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    • One major mode of bioelectric action is that of an

    2022-05-25

    (2) One major mode of bioelectric action is that of an amplifier of subcellular symmetry-breaking mechanisms into tissue-wide order. An example is the patterning of the embryonic left (L)-right (R) axis reviewed in References [57, 58]; the following scheme describes the data obtained largely from the frog model, although the roles of ion channels, gap junctions, and serotonergic machinery are conserved to a number of other vertebrate and invertebrate taxa [59,60]. The invariant asymmetry of the embryonic organs (heart, gut and other visceral organs, and brain) is driven by asymmetric expression of genes such as Nodal during early organogenesis. Upstream of the asymmetric gene cascade [61] that determines L-R-sided transcription lies the system of embryo-wide gap junction-mediated communication which is required for the assignment of transcriptional laterality [62,63]. This long-range communication is implemented via serotonin transport [[64], [65], [66]] driven by a bioelectric gradient. The direction of the electrophoretic force is invariant due to the very early (2–4 cell) localization of specific NHS-LC-Biotin on the right side of the embryo, which in turn results from the asymmetric transport of molecular motor proteins (and their channel and pump cargo) by the cytoskeletal machinery [67]. The invariant L-R chirality of the cytoskeletal intracellular transport system is established by the microtubule organizing center, oriented at the time of sperm entry with respect to the other two axes [68]. Thus, the activity of ion transport at the cellular level is able to convert microscopic symmetry-breaking into positional identity at the level of a major body axis and provides geometric information to transcriptional cascades that on their own could not tell left from right [[69], [70], [71]]. A similar electrophoretic system is used to load charged maternal determinants along the nurse cell-oocyte axis in insects [27,72]. (3) Another fundamental role for bioelectric signals is the establishment of pre-patterns –regionalizations of membrane potentials and gap junctions that demarcate embryonic compartments [73,74]. For example, in the frog embryo (Fig. 1L–M) the development of the eye is presaged by a characteristic pattern of hyperpolarized cells in the nascent face ectoderm [8]. Artificial depolarization of this region prevents eye formation, while, strikingly, establishment of this pattern elsewhere in the body via channel misexpression induces eyes to form outside the anterior neural field (in the tail, gut, etc.) [8] (Fig. 1N). Indeed, the other components of the face likewise have distinct bioelectric precursors: the “electric face” [75] is a pre-pattern observable by voltage reporter dyes that indicate where the different gene expression domains (e.g., Frizzled) are going to be expressed (Fig. 1M). Optogenetic methods for moving the bioelectric boundaries result in predictable changes in downstream gene expression and thus in subsequent facial anatomy [76]. In addition to the face, the brain likewise relies on a characteristic pre-pattern to dictate its size and morphology [9]; the pattern is instructive not only because altering it can produce defects or ectopic brain tissue, but also because enforcing it can over-ride otherwise devastating teratogenic influences. Artificially strengthening the endogenous brain pre-pattern (Fig. 1L) can rescue embryos expressing a mutated form of the Notch protein (a key brain pattering gene which, when mutated, results in strong defects of brain patterning) [9] or exposed to powerful chemical teratogens [77], resulting in near normal morphology, gene expression, and learning behavior. Interestingly, the pre-pattern responsible for brain morphogenesis is not entirely local, since disruptions in the state of remote cells (e.g., those in the gut) may result in defects in brain size [9,77]. Thus, bioelectric pre-patterns can serve as instructive scaffolds that help establish the locations, size, and shape of numerous body organs including the eye, brain, and heart [78] in Xenopus, the fins in zebrafish [79], the wing in Drosophila [80,81], and the head in planaria [10].