Ectopic Expression of Select Innexins in Individual Central Neurons Couples Them to Pre-Existing Neuronal or Glial Networks That Express the Same Innexin

Abstract

Fifteen of the 21 innexin (Inx) genes (Hve-inx) found in the genome of the medicinal leech, Hirudo verbana, are expressed in the CNS (Kandarian et al., 2012). Two are expressed pan-neuronally, while the others are restricted in their expression to small numbers of cells, in some cases reflecting the membership of known networks of electrically coupled and dye-coupled neurons or glial cells. We report here that when Hve-inx genes characteristic of discrete coupled networks were expressed ectopically in neurons known not to express them, the experimental cells were found to become dye coupled with the other cells in that network. Hve-inx6 is normally expressed by only three neurons in each ganglion, which form strongly dye-coupled electrical connections with each other [Shortening-Coupling interneuron (S-CI) network] (Muller and Scott, 1981; Dykes and Macagno, 2006). But when Hve-inx6 was ectopically expressed in a variety of central embryonic neurons, those cells became dye coupled with the S-CI network. Similarly, Hve-inx2 is normally uniquely expressed by the ganglion’s large glial cells, but when it was ectopically expressed in different central neurons, they became dye coupled to the glial cells. In contrast, overexpression of the pan-neuronal Inx genes Hve-inx1 and Hve-inx14 did not yield any novel instances of dye coupling to pre-existent neuronal networks. These results reveal that expression of certain innexins is sufficient to couple individual neurons to pre-existing networks in the CNS. We propose that a primary determinant of selective neuronal connectivity and circuit formation in the leech is the surface expression of unique subsets of gap junctional proteins.

Endogenous Voltage Potentials and the Microenvironment: Bioelectric Signals that Reveal, Induce and Normalize Cancer

“…“Glioma cells are ill-equipped to participate in ion and amino acid homeostasis, those important altruistic tasks performed by their nonmalignant counterparts. Instead, gliomas are more concerned about their relentless growth and invasive migration” [450]. Interestingly, cooperation occurs among the tumor cells that can be analyzed via the same mathematical tools that explain cooperation and competition among somatic cells and members of societal groups [339,451,452].…”

Control of neuronal morphology and connectivity: Emerging developmental roles for gap junctional proteins

“…In the leech, with two notable exceptions presented below, over‐expression or ectopic expression of a single innexin in single neurons did not appear to cause a change in cellular coupling [67]. A similar finding was also recently reached while overexpressing different single innexins in C. elegans muscle cells [58].…”

“…(B) Neurobiotin tracer coupling reveals normal coupling to a set of small‐unidentified interneurons (arrowheads). (C–F) Ectopic expression of Inx6 by select neurons leads to their inclusion into the S–CI circuit (data taken from [67]). (C) Control Neurobiotin and Alexa 488 Dextran (MW 10 000) injection of a single S cell shows tracer‐coupling only to the bilateral CIs, all of which normally express Inx6 [63].…”

Intracellular Recording, Sensory Field Mapping, and Culturing Identified Neurons in the Leech, <i>Hirudo medicinalis</i>

“…The gene gun technique has recently been used to express invertebrate connexin (innexin) proteins in leech neurons. Expression of an innexin from Hirudo verbana (Hve-inx6) is sufficient for the formation of ectopic gap junctions in the medicinal leech 29 . The medicinal leech-or H. verbena as it were 30 is sure to advance our understanding of the physiology and evolution of electrical synapses over the next few years.…”