Chemotropic guidance of developing axons in the mammalian central nervous system

In the developing nervous system, axons project considerable distances along stereotyped pathways to reach their targets. Axon guidance depends partly on the recognition of cell-surface and extracellular matrix cues derived from cells along the pathways. It has also been proposed that neuronal growth cones are guided by gradients of chemoattractant molecules emanating from their intermediate or final cellular targets. Although there is evidence that the axons of some peripheral neurons in vertebrates are guided by chemotropism and the directed growth of some central axons to their targets is consistent with such a mechanism, it remains to be determined whether chemotropism operates in the central nervous system. During development of the spinal cord, commissural axons are deflected towards a specialized set of midline neural epithelial cells, termed the floor plate, which could reflect guidance by substrate cues or by diffusible chemoattractant molecules. Here we provide evidence in support of chemotropic guidance by demonstrating that the rat floor-plate cells secrete a diffusible factor(s) that influences the pattern and orientation of commissural axon growth in vitro without affecting other embryonic spinal cord axons. These findings support the hypothesis that chemotropic mechanisms guide developing axons to their intermediate targets in the vertebrate CNS.     

Loss of autophagy in the central nervous system causes neurodegeneration in mice

Protein quality-control, especially the removal of proteins with aberrant structures, has an important role in maintaining the homeostasis of non-dividing neural cells. In addition to the ubiquitin-proteasome system, emerging evidence points to the importance of autophagy--the bulk protein degradation pathway involved in starvation-induced and constitutive protein turnover--in the protein quality-control process. However, little is known about the precise roles of autophagy in neurons. Here we report that loss of Atg7 (autophagy-related 7), a gene essential for autophagy, leads to neurodegeneration. We found that mice lacking Atg7 specifically in the central nervous system showed behavioural defects, including abnormal limb-clasping reflexes and a reduction in coordinated movement, and died within 28 weeks of birth. Atg7 deficiency caused massive neuronal loss in the cerebral and cerebellar cortices. Notably, polyubiquitinated proteins accumulated in autophagy-deficient neurons as inclusion bodies, which increased in size and number with ageing. There was, however, no obvious alteration in proteasome function. Our results indicate that autophagy is essential for the survival of neural cells, and that impairment of autophagy is implicated in the pathogenesis of neurodegenerative disorders involving ubiquitin-containing inclusion bodies.     

Regenerating the damaged central nervous system

It is self-evident that the adult mammalian brain and spinal cord do not regenerate after injury, but recent discoveries have forced a reconsideration of this accepted principle. Advances in our understanding of how the brain develops have provided a rough blueprint for how we may bring about regeneration in the damaged brain. Studies in developmental neurobiology, intracellular signalling and neuroimmunology are bringing the regeneration field closer to success. Notwithstanding these advances, clear and indisputable evidence for adult functional regeneration remains to be shown.     

Visual processing in the leech central nervous system

In animals with complex visual systems spatial contrast is enhanced by mutual inhibition between retinal neurones monitoring different fields. By analogy, if animals like leeches and some caterpillars, that have several simple (non-image-forming) eyes aimed in different directions, are capable of rudimentary form detection, one might predict mutual antagonism between eyes monitoring different fields. In support of this prediction, I report here a paired interneurone in the central nervous system (CNS) of the leech which is stimulated by eyes on one side of the animal and inhibited by eyes on the other. There are striking parallels between these neurones and other integrating neurones, in particular those processing bilateral auditory input in crickets, suggesting that the visual system of the leech may be representative of a general class of sensory processing systems.     

中枢神经系统内的胰岛素及其受体

众所周知,胰岛素是一个小分子的蛋白质类激素,其主要作用是维持外周的血糖平衡,而对于中枢神经系统内胰岛素的研究则相对较少。大脑中非胰岛素依赖性葡萄糖的摄取,是大家认为脑是胰岛素不敏感器官的主要原因。然而,最近的研究结果显示,大脑的提取物中有高浓度的胰岛素;外周分泌的胰岛素,可以经过特异的转运系统穿过血脑屏障进入中枢神经系统。与大脑中胰岛素的来源、定位和功能的研究相比,中枢神经系统中胰岛素受体的表达也得到了相当高的关注。将从中枢神经系统尤其是脑内的胰岛素及其受体,中枢神经系统内胰岛素的功能,包括繁殖、食物摄入与体重控制、糖代谢调节和记忆等方面进行简要的综述。     

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.     

