Alterations in Purkinje cell spines of calbindin D-28 k and parvalbumin knock-out mice.

The second messenger Ca2+ is known to act in a broad spectrum of fundamental cell processes, including modifications of cell shape and motility, through the intermediary of intracellular calcium-binding proteins. The possible impact of the lack of the intracellular soluble Ca2+-binding proteins parvalbumin (PV) and calbindin D-28 k (CB) was tested on spine morphology and topology in Purkinje cell dendrites of genetically modified mice. Three different genotypes were studied, i.e. PV or CB single knock-out (PV-/-, CB-/-) and PV and CB double knock-out mice (PV-/-CB-/-). Purkinje cells were microinjected with Lucifer Yellow and terminal dendrites scanned at high resolution with a confocal laser microscope followed by three-dimensional (3-D) reconstruction. The absence of PV had no significant effect on spine morphology, whereas the absence of CB resulted in a slight increase of various spine parameters, most notably spine length. In double knock-out mice, the absence of both PV and CB entailed a doubling of spine length, an increase in spine volume and spine surface, a higher spine density along the dendrites, as well as a more clustered spine distribution. In all three genotypes, a reduction in the number of stubby spines was observed compared with wild-type animals. These results suggest a morphological compensation for the lack of the soluble calcium buffers in the cytoplasm of Purkinje cell dendritic spines. The increase in various spine parameters, particularly volume, may counteract the lack of the calcium buffers, such as to adjust Ca2+-transients at the transitional zone between spines and dendrites.