Interplay between electrical activity and bone morphogenetic protein signaling regulates spinal neuron differentiation

Spinal neuron diversity scales exponentially with swim-to-limb transformation during frog metamorphosis

Vertebrates exhibit a wide range of motor behaviors, ranging from swimming to complex limb-based movements. Here we take advantage of frog metamorphosis, which captures a swim-to-limb-based movement transformation during the development of a single organism, to explore changes in the underlying spinal circuits. We find that the tadpole spinal cord contains small and largely homogeneous populations of motor neurons (MNs) and V1 interneurons (V1s) at early escape swimming stages. These neuronal populations only modestly increase in number and subtype heterogeneity with the emergence of free swimming. In contrast, during frog metamorphosis and the emergence of limb movement, there is a dramatic expansion of MN and V1 interneuron number and transcriptional heterogeneity, culminating in cohorts of neurons that exhibit striking molecular similarity to mammalian motor circuits. CRISPR/Cas9-mediated gene disruption of the limb MN and V1 determinants FoxP1 and Engrailed-1, respectively, results in severe but selective deficits in tail and limb function. Our work thus demonstrates that neural diversity scales exponentially with increasing behavioral complexity and illustrates striking evolutionary conservation in the molecular organization and function of motor circuits across species.

Noncanonical function of folate through folate receptor 1 during neural tube formation

Folate supplementation reduces the occurrence of neural tube defects (NTDs), birth defects consisting in the failure of the neural tube to form and close. The mechanisms underlying NTDs and their prevention by folate remain unclear. Here we show that folate receptor 1 (FOLR1) is necessary for the formation of neural tube-like structures in human-cell derived neural organoids. FOLR1 knockdown in neural organoids and in Xenopus laevis embryos leads to NTDs that are rescued by pteroate, a folate precursor that is unable to participate in metabolism. We demonstrate that FOLR1 interacts with and opposes the function of CD2-associated protein, molecule essential for apical endocytosis and turnover of C-cadherin in neural plate cells. In addition, folates increase Ca2+ transient frequency, suggesting that folate and FOLR1 signal intracellularly to regulate neural plate folding. This study identifies a mechanism of action of folate distinct from its vitamin function during neural tube formation.

Deciphering the Calcium Code: A Review of Calcium Activity Analysis Methods Employed to Identify Meaningful Activity in Early Neural Development

The intracellular and intercellular flux of calcium ions represents an ancient and universal mode of signaling that regulates an extensive array of cellular processes. Evidence for the central role of calcium signaling includes various techniques that allow the visualization of calcium activity in living cells. While extensively investigated in mature cells, calcium activity is equally important in developing cells, particularly the embryonic nervous system where it has been implicated in a wide variety array of determinative events. However, unlike in mature cells, where the calcium dynamics display regular, predictable patterns, calcium activity in developing systems is far more sporadic, irregular, and diverse. This renders the ability to assess calcium activity in a consistent manner extremely challenging, challenges reflected in the diversity of methods employed to analyze calcium activity in neural development. Here we review the wide array of calcium detection and analysis methods used across studies, limiting the extent to which they can be comparatively analyzed. The goal is to provide investigators not only with an overview of calcium activity analysis techniques currently available, but also to offer suggestions for future work and standardization to enable informative comparative evaluations of this fundamental and important process in neural development.

Non-canonical function of folate/folate receptor 1 during neural tube formation

Folate supplementation reduces the occurrence of neural tube defects, one of the most common and serious birth defects, consisting in the failure of the neural tube to form and close early in pregnancy. The mechanisms underlying neural tube defects and folate action during neural tube formation remain unclear. Here we show that folate receptor 1 (FOLR1) is necessary for the formation of neural tube-like structures in human-cell derived neural organoids. Knockdown of FOLR1 in human neural organoids as well as in the Xenopus laevis in vivo model leads to neural tube defects that are rescued by pteroate, a folate precursor that binds to FOLR1 but is unable to participate in metabolic pathways. We demonstrate that FOLR1 interacts with and opposes the function of CD2-associated protein (CD2AP), a molecule that we find is essential for apical endocytosis and the spatiotemporal turnover of the cell adherens junction component C-cadherin in neural plate cells. The counteracting action of FOLR1 on these processes is mediated by regulating CD2AP protein level via a degradation-dependent mechanism. In addition, folate and pteroate increase Ca 2+ transient frequency in the neural plate in a FOLR1-dependent manner, suggesting that folate/FOLR1 signal intracellularly to regulate neural plate folding. This study identifies a mechanism of action of folate distinct from its vitamin function during neural tube formation.

Calcium dynamics at the neural cell primary cilium regulate Hedgehog signaling-dependent neurogenesis in the embryonic neural tube

The balance between neural stem cell proliferation and neuronal differentiation is paramount for the appropriate development of the nervous system. Sonic hedgehog (Shh) is known to sequentially promote cell proliferation and specification of neuronal phenotypes, but the signaling mechanisms responsible for the developmental switch from mitogenic to neurogenic have remained unclear. Here, we show that Shh enhances Ca2+ activity at the neural cell primary cilium of developing Xenopus laevis embryos through Ca2+ influx via transient receptor potential cation channel subfamily C member 3 (TRPC3) and release from intracellular stores in a developmental stage-dependent manner. This ciliary Ca2+ activity in turn antagonizes canonical, proliferative Shh signaling in neural stem cells by down-regulating Sox2 expression and up-regulating expression of neurogenic genes, enabling neuronal differentiation. These discoveries indicate that the Shh-Ca2+-dependent switch in neural cell ciliary signaling triggers the switch in Shh action from canonical-mitogenic to neurogenic. The molecular mechanisms identified in this neurogenic signaling axis are potential targets for the treatment of brain tumors and neurodevelopmental disorders.

