Contact-dependent regulation of N-type calcium channel subunits during synaptogenesis

Regulation of N-type voltage-gated calcium channels (Cav2.2) and transmitter release by collapsin response mediator protein-2 (CRMP-2) in sensory neurons

Collapsin response mediator proteins (CRMPs) mediate signal transduction of neurite outgrowth and axonal guidance during neuronal development. Voltage-gated Ca(2+) channels and interacting proteins are essential in neuronal signaling and synaptic transmission during this period. We recently identified the presynaptic N-type voltage-gated Ca(2+) channel (Cav2.2) as a CRMP-2-interacting partner. Here, we investigated the effects of a functional association of CRMP-2 with Cav2.2 in sensory neurons. Cav2.2 colocalized with CRMP-2 at immature synapses and growth cones, in mature synapses and in cell bodies of dorsal root ganglion (DRG) neurons. Co-immunoprecipitation experiments showed that CRMP-2 associates with Cav2.2 from DRG lysates. Overexpression of CRMP-2 fused to enhanced green fluorescent protein (EGFP) in DRG neurons, via nucleofection, resulted in a significant increase in Cav2.2 current density compared with cells expressing EGFP. CRMP-2 manipulation changed the surface levels of Cav2.2. Because CRMP-2 is localized to synaptophysin-positive puncta in dense DRG cultures, we tested whether this CRMP-2-mediated alteration of Ca(2+) currents culminated in changes in synaptic transmission. Following a brief high-K(+)-induced stimulation, these puncta became loaded with FM4-64 dye. In EGFP and neurons expressing CRMP-2-EGFP, similar densities of FM-loaded puncta were observed. Finally, CRMP-2 overexpression in DRG increased release of the immunoreactive neurotransmitter calcitonin gene-related peptide (iCGRP) by approximately 70%, whereas siRNA targeting CRMP-2 significantly reduced release of iCGRP by approximately 54% compared with control cultures. These findings support a novel role for CRMP-2 in the regulation of N-type Ca(2+) channels and in transmitter release.

straightjacket is required for the synaptic stabilization of cacophony, a voltage-gated calcium channel alpha1 subunit

In a screen to identify genes involved in synaptic function, we isolated mutations in Drosophila melanogaster straightjacket (stj), an α₂δ subunit of the voltage-gated calcium channel. stj mutant photoreceptors develop normal synaptic connections but display reduced “on–off” transients in electroretinogram recordings, indicating a failure to evoke postsynaptic responses and, thus, a defect in neurotransmission. stj is expressed in neurons but excluded from glia. Mutants exhibit endogenous seizure-like activity, indicating altered neuronal excitability. However, at the synaptic level, stj larval neuromuscular junctions exhibit approximately fourfold reduction in synaptic release compared with controls stemming from a reduced release probability at these synapses. These defects likely stem from destabilization of Cacophony (Cac), the primary presynaptic α₁ subunit in D. melanogaster. Interestingly, neuronal overexpression of cac partially rescues the viability and physiological defects in stj mutants, indicating a role for the α₂δ Ca²⁺ channel subunit in mediating the proper localization of an α₁ subunit at synapses.

Role of the synprint site in presynaptic targeting of the calcium channel Ca V 2.2 in hippocampal neurons

Sequences in the cytoplasmic II-III loop of CaV2 voltage-gated calcium channels, termed the synaptic protein interaction (synprint) site, are considered important for the functional incorporation of presynaptic calcium channels into the synaptic vesicle fusion apparatus. Two novel CaV2.2 splice variants lack large parts of the cytoplasmic II-III loop (Delta1 R756-L1139, Delta2 K737-A1001) including the synprint protein-protein interaction domain. Here we expressed green fluorescent protein (GFP)-alpha1B subunit fusion constructs of CaV2.2 splice variants in mouse hippocampal neurons to study their distribution in distinct neuronal compartments and to address the question of whether and how the synprint site functions in the presynaptic targeting of N-type calcium channels. Similar to full-length GFP-alpha1B but divergent from the somatodendritic alpha1C-HA (CaV1.2) channel type, the splice variants GFP-alpha1B-Delta1 and GFP-alpha1B-Delta2 were targeted into the axons. Nevertheless, their ability to form bona fide presynaptic clusters was almost abolished for GFP-alpha1B-Delta1 and significantly reduced for GFP-alpha1B-Delta2. Thus, the synprint site is important for normal synaptic targeting of CaV2.2 but not essential. Conversely, insertion of the synprint site into the II-III loop of alpha1C-HA did not restore axonal targeting or synaptic clustering. Together these results indicate that protein-protein interactions with the synprint site must cooperate with other targeting mechanisms in the incorporation of CaV2.2 into presynaptic specializations of hippocampal neurons but are neither necessary nor sufficient for axonal targeting. The unique targeting properties of the splice variants lacking the synprint site are suggestive of specific functions of these calcium channels apart from activating fast synaptic transmission.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

