RNA synthesis dependence of action potential development in spinal cord neurones

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.

BMP gradients steer nerve growth cones by a balancing act of LIM kinase and Slingshot phosphatase on ADF/cofilin

Bone morphogenic proteins (BMPs) are involved in axon pathfinding, but how they guide growth cones remains elusive. In this study, we report that a BMP7 gradient elicits bidirectional turning responses from nerve growth cones by acting through LIM kinase (LIMK) and Slingshot (SSH) phosphatase to regulate actin-depolymerizing factor (ADF)/cofilin-mediated actin dynamics. Xenopus laevis growth cones from 4–8-h cultured neurons are attracted to BMP7 gradients but become repelled by BMP7 after overnight culture. The attraction and repulsion are mediated by LIMK and SSH, respectively, which oppositely regulate the phosphorylation-dependent asymmetric activity of ADF/cofilin to control the actin dynamics and growth cone steering. The attraction to repulsion switching requires the expression of a transient receptor potential (TRP) channel TRPC1 and involves Ca²⁺ signaling through calcineurin phosphatase for SSH activation and growth cone repulsion. Together, we show that spatial regulation of ADF/cofilin activity controls the directional responses of the growth cone to BMP7, and Ca²⁺ influx through TRPC tilts the LIMK-SSH balance toward SSH-mediated repulsion.

Differentiation of electrical excitability in motoneurons

Investigation of the differentiation of electrical properties of motoneurons has been stimulated by the importance of these neurons for embryonic behavior and facilitated by their experimental accessibility. In this review, we examine the development of different patterns of excitability and their functions, and discuss the emergence of repetitive firing and localization of ion channels in axons and dendrites. Finally, we summarize studies of the role of extrinsic factors in differentiation. These changes associated with differentiation of young motoneurons may presage those occurring later in the context of plasticity in the mature nervous system.

Potassium currents in developing neurons

In Xenopus spinal neurons, delayed rectifier type voltage-dependent potassium currents (IKv) are developmentally regulated. These currents play a pivotal role in maturation of the action potential from a long-duration calcium-dependent impulse to a brief sodium-dependent one. Although spinal neurons are heterogeneous, IKv undergoes a synchronized and homogeneous developmental functional up-regulation across this diverse population of motor, sensory, and interneurons. This finding suggested that the diverse population of neurons expressed a common potassium channel. Thus, recent efforts have been directed towards cloning the relevant potassium channel gene. However, these molecular studies reveal an unsuspected heterogeneity in the molecular components of voltage-dependent potassium channels. Further, synchronous differentiation of IKv is achieved via heterogeneous Kv channel gene expression.

Requirement of RNA synthesis for pathfinding by growing axons

The effects of actinomycin D were studied in cultured grasshopper embryos at different stages of development by following the outgrowth patterns of identified neurones known as aCC, pCC, and Q1. When administered at stages occurring before 31% of embryonic development, actinomycin D (0.05-0.10 microM for 24-48 hours) prevented axon extension, whereas it did not affect the development of the nervous system in embryos older than 34% of development. At 31-34% of development, actinomycin D perturbed pathfinding of aCC without blocking axon extension. Thus, only 22% of the aCCs (n = 271) in embryos treated with actinomycin D extended an axon along the intersegmental nerve as in control embryos. In the remaining embryos, aCC failed to turn into the intersegmental nerve root; its growth cone remained in the longitudinal connective, above or below the turning point. Neurones of the group caudal to the intersegmental nerve root could extend along either the anterior or posterior commissure of the next posterior segment. In contrast to the observations made with aCC, only 1.2% of pCC (n = 166) and 0.0% of Q1 (n = 45) in embryos treated with actinomycin D showed axon growth along aberrant pathways. The position of the growth cones of most pCCs and all Q1s observed were in various points along their normal pathway. Both pCC and Q1, as a population, showed an extension rate significantly lower than that of their control counterparts. The effect of actinomycin D on aCC pathway choice was probably mediated by inhibition of RNA synthesis, because incorporation of uridine into RNA was reduced by 40%. The labelling of several monoclonal antibodies (1C10, 3B11, 7F7) that recognise surface glycoproteins (lachesin, fasciclin I, and REGA-1) involved in nervous system development of grasshopper embryos was suppressed. Our results suggest that the navigation of some axons along different pathways requires the synthesis of new mRNA.

