Biomembranes: Electrical Synapses/Gap Junctions (PDF)

Biomembranes Electrical Synapses/Gap Junctions Part I  Learning Objectives You will be able to…… Describe the structure and discuss the importance of gap junctions Describe techniques for the study of gap junctions Compare electrical and chemical synapses Describe the p...

Biomembranes Electrical Synapses/Gap Junctions Part I  Learning Objectives You will be able to…… Describe the structure and discuss the importance of gap junctions Describe techniques for the study of gap junctions Compare electrical and chemical synapses Describe the properties of electrical synapses that form the basis for coupled networks in the brain Explain the basis for the deficits seen in the retina and cerebellum of Cx36 knockout mice  General References Synchrony and so much more: Diverse roles for electrical synapses in neural circuits. Connors BW. Dev Neurobiol. 2017 May;77(5):610-624. Electrical synapses in the mammalian brain. Connors BW, Long MA. Annu Rev Neurosci. 2004;27:393-418. Electrical synapses: a dynamic signaling system that shapes the activity of neuronal networks. Hormuzdi SG, Filippov MA, Mitropoulou G, Monyer H, Bruzzone R. Biochim Biophys Acta. 2004 Mar 23;1662(1-2):113-37. Electrical Synapses/Gap Junctions Part I What are Gap Junctions? What are Gap Junctions? An array of intercellular channels for direct cell-to-cell communication but may also connect with the extracellular space as hemichannels  What are they made of? Gap junctions are Composed of Connexins Cell 2 Cell 1 connexon connexins  The connexin family HCx59 HCx43 HCx62 rCx33 HCx46 HCx37 HCx50 HCx40 HCx30 HCx30.2 HCx26 Numerous connexin genes HCx32 HCx45 Expressed by most cell types HCx31.1 HCx46.6 Also in invertebrates (called innexins HCx30.3 HCx36 HCx31 HCx25 HCx31.9 HCx40.1 Co-expression of connexins can lead to complex assemblies of subunits 21 connexin genes are expressed in the human genome  What do they do? Gap junction channels are permeable to inorganic ions (K+, Na+, Cl-, HCO3-), small organic (signalling) molecules (cAMP, IP3), dyes, and metabolites (glucose) Gap junctions coordinate the biochemical and electrical activities of coupled populations in a cell-to-cell fashion ie materials pass from cell to cell within a whole network so every cell within a network has a ‘taste’ of every other cell. This may coordinate their function and biochemical state. In neurons, they do not generate current fluxes de novo  Intercellular channels at gap junctions are densely packed Electrical transmission between MesV neurons is mediated by connexin36 (Cx36)-containing gap junctions located at somato-somatic contacts – only 0.1% of channels are conductive! J Membr Biol. 2012 Jun;245(5-6):283-90.  Gap Junctions are ubiquitous The targeted arrangement of gap junctions and connexin subunits ensures the generation of cell-specific assemblies Gap junctions mediate bidirectional And between signaling between oocytes and epithelial cells of the granulosa cells. gut Gap junctions are important European Journal of Clinical Investigation Vol 41 The distribution of connexins in the gap junctions of the brain HCx59 HCx43 HCx62 Numerous connexin genes rCx33 HCx46 HCx37 HCx50 Expressed by most cell types HCx40 HCx30 HCx30.2 HCx26 Also in invertebrates HCx32 Inherited human disorders HCx45 resulting from connexin HCx31.1 mutations HCx46.6 HCx30.3 Cataracts HCx36 (Cx46, Cx50) HCx31 HCx25 Hearing impairment HCx31.9 (Cx26,Cx30, Cx31) HCx40.1 CMTX Astrocytes (Cx32) Meninges Neurons Oligodendrocytes Electrical Synapses: Their Discovery  Electrical Synapses: Nomenclature Electrical synapses demonstrated in 1959 (Fursham and Potter) TRANSMISSION AT THE GIANT MOTOR SYNAPSES OF THE CRAYFISH. FURSHPAN EJ, POTTER DD. Gap junctions discovered in 1967 HEXAGONAL ARRAY OF SUBUNITS IN INTERCELLULAR JUNCTIONS OF THE MOUSE HEART AND LIVER J. P. REVEL AND M. J. KARNOVSKY Nomenclature Electrical Synapses Electrotonic