Antisense suppression of potassium channel expression demonstrates its role in maturation of the action potential

A developmental increase in delayed rectifier potassium current (I(Kv)) in embryonic Xenopus spinal neurons is critical for the maturation of excitability and action potential waveform. Identifying potassium channel genes that generate I(Kv) is essential to understanding the mechanisms by which they are controlled. Several Kv genes are upregulated during embryogenesis in parallel with increases in I(Kv) and produce delayed rectifier current when heterologously expressed, indicating that they could encode channels underlying this current. We used antisense (AS) cRNA to test the contribution of xKv3.1 to the maturation of I(Kv), because xKv3.1 AS appears to suppress specifically heterologous expression of potassium current by xKv3.1 mRNA. The injection of xKv3.1 AS into embryos reduces endogenous levels of xKv3.1 mRNA in the developing spinal cord and reduces the amplitude and rate of activation of I(Kv) in 40% of cultured neurons, similar to the percentage of neurons in which endogenous xKv3.1 transcripts are detected. The current in these mature neurons resembles that at an earlier stage of differentiation before the appearance of xKv3.1 mRNA. Furthermore, AS expression increases the duration of the action potential in 40% of the neurons. No change in voltage-dependent calcium current is observed, suggesting that the decrease in I(Kv) is sufficient to account for lengthening of the action potential. Computer-simulated action potentials incorporating observed reductions in amplitude and rate of activation of I(Kv) exhibit an increase in duration similar to that observed experimentally. Thus xKv3.1 contributes to the maturation of I(Kv) in a substantial percentage of these developing spinal neurons.

Sustained upregulation in embryonic spinal neurons of a Kv3.1 potassium channel gene encoding a delayed rectifier current

Differentiation of electrical excitability entails changes in the currents that generate action potentials in spinal neurons of Xenopus embryos, resulting in reduced calcium entry during impulses generated at later stages of development. A dramatic increase in delayed rectifier current (I(Kv)) during the first day of development plays the major role in this process. Identification of potassium channel genes responsible for the increase in I(Kv) is critical to understanding the molecular mechanisms involved. Several members of the Shaw Kv3 gene subfamily encode delayed rectifier currents, indicating that they could contribute to the upregulation of I(Kv) that reduces the duration of action potentials. We isolated a Xenopus (x) Kv3.1 gene whose expression is restricted to the central nervous system, which is upregulated throughout the period during which I(Kv) develops in vivo. The fraction of neurons in which transcripts of this gene are detected by single-cell RT-PCR increases to 40% with time in culture, paralleling the development of I(Kv) in neurons in vitro. Expression of xKv3.1 mRNA generates a delayed rectifier potassium current in oocytes, suggesting that xKv3. 1 contributes to the maturation of I(Kv) and shortening of the action potential.

Regulation of calcineurin by growth cone calcium waves controls neurite extension

Growth cones generate spontaneous transient elevations of intracellular Ca(2+) that regulate the rate of neurite outgrowth. Here we report that these Ca(2+) waves inhibit neurite extension via the Ca(2+)-dependent phosphatase calcineurin (CN) in Xenopus spinal neurons. Pharmacological blockers of CN (cyclosporin A and deltamethrin) and peptide inhibitors of CN [the Xenopus CN (xCN) autoinhibitory domain and African swine fever virus protein A238L] block the Ca(2+)-dependent reduction of neurite outgrowth in cultured neurons. Time-lapse microscopy of growing neurites demonstrates directly that the reduction in the rate of outgrowth by Ca(2+) transients is blocked by cyclosporin A. In contrast, expression of a constitutively active form of xCN in the absence of waves results in shorter neurite lengths similar to those seen in the presence of waves. The developmental expression pattern of xCN transcripts in vivo coincides temporally with axonal pathfinding by spinal neurons, supporting a role of CN in regulating Ca(2+)-dependent neurite extension in the spinal cord. Ca(2+) wave frequency and Ca(2+)-dependent expression of GABA are not affected by inhibition or activation of CN. However, phosphorylation of the cytoskeletal element GAP-43, which promotes actin polymerization, is reduced by Ca(2+) waves and enhanced by suppression of CN activity. CN ultimately acts on the growth cone actin cytoskeleton, because disrupting actin microfilaments with cytochalasin D or stabilizing them with jasplakinolide negates the effects of suppressing or activating CN. Destabilization or stabilization of microtubules with colcemide or taxol results in Ca(2+)-independent inhibition of neurite outgrowth. The results identify components of the cascade by which Ca(2+) waves act to regulate neurite extension.

