An increase in intracellular pH during neural induction in Xenopus

Transmembrane H+ fluxes and the regulation of neural induction in Xenopus laevis

It has previously been reported that in ex vivo planar explants prepared from Xenopus laevis embryos, the intracellular pH (pHi) increases in cells of the dorsal ectoderm from stage 10.5 to 11.5 (i.e. 11-12.5 hpf). It was proposed that such increases (potentially due to H+ being extruded, sequestered, or buffered in some manner), play a role in regulating neural induction. Here, we used an extracellular ion-selective electrode to non-invasively measure H+ fluxes at eight locations around the equatorial circumference of intact X. laevis embryos between stages 9-12 (˜7-13.25 hpf). We showed that at stages 9-11, there was a small H+ efflux recorded from all the measuring positions. At stage 12 there was a small, but significant, increase in the efflux of H+ from most locations, but the efflux from the dorsal side of the embryo was significantly greater than from the other positions. Embryos were also treated from stages 9-12 with bafilomycin A1, to block the activity of the ATP-driven H+ pump. By stage 22 (24 hpf), these embryos displayed retarded development, arresting before the end of gastrulation and therefore did not display the usual anterior and neural structures, which were observed in the solvent-control embryos. In addition, expression of the early neural gene, Zic3, was absent in treated embryos compared with the solvent controls. Together, our new in vivo data corroborated and extended the earlier explant-derived report describing changes in pHi that were suggested to play a role during neural induction in X. laevis embryos.

V-ATPase-dependent ectodermal voltage and pH regionalization are required for craniofacial morphogenesis

Using voltage and pH reporter dyes, we have discovered a never-before-seen regionalization of the Xenopus ectoderm, with cell subpopulations delimited by different membrane voltage and pH. We distinguished three courses of bioelectrical activity. Course I is a wave of hyperpolarization that travels across the gastrula. Course II comprises the appearance of patterns that match shape changes and gene expression domains of the developing face; hyperpolarization marks folding epithelium and both hyperpolarized and depolarized regions overlap domains of head patterning genes. In Course III, localized regions of hyperpolarization form at various positions, expand, and disappear. Inhibiting H(+) -transport by the H(+) -V-ATPase causes abnormalities in: (1) the morphology of craniofacial structures; (2) Course II voltage patterns; and (3) patterns of sox9, pax8, slug, mitf, xfz3, otx2, and pax6. We conclude that this bioelectric signal has a role in development of the face. Thus, it exemplifies an important, under-studied mechanism of developmental regulation.

SDF-1 alpha regulates mesendodermal cell migration during frog gastrulation

During frog gastrulation, mesendodermal cells become apposed to the blastocoel roof (BCR) by endoderm rotation, and migrate towards the animal pole. The leading edge of the mesendodermal cells (LEM) contributes to the directional migration of involuting marginal zone (IMZ) cells, but the molecular mechanism of this process is not well understood. Here we show that CXCR4/SDF-1 signaling mediates the directional movement of the LEM in Xenopus embryos. Expression of xCXCR4 was detected in the IMZ, and was complemented by xSDF-1alpha expression in the inner surface of the BCR. Over-expression of xCXCR4 and xSDF-1alpha caused gastrulation defects. An xCXCR4 N-terminus deletion construct and xSDF-1alpha-MO also inhibited gastrulation. Furthermore, explants of LEM migrate towards the dorsal BCR in the presence of xSDF-1alpha, and altered xCXCR4 expression in the LEM inhibited LEM migration. These results suggest that CXCR4/SDF-1 signaling is necessary for the migrations of massive numbers of cells during gastrulation.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

FGF Signal Interpretation Is Directed by Sprouty and Spred Proteins during Mesoderm Formation

Vertebrate gastrulation requires coordination of mesoderm specification with morphogenetic movements. While both of these processes require FGF signaling, it is not known how mesoderm specification and cell movements are coordinated during gastrulation. The related Sprouty and Spred protein families are recently discovered regulators of receptor tyrosine kinase signaling. We identified two genes for each family in Xenopus tropicalis: Xtsprouty1, Xtsprouty2, Xtspred1, and Xtspred2. In gain- and loss-of-function experiments we show that XtSprouty and XtSpred proteins modulate different signaling pathways downstream of the FGF receptor (FGFR), and consequently different developmental processes. Notably, XtSproutys inhibit morphogenesis and Ca(2+) and PKCdelta signaling, leaving MAPK activation and mesoderm specification intact. In contrast, XtSpreds inhibit MAPK activation and mesoderm specification, with little effect on Ca(2+) or PKCdelta signaling. These differences, combined with the timing of their developmental expression, suggest a mechanism to switch FGFR signal interpretation to coordinate mesoderm formation and cell movements during gastrulation.

