Regulation of tight junction assembly and epithelial polarity by a resident protein of apical endosomes.

The establishment of tight junctions and cell polarity is an essential process in all epithelia. Endotubin is an integral membrane protein found in apical endosomes of developing epithelia when tight junctions and epithelial polarity first arise. We found that the disruption of endotubin function in cells in culture by siRNA or overexpression of the C-terminal cytoplasmic domain of endotubin causes defects in organization and function of tight junctions. We observe defects in localization of tight junction proteins, reduced transepithelial resistance, increased lanthanum penetration between cells and reduced ability of cells to form cysts in three-dimensional culture. In addition, in cells overexpressing the C-terminal domain of endotubin, we observe a delay in re-establishing the normal distribution of endosomes after calcium switch. These results suggest that endotubin regulates trafficking of polarity proteins and tight junction components out of the endosomal compartment, thereby providing a critical link between a resident protein of apical endosomes and tight junctions.

Amelioration of enteric neuropathology in a mouse model of Niemann-Pick C by Npc1 expression in enteric glia.

Niemann-Pick C (NPC) disease is an autosomal recessive, lethal, neurodegenerative disorder caused by mutations in NPC1. By using the glial fibrillary acidic protein (GFAP) promoter, we demonstrated previously that astrocyte-specific expression of Npc1 decreased neuronal storage of cholesterol in Npc1(-/-) mice; reduced numbers of axonal spheroids; and produced less degeneration of neurons, reactive astrocytes, and loss of myelin tracts in the central nervous system. GFAP-Npc1, Npc1(-/-) mice exhibited markedly enhanced survival, and death was not associated with the severe terminal weight loss observed in Npc1(-/-) mice. Intestinal transit is delayed in Npc1(-/-) mice but is normal in GFAP-NPC1, Npc1(-/-) until late in the course of their disease. Because glia play an important role in the enteric nervous system, we studied morphology and cholesterol content of intestines from Npc1(-/-) mice and examined the effect of GFAP-promoted restoration of Npc1 in enteric glia. Although the number of neurons was not altered, the total amount of cholesterol stored in the small intestine was decreased, as were the number of neurons with inclusions and the number of inclusions per neuron. We conclude that expression of Npc1 by enteric glial cells can ameliorate the enteric neuropathology, and we speculate that dysfunction of the enteric nervous system contributes to the retarded intestinal transit, weight loss, and demise of Npc1(-/-) mice.

Astrocyte-only Npc1 reduces neuronal cholesterol and triples life span of Npc1-/- mice.

Niemann-Pick type C (NPC) disease is an autosomal recessive, lethal neurodegenerative disorder. Although neurodegeneration of Purkinje cells in the mouse model (Npc1(-/-)) is thought to be autonomous, the basis of neuronal death in other regions of the brain remains elusive. We addressed this issue in vivo by using the glial fibrillary acidic protein (GFAP) promoter to direct astrocyte-specific, replacement expression of Npc1 in Npc1(-/-) mice. These mice showed enhanced survival, decreased neuronal storage of cholesterol associated with less accumulation of axonal spheroids, lower numbers of degenerated neurons and reactive astrocytes, and restoration of myelin tracts. Their death was not associated with the usual terminal decline in weight but instead with a loss of Purkinje cells and motor coordination. We conclude that neurodegeneration of Npc1(-/-) mice is greatly affected by the loss of fibrillary astrocyte function.