Ca(2+)-permeable AMPA receptors and spontaneous presynaptic transmitter release at developing excitatory spinal synapses

At many mature vertebrate glutamatergic synapses, excitatory transmission strength and plasticity are regulated by AMPA and NMDA receptor (AMPA-R and NMDA-R) activation and by patterns of presynaptic transmitter release. Both receptors potentially direct neuronal differentiation by mediating postsynaptic Ca(2+) influx during early development. However, the development of synaptic receptor expression and colocalization has been examined developmentally in only a few systems, and changes in release properties at neuronal synapses have not been characterized extensively. We recorded miniature EPSCs (mEPSCs) from spinal interneurons in Xenopus embryos and larvae. In mature 5-8 d larvae, approximately 70% of mEPSCs in Mg(2+)-free saline are composed of both a fast AMPA-R-mediated component and a slower NMDA-R-mediated decay, indicating receptor colocalization at most synapses. By contrast, in 39-40 hr embryos approximately 65% of mEPSCs are exclusively fast, suggesting that these synapses initially express predominantly AMPA-R. In a physiological Mg(2+) concentration (1 mM), mEPSCs throughout development are mainly AMPA-R-mediated at negative potentials. Embryonic synaptic AMPA-R are highly Ca(2+)-permeable, mEPSC amplitude is over twofold larger than at mature synapses, and mEPSCs frequently occur in bursts consistent with asynchronous multiquantal release. AMPA-R function in this motor pathway thus appears to be independent of previous NMDA-R activation, unlike other regions of the developing nervous system, ensuring a greater reliability for embryonic excitatory transmission. Early spontaneous excitatory activity is specialized to promote AMPA-R-mediated synaptic Ca(2+) influx, which likely has significant roles in neuronal development.

Calcium signaling in the developing Xenopus myotome

Embryonic Xenopus myocytes generate spontaneous calcium (Ca(2+)) transients during differentiation in culture. Suppression of these transients disrupts myofibril organization and the formation of sarcomeres through an identified signal transduction cascade. Since transients often occur during myocyte polarization and migration in culture, we hypothesized they might play additional roles in vivo during tissue formation. We have tested this hypothesis by examining Ca(2+) dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca(2+) transients occur in cells of the developing myotome with characteristics remarkably similar to those in cultured myocytes. Transients produced within the myotome are correlated with somitogenesis as well as myocyte maturation. Since transients arise from intracellular stores in cultured myocytes, we examined the functional distribution of both IP(3) and ryanodine receptors in the intact myotome by eliciting Ca(2+) elevations in response to photorelease of caged IP(3) and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca(2+) release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.

In vivo regulation of axon extension and pathfinding by growth-cone calcium transients

Growth cones at the tips of extending neurites migrate through complex environments in the developing nervous system and guide axons to appropriate target regions using local cues. The intracellular calcium concentration ([Ca2+]i) of growth cones correlates with motility in vitro, but the physiological links between environmental cues and axon growth in vivo are unknown. Here we report that growth cones generate transient elevations of [Ca2+]i as they migrate within the embryonic spinal cord and that the rate of axon outgrowth is inversely proportional to the frequency of transients. Suppressing Ca2+ transients by photorelease of a Ca2+ chelator accelerates axon extension, whereas mimicking transients with photorelease of Ca2+ slows otherwise rapid axonal growth. The frequency of Ca2+ transients is cell-type specific and depends on the position of growth cones along their pathway. Furthermore, growth-cone stalling and axon retraction, which are two important aspects of pathfinding, are associated with high frequencies of Ca2+ transients. Our results indicate that environmentally regulated growth-cone Ca2+ transients control axon growth in the developing spinal cord.

