Elsevier

Neuroscience

Volume 268, 30 May 2014, Pages 33-47
Neuroscience

Localization and expression of CaBP1/caldendrin in the mouse brain

https://doi.org/10.1016/j.neuroscience.2014.02.052Get rights and content

Highlights

  • CaBP1 and caldendrin are alternate splice variants of Ca2+ binding proteins related to calmodulin.

  • Caldendrin is the major variant and is developmentally upregulated in the cerebral cortex.

  • Antibodies recognizing both variants label select groups of excitatory and inhibitory neurons.

  • CaBP1/caldendrin immunostaining is absent in CaBP1/caldendrin knockout mice.

  • CaBP1/caldendrin may regulate Ca2+ signaling from both pre- and post- synaptic sites of action.

Abstract

Ca2+ binding protein 1 (CaBP1) and caldendrin are alternatively spliced variants of a subfamily of CaBPs with high homology to calmodulin. Although CaBP1 and caldendrin regulate effectors including plasma membrane and intracellular Ca2+ channels in heterologous expression systems, little is known about their functions in vivo. Therefore, we generated mice deficient in CaBP1/caldendrin expression (C-KO) and analyzed the expression and cellular localization of CaBP1 and caldendrin in the mouse brain. Immunoperoxidase labeling with antibodies recognizing both CaBP1 and caldendrin was absent in the brain of C-KO mice, but was intense in multiple brain regions of wild-type mice. By Western blot, the antibodies detected two proteins that were absent in the C-KO mouse and consistent in size with caldendrin variants originating from alternative translation initiation sites. By quantitative PCR, caldendrin transcript levels were far greater than those for CaBP1, particularly in the cerebral cortex and hippocampus. In the frontal cortex but not in the hippocampus, caldendrin expression increased steadily from birth. By double-label immunofluorescence, CaBP1/caldendrin was localized in principal neurons and parvalbumin-positive interneurons. In the cerebellum, CaBP1/caldendrin antibodies labeled interneurons in the molecular layer and in basket cell terminals surrounding the soma and axon initial segment of Purkinje neurons, but immunolabeling was absent in Purkinje neurons. We conclude that CaBP1/caldendrin is localized both pre- and postsynaptically where it may regulate Ca2+ signaling and excitability in select groups of excitatory and inhibitory neurons.

Introduction

In the nervous system, the generation and propagation of Ca2+ signals is crucial for the control of neuronal excitability, gene expression, and synaptic plasticity (Augustine et al., 2003). The ubiquitous Ca2+-sensing protein calmodulin (CaM) is thought to be a major player in this process. CaM is the prototypical member of the superfamily of EF-hand Ca2+ binding proteins (CaBPs), so named because they contain one or more helix-loop-helix domains that coordinate a Ca2+ ion (Kretsinger, 1976). CaM modulates the activity of numerous effector proteins (Chin and Means, 2000) and is an essential regulator of neuronal development (Bolsover, 2005) and synaptic plasticity (Xia and Storm, 2005).

A subset of EF-hand proteins related to CaM is expressed mainly in neurons (Burgoyne et al., 2004). Most similar to CaM are the CaBPs (CaBP1–8), which are found primarily in the brain, retina, and inner ear (Haeseleer et al., 2000, Laube et al., 2002, Yang et al., 2006, Cui et al., 2007). A wealth of evidence supports a role for CaBPs as regulators of voltage-gated Cav Ca2+ channels. CaBP4 potentiates the activity of Cav1.4 (Haeseleer et al., 2004, Shaltiel et al., 2012) and Cav1.3 Ca2+ channels (Yang et al., 2006, Cui et al., 2007, Lee et al., 2007), and CaBP2 enhances the function of Cav1.3 channels (Schrauwen et al., 2012). Cav1.4 and Cav1.3 are the primary Cav channels that regulate neurotransmitter release from retinal photoreceptors and cochlear inner hair cells, respectively (Platzer et al., 2000, Mansergh et al., 2005). Human mutations in the genes encoding CaBP4 and CaBP2 impair Cav1 regulation and cause vision and hearing impairment, respectively (Zeitz et al., 2006, Littink et al., 2009, Schrauwen et al., 2012, Shaltiel et al., 2012).