The myeloid cells of the central nervous system parenchyma

A microglial cell is both a glial cell of the central nervous system and a mononuclear phagocyte, which belongs to the haematopoietic system and is involved in inflammatory and immune responses. As such, microglia face a challenging task. The neurons of the central nervous system cannot divide and be replenished, and therefore need to be protected against pathogens, which is a key role of the immune system, but without collateral damage. In addition, after physical injury, neural cells need restorative support, which is provided by inflammatory responses. Excessive or chronic inflammatory responses can, however, be harmful. How microglia balance these demands, and how their behaviour can be modified to ameliorate disorders of the central nervous system, is becoming clear.     

A physiological role for GABAB receptors in the central nervous system

The role of GABA in synaptic transmission in the mammalian central nervous system is more firmly established than for any other neurotransmitter. With virtually every neuron studied, the synaptic action of GABA is mediated by bicuculline-sensitive GABAA receptors which selectively increase chloride conductance. However, it has been shown that GABA has a presynaptic inhibitory action on transmitter release that is insensiive to bicuculline and is selectively mimicked by baclofen. The receptors involved in this action are referred to as GABAB receptors, to distinguish them from the classic bicuculline-sensitive GABAA receptors. In hippocampal pyramidal cells an additional postsynaptic action of GABA and baclofen has been reported that is also insensitive to GABAA antagonists, and may be mediated by GABAB receptors on the postsynaptic neuron. This action of GABA and baclofen involves an increase in potassium conductance. Synaptic activation of pathways converging on hippocampal pyramidal cells results in a slow inhibitory postsynaptic potential which involves an increase in potassium conductance, and it has been suggested that GABAB receptors might be responsible for this synaptic potential. However, to establish convincingly that GABAB receptors are physiologically important in the central nervous system, a selective GABAB antagonist is required. Here we provide this missing evidence. Using the hippocampal slice preparation, we now report that the phosphonic acid derivative of baclofen, phaclofen, is a remarkably selective antagonist of both the postsynaptic action of baclofen and the bicuculline-resistant action of GABA, and that it selectively abolishes the slow inhibitory postsynaptic potential in pyramidal cells.     

Chemotropic effect of specific target epithelium in the developing mammalian nervous system

Developing nerve fibres are guided to their targets by specific directional cues which are thought to be expressed in the tissues along the route and may involve the extracellular matrix. Another possibility, that directional cues emanate from the target itself, is consistent with the recent demonstration of homing behaviour by ectopic retinal ganglion axons and our previous demonstration that early trigeminal neurites grow directly to their virgin peripheral target in vitro. Here we show that this chemotropic effect is precisely limited to the trigeminal system; trigeminal ganglion neurites grow directly to their own target field but not to the adjoining field, normally innervated by the geniculate ganglion; furthermore, the trigeminal field does not influence the growth of geniculate neurites. Also, when trigeminal ganglia are co-cultured with isolated tissue layers of their target, neurites grow only towards the epithelial and not the mesenchymal component. These findings suggest that trigeminal epithelium is specified to attract correct innervation and that pathway mesenchyme, in which preformed guidance cues have been postulated, may provide favourable conditions for nerve fibre growth but not govern its direction.     

Sensory neurones recognise defined pathways in Drosophila central nervous system

An analysis of the central projection of various sensory neurones in the homoeotic mutant bithorax postbithorax, and in flies where an adult nerve had been experimentally misrouted, reveals that neurones are able to develop a normal projection even if they enter the central nervous system at an unusual place.     

ATP is a mediator of chemosensory transduction in the central nervous system

Extracellular signalling by the purine nucleotide ATP has long been associated with sensory function. In the periphery, ATP mediates nociception, mechanosensitivity, thermal sensitivity and O2 chemosensitivity. These processes share a common mechanism that involves the release of ATP to excite afferent fibres via activation of ionotropic P2X and/or metabotropic P2Y receptors. Chemosensors located in the brainstem are crucial for the maintenance of physiological levels of blood gases through the regulation of breathing. Here we show that an increase in pCO2 in the arterial blood triggers the immediate release of ATP from three chemosensitive regions located on the ventral surface of the medulla oblongata. Blockade of ATP receptors at these sites diminishes the chemosensory control of breathing, suggesting that ATP release constitutes a key step in central chemosensory transduction. These new data suggest that ATP, a phylogenetically ancient, unique and simple molecule, has been widely used in the evolution of afferent systems to mediate distinct forms of sensory transduction not only in the periphery but also within the central nervous system.     

