Immunocytochemical localization of the intrinsic factor-cobalamin receptor in dog-ileum: distribution of intracellular receptor during cell maturation.

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Abstract
Absorption of cobalamin is facilitated by the binding of the intrinsic factor-cobalamin complex (IF-cbl) to specific receptors in the ileum. The physical and biochemical characteristics of this ligand-receptor binding reaction were extensively studied, but little is known about the cellular mechanisms or receptor synthesis, intracellular transport and expression on the microvillus surface membrane. These mechanisms were investigated by using ultrastructural immunocytochemistry to localize the IF-cbl receptor in the crypt, mid-villus and villus tip regions of mucosal biopsies obtained from the ileum of anesthetized dogs. Prior to initiating the ileal localization studies, the antisera to purified canine IF-cbl receptor that was employed in these studies had specificity for site (e.g., ileal enterocytes vs. other cells within the gastrointestinal tract) and immunohistochemical specificity. Receptor synthesis in endoplasmic reticulum begins in crypt enterocytes, but continues in cells throughout the villus. In the mid-villus region synthesized receptor translocates vectorially to the microvillus surface associated with membranous vesicles and then inserts into the microvillus pit. Receptor remains fixed to the microvillus pit and does not distribute uniformly over the brush border membrane. All villus tip enterocytes contained IF-cbl receptor in mcirovillus pits, vesicles and endoplasmic reticulum, but in addition extensive perinuclear membrane staining was evident as well as reinternalized receptor associated with multivesicular bodies. Basolateral membranes contained no receptor at any level of the villus. The IF-cbl receptor apparently translocates to the apical cell surface at the mid-villus region by transport in vesicles, directly inserts into and then remains fixed in microvillus pits, is elaborated on the luminal surface most extensively in villus tip cells, and, although reinternalized, does not move IF and/or cbl to the basolateral cell surface.

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