synapses Electrical or electrotonic coupling Gap junctions: the structural correlate of electrical synapses Connexins, innexins, pannexins: the proteins that comprise gap junctions Electrical synapses in the brain are composed of connexin 36 (Cx36) Electrical Synapses/Gap Junctions Part II Physiology  Cellular Properties of Connexin Intercellular Channels A different kind of synapse ……….chemical vs electrical synapse  Electrical synapses: assaying function Dye coupling Easier, less technically challenging Is not reliable, may show some specificity for neuronal or connexin type (???!!!) Less information derived pertaining to the electrical synapse, cell, network Ganglion cell to AII Amacrine cells Is a terminal procedure, once Dual cell electrophysiology applied cannot be reused Cell 1 20 mV Cell 2 4 mV  Electrical synaptic transmission Electrical synapses can pass subthreshold current Action potentials result in strongly attenutated postsynaptic responses called spikelets Electrical synapses are bidirectional Show no preference for depolarizing or hyperpolarizing responses Electrical synapses are sign preserving COUPLING COEFFICIENT = The ratio between the voltage change observed in the non-injected and the injected neurons.  Electrical vs Chemical Structural differences; close Electrical transmission is ionic opposition of membranes for current whereas chemical requires electrical neurotransmitter release and Electrical synapses activate binding faster than chemical ones: Electrical synapses are almost synaptic delay always bidirectional; chemical synapses not Chemical synapses are de/hyperpolarizing; electrical synapses show no such specificity Electrical synapses are sign preserving; chemical are not Electrical synapses are reliable; reliability at chemical synapses varies Chemical synapses are metabolically expensive; 47% of energy at synapses linked to action potentials, 34% of biochemcial processes involved in Network Properties  Relevant Properties Important Properties C e ll 2 Bidirectionality Shorter synaptic delay C e ll 1 Sign preservation Mediates both hyperpolarizing and depolarizing responses 20 m V Facilitates synchrony (both, sub- and supra-threshold) C e ll 2 and promotes action potentials Coordinates activity in cell-to-cell fashion of a large population! C e ll 1  Electrical Synapses create a network of synchronously coactive neurons Cx36 can create multiple groups of coupled interneurons In many brain regions Cx36 expression is restricted to interneurons Interneurons comprise many different subtypes Cx36 typically couples only similar interneuron subtypes Thus, electrical synapses create electrically coupled homocellular assemblies Hestrin S, Galarreta M.; Trends Neurosci. 2005 Jun;28(6):304-9.  Specificity in Cx36 assembly creates multiple networks of synchronously coactive neurons Synchrony generates brain rhythms, perhaps Cx36 is vital  Electrical Synapses/Gap Junctions Part III Importance: Lessons from the KO mouse Synchrony generates brain rhythms, perhaps Cx36 is vital Lessons from the knockout Cause for the motor impairment? I local circuit neuron  Cause for the motor impairment? Cerebellar Circuitry II Cause for the motor impairment? III Cause for the motor impairment? IV Cause for the motor impairment? I Cause for the motor impairment? V Role in the retina  Summary In mammals, most electrical synapses are comprised of Cx36 but other connexins and pannexins may also play a role…….electrical synapses in the retina are comprised of other connexins as well Electrical synapses usually allow ionic current and small organic molecules to pass reliably in both directions There are considerable differences between electrical and chemical synapses and they together generate the complex electrical activity that encodes brain function Studies in the Cx36 knockout have clearly demonstrated its importance in a variety of important functions in the brain retina (and pancreas!)