Renate Gauss

Molecular identification of a hyperpolarization-activated channel in sea urchin sperm

Sea urchin eggs attract sperm through chemotactic peptides, which evoke complex changes in membrane voltage and in the concentrations of cyclic AMP, cyclic GMP and Ca2+ ions (see ref. 1 for a review). The intracellular signalling pathways and their cellular targets are largely unknown. We have now cloned, from sea urchin testis, the complementary DNA encoding a channel polypeptide, SPIH. Functional expression of SPIH gives rise to weakly K+-selective hyperpolarization-activated channels, whose activity is enhanced by the direct action of cAMP. Thus, SPIH is under the dual control of voltage and cAMP. The SPIH channel, which is confined to the sperm flagellum, may be involved in the control of flagellar beating. SPIH currents exhibit all the hallmarks of hyperpolarization-activated currents (Ih),, which participate in the rhythmic firing of central neurons, control pacemaking in the heart, and curtail saturation by bright light in retinal photoreceptors,. Because of their sequence and functional properties, Ih channels form a class of their own within thesuperfamily of voltage-gated and cyclic-nucleotide-gated channels.

Hyperpolarization-activated channels HCN1 and HCN4 mediate responses to sour stimuli

Sour taste is initiated by protons acting at receptor proteins or channels. In vertebrates, transduction of this taste quality involves several parallel pathways. Here we examine the effects of sour stimuli on taste cells in slices of vallate papilla from rat. From a subset of cells, we identified a hyperpolarization-activated current that was enhanced by sour stimulation at the taste pore. This current resembled Ih found in neurons and cardio-myocytes, a current carried by members of the family of hyperpolarization-activated and cyclic-nucleotide-gated (HCN) channels. We show by in situ hybridization and immunohistochemistry that HCN1 and HCN4 are expressed in a subset of taste cells. By contrast, gustducin, the G-protein involved in bitter and sweet taste, is not expressed in these cells. Lowering extracellular pH causes a dose-dependent flattening of the activation curve of HCN channels and a shift in the voltage of half-maximal activation to more positive voltages. Our results indicate that HCN channels are gated by extracellular protons and may act as receptors for sour taste.

Pacemaker oscillations in heart and brain: a key role for hyperpolarization-activated cation channels.

Rhythmic activity of single cells or multicellular networks is a common feature of all organisms. The oscillatory activity is characterized by time intervals of several seconds up to many hours. Cellular rhythms govern the beating of the heart, the swimming behavior of sperm, cycles of sleep and wakefulness, breathing, and the release of hormones. Many neurons in the brain and cardiac cells are characterized by endogenous rhythmic activity, which relies on a complex interplay between several distinct ion channels. In particular, one type of ion channel plays a prominent role in the control of rhythmic electrical activity since it determines the frequency of the oscillations. The activity of the channels is thus setting the “pace” of the oscillations; therefore, these channels are often referred to as “pacemaker” channels. Despite their obvious important physiological function, it was not until recently that genes encoding pacemaker channels have been identified. Because both hyperpolarization and cyclic nucleotides are key elements that control their activity, pacemaker channels have now been designated hyperpolarization-activated and cyclic nucleotide–gated (HCN) channels. The molecular identification of the channels and the upcoming studies on their properties in heterologous systems will certainly enhance our understanding of “pacemaking” in physiological systems. This review gives a brief insight into the physiological importance of these channels and sums up what we have learned since the first cloning of genes succeeded (for recent reviews, see also Clapham 1998; Luüthi and McCormick 1998a; Biel et al. 1999; Ludwig, Zong, Hofmann, et al. 1999; Santoro and Tibbs 1999). (Chronobiology International, 17(4), 453–469, 2000)

Molecular identification and functional characterization of an adenylyl cyclase from the honeybee

Cyclic AMP (cAMP) serves as an important messenger in virtually all organisms. In the honeybee (Apis mellifera), cAMP‐dependent signal transduction has been implicated in behavioural processes as well as in learning and memory. Key components of cAMP‐signalling cascades are adenylyl cyclases. However, the molecular identities and biochemical properties of adenylyl cyclases are completely unknown in the honeybee. We have cloned a cDNA (Amac3) from honeybee brain that encodes a membrane‐bound adenylyl cyclase. The Amac3 gene is an orthologue of the Drosophila ac39E gene. The corresponding proteins share an overall amino acid similarity of approximately 62%. Phylogenetically, AmAC3 belongs to group 1 adenylyl cyclases. Heterologously expressed AmAC3 displays basal enzymatic activity and efficient coupling to endogenous G protein signalling pathways. Stimulation of β‐adrenergic receptors induces AmAC3 activity with an EC50 of about 3.1 µm. Enzymatic activity is also increased by forskolin (EC50 approximately 15 µm), a specific agonist of membrane‐bound adenylyl cyclases. Similar to certain biogenic amine receptor genes of the honeybee, Amac3 transcripts are expressed in many somata of the brain, especially in mushroom body neurones. These results suggest that the enzyme serves in biogenic amine signal transduction cascades and in higher brain functions that contribute to learning and memory of the bee.