Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press The Diversity of Calcium Sensor Proteins in the Regulation of Neuronal Function Hannah V. McCue, Lee P. Haynes, and Robert D. Burgoyne The Physiological Laboratory, School of Biomedical Sciences, University of Liverpool, Crown Street, Liverpool L69 3BX, United Kingdom Correspondence: [email protected] Calcium signaling in neurons as in other cell types mediates changes in gene expression, cell growth, development, survival, and cell death. However, neuronal Ca2þ signaling processes have become adapted to modulate the function of other important pathways including axon outgrowth and changes in synaptic strength. Ca2þ plays a key role as the trigger for fast neuro- transmitter release. The ubiquitous Ca2þ sensor calmodulin is involved in various aspects of neuronal regulation. The mechanisms by which changes in intracellular Ca2þ concentration in neurons can bring about such diverse responses has, however, become a topic of wide- spread interest that has recently focused on the roles of specialized neuronal Ca2þ sensors. In this article, we summarize synaptotagmins in neurotransmitter release, the neur- onal roles of calmodulin, and the functional significance of the NCS and the CaBP/ calneuron protein families of neuronal Ca2þ sensors. alcium signaling in many cell types can processes have been shown to be dependent Cmediate changes in gene expression, cell upon the particular route of Ca2þ entry into growth, development, survival, and cell death. the cell. It has long been known that the physio- However, neuronal calcium signaling processes logical outcome from a change in [Ca2þ]i have become adapted to modulate the function depends on its location, amplitude, and dura- of important pathways in the brain, including tion. The importance of location becomes neuronal survival, axon outgrowth, and changes even more pronounced in neurons because of in synaptic strength. Changes in the concentra- their complex and extended morphologies. tion of intracellular free Ca2þ ([Ca2þ]i) are [Ca2þ]i also regulates neuronal development essential for the transmission of information and neuronal survival (Spitzer 2006). In addi- through the nervous system as the trigger for tion, modifications to Ca2þ signaling pathways neurotransmitter release at synapses. In addi- have been suggested to underlie various neuro- tion, alterations in [Ca2þ]i can lead to a wide pathological disorders (Braunewell 2005; Ber- range of different physiological changes that ridge 2010). can modify neuronal functions over time scales Highly localized Ca2þ elevations (Augustine of milliseconds through tens of minutes to et al. 2003) formed following Ca2þ entry though days or longer (Berridge 1998). Many of these voltage-gated Ca2þ channels (VGCCs) lead to Editors: Martin Bootman, Michael J. Berridge, James W. Putney, and H. Llewelyn Roderick Additional Perspectives on Calcium Signaling available at www.cshperspectives.org Copyright # 2010 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a004085 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a004085 1 Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press H.V. McCue, L.P. Haynes, and R.D. Burgoyne synaptic vesicle fusion with the presynaptic role of synaptotagmins in neurotransmitter membrane and thereby allow neurotransmitter release has been the subject of intense investiga- release within less than a millisecond. Differ- tions, which have been extensively reviewed ently localized and timed Ca2þ signals can, for (Chapman 2008; Rizo and Rosenmund 2008; example, result in changes to the properties of Sudhof and Rothman 2009) and so only a brief the VGCCs (Catterall and Few 2008) or lead to outline is given here. Synaptotagmins bind changes in gene expression (Bito et al. 1997). Ca2þ with relatively low affinity (Kd . 10 mM) Postsynaptic Ca2þ signals arising from activa- through their two C2 domains (C2A and C2B) tion of NMDA receptors give rise to two impor- (Shao et al. 1998; Fernandez et al. 2001), which tant processes in synaptic plasticity, long term are functional in many but not all synaptotag- potentiation (LTP) and long term depression min isoforms. Ca2þ binding by C2 domains (LTD). LTP and LTD are examples of the way requires coordination of Ca2þ by both the synaptic transmission can change synaptic effi- protein and membrane lipids and this lipid cacy and are thought to be important in modu- interaction is a key aspect for its function. In lating learning and memory. Importantly, the synaptotagmin I, the C2A and C2B domains Ca2þ signals that bring about either LTP or (Fig. 1) bind three and two Ca2þ ions, respec- LTD differ only in their timing and duration. tively (Shao et al. 1998; Fernandez et al. 2001). LTP is triggered by Ca2þ signals on the micro- It is now well established that synaptotagmin I molar scale for shorter durations, whereas LTD is a key sensor for evoked, synchronous neuro- is triggered by changes in [Ca2þ]i on the