Simple rules for a “simple” nervous system? Molecular and biomathematical approaches to enteric nervous system formation and malformation

We review morphogenesis of the enteric nervous system from migratory neural crest cells, and defects of this process such as Hirschsprung disease, centering on cell motility and assembly, and cell adhesion and extracellular matrix molecules, along with cell proliferation and growth factors. We then review continuum and agent-based (cellular automata) models with rules of cell movement and logistical proliferation. Both movement and proliferation at the individual cell level are modeled with stochastic components from which stereotyped outcomes emerge at the population level. These models reproduced the wave-like colonization of the intestine by enteric neural crest cells, and several new properties emerged, such as colonization by frontal expansion, which were later confirmed biologically. These models predict a surprising level of clonal heterogeneity both in terms of number and distribution of daughter cells. Biologically, migrating cells form stable chains made up of unstable cells, but this is not seen in the initial model. We outline additional rules for cell differentiation into neurons, axon extension, cell-axon and cell–cell adhesions, chemotaxis and repulsion which can reproduce chain migration. After the migration stage, the cells re-arrange as a network of ganglia. Changes in cell adhesion molecules parallel this, and we describe additional rules based on Steinberg's Differential Adhesion Hypothesis, reflecting changing levels of adhesion in neural crest cells and neurons. This was able to reproduce enteric ganglionation in a model. Mouse mutants with disturbances of enteric nervous system morphogenesis are discussed, and these suggest future refinement of the models. The modeling suggests a relatively simple set of cell behavioral rules could account for complex patterns of morphogenesis. The model has allowed the proposal that Hirschsprung disease is mostly an enteric neural crest cell proliferation defect, not a defect of cell migration. In addition, the model suggests an explanations for zonal and skip segment variants of Hirschsprung disease, and also gives a novel stochastic explanation for the observed discordancy of Hirschsprung disease in identical twins.     

Target cells for HIV in the central nervous system: macrophages or glial cells?

Infection of foetal or embryonic brain cells and cell lines from human astrocytomas and gliomas with HIV1 derived from T-lymphoma cultures leads to the expression of HIV in about 1 to 2% of the cells in culture. Single-cell cloning of astrocytoma cells shortly after infection resulted in the establishment of persistently HIV1-infected cell lines. These cultures were characterized by low production of virus and moderate intra- and extracellular expression of structural proteins. However, high expression of the nef regulatory protein was found. The virus could be rescued by cocultivation with T cells and primary macrophages giving rise to typical syncytia formation.In contrast to infection with HIV-infected T-lymphoma lines, cocultivation with HIV1-infected primary macrophages or monocytic cell lines induced a reduction in the growth of astrocytes and failed to induce productive infection. These in vitro observations support the hypothesis that astrocytes and glial cells may be a reservoir for HIV in the central nervous system and that macrophages may not carry the virus to the brain, but rather may be infected in the brain after having penetrated the blood-brain barrier.     

Vascular damage in the central nervous system: a multifaceted role for vascular-derived TGF-β

The brain function depends on a continuous supply of blood. The blood–brain barrier (BBB), which is formed by vascular cells and glia, separates components of the circulating blood from neurons and maintains the precisely regulated brain milieu required for proper neuronal function. A compromised BBB alters the transport of molecules between the blood and brain and has been associated with or shown to precede neurodegenerative disease. Blood components immediately leak into the brain after mechanical damage or as a consequence of a compromised BBB in brain disease changing the extracellular environment at sites of vascular damage. It is intriguing how blood-derived components alter the cellular and molecular constituents of the neurovascular interface after BBB opening. We recently identified an unexpected role for the blood protein fibrinogen, which is deposited in the nervous system promptly after vascular damage, as an initial scar inducer by promoting the availability of active TGF-β. Fibrinogen-bound latent TGF-β interacts with astrocytes, leading to active TGF-β formation and activation of the TGF-β/Smad signaling pathway. Here, we discuss the pleiotropic effects of potentially vascular-derived TGF-β on cells at the neurovascular interface and we speculate how these biological effects might contribute to degeneration and regeneration processes. Summarizing the effects of the components derived from the brain vascular system on nervous system regeneration might support the development of new therapeutic approaches.     

