Developmental changes in the subcellular localization of calretinin.

Brainstem auditory neurons in the chick nucleus magnocellularis (NM) express high levels of the neuron-specific calcium-binding protein calretinin (CR). CR has heretofore been considered a diffusible calcium buffer that is dispersed uniformly throughout the cytosol. Using high-resolution confocal microscopy and complementary biochemical analyses, we have found that during the development of NM neurons, CR changes from being expressed diffusely at low concentrations to being highly concentrated beneath the plasma membrane. This shift in CR localization occurs at the same time as the onset of spontaneous activity, synaptic transmission, and synapse refinement in NM. In the chick brainstem auditory pathway, this subcellular localization appears to occur only in NM neurons and only with respect to CR, because calmodulin remains diffusely expressed in NM. Biochemical analyses show the association of calretinin with the membrane is detergent-soluble and calcium-independent. Because these are highly active neurons with a large number of Ca2+-permeable synaptic AMPA receptors, we hypothesize that localization of CR beneath the plasma membrane is an adaptation to spatially restrict the calcium influxes.