Friso R. Postma

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]P2) from the plasma membrane. Knockdown of phospholipase Cβ3 (PLCβ3) inhibits PtdIns(4,5)P2 hydrolysis and keeps Cx43 channels open after receptor activation. Using a translocatable 5-phosphatase, we show that PtdIns(4,5)P2 depletion is sufficient to close Cx43 channels. When PtdIns(4,5)P2 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 PLCβ3 via its third PDZ domain. ZO-1 is essential for PtdIns(4,5)P2-hydrolyzing receptors to inhibit cell–cell communication, but not for receptor–PLC coupling. Our results show that PtdIns(4,5)P2 is a key regulator of Cx43 channel function, with no role for other second messengers, and suggest that ZO-1 assembles PLCβ3 and Cx43 into a signaling complex to allow regulation of cell–cell communication by localized changes in PtdIns(4,5)P2.

Electrical synapses formed by connexin36 regulate inhibition- and experience-dependent plasticity

The mammalian brain constantly adapts to new experiences of the environment, and inhibitory circuits play a crucial role in this experience-dependent plasticity. A characteristic feature of inhibitory neurons is the establishment of electrical synapses, but the function of electrical coupling in plasticity is unclear. Here we show that elimination of electrical synapses formed by connexin36 altered inhibitory efficacy and caused frequency facilitation of inhibition consistent with a decreased GABA release in the inhibitory network. The altered inhibitory efficacy was paralleled by a failure of theta-burst long-term potentiation induction and by impaired ocular dominance plasticity in the visual cortex. Together, these data suggest a unique mechanism for regulating plasticity in the visual cortex involving synchronization of inhibitory networks via electrical synapses.

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.

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.