Bioelectromagnetics in morphogenesis

Understanding the factors that allow biological systems to reliably self-assemble consistent, highly complex, four dimensional patterns on many scales is crucial for the biomedicine of cancer, regeneration, and birth defects. The role of chemical signaling factors in controlling embryonic morphogenesis has been a central focus in modern developmental biology. While the role of tensile forces is also beginning to be appreciated, another major aspect of physics remains largely neglected by molecular embryology: electromagnetic fields and radiations. The continued progress of molecular approaches to understanding biological form and function in the post genome era now requires the merging of genetics with functional understanding of biophysics and physiology in vivo. The literature contains much data hinting at an important role for bioelectromagnetic phenomena as a mediator of morphogenetic information in many contexts relevant to embryonic development. This review attempts to highlight briefly some of the most promising (and often underappreciated) findings that are of high relevance for understanding the biophysical factors mediating morphogenetic signals in biological systems. These data originate from contexts including embryonic development, neoplasm, and regeneration.

Developmental changes in the management of acid loads during preimplantation mouse development

Intracellular pH recovery in Quackenbush Swiss mouse preimplantation embryos following acid loading was investigated under conditions of H+-monocarboxylate cotransporter inactivity. Isoform-sensitive inhibitors of Na+-H+ exchange (NHE) were used to block the Na+-dependent component of the response. A biphasic dose-response curve for HOE-694 and N-methylisopropylamiloride (MIA) suggested that two isoforms (putatively NHE1 and NHE3) are active in the oocyte, 1-cell, and 2-cell stages. By the blastocyst stage, loss of one of the MIA-sensitive NHE activities (putatively NHE3) was observed in isolated inner cell masses, and an MIA-resistant component of the recovery was identified. The MIA-resistant component was inhibited by 2 mM amiloride and enhanced by external K+ and by 4,4'-diisothiocyanostilbene-2,2'-disulfonate, suggesting NHE4 activity. However, unlike NHE4 in other tissues, the MIA-resistant component did not transport Li+ in exchange for H+, and reverse transcription-polymerase chain reaction detected NHE4 mRNA in the oocyte but not in later stages. Trophoblast, whether in intact or collapsed blastocysts, did not show measurable NHE activity or MIA-sensitive activity during recovery from acid load. Both trophoblast and pluriblast manifested an H+ conductance in response to acid load. This H+ conductance was first detected at the 8-cell stage and was blocked by zinc in the isolated inner cell mass but not in trophoblast. No other effective inhibitors of its activity were found.

The planar cell polarity gene strabismus regulates convergence and extension and neural fold closure in Xenopus

We cloned Xenopus Strabismus (Xstbm), a homologue of the Drosophila planar cell or tissue polarity gene. Xstbm encodes four transmembrane domains in its N-terminal half and a PDZ-binding motif in its C-terminal region, a structure similar to Drosophila and mouse homologues. Xstbm is expressed strongly in the deep cells of the anterior neural plate and at lower levels in the posterior notochordal and neural regions during convergent extension. Overexpression of Xstbm inhibits convergent extension of mesodermal and neural tissues, as well as neural tube closure, without direct effects on tissue differentiation. Expression of Xstbm(DeltaPDZ-B), which lacks the PDZ-binding region of Xstbm, inhibits convergent extension when expressed alone but rescues the effect of overexpressing Xstbm, suggesting that Xstbm(DeltaPDZ-B) acts as a dominant negative and that both increase and decrease of Xstbm function from an optimum retards convergence and extension. Recordings show that cells expressing Xstbm or Xstbm(DeltaPDZ-B) fail to acquire the polarized protrusive activity underlying normal cell intercalation during convergent extension of both mesodermal and neural and that this effect is population size-dependent. These results further characterize the role of Xstbm in regulating the cell polarity driving convergence and extension in Xenopus.

