Tridimensional organization of Purkinje neuron cisternal stacks, a specialized endoplasmic reticulum subcompartment rich in inositol 1,4,5-trisphosphate receptors

Summary

Stacks of regularly spaced, flat, smooth-surfaced endoplasmic reticulum cisternae, frequently observed in both the cell body and dendrites of cerebellar Purkinje neurons, were previously shown by immunocytochemistry to be highly enriched in receptors for the second messenger, inositol 1,4,5-trisphosphate. Morphometric analyses have been carried out on randomly selected thin section images of rat Purkinje neurons to reveal the tridimensional organization of these structures. Individual stacked cisternae (on the average ≈ 3.5 per stack) were shown to be separated from each other by a 23.5 nm space occupied by perpendicular bridges, ≈ 20 nm in diameter, most probably composed by two apposed receptor homotetramer molecules, inserted into the parallel membranes in their hydrophobic domains. In the stacked membranes the density of the bridges was ≈ 500 μm⁻², corresponding to ≈ 15% of the surface area. The lateral distribution 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 conventional elements of the endoplasmic reticulum, whereas the internal cisternae were not. Since continuities between stacked cisternae were never observed, the results indicate that the internal cisternae are at least transitorily discrete, i.e. they are not in permanent lumenal continuity 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 the biogenesis of cisternal stacks, based on the head-to-head binding and lateral interaction of the inositol 1,4,5-trisphosphate receptor molecules in the plane of the interacting membranes, is proposed and critically discussed.