Unlike CaBP4 and CaBP2, CaBP1 is highly expressed in the brain (Haeseleer et al., 2000). Alternative splicing of the N-terminal domain gives rise to multiple variants (CaBP1-S, CaBP1-L, and caldendrin) (Seidenbecher et al., 1998, Haeseleer et al., 2000). CaBP1 has powerful effects in opposing CaM regulation of Cav channels (Lee et al., 2002). For the major Cav1 channels in the brain (Cav1.2 and Cav1.3), CaBP1 competes with CaM for binding to the channel and facilitates channel opening by preventing Ca2+/CaM-dependent inactivation (Lee et al., 2002, Zhou et al., 2004, Zhou et al., 2005, Yang et al., 2006, Cui et al., 2007, Findeisen and Minor, 2010, Oz et al., 2011). CaBP1 also binds to and regulates inositol 1,4,5-trisphosphate receptors (IP3Rs) (Yang et al., 2002, Haynes et al., 2004). The largest CaBP1 variant, caldendrin, was initially discovered as a component of the postsynaptic membrane of excitatory synapses in the brain (Seidenbecher et al., 1998), where it may interact with the cytoskeleton (Seidenbecher et al., 2004), A-kinase anchoring protein (AKAP)79/150 (Gorny et al., 2012), Cav1.2 (Tippens and Lee, 2007), and/or Jacob, a protein that couples N-methyl-d-aspartate (NMDA) receptor signaling to the nucleus (Dieterich et al., 2008).

The above findings suggest a role for CaBP1 and caldendrin in regulating neuronal Ca2+ signaling, but the neurophysiological significance of these proteins in vivo is unknown. To fill this gap in knowledge, we generated mice with targeted inactivation of the CaBP1/caldendrin gene (C-KO). While previous studies have characterized the distribution of CaBP1 and caldendrin in the nervous system of rat (Laube et al., 2002) and human (Bernstein et al., 2003), similar analyses in the mouse are necessary to understand phenotypes of the C-KO mice. Therefore, we used molecular, biochemical, and immunocytochemical strategies to characterize the expression and localization of CaBP1 and caldendrin in the mouse brain.

Section snippets

Generation of CaBP1/caldendrin knock-out mice

Genetic inactivation of CaBP1/caldendrin expression was accomplished with the assistance of the University of Iowa Gene Transfer Core. The strategy involved replacing exon 1 and exon 1b of the mouse CaBP1/caldendrin gene with sequences corresponding to mCherry and neomycin resistance gene. The targeting vector was constructed by amplifying a 2.4 kilobase (kb) DNA fragment upstream of exon 1 and a 5.8-kb fragment found downstream from exon 1b as the short and long arms, respectively. The 2.4-kb

Generation of CaBP1/caldendrin knock-out mice

To generate C-KO mice, a targeting construct was designed in which the DNA sequences encoding mCherry fluorescent protein and the neomycin resistance cassette would be expressed in place of exon 1a of caldendrin and exon 1b of CaBP1 upon homologous recombination (Fig. 1A). C-KO mice were viable and bred normally, with no differences in survival or apparent physiological deficits. Body weight measured between 3 and 21 weeks of age was similar to that of WT mice (data not shown). The presence of

Discussion

Our study provides the first report of the expression and localization of CaBP1 and caldendrin in the mouse brain. Our results largely agree with those of Laube and colleagues in the rat brain (Laube et al., 2002), but add the following new insights. First, we verify that the labeling pattern obtained with our CaBP1/CD antibodies is specific by its absence in the C-KO mouse brain. Second, we confirm quantitatively at the transcriptional level that caldendrin is the major CaBP1 variant expressed

Acknowledgments

This work was supported by the National Institutes of Health (DC009433, NS084190 to A.L.; DC010362 to the Iowa Center for Molecular Auditory Neuroscience; EY020850 to F.H.) and a Carver Research Program of Excellence Award to A.L. The authors thank Baoli Yang (U. Iowa Gene Targeting Core) for guidance on generating CaBP1/caldendrin KO mice, Daniel Soh for care of mouse colonies and DP Mohapatra for support and the use of equipment.

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