TRIDIMENSIONAL ORGANIZATION OF PURKINJE NEURON CISTERNAL STACKS, A SPECIALIZED ENDOPLASMIC-RETICULUM SUBCOMPARTMENT RICH IN INOSITOL 1,4,5-TRISPHOSPHATE RECEPTORS

Stacks of regularly spaced, flat, smooth-surfaced endoplasmic reticulu m cisternae . frequently observed in both the cell body and dendrites of cerebellar Purkinje neurons, were previously shown by immunocytoche mistry to be highly enriched in receptors for the second messenger, in ositol 1,4,5-trisphosphate. Morphometric analyses have been carried ou t on randomly selected thin section images of rat Purkinje neurons to reveal the tridimensional organization of these structures. Individual stacked cisternae (on the average almost-equal-to 3.5 per stack) were shown to be separated from each other by a 23.5 nm space occupied by perpendicular bridges, almost-equal-to 20 nm in diameter, most probabl y composed by two apposed receptor homotetramer molecules, inserted in to the parallel membranes in their hydrophobic domains. In the stacked membranes the density of the bridges was almost-equal-to 500 mum-2, c orresponding to almost-equal-to 15% of the surface area. The lateral d istribution of bridges was not random, but revealed regular distances that might correspond to unoccupied receptor slots. In each stack, the external cisternae were often in direct lumenal continuity with conve ntional elements of the endoplasmic reticulum, whereas the internal ci sternae were not. Since continuities between stacked cisternae were ne ver observed, the results indicate that the internal cisternae are at least transitorily discrete, i.e. they are not in permanent lumenal co ntinuity with the rest of the endoplasmic reticulum. To our knowledge this is the first demonstration of a physical subcompartmentalization of the latter endomembrane system in a non-mitotic cell. A model for t he biogenesis of cisternal stacks, based on the head-to-head binding a nd lateral interaction of the inositol 1,4,5-trisphosphate receptor mo lecules in the plane of the interacting membranes, is proposed and cri tically discussed.