A single pulse of nerve growth factor triggers long-term neuronal excitability through sodium channel gene induction

The continuous presence of nerve growth factor (NGF) is thought to be required for the elaboration of neuronal-like traits in PC12 cells. Surprisingly, we find that a 1 min exposure to NGF is sufficient to engage a longer-term genetic program leading to the acquisition of membrane excitability. Whereas continuous exposure to NGF causes the induction of a family of sodium channels, the effect of a brief exposure is to induce selectively expression of the peripheral nerve-type sodium channel gene PN1, through a distinct signaling pathway requiring immediate-early genes. A 1 min exposure of PC12 cells to interferon-gamma also causes PN1 gene induction, suggesting that the "triggered" NGF and interferon-gamma signaling pathways share common molecular intermediates.

Potentiation of transmitter release by ciliary neurotrophic factor requires somatic signaling

Neurotrophic factors participate in the development and maintenance of the nervous system. Application of ciliary neurotrophic factor (CNTF), a protein that promotes survival of motor neurons, resulted in an immediate potentiation of spontaneous and impulse-evoked transmitter release at developing neuromuscular synapses in Xenopus cell cultures. When CNTF was applied at the synapse, the onset of the potentiation was slower than that produced by application at the cell body of the presynaptic neuron. The potentiation effect was abolished when the neurite shaft was severed from the cell body. Thus, transmitter secretion from the nerve terminals is under immediate somatic control and can be regulated by CNTF.

Aspects of early postnatal development of cortical neurons that proceed independently of normally present extrinsic influences

To examine the contribution of local versus extrinsic influences on postnatal development of cortical neurons, we compared the maturation of deep (infragranular) layer neurons in isolated slices of neocortex grown in organotypic culture to a similar population of neurons developing in vivo. All slice cultures were prepared from sensorimotor cortices of newborn mice (P0) and neurons in these cultures were examined at daily intervals during the first 9 days in vitro (DIV). The maturational state of neurons developing in vivo over this same time period was assessed in acute slices prepared from animals of equivalent postnatal age, P1-P9. Electrophysiological recordings were obtained from neurons in both cultured and acute slices, using Lucifer yellow filled whole-cell recording electrodes, enabling subsequent morphometric analysis of the labeled cells. We report significant changes in both cellular morphology and electrical membrane properties of these deep layer cortical neurons during the first week in culture. Morphological maturation over this time period was characterized by a two- to three-fold increase in cell body size and total process length, and an increase in dendritic complexity. In this same population of cells a three-fold decrease in input resistance and changes in the action potential waveform, including a two-fold decrease in the AP duration, also occur. The degree of morphological and electrophysiological differentiation of individual neurons was highly correlated across developmental ages, suggesting that the maturational state of a cell is reflected in both cellular morphology and intrinsic membrane properties. A remarkably similar pattern of neuronal maturation was observed in neurons in layers V, VI/SP examined in acute slices prepared from animals between P1-P9. Because our culture system preserves many aspects of the local cortical environment while eliminating normal extrinsic influences (including thalamic, brainstem, and callosal connections), our findings argue that this early phase of neuronal differentiation, including the rate and extent of dendritic growth and development of AP waveform, results from instructive and/or permissive local influences, and appears to proceed independently of the many normally present extrinsic factors.

Role of calcium and protein kinase C in development of the delayed rectifier potassium current in Xenopus spinal neurons

The delayed rectifier current of embryonic Xenopus spinal neurons plays the central role in developmental conversion of calcium-dependent action potentials to sodium-dependent spikes. During its maturation, this potassium current undergoes a pronounced increase in rate of activation. The mechanism underlying the change in kinetics was analyzed with whole-cell voltage clamp of neurons cultured under various conditions. Calcium is necessary at an early stage of development, to permit influx that triggers subsequent release of calcium from intracellular stores. Its action is prevented by depletion of protein kinase C and mimicked by stimulation of the kinase. Calcium influx through voltage-dependent channels at early stages of development regulates the differentiation of potassium current kinetics and modulation of the ionic dependence of action potentials.

