A Novel Method for Inducing Nerve Growth via Modulation of Host Resting Potential: Gap Junction-Mediated and Serotonergic Signaling Mechanisms

Abstract

SummaryA major goal of regenerative medicine is to restore the function of damaged or missing organs through the implantation of bioengineered or donor-derived components. It is necessary to understand the signals and cues necessary for implanted structures to innervate the host, as organs devoid of neural connections provide little benefit to the patient. While developmental studies have identified neuronal pathfinding molecules required for proper patterning during embryogenesis, strategies to initiate innervation in structures transplanted at later times or alternate locations remain limited. Recent work has identified membrane resting potential of nerves as a key regulator of growth cone extension or arrest. Here, we identify a novel role of bioelectricity in the generation of axon guidance cues, showing that neurons read the electric topography of surrounding cells, and demonstrate these cues can be leveraged to initiate sensory organ transplant innervation. Grafts of fluorescently labeled embryological eye primordia were used to produce ectopic eyes in Xenopus laevis tadpoles. Depolarization of host tissues through anion channel activation or other means led to a striking hyperinnervation of the body by these ectopic eyes. A screen of possible transduction mechanisms identified serotonergic signaling to be essential for hyperinnervation to occur, and our molecular data suggest a possible model of bioelectrical control of the distribution of neurotransmitters that guides nerve growth. Together, these results identify the molecular components of bioelectrical signaling among cells that regulates axon guidance, and suggest novel biomedical and bioengineering strategies for triggering neuronal outgrowth using ion channel drugs already approved for human use.Electronic supplementary materialThe online version of this article (doi:10.1007/s13311-014-0317-7) contains supplementary material, which is available to authorized users.

Neurotransmitter signaling pathways required for normal development inXenopus laevisembryos: a pharmacological survey screen

“…; Blackiston et al. ), and it is tempting to speculate that neurotransmitters could play a conserved role as a layer of chemical signaling mediating between endogenous bioelectric gradients and downstream transcriptional cascades that regulate cell behavior and thus anatomy.…”

Local and long-range endogenous resting potential gradients antagonistically regulate apoptosis and proliferation in the embryonic CNS

“…Cell autonomous local bioelectric signaling has been observed in eye (Pai et al, 2012a) and brain (Pai et al, 2015) patterning, and is mainly transduced by intracellular calcium signaling and GJCs to regulate cellular behavior. Non-cell-autonomous bioelectric signaling has been observed in brain development (Pai et al, 2015), tumor suppression , ectopic innervation (Blackiston et al, 2015a), and left-right patterning (Levin and Mercola, 1999). While the transduction mechanisms of such effects are beginning to be understood, the extent of such signaling during normal development has not been explored.…”

“…Movement of serotonin through GJCs has also been shown to be an important component of distant bioelectric communication guiding ectopic nerve growth (Blackiston et al, 2015a). Finally, modeling and experimental testing of model predictions has revealed that the left and right sides of the Xenopus embryo produce a coherent long-range bioelectric growth-control signal that is coordinated by the action of GJCs .…”

“…Cell to cell gap junction connections are quite dynamic during embryonic development (Anava et al, 2013, Warner, 1985, and the sensitivity of GJs to transjunctional voltage (Palacios-Prado andBukauskas, 2009, Verselis et al, 1991) suggest gap junctions as the ideal candidate to mediate cells' ability to compare their voltage with neighboring cell groups. The molecular identity of the signal (ionic, biochemical, or both) passing through GJCs communicating this information is not yet known, although prior work suggests neurotransmitters such as serotonin as a likely candidate for subsequent analysis (Blackiston et al, 2011, Blackiston et al, 2015b, Fukumoto et al, 2005, Lobikin et al, 2012a. Understanding the precise transduction mechanism of local and distant bioelectric signals in regulating brain patterning remains an active area of investigation.…”

Interplay between morphogen‐directed positional information systems and physiological signaling

“…Work done in Xenopus and chick provides insight to this occurrence where serotonin (5‐HT), an endogenous neurotransmitter, establishes left‐right patterning in embryos through regulation of ion fluxes . A follow‐up study showed that extracellular 5‐HT availability drives innervation through tissues via gap junctional communication modulation . Combining their findings, Levin and colleagues propose that extracellular 5‐HT, a positively charged molecule, navigates through gap junctions to accumulate in hyperpolarized cells, mimicking the reuptake function of 5‐HT transporters (SERTs).…”