Properties of ectopic neurons induced by Xenopus neurogenin1 misexpression

We have examined cells cultured from ectoderm-misexpressing Neurogenin1 (Ngn1) to describe better the extent to which this gene can control aspects of neuronal phenotype including motility, morphology, excitability, and synaptic properties. Like primary spinal neurons which normally express Ngn1, cells in Ngn1-misexpressing cultures exhibit a motility-correlated behavior called circus movements prior to neuritogenesis. Misexpression of NeuroD also causes circus movements and later neuronal differentiation. GSK3beta, which inhibits NeuroD function in vivo, blocks both Ngn1-induced and NeuroD-induced neuronal differentiation, while Notch signaling inhibits only Ngn1-induced neuronal differentiation, confirming that NeuroD is downstream of Ngn1 and insensitive to Notch inhibition. While interfering with NeuroD function in ventral ectoderm inhibits both circus movements and neuronal differentiation, such inhibition in the neural plate inhibits only neuronal differentiation, suggesting that additional factors regulate circus movements in the neural ectoderm. Ngn1-misexpressing cells extend N-tubulin-positive neurites and exhibit tetrodotoxin-sensitive action potentials. Unlike the majority of cultured spinal neurons, however, Ngn1-misexpressing cells do not respond to glutamate and do not form functional synapses with myocytes, suggesting that these cells are either like Rohon-Beard sensory neurons or are not fully differentiated.

Development of electrical excitability in embryonic neurons: mechanisms and roles

Xenopus spinal neurons serve as a nearly ideal population of excitable cells for study of developmental regulation of electrical excitability. On the one hand, the firing properties of these neurons can be directly examined at early stages of differentiation and membrane excitability changes as neurons mature. Underlying changes in voltage-dependent ion channels have been characterized and the mechanisms that bring about these changes are being defined. On the other hand, these neurons have been shown to be spontaneously active at stages when action potentials provide significant calcium entry. Calcium entry provokes further elevation of intracellular calcium via release from intracellular stores. The resultant transient elevations of intracellular calcium encode differentiation in their frequency. Recent studies have shown that different neuronal subpopulations enlist distinct mechanisms for regulation of excitability and recruit specific programs of differentiation by particular patterns of activity.

A calcium signaling cascade essential for myosin thick filament assembly in Xenopus myocytes

Spontaneous calcium release from intracellular stores occurs during myofibrillogenesis, the process of sarcomeric protein assembly in striated muscle. Preventing these Ca2+ transients disrupts sarcomere formation, but the signal transduction cascade has not been identified. Here we report that specific blockade of Ca2+ release from the ryanodine receptor (RyR) activated Ca2+ store blocks transients and disrupts myosin thick filament (A band) assembly. Inhibition of an embryonic Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) by blocking the ATP-binding site, by allosteric phosphorylation, or by intracellular delivery of a pseudosubstrate peptide, also disrupts sarcomeric organization. The results indicate that both RyRs and MLCK, which have well-described calcium signaling roles in mature muscle contraction, have essential developmental roles during construction of the contractile apparatus.

AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons

N-methyl--aspartate (NMDA) receptors are often the first ionotropic glutamate receptors expressed at early stages of development and appear to influence neuronal differentiation by mediating Ca2+ influx. Although less well studied, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors also can generate Ca2+ elevations and may have developmental roles. We document the presence of AMPA and NMDA class receptors and the absence of kainate class receptors with whole cell voltage-clamp recordings from Xenopus embryonic spinal neurons differentiated in vitro. Reversal potential measurements indicate that AMPA receptors are permeable to Ca2+ both in differentiated neurons and at the time they first are expressed. The PCa/Pmonocation of 1.9 is close to that of cloned Ca2+-permeable AMPA receptors expressed in heterologous systems. Ca2+ imaging reveals that Ca2+ elevations are elicited by AMPA or NMDA in the absence of Mg2+. The amplitudes and durations of these agonist-induced Ca2+ elevations are similar to those of spontaneous Ca2+ transients known to act as differentiation signals in these cells. Two sources of Ca2+ amplify AMPA- and NMDA-induced Ca2+ elevations. Activation of voltage-gated Ca2+ channels by AMPA- or NMDA-mediated depolarization contributes approximately 15 or 30% of cytosolic Ca2+ elevations, respectively. Activation of either class of receptor produces elevations of Ca2+ that elicit further release of Ca2+ from thapsigargin-sensitive but ryanodine-insensitive stores, contributing an additional approximately 30% of Ca2+ elevations. Voltage-clamp recordings and Ca2+ imaging both show that these spinal neurons express functional AMPA receptors soon after neurite initiation and before expression of NMDA receptors. The Ca2+ permeability of AMPA receptors, their ability to generate significant elevations of [Ca2+]i, and their appearance before synapse formation position them to play roles in neural development. Spontaneous release of agonists from growth cones is detected with glutamate receptors in outside-out patches, suggesting that spinal neurons are early, nonsynaptic sources of glutamate that can influence neuronal differentiation in vivo.