Development of anomalous rectification (Ih) and of a tetrodotoxin-resistant sodium current in embryonic quail neurones

1. The developmental expression of an inwardly rectifying current activated by membrane hyperpolarization (Ih) and of a tetrodotoxin (TTX)-resistant Na+ current (INa(TR)) was studied using freshly dissociated ganglionic quail neurones of various embryonic ages. This work was carried out on parasympathetic (ciliary) and sensory (trigeminal and dorsal root) ganglion neurones with the whole-cell configuration of the patch-clamp technique. 2. In sensory and parasympathetic neurones, Ih was activated at potentials more negative than -60 mV and displayed strong inward rectification. No sign of time- or voltage-dependent inactivation was apparent. Ih was carried by both Na+ and K+ ions and was selectively and reversibly blocked by extracellular Cs+. 3. During the development of sensory neurones, Ih was observed for the first time between embryonic day 10 (E10) and E11 and the percentage of neurones expressing the current increased subsequently, reaching a plateau level of about 80% at E14. In the parasympathetic neurones of the ciliary ganglion, Ih was already detected at E10 and the percentage of neurones possessing the current increased until E16, a stage at which all neurones were found to express Ih. 4. In the presence of TTX (1 microM), an inward Na+ current, INa(TR), was recorded in sensory neurones after E12. This current was activated at potentials more depolarized than -30 mV and its amplitude was maximal at +5 mV. INa(TR) showed time- and voltage-dependent inactivation. Half-maximal steady-state inactivation was observed at -40 mV. 5. INa(TR) was observed for the first time after E12 in sensory neurones and the percentage of neurones with INa(TR) increased until E14. Thereafter, 80% of the neurones had the current. In contrast, INa(TR) was never observed in the parasympathetic neurones of the ciliary ganglion during embryonic development. 6. Our results with parasympathetic and sensory neurones suggest that the expression of INa(TR) is linked to the phenotype and not to the embryonic origin of a neurone.

Differentiation of delayed rectifier potassium current in embryonic amphibian myocytes

The developmentally regulated expression of prolonged outward potassium currents influences the extent to which sustained inward currents contribute to the action potential at early stages of differentiation. In amphibian spinal neurons, the long duration and calcium dependence of the embryonic action potential and the amount of calcium influx are largely determined by the extent of maturation of the delayed rectifier potassium current (IKv). We have undertaken a parallel study of differentiation of myocytes, in which action potentials are brief and sodium-dependent even at early stages. The early expression of electrical excitability in embryonic amphibian myocytes growing in culture has been examined previously using intracellular voltage recording techniques. The membrane exhibits a delayed rectification in response to depolarization at times earlier than those at which impulses can first be generated. We have examined the differentiation of this outward current in embryonic myocytes developing in vitro, using whole cell voltage clamp. IKv is initially absent. When first recorded it is small and slowly activating but undergoes sixfold increases in both density and rate of activation during the first day in culture. This maturation is dependent upon transcription, and both rate and density are influenced by the presence of other cell types. The large amplitude of the outward delayed rectifier prevents expression of long duration action potentials.

A critical period of transcription required for differentiation of the action potential of spinal neurons

The early development of excitability of amphibian spinal neurons is characterized by a change from a long Ca2(+)-dependent action potential to a brief Na(+)-dependent impulse. The delayed rectifier K+ current plays a major role in this cell autonomous differentiation. Here we show that the maturation of the delayed rectifier current, and hence the action potential, involves a critical period of mRNA synthesis. It is blocked by inhibition of transcription during an early period of development in culture and fails to develop following removal of the inhibitor and resumption of RNA synthesis. However, the development of an inactivating K+ A-current recovers in these neurons, indicating that some programs of neuronal development are affected during this critical period, while others are spared.

Changes in densities and kinetics of delayed rectifier potassium channels during neuronal differentiation

Single-channel K+ currents were recorded from young and mature spinal neurons cultured from Xenopus embryos to examine the bases of the developmental increases in density and in rate of activation of the macroscopic voltage-dependent delayed rectifier K+ current (IKv). K+ channels of three conductance classes (integral of 80, 30, and 15 pS) are present at both ages, but only the intermediate and small conductance classes are voltage-dependent and thus underlie IKv. The increase in the density of IKv is due to increases in the numbers of intermediate and small channels per cell, but not to changes in their open probabilities. The increase in rate of activation of IKv results from a change in the activation kinetics of the intermediate channel class alone.

