Evolution of nerve development in frogs. I. The development of the peripheral nervous system in Discoglossus pictus (Discoglossidae)

The gross anatomical development of the peripheral nervous system (PNS) during embryogenesis and metamorphosis in the frog Discoglossus pictus is described based on whole-mount immunostaining for nerves and muscles. In the head, neurite outgrowth starts with the mandibular ramus of the trigeminal nerve at the tailbud stage. Cranial muscles are innervated as soon as they differentiate, beginning at mid-embryonic stages. During late embryonic stages, the course of the trigeminal and facial nerves becomes greatly distorted and changes again drastically during metamorphosis accompanying the reorganization of the jaw muscles. Two occipital somites and nerves develop transitorily but degenerate at late embryonic stages. The hypoglossal nerve develops by fusion of the first and second spinal nerves and receives a transitory contribution of the third and fourth spinal nerve at embryonic stages. In the trunk, several classes of Rohon-Beard neurites could be identified at embryonic stages, one of which forms intersegmental sensory nerves that prefigure the course of the sensory rami of spinal nerves at later stages. We give detailed schedules of PNS and cranial muscle development which, in comparison with data on other frog species described in a companion paper, will serve as a basis to evaluate heterochronic shift during evolution of PNS development in frogs.

Evolution of nerve development in frogs. II. Modified development of the peripheral nervous system in the direct-developing frog Eleutherodactylus coqui (Leptodactylidae)

We use whole-mount immunohistochemistry to describe the pattern of development of cranial nerves and muscles in the direct-developing leptodactylid frog Eleutherodactylus coqui. Comparison with nerve development in the biphasically developing frogs Physalaemus pustulosus (Leptodactylidae) and Discoglossus pictus (Discoglossidae; described in a companion paper) allows us to infer the ancestral leptodactylid ontogenetic pattern and the extent to which it has been modified during the evolution of direct development in Eleutherodactylus. While early embryonic development of cranial nerves and muscles is remarkably conserved in E. coqui, most transitory embryonic and larval characters (e.g., occipital and spinal myotomes together with their innervation, the distorted course of trigeminal and facial nerves, ventral branchial arch muscles, a subset of branchial-nerve rami and the lateral-line system) never develop. However, a few larva-typical characters are recapitulated, including Rohon-Beard cells and an anastomosis between the vagal and hypoglossal nerve. In addition to the abbreviation of ontogeny by loss of larva-specific traits, dramatic dissociations and heterochronic shifts of characters can be observed in E. coqui. The onset of development of limb and trunk innervation has been pre-displaced to early embryonic stage. Moreover, the reorientation of cranial muscles and nerves corresponding to late metamorphic events in biphasically developing anurans occurs relatively much earlier and is less pronounced in E. coqui resulting in an extreme condensation of ontogeny.

Pretecto-tectal interactions: effects of lesioning and stimulating the pretectum on field potentials in the optic tectum of salamanders in vitro

Interactions between pretectum and optic tectum of salamanders were analyzed by recording evoked potentials (EPs) in the optic tectum in vitro in response to stimulation of the contralateral optic nerve. Neither lesioning of the pretectum nor ablation of the medulla oblongata including the nucleus isthmi altered the shape of tectal EPs, suggesting that the tectal EP in this preparation reflects activation of tectal circuitry by retinal afferents without a major contribution from non-retinal afferents. To analyze the effect of stimulation of the pretectum on the tectal EP, we stimulated the pretectal area pharmacologically. Amplitudes of tectal EPs decreased rapidly after stimulation of the pretectum and recovered within minutes (glutamate) or hours (kainic acid). The pretectal influence on the tectal EP might act presynaptically on retinal afferents or by modulating the response of inhibitory interneurons.

Fasciculation frequency changes at different length of relaxed muscle

We have studied, in chronically denervated muscles, fasciculation potentials (FPs) which were clearly identified when the muscles were either relaxed or passively stretched. The frequency of 10 FPs was determined in both positions in seven patients. In stretched muscles, two FPs were no longer recorded, and the mean FP frequency decreased significantly (P < .01). There was also a decrease of the FP amplitude. As FPs are frequently associated with ordinary muscle cramps, stretching of the muscle may lessen the frequency of FPs by the same mechanism which stops cramps.

