Characterization of calcium binding protein 1 (CaBP1/CD) expression and localization in the mouse brain

Ca²⁺-binding proteins (CaBP) alter Ca²⁺ signals, triggering cellular processes such as gene transcription regulation in neurons. CaBP1/CD is a calmodulin (CaM)-like Ca²⁺ binding protein that may regulate neuronal functions through interactions with effectors such as voltage-gated Ca²⁺ (Ca_v) channels and inositol trisphosphate receptors (InsP₃Rs). To gain insight into the potential cellular functions of CaBP1/CD, we analyzed the expression and localization of CaBP1/CD variants in mouse brain. Of the three CaBP1/CD splice variants that have been characterized (CaBP1-S, CaBP1-L, and caldendrin (CD)), CD was the major variant expressed in mouse brain by western blot and quantitative polymerase chain reaction. These results reflected the expression of CaBP1/CD since they were not reproduced in mice with targeted disruption of the gene encoding CaBP1/CD (CaBP1 knock-out). By immunoperoxidase labeling, CaBP1/CD was localized in multiple cell-types including pyramidal cells in the cerebral cortex and hippocampal CA3 neurons and inhibitory neurons in the cerebellum. In the cerebellum, CaBP1/CD was not detected in Purkinje neurons but strongly colocalized with voltage-sensitive Shaker-type potassium channel, K_v1.2, in the pinceau formation formed between basket cells and the Purkinje cell axon initial segment. We conclude that CaBP1/CD is expressed in a subset of principal neurons where it may regulate Ca²⁺ signaling and neuronal excitability.