Perspectives on developmental neurobiology最新文献

Navigating growth cones need signal transduction machinery to amplify and transmit the effects of extracellular signals throughout the growth cone. In culture, many drugs that affect second messengers are known to modulate neurite extension (with different effects on different neurons), and gradients of calcium influx and cyclic nucleotide analogs can cause growth cones to turn. However, it is not clear which of these responses are physiologically relevant, as axons grow through much more complex environments in vivo. The "exposed brain" preparation in Xenopus embryos provides an experimentally tractable system in which it is possible to study growth, pathfinding, and target recognition of retinal growth cones in vivo, while pharmacologically manipulating their signal transduction systems. These growth cones can also be easily studied in explant culture. We describe preliminary results of parallel in vivo and in vitro experiments using an array of drugs that perturb transduction molecules. Surprisingly, calcium ionophores and cyclic nucleotide analogs have no significant effect on retinal axon growth or pathfinding. Several agents including herbimycin A, ML-7, mastoparan, and RHC80267 inhibit retinal axon growth, both in vivo and in vitro, suggesting that tyrosine kinases, myosin, heterotrimeric G-proteins, and diacylglycerol lipase are important for retinal growth cones navigating in the optic pathway.

Axonal pathfinding occurs through detection of environmental cues by cytoskeletal machinery that is responsible for growth cone migration. The cycle of filopodial and lamellipodial protrusion, adhesion, and generation of tensions to advance a growth cone result from concerted actions of ABPs to regulate actin filament polymerization, assembly into networks and bundles, and production of tension to move the growth cone and its contents. The direction of neurite elongation is controlled by forward movement of microtubules in growth cones, which is pioneered by the advance of microtubules into P domain of the leading margin. Actin filaments both promote and impede this advance of microtubules in several ways. This cytoskeletal machinery is controlled by major signaling mechanisms. To understand growth cone guidance we must reveal the spatial and temporal changes generated in [Ca++]i, phospholipids, and protein phosphorylation and dephosphorylation, and then identify the ABPs and MAPs that are their targets.

Axonogenesis is the earliest step in acquisition of neuronal polarity. The subcellular mechanisms underlying this pivotal event are unknown. Because of the abundant presence and functional necessity of microtubule-associated proteins in growing neurites, a large effort has been directed at characterizing their role in establishment and maintenance of neuronal polarity. One unsolved puzzle is how MAPs, most of which are unpolarized in early stages of development, can locally influence microdifferentiation of axons and dendrites. In this review, we discuss recent evidence suggesting that locally controlled phosphorylation of microtubule-associated proteins tau and MAP1B may play a role in establishment of polarity and early axonal outgrowth.

Protein-tyrosine kinase, such as those of the trk and Eph families, serve as membrane receptors for extracellular cues which regulate the rate and direction of growth of numerous groups of axons. Certain cytoplasmic protein-tyrosine kinases, such as src, are also abundant in growth cones. But, how protein-tyrosine phosphorylation regulates the growth cone is poorly understood. We discuss here potential roles for tyrosine phosphorylation in the protrusive structures of the growth cone, especially filopodia, which are important in detecting cues. A particular focus is the integrin receptor for substrate-bound growth promoters like laminin. Changes in tyrosine phosphorylation may be important in both facilitating and mediating the interaction of filopodia with these growth promoters.

Adult peripheral neurons undergo dramatic shifts in gene expression following axotomy that are collectively referred to as the cell body reaction. Changes in neuropeptide expression are a prominent feature of these axotomized neurons. For example, while sympathetic, sensory, and motor neurons do not normally express the neuropeptides galanin and vasoactive intestinal peptide, they begin to do so within days after axotomy. In contrast, the expression of other peptides, which these neurons normally express, such as neuropeptide Y in sympathetic neurons and substance P in sensory neurons, is decreased. Recent studies in sympathetic neurons have demonstrated that leukemia inhibitory factor plays an important role in triggering these changes in neuropeptide phenotype in adult neurons. Future studies will be directed at determining to what extent LIF triggers the many other changes in gene expression after sympathetic axotomy and whether this cytokine plays a similar role in sensory and motor neurons.

Cultured Xenopus spinal neurons exhibit chemotropic turning toward the source of neurotransmitters acetylcholine and glutamate. Here we review the experimental evidence that transmitter-induced turning of the growth cone is mediated by an influx of Ca2+, that a gradient of intracellular Ca2+ within the growth cone is responsible for the directional growth cone response, and that asymmetric filopodia formation precedes and is essential for the turning response.