High frequency discharge of a fraction (f) of motor unit action potential

Two repetitive discharges, firing at 105 and 180 Hz, were evoked in muscles with chronic denervation. They were delayed following a motor unit action potential (M), and their maximum duration was 800 and 50 ms, respectively. The potential of both high frequency discharges was of low amplitude and short duration. It was considered to be a fraction (f) of the motor unit potential, and to depend on muscle fibres re-innervated by an axonal branch. The repetitive mechanism was tested by double stimulation. It seemed to be an ephaptic re-excitation of the axonal branch by a sprout rather than an ectopic trigger locus on the latter. The antidromic waves, associated with the repetitive discharges on the axonal branch, failed to be transmitted to the main axon. This failure, assimilated to a conduction block, was complete in the first case as M was not repetitive. It was intermittent in the second case as M was firing intermittently. Repetitive activity associated with 'terminal multifocal block' could be relevant to the fractional activation observed by others in some disease states.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

A patient-derived cytotoxic T-lymphocyte clone and two peptide-dependent monoclonal antibodies recognize HLA-B27-peptide complexes with low stringency for peptide sequences

HLA-B27 molecules expressed on the T2 mutant cell line do not have peptides. Such empty HLA-B27 molecules were not recognized by an HLA-B27-restricted cytotoxic T-lymphocyte (CTL) clone (auto-1) derived from synovial fluid. To test for peptide dependency of the clone, B27-T2 cells were incubated with a panel of 48 different peptides. This lack of stringency was compared with that of a peptide-dependent monoclonal antibody, B27.M2. Positive B27.M2 reactivity resulted when the B27-T2 cells were incubated with two peptides: RRKAMFEDI and RRMGPPVGHR, derived from Chlamydia HSP60 and human ribonucleoprotein, respectively. Because of the limited availability of CTL versus monoclonal antibody, the specificity of B27.M2 was studied in greater detail. The importance of the HLA-B27 heavy chain in antibody recognition of class I-peptide complexes was demonstrated by site-directed mutagenesis. The stringency of the peptide residues was tested by making analogs of each of the nine residues in RRKAMFEDI, creating a panel of 180 analogs. Although stringency was highest for the sixth position, as many as six different amino acids provided positive reactivity. These results indicate that immune recognition of HLA-B27-peptide complexes might have rather low stringency for the peptide sequences. In theory, then, pathogen-derived peptides which induce autoimmunity by generating autoreactive CTL might not share much sequence similarity with the responsible self peptides.

Similarities and differences in the cytoarchitecture of the tectum of frogs and salamanders

Frogs exhibit a morphologically complex (multiply laminated) optic tectum, while salamanders have one of the morphologically simplest tecta among vertebrates. In a comparative approach, the morphology of tectal projection neurons is investigated in three salamander species, Hydromantes italicus, H. genei and Plethodon jordani, and two frog species, Discoglossus pictus and Eleutherodactylus coqui, by means of retrograde Biocytin labeling complemented by intracellular Biocytin staining of cells. Despite striking differences in the gross anatomy of the tectum, salamanders and frogs have the same types of tectal neurons with respect to their dendritic arborization and the pattern of ipsilaterally and bilaterally ascending (to praetectum and thalamus) and ipsilaterally or contralaterally descending projections (to nucleus isthmi, medulla oblongata and rostral spinal cord). In the light of these findings, the relationship between morphological complexity of the tectum and behavioral complexity (feeding behavior) is discussed.