Phagocytic functions of microglial cells in the central nervous system and their importance in two neurodegenerative diseases: multiple sclerosis and Alzheimer’s disease

Microglial cells are the resident phagocytic cells of the central nervous system (CNS). They possess a wide range of receptors allowing them to identify and internalize numerous pathogens. We will discuss here the role of the most important receptors of microglia involved in non-opsonin-dependent phagocytosis (mannose receptor, β-glucan receptor, scavenger receptor) and that of receptors involved in the opsonin-dependent phagocytosis, namely the complement 3 (CR3) and the Fcγ receptors (FcγR). First, the molecular and cellular mechanisms induced when these receptors are conducting a phagocytic event are presented. In the second part, we will discuss the role these receptors may play in multiple sclerosis and Alzheimer’s disease, in the elimination by phagocytosis of myelin and beta amyloid peptide respectively. The first two authors contributed equally to this work.     

The glial regenerative response to central nervous system injury is enabled by pros-notch and pros-NFκB feedback

Organisms are structurally robust, as cells accommodate changes preserving structural integrity and function. The molecular mechanisms underlying structural robustness and plasticity are poorly understood, but can be investigated by probing how cells respond to injury. Injury to the CNS induces proliferation of enwrapping glia, leading to axonal re-enwrapment and partial functional recovery. This glial regenerative response is found across species, and may reflect a common underlying genetic mechanism. Here, we show that injury to the Drosophila larval CNS induces glial proliferation, and we uncover a gene network controlling this response. It consists of the mutual maintenance between the cell cycle inhibitor Prospero (Pros) and the cell cycle activators Notch and NFκB. Together they maintain glia in the brink of dividing, they enable glial proliferation following injury, and subsequently they exert negative feedback on cell division restoring cell cycle arrest. Pros also promotes glial differentiation, resolving vacuolization, enabling debris clearance and axonal enwrapment. Disruption of this gene network prevents repair and induces tumourigenesis. Using wound area measurements across genotypes and time-lapse recordings we show that when glial proliferation and glial differentiation are abolished, both the size of the glial wound and neuropile vacuolization increase. When glial proliferation and differentiation are enabled, glial wound size decreases and injury-induced apoptosis and vacuolization are prevented. The uncovered gene network promotes regeneration of the glial lesion and neuropile repair. In the unharmed animal, it is most likely a homeostatic mechanism for structural robustness. This gene network may be of relevance to mammalian glia to promote repair upon CNS injury or disease.     

Nature brings new avenues to the therapy of central nervous system diseases——an overview of possible treatments derived from natural products

Natural products(NPs), including traditional Chinese medicine(TCM), have been long and widely used in the prevention and treatment of central nervous system(CNS) diseases by virtue of their abundant sources, diverse structures, and novel activities.In this review article, we intend to summarize and discuss the situation or status of the clinical employments or trials of the NPs and their derivatives with CNS activities. NPs that have been extensively studied in preclinical research in recent years are also included. The compounds presented in this review are classified according to their indications and followed by details such as natural sources, possible biological mechanisms, and development status, while a considerable proportion of them are found in TCM. In addition, some drug combinations with synergistic effects are also mentioned. According to their impressive therapeutic effects and novel chemical structures, NPs are not only effective therapeutic remedies in clinic, but also lead compounds for structural modification, which indicate that nature brings new avenues to the therapy of CNS diseases.     

Cellular and cytokine responses of the human central nervous system to intracranial administration of tumor necrosis factor α for the treatment of malignant gliomas

To elucidate the role of tumor necrosis factor (TNF) as a biological response modifier, we studied cellular and cytokine responses of the central nervous system to TNF administered intracranially in a phase I clinical trial for patients with malignant gliomas. Six patients received injections of TNF (1.25×10³–10×10³ U/injection) into the tumor cavities, and regional fluids (RF) and lumbar cerebrospinal fluids (CF) were serially sampled before and after the injections. Recruitment of neutrophils occurred, mostly peaking 8 h after TNF injection, and fewer numbers of CD4⁺ T cells and monocytes/macrophages migrated, subsequently peaking at 24 h. The CF leukocytosis persisted for 48 h and was associated with an increased level of neutrophil chemotactic activity in the CF. This neutrophil chemotactic activity was attributed to interleukin-8 (IL-8) by HPLC. The level of IL-6 activity in the CF and RF consistently increased; beginning 2 h after TNF injection and reaching the maximum between 8 h and 12 h. It returned to the basal level within 48 h. IL-1 was detected in the CF of three patients, its level peaking at 8 h. Prostaglandin E₂ also increased after injection of TNF, peaking between 4 h and 12 h and then gradually decreasing. Transforming growth factor was found in all cases tested and one patient showed a significant change after TNF injection. IL-2 activity, interferon (INF) activity, IFN, and granulocyte/macrophage-colony-stimulating factor were not detected in the CF or RF. In conclusion, TNF is biologically effective in inducing migration of immune cells and generating multiple cytokine responses in the human central nervous system.     