Myocardial Afterload Is a Key Biomechanical Regulator of Atrioventricular Myocyte Differentiation in Zebrafish

Heart valve development is governed by both genetic and biomechanical inputs. Prior work has demonstrated that oscillating shear stress associated with blood flow is required for normal atrioventricular (AV) valve development. Cardiac afterload is defined as the pressure the ventricle must overcome in order to pump blood throughout the circulatory system. In human patients, conditions of high afterload can cause valve pathology. Whether high afterload adversely affects embryonic valve development remains poorly understood. Here we describe a zebrafish model exhibiting increased myocardial afterload, caused by vasopressin, a vasoconstrictive drug. We show that the application of vasopressin reliably produces an increase in afterload without directly acting on cardiac tissue in zebrafish embryos. We have found that increased afterload alters the rate of growth of the cardiac chambers and causes remodeling of cardiomyocytes. Consistent with pathology seen in patients with clinically high afterload, we see defects in both the form and the function of the valve leaflets. Our results suggest that valve defects are due to changes in atrioventricular myocyte signaling, rather than pressure directly acting on the endothelial valve leaflet cells. Cardiac afterload should therefore be considered a biomechanical factor that particularly impacts embryonic valve development.

Injury-induced Erk1/2 signaling tissue-specifically interacts with Ca2+ activity and is necessary for regeneration of spinal cord and skeletal muscle

The transition of stem cells from quiescence to proliferation enables tissues to self-repair. The signaling mechanisms driving these stem-cell-status decisions are still unclear. Ca²⁺ and the extracellular signal-regulated kinase (Erk1/2) are two signaling pathways that have the potential to coordinate multiple signals to promote a specific cellular response. They both play important roles during nervous system development but their roles during spinal cord and muscle regeneration are not fully deciphered. Here we show in Xenopus laevis larvae that both Ca²⁺ and Erk1/2 signaling pathways are activated after tail amputation. In response to injury, we find that Erk1/2 signaling is activated in neural and muscle stem cells and is necessary for spinal cord and skeletal muscle regeneration. Finally, we show in vivo that Erk1/2 activity is necessary for an injury-induced increase in intracellular store-dependent Ca²⁺ dynamics in skeletal muscle-associated tissues but that in spinal cord, injury increases Ca²⁺ influx-dependent Ca²⁺ activity independent of Erk1/2 signaling. This study suggests that precise temporal and tissue-specific activation of Ca²⁺ and Erk1/2 pathways is essential for regulating spinal cord and muscle regeneration.

Homeostatic Regulation of Motoneuron Properties in Development

Homeostatic plasticity represents a set of compensatory mechanisms that are engaged following a perturbation to some feature of neuronal or network function. Homeostatic mechanisms are most robustly expressed during development, a period that is replete with various perturbations such as increased cell size and the addition/removal of synaptic connections. In this review we look at numerous studies that have advanced our understanding of homeostatic plasticity by taking advantage of the accessibility of developing motoneurons. We discuss the homeostatic regulation of embryonic movements in the living chick embryo and describe the spinal compensatory mechanisms that act to recover these movements (homeostatic intrinsic plasticity) or stabilize synaptic strength (synaptic scaling). We describe the expression and triggering mechanisms of these forms of homeostatic plasticity and thereby gain an understanding of their roles in the motor system. We then illustrate how these findings can be extended to studies of developing motoneurons in other systems including the rodents, zebrafish, and fly. Furthermore, studies in developing drosophila have been critical in identifying some of the molecular signaling cascades and expression mechanisms that underlie homeostatic intrinsic membrane excitability. This powerful model organism has also been used to study a presynaptic form of homeostatic plasticity where increases or decreases in synaptic transmission are associated with compensatory changes in probability of release at the neuromuscular junction. Further, we describe studies that demonstrate homeostatic adjustments of ion channel expression following perturbations to other kinds of ion channels. Finally, we discuss work in xenopus that shows a homeostatic regulation of neurotransmitter phenotype in developing motoneurons following activity perturbations. Together, this work illustrates the importance of developing motoneurons in elucidating the mechanisms and roles of homeostatic plasticity.

Novel method to repair articular cartilage by direct reprograming of prechondrogenic mesenchymal stem cells

Age-related cartilage loss is worsened by the limited regenerative capacity of chondrocytes. The role of cell-based therapies using mesenchymal stem cells is gaining interest. Adipose tissue-derived mesenchymal stem cells (ADSCs) are an attractive source to generate the optimal number of chondrocytes required to repair a cartilage defect and regenerate hyaline articular cartilage. Here, we report an outstanding technique to prepare chondrocytes for cartilage repair using canine ADSCs. We hypothesized that external electrical fields promote prechondrogenic condensation without requiring genetic modifications or exogenous factors. We analyzed the effect of electrical stimulation (ES) on the differentiation of ADSC micromass into chondrocytes. Highly compact structures were formed within 3 days of ES of canine ADSC micromass. The expression of type I collagen gene was abolished in these cells compared with that in control micromass cultures and monolayer cultures. We further found that ES enhanced the production of proteoglycan, a highly produced extracellular matrix component in chondrocytes. Additionally, single-cell RNA sequencing analysis showed that canine ADSC micromass undergoing ES developed a prechondrogenic cell aggregation, suggesting their metabolic conversion, biogenesis, and calcium ion change. Collectively, our findings demonstrate the capacity of ES to drive the chondrogenesis of ADSCs in the absence of exogenous factors and confirm its commercial potential as a budget-friendly therapy for the repair of cartilage defects.