Distribution of cranial and rostral spinal nerves in tadpoles of the frog Discoglossus pictus (Discoglossidae)

We studied the peripheral nervous system of early tadpoles of the frog Discoglossus pictus using whole-mount immunohistochemistry. Double-labeling of muscles and nerves allowed us to determine the innervation of all cranial muscles supplied by the trigeminal, facial, glossopharyngeal, vagal, and hypoglossal nerves. The gross anatomical pattern of visceral, cutaneous, and lateral-line innervation was also assessed. Most muscles of the visceral arches are exclusively supplied by posttrematic rami of the corresponding branchiomeric nerves, the only exceptions being some ventral muscles (intermandibular, interhyoid, and subarcual rectus muscles). In the mandibular arch, the pattern of motor ramules of the trigeminal nerve prefigures in a condensed form the adult pattern, but the muscles of the hyoid arch are innervated by ramules of the facial nerve in a pattern that differs from that of postmetamorphic frogs. With respect to the nerves of the branchial arches, pretrematic visceral rami, typical of other gnathostomes, are absent in D. pictus. Instead, we find a separate series of posttrematic profundal visceral rami. Pharyngeal rami of all branchial nerves contribute to Jacobson's anastomosis. We provide a detailed description of the lateral-line innervation and describe a new ramus of the middle lateral-line nerve (ramus suprabranchialis). We confirm the presence of a first spinal nerve and its contribution to the hypoglossal nerve in D. pictus tadpoles.

Immunohistological localization of tenascin-C in the developing and regenerating retinotectal system of two amphibian species

The expression pattern of the extracellular matrix molecule tenascin-C was investigated in the retinotectal system of the frog Discoglossus pictus and the salamander Pleurodeles waltl during development and optic nerve regeneration in the adult. In both species, the retina was devoid of tenascin-C immunoreactivity at all ages studied. During development, tenascin-C was distributed in a gradient in the optic nerve, with the highest immunoreactivity in the eye near part of the optic nerve. The myelin-associated glycoprotein was distributed in a gradient with opposite polarity. In Discoglossus, but not Pleurodeles, tenascin-C was detected in the anterior chiasm. In the tectum of both species, tenascin-C was observed in deep cellular and fiber layers but not in the layers receiving optic fibers or proliferative zones. The distribution patterns of tenascin-C were the same during development and in the adult, except for a disappearance of the molecule from the intraocular part of the optic nerve. After lesioning the optic nerve of adult animals, tenascin-C was strongly reexpressed in the intraocular part of the optic nerve but was only weakly upregulated in the distal optic nerve stump. In contrast, a chondroitin sulfate epitope was strongly upregulated in the distal optic nerve stump. These observations suggest that during development, tenascin-C serves as an attenuating barrier for myelinating cells in the optic nerve and contributes to the guidance of growing retinal ganglion cell axons. Due to its sustained expression in the adult, tenascin-C may have similar functions during regeneration of the lesioned adult retinotectal system.

Cancellation of single F wave by double stimulation in case of chronic denervation

Single F waves evoked by stimulation with above threshold intensity are cancelled by a second shock, as was previously demonstrated with maximal or sub-maximal intensity. Double stimulation of any intensity thus makes it possible to detect a possible reflex component in the spinal response, by elimination of the F wave. As an H or a heteronymous H reflex may be a sign of a disordered central motor system state, this fast method has a direct clinical utility.

Brain size and morphology in miniaturized plethodontid salamanders

In six miniaturized salamanders of the family Plethodontidae, including one of the smallest tetrapod vertebrates. Thorius pennatulus, the anatomical consequences of miniaturization for the brain were investigated. We determined (1) absolute and relative size of the brain, major parts of the brain, the tectum and tectal gray matter, (2) nerve cell size and density, and (3) the number of cells within the visual and visuomotor centers (thalamus, tectum/praetectum and tegmentum). No common compensatory strategy for the brain among the miniaturized salamanders was found. Except for the smallest species, T. pennatulus, only some of the expected compensatory processes (increase in relative size of the brain, relative size of visual centers, relative amount of gray matter or relative density of cell packing density) are found in any species, and these occur in different combinations and degrees. The most decisive factor for maximizing cell number was cell size. Miniaturized species with small cells also have many visual cells, regardless of the other factors. In contrast, the minimum number of visual neurons is found in miniaturized salamanders with large cells. It is concluded that the neuroanatomical traits investigated exert different degrees of resistance to adaptive compensatory processes. Cell size seems to be the most resistant parameter and is strictly dependent on genome size.