nano- transmitter release in many classes of neurons molar scale for longer durations (Yang et al. (Fernandez-Chacon et al. 2001). Structure– 1999). Specific Ca2þ signals are likely to be function studies based on expression of specific decoded by different Ca2þ sensor proteins. mutants have been carried out in mice, worms, These are proteins that undergo a conforma- and flies. For example, disruption of Ca2þ bind- tional change on Ca2þ binding and then interact ing to the C2B domain of synaptotagmin I has with and regulate various target proteins. Among those Ca2þ sensors that are important for neuronal function are the synaptotagmins that control neurotransmitter release (Chap- man 2008), the ubiquitous EF-hand contain- ing sensor calmodulin that has many neuronal roles, and the more recently discovered neuronal EF-hand containing proteins, including the neuronal calcium sensor (NCS) protein (Bur- goyne 2007) and the calcium-binding protein (CaBP)/calneuron (Haeseleer et al. 2002) fam- ilies. We will briefly review synaptotagmins and the neuronal functions of calmodulin but concentrate on the NCS and CaBP families of Ca2þ sensors. SYNAPTOTAGMINS AND NEUROTRANSMITTER RELEASE Synaptotagmins are transmembrane proteins Figure 1. Structures of the C2A and C2B domains of synaptotagmin I. The structures show the isolated C2 mostly found associated with synaptic and domains in their Ca2þ-loaded state with the bound secretory vesicles. There are multiple known Ca2þ ions shown in green. The coordinates for the isoforms of synaptotagmin (Craxton 2004) of structures for the C2A and C2B domains come which synaptotagmin I is the best studied. The from the PDB files 1BYN and 1K5W, respectively. 2 Cite this article as Cold Spring Harb Perspect Biol 2010;2:a004085 Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press Calcium Sensor Proteins in Neuronal Function been shown to have a more deleterious effect NEURONAL FUNCTIONS OF CALMODULIN than disruption of Ca2þ binding to its C2A domain (Mackler et al. 2002; Robinson et al. Calmodulin is a ubiquitously expressed Ca2þ 2002). The details of exactly how it triggers exo- -binding protein that can bind four Ca2þ ions cytosis and the function of other syntaptotag- through its four EF-hand domains (Chattopad- min isoforms remain controversial. Membrane hyaya et al. 1992). This protein has been highly fusion requires the pairing and interaction of conserved throughout evolution, is found in all so-called SNARE proteins on vesicle and target eukaryotes, and is 100% identical across all ver- membranes (Sollner et al. 1993). These can tebrates at the amino acid level. It is involved in assemble into a SNARE complex that may the regulation of many essential physiological form the minimal fusion machinery. For synap- processes including cell motility, exocytosis, tic vesicle and neuroendocrine exocytosis, the cytoskeletal assembly, and modulation of intra- SNARE proteins are SNAP-25, syntaxin 1, and cellular Ca2þ concentrations. The first two EF- synaptobrevin. In the case of regulated exocy- hands of calmodulin form an amino-terminal tosis, such as in neurotransmitter release, ves- globular domain that is joined by a flexible icle fusion is tightly regulated and requires a linker to a highly homologous carboxy-termi- Ca2þ signal for activation. Ca2þ entry through nal region encompassing the third and fourth VGCCs leading to Ca2þ elevation in local EF-hands. The carboxy-terminal pair of EF- microdomains close to the mouth of the Ca2þ hands has a much higher affinity for Ca2þ channel is able to trigger very rapid (within than the amino-terminal pair, which allows less than 1 ms) fusion of synaptic vesicles. Syn- the two domains to behave independently at aptotagmin can bind to both syntaxin and varying Ca2þ concentrations (Tadross et al. SNAP-25, and fast neurotransmitter release 2008). The highly flexible linker between the requires synaptotagmin (Geppert et al. 1994) two domains can be bent dramatically upon probably prebound to assembled or partially binding to target proteins (Fig. 2) and is an assembled SNARE complexes (Schiavo et al. essential property of calmodulin, which permits 1997; Rickman et al. 2006) so that Ca2þ- this protein to interact with a large and diverse induced interaction with phospholipids can array of interacting partners. The significant occur rapidly (Xue et al. 2008). It is still under conformational changes on binding to its tar- debate how important synaptotagmin is in gets (Fallon et al. 2005) can increase its affinity vesicle docking (de Wit et al. 2009) and how it for Ca2þ. acts at the plasma membrane in fusion itself Calmodulin is present in brain at high con- (Tang et al. 2006; Hui et al. 2009). Synaptotag- centrations (up to 100 mM). In addition to min could act as a brake on fusion that is its more general functions, calmodulin also has relieved on Ca2þ binding or have a positive a series of specific roles in transducing Ca2þ sig- role in membrane fusion (Chicka et al.