Selective induction of brain type II Na+ channels by nerve growth factor

Cells derived from a rat pheochromocytoma (PC12 cells) can generate an action potential only upon treatment with nerve growth factor. Using electrophysiological methods, we found that the appearance of action potentials in nerve growth factor-treated PC12 cells can be explained by an increase in the density of Na+ channels. The functional properties of Na+ channels in PC12 cells are similar to those described for peripheral nerves but appear to be different from Na+ channels synthesized in Xenopus oocytes injected with brain type II Na+ -channel mRNA. To determine if PC12 cells express the brain type II Na+ -channel gene, we performed RNase-protection analyses using probes that can distinguish between the brain type I and type II Na+ -channel mRNAs. The results from these studies indicate that undifferentiated PC12 cells express the type II but not the type I Na+ -channel gene. Treatment with nerve growth factor increases expression of the type II Na+ -channel gene but has no effect on type I gene expression. Our findings suggest that Na+ -channel excitability in PC12 cells is due to the specific induction of the brain type II gene by nerve growth factor.

Development of the fast, transient outward K+ current in embryonic sympathetic neurones

Voltage-activated outward potassium (K+) currents in developing sympathetic neurones, dissociated from the rat superior cervical ganglion (SCG), were studied using the whole-cell patch clamp recording technique. In voltage-clamped neonatal SCG cells, two voltage-dependent K+ currents were measured: the fast, transient K+ current, IA; and, the slower activating, non-inactivating delayed rectifier, IK. Only IK, however, appeared to be present in SCG neurones isolated from early embryonic (E14.5-16.5) rat pups; IA was not observed in these cells. When these embryonic neurones were maintained in cell culture, IA developed over a time course (approximately 4-6 days) similar to that seen in vivo. IA, therefore, which appears to facilitate the fast repolarization phase of the action potential in rat SCG neurones, is the last voltage-activated current to develop in these cells.

Differentiation of voltage-gated potassium current and modulation of excitability in cultured amphibian spinal neurones

Gigaohm-seal whole-cell voltage-clamp techniques were used to study the development of ionic currents in the membrane of embryonic amphibian (Ambystoma) spinal neurones during in vitro differentiation. Dissociated neural plate cells, some of which are neuronal precursor cells, were placed into culture. Cells became excitable at the time of neurite outgrowth, 2-3 days later, and over the next 2-10 days the duration of the action potential shortened from about 100 ms to about 1 ms. Voltage-clamp recordings demonstrated that at the time of appearance of neurites, activatable Na, Ca and voltage-gated K channels were present in the membrane (Ca-dependent K channels were not studied). Over succeeding days in culture, records of total membrane current indicated that the amplitudes of peak inward and steady-state outward currents both increased. As a result of these increases, the pattern of total membrane current came to be increasingly dominated by outward currents. With inward Na and Ca currents blocked, a voltage-gated K current (IK(V] could be studied in isolation. The reversal potential of this current varied in good agreement with the equilibrium potential for K ions predicted by the Nernst relation. The wave form of IK(V) activation was sigmoidal. Activation was more rapid at more positive voltages (relative to the usual holding potential of -70 mV), and deactivation was more rapid at more negative voltages. The amplitude of IK(V) increased during neural development, while cell size remained approximately constant. Increases in rates of activation and deactivation were observed in parallel with the increase in current density. When measured at 0 mV, cells studied on day 4 of culture or earlier showed steady-state chord conductances (gK(V] of less than 20 nS, and one-half activation times (t1/2) of 2 X 5-10 ms. Older cells showed gK(V)s of 10-80 nS, and t1/2s of 0 X 8-2 X 5 ms. As Na, and to a lesser extent Ca, current amplitudes were also increasing during differentiation, these observations concerning IK(V) suggested that its amplitude and kinetic changes might in part be responsible for the observed decrease in action potential duration during development. This hypothesis was tested by modelling Na, Ca and voltage-gated K currents and testing the effects of changes in amplitude and kinetics of IK(V) on the duration and ionic dependence of reconstructed action potentials. The results obtained using this model suggested that the increase in IK(V) amplitude and activation rate was sufficient to change action potential duration and apparent ionic sensitivity.

Autonomous early differentiation of neurons and muscle cells in single cell cultures

The extent to which early differentiation of neurons and muscle cells is autonomous or governed by soluble factors released from other cells has been examined by following development of single cells plated alone in a simple, defined culture medium. The differentiation of electrical excitability and sensitivity to neurotransmitters of amphibian spinal neurons and trunk muscle in Xenopus embryos has already been described. For both cell types, differentiation in cultures containing relatively large numbers of dispersed cells parallels development in vivo, with respect to qualitative changes in membrane properties and the time course of development. Cell contacts are not required for this process. Here we show that the differentiation of membrane properties of single, isolated cells exhibits a similar set of changes, although muscle cells develop more slowly in some respects and all cells survive for a shorter period of time. The results suggest that the continued presence of specific extracellular differentiation-promoting factors is not required for these early steps of neuronal development, although a role for such factors in development of myocytes cannot be excluded. In contrast, survival factors secreted by other cells may be necessary to prolong the lifetimes of dissociated cells.