From Neural Tube Formation Through the Differentiation of Spinal Cord Neurons: Ion Channels in Action During Neural Development

Ion channels are expressed throughout nervous system development. The type and diversity of conductances and gating mechanisms vary at different developmental stages and with the progressive maturational status of neural cells. The variety of ion channels allows for distinct signaling mechanisms in developing neural cells that in turn regulate the needed cellular processes taking place during each developmental period. These include neural cell proliferation and neuronal differentiation, which are crucial for developmental events ranging from the earliest steps of morphogenesis of the neural tube through the establishment of neuronal circuits. Here, we compile studies assessing the ontogeny of ionic currents in the developing nervous system. We then review work demonstrating a role for ion channels in neural tube formation, to underscore the necessity of the signaling downstream ion channels even at the earliest stages of neural development. We discuss the function of ion channels in neural cell proliferation and neuronal differentiation and conclude with how the regulation of all these morphogenetic and cellular processes by electrical activity enables the appropriate development of the nervous system and the establishment of functional circuits adapted to respond to a changing environment.

Differentiation of Mesenchymal Stem Cells to Neuroglia: in the Context of Cell Signalling

The promise of engineering specific cell types from stem cells and rebuilding damaged or diseased tissues has fascinated stem cell researchers and clinicians over last few decades. Mesenchymal Stem Cells (MSCs) have the potential to differentiate into non-mesodermal cells, particularly neural-lineage, consisting of neurons and glia. These multipotent adult stem cells can be used for implementing clinical trials in neural repair. Ongoing research identifies several molecular mechanisms involved in the speciation of neuroglia, which are tightly regulated and interconnected by various components of cell signalling machinery. Growing MSCs with multiple inducers in culture media will initiate changes on intricately interlinked cell signalling pathways and processes. Net result of these signal flow on cellular architecture is also dependent on the type of ligands and stem cells investigated in vitro. However, our understanding about this dynamic signalling machinery is limited and confounding, especially with spheroid structures, neurospheres and organoids. Therefore, the results for differentiating neurons and glia in vitro have been inconclusive, so far. Added to this complication, we have no convincing evidence about the electrical conductivity and functionality status generated in differentiating neurons and glia. This review has taken a step forward to tailor the information on differentiating neuroglia with the common methodologies, in practice.

Toward Decoding Bioelectric Events in Xenopus Embryogenesis: New Methodology for Tracking Interplay Between Calcium and Resting Potentials In Vivo

Although chemical signaling during embryogenesis is readily addressed by a plethora of available techniques, the developmental functions of ionic signaling are still poorly understood. It is increasingly realized that bioelectric events in nonneural cells are critical for pattern regulation, but their study has been hampered by difficulties in monitoring and manipulating them in vivo. Recent developments in visualizing electrical signaling dynamics in the field of neuroscience have facilitated functional experiments that reveal instructive developmental bioelectric signals. However, there is a pressing need for additional tools to explore time-dependent ionic signaling to understand complex endogenous dynamics. Here, we present methodological advances, including 4D imaging and data analysis, for improved tracking of calcium flux in the Xenopus laevis embryo, lowering the barrier for in vivo physiology work in this important model system. Using these techniques, we investigated the relationship between bioelectric ion channel activity and calcium, finding that cell hyperpolarization and depolarization both induce persistent static elevation of cytoplasmic calcium levels that fade over developmental time. These calcium changes correlate with increased cell mobility in early embryos and abnormal craniofacial morphology in later embryos. We thus highlight membrane potential modulation as a tractable tool for modulation of signaling cascades that rely on calcium as a transduction mechanism. The methods we describe facilitate the study of important novel aspects of developmental physiology, are extendable to numerous classes of existing and forthcoming fluorescent physiological reporters, and establish highly accessible, inexpensive protocols for their investigation.

Ca2+ Signaling and Regeneration

Regeneration is the process by which lost or damaged tissue is replaced in adult organisms. Some organisms exhibit robust regenerative capabilities, while others, including humans, do not. Understanding the molecular principles governing the regenerative malleability of different organisms is of fundamental biological interest. Further, this problem has clear impact for the field of "regenerative medicine," which aspires to understand how human cells, tissues, and organs may be restored to normal function in scenarios of disease, damage, or age-related decline. This review will focus on the planarian flatworm as a powerful model system for studying the role of Ca²⁺ signals in regeneration. These invertebrate animals display an astounding innate regenerative capacity capable of regenerating complete organisms from tiny, excised fragments. New knowledge and methodological capabilities in this system highlight the potential for studying the role of Ca²⁺ signaling at multiple stages of the regenerative blueprint that controls stem cell behavior in vivo.

