Gap Junctions Contribute to Differential Light Adaptation across Direction-Selective Retinal Ganglion Cells.

Direction-selective ganglion cells (DSGCs) deliver signals from the retina to multiple brain areas to indicate the presence and direction of motion. Delivering reliable signals in response to motion is critical across light levels. Here we determine how populations of DSGCs adapt to changes in light level, from moonlight to daylight. Using large-scale measurements of neural activity, we demonstrate that the population of DSGCs switches encoding strategies across light levels. Specifically, the direction tuning of superior (upward)-preferring ON-OFF DSGCs becomes broader at low light levels, whereas other DSGCs exhibit stable tuning. Using a conditional knockout of gap junctions, we show that this differential adaptation among superior-preferring ON-OFF DSGCs is caused by connexin36-mediated electrical coupling and differences in effective GABAergic inhibition. Furthermore, this adaptation strategy is beneficial for balancing motion detection and direction estimation at the lower signal-to-noise ratio encountered at night. These results provide insights into how light adaptation impacts motion encoding in the retina.

Reversal of disease-related pathologies in the fragile X mouse model by selective activation of GABAB receptors with arbaclofen.

Fragile X syndrome (FXS), the most common inherited cause of intellectual disability and autism, results from the transcriptional silencing of FMR1 and loss of the mRNA translational repressor protein fragile X mental retardation protein (FMRP). Patients with FXS exhibit changes in neuronal dendritic spine morphology, a pathology associated with altered synaptic function. Studies in the mouse model of fragile X have shown that loss of FMRP causes excessive synaptic protein synthesis, which results in synaptic dysfunction and altered spine morphology. We tested whether the pharmacologic activation of the γ-aminobutyric acid type B (GABA(B)) receptor could correct or reverse these phenotypes in Fmr1-knockout mice. Basal protein synthesis, which is elevated in the hippocampus of Fmr1-knockout mice, was corrected by the in vitro application of the selective GABA(B) receptor agonist STX209 (arbaclofen, R-baclofen). STX209 also reduced to wild-type values the elevated AMPA receptor internalization in Fmr1-knockout cultured neurons, a known functional consequence of increased protein synthesis. Acute administration of STX209 in vivo, at doses that modify behavior, decreased mRNA translation in the cortex of Fmr1-knockout mice. Finally, the chronic administration of STX209 in juvenile mice corrected the increased spine density in Fmr1-knockout mice without affecting spine density in wild-type mice. Thus, activation of the GABA(B) receptor with STX209 corrected synaptic abnormalities considered central to fragile X pathophysiology, a finding that suggests that STX209 may be a potentially effective therapy to treat the core symptoms of FXS.

Regulation of connexin43 gap junctional communication by phosphatidylinositol 4,5-bisphosphate.

Cell-cell communication through connexin43 (Cx43)-based gap junction channels is rapidly inhibited upon activation of various G protein-coupled receptors; however, the mechanism is unknown. We show that Cx43-based cell-cell communication is inhibited by depletion of phosphatidylinositol 4,5-bisphosphate (PtdIns[4,5]P(2)) from the plasma membrane. Knockdown of phospholipase Cbeta3 (PLCbeta3) inhibits PtdIns(4,5)P(2) hydrolysis and keeps Cx43 channels open after receptor activation. Using a translocatable 5-phosphatase, we show that PtdIns(4,5)P(2) depletion is sufficient to close Cx43 channels. When PtdIns(4,5)P(2) is overproduced by PtdIns(4)P 5-kinase, Cx43 channel closure is impaired. We find that the Cx43 binding partner zona occludens 1 (ZO-1) interacts with PLCbeta3 via its third PDZ domain. ZO-1 is essential for PtdIns(4,5)P(2)-hydrolyzing receptors to inhibit cell-cell communication, but not for receptor-PLC coupling. Our results show that PtdIns(4,5)P(2) is a key regulator of Cx43 channel function, with no role for other second messengers, and suggest that ZO-1 assembles PLCbeta3 and Cx43 into a signaling complex to allow regulation of cell-cell communication by localized changes in PtdIns(4,5)P(2).

Multiple connexins contribute to intercellular communication in the Xenopus embryo.

To explore the role of gap junctional intercellular communication (GJIC) during Xenopus embryogenesis, we utilized the host-transfer and antisense techniques to specifically deplete Cx38, the only known maternally expressed connexin. Cx38-depleted embryos developed normally but displayed robust GJIC between blastomeres at 32-128 cell stages, suggesting the existence of other maternal connexins. Analysis of embryonic cDNA revealed maternal expression of two novel connexins, Cx31 and Cx43.4, and a third, Cx43, that had been previously identified as a product of zygotic transcription. Thus, the early Xenopus embryo contains at least four maternal connexins. Unlike Cx38, expression of Cx31, Cx43 and Cx43.4 continue zygotically. Of these, Cx43.4 is the most abundant, accumulating significantly in neural structures including the brain, the eyes and the spinal cord.

Connexin29 is uniquely distributed within myelinating glial cells of the central and peripheral nervous systems.

Although both Schwann cells and oligodendrocytes express connexin32 (Cx32), the loss of this connexin causes demyelination only in the PNS. To determine whether oligodendrocytes might express another connexin that can function in place of Cx32, we searched for novel CNS-specific connexins using reverse transcriptase-PCR and degenerate primers. We identified Cx29, whose transcript was restricted to brain, spinal cord, and sciatic nerve. Developmental expression of Cx29 mRNA in the CNS paralleled that of other myelin-related mRNAs, including Cx32. In the CNS, Cx29 antibodies labeled the internodal and juxtaparanodal regions of small myelin sheaths, whereas Cx32 staining was restricted to large myelinated fibers. In the PNS, Cx29 expression preceded that of Cx32 and declined to lower levels than Cx32 in adulthood. In adult sciatic nerve, Cx29 was primarily localized to the innermost aspects of the myelin sheath, the paranode, the juxtaparanode, and the inner mesaxon. Cx29 displayed a striking coincidence with Kv1.2 K(+) channels, which are localized in the axonal membrane. Both Cx29 and Cx32 were found in the incisures. Cx29 expressed in N2A cells did not induce intercellular conductances but did participate in the formation of active channels when coexpressed with Cx32. Together, these data show that Cx29 and Cx32 are expressed by myelinating glial cells with distinct distributions.