Cells remain competent to respond to mesoderm-inducing signals present during gastrulation in Xenopus laevis

During gastrulation, the vertebrate embryo is patterned and shaped by complex signaling pathways and morphogenetic movements. One of the first regions defined during gastrulation is the prospective notochord, which exhibits specific cell behaviors that drive the extension of the embryonic axis. To examine the signals involved in notochord formation in Xenopus laevis and the competence of cells to respond to these signals, we performed cell transplantation experiments during gastrulation. Labeled cells from the prospective notochord, somitic mesoderm, ventrolateral mesoderm, neural ectoderm, and epidermis, between stages 9 (pregastrulation) and 12 (late gastrulation), were grafted into the prospective notochord region of the early gastrula. We show that cells from each region are competent to respond to notochord-inducing signals and differentiate into notochordal tissue. Cells from the prospective neural ectoderm are the most responsive to notochord-inducing signals, whereas cells from the ventrolateral and epidermal regions are the least responsive. We show that at the end of gastrulation, while transplanted cells lose their competence to form notochord, they remain competent to form somites. These results demonstrate that at the end of gastrulation cell fates are not restricted within germ layers. To determine whether notochord-inducing signals are present throughout gastrulation, grafts were made into progressively older host embryos. We found that regardless of the age of the host, grafted cells from each region give rise to notochordal tissue. This indicates that notochord-inducing signals are present throughout gastrulation and that these signals overlap with somite-inducing signals at the end of gastrulation. We conclude that it is the change of competence that restricts cells to specific tissues rather than the regulation of the inducing signals.

Ontogeny of cation-Cl- cotransporter expression in rat neocortex

Neuronal precursors and immature cortical neurons actively accumulate Cl- and as a consequence depolarize in response to GABAA receptor activation. With maturity, intracellular Cl- decreases resulting in a shift towards GABAA inhibition. These observations suggest that changes in expression of cation-Cl- cotransporters may have a significant role in the ontogeny of neuronal Cl- homeostasis. Using ribonuclease protection analysis and in situ hybridization we examined the developmental expression of all presently known members of the cation-Cl- cotransporter gene family in rat brain. Of the inwardly directed cotransporters, NKCC-1, NKCC-2, and NCC-1, only NKCC-1 was detected at significant levels in brain. NKCC-1 was expressed in neurons, appearing first in cortical plate but not in ventricular or subventricular zone. Expression levels peaked by the third postnatal week and were maintained into adulthood. The outwardly directed cotransporters, KCC-1 and KCC-2, demonstrated significantly different levels and time courses of expression. KCC-1 was expressed prenatally at very low levels which increased little over the course of development. In contrast, KCC-2 expression appeared perinatally and increased dramatically after the first week of postnatal life. Differential changes in expression of this gene family occurred during periods of critical shifts in chloride homeostasis and GABA response suggestive of a role in these processes. Furthermore the absence of expression of known inwardly directed cotransporters in Cl- accumulating neuroepithelia and lack of evidence for glial expression suggests that as yet unidentified members of this gene family may be involved in chloride homeostasis in immature neuronal precursors and neuroglia.

Developmental expression of C1C-2 in the rat nervous system

Regulation of expression of the voltage-gated chloride channel, C1C-2, was investigated during development and adult life in rat brain. RNase protection assays demonstrated a marked increase in levels of expression of C1C-2 in brain during early postnatal development which was also detected in adult brain. In situ hybridization of E15 and E18 rat brains demonstrated C1C-2 expression in deep brain nuclei and scattered cells within the neuroepithelial layers, but not in the regions of subventricular zone that primarily give rise to glial populations. By E18 all neurons within the emerging cortical plate and its equivalent in other areas of the CNS were heavily labeled. During the first postnatal week, C1C-2 was highly expressed in most neurons. By P7 a pattern of differential expression emerged with evidence of decreased expression of C1C-2 mRNA in many neuronal populations. In adult rat brain, C1C-2 was expressed at highest levels in large neurons as found within layer V of cortex, Ammon's Horn of hippocampus, or mitral cells of the olfactory bulb and Purkinje cells within the cerebellum. Many smaller neurons within the diencephalon maintained significant levels of expression. A functional conductance was readily detected in hippocampal neurons during the first postnatal week, which had the same characteristic properties as the conductance observed in adult neurons. The observed expression and functional presence of C1C-2 suggest a widespread role in neuronal chloride homeostasis in early postnatal life, and demonstrated that cell specific shut-down resulted in the adult pattern of expression.

Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

1. The effects of intracellular pH changes on the acetylcholine (ACh) release and cytoplasmic Ca2+ concentration at developing neuromuscular synapses were studied in Xenopus nerve-muscle co-cultures. 2. Spontaneous and evoked ACh release of motoneurons was monitored by using whole-cell voltage-clamped myocytes. Intracellular alkalinization with 15 mM NH4Cl slightly reduced the frequency of spontaneous synaptic currents (SSCs). However, cytosolic acidification following withdrawal of extracellular NH4Cl caused a marked and transient increase in spontaneous ACh release. 3. Another method of cytosolic acidification was used in which NaCl in Ringer solution was replaced with weak organic acids. The increase in spontaneous ACh release paralleled the level of intracellular acidification resulting from addition of these organic acids. Acetate and propionate but not isethionate, methylsulphate and glucuronate, caused an increase in intracellular pH and a marked increase in spontaneous ACh release. 4. Impulse-evoked ACh release was slightly augmented by intracellular alkalinization and inhibited by cytosolic acidification. 5. Cytosolic acidification was accompanied by an elevation in the cytoplasmic Ca2+ concentration ([Ca2+]i), resulting from both external Ca2+ influx and intracellular Ca2+ mobilization. In contrast, the increase in [Ca2+]i induced by high K+ was inhibited by cytosolic acidification. 6. We conclude that cytosolic acidification regulates spontaneous and evoked ACh release differentially in Xenopus motoneurons, increasing spontaneous ACh release but inhibiting evoked ACh release.

Regulation of ectodermal differentiation in Xenopus laevis animal caps treated with TPA and ammonium chloride

Animal caps isolated from Xenopus laevis embryos at the blastula stage were treated sequentially with NH4Cl, a known cement gland inducer, and with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a known neural inducer. The two artificial inducers were also used in reverse order to see if they can mimic the natural inducers acting during the progressive determination of the ectodermal organ. Immunofluorescence and whole-mount in situ hybridization were used to study the expression of tubulin, taken to indicate an early step on the pathway of cell elongation, and neural cell adhesion molecule (N-CAM) taken to indicate an early step in the determination of the nervous system. The expression of XCG-1, a marker of early specification of the cement gland, was also studied. The results showed that the two artificial inducers can mimic the effects of the natural inducers in animal cap explants. The TPA behaves like a neural inducer, reducing the number and the extension of the cement gland when added to the medium in addition to NH4Cl, before or after NH4Cl treatment. In the process of cement gland/neural induction, it is possible to redirect the ectoderm already specified as cement gland to neural tissue, but it does not seem possible to respecify the neural tissue as cement gland. Moreover, the animal caps were also cut into dorsal and ventral parts and the two halves were treated separately. The results were similar to those obtained with treatment of the entire animal cap, suggesting that a dorsal-ventral pattern is not yet established before the gastrula stage, and that in normal embryos there are boundaries between the effects of different inducers.

The role of intracellular alkalinization in the establishment of anterior neural fate in Xenopus

Our previous work demonstrated that Xenopus ectoderm cells undergo an alkalinization in response to planar inductive signals during neural induction in explants. We have examined the role of intracellular alkalinization in the establishment of anterior neural fate. First, RT-PCR was used to examine neural-specific gene expression in planar explants in which the alkalinization is prevented by treatment with 4,4'-dihydrodiisothiocyanatostilbene-2,2'-disulfonate (H2DIDS). In explants cultured in the presence of H2DIDS, expression of NCAM and the anterior neural gene otx2 is greatly reduced or absent. Second, neural-specific gene expression was examined in isolates of uninduced animal cap ectoderm cultured in the presence of either methylamine or ammonium chloride. NCAM, otx2, and the anterior neural inducer noggin were expressed in alkalinized ectoderm, while the more posterior neural markers krox-20 and Hox B9 were undetectable. Expression of NCAM, otx2, and noggin was observed at stage 11 in both alkalinized ectoderm and the newly induced neural plate, suggesting that intracellular alkalinization could contribute to propagation of noggin signaling through the dorsal ectoderm. Alkalinization of uninduced ectoderm at stage 10.5 led to an upregulation of otx2 within 15 min. Activation of NCAM expression in alkalinized dissociated cells was identical to that observed in intact animal caps, indicating that alkalinization-mediated changes in gene expression do not require cell-cell contact. Finally, the effects of intracellular alkalinization on protein tyrosine phosphorylation were investigated using 2D gel electrophoresis and immunoblots probed with an antiphosphotyrosine antibody. Several phosphorylated protein detected in induced and alkalinized ectoderm were greatly reduced or absent in uninduced ectoderm, indicating that alkalinization elicits alterations in tyrosine phosphorylation similar to some of those observed during neural induction in vivo. Our results indicate that intracellular alkalinization plays a critical role in the activation of anterior neural-specific gene expression and that alkalinization may act by regulating the activity of a tyrosine kinase or phosphatase.