Growth at Cold Temperature Increases the Number of Motor Neurons to Optimize Locomotor Function

During vertebrate development, spinal neurons differentiate and connect to generate a system that performs sensorimotor functions critical for survival. Spontaneous Ca²⁺ activity regulates different aspects of spinal neuron differentiation. It is unclear whether environmental factors can modulate this Ca²⁺ activity in developing spinal neurons to alter their specialization and ultimately adjust sensorimotor behavior to fit the environment. Here, we show that growing Xenopus laevis embryos at cold temperatures results in an increase in the number of spinal motor neurons in larvae. This change in spinal cord development optimizes the escape response to gentle touch of animals raised in and tested at cold temperatures. The cold-sensitive channel TRPM8 increases Ca²⁺ spike frequency of developing ventral spinal neurons, which in turn regulates expression of the motor neuron master transcription factor HB9. TRPM8 is necessary for the increase in motor neuron number of animals raised in cold temperatures and for their enhanced sensorimotor behavior when tested at cold temperatures. These findings suggest the environment modulates neuronal differentiation to optimize the behavior of the developing organism.

Spencer et al. discover that Xenopus larvae reared in cold temperature are better equipped to escape upon touch at cold temperature relative to warm-grown siblings. This advantage is dependent on the cold-sensitive channel TRPM8, which is necessary for increased Ca²⁺ spike frequency in embryonic spinal neurons, their differentiation, and survival.

NMDA Receptor Signaling Is Important for Neural Tube Formation and for Preventing Antiepileptic Drug-Induced Neural Tube Defects

Failure of neural tube closure leads to neural tube defects (NTDs), which can have serious neurological consequences or be lethal. Use of antiepileptic drugs (AEDs) during pregnancy increases the incidence of NTDs in offspring by unknown mechanisms. Here we show that during Xenopus laevis neural tube formation, neural plate cells exhibit spontaneous calcium dynamics that are partially mediated by glutamate signaling. We demonstrate that NMDA receptors are important for the formation of the neural tube and that the loss of their function induces an increase in neural plate cell proliferation and impairs neural cell migration, which result in NTDs. We present evidence that the AED valproic acid perturbs glutamate signaling, leading to NTDs that are rescued with varied efficacy by preventing DNA synthesis, activating NMDA receptors, or recruiting the NMDA receptor target ERK1/2. These findings may prompt mechanistic identification of AEDs that do not interfere with neural tube formation.SIGNIFICANCE STATEMENT Neural tube defects are one of the most common birth defects. Clinical investigations have determined that the use of antiepileptic drugs during pregnancy increases the incidence of these defects in the offspring by unknown mechanisms. This study discovers that glutamate signaling regulates neural plate cell proliferation and oriented migration and is necessary for neural tube formation. We demonstrate that the widely used antiepileptic drug valproic acid interferes with glutamate signaling and consequently induces neural tube defects, challenging the current hypotheses arguing that they are side effects of this antiepileptic drug that cause the increased incidence of these defects. Understanding the mechanisms of neurotransmitter signaling during neural tube formation may contribute to the identification and development of antiepileptic drugs that are safer during pregnancy.

The bioelectric code: An ancient computational medium for dynamic control of growth and form

What determines large-scale anatomy? DNA does not directly specify geometrical arrangements of tissues and organs, and a process of encoding and decoding for morphogenesis is required. Moreover, many species can regenerate and remodel their structure despite drastic injury. The ability to obtain the correct target morphology from a diversity of initial conditions reveals that the morphogenetic code implements a rich system of pattern-homeostatic processes. Here, we describe an important mechanism by which cellular networks implement pattern regulation and plasticity: bioelectricity. All cells, not only nerves and muscles, produce and sense electrical signals; in vivo, these processes form bioelectric circuits that harness individual cell behaviors toward specific anatomical endpoints. We review emerging progress in reading and re-writing anatomical information encoded in bioelectrical states, and discuss the approaches to this problem from the perspectives of information theory, dynamical systems, and computational neuroscience. Cracking the bioelectric code will enable much-improved control over biological patterning, advancing basic evolutionary developmental biology as well as enabling numerous applications in regenerative medicine and synthetic bioengineering.

Bioelectric signaling in regeneration: Mechanisms of ionic controls of growth and form

The ability to control pattern formation is critical for the both the embryonic development of complex structures as well as for the regeneration/repair of damaged or missing tissues and organs. In addition to chemical gradients and gene regulatory networks, endogenous ion flows are key regulators of cell behavior. Not only do bioelectric cues provide information needed for the initial development of structures, they also enable the robust restoration of normal pattern after injury. In order to expand our basic understanding of morphogenetic processes responsible for the repair of complex anatomy, we need to identify the roles of endogenous voltage gradients, ion flows, and electric fields. In complement to the current focus on molecular genetics, decoding the information transduced by bioelectric cues enhances our knowledge of the dynamic control of growth and pattern formation. Recent advances in science and technology place us in an exciting time to elucidate the interplay between molecular-genetic inputs and important biophysical cues that direct the creation of tissues and organs. Moving forward, these new insights enable additional approaches to direct cell behavior and may result in profound advances in augmentation of regenerative capacity.