Ontogenetic expression of CART-peptides in the central nervous system and the periphery: a possible neurotrophic role?

Little attention has been devoted to the expression of CART during development. However, a few studies in the central nervous system and periphery provide a clear indication that these peptides may play significant roles during histogenesis, and may have trophic actions.     

D18G transthyretin is monomeric,aggregation prone,and not detectable in plasma and cerebrospinal fluid: a prescription for central nervous system amyloidosis?

Over 70 transthyretin (TTR) mutations facilitate amyloidosis in tissues other than the central nervous system (CNS). In contrast, the D18G TTR mutation in individuals of Hungarian descent leads to CNS amyloidosis. D18G forms inclusion bodies in Escherichia coli, unlike the other disease-associated TTR variants overexpressed to date. Denaturation and reconstitution of D18G from inclusion bodies afford a folded monomer that is destabilized by 3.1 kcal/mol relative to an engineered monomeric version of WT TTR. Since TTR tetramer dissociation is typically rate limiting for amyloid formation, the monomeric nature of D18G renders its amyloid formation rate 1000-fold faster than WT. It is perplexing that D18G does not lead to severe early onset systemic amyloidosis, given that it is the most destabilized TTR variant characterized to date, more so than variants exhibiting onset in the second decade. Instead, CNS impairment is observed in the fifth decade as the sole pathological manifestation; however, benign systemic deposition is also observed. Analysis of heterozygote D18G patient's serum and cerebrospinal fluid (CSF) detects only WT TTR, indicating that D18G is either rapidly degraded postsecretion or degraded within the cell prior to secretion, consistent with its inability to form hybrid tetramers with WT TTR. The nondetectable levels of D18G TTR in human plasma explain the absence of an early onset systemic disease. CNS disease may result owing to the sensitivity of the CNS to lower levels of D18G aggregate. Alternatively, or in addition, we speculate that a fraction of D18G made by the choroid plexus can be transiently tetramerized by the locally high thyroxine (T(4)) concentration, chaperoning it out into the CSF where it undergoes dissociation and amyloidogenesis due to the low T(4) CSF concentration. Selected small molecule tetramer stabilizers can transform D18G from a monomeric aggregation-prone state to a nonamyloidogenic tetramer, which may prove to be a useful therapeutic strategy against TTR-associated CNS amyloidosis.     

Microglia and reactive “M” cells of degenerating central nervous system: Does similar morphology and function imply a common origin?

Summary Penetration of the median eminence and ventrolateral walls of the III ventricle by intraventricularly injected horseradish peroxidase was studied in rats. Experimental times varied between 10 to 70 min (short-term experiments) and 12 hrs to 30 days (long-term experiments).In short-term experiments, the median eminence was found to be completely stained whereas the lateral walls of the III ventricle were penetrated only up to 1 mm in depth. Spreading of the tracer takes place predominantly through the extracellular space and cellular uptake and transport do not seem to play a role during the first 70 min following the injection.In long-term experiments, the tanycytes exhibit a variety of intracellular inclusions marked by HRP precipitate. Tanycytic perikarya contain dense bodies and lipofuscin-like aggregates. Lipoprotein granules are thought to arise from these and are interpreted as lysosomal residual bodies. In tanycytic processes and perivascular endfeet, accumulations of HRP-containing tubules and polymorphous granules are encountered suggesting a transport towards the blood vessels of the portal plexus or of the arcuate and ventromedial areas, respectively. Sporadic tanycytes which are completely and evenly stained from the ventricular surface to the end of their processes were observed in the arcuate and ventromedial area. Whether this appearance can be taken as a sign of normal cell function seems doubtful. Some possible routes of transport through the median eminence—extracellular and transcellular—are summarized in a schematic drawing, taking into account the present findings and those published elsewhere in the literature.We gratefully acknowledge an Anglo-German Collaboration Grant of the Wellcome Trust enabling stimulating discussions with other workers in the field.     

Charles-Edouard Brown-Séquard: an eventful life and a significant contribution to the study of the nervous system

This is an account of the life of a 19th-century physiologist who was born in 1817 in Port-Louis (Mauritius Island, formerly 'Ile de France') and died in Paris in 1894. His mother tongue, education and medical training were French, but as the 'Ile de France' had become British a few years before his birth, he was a British citizen and therefore ineligible for a permanent position in a French institution. This explains, partly at least, his eventful life, during which he restlessly wandered during several decades between France, the United States, Great Britain and Mauritius, without ever finding a position that would satisfy him. This difficult period lasted until 1879 when, having finally acquired French nationality, he succeeded Claude Bernard in the chair of experimental medicine at the 'Collège de France'. Some of his contributions to the physiology of the nervous system are analysed: sensory pathways in the spinal cord, vasoconstrictor innervation, nervous inhibition and experimental epilepsy.     

