Epithelial transport and intracellular trafficking: Physiology and pathophysiology

Abstract

Important for the normal physiological function of several membrane transporters is not only that they are expressed in a particular cell at all, moreover, the membrane transporter has to be trafficked at the appropriate time to the appropriate cellular location. Cystic fibrosis, the most frequent inherited disease in the white population, is caused by a mislead chloride channel, which is expressed in bronchial epithelial cells, but never attains the cell surface. Inherited forms of intrahepatic cholestasis are due to mislead ATP binding cassette transporter, which are involved in bile formation in the canalicular membrane of hepatocytes. The glucose-galactose malabsorption syndrome is caused by false intracellular sorting of SGLT1 in enterocytes. Besides these examples there is a growing number of diseases, which were identified to be caused by deranged intracellular sorting of membrane transporters (trafficking diseases). In several instances, point mutations in the genes for misguided membrane transporters have been identified, resulting in an exchange of a single amino acid in the transporter protein. However, the cascade of molecular events leading from a point mutation to false trafficking and ultimately to the full blown picture of the disease, is in all instances only poorly understood. Therefore, elucidation of the intracellular pathways and regulation of membrane transporters may help to understand the cause of trafficking diseases at a molecular level and provide clues for novel therapies.