Neuronal calcium sensor proteins: generating diversity in neuronal Ca2+ signalling

Abstract

In neurons, intracellular calcium signals have crucial roles in activating neurotransmitter release and in triggering alterations in neuronal function. Calmodulin has been widely studied as a Ca(2+) sensor that has several defined roles in neuronal Ca(2+) signalling, but members of the neuronal calcium sensor protein family have also begun to emerge as key components in a number of regulatory pathways and have increased the diversity of neuronal Ca(2+) signalling pathways. The differing properties of these proteins allow them to have discrete, non-redundant functions.

Figure 1

Diversity of Ca²⁺ signals in neurons. A wide variety of different Ca²⁺ signals that range from very local pre- or post-presynaptic signals to an increase in Ca²⁺ concentration throughout the neuron can be generated in mature neurons. Three types of Ca²⁺ signal are shown schematically. a Local Ca²⁺ signals that are generated and remain in the pre-synaptic nerve terminal or the post-synaptic dendritic spine as very local Ca²⁺ elevations near to Ca²⁺ channels (nano- or micro-domains) are shown on the left of the figure; more diffuse Ca²⁺ elevations that fill the terminals or spines are shown on the right. b A Ca²⁺ signal that has propagated into part of the dendritic tree close to the site of synaptic inputs but that dissipates before reaching the cell body is shown. c A global Ca²⁺ signal is shown that would occur following more extensive stimulation owing to multiple active synaptic inputs along the dendrite with the generation of a propagating Ca²⁺ wave throughout the neuron (including the nucleus - labelled N). d Different Ca²⁺-regulated processes occur in neurons over a wide range of timescales and their selective activation will depend on the temporal nature of the Ca²⁺ signal.

Figure 2

The structure of NCS proteins . The structure and Ca²⁺-binding properties of NCS proteins, in comparison to calmodulin, are shown. a A schematic representation of the general domain structure of NCS proteins. These proteins contain 4 EF hands, the first of which cannot bind Ca²⁺. Many family members possess an N-terminal myristoyl group. b The crystal structure of human NCS-1 in the Ca²⁺-bound form (PDB ID code 1G8I) showing the presence of three bound Ca²⁺ ions (green spheres) in EF hands 2-4. c The crystal structure of rat calmodulin in Ca²⁺-loaded form (PDB ID code 3CLN) with four bound Ca²⁺ ions. d Comparison of the Ca²⁺ affinities of NCS proteins and that of calmodulin. The figures show schematic binding curves which are modelled on the Ca²⁺ -dependency of the Ca²⁺/myristoyl switch of hippocalcin in live cells , the in vitro binding of Ca²⁺ to NCS-1 and for calmodulin is based on Ca²⁺ binding in vitro to calmodulin alone or with a bound peptide from its target CaMKII . e A comparison of the Ca²⁺ -dependency of activation of retinal guanylyl cyclase in the presence of 1 mM Ca²⁺. The schematic binding curves are modelled on in vitro data . The range of measured Ca²⁺ concentrations in mouse rod photoreceptors is shaded in blue.

Figure 3

A protein-protein interaction map showing the known interactions of NCS-1. The figure summarises data on the known protein interactions made by NCS-1 proteins and indicates whether these are Ca²⁺-dependent or -independent. AP1, clathrin adaptor protein 1 ; AP2, clathrin adaptor protein 2 ; ARF1, ADP ribosylation factor 1 ; CAPS, Ca²⁺-dependent activator protein of secretion ; calcineurin ; D2R, dopamine receptor type 2 ; GRK2, G-protein-coupled receptor kinase 2 ; IL1RAPL, interleukin 1 receptor associated protein-like protein ; IP₃R, inositol 1,4,5 trisphophate receptor ; Kv4.2, Kv4.2 potassium channel subunit , ; PDE, cyclic nucleotide phosphodiesterase ; PI4K, phosphatidylinositol 4-kinase type IIIβ , ; TGFβR1, the type I receptor for transforming growth factor β ; TRPC5, transient receptor potential channel 5 .

Figure 4

The KChIP class of NCS proteins and their splice variants a Alternative splicing of the four human KChIP genes generates a series of variants with distinct N-terminal domains. Some of the isoforms possess myristoylation or palmitoylation sites (as indicated) that could confer membrane targeting and localization. The isoforms shown here have been shown to be expressed in human tissues There is no generally agreed terminology for KChIP isoforms and those shown are based on the following sequences (Human KChIP1 isoforms: GenBank accession number DQ148478 (1.1), DQ148477 (1.2), DQ148476 (1.3). Human KChIP2 isoforms: NM_01491 (2.1), DQ148480 (2.2), DQ148481 (2.3), DQ148482 (2.4), DQ148483 (2.5). Human KChIP3 isoforms: DQ148485 (3.1), DQ148486 (3.2). Human KChIP4 isoforms: DQ148487 (4.1), DQ148488 (4.2), DQ148491 (4.3), DQ148489 (4.4), DQ148490 (4.5), DQ148492 (4.6)). b When expressed in COS-7 cells as fluorescently tagged proteins, the isoforms of each of the four of the KChIP genes show different localization patterns. KChIP1.2 is present on vesicular structures, KChIP2.3 is expressed on the plasma membrane, and KCHIPs 3.1 and 4.1 are diffuse and cytosolic.