Purposeful patterns of spontaneous calcium transients in embryonic spinal neurons

Embryonic cultured Xenopus spinal neurons generate two types of spontaneous elevation of intracellular calcium that encode developmental information in the frequency with which they are produced. Calcium spikes regulate the appearance of GABA and maturation of potassium current. Calcium waves in growth cones regulate neurite extension. Spikes and waves are also observed in neurons differentiating in situ. Because differentiation is dependent on the frequency of calcium transients, neurons that are coactive and fire spikes in concert would be expected to differentiate together. Consistent with this prediction, segmentally arrayed clusters of putative motoneurons on the ventral aspect of the neural tube fire together during development.

Breaking the code: regulation of neuronal differentiation by spontaneous calcium transients

Calcium ions play critical roles in neuronal development. Stimulation of transient elevations of intracellular calcium (Ca2+i) activates protein kinases, regulates transcription and influences motility and morphology. Embryonic Xenopus spinal neurons exhibit a Ca(2+)-sensitive period in culture; removing extracellular Ca2+ during this period affects several aspects of neuronal differentiation. However, both the mechanisms that generate natural fluctuations in Ca2+i and the signals they transduce are not well understood. Spontaneous, transient and repeated elevations of Ca2+i in embryonic Xenopus spinal neurons have been observed over periods up to 5 h in vitro and in vivo, confocally imaging fluo 3-loaded cells. Developing neurons generate two distinctive types of spontaneous Ca2+i transients, calcium spikes and calcium waves. We have investigated the mechanisms by which they are generated and their roles in directing neuronal differentiation. Spikes are generated by spontaneous action potentials, and thus are rapidly propagated throughout entire neurons. Ca2+ entry triggers Ca2+ release from intracellular stores, and spikes have a characteristic double exponential decay. In contrast, the generation of waves does not involve conventional voltage-dependent Ca2+ channels, but an unknown Ca2+ entry pathway that can be blocked by Ni2+ at a higher concentration than required to block classical voltage-dependent Ca2+ channels. Waves rise and decay slowly, and unlike spikes, are local events. However both spikes and waves are abolished by removal of extracellular Ca2+. Developmentally, the incidence and frequency of spikes decrease while the incidence and frequency of waves are constant. To study the roles of Ca2+ transients, we have imaged Ca2+ in spinal neurons throughout an extended period of early development, and find that spikes and waves are expressed at distinct frequencies. Neuronal differentiation is altered when they are eliminated by preventing Ca2+ influx. By reimposing different frequency patterns of Ca2+ transients, we demonstrate that natural spike activity is sufficient to promote normal neurotransmitter expression and channel maturation, while wave activity at growth cones is sufficient to regulate neurite extension. On the other hand, suppression of spontaneous Ca2+ elevations with BAPTA, a rapid Ca2+ chelator, indicates that they are also necessary to direct differentiation. Ca2+ transients appear to encode information in their frequency. Thus, they act like action potentials, although they are 10(4) times longer in duration and less frequent and implement an intrinsic development program.

Mitochondrial dysfunction is a primary event in glutamate neurotoxicity

Excitotoxic neuronal death, associated with neurodegenerative disorders and hypoxic insults, results from excessive exposure to excitatory neurotransmitters. Glutamate neurotoxicity is triggered primarily by massive Ca2+ influx arising from overstimulation of the NMDA subtype of glutamate receptors. The underlying mechanisms, however, remain elusive. We have tested the hypothesis that mitochondria are primary targets in excitotoxicity by confocal imaging of intracellular Ca2+ ([Ca2+]i) and mitochondrial membrane potential (delta psi) on cultured rat hippocampal neurons. Sustained activation of NMDA receptors (20 min) elicits reversible elevation of [Ca2+]i. Longer activation (50 min) renders elevation of [Ca2+]i irreversible (Ca2+ overload). Susceptibility to NMDA-induced Ca2+ overload is increased when the 20 min stimuli are applied to neurons pretreated with electron transport chain inhibitors, thereby implicating mitochondria in [Ca2+]i homeostasis during excitotoxic challenges. Remarkably, delta psi exhibits prominent and persistent depolarization in response to NMDA, which closely parallels the incidence of neuronal death. Blockade of the mitochondrial permeability transition pore by cyclosporin A allows complete recovery of delta psi and prevents cell death. These results suggest that early mitochondrial damage plays a key role in induction of glutamate neurotoxicity.

