Neuronal Ca2+-activated Cl− channels — homing in on an elusive channel species
Introduction
Cl− channels are a large, ubiquitous and highly diverse group of ion channels involved in many physiological key processes including: regulation of electrical excitability; muscle contraction; secretion; and sensory signal transduction. Cl− channels belonging to three distinct channel families have been characterized in detail, including molecular structure and tissue distribution [for review, see Jentsch and Günther (1997)]: voltage-gated Cl− channels, the adenosine 3′:5′-cyclic monophosphate (cAMP)-regulated channel CFTR (cystic fibrosis transmembrane conductance regulator), and ligand-gated Cl− channels that open upon binding of the neurotransmitters γ-aminobutyric acid (GABA) or glycine. A fourth family of Cl− channels is regulated by the cytosolic Ca2+ concentration, [Ca2+]i, and much less is known about structure and function of this group of Cl− channels. Ca2+-activated Cl− channels have been identified in a number of peripheral and central populations of neurons. They open in response to an increase of [Ca2+]i that follows Ca2+ influx during electrical excitation. Biophysical characterization of Ca2+-activated Cl− channels has been hampered by several factors intrinsic to the channel proteins (small channel conductance, loss of channel activity in cell-free preparations) as well as by limited experimental access to neuronal cell compartments that express these channels (dendritic and ciliary processes). Furthermore, the physiological functions that these channels subserve in signal processing are mostly unclear because the Cl− equilibrium potential, ECl, is not known so that polarity and amplitude of Cl− currents cannot be assessed. This state of affairs is particularly lamentable because the membrane conductance induced by Ca2+-activated Cl− channels can be large and is expected to profoundly affect electrical excitability of the neurons.
An exciting new development in this field is the discovery and biophysical characterization of Ca2+-activated Cl− channels in vertebrate olfactory sensory neurons which are the first neuronal channels of this family that have been investigated in some detail. These channels are expressed in the sensory cilia of olfactory neurons where they are involved in generating the receptor current during odor detection. Electrophysiological recordings have revealed some of the key properties, including ion selectivity, channel conductance, and Ca2+ sensitivity. Furthermore, an estimate has been obtained for ECl which governs Cl− flux through the channels in situ. With this set of data, a concept has been developed for the role of this channel in sensory signal transduction that may also be helpful for understanding the function of Ca2+-activated Cl− channels in other neurons. However, in the absence of any structural information about the channel protein, investigation of more complex questions, including regulation of channel expression and channel activity, is difficult.
Research into Ca2+-activated Cl− channels in non-neuronal tissues, in particular epithelia and smooth muscle, has been very successful in recent years and has led to the cloning of the first genes encoding such channels in secretory epithelia. We expect that the advanced knowledge of these Ca2+-activated Cl− channels will stimulate research of neuronal channels. To help with this process, we collect in the first part of this review published data about Ca2+-activated Cl− channels from various neuronal and non-neuronal tissues and try to establish common as well as diverse features characteristic of this channel family. This synopsis may be useful to extract information about the molecular properties of the channels and may aid neurophysiologists in devising experiments that lead to further understanding of structure and function. In the second part, we describe the Ca2+-activated Cl− channels of olfactory sensory neurons. We show how the channels are activated during odor detection and how they contribute to the receptor current and to electrical excitation of the cell. We then review the literature on Ca2+-activated Cl− channels in other sensory neurons, as well as in spinal cord neurons and neurons of the autonomic nervous system. Finally, we discuss various aspects of ECl, which determines direction and amplitude of Cl− currents conducted by Ca2+-activated Cl− channels and is, therefore, a key parameter for understanding their physiological function.
The family of Ca2+-activated Cl− channels comprises at least two functionally distinct groups of channels:
- 1.
Ca2+-gated Cl− channels open when Ca2+ ions bind to specific Ca2+-binding sites on the cytosolic side of the channel protein. Ca2+ is the only factor necessary to induce channel opening. Thus, Ca2+-gated Cl− channels are ligand-operated channels.
- 2.
Ca2+/calmodulin-dependent protein kinase II (CaMK II)-activated Cl− channels also open in response to an increase of [Ca2+]i, but channel activation is mediated by CaMK II. These channels may not possess a Ca2+-binding site; they are probably activated upon phosphorylation of the channel protein.
A pertinent test to distinguish between these gating mechanisms is to separate channels from the cytosol by excising membrane patches or by reconstituting native channels in artificial lipid bilayers. If the channels open upon application of Ca2+ in the absence of any other cellular constituents, in particular kinases and adenosine 5′-triphosphate (ATP), they are identified as Ca2+-gated Cl− channels. CaMK II-activated Cl− channels can be identified by recording Cl− channels from intact cells when channel activity is induced during elevation of the cytosolic Ca2+ concentration. The dependence on kinase activity can be probed with various CaMK II inhibitors and by using a Ca2+-independent form of CaMK II. However, on the basis of currently available data, a clear-cut distinction as to which of the two groups a particular channel belongs can often not be made. Firstly, channels in only a few cell types have been rigorously investigated with regard to their gating mechanism. Secondly, it will be shown that Ca2+-gated Cl− channels may also be a substrate for CaMK II, and that phosphorylation can profoundly alter Ca2+ sensitivity. These channels may only respond to physiological levels of [Ca2+]i when they are phosphorylated by CAMK II. Thus, we use the term Ca2+-gated Cl− channels only in cases where channel activation by Ca2+ alone has been demonstrated. We refer to all other channels as Ca2+-activated Cl− channels.
As this review is on neuronal channels, we will not describe the functional tasks that Ca2+-activated Cl− channels perform in non-neuronal cell types. For discussion of these physiological aspects we refer the reader to recent reviews by Scott et al., 1995, Carl et al., 1996, and Large and Wang (1996).
Section snippets
Ion selectivity
The ability of Ca2+-activated Cl− channels to discriminate between anions and cations has been assessed by applying defined concentrations of Cl− and Na+ or K+ to both sides of the membrane and measuring the reversal voltage Vrev of the channel current. In a simple concept for ion permeation, Vrev is related to ion concentrations according to:where R is the molar gas constant, T the absolute temperature, F the Faraday constant, and the indices i
Olfactory signal transduction
A Ca2+-gated Cl− channel plays a central role in transduction of odorous stimuli in vertebrates. Although little is known about the structure of this channel, its physiological function is now well established.
In vertebrates, transduction of an odorous stimulus into an electrical signal occurs in olfactory receptor neurons. The neurons lie in an olfactory epithelium that lines parts of the nasal cavity. Two processes extend from the soma of each olfactory receptor neuron (Fig. 14). A single,
Cl− homeostasis: three cellular strategies
As discussed in the preceding sections, it is often difficult to assess the precise physiological function of Ca2+-activated Cl− channels for lack of information about the Cl− equilibrium potential ECl that, together with the membrane voltage Vm, constitutes the driving force for current flow through these channels. In contrast to permeant cations which show an almost stereotypical distribution across the plasma membrane of most cell types, Cl− concentration gradients can be quite diverse [for
Perspectives
Ca2+-activated Cl− channels appear to participate in signal processing of various sensory modalities, including at least some of the somatic senses as well as visual, gustatory and olfactory perception. The channels are also present in non-sensory neurons of the spinal cord and the autonomic nervous system, and further investigation may demonstrate an even more widespread expression in the nervous system. Of particular interest is the subcellular localization: channels may be located in the
Acknowledgements
The authors thank Dieter Grammig for skilful help with the art work. This work was supported by the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm `Molekulare Sinnesphysiologie'.
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