Phelan P et al. (2008), Molecular mechanism of rectification at identif...

XB-ART-39472

Curr Biol 2008 Dec 23;1824:1955-60. doi: 10.1016/j.cub.2008.10.067.

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Molecular mechanism of rectification at identified electrical synapses in the Drosophila giant fiber system.

Phelan P , Goulding LA , Tam JL , Allen MJ , Dawber RJ , Davies JA , Bacon JP .

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Electrical synapses are neuronal gap junctions that mediate fast transmission in many neural circuits. The structural proteins of gap junctions are the products of two multigene families. Connexins are unique to chordates; innexins/pannexins encode gap-junction proteins in prechordates and chordates. A concentric array of six protein subunits constitutes a hemichannel; electrical synapses result from the docking of hemichannels in pre- and postsynaptic neurons. Some electrical synapses are bidirectional; others are rectifying junctions that preferentially transmit depolarizing current anterogradely. The phenomenon of rectification was first described five decades ago, but the molecular mechanism has not been elucidated. Here, we demonstrate that putative rectifying electrical synapses in the Drosophila Giant Fiber System are assembled from two products of the innexin gene shaking-B. Shaking-B(Neural+16) is required presynaptically in the Giant Fiber to couple this cell to its postsynaptic targets that express Shaking-B(Lethal). When expressed in vitro in neighboring cells, Shaking-B(Neural+16) and Shaking-B(Lethal) form heterotypic channels that are asymmetrically gated by voltage and exhibit classical rectification. These data provide the most definitive evidence to date that rectification is achieved by differential regulation of the pre- and postsynaptic elements of structurally asymmetric junctions.

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Species referenced: Xenopus
Genes referenced: lgals4.2

