Alterations in the biosynthesis of cholesterol, dolichol and dolichyl-P in the genetic cholesterol homeostasis disorder, Niemann-Pick type C disease

Arabidopsis DOK1 encodes a functional dolichol kinase involved in reproduction

Dolichol phosphate (Dol-P) serves as a carrier of complex polysaccharides during protein glycosylation. Dol-P is synthesized by the phosphorylation of dolichol or the monodephosphorylation of dolichol pyrophosphate (Dol-PP); however, the enzymes that catalyze these reactions remain unidentified in Arabidopsis thaliana. We performed a genome-wide search for cytidylyltransferase motif-containing proteins in Arabidopsis, and found that At3g45040 encodes a protein homologous with Sec59p, a dolichol kinase (DOK) in Saccharomyces cerevisiae. At3g45040, designated AtDOK1, complemented defects in the growth and N-linked glycosylation of the S. cerevisiae sec59 mutant, suggesting that AtDOK1 encodes a functional DOK. To characterize the physiological roles of AtDOK1 in planta, we isolated two independent lines of T-DNA-tagged AtDOK1 mutants, dok1-1 and dok1-2. The heterozygous plants showed developmental defects in male and female gametophytes, including an aberrant pollen structure, low pollen viability, and short siliques. Additionally, the mutations had incomplete penetrance. These results suggest that AtDOK1 is a functional DOK required for reproductive processes in Arabidopsis.

Abnormal gene expression in cerebellum of Npc1-/- mice during postnatal development

Niemann-Pick Type C (NPC) disease is an autosomal recessive neurodegenerative disorder with abnormal lipid storage as the major cellular pathologic hallmark. Genetic analyses have identified mutations in NPC1 gene in the great majority of cases, while mutations in NPC2 account for the remainders. Yet little is known regarding the cellular mechanisms responsible for NPC pathogenesis, especially for neurodegeneration, which is the usual cause of death. To identify critical steps that could account for the pathological manifestations of the disease in one of the most affected brain structures, we performed global gene expression analysis in the cerebellum from 3-week old Npc1+/+ and Npc1-/- mice with two different microarray platforms (Agilent and Illumina). Differentially expressed genes identified by both microarray platforms were then subjected to KEGG pathway analysis. Expression of genes in six pathways was significantly altered in Npc1-/- mice; functionally, these signaling pathways belong to the following three categories: (1) steroid and terpenoid biosynthesis, (2) immune response, and (3) cell adhesion/motility. In addition, the expression of several proteins involved in lipid transport was significantly altered in Npc1-/- mice. Our results provide novel molecular insight regarding the mechanisms of pathogenesis in NPC disease and reveal potential new therapeutic targets.

Polyisoprenoid alcohols--recent results of structural studies

Polyisoprenoid alcohols (polyprenols and dolichols) are linear polymers of from several up to more than 100 isoprene units identified in almost all living organisms. Studies of their chemical structures have resulted in the discovery of new variants such as the recently described alloprenols with reversed configuration of the double bond in the alpha-isoprene unit. In parallel, structural elucidation of metabolically labeled plant dolichols has indicated that both the mevalonate and methylerythritol phosphate pathways are involved in the biosynthesis of dolichols in roots, leading to the construction of a spatial model of their biosynthesis. According to this model, in root cells, synthesis of the dolichol molecule is initiated in the plastids, and the resulting intermediates, oligoprenyl diphosphates, are exported to the cytoplasm and are elongated up to the desired chain length. The metabolic consequences of this putative model are discussed in the context of the enzymatic machinery involved.

In vivo interaction between the human dehydrodolichyl diphosphate synthase and the Niemann-Pick C2 protein revealed by a yeast two-hybrid system

Dehydrodolichyl diphosphate (DedolPP) synthase catalyzes the sequential condensation of isopentenyl diphosphate with farnesyl diphosphate to synthesize DedolPP, a biosynthetic precursor for dolichol which plays an important role as a sugar-carrier lipid in the biosynthesis of glycoprotein in eukaryotic cells. During certain pathological processes like Alzheimer's disease or some neurological disorders, dolichol has been shown to accumulate in human brain. In order to understand the regulatory mechanism of dolichol in eukaryotes, we performed a yeast two-hybrid screen using full length human DedolPP synthase gene [Endo et al. BBA 1625 (2003) 291] as a bait to find some proteins specifically interacting with the enzyme. We identified Niemann-Pick Type C2 protein (NPC2) to show a specific interaction with human DedolPP synthase. This interaction was further confirmed by in vitro co-immunoprecipitation experiment, indicating the possible physiological interaction between NPC2 and DedolPP synthase proteins in human.

Cholesterol-dependent modulation of tau phosphorylation in cultured neurons

One of the hallmarks of Alzheimer's disease (AD) is the abnormal state of tau. It is both highly phosphorylated and aggregated into paired helical filaments (PHFs) in neurofibrillary tangles (NFTs). However, the mechanism underlying the hyperphosphorylation of tau in NFTs and neuronal degeneration in AD remains to be elucidated. The fact that hyperphosphorylation of tau in NFTs are also found in the patients with Niemann-Pick disease, type C (NPC), which is a cholesterol storage disease associated with defective intracellular trafficking of exogenous cholesterol, implies that perturbation of cholesterol metabolism may be involved in tau phosphorylation and neurodegeneration. Here, we report that cholesterol deficiency induced by inhibition of cholesterol biosynthesis in cultured neurons results in hyperphosphorylation of tau, accompanied by axonal degeneration associated with microtubule depolymerization. These changes were prevented by concurrent treatment with beta-migrating very low-density lipoprotein (beta-VLDL) or cholesterol. We propose that intracellular cholesterol plays an essential role in the modulation of tau phosphorylation and the maintenance of microtubule stability.