Parvalbumin is freely mobile in axons, somata and nuclei of cerebellar Purkinje neurones

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The Ca2+-binding protein (CaBP) parvalbumin (PV) is strongly expressed in cerebellar Purkinje neurones (PNs). It is considered a pure Ca2+ buffer, lacking any Ca2+ sensor function. Consistent with this notion, no PV ligand was found in dendrites of PNs. Recently, however, we observed for a related CaBP that ligand-targeting differs substantially between dendrites and axons. Thus, here we quantified the diffusion of dye-labelled PV in axons, somata and nuclei of PNs by two-photon fluorescence recovery after photobleaching (FRAP). In all three compartments the fluorescence rapidly returned to baseline, indicating that no large or immobile PV ligand was present. In the axon, FRAP was well described by a one-dimensional diffusion equation and a diffusion coefficient (D) of 12 (IQR 6–20) μm2/s. For the soma and nucleus a three-dimensional model yielded similar D values. The diffusional mobility in these compartments was ∼3 times smaller than in dendrites. Based on control experiments with fluorescein dextrans, we attributed this reduced mobility of PV to different cytoplasmic properties rather than to specific PV interactions in these compartments. Our findings support the notion that PV functions as a pure Ca2+ buffer and will aid simulations of neuronal Ca2+ signalling.

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