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	Figure 1. Neurons and Muscles of the Drosophila Giant Fiber SystemThe Giant Fiber System mediates escape behavior in the fly. Visual stimuli activate a pair of large interneurons, the Giant Fibers (GF, red). In the brain, the GFs form synaptic connections with the giant commissural interneurons (GCI, orange). The GF axons descend to the mesothoracic ganglion, where they terminate in characteristic lateral bends. The GF bends form extensive synaptic connections with the medially directed dendrites of the motorneurons (TTMn, blue) of the tergotrochanteral (jump) muscles (TTM, brown; left) of the middle leg. Just before the bends, the GF axons synapse with the peripherally synapsing interneurons (PSI, green), which, in turn, innervate the motorneurons (DLMn, yellow) of the dorsal longitudinal (flight) muscles (DLM, pink; right). The PSI axons and terminal tips of the TTMn medial dendrites synapse with one another at the midline. The central synapses are electrical or mixed electrochemical synapses. The neuromuscular junctions, which, for clarity, are shown on one side only of the bilaterally symmetrically pathway, are chemical synapses. Reprinted from [13] with permission from Elsevier.
	Figure 2. ShakB Immunolabeling Is Restored at GFS Synapses in Transgenic shakB2 FliesCorresponding regions of the mesothoracic neuromere of the adult CNS labeled with antisera that detect all ShakB proteins. Images are projections of confocal z series.(A) Wild-type. The midline anterior cluster of staining (arrows) corresponds to the region of contact between axons of the GFs and PSIs and the tips of the TTMn medial dendrites. More posteriorly, the bilateral tracts of staining (arrowheads) profile regions of contact between the terminal bend of the GF axon and the medial dendrite of the TTMn.(B) shakB2 mutants. Immunolabeling is abolished with the exception of discrete spots at the region of the GF-PSI-TTMn contacts.(C–E) Transgenic mutant lines. A307-GAL4-directed (C and E) or c17-GAL4-directed (D) expression of UAS-shakB(n+16) (C and D) and UAS-shakB(n) (E) transgenes in a shakB2 mutant background restores ShakB expression at GFS synapses. In A307 transgenic lines (C and E), immunolabeling is restored at the GF-TTMn and, to a lesser extent, at the GF-PSI synapses. In c17 transgenic lines (D), labeling is restored at the GF-PSI-TTMn midline contacts, but not, to detectable levels, at the GF-TTMn synapses. Arrowheads in (D) indicate non-GFS sensory neurons known to express c17[29]. The genotypes of transgenic flies are as follows: (C) shakB2; A307-GAL4/UAS-shakB(n+16), (D) shakB2; c17-GAL4/UAS-shakB(n+16) or shakB2; c17-GAL4/c17-GAL4; UAS-shakB(n+16), and (E) shakB2; A307-GAL4; UAS-shakB(n).The scale bar represents 10 μm and applies to all panels.
	Figure 3. Targeted Expression of shakB(n+16), but Not shakB(n), Rescues Dye Coupling and Synaptic Activity in the GFS of shakB2 Mutants(A–J) Dye coupling between neurons of the GFS. Lucifer yellow was injected into one GF axon and visualized directly (A–E, G, and I) or after labeling with antilucifer yellow antibody (F, H, and J). Images are projections of confocal Z series.(A and B) Wild-type. Lucifer yellow diffuses from the injected GF (A, arrow) into the GCIs in the brain (A, arrowhead) and into the ipsilateral TTMn (B, arrowhead), both PSIs (B, arrows), and other neurons in the mesothoracic ganglion. Weak coupling to the contralateral GF is also observed.(C and D) Dye coupling in the brain (C) and thoracic ganglion (D) is eliminated in shakB2 mutants.(E and F) A307-directed expression of shakB(n+16) in shakB2 mutants restores coupling to the GCIs (E) and the TTMn (F, arrowhead).(G and H) c17-directed expression of shakB(n+16) in shakB2 mutants rescues coupling to the PSI (H, arrows).(I and J) Expression of shakB(n) in shakB2 mutants with A307 fails to restore dye coupling to neurons in the brain (I) or mesothoracic ganglion (J).Scale bars represent 40 μm; (B–F), (I), and (J) are as for (A).(K–N) Recordings from the TTM muscle in response to GF simulation.(K and L) Traces from individual flies showing (K) response latency to a single stimulus and (L) responses to a train of ten stimuli delivered at a frequency of 100 Hz. In controls, response latency is short (<1 ms), and ten out of ten stimuli elicit a response. shakB2 mutants exhibit long latency responses and poor following frequency. A307- or c17-directed expression of shakB(n+16) in shakB2 mutants partially rescues response latency in some flies and increases the response to repetitive stimuli in most flies. Rescue is never observed in shakB2 mutants expressing shakB(n) under the control of A307.(M and N) Mean ± SEM data for response latency (M) and following frequency (N) for n flies recorded. ∗p < 0.05 in unpaired Student's t test.Genotypes of transgenic flies are as in Figure 2. In (K)–(N), controls are shakB2/+; UAS-shakB(n+16); shakB2 are shakB2/Y; UAS-shakB(n+16).
	Figure 4. ShakB(N+16) and ShakB(L) Form Heterotypic Channels that Are Asymmetrically Gated by Voltage, Whereas Homotypic Channels Exhibit Symmetrical Voltage ResponsesXenopus oocytes injected with shakB RNAs were paired and recorded by dual voltage clamp. Both cells of a pair were clamped at a holding potential of −80 mV. Transjunctional voltage steps (Vj, mV) were then applied to one cell while the current (Ij, nA) required to maintain the other cell at the holding potential was recorded. Junctional conductance (Gj, μS) is Ij/Vj.(A–E) Heterotypic cell pairs.(A–C) Representative traces from cell pairs injected with 0.25 ng RNA. Junctional currents (A and C) were elicited by application of Vj steps (B) to (A) the ShakB(N+16)-expressing cell or (C) the ShakB(L)-expressing cell. Gj is shown for depolarizing (below baseline) and hyperpolarizing (above baseline) 10 mV steps. Mean values are provided in Table S3.(D–E) Gj/Vj plots. Initial (open symbols) and steady-state (closed symbols) Gjs recorded in response to application of Vj steps to (D) the ShakB(N+16)-expressing cell or (E) the ShakB(L)-expressing cell. Gjs, normalized to their values at −10 mV (D) and 10 mV (E), are mean ± SD for n = 4 pairs injected with 0.1–0.25 ng RNA. Steady-state data are fitted to a Boltzmann equation (parameters in Table S3). Heterotypic channels respond asymmetrically to applied voltage.(F–J) Homotypic cell pairs.(F–H) Typical recordings and Gj/Vj plots (I and J) for oocyte pairs in which both cells expressed (F and I) ShakB(N+16) (0.5–2 ng RNA) or (H and J) ShakB(L) (0.05–0.25 ng RNA). (F and H) Gj is shown for depolarizing and hyperpolarizing 10 mV steps. (I and J) Initial (open symbols) and steady-state (closed symbols) Gjs normalized to their values at Vj = ±10 mV are mean ± SD for n = 8 (I) and n = 3 (J) pairs. Curves in (J) are Boltzmann fits of the data. ShakB(N+16) channels show no significant voltage sensitivity. ShakB(L) channels exhibit a symmetrical response to applied voltage. Mean Gjs and Boltzmann parameters are in Table S3.

References [+] :

Allen, Targeted expression of truncated glued disrupts giant fiber synapse formation in Drosophila. 1999, Pubmed