Regulation of the human IgE receptor (Fc epsilon RII/CD23) by EBV. Localization of an intron EBV-responsive enhancer and characterization of its cognate GC-box binding factors

EBV infection of human B lymphocytes induces expression of the low affinity IgE receptor, Fc epsilon RII/CD23. CD23 is constitutively expressed in EBV-immortalized B cells and may play an essential role in immortalization. We previously explored the regulation of CD23 by EBV, showing that induction results from transcriptional activation that is mediated, in part, by an EBV-responsive transcriptional regulatory element in the 5' region of CD23 (-229 to +305 relative to the type a promoter). We now report the localization of the regulatory element and characterization of its cognate DNA-binding proteins. Reporter gene assays in EBV-positive and -negative lines localized a functional EBV-responsive enhancer to a 37-bp fragment (+248 to +284) that contains a GC-rich sequence (GC box) within intron I of type a CD23. This fragment was shown by mobility shift assays to specifically bind nuclear protein(s) from EBV-positive lines, but not EBV-negative lines. Mutation of the GC box resulted in a loss of protein-binding activity, implicating involvement of a GC box-binding protein in the DNA/protein interaction. Supershift assays suggested that the ubiquitous GC box-binding transcription factor, Sp1, is not a part of the complex, and UV-crosslinking studies demonstrated that the DNA/protein complex contains at least two proteins that differ in size from other known GC box-binding proteins. Binding of these proteins to the enhancer element requires phosphorylation, because phosphatase treatment of nuclear extracts abolished formation of the DNA/protein complex. These studies reveal the presence of an EBV-responsive enhancer element in intron I of type a CD23 and implicate a GC box-binding transcription factor in the activation of CD23 by EBV.

Regulation of the human IgE receptor (Fc epsilon RII/CD23) by EBV. Localization of an intron EBV-responsive enhancer and characterization of its cognate GC-box binding factors

EBV infection of human B lymphocytes induces expression of the low affinity IgE receptor, Fc epsilon RII/CD23. CD23 is constitutively expressed in EBV-immortalized B cells and may play an essential role in immortalization. We previously explored the regulation of CD23 by EBV, showing that induction results from transcriptional activation that is mediated, in part, by an EBV-responsive transcriptional regulatory element in the 5' region of CD23 (-229 to +305 relative to the type a promoter). We now report the localization of the regulatory element and characterization of its cognate DNA-binding proteins. Reporter gene assays in EBV-positive and -negative lines localized a functional EBV-responsive enhancer to a 37-bp fragment (+248 to +284) that contains a GC-rich sequence (GC box) within intron I of type a CD23. This fragment was shown by mobility shift assays to specifically bind nuclear protein(s) from EBV-positive lines, but not EBV-negative lines. Mutation of the GC box resulted in a loss of protein-binding activity, implicating involvement of a GC box-binding protein in the DNA/protein interaction. Supershift assays suggested that the ubiquitous GC box-binding transcription factor, Sp1, is not a part of the complex, and UV-crosslinking studies demonstrated that the DNA/protein complex contains at least two proteins that differ in size from other known GC box-binding proteins. Binding of these proteins to the enhancer element requires phosphorylation, because phosphatase treatment of nuclear extracts abolished formation of the DNA/protein complex. These studies reveal the presence of an EBV-responsive enhancer element in intron I of type a CD23 and implicate a GC box-binding transcription factor in the activation of CD23 by EBV.

Some remarks on alleged qualitative differences between anamniote and amniote nervous systems

There are no 'lower' compared to 'higher' vertebrates (craniates) placed along a linear process of evolution. The different vertebrate/craniate groups evolved independently from a common ancestor, and during this evolution, increases in the complexity of nervous systems and of behaviour are as common as simplification (e.g., in myxinoids, lungfishes and amphibians). The organization of premotor networks is basically the same among tetrapods. Similar to the afferent sensory systems (e.g., the visual system), the premotor system--at least in the context of feeding and calling--is organized in a parallel fashion, with separate channels carrying different information to motor centres, which are the level of integration. A command neuron or command neuron ensemble system exists neither in anamniotes nor in amniotes. At feeding or calling, anamniotes exhibit no stereotyped behaviour or 'fixed action pattern' compared to the plastic behaviour of amniotes; differences are only quantitative. All vertebrates possess bulbar and spinal motor program modules, which can be controlled in a flexible way by descending pathways and reafferent circuits.