T cells become licensed in the lung to enter the central nervous system

The blood–brain barrier (BBB) and the environment of the central nervous system (CNS) guard the nervous tissue from peripheral immune cells. In the autoimmune disease multiple sclerosis, myelin-reactive T-cell blasts are thought to transgress the BBB and create a pro-inflammatory environment in the CNS, thereby making possible a second autoimmune attack that starts from the leptomeningeal vessels and progresses into the parenchyma. Using a Lewis rat model of experimental autoimmune encephalomyelitis, we show here that contrary to the expectations of this concept, T-cell blasts do not efficiently enter the CNS and are not required to prepare the BBB for immune-cell recruitment. Instead, intravenously transferred T-cell blasts gain the capacity to enter the CNS after residing transiently within the lung tissues. Inside the lung tissues, they move along and within the airways to bronchus-associated lymphoid tissues and lung-draining mediastinal lymph nodes before they enter the blood circulation from where they reach the CNS. Effector T cells transferred directly into the airways showed a similar migratory pattern and retained their full pathogenicity. On their way the T cells fundamentally reprogrammed their gene-expression profile, characterized by downregulation of their activation program and upregulation of cellular locomotion molecules together with chemokine and adhesion receptors. The adhesion receptors include ninjurin 1, which participates in T-cell intravascular crawling on cerebral blood vessels. We detected that the lung constitutes a niche not only for activated T cells but also for resting myelin-reactive memory T cells. After local stimulation in the lung, these cells strongly proliferate and, after assuming migratory properties, enter the CNS and induce paralytic disease. The lung could therefore contribute to the activation of potentially autoaggressive T cells and their transition to a migratory mode as a prerequisite to entering their target tissues and inducing autoimmune disease.     

Transvascular delivery of small interfering RNA to the central nervous system

A major impediment in the treatment of neurological diseases is the presence of the blood-brain barrier, which precludes the entry of therapeutic molecules from blood to brain. Here we show that a short peptide derived from rabies virus glycoprotein (RVG) enables the transvascular delivery of small interfering RNA (siRNA) to the brain. This 29-amino-acid peptide specifically binds to the acetylcholine receptor expressed by neuronal cells. To enable siRNA binding, a chimaeric peptide was synthesized by adding nonamer arginine residues at the carboxy terminus of RVG. This RVG-9R peptide was able to bind and transduce siRNA to neuronal cells in vitro, resulting in efficient gene silencing. After intravenous injection into mice, RVG-9R delivered siRNA to the neuronal cells, resulting in specific gene silencing within the brain. Furthermore, intravenous treatment with RVG-9R-bound antiviral siRNA afforded robust protection against fatal viral encephalitis in mice. Repeated administration of RVG-9R-bound siRNA did not induce inflammatory cytokines or anti-peptide antibodies. Thus, RVG-9R provides a safe and noninvasive approach for the delivery of siRNA and potentially other therapeutic molecules across the blood-brain barrier.     

Segment-specific expression of a zinc-finger gene in the developing nervous system of the mouse

The process of segmentation, in which repeated homologous structures are generated along the anterior-posterior axis of the embryo is a widespread mechanism in animal development. In vertebrates, segmentation is most apparent in the somites and the peripheral nervous system, but the existence of repetitive bulges, termed neuromeres, in the early neural epithelium of vertebrates suggests that the CNS may also be segmented. Consistent with this, cranial ganglia and certain neurons are associated with specific hindbrain neuromeres. Here, we report that Krox-20, a zinc-finger gene, is expressed in two alternate neuromeres in the mouse early hindbrain. This pattern subsequently decays and Krox-20 is transiently expressed in specific hindbrain nuclei. In addition, Krox-20 is expressed in early neural crest cells, and then in the neural crest-derived boundary caps, glial components of the cranial and spinal ganglia. The demonstration that neuromeres are domains of gene expression provides molecular evidence for the segmentation of the CNS.