Polarized endocytosis and transcytosis in the hypothalamic tanycytes of the rat

Four types of tanycytes can be distinguished in the rat hypothalamus: alpha(1) and alpha(2) tanycytes establish an anatomical link between the ventricular cerebrospinal fluid (CSF) and the arcuate nucleus, whereas beta(1) and beta(2) tanycytes establish a link between CSF and portal blood. Endocytosis and transcytosis in these cells have been investigated by (1) immunocytochemistry with antibodies against molecular markers of the endocytotic and transcytotic pathways; (2) the administration of wheat germ agglutinin (WGA) into the ventricular or subarachnoidal CSF and following its internalisation by and its routing through tanycytes. The four populations of tanycytes show marked differences concerning the expression and subcellular location of proteins involved in endocytosis and transcytosis, such as clathrin, caveolin-1, Rab4 and ARF6. Thus, beta(1,2) tanycytes express caveolin-1 at the ventricular cell pole and at their terminals contacting the portal capillaries, whereas alpha(1,2) tanycytes do not, suggesting that caveolae-dependant endocytosis does not occur in the latter and that, in beta(1,2) tanycytes, it may occur at both cell poles. In beta(1,2) tanycytes, clathrin is only expressed at the ventricular cell pole indicating that clathrin-dependant endocytosis operates for compounds present in the ventricular CSF and not for those exposed to the terminals. This agrees with the property of beta(1,2) tanycytes of internalising WGA through the ventricular cell pole but not through the terminals. The subcellular distribution in beta(1,2) tanycytes of WGA and of the proteins clathrin and Rab4 indicates that part of the internalised WGA follows the degradative pathway and part is sorted to a transcytotic pathway and that the transcytotic and the secretory pathways might intersect.