Development of a high-affinity GABA uptake system in embryonic amphibian spinal neurons

High-affinity uptake systems for amino acid neurotransmitter precursors have been highly correlated with the use of the particular amino acid or its derivative as a transmitter. We have found interneurons in the Xenopus embryo spinal cord which accumulate GABA by a high-affinity uptake system. They originate near the end of gastrulation and their ability to accumulate GABA first appears at the early tail bud stage. By position and appearance they are comparable to some of the embryonic interneurons described by A. Roberts and J. D. W. Clarke (1982, Phil. Trans. R. Soc. London Ser. B 296, 195-212). GABA-accumulating neurons also develop in dissociated cell cultures made from the presumptive spinal cord of neural plate stage Xenopus embryos. GABA accumulation in cultured neurons, as in cells in vivo, occurs via a high-affinity uptake system; GABA-accumulating cells have the same time of origin as the cells in vivo, and the ability to accumulate GABA in the population of cultured neurons appears at a time equivalent to that observed in intact sibling embryos. Thus it seems likely that the population of GABA-accumulating neurons developing in cell culture corresponds to the GABA-accumulating interneurons in vivo. The development of these neurons in dissociated cell cultures permits perturbation experiments that would be difficult to perform in vivo. We have examined the development of high-affinity GABA uptake in conditions that permit no electrical impulse activity in the cultures. The onset and extent of development of GABA accumulation in the neuronal population are normal under these conditions.

Developmental acquisition of Ca2+-sensitivity by K+ channels in spinal neurones

A developmental change in the ionic basis of the inward current of action potentials has been observed in many excitable cells. In cultured spinal neurones of Xenopus, the timing of the development of the action parallels that seen in vivo. In vitro, as in vivo, neurones initially produce action potentials of long duration which are principally Ca-dependent; after 1 day of development the impulse is brief and primarily Na-dependent. At both ages, however, both inward components are present and the mechanism underlying shortening of the action potential is unknown. One possibility is that the outward currents change during development. Using the patch-clamp technique, we have recorded single K+-channel currents in membrane patches isolated from the cell bodies of cultured embryonic neurones. The unitary conductance of one class of K+ channels was approximately 155 pS and depolarization increased the probability of a channel being open. Neither conductance nor voltage dependence seemed to change with time in culture; in contrast, the Ca2+-sensitivity of this K+ channel increased. In younger neurones, Ca2+-sensitivity was greatly reduced or absent, whereas in more mature neurones, the activity of this channel was Ca-dependent. Such a change could account for the shortening of the action potential duration by increasing the relative contribution of outward currents.

Early differentiation of vertebrate spinal neurons in the absence of voltage-dependent Ca2+ and Na+ influx

The development of the action potential and responses to neurotransmitters have been described for a population of embryonic spinal neurons developing in vivo. A comparable pattern is seen for spinal neurons developing in dissociated cell culture. The impulse appears very early in this developmental sequence, and the action potential involves a large inward Ca2+ current. Since Ca2+ is a ubiquitous intracellular regulator, we questioned whether a large influx of Ca2+ is necessary for the subsequent differentiation of membrane properties. Embryonic Xenopus neurons grown in normal culture medium do not make Ca2+- or Na+-dependent action potentials in their cell bodies in a Ca2+-free saline containing tetrodotoxin (TTX). To achieve a chronic blockade of impulse activity, neurons were grown in a medium in which Ca2+ was replaced by Mg2+, and to which 1 mM EGTA was added. In some instances TTX was present. Neurons grown in these experimental culture media extend neurites more rapidly than controls. Action potentials cannot be elicited from neurons when examined in experimental medium. However, examination in saline reveals that the change in the ionic dependence of the impulse is indistinguishable from that observed in neurons grown in normal medium. Furthermore, the time of onset of responses to GABA is unaffected by this experimental treatment. Thus the expression of Ca2+- and Na+-dependent action potentials seems not to play a part in the early differentiation of these membrane properties. However, the later development of GABA sensitivity is reduced.