A tctex1-Ca2+ channel complex for selective surface expression of Ca2+ channels in neurons

Voltage-gated Ca(2+) channels (VGCCs) are important in regulating a variety of cellular functions in neurons. It remains poorly understood how VGCCs with different functions are sorted within neurons. Here we show that the t-complex testis-expressed 1 (tctex1) protein, a light-chain subunit of the dynein motor complex, interacts directly and selectively with N- and P/Q-type Ca(2+) channels, but not L-type Ca(2+) channels. The interaction is insensitive to Ca(2+). Overexpression in hippocampal neurons of a channel fragment containing the binding domain for tctex1 significantly decreases the surface expression of endogenous N- and P/Q-type Ca(2+) channels but not L-type Ca(2+) channels, as determined by immunostaining. Furthermore, disruption of the tctex1-Ca(2+) channel interaction significantly reduces the Ca(2+) current density in hippocampal neurons. These results underscore the importance of the specific tctex1-channel interaction in determining sorting and trafficking of neuronal Ca(2+) channels with different functionalities.

Differential targeting of the L-type Ca2+ channel alpha1C (CaV1.2) to synaptic and extrasynaptic compartments in hippocampal neurons

In central nervous system neurons L-type Ca2+ channels are involved in developmental processes, the integration and conduction of postsynaptic electric activity and synaptic plasticity. However, little is known about the channel isoforms underlying each of these functions or about the exact localization and targeting properties of the major L-type channel isoform alpha1C (CaV1.2) in neurons. We addressed these questions using high-resolution immunofluorescence analysis of the endogenous alpha1C and epitope-tagged recombinant channel isoforms expressed in mouse hippocampal neurons. Endogenous alpha1C and surface-expressed hemagglutinin (HA)-tagged alpha1C-HA were localized in small clusters distributed between the axon initial segment and the apical branches of the dendritic tree. The average cluster size was estimated to be eight channels per alpha1C-HA cluster. Analysis of the subcellular localization of alpha1C-HA clusters relative to known synaptic markers suggested the existence of two distinct populations of alpha1C clusters, extrasynaptic and synaptic, the latter associated with glutamatergic synapses in dendritic spines. Both glutamatergic and GABAergic neurons expressed alpha1C in the soma and dendrites. In contrast to the N-type channel GFP-alpha1B, GFP-alpha1C was excluded from distal axons and nerve terminals of mature neurons. In developing neurons, however, alpha1C and alpha1C-HA were robustly expressed in the growth cone, indicating that specific targeting properties of neuronal compartments change during differentiation. Synaptic and extrasynaptic localizations of alpha1C correspond to putative roles of L-type Ca2+ currents in synaptic modulation and in the propagation of dendritic Ca2+ spikes, respectively. The transiently expressed alpha1C in the growth cone may be involved in neurite extension and axonal pathfinding.

