Calcium concentration jumps reveal dynamic ion selectivity of calcium-activated chloride currents in mouse olfactory sensory neurons and TMEM16b-transfected HEK 293T cells

The anoctamin family: TMEM16A and TMEM16B as calcium-activated chloride channels

The Ca(2+)-activated Cl(-) channels (CaCCs) are involved in a variety of physiological functions, such as transepithelial anion transport, smooth muscle contraction and olfaction. Recently, the question of the molecular identity of CaCCs has apparently been resolved with the identification of TMEM16A protein (also known as anoctamin-1). Expression of TMEM16A is associated with the appearance of Ca(2+)- and voltage-dependent Cl(-) currents with properties similar to those of native CaCCs. The putative structure of TMEM16A consists of eight transmembrane domains, with both the amino- and the carboxy-terminus protruding in the cytosol. TMEM16A is also characterized by the existence of different protein variants generated by alternative splicing. A close paralogue of TMEM16A, TMEM16B (anoctamin-2), is also associated with CaCC activity, although with different properties. The TMEM16B-dependent channels require higher intracellular Ca(2+) concentrations and have faster activation and deactivation kinetics. Expression of other anoctamins is devoid of detectable channel activity. These proteins, such as TMEM16F (anoctamin-6), may have different functions.

Sex differences in the inflammatory mediator-induced sensitization of dural afferents

Approximately 20% of the adult population suffers from migraine. This debilitating pain disorder is three times more prevalent in women than in men. To begin to evaluate the underlying mechanisms that may contribute to this sex difference, we tested the hypothesis that there is a sex difference in the inflammatory mediator (IM)-induced sensitization of dural afferents. Acutely dissociated retrogradely labeled dural afferents from adult Sprague-Dawley rats were examined with whole cell patch-clamp recordings. Baseline passive and active electrophysiological properties of dural afferents from both sexes were comparable. However, while IM-induced increases in the excitability of dural afferents from male and female rats were also comparable, the proportion of dural afferents from female rats sensitized by IM (~100%) was significantly greater than that of dural afferents from male rats (~50%). This appeared to be due to differences downstream of IM receptors, as tetrodotoxin-resistant sodium current was increased by IM in a majority of male dural afferents (13/14). These data indicate that there are both quantitative and qualitative differences in the IM-induced sensitization of dural afferents that may contribute to the sex difference in the manifestation of migraine.

Physiological roles and diseases of Tmem16/Anoctamin proteins: are they all chloride channels?

The Tmem16 gene family was first identified by bioinformatic analysis in 2004. In 2008, it was shown independently by 3 laboratories that the first two members (Tmem16A and Tmem16B) of this 10-gene family are Ca(2+)-activated Cl(-) channels. Because these proteins are thought to have 8 transmembrane domains and be anion-selective channels, the alternative name, Anoctamin (anion and octa=8), has been proposed. However, it remains unclear whether all members of this family are, in fact, anion channels or have the same 8-transmembrane domain topology. Since 2008, there have been nearly 100 papers published on this gene family. The excitement about Tmem16 proteins has been enhanced by the finding that Ano1 has been linked to cancer, mutations in Ano5 are linked to several forms of muscular dystrophy (LGMDL2 and MMD-3), mutations in Ano10 are linked to autosomal recessive spinocerebellar ataxia, and mutations in Ano6 are linked to Scott syndrome, a rare bleeding disorder. Here we review some of the recent developments in understanding the physiology and structure-function of the Tmem16 gene family.

A minimal isoform of the TMEM16A protein associated with chloride channel activity

TMEM16A protein, also known as anoctamin-1, has been recently identified as an essential component of Ca(2+)-activated Cl(-) channels. We previously reported the existence of different TMEM16A isoforms generated by alternative splicing. In the present study, we have determined the functional properties of a minimal TMEM16A protein. This isoform, called TMEM16A(0), has a significantly shortened amino-terminus and lacks three alternative segments localized in the intracellular regions of the protein (total length: 840 amino acids). TMEM16A(0) expression is associated with Ca(2+)-activated Cl(-) channel activity as measured by three different functional assays based on the halide-sensitive yellow fluorescent protein, short-circuit current recordings, and patch-clamp technique. However, compared to a longer isoform, TMEM16(abc) (total length: 982 amino acids), TMEM16A(0) completely lacks voltage-dependent activation. Furthermore, TMEM16A(0) and TMEM16A(abc) have similar but not identical responses to extracellular anion replacement, thus suggesting a difference in ion selectivity and conductance. Our results indicate that TMEM16A(0) has the basic domains required for anion transport and Ca(2+)-sensitivity. However, the absence of alternative segments, which are present in more complex isoforms of TMEM16A, modifies the channel gating and ion transport ability.

Ca2+-activated Cl− currents are dispensable for olfaction

Canonical olfactory signal transduction involves the activation of cyclic AMP-activated cation channels that depolarize the cilia of receptor neurons and raise intracellular calcium. Calcium then activates Cl(-) currents that may be up to tenfold larger than cation currents and are believed to powerfully amplify the response. We identified Anoctamin2 (Ano2, also known as TMEM16B) as the ciliary Ca(2+)-activated Cl(-) channel of olfactory receptor neurons. Ano2 is expressed in the main olfactory epithelium (MOE) and in the vomeronasal organ (VNO), which also expresses the related Ano1 channel. Disruption of Ano2 in mice virtually abolished Ca(2+)-activated Cl(-) currents in the MOE and VNO. Ano2 disruption reduced fluid-phase electro-olfactogram responses by only ∼40%, did not change air-phase electro-olfactograms and did not reduce performance in olfactory behavioral tasks. In contrast with the current view, cyclic nucleotide-gated cation channels do not need a boost by Cl(-) channels to achieve near-physiological levels of olfaction.

Sensory transduction, the gateway to perception: mechanisms and pathology

The 102nd biannual Boehringer Ingelheim Fonds International Titisee Conference took place in October 2010. In the welcoming atmosphere of the small lakeside resort in the Black Forest, southern Germany, scientists from around the world gathered to discuss current topics and challenges in the area of sensory biology. The research presented covered all of the classical Aristotelian senses (and beyond) and provided a glimpse at recent progress and recurring themes in the sensory systems.