Cross-limb communication during Xenopus hind-limb regenerative response: non-local bioelectric injury signals

Regeneration of damaged body-parts requires coordination of size, shape, location, and orientation of tissue with the rest of the body. It is not currently known how far injury sites communicate with the remaining soma during repair, or what information may emanate from the injury site to other regions. We examined the bioelectric properties (resting potential gradients in the epidermis) of Xenopus froglets undergoing hind-limb amputation and observed that the contralateral (un-damaged) limb exhibits apparent depolarization signals immediately after the opposite hind-limb is amputated. The pattern of depolarization matches that of the amputated limb and is correlated to the position and type of injury, revealing that information about damage is available to remote body tissues and is detectable non-invasively in vivo by monitoring of the bioelectric state. These data extend knowledge about the electrophysiology of regenerative response, identify a novel communication process via long-range spread of injury signaling, a phenomenon which we call bioelectric injury mirroring (BIM), and suggests revisions to regenerative medicine and diagnostic strategies focused entirely on the wound site and to the use of contralateral limbs as controls.

Xenopus laevis as a Model Organism for the Study of Spinal Cord Formation, Development, Function and Regeneration

The spinal cord is the first central nervous system structure to develop during vertebrate embryogenesis, underscoring its importance to the organism. Because of its early formation, accessibility to the developing spinal cord in amniotes is challenging, often invasive and the experimental approaches amenable to model systems like mammals are limited. In contrast, amphibians, in general and the African-clawed frog Xenopus laevis, in particular, offer model systems in which the formation of the spinal cord, the differentiation of spinal neurons and glia and the establishment of spinal neuron and neuromuscular synapses can be easily investigated with minimal perturbations to the whole organism. The significant advances on gene editing and microscopy along with the recent completion of the Xenopus laevis genome sequencing have reinvigorated the use of this classic model species to elucidate the mechanisms of spinal cord formation, development, function and regeneration.

Inwardly rectifying potassium channels influence Drosophila wing morphogenesis by regulating Dpp release

Loss of embryonic ion channel function leads to morphological defects, but the underlying reason for these defects remains elusive. Here, we show that inwardly rectifying potassium (Irk) channels regulate release of the Drosophila bone morphogenetic protein Dpp in the developing fly wing and that this is necessary for developmental signaling. Inhibition of Irk channels decreases the incidence of distinct Dpp-GFP release events above baseline fluorescence while leading to a broader distribution of Dpp-GFP. Work by others in different cell types has shown that Irk channels regulate peptide release by modulating membrane potential and calcium levels. We found calcium transients in the developing wing, and inhibition of Irk channels reduces the duration and amplitude of calcium transients. Depolarization with high extracellular potassium evokes Dpp release. Taken together, our data implicate Irk channels as a requirement for regulated release of Dpp, highlighting the importance of the temporal pattern of Dpp presentation for morphogenesis of the wing.

Endogenous Bioelectric Signaling Networks: Exploiting Voltage Gradients for Control of Growth and Form

Living systems exhibit remarkable abilities to self-assemble, regenerate, and remodel complex shapes. How cellular networks construct and repair specific anatomical outcomes is an open question at the heart of the next-generation science of bioengineering. Developmental bioelectricity is an exciting emerging discipline that exploits endogenous bioelectric signaling among many cell types to regulate pattern formation. We provide a brief overview of this field, review recent data in which bioelectricity is used to control patterning in a range of model systems, and describe the molecular tools being used to probe the role of bioelectrics in the dynamic control of complex anatomy. We suggest that quantitative strategies recently developed to infer semantic content and information processing from ionic activity in the brain might provide important clues to cracking the bioelectric code. Gaining control of the mechanisms by which large-scale shape is regulated in vivo will drive transformative advances in bioengineering, regenerative medicine, and synthetic morphology, and could be used to therapeutically address birth defects, traumatic injury, and cancer.

Folate receptor 1 is necessary for neural plate cell apical constriction during Xenopus neural tube formation

Folate supplementation prevents up to 70% of neural tube defects (NTDs), which result from a failure of neural tube closure during embryogenesis. The elucidation of the mechanisms underlying folate action has been challenging. This study introduces Xenopus laevis as a model to determine the cellular and molecular mechanisms involved in folate action during neural tube formation. We show that knockdown of folate receptor 1 (Folr1; also known as FRα) impairs neural tube formation and leads to NTDs. Folr1 knockdown in neural plate cells only is necessary and sufficient to induce NTDs. Folr1-deficient neural plate cells fail to constrict, resulting in widening of the neural plate midline and defective neural tube closure. Pharmacological inhibition of folate action by methotrexate during neurulation induces NTDs by inhibiting folate interaction with its uptake systems. Our findings support a model in which the folate receptor interacts with cell adhesion molecules, thus regulating the apical cell membrane remodeling and cytoskeletal dynamics necessary for neural plate folding. Further studies in this organism could unveil novel cellular and molecular events mediated by folate and lead to new ways of preventing NTDs.

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action.

Ion channels in development and cancer

Ion channels have emerged as regulators of developmental processes. In model organisms and in people with mutations in ion channels, disruption of ion channel function can affect cell proliferation, cell migration, and craniofacial and limb patterning. Alterations of ion channel function affect morphogenesis in fish, frogs, mammals, and flies, demonstrating that ion channels have conserved roles in developmental processes. One model suggests that ion channels affect proliferation and migration through changes in cell volume. However, ion channels have not explicitly been placed in canonical developmental signaling cascades until recently. This review gives examples of ion channels that influence developmental processes, offers a potential underlying molecular mechanism involving bone morphogenetic protein (BMP) signaling, and finally explores exciting possibilities for manipulating ion channels to influence cell fate for regenerative medicine and to impact disease.

