The formation and function of cilia involves the movement of intraflagellar transport (IFT) particles underneath the ciliary membrane, along axonemal microtubules. Although this process has been studied extensively, its molecular basis remains incompletely understood. For example, it is unknown how the IFT particle interacts with transmembrane proteins. To study the IFT particle further, we examined elipsa, a locus characterized by mutations that cause particularly early ciliogenesis defects in zebrafish. We show here that elipsa encodes a coiled-coil polypeptide that localizes to cilia. Elipsa protein binds to Ift20, a component of IFT particles, and Elipsa homologue in Caenorhabditis elegans, DYF-11, translocates in sensory cilia, similarly to the IFT particle. This indicates that Elipsa is an IFT particle polypeptide. In the context of zebrafish embryogenesis, Elipsa interacts genetically with Rabaptin5, a well-studied regulator of endocytosis, which in turn interacts with Rab8, a small GTPase, known to localize to cilia. We show that Rabaptin5 binds to both Elipsa and Rab8, suggesting that these proteins provide a bridging mechanism between the IFT particle and protein complexes that assemble at the ciliary membrane.

A systematic analysis of the Drosophila genome data reveals the existence of pHCl, a novel member of ligand-gated ion channel subunits. pHCl shows nearly identical similarity to glutamate-, glycine-, and histamine-gated ion channels, does however not belong to any of these ion channel types. We identified three different sites, where splicing generates multiple transcripts of the pHCl mRNA. The pHCl is expressed in Drosophila embryo, larvae, pupae, and the adult fly. In embryos, in situ hybridization detected pHCl in the neural cord and the hindgut. Functional expression of the three different splice variants of pHCl in oocytes of Xenopus laevis and Sf9 cells induces a chloride current with a linear current-voltage relationship that is inhibited by extracellular protons and activated by avermectins in a pH-dependent manner. Further, currents through pHCl channels were induced by a raise in temperature. Our data give genetic and electrophysiological evidence that pHCl is a member of a new branch of ligand-gated ion channels in invertebrates with, however, a hitherto unique combination of pharmacological and biophysical properties.Ligand-gated ion channels (LGICs) 1 mediate the fast inhibitory and excitatory responses of neuronal and muscle cells to neurotransmitters. A universal feature of the type of "Cys-loop" class of LGIC is a common topology of four membrane-spanning segments (M1-M4) and a huge N-terminal extracellular domain with a hyperconservated cysteinebridge motive (1). In vertebrates this "Cys-bridge" family of phylogenetically related genes codes for cation channels activated by acetylcholine and serotonin or for anion channels activated by GABA and glycine (1). In addition, glutamateand serotonin-gated anion channel genes are known in invertebrates (2, 3). Recently, genes for histamine-gated chloride channels and GABA-gated cation channels were identified in invertebrates (4 -7). The molecular basis of further channel types like acetylcholine-gated chloride channels in invertebrates is, however, still unknown (8). Information from the Drosophila melanogaster genome sequencing project allows identifying all members of the superfamily of ligand-gated ion channels occurring in this species by bioinformatic analysis of new homologous genes. The summarized data obtained from several published bioinformatic analyses (5,6,9,10) show that the group of ligand-gated "chloride" channels consists of 12 genes that are coding for GABA, histamine, and glutamate receptors or new, homologous ion channel types. Four members of this group cannot be directly assigned to the GABA, glutamate, or histamine branches and thus code for putative new types of ligand-gated chloride channels with yet unknown function. In a systematic expression approach of these predicted novel types of ion channels in Xenopus oocytes, it was found that none of the typical neurotransmitters activated these novel types of channels (6). Therefore, we extended the molecular biological analysis of the mRNA and found that the gene CG6112 encodes fo...

Histamine is not only a crucial cytokine in the periphery but also an important neurotransmitter and neuromodulator in the brain. It is known to act on metabotropic H1-H4 receptors, but the existence of directly histamine-gated chloride channels in mammals has been suspected for many years. However, the molecular basis of such mammalian channels remained elusive, whereas in invertebrates, genes for histamine-gated channels have been already identified. In this report, we demonstrated that histamine can directly open vertebrate ion channels and identified ␤ subunits of GABA A receptors as potential candidates for histamine-gated channels. In Xenopus oocytes expressing homomultimeric ␤ channels, histamine evoked currents with an EC 50 of 212 M (␤ 2 ) and 174 M (␤ 3 ), whereas GABA is only a very weak partial agonist. We tested several known agonists and antagonists for the histamine-binding site of H1-H4 receptors and described for ␤ channels a unique pharmacological profile distinct from either of these receptors. In heteromultimeric channels composed of ␣ 1 ␤ 2 or ␣ 1 ␤ 2 ␥ 2 subunits, we found that histamine is a modulator of the GABA response rather than an agonist as it potentiates GABA-evoked currents in a ␥ 2 subunit-controlled manner. Despite the vast number of synthetic modulators of GABA A receptors widely used in medicine, which act on several distinct sites, only a few endogenous modulators have yet been identified. We show here for the first time that histamine modulates heteromultimeric GABA A receptors and may thus represent an endogenous ligand for an allosteric site.Ligand-gated ion channels mediate the fast responses of cells to neurotransmitters (1). A universal feature of ligand-gated ion channels subunits is a common topology, comprising four membrane-spanning segments (M1-M4) and a huge N-terminal extracellular domain with a hyperconserved cysteine loop motif. In vertebrates, this "Cys loop" family of phylogenetically related genes codes for anion and cation channels activated by acetylcholine and serotonin (cation channels) or GABA 3 and glycine (anion channels) (1, 2). Despite the many years of intensive research on such ion channels, recent reports revealed unexpected new findings about this channel family. In vertebrates, a gene for zinc-gated ion channels was recently discovered (3). In insects, new classes of ligand-gated chloride channels gated by histamine or pH and cation channels gated by GABA were reported (4 -7

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We examined seizure-susceptibility in a model of human epilepsy using optogenetic stimulation of (red-activatable channelrhodopsin). Photostimulation of the seizure-sensitive mutant causes behavioral paralysis that resembles paralysis caused by mechanical stimulation, in many aspects. Electrophysiology shows that photostimulation evokes abnormal seizure-like neuronal firing in followed by a quiescent period resembling synaptic failure and apparently responsible for paralysis. The pattern of neuronal activity concludes with seizure-like activity just prior to recovery. We tentatively identify the mushroom body as one apparent locus of optogenetic seizure initiation. The α/β lobes may be primarily responsible for mushroom body seizure induction.