Development of Ca2+-channel and BK-channel expression in embryos and larvae of Xenopus laevis

The patterns and density of channels expressed in neurons critically determine their electrical properties. We have examined developmental regulation of Ca2+-channel expression during the maturation of the spinal motor circuits in Xenopus as it develops from an embryo to a larva. In embryonic neurons approximately 60% of the current is carried by N-type channels, 8% by l-type channels and the remainder by an unidentified channel. As the embryo matures, omega-agatoxin-sensitive P/Q channels are gradually expressed and replace the unidentified HVA channel such that at stage 42 approximately 25% of the current is carried by P/Q channels. We have used fluorescent labelling of selective channel toxins to directly observe the distribution of P/Q, N and BK channels. The P/Q channel distribution was most prevalent on the cell surface proximal to the areas of the soma where processes emerge. Both N and BK channels were distributed throughout the soma but still exhibited concentration around the areas adjacent to the emergence of processes from the soma. The patterns of fluorescence labelling during development mirrored the development of the respective ionic currents. Both N and P/Q channels contribute roughly equally to activation of the BK current, suggesting that overlap in the distribution of the N, P/Q and BK channels is important in their functional interdependence. The newly expressed P/Q channels play a role in spike initiation and repetitive firing in larval spinal neurons and contribute to burst generation during swimming in the larva.

Cell-cell signaling during synapse formation in the CNS

Synapses join individual nerve cells into a functional network. Specific cell-cell signaling events regulate synapse formation during development and thereby generate a highly reproducible connectivity pattern. The accuracy of this process is fundamental for normal brain function, and aberrant connectivity leads to nervous system disorders. However, despite the overall precision with which neuronal circuits are formed, individual synapses and synaptic networks are also plastic and can readily adapt to external stimuli or perturbations. In recent studies, several trans-synaptic signaling systems have been identified that can mediate various aspects of synaptic differentiation in the central nervous system. It appears that these individual pathways functionally cooperate, thereby generating robustness and flexibility, which ensure normal nervous system function.

Astrocytes selectively enhance N-type calcium current in hippocampal neurons

Astrocytes influence neuronal development, synapse formation, and synaptic transmission, partly through affecting neuronal calcium signals. In order to elucidate the extent to which astrocytes modulate neuronal voltage-gated calcium currents, we performed a whole-cell patch clamp analysis of neurons in astrocyte-deplete and astrocyte-enriched conditions. We demonstrate that hippocampal neurons in an astrocyte-enriched environment show augmentation of voltage-gated calcium current at 1-3 days in vitro. Further study in pairs of adjacent neurons showed that the augmentation in calcium current was dependent on direct contact with the astrocyte. Pharmacological analysis demonstrated the augmentation is selective for the N-type calcium current, although immunochemical labeling of the alpha1(B) subunit of the N-type calcium channel was unchanged. These findings show that astrocytes regulate neuronal voltage-gated calcium currents in a contact-dependent manner. The specificity of the effect for the N-type calcium current at early days in culture has special significance regarding the role of astrocytes in hippocampal synaptogenesis.

Synaptic targeting of N-type calcium channels in hippocampal neurons

N-type calcium (Ca2+) channels play a critical role in synaptic function, but the mechanisms responsible for their targeting in neurons are poorly understood. N-type channels are formed by an alpha(1B) (Ca(V)2.2) pore-forming subunit associated with beta and alpha2delta auxiliary subunits. By expressing epitope-tagged recombinant alpha1B subunits in rat hippocampal neuronal cultures, we demonstrate here that synaptic targeting of N-type channels depends on neuronal contacts and synapse formation. We also establish that the C-terminal 163 aa (2177-2339) of the alpha1B-1 (Ca(V)2.2a) splice variant contain sequences that are both necessary and sufficient for synaptic targeting. By site-directed mutagenesis, we demonstrate that postsynaptic density-95/discs large/zona occludens-1 and Src homology 3 domain-binding motifs located within this region of the alpha1B subunit (Maximov et al., 1999) act as synergistic synaptic targeting signals. We also show that the recombinant modular adaptor proteins Mint1 and CASK colocalize with N-type channels in synapses. We found that the alpha1B-2 (Ca(V)2.2b) splice variant is restricted to soma and dendrites and postulated that somatodendritic and axonal/presynaptic isoforms of N-type channels are generated via alternative splicing of alpha1B C termini. These data lead us to propose that during synaptogenesis, the alpha1B-1 (Ca(V)2.2a) splice variant of the N-type Ca2+ channel pore-forming subunit is recruited to presynaptic locations by means of interactions with modular adaptor proteins Mint1 and CASK. Our results provide a novel insight into the molecular mechanisms responsible for targeting of Ca2+ channels and other synaptic proteins in neurons.