Cellular Organization of Neuroimmune Interactions in the Gastrointestinal Tract

The gastrointestinal (GI) tract is the largest immune organ; in vertebrates, it is the only organ whose function is controlled by its own intrinsic enteric nervous system (ENS), but it is additionally regulated by extrinsic (sympathetic and parasympathetic) innervation. The GI nervous and immune systems are highly integrated in their common goal, which is to unite digestive functions with protection from ingested environmental threats. This review discusses the physiological relevance of enteric neuroimmune integration by summarizing the current knowledge of evolutionary and developmental pathways, cellular organization, and molecular mechanisms of neuroimmune interactions in health and disease.

Calcium signaling in skeletal muscle development, maintenance and regeneration

Skeletal muscle-specific stem cells are pivotal for tissue development and regeneration. Muscle plasticity, inherent in these processes, is also essential for daily life activities. Great advances and efforts have been made in understanding the function of the skeletal muscle-dedicated stem cells, called muscle satellite cells, and the specific signaling mechanisms that activate them for recruitment in the repair of the injured muscle. Elucidating these signaling mechanisms may contribute to devising therapies for muscular injury or disease. Here we review the studies that have contributed to our understanding of how calcium signaling regulates skeletal muscle development, homeostasis and regeneration, with a focus on the calcium dynamics and calcium-dependent effectors that participate in these processes.

Crosstalk among electrical activity, trophic factors and morphogenetic proteins in the regulation of neurotransmitter phenotype specification

Morphogenetic proteins are responsible for patterning the embryonic nervous system by enabling cell proliferation that will populate all the neural structures and by specifying neural progenitors that imprint different identities in differentiating neurons. The adoption of specific neurotransmitter phenotypes is crucial for the progression of neuronal differentiation, enabling neurons to connect with each other and with target tissues. Preliminary neurotransmitter specification originates from morphogen-driven neural progenitor specification through the combinatorial expression of transcription factors according to morphogen concentration gradients, which progressively restrict the identity that born neurons adopt. However, neurotransmitter phenotype is not immutable, instead trophic factors released from target tissues and environmental stimuli change expression of neurotransmitter-synthesizing enzymes and specific vesicular transporters modifying neuronal neurotransmitter identity. Here we review studies identifying the mechanisms of catecholaminergic, GABAergic, glutamatergic, cholinergic and serotonergic early specification and of the plasticity of these neurotransmitter phenotypes during development and in the adult nervous system. The emergence of spontaneous electrical activity in developing neurons recruits morphogenetic proteins in the process of neurotransmitter phenotype plasticity, which ultimately equips the nervous system and the whole organism with adaptability for optimal performance in a changing environment.

Genome-wide analysis reveals conserved transcriptional responses downstream of resting potential change in Xenopus embryos, axolotl regeneration, and human mesenchymal cell differentiation

Endogenous bioelectric signaling via changes in cellular resting potential (V_mem) is a key regulator of patterning during regeneration and embryogenesis in numerous model systems. Depolarization of V_mem has been functionally implicated in dedifferentiation, tumorigenesis, anatomical re‐specification, and appendage regeneration. However, no unbiased analyses have been performed to understand genome‐wide transcriptional responses to V_mem change in vivo. Moreover, it is unknown which genes or gene networks represent conserved targets of bioelectrical signaling across different patterning contexts and species. Here, we use microarray analysis to comparatively analyze transcriptional responses to V_mem depolarization. We compare the response of the transcriptome during embryogenesis (Xenopus development), regeneration (axolotl regeneration), and stem cell differentiation (human mesenchymal stem cells in culture) to identify common networks across model species that are associated with depolarization. Both subnetwork enrichment and PANTHER analyses identified a number of key genetic modules as targets of V_mem change, and also revealed important (well‐conserved) commonalities in bioelectric signal transduction, despite highly diverse experimental contexts and species. Depolarization regulates specific transcriptional networks across all three germ layers (ectoderm, mesoderm, and endoderm) such as cell differentiation and apoptosis, and this information will be used for developing mechanistic models of bioelectric regulation of patterning. Moreover, our analysis reveals that V_mem change regulates transcripts related to important disease pathways such as cancer and neurodegeneration, which may represent novel targets for emerging electroceutical therapies.

Neurotransmitter Switching? No Surprise

Among the many forms of brain plasticity, changes in synaptic strength and changes in synapse number are particularly prominent. However, evidence for neurotransmitter respecification or switching has been accumulating steadily, both in the developing nervous system and in the adult brain, with observations of transmitter addition, loss, or replacement of one transmitter with another. Natural stimuli can drive these changes in transmitter identity, with matching changes in postsynaptic transmitter receptors. Strikingly, they often convert the synapse from excitatory to inhibitory or vice versa, providing a basis for changes in behavior in those cases in which it has been examined. Progress has been made in identifying the factors that induce transmitter switching and in understanding the molecular mechanisms by which it is achieved. There are many intriguing questions to be addressed.