Evaluation of genetic tests for susceptibility to common complex diseases: why, when and how?

Recent research into the human genome has generated a wealth of scientific knowledge and increased both public and professional interest in the concept of personalised medicine. Somewhat unexpectedly, in addition to increasing our understanding about the genetic basis for numerous diseases, these new discoveries have also spawned a burgeoning new industry of ‘consumer genetic testing’. In this paper, we present the principles learnt though the evaluation of tests for single gene disorders and suggest a comparable framework for the evaluation of genetic tests for susceptibility to common complex diseases. Both physicians and the general public will need to be able to assess the claims made by providers of genetic testing services, and ultimately policy-makers will need to decide if and when such tests should be offered through state funded healthcare systems.     

Combination of solid phase extraction and a microalgal test system based on pulse-amplitude modulated fluorescence to detect photosystem 11 herbicides up to 0.05 μeq l−1

This note reports a microalgal test system which usesin-vivo chlorophyll a-fluorescence in combination with a solid phase extraction procedure to monitor photosystem II-herbicides in natural waters up to 0.05 g L^–1.     

Contribution of nicotinic receptors to the function of synapses in the central nervous system: The action of choline as a selective agonist of α7 receptors

The α7-nicotinic receptor (nAChR)-selective agonist choline and nAChR-subtype-selective antagonists led to the discovery that activation of both α7 and α4β2 nAChRs located in CA1 interneurons in slices taken from the rat hippocampus facilitates the tetrodotoxin (TTX)-sensitive release of γ-aminobutyric acid (GABA). Experiments carried out in cultured hippocampal neurons not only confirmed that preterminal α7 and α4β2 nAChRs modulate the TTX-sensitive release of GABA, but also demonstrated that evoked release of GABA is reduced by rapid exposure of the neurons to acetylcholine (ACh, 10 μM-1 mM) in the presence of the muscarinic receptor antagonist atropine (1 μM). This effect of ACh, which is fully reversible and concentration-dependent, is partially blocked by superfusion of the cultured neurons with external solution containing either the α7-nAChR-selective antagonist methyllycaconitine (MLA, 1 nM) or the α4β2-nAChR-selective antagonist dihydro-β-erythroidine (DHβE, 100 nM). A complete blockade of ACh-induced reduction of evoked release of GABA was achieved only when the neurons were perfused with external solution containing both MLA and DHβE, suggesting that activation of both α7 and α4β2 nAChRs modulates the evoked release of GABA from hippocampal neurons. Such mechanisms may account for the apparent involvement of nAChRs in the psychological effects of tobacco smoking, in brain disorders (e.g., schizophrenia and epilepsy), and in physiological processes, including cognition and nociception.     

Na+-K+-Cl− cotransport system in brain capillary endothelial cells: Response to endothelin and hypoxia

Effect of endothelin-1 and chemically induced hypoxia on Na⁺−K⁺−Cl⁻ cotransport activity in cultured rat brain capillary endothelial cells was examined by using⁸⁶Rb⁺ as a tracer for K⁺; bumetanide-sensitive K⁺ uptake was defined as Na⁺−K⁺−Cl⁻ cotransport activity. Endothelin-1, phorbol 12-myristate 13-acetate (PMA), or thapsigargin increased Na⁺−K⁺−Cl⁻ cotransport activity. A protein kinase C inhibitor, bisindolylmaleimide, inhibited PMA- and endothelin-1- (but not thapsigargin-) induced Na⁺−K⁺−Cl⁻ cotransport activity, indicating the presence of both protein kinase C-dependent regulatory mechanisms and protein kinase C-independent mechanisms which involve intracellular Ca²⁺. Oligomycin, sodium azide, or antimycin A increased Na⁺−K⁺−Cl⁻ cotransport activity by 80–200%. Oligomycin-induced Na⁺−K⁺−Cl⁻ cotransport activity was reduced by an intracellular Ca²⁺ chelator (BAPTA/AM) but not affected by bisindolylmaleimide, suggesting the involvement of intracellular Ca²⁺, and not protein kinase C, in hypoxia-induced Na⁺−K⁺−Cl⁻ cotransport activity. Portions were presented at “27th Annual Meeting, The American Society for Neurochemistry” Philadelphia, Pennsylvania, March 2–6, 1996.