Spontaneous calcium transients regulate myofibrillogenesis in embryonic Xenopus myocytes

Spontaneous transient elevations of intracellular calcium (Ca2+i) have functional roles in the development of Xenopus spinal neurons. However, less is known about the roles of elevations of Ca2+i in the differentiation of other cell types. We have examined Xenopus myocytes as a first step in determining if Ca2+i transients are a more general feature of differentiation in excitable cells. We find that cultured myocytes, like neurons, exhibit spontaneous Ca2+i transients during an early developmental period. These transients average 1.4 min in duration and occur at an average frequency of 6/hr in cultures containing myocytes and neurons. Culture conditions can influence transient production; for example, myocyte-enriched cultures have a lower incidence of transient-producing cells. Transients persist in 0-Ca2+ medium, indicating that they arise from intracellular stores. Caffeine-sensitive Ca2+ stores are present in these cells, and depletion or block of these stores eliminates transient production. To determine if transients play a functional role during development, we blocked their production with intracellular BAPTA, a rapid Ca2+ chelator. Cellular differentiation is significantly inhibited only when BAPTA is applied early in development, during the period of transient production, while later BAPTA treatments have no effect. Blocking transient production severely perturbed myofibril organization and sarcomere assembly. However, other aspects of myocyte differentiation were not affected by transient blockade, indicating that not all myogenic differentiation programs are regulated in this manner. Our results suggest that spontaneous Cai2+ transients play a role in cytoskeletal organization during myofibrillogenesis.

Temporal regulation of Shaker- and Shab-like potassium channel gene expression in single embryonic spinal neurons during K+ current development

A developmental increase in density of delayed rectifier potassium current (IKv) in embryonic Xenopus spinal neurons shortens action potential durations and limits calcium influx governing neuronal differentiation. Although previous work demonstrates that maturation of IKv depends on general mRNA synthesis, it is not known whether increases in K+ channel gene transcripts direct maturation of the current. Accordingly, the developmental appearance of specific Kv potassium channel genes was determined using single-cell reverse transcription-PCR techniques after whole-cell recording of IKv during the period of its development. Detection of a coexpressed housekeeping gene along with the potassium channel gene controlled for successful aspiration of cellular mRNA and allowed scoring of cells in which Kv gene transcripts were not detected. Diverse types of Xenopus spinal neurons exhibit homogeneous development of IKv both in vivo and in culture. In contrast, transcripts of two genes encoding delayed rectifier current, Kv1.1 (Shaker) and Kv2.2 (Shab), are expressed heterogeneously during the period in which the current develops. Kv1.1 mRNA achieves maximal appearance in approximately 30% of cells, while IKv is immature; Kv2.2 mRNA appears later in approximately 60% of mature neurons. Kv1.1 and 2.2 are thus candidates for generation of IKv, and spinal neurons are a heterogeneous population with respect to potassium channel gene expression. Moreover, correlation of gene expression with current properties shows that neurons lacking Kv2.2 have a characteristic voltage dependence of activation of IKv.

Regulation of intracellular Cl- levels by Na(+)-dependent Cl- cotransport distinguishes depolarizing from hyperpolarizing GABAA receptor-mediated responses in spinal neurons

Rohon-Beard (RB) spinal neurons of Xenopus larvae are depolarized by GABA. To study the mechanisms underlying this distinctive response, intracellular and patch-clamp recordings were made from RB neurons in situ. The intracellularly recorded GABA reversal potential (EREV) was near -30 mV in normal saline and was approximately 25 mV more negative in Na(+)-free saline. Whole-cell recordings from RB neurons and from neighboring dorsolateral interneurons (DLi) revealed that GABA responses of both cells were mediated by GABAA receptors. Currents elicited by GABA were mimicked by muscimol and reversibly blocked by bicuculline, and EREV shifted with changes in Cl- concentration ([Cl]) in agreement with Cl- selectivity. In perforated patch recordings, EREV for RB cells was significantly more positive than for DLi cells (-38 vs -63 mV), indicating that intact RB cells maintain higher levels of intracellular Cl-. Replacement of external Na+ or exposure to the Cl- transport inhibitor bumetanide (100 microM) shifted RB cell EREV to move negative values, consistent with Na+(-)dependent Cl cotransport contributing to higher internal [Cl]. In contrast, these treatments did not change DLi cell EREV. The results indicate that a Na+(-)dependent Cl- transport mechanism underlies GABAA receptor-mediated depolarizing Cl- conductances in RB neurons. Thus, both inhibitory and excitatory GABA responses appear to be present during the same developmental period in vivo. GABA may stimulate Ca2+ influx in RB neurons because the intracellular GABA EREV is above the threshold for low voltage-activated Ca2+ channels.