Molecular bioelectricity: how endogenous voltage potentials control cell behavior and instruct pattern regulation in vivo

In addition to biochemical gradients and transcriptional networks, cell behavior is regulated by endogenous bioelectrical cues originating in the activity of ion channels and pumps, operating in a wide variety of cell types. Instructive signals mediated by changes in resting potential control proliferation, differentiation, cell shape, and apoptosis of stem, progenitor, and somatic cells. Of importance, however, cells are regulated not only by their own V_mem but also by the V_mem of their neighbors, forming networks via electrical synapses known as gap junctions. Spatiotemporal changes in V_mem distribution among nonneural somatic tissues regulate pattern formation and serve as signals that trigger limb regeneration, induce eye formation, set polarity of whole-body anatomical axes, and orchestrate craniofacial patterning. New tools for tracking and functionally altering V_mem gradients in vivo have identified novel roles for bioelectrical signaling and revealed the molecular pathways by which V_mem changes are transduced into cascades of downstream gene expression. Because channels and gap junctions are gated posttranslationally, bioelectrical networks have their own characteristic dynamics that do not reduce to molecular profiling of channel expression (although they couple functionally to transcriptional networks). The recent data provide an exciting opportunity to crack the bioelectric code, and learn to program cellular activity at the level of organs, not only cell types. The understanding of how patterning information is encoded in bioelectrical networks, which may require concepts from computational neuroscience, will have transformative implications for embryogenesis, regeneration, cancer, and synthetic bioengineering.

Inversion of Sonic hedgehog action on its canonical pathway by electrical activity

Sonic hedgehog (Shh) is a morphogenic protein that operates through the Gli transcription factor-dependent canonical pathway to orchestrate normal development of many tissues. Because aberrant levels of Gli activity lead to a wide spectrum of diseases ranging from neurodevelopmental defects to cancer, understanding the regulatory mechanisms of Shh canonical pathway is paramount. During early stages of spinal cord development, Shh specifies neural progenitors through the canonical signaling. Despite persistence of Shh as spinal cord development progresses, Gli activity is switched off by unknown mechanisms. In this study we find that Shh inverts its action on Gli during development. Strikingly, Shh decreases Gli signaling in the embryonic spinal cord by an electrical activity- and cAMP-dependent protein kinase-mediated pathway. The inhibition of Gli activity by Shh operates at multiple levels. Shh promotes cytosolic over nuclear localization of Gli2, induces Gli2 and Gli3 processing into repressor forms, and activates cAMP-responsive element binding protein that in turn represses gli1 transcription. The regulatory mechanisms identified in this study likely operate with different spatiotemporal resolution and ensure effective down-regulation of the canonical Shh signaling as spinal cord development progresses. The developmentally regulated intercalation of electrical activity in the Shh pathway may represent a paradigm for switching from canonical to noncanonical roles of developmental cues during neuronal differentiation and maturation.

Endogenous gradients of resting potential instructively pattern embryonic neural tissue via Notch signaling and regulation of proliferation

Biophysical forces play important roles throughout embryogenesis, but the roles of spatial differences in cellular resting potentials during large-scale brain morphogenesis remain unknown. Here, we implicate endogenous bioelectricity as an instructive factor during brain patterning in Xenopus laevis. Early frog embryos exhibit a characteristic hyperpolarization of cells lining the neural tube; disruption of this spatial gradient of the transmembrane potential (Vmem) diminishes or eliminates the expression of early brain markers, and causes anatomical mispatterning of the brain, including absent or malformed regions. This effect is mediated by voltage-gated calcium signaling and gap-junctional communication. In addition to cell-autonomous effects, we show that hyperpolarization of transmembrane potential (Vmem) in ventral cells outside the brain induces upregulation of neural cell proliferation at long range. Misexpression of the constitutively active form of Notch, a suppressor of neural induction, impairs the normal hyperpolarization pattern and neural patterning; forced hyperpolarization by misexpression of specific ion channels rescues brain defects induced by activated Notch signaling. Strikingly, hyperpolarizing posterior or ventral cells induces the production of ectopic neural tissue considerably outside the neural field. The hyperpolarization signal also synergizes with canonical reprogramming factors (POU and HB4), directing undifferentiated cells toward neural fate in vivo. These data identify a new functional role for bioelectric signaling in brain patterning, reveal interactions between Vmem and key biochemical pathways (Notch and Ca(2+) signaling) as the molecular mechanism by which spatial differences of Vmem regulate organogenesis of the vertebrate brain, and suggest voltage modulation as a tractable strategy for intervention in certain classes of birth defects.

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

Bioelectric signals, particularly transmembrane voltage potentials (Vmem), play an important role in large-scale patterning during embryonic development. Endogenous bioelectric gradients across tissues function as instructive factors during eye, brain, and other morphogenetic processes. An important and still poorly-understood aspect is the control of cell behaviors by the voltage states of distant cell groups. Here, experimental alteration of endogenous Vmem was induced in Xenopus laevis embryos by misexpression of well-characterized ion channel mRNAs, a strategy often used to identify functional roles of Vmem gradients during embryonic development and regeneration. Immunofluorescence analysis (for activated caspase 3 and phosphor-histone H3P) on embryonic sections was used to characterize apoptosis and proliferation. Disrupting local bioelectric signals (within the developing neural tube region) increased caspase 3 and decreased H3P in the brain, resulting in brain mispatterning. Disrupting remote (ventral, non-neural region) bioelectric signals decreased caspase 3 and highly increased H3P within the brain, with normal brain patterning. Disrupting both the local and distant bioelectric signals produced antagonistic effects on caspase 3 and H3P. Thus, two components of bioelectric signals regulate apoptosis-proliferation balance within the developing brain and spinal cord: local (developing neural tube region) and distant (ventral non-neural region). Together, the local and long-range bioelectric signals create a binary control system capable of fine-tuning apoptosis and proliferation with the brain and spinal cord to achieve correct pattern and size control. Our data suggest a roadmap for utilizing bioelectric state as a diagnostic modality and convenient intervention parameter for birth defects and degenerative disease states of the CNS.