Epithelial cell wedging and neural trough formation are induced planarly in Xenopus, without persistent vertical interactions with mesoderm

In this study we investigate the induction of the cell behaviors underlying neurulation in the frog, Xenopus laevis. Although planar signals from the organizer can induce convergent extension movements of the posterior neural tissue in explants, the remaining morphogenic processes of neurulation do not appear to occur in absence of vertical interactions with the organizer (R. Keller et al. , 1992, Dev. Dyn. 193, 218-234). These processes include: (1) cell elongation perpendicular to the plane of the epithelium, forming the neural plate; (2) cell wedging, which rolls the neural plate into a trough; (3) intercalation of two layers of neural plate cells to form one layer; and (4) fusion of the neural folds. To allow planar signaling between all the inducing tissues of the involuting marginal zone and the responding prospective ectoderm, we have designed a "giant sandwich" explant. In these explants, cell elongation and wedging are induced in the superficial neural layer by planar signals without persistent vertical interactions with underlying, involuted mesoderm. A neural trough forms, and neural folds form and approach one another. However, the neural folds do not fuse with one another, and the deep cells of these explants do not undergo their normal behaviors of elongation, wedging, and intercalation between the superficial neural cells, even when planar signals are supplemented with vertical signaling until the late midgastrula (stage 11.5). Vertical interactions with mesoderm during and beyond the late gastrula stage were required for expression of these deep cell behaviors and for neural fold fusion. These explants offer a way to regulate deep and superficial cell behaviors and thus make possible the analysis of the relative roles of these behaviors in closing the neural tube.

Control of cytosolic pH in two-cell mouse embryos: roles of H(+)-lactate cotransport and Na+/H+ exchange

In this study we used imaging techniques with the fluorescent pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to investigate the control of cytosolic pH (pHi) in two-cell mouse embryos in nominally HCO3(-)-free conditions. We found that the resting pHi of two-cell embryos (40-50 h after human chorionic gonadotropin) in HCO3(-)-free M2 was 7.31 +/- 0.01 (n = 172 embryos), which is significantly above the level predicted if H+ is at electrochemical equilibrium. We showed that two-cell embryos contain a H(+)-monocarboxylate cotransport system with apparent Michaelis constants for D-lactate, L-lactate, and pyruvate of 11.5, 3.7, and 3.5 mM, respectively. It is inhibited by p-chloromercuribenzoic acid (300 microM), p-chloromercuriphenylsulfonic acid (300 microM), and alpha-cyano-4-hydroxycinnamate (1 mM) and is insensitive to 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid (500 microM). We also showed that the pHi response to the acid load produced by an NH4Cl pulse has two components, one due to H(+)-monocarboxylate cotransport and the other due to Na+/H+ exchange. We found no evidence that a H+ conductance was responsible in these cells for the recovery in pHi after an acid load.