Retinal ganglion cell axons become reordered as they pass through the chiasmatic region of the optic pathway. Studies in carnivores and rodents show that the fiber order established in the optic tract is a chronological index of their arrival time during development and that the cause of the reordering may relate to the changing glial environment, as well as to the spatial and temporal distribution of proteoglycans within the developing pathway. Primate optic axons become similarly reordered, allowing one to predict a developmental sequence of ganglion cell genesis from fiber position within the mature optic tract. Fiber position within the tract also anticipates the pattern of geniculate innervation, but a prominent exception to this rule is found in the prosimian Galago. The chronotopic reordering is found in every mammalian species that has been examined, including eutherians and metatherians, suggesting that the mechanism producing it is evolutionarily conserved.

The identification of compounds that can protect cells against death induced by exposure to noxious stimuli and against programmed cell death (apoptosis) associated with exposure to inadequate amounts of trophic factors is of great interest in contemporary biology. We have found that N-acetyl-L-cysteine (NAC) is able to promote cell survival in these two distinct experimental paradigms of, respectively, "death by murder" and "death by neglect." In the former case, NAC prevented the death of oligodendrocytes induced by glutamate or tumor necrosis factor-alpha (TNF-alpha), and also prevented TNF-alpha-induced death of L929 cells. NAC also acted in synergy with ciliary neurotrophic factor (CNTF) to prevent killing of oligodendrocytes by TNF-alpha. In analysis of "death by neglect," NAC markedly enhanced the extent of spinal ganglion neuron survival obtained with suboptimal concentrations of nerve growth factor and of oligodendrocyte survival obtained with suboptimal concentrations of CNTF or insulin-like growth factor-1. Surprisingly, significant rescue of oligodendrocytes from apoptosis was also observed with combinations of NAC with progesterone, vitamin C, or Trolox, a water-soluble vitamin E analogue, although not with any of these compounds applied individually. These results demonstrate that cocktails of small molecules such as those we have studied may have beneficial effects not predictable from the action of any individual member of the cocktail. In light of the long clinical history of therapeutic use of NAC and the other compounds identified in our studies, we suggest that it may be of interest to examine use of NAC alone, or combinations of NAC with the other small molecules we have studied, in conditions in which certain toxin-mediated forms of cell death or apoptosis contribute significantly to disease.

Many growth factors that have effects on neurons are present in the developing and mature central nervous system where their functions are poorly understood. In the peripheral nervous system, target-derived growth factors can regulate the survival of developing afferent neurons. Recent studies suggest that neurotrophins derived from target neurons may also regulate the survival of afferent basal forebrain cholinergic neurons during development of the central nervous system. However, in mature animals, these cholinergic neurons do not appear to require target-derived neuroptrophins for survival. Similar findings have also been reported for dopaminergic neurons of the substantia nigra. Although further studies are required, available experimental evidence suggests that target-derived growth factors may influence neuronal phenotypes such as axonal sprouting or transmitter production instead of survival in mature animals.

An intricate interplay between neurotrophic factor and excitatory transmitter signaling systems exists in both the developing and adult brain. Interactions between these signaling systems appears to be a fundamental mechanism regulating adaptive neuritic pruning and cell death. Accordingly, genetically and environmentally induced imbalances in this regulatory system are implicated in the pathogenesis of a variety of acute (such as stroke and traumatic brain injury) and chronic (such as Alzheimer's and Parkinson's diseases) neurodegenerative disorders. Neurons exhibit both acute and delayed responses to neurotrophic factors and excitatory transmitters; acute responses include rapid structural remodeling of growth cones and synaptic contacts, and delayed responses include induction or suppression of the expression of gene products involved in neuroprotection. Intracellular free Ca2+ and free radicals appear to play key roles as mediators of both acute and delayed responses of neurons to excitatory transmitters and neurotrophic factors. For example, the delayed response to bFGF includes stabilization of Ca2+ homeostasis and induction of antioxidant enzymes; both of these actions of bFGF antagonize the dendrite outgrowth-stabilizing and excitotoxic actions of glutamate. Intricate regulatory interactions exist between glutamate and neurotrophic factor signaling systems so that glutamate can induce the expression of neurotrophic factors and their receptors, and neurotrophic factors modulate the expression of exitatory transmitter receptors. A novel signaling system that can interact with both glutamate and neurotrophic factor systems is that of the beta-amyloid precursor protein, which appears to play important roles in neuronal plasticity and survival. A working model for the regulation of neuronal survival and connectivity is presented, which considers spatial and temporal constraints on release of, and receptors for, neurotrophic factors and excitatory transmitters.