Contact-dependent aggregation of functional Ca2+ channels, synaptic vesicles and postsynaptic receptors in active zones of a neuromuscular junction

To examine whether Ca2+ channels aggregate in a contact-dependent manner, we characterized the distribution of synaptic vesicles and postsynaptic receptors, and compared it to the location of Ca2+ entry sites, in a Xenopus laevis nerve-muscle coculture preparation using a localized Ca2+ detection method. The majority (75%) of Ca2+ entry sites at spontaneously formed nerve-muscle contacts were associated with enhanced immunofluorescence to the synaptic vesicle protein, SV2. In contrast, only 11% of recorded sites without Ca2+ transients exhibited significant SV2 immunofluorescence. When comparing the spatial distribution of synaptic markers with that of Ca2+ entry sites, we found that the majority of Ca2+ entry sites (61%) were associated with both enhanced SV2 immunofluorescence and R-BTX fluorescence, thereby identifying putative neurotransmitter release sites where Ca2+ channels, synaptic vesicles and postsynaptic receptors are colocalized. Using polystyrene beads coated with a heparin binding protein known to mediate in vitro postsynaptic receptor clustering, we show that the location of Ca2+ domains was associated with enhanced SV2 immunofluorescence at neurite-to-bead contacts. We conclude that the localization of functional Ca2+ channels to putative active zones follows a contact-dependent signalling mechanism similar to that known to mediate vesicle aggregation and AChR clustering.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Calcium channels coupled to neurotransmitter release at dually innervated neuromuscular junctions in the newborn rat

We studied the effect of several calcium channel blockers (omega-Conotoxin-GVIA, 1 and 3microM; omega-Agatoxin-IVA, 100nM; Nitrendipine, 1 and 10microM) on evoked transmitter release at singly and dually innervated endplates of the levator auris longus muscle from three- to six-day-old rats. In dually innervated fibers, a second endplate potential may appear after the first one when we increase the stimulation intensity. The lowest and highest endplate potential amplitudes are designated "small endplate potential" and "large endplate potential", respectively. The percentage of doubly innervated junctions remains almost constant throughout the age range examined. Nevertheless, the percentage of junctions innervated by three or more terminal axons drops, whereas the singly innervated junctions increase. Therefore, between postnatal days 3 and 6, roughly half the neuromuscular junctions may experience the final process of axonal elimination. The synaptic efficacy of the large endplate potential in dual junctions, measured as the mean amplitude of the synaptic potential and mean quantal content, was the same as in the junctions that had become recently mono-innervated in the same postnatal period. In singly innervated fibers, the endplate potential size was strongly reduced by both the P/Q-type voltage-dependent calcium channel blocker omega-Agatoxin-IVA (79.17+/-4.02%; P < 0.05) and the N-type voltage-dependent calcium channel blocker omega-Conotoxin-GVIA (56.31+/-7.80%; P < 0.05), whereas endplate potential amplitude was not significantly changed by the L-type voltage-dependent calcium channel blocker Nitrendipine. In dually innervated fibers, the P/Q-type voltage-dependent calcium channel blocker omega-Agatoxin-IVA and L-type voltage-dependent calcium channel blocker Nitrendipine increased the size of the small endplate potential (161.29+/-47.87% and 109.32+/-11.03%, respectively; P < 0.05 in both cases) and reduced the large endplate potential (74.42+/-15.32% and 70.91+/-10.04%, respectively; P < 0.05 in both cases). The N-type voltage-dependent calcium channel blocker omega-Conotoxin-GVIA significantly increased the small endplate potential in the first few minutes after toxin application (at 10min: 90.23+/-17.38%; P < 0.05). This increase was not maintained, while the large endplate potential was strongly inhibited (69.25+/-7.5%; P < 0.05). In conclusion, in the dually innervated endplates of the newborn rat, presynaptic calcium channel types can have different roles in transmitter release from each of the two inputs, which suggests that nerve terminal voltage-dependent calcium channels are involved in neonatal synaptic maturation.