Functional cardiomyocytes derived from Isl1 cardiac progenitors via Bmp4 stimulation

As heart failure due to myocardial infarction remains a leading cause of morbidity worldwide, cell-based cardiac regenerative therapy using cardiac progenitor cells (CPCs) could provide a potential treatment for the repair of injured myocardium. As adult CPCs may have limitations regarding tissue accessibility and proliferative ability, CPCs derived from embryonic stem cells (ESCs) could serve as an unlimited source of cells with high proliferative ability. As one of the CPCs that can be derived from embryonic stem cells, Isl1 expressing cardiac progenitor cells (Isl1-CPCs) may serve as a valuable source of cells for cardiac repair due to their high cardiac differentiation potential and authentic cardiac origin. In order to generate an unlimited number of Isl1-CPCs, we used a previously established an ESC line that allows for isolation of Isl1-CPCs by green fluorescent protein (GFP) expression that is directed by the mef2c gene, specifically expressed in the Isl1 domain of the anterior heart field. To improve the efficiency of cardiac differentiation of Isl1-CPCs, we studied the role of Bmp4 in cardiogenesis of Isl1-CPCs. We show an inductive role of Bmp directly on cardiac progenitors and its enhancement on early cardiac differentiation of CPCs. Upon induction of Bmp4 to Isl1-CPCs during differentiation, the cTnT+ cardiomyocyte population was enhanced 2.8±0.4 fold for Bmp4 treated CPC cultures compared to that detected for vehicle treated cultures. Both Bmp4 treated and untreated cardiomyocytes exhibit proper electrophysiological and calcium signaling properties. In addition, we observed a significant increase in Tbx5 and Tbx20 expression in differentiation cultures treated with Bmp4 compared to the untreated control, suggesting a link between Bmp4 and Tbx genes which may contribute to the enhanced cardiac differentiation in Bmp4 treated cultures. Collectively these findings suggest a cardiomyogenic role for Bmp4 directly on a pure population of Isl1 expressing cardiac progenitors, which could lead to enhancement of cardiac differentiation and engraftment, holding a significant therapeutic value for cardiac repair in the future.

Spatiotemporal integration of developmental cues in neural development

Nervous system development relies on the generation of neurons, their differentiation and establishment of synaptic connections. These events exhibit remarkable plasticity and are regulated by many developmental cues. Here, we review the mechanisms of three classes of these cues: morphogenetic proteins, electrical activity, and the environment. We focus on second messenger dynamics and their role as integrators of the action of diverse cues, enabling plasticity in the process of neural development.

Crosstalk between muscularis macrophages and enteric neurons regulates gastrointestinal motility

Intestinal peristalsis is a dynamic physiologic process influenced by dietary and microbial changes. It is tightly regulated by complex cellular interactions; however, our understanding of these controls is incomplete. A distinct population of macrophages is distributed in the intestinal muscularis externa. We demonstrate that, in the steady state, muscularis macrophages regulate peristaltic activity of the colon. They change the pattern of smooth muscle contractions by secreting bone morphogenetic protein 2 (BMP2), which activates BMP receptor (BMPR) expressed by enteric neurons. Enteric neurons, in turn, secrete colony stimulatory factor 1 (CSF1), a growth factor required for macrophage development. Finally, stimuli from microbial commensals regulate BMP2 expression by macrophages and CSF1 expression by enteric neurons. Our findings identify a plastic, microbiota-driven crosstalk between muscularis macrophages and enteric neurons that controls gastrointestinal motility. PAPERFLICK:

Spontaneous calcium transients manifest in the regenerating muscle and are necessary for skeletal muscle replenishment

Tissue regeneration entails replenishing of damaged cells, appropriate cell differentiation and inclusion of regenerated cells into functioning tissues. In adult humans, the capacity of the injured spinal cord and muscle to self-repair is limited. In contrast, the amphibian larva can regenerate its tail after amputation with complete recovery of muscle, notochord and spinal cord. The cellular and molecular mechanisms underlying this phenomenon are still unclear. Here we show that upon injury muscle cell precursors exhibit Ca(2+) transients that depend on Ca(2+) release from ryanodine receptor-operated stores. Blockade of these transients impairs muscle regeneration. Furthermore, inhibiting Ca(2+) transients in the regenerating tail prevents the activation and proliferation of muscle satellite cells, which results in deficient muscle replenishment. These findings suggest that Ca(2+)-mediated activity is critical for the early stages of muscle regeneration, which may lead to developing effective therapies for tissue repair.

Dynamic regulation of neurotransmitter specification: relevance to nervous system homeostasis

During nervous system development the neurotransmitter identity changes and coexpression of several neurotransmitters is a rather generalized feature of developing neurons. In the mature nervous system, different physiological and pathological circumstances recreate this phenomenon. The rules of neurotransmitter respecification are multiple. Among them, the goal of assuring balanced excitability appears as an important driving force for the modifications in neurotransmitter phenotype expression. The functional consequences of these dynamic revisions in neurotransmitter identity span a varied range, from fine-tuning the developing neural circuit to modifications in addictive and locomotor behaviors. Current challenges include determining the mechanisms underlying neurotransmitter phenotype respecification and how they intersect with genetic programs of neuronal specialization. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.