pH regulation and proton signalling by glial cells

The regulation of H+ in nervous systems is a function of several processes, including H+ buffering, intracellular H+ sequestering, CO2 diffusion, carbonic anhydrase activity and membrane transport of acid/base equivalents across the cell membrane. Glial cells participate in all these processes and therefore play a prominent role in shaping acid/base shifts in nervous systems. Apart from a homeostatic function of H(+)-regulating mechanisms, pH transients occur in all three compartments of nervous tissue, neurones, glial cells and extracellular spaces (ECS), in response to neuronal stimulation, to neurotransmitters and hormones as well as secondary to metabolic activity and ionic membrane transport. A pivotal role for H+ regulation and shaping these pH transients must be assigned to the electrogenic and reversible Na(+)-HCO3-membrane cotransport, which appears to be unique to glial cells in nervous systems. Activation of this cotransporter results in the release and uptake of base equivalents by glial cells, processes which are dependent on the glial membrane potential. Na+/H+ and Cl-/HCO3-exchange, and possibly other membrane carriers, accomplish the set of tools in both glial cells and neurones to regulate their intracellular pH. Due to the pH dependence of a great variety of processes, including ion channel gating and conductances, synaptic transmission, intercellular communication via gap junctions, metabolite exchange and neuronal excitability, rapid and local pH transients may have signalling character for the information processing in nervous tissue. The impact of H+ signalling under both physiological and pathophysiological conditions will be discussed for a variety of nervous system functions.

Induction of notochord cell intercalation behavior and differentiation by progressive signals in the gastrula of Xenopus laevis

We show that notochord-inducing signals are present during Xenopus laevis gastrulation and that they are important for both inducing and organizing cell behavior and differentiation in the notochord. Previous work showed that convergent extension of prospective notochordal and somitic mesoderm occurs by mediolateral cell intercalation to produce a longer, narrower tissue. Mediolateral cell intercalation is driven by bipolar, mediolaterally directed protrusive activity that elongates cells and then pulls them between one another along the mediolateral axis. This cell behavior, and subsequent notochordal cell differentiation, begins anteriorly and spreads posteriorly along the notochordal-somitic boundary, and from this lateral boundary progresses medially towards the center of the notochord field. To examine whether these progressions of cell behaviors and differentiation are induced and organized during gastrulation, we grafted labeled cells from the prospective notochordal, somitic and epidermal regions of the gastrula into the notochordal region and monitored their behavior by low light, fluorescence videomicroscopy. Prospective notochordal, epidermal and somitic cells expressed mediolateral cell intercalation behavior in an anterior-to-posterior and lateral-to-medial order established by the host notochord. Behavioral changes were induced first and most dramatically among cells grafted next to the notochordal-somitic boundary, particularly those in direct contact with the boundary, suggesting that the boundary may provide signals that both induce and organize notochordal cell behaviors. By physically impeding normal convergent extension movements, notochordal cell behaviors and differentiation were restricted to the anteriormost notochordal region and to the lateral notochordal-somitic boundary. These results show that mediolateral cell intercalation behavior and notochordal differentiation can be induced in the gastrula stage, among cells not normally expressing these characteristics, and that these characteristics are induced progressively, most likely by signals emanating from the notochordal-somitic boundary. In addition, they show that morphogenetic movements during gastrulation are necessary for complete notochord formation and that the prospective notochord region is not determined by the onset of gastrulation.

Intracellular acidification of gastrula ectoderm is important for posterior axial development in Xenopus

There is evidence suggesting that pHi elevation can induce differentiation to cement gland, an extremely anterior structure, during the early development of Xenopus laevis (Picard, J. J. (1975) J. Embryol. exp. Morphol. 33, 957–967; Sive, H. L., Hattori, K. and Weintraub, H. (1989) Cell 58, 171–180). We wanted to investigate whether axial development or neural induction are mediated in Xenopus via regulation of pHi. Our interest was stimulated further because certain signal transduction pathways, which are thought to mediate anterior neural induction (Otte, A. P., Van Run, P., Heideveld, M., Van Driel, R. and Durston, A. J. (1989) Cell 58, 641–648; Durston and Otte (1991), Cell-Cell Interactions in Early Development, pp. 109–127), are also known to modify the activity of proton extruders (Mitsuka and Berk (1991) Am. J. Physiol. 260, C562-C569; Wakabayashi, S., Sardet, C., Fafournoux, P., Counillon, L., Meloche, S., Pages, G. and Pouyssegur, J. (1993) Rev. Physiol. Biochem. Pharmacol. Vol. 119, pp. 157–186). We therefore measured pHi in explants of gastrula ectoderm and neurectoderm and identified ion exchangers that regulate pHi in these tissues. The measurements showed that pHi decreases in explants of both neurectoderm and uninduced ectoderm during the time course of gastrulation, this pHi decrease thus fails to correlate with neural induction. One important regulator of this cytoplasmic acidification is the Na+/H+ exchanger. The pHi set point, at which the acid extrusion activity of this alkalizing exchanger is shut off, shifts to more acidic values during the time course of gastrulation, thus permitting cytoplasmic acidification. We found also that preventing cytoplasmic acidification and thereby elevating pHi in late gastrula cells led to the specific suppression of posterior development. Neural induction and anterior development were unaffected by treatments leading either to an elevation of or a decrease in pHi. These findings indicate that the cellular processes mediating anterior development and neural induction are pHi tolerant, while the signals mediating posterior development require a sustained pHi decrease for their action, suggesting that downregulation of pHi is necessary for posterior axial development.

