan and microbial models of Niemann–Pick Type C disease by Katsumi Higaki; Dorca Almanzar-Paramio; Stephen L. Sturley (38-47).
Niemann–Pick Type C (NP-C) disease compellingly provides insight into lipid transport and the association of this process with severe neuronal dysfunction. The two genes that define this syndrome, NPC1 and NPC2, are conserved throughout much of eukaryotic evolution, to the extent that the yeast and mammalian NPC1 genes are functionally interchangeable. We present here an evolutionary perspective of the genes defective in NP-C disease. We will describe how conservation of sequences and their biological roles in a variety of microbial and metazoan model systems may act as roadmaps to understanding this syndrome in humans.
Keywords: Cholesterol; Sphingolipid; Niemann–Pick type C; Yeast; NPC1; NPC2; NCR1;

Consequences of NPC1 and NPC2 loss of function in mammalian neurons by Steven U. Walkley; Kinuko Suzuki (48-62).
Genetic deficiency of NPC1 or NPC2 results in a devastating cholesterol-glycosphingolipidosis of brain and other organs known as Niemann–Pick type C (NPC) disease. While NPC1 is a transmembrane protein believed involved in retroendocytic shuttling of substrate(s) to the Golgi and possibly elsewhere in cells as part of an essential recycling/homeostatic control mechanism, NPC2 is a soluble lysosomal protein known to bind cholesterol. The precise role(s) of NPC1 and NPC2 in endosomal–lysosomal function remain unclear, nor is it known whether the two proteins directly interact as part of this function. The pathologic features of NPC disease, however, are well documented. Brain cells undergo massive intracellular accumulation of glycosphingolipids (lactosylceramide, glucosylceramide, GM2 and GM3 gangliosides) and cholesterol and concomitant distortion of neuron shape (meganeurite formation). In neurons from humans with NPC disease the metabolic defects and storage often lead to extensive growth of new, ectopic dendrites (possibly linked to ganglioside sequestration) as well as formation of neurofibrillary tangles (NFTs) (possibly linked to dysregulation of cholesterol metabolism). Other features of cellular pathology in NPC disease include fragmentation of the Golgi apparatus and neuroaxonal dystrophy, though reasons for these changes remain largely unknown. As the disease progresses, neurodegeneration is also apparent for neurons in some brain regions, particularly Purkinje cells of the cerebellum, but the basis of this selective neuronal vulnerability is unknown. The NPC1 protein is evolutionarily conserved with homologues reported in yeast to humans; NPC2 is reported in C. elegans to humans. While neurons in mammalian models of NPC1 and NPC2 diseases exhibit many changes that are remarkably similar to those in humans (e.g., endosomal/lysosomal storage, Golgi fragmentation, neuroaxonal dystrophy, neurodegeneration), a reduced degree of ectopic dendritogenesis and an absence of NFTs in these species suggest important differences in the way lower mammalian neurons respond to NPC1/NPC2 loss of function.
Keywords: Niemann–Pick type C; Endosome; Lysosome; Axonal spheroid; Dendritogenesis; Neurodegeneration; Neurofibrillary tangle; Golgi fragmentation;

Before the loss: neuronal dysfunction in Niemann–Pick Type C disease by Colleen A. Paul; Aimee K. Boegle; Robert A. Maue (63-76).
Niemann–Pick Type C (NPC) disease is an autosomal recessive disorder caused by mutations in either the NPC1 or HE1 genes. Hallmarks of this presently incurable disease include abnormal intracellular accumulation of cholesterol and glycosphingolipids, progressive neuropathology and neurodegeneration, and premature death. There have been increased efforts to understand the effects of NPC disease on neurons of the brain, in part due to the recent development of improved research tools and reagents, and in part due to the rapidly growing appreciation of the importance of cholesterol and lipoproteins in the brain during neuronal development, function, and degeneration. Here, we highlight fundamental aspects of neurons that appear to be affected by NPC disease, including their morphology, metabolism, intracellular transport, electrical signaling, and response to environmental factors, and suggest other potentially important areas for future investigation. This provides a framework for acquiring additional insight to this disorder and shaping new therapeutic approaches to NPC disease.
Keywords: Niemann–Pick Type C disease; Neuron; Cholesterol; Neurotrophic factor; Morphology; Excitability; Transport;

Therapy of Niemann–Pick disease, type C by Marc C. Patterson; Frances Platt (77-82).
Niemann–Pick disease, type C (NPC) is a progressive autosomal recessive neurodegenerative disease, characterized by late endosomal–lysosomal accumulation of multiple lipid molecules in association with abnormal tubulovesicular trafficking. The major gene product, NPC1 protein, is not suitable for transduction therapies, and gene replacement or repair is not yet practicable for NPC and related disorders. Attempts at therapy to date have focused on reduction of the accumulating molecules that are presumed to have direct or indirect toxic effects. More recent insights into the pathophysiology of NPC raise the possibility of small molecule therapies to interdict pathways triggering apoptosis and related routes to cell death and dysfunction.
Keywords: Therapy; Niemann–Pick disease; Gene;

The pathophysiology and mechanisms of NP-C disease by Stephen L. Sturley; Marc C. Patterson; William Balch; Laura Liscum (83-87).
The molecular isolation of NPC1 and NPC2, the genes defective in patients with Niemann–Pick disease type C (NP-C), has heralded in an exponential increase in our understanding of this syndrome and thus of human intracellular sterol transport. Despite this, neither the mechanisms of action nor the substrates for these putative transporters have been defined. In this overview, we describe our perspectives on the current awareness of the genetic determination and cellular biology of this syndrome, with emphasis on the underlying events that lead to neurodegeneration and the manner in which they might eventually be treated.
Keywords: Neurodegeneration; Cholesterol; Sphingolipid; Niemann–Pick type C; NPC1; NPC2;

Author Index (89).