On the roles and sub-cellular localisations of lipids in neurodegenerative disorders

Niemann-Pick type C disease (NPCD) is a ruinous condition that mostly affects children. Although stemming from a single genetic error its pathology involves dysfunction of multiple organs especially liver, spleen and brain. Life expectancy is dramatically reduced. NPCD is usually caused by a mutation in NPC1 a protein located in the lysosome, the cell’s recycling centre, and believed to export cholesterol from there for use elsewhere in the cell. Consequently NPCD patients accumulate cholesterol in their lysosomes. Extensive research has found that mutation of this single protein restricted to one organelle has effects that encompass almost every aspect of cellular function. This is rather surprising so chapter 1 attempts to piece together what we know to give as coherent and complete an account as possible of the cellular pathology of this disease including insights gathered from the various treatment approaches tried to date. Using a combined docking-molecular dynamics approach Chapter 2 supports the idea, previously advanced from in vitro experiments, that NPC1 and its partner NPC2 may also bind sphingosine, the simplest sphingolipid. This chapter also uses docking to understand some previously observed, but not fully explained, aspects of lysosomal dysfunction in NPCD including reduced big potassium channel activity, annexin mislocalisation and impaired SNARE recycling. These errors are traced ultimately to cholesterol accumulation but pathological roles are found for other lipids which also accumulate in this disease Chapter 3 finds that lysosomal pH is increased in fibroblasts from NPC1-deficient patient fibroblasts; inhibtion of glucocerbrosidase 2 (GBA2), the only clinically approved NPCD treatment, corrects this error. The same treatment is not effective in other Niemann-Pick variants. Only limited and preliminary success was encounted when GBA2 inhibition was examined as a potential treatment for related diseases. Chapter 4 finds that the mitochondrial defect in NPCD is not responsive to GBA2 inhibition nor to manipulation of sphingolipids more widely. Attempts at correcting this error by manipulating mitochondrial cholesterol, widely believed to be elevated to levels toxic to this organelle, were not clearly effective. Chapter 5 draws the various threads of this thesis together and combines them with very recent work published elsewhere to sketch an integrated cellular pathology of NPCD.