In vivo labelling of L-type Ca2+ channels by fluorescent dihydropyridine: correlation between ontogenesis of the channels and the acquisition of neural competence in ecotderm cells from Pleurodeles waltl embryos

The ability of the ectodermal cells to be induced and to differentiate toward neural tissue, called neural competence, is acquired shortly before gastrulation and lost during late gastrula stages in Pleurodeles waltl embryos. We have examined ectodermal cells' neural competence in relation to the evolution of the density of L-type calcium channels using the fluorescent labelled dihydropyridine probe (STBodipy-DHP). We find that the appearance of dihydropyridine sensitive calcium channels (L-type Ca2+ channels) is correlated with the acquisition of neural competence by the ectoderm cells. The highest density of these channels is reached when competence of the ectoderm is optimal. Conversely, the decrease of L-type Ca2+ channel density occurs simultaneously with the normal loss of competence. In addition, we show that these channels are functional since stimulation by S(-)-Bay K 8644 triggered an increase in [Ca2+]i revealed by fluorescence measurements using Fluo-3. This increase in [Ca2+]i is a function of the L-type Ca2+ channels' density. We propose that the molecular basis of the gain and loss of neural competence is linked to the presence of L-type Ca2+ channels in ectodermal cell membranes of Pleurodeles waltl embryos.

Differential effects of cytotactin/tenascin fusion proteins on intracellular pH and cell morphology

Cytotactin/tenascin is a multidomain extracellular matrix protein that inhibits both cell spreading and intracellular alkalinization. The protein has multiple different domains which are homologous to regions in epidermal growth factor, fibronectin, and fibrinogen. In previous studies, we produced nonoverlapping fusion proteins corresponding to these domains and examined their effects on cell attachment and spreading. Based on their ability either to promote or to inhibit cell attachment, two of these fusion proteins were shown to be adhesive and two were shown to be counteradhesive. To determine how the adhesive and counteradhesive activities of different cytotactin/tenascin domains alter intracellular pH (designated pHi), we have measured pHi, in NIH3T3 and U251MG cells in the presence of the cytotactin/tenascin fusion proteins and intact cytotactin/tenascin, as well as fibronectin. Cells incubated in the presence of intact cytotactin/tenascin or of the counteradhesive fusion proteins had a pHi lower than control cells. In contrast, the presence of the adhesive fusion proteins or of fibronectin caused cells to have higher pHi values than control cells. When two fragments were simultaneously presented, one of which alone increased pHi and the other of which alone decreased pHi, the predominant effect was that of lowered pHi. Incubation with an RGD-containing peptide derived from the cytotactin/tenascin sequence inhibited alkalinization promoted by the adhesive fragment containing the second through sixth fibronectin type III repeats that was known to bind to integrins. Incubation of the cells with heparinase I or III inhibited the intracellular alkalinization of cells plated in the presence of the other adhesive fusion protein containing the fibrinogen domain, suggesting that heparan sulfate proteoglycans were involved in these pHi changes. The activity of protein kinase C appeared to be important for the changes in pHi mediated by all of the proteins. The protein kinase C inhibitor Calphostin C blocked the rise in pHi elicited by the adhesive fusion proteins and by fibronectin. Moreover, activation of protein kinase C by the addition of phorbol esters increased the pHi in cells plated on cytotactin/tenascin or counteradhesive fusion proteins and reversed their effects. The results of this study support the hypothesis that cytotactin/tenascin can bind to multiple cell surface receptors and thereby elicit different physiological responses. Decreases in pHi are correlated with the phenomenon of counteradhesion whereas the ability to increase pHi is associated with cell attachment via at least two different types of cell surface receptors. The data raise the possibility that binding of cytotactin/tenascin may influence primary cellular processes such as migration and proliferation through the differential regulation of pHi.