Class D L-Type Ca2+ Channels

Research project P 14820 Class D L-Typ Ca2+ Channels Jörg STRIESSNIG 27.11.2000 The aim of this project is to assess the pharmacotherapeutic potential of class D L-type Ca2+ channels (LTCCs) by studying their functional properties, their expression in various tissues and their modulation by drugs. It directly continues our successful work from FWF project P12641-MED ("Targeted Disruption of Class C Ca2+ Channel Dihydropyridine Sensitivity"; expiring November 2000). Using homologous recombination in mice we successfully generated two mouse models, alpha1D-deficient mice (alpha1D-l-) and mice with dihydropyridineinsensitive a I C subunits (a I cll-). These are now available for studying class D Ca2+ channel function in detail. We already discovered an important physiological role of class D channels for stimulus-secretion coupling in cochlear inner hair cells and the pacemaker activity of the sinoatrial node. In this project we will study the expression, functional and pharmacological properties of D-LTCCs in neurons, the cardiovascular system (including sinoatrial node cells) and sensory cells in vitro. Using a1CDHP- mice the role of class D L-type Ca2+ channels for central nervous system, cardiovascular, sensory function and insulin secretion can be studied in vivo and their relative contribution to total Ca2+ current in different cells can be determined. Our previous studies have revealed unusual gating kinetics and drug sensitivity of class D channels. Therefore the effect of alternative splicing on gating and modulation by Ca2+ channel blockers will be investigated in recombinant class D channels. Our experiments will answer the important question if selective modulators of these channels comprise interesting drugs for the clinical therapy, such as the of hearing disorders or cardiovascular diseases.

The aim of this project was to investigate the physiological role of voltage-gated L-type calcium channels. These channels mediate the selective influx of calcium ions from the extracellular fluid into the cell upon membrane depolarization and thereby control important physiological processes, such as muscle contraction, hormone and neurotransmitter secretion and processes of neuronal plasticity. It is known that L-type calcium channels can be formed by four different isoforms. We therefore developed animal models that allowed to pharmacologically and biochemically dissect the physiological significance of these different isoforms. This has important therapeutic implications because once the physiological role of individual channel isoforms is known, isoform-selective drugs may allow to affect defined body functions. Using twow newly developed mouse models we could demonstrate for the first time that the inhibition of Cav1.2 channels in the brain causes antidepressant-like effects in mice, whereas activation of Cav1.3 is sufficient to induce depression-like behavior. This clearly demonstrates the role of these channels for the control of mood behavior. We found that Cav1.3 but not Cav1.2 is important for the control of the heart rate in the sinoatrial node, that Cav1.2 but not Cav1.3 controls insulin release from pancreatic beta cells and that Cav1.3 forms the majority of L-type channel currents in the inner and outer hair cells of the cochlea. We also found that a particular amino acid residue of the so-called a1 subunit of these calcium channels plays an important role for the block and activation of these channels by drugs. In the retina Cav1.4 channels are the most importnat calcium channel isoform. We succeeded in analyzing the functional properties of this channel which is difficult to study in vitro. We found that this channel possesses unique functional properties which make it highly suitable to support tonic neurotransmitter release from retinal photoreceptors in response to changes in illumination. From our experiments we can conclude that Cav1.2 selective modulators with efficient permeation into the CNS are likely to exert antidepressant effects. Alternatively, Cav1.3 selective inhibitors would be suitable to lower heart rate without affecting cardiac contractile force and could thus be suitable to treat patients with heart failure. Ongoing studies aim at revealing the molecular mechanisms through which L-type calcium channels exert their neuromodulatory effects and to understand their fine-tuning by other signaling pathways.