Alternative splicing in intracellular loop connecting domains II and III of the alpha 1 subunit of Cav1.2 Ca2+ channels predicts two-domain polypeptides with unique C-terminal tails

Novel splice variants of the alpha(1) subunit of the Ca(v)1.2 voltage-gated Ca(2+) channel were identified that predicted two truncated forms of the alpha(1) subunit comprising domains I and II generated by alternative splicing in the intracellular loop region linking domains II and III. In rabbit heart splice variant 1 (RH-1), exon 19 was deleted, which resulted in a reading frameshift of exon 20 with a premature termination codon and a novel 19-amino acid carboxyl-terminal tail. In the RH-2 variant, exons 17 and 18 were deleted, leading to a reading frameshift of exons 19 and 20 with a premature stop codon and a novel 62-amino acid carboxyl-terminal tail. RNase protection assays with RH-1 and RH-2 cRNA probes confirmed the expression in cardiac and neuronal tissue but not skeletal muscle. The deduced amino acid sequence from full-length cDNAs encoding the two variants predicted polypeptides of 99.0 and 99.2 kDa, which constituted domains I and II of the alpha(1) subunit of the Ca(v)1.2 channel. Antipeptide antibodies directed to sequences in the second intracellular loop between domains II and III identified the 240-kDa Ca(v)1.2 subunit in sarcolemmal and heavy sarcoplasmic reticulum (HSR) membranes and a 99-kDa polypeptide in the HSR. An antipeptide antibody raised against unique sequences in the RH-2 variant also identified a 99-kDa polypeptide in the HSR. These data reveal the expression of additional Ca(2+) channel structural units generated by alternative splicing of the Ca(v)1.2 gene.

Different localizations and functions of L-type and N-type calcium channels during development of hippocampal neurons

Using immunocytochemical assays and patch-clamp and calcium-imaging recordings, we demonstrate that L-type and N-type calcium channels have distinct patterns of expression and distribution and play different functional roles during hippocampal neuron differentiation. L-type channels, which support the depolarization-induced calcium influx in neurons from the very early developmental stages, are functionally restricted to the somatodendritic compartment throughout neuronal development and play a crucial role in supporting neurite outgrowth at early developmental stages. N-type channels, which start contributing at later neuronal differentiation stages (3-4 DIV), are also functionally expressed in the axons of immature neurons. At this developmental stage preceding synaptogenesis, N-type (but not L-type) channels are involved in controlling synaptic vesicle recycling. It is only at later developmental stages (10-12 DIV), when the neurons have established a clear axodendritic polarity and form synaptic contacts, that N-type channels are progressively excluded from the axon. Electrophysiological recordings of single neurons growing in microislands revealed that synaptic maturation coincides with a progressive increase in N-type channels in the somatodendritic region and a progressive decrease in the N-type channels supporting glutamate release from the presynaptic terminal. These results indicate that L-type and N-type calcium channels undergo dynamic, developmentally regulated rearrangements in regional distribution and function and also suggest that different mechanisms may be involved in the sorting and/or stabilization of these two types of channels in different plasma membrane domains during neuronal differentiation.

Regeneration as an application of gastropod neural plasticity

Gastropod research is providing many insights into mechanisms of neural regeneration. These observations were made possible by the pioneering work of individuals who described the nervous systems of gastropods, mapped prominent neurons and determined their roles and connections, and developed the techniques for culturing them. This information has allowed questions about injury responses, target selection, and pathway cues to be explored at the level of individually identified neurons. Because of gastropod studies, more is known about axon sealing, growth cone formation and behavior, signals that travel from the site of axotomy to the soma, and the second messengers that are activated there. The responses in neurons and non-neuronal cells during neural development and injury are coordinated by chemical messenger systems that are highly conserved, including neurotransmitters, cytokines, and neurotrophins. The nervous system is modified in learning paradigms by some of the same messenger systems activated by injury, because learning and injury both challenge neurons to change. The conservation of basic mechanisms that coordinate neuronal plasticity allows us to approach basic questions in relatively simple nervous systems with reasonable confidence that the findings will be relevant for other nervous systems, including possible applications to the mammalian nervous system.