Depth perception in salamanders: the wiring of visual maps

Most members of the salamander family Plethodontidae exhibit fast and precise prey localization and are likely to use stereopsis for this task. Tectal and isthmic neurons were stained iontophoretically with biocytin after intracellular recording in order to investigate the fine structure and arborization pattern of dendritic and telodendritic trees. The back-projection of isthmic neurons to both tectal hemispheres exhibits a conspicuous and spatially restricted connection with tectal neurons and the afferent fiber layers, which probably plays an important role in the context of stereoscopic depth perception and detection of retinal disparities.

Tectal activation of premotor and motor networks during feeding in salamanders

In salamanders, three separate pathways, a crossed and two uncrossed ones, extend from the tectum mesencephali to the brain stem and spinal cord. These pathways arise from different types of tectal neurons; their dendrites arborize in different layers of retinal afferents and, thus, receive different types of retinal information. Collaterals of descending axons extend in regions, where motoneurons and interneurons related to prey capture are situated. The response properties of tectal neurons, interneurons and motoneurons related to prey capture were revealed by intracellular recording with subsequent dye (biocytin) injection. Most tectal neurons exhibit long latencies after optic-nerve stimulation, which indicates a complex processing of visual information inside the tectum. Our data show that no one-way connection exists between the tectum and motor nuclei; rather, these centres, together with a number of interneurons, exhibit a complex interaction during feeding.

Epstein-Barr viral nuclear antigen 1 antisense oligodeoxynucleotide inhibits proliferation of Epstein-Barr virus-immortalized B cells

The Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1) is a latent viral protein that is expressed in all EBV-immortalized lymphocytes and plays an essential role in immortalization y EBV. EBNA-1 protein is required for replication and maintenance of the episomal viral genome in latently infected, immortalized cells. Given the essential function of EBNA-1 in immortalization, we have examined the effect of EBNA-1 antisense oligodeoxynucleotides on expression of EBNA-1 protein and proliferation in EBV-immortalized lymphoblastoid cells. We have shown that exposure to unmodified antisense oligodeoxynucleotide of codons 6 through 10 of EBNA-1 partially suppressed EBNA-1 protein expression in EBV-immortalized lymphoblastoid cells relative to untreated cells or cells exposed to two scrambled sequences of the EBNA-1 antisense. Furthermore, EBNA-1 antisense inhibited proliferation of EBV-immortalized cells by at least 50% compared with the scrambled antisense sequences. There was no difference in the effect of antisense and scrambled antisense oligodeoxynucleotides on the proliferation of EBV-negative cells, indicating that the antiproliferative effect of EBNA-1 antisense was EBV-specific. These findings underscore the essential role of EBNA-1 in immortalization and, furthermore, have potential therapeutic implications for EBV-associated neoplastic diseases.

Cell size predicts morphological complexity in the brains of frogs and salamanders

The morphological organization of the brain of frogs and salamanders varies greatly in the degree to which it is subdivided and differentiated. Members of these taxa are visually oriented predators, but the morphological complexity of the visual centers in the brain varies interspecifically. We give evidence that the morphological complexity of the amphibian tectum mesencephali, the main visual center, can be predicted from knowledge of cell size, which varies greatly among these taxa. Further, cell size is highly correlated with genome size. Frogs with small cells have more complex morphologies of the tectum than do those with large cells independent of body and brain size. In contrast, in salamanders brain-body size relationships also are correlated with morphological complexity of the brain. Small salamanders with large cells have the simplest tecta, whereas large salamanders with small cells exhibit the most complex tectal morphologies. Increases in genome, and consequently cell size, are associated with a decrease in the differentiation rate of nervous tissue, which leads to the observed differences in brain morphology. On the basis of these findings we hypothesize that important features of the structure of the brain can arise independently of functional demands, from changes at a lower level of organismal organization--in this case increase in genome size, which induces simplification of brain morphology.