Activity-dependent gene expression triggered by Ca2+ entry into neurons is critical for learning and memory, but whether specific sources of Ca2+ act distinctly or merely supply Ca2+ to a common pool remains uncertain. Here we report that both signaling modes co-exist and pertain to CaV1 and CaV2 channels, respectively, coupling membrane depolarization to CREB phosphorylation and gene expression. CaV1 channels are advantaged in their voltage-dependent gating and use nanodomain Ca2+ to drive local CaMKII aggregation and trigger communication with the nucleus. By contrast, CaV2 channels must elevate [Ca2+]i microns away and promote CaMKII aggregation at CaV1 channels. Consequently, CaV2 channels are ∼10-fold less effective in signaling to the nucleus than CaV1 channels for the same bulk [Ca2+]i increase. Furthermore, CaV2-mediated Ca2+ rises are preferentially curbed by uptake into the endoplasmic reticulum and mitochondria. This source-biased buffering limits the spatial spread of Ca2+, further attenuating CaV2-mediated gene expression.

Communication between cell surface proteins and the nucleus is integral to many cellular adaptations. In the case of ion channels in excitable cells, the dynamics of signaling to the nucleus are particularly important because the natural stimulus, surface membrane depolarization, is rapidly pulsatile. To better understand excitation–transcription coupling we characterized the dependence of cAMP response element–binding protein phosphorylation, a critical step in neuronal plasticity, on the level and duration of membrane depolarization. We find that signaling strength is steeply dependent on depolarization, with sensitivity far greater than hitherto recognized. In contrast, graded blockade of the Ca2+ channel pore has a remarkably mild effect, although some Ca2+ entry is absolutely required. Our data indicate that Ca2+/CaM-dependent protein kinase II acting near the channel couples local Ca2+ rises to signal transduction, encoding the frequency of Ca2+ channel openings rather than integrated Ca2+ flux—a form of digital logic.

Ca2؉ enters pituitary and pancreatic neuroendocrine cells through dihydropyridine-sensitive channels triggering hormone release. Inhibitory metabotropic receptors reduce Ca 2؉ entry through activation of pertussis toxin-sensitive G proteins leading to activation of K ؉ channels and voltage-sensitive inhibition of L-type channel activity. Despite the cloning and functional expression of several Ca 2؉ channels, those involved in regulating hormone release remain unknown. Using reverse transcription-polymerase chain reaction we identified mRNAs encoding three ␣ 1 (␣ 1A , ␣ 1C , and ␣ 1D ), four ␤, and one ␣ 2 -␦ subunit in rat pituitary GH 3 cells; ␣ 1B and ␣ 1S transcripts were absent. GH 3 cells express multiple alternatively spliced ␣ 1D mRNAs. Many of the ␣ 1D transcript variants encode "short" ␣ 1D (␣ 1D-S ) subunits, which have a QXXER amino acid sequence at their C termini, a motif found in all other ␣ 1 subunits that couple to opioid receptors. The other splice variants identified terminate with a longer C terminus that lacks the QXXER motif (␣ 1D-L ). We cloned and expressed the predominant ␣ 1D-S transcript variants in rat brain and GH 3 cells and their ␣ lD-L counterpart in GH 3 cells. Unlike ␣ 1A channels, ␣ 1D channels exhibited current-voltage relationships similar to those of native GH 3 cell Ca 2؉ channels, but lacked voltage-dependent G protein coupling. Our data demonstrate that alternatively spliced ␣ 1D transcripts form functional Ca 2؉ channels that exhibit voltage-dependent, G protein-independent facilitation. Furthermore, the QXXER motif, located on the C terminus of ␣ 1D-S subunit, is not sufficient to confer sensitivity to inhibitory G proteins.

Voltage-gated Ca 2؉ channels (VGCCs) are membrane proteins that determine the activity and survival of neurons, and mutations in the P/Q-type VGCCs are known to cause cerebellar ataxia. VGCC dysfunction may also underlie acquired peripheral and central nervous system diseases associated with small-cell lung cancer, including Lambert-Eaton myasthenic syndrome (LEMS) and paraneoplastic cerebellar ataxia (PCA). The pathogenic role of anti-VGCC antibody in LEMS is well established. Although anti-VGCC antibody is also found in a significant fraction of PCA patients, its contribution to PCA is unclear. Using a polyclonal peptide antibody against a major immunogenic region in P/Q-type VGCCs (the extracellular Domain-III S5-S6 loop), we demonstrated that such antibody was sufficient to inhibit VGCC function in neuronal and recombinant VGCCs, alter cerebellar synaptic transmission, and confer the phenotype of cerebellar ataxia. Our data support the hypothesis that anti-VGCC antibody may play a significant role in the pathogenesis of cerebellar dysfunction in PCA.Lambert-Eaton myasthenic syndrome ͉ paraneoplastic ͉ P/Q-type ͉ N-type ͉ neurotransmission T he association between anti-voltage-gated Ca 2ϩ channel (VGCC) antibody and paraneoplastic cerebellar ataxia (PCA) dates back several decades to clinical observations of the coexistence of small-cell lung cancer with either cerebellar ataxia, Lambert-Eaton myasthenic syndrome (LEMS), or both (1, 2). The majority of these cancer patients with neurological symptoms have antibody against different types of VGCCs, especially P/Q-and N-type (3-5). The presence of different antibodies may be the consequence of an autoimmune response against the cancer cells (6, 7), known to express different VGCCs (8). There is conclusive evidence that the peripheral disease LEMS is caused by anti-VGCC antibodies, which diminish the availability of P/Q-type channels of the motor nerve terminals (9, 10).In contrast, much less is known about the origin of cerebellar ataxia associated with anti-VGCC antibody, although VGCCs are prominent in cerebellar neurons (11,12), and mutations in the P/Q-type VGCC cause ataxia (13). PCA patients have a high titer of anti-VGCC antibody (14-17) and undergo a selective loss of P/Q-type VGCC-containing cerebellar neurons (2, 18). Sera from LEMS patients, known to contain anti-VGCC antibodies, reduce P/Q-type VGCC surface expression in cerebellar granule and Purkinje neurons (19), consistent with an overlap of clinical syndromes between LEMS and PCA and, possibly, of pathogenic mechanism. There is no evidence to date that passive transfer of sera from LEMS or PCA patients is sufficient to cause central nervous system disease. Based on epitope mapping of antibody repertoire in patients with paraneoplastic neurological syndromes and small-cell lung cancer, we generated an antibody against a major epitope in the P/Q-type VGCC and evaluated its ability to affect cerebellar VGCC function and motor behavior. ResultsFunctional Effects of Anti-VGCC Antibody. We looked for...

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