Perspectives on developmental neurobiology最新文献

Ciliary neurotrophic factor (CNTF) was first identified as a trophic activity that was able to support the survival of chick ciliary ganglion (CG) neurons in vitro. CNTF from rabbit and rat were subsequently purified from sciatic nerve and their cDNA sequences cloned. Another trophic molecule for CG neurons was identified as a growth promoting activity (GPA). GPA was purified from chicken sciatic nerve and cloned from embryonic chicken eye. The rat and rabbit CNTFs have a considerable amount of structural homology and are not secreted in significant quantities, whereas GPA is less similar in that it is only 49% homologous with rabbit and rat CNTF and is secreted by cells. This review discusses other similarities and differences in biological activities, molecular structure, receptor signaling and cellular distribution between CNTF and GPA and suggests that these molecules may have different functions in rodents and birds.

Ciliary neurotrophic factor (CNTF) and growth promoting activity (GPA) are two members of a family of structurally and functionally related cytokines. Although the primary sequences of these proteins are only distantly related, many share striking functional similarities. The question of the potential existence of more, as yet undiscovered, members of this family, especially those most related to CNTF, is discussed. There are several biological systems which exhibit unexplained CNTF-like activities. This has led to speculation that there are indeed other CNTF-like proteins to be found. Because of the poor primary sequence conservation among known members of this family, even those sharing strong functional similarities, it is unlikely that a cloning approach based on sequence homology will find these putative new members of the family. Instead, a more biological approach, based on functional similarities, is more likely to succeed.

Recent studies dealing with the organization of retinal projections in the developing rhesus monkey brain have revealed a high degree of developmental specificity. This is demonstrated by the ingrowth patterns of the initial contingents of crossed and uncrossed fibers that form the primordial optic tract as well as by the adult-like nasotemporal retinal decussation pattern evident even before the period of ganglion cell death. On the basis of these observations, it is suggested that early generated retinal fibers are guided through the optic chiasm by a transiently expressed decussation signal, and that later generated fibers utilize retinal position-dependent cues to innervate the appropriate hemisphere. Furthermore, the first retinal fibers to arrive at the dorsal lateral geniculate nucleus invade only the presumed parvocellular layers. Thus, the initial innervation of the lateral geniculate nucleus appears to reflect the birth order of retinal ganglion cell classes. It is suggested that the high degree of precision evident in the macaque monkey nasotemporal retinal decussation pattern relates to the adultlike distribution of callosal projection neurons in the developing striate cortex of the primate.

N-syndecan is a member of the syndecan family of transmembrane heparan sulfate proteoglycans that was cloned initially from neonatal rat Schwann cells and is the principal syndecan expressed during early postnatal development in the central and peripheral nervous systems. Purified N-syndecan binds in vitro with high affinity to several extracellular regulatory ligands, including basic fibroblast growth factor, the secreted adhesive protein heparin binding growth-associated molecule, and a novel collagen-like protein secreted by Schwann cells. These extracellular ligands utilize the heparan sulfate chains of N-syndecan for binding. Based on the striking amino acid sequence homology of the cytoplasmic domain of N-syndecan to syndecan-1, it is proposed that N-syndecan associates with the actin-based cytoskeleton. N-syndecan core proteins self associate by means of an unusual dimerization motif comprised of the transmembrane domain and a short flanking sequence in the ectodomain. Similar to other single transmembrane domain receptor proteins, it is suggested that ligand-regulated dimerization of N-syndecan represents a mechanism for regulating downstream signaling activities. In rat brain tissue a significant fraction of the N-syndecan molecules are present in a soluble form, presumably as a result of proteolytic membrane shedding. A model is presented for morphoregulatory activity of N-syndecan in which extracellular ligand-induced clustering of N-syndecan molecules on the cell surface promotes cytoskeletal association and reorganization. Membrane shedding separates the functional domains of the proteoglycan and terminates this activity.

During development, the extracellular matrix (ECM) is a complex dynamic structure whose components and organization help to establish the requisite position and state of differentiation. Until recently, the large chondroitin sulfate proteoglycan, aggrecan, has been localized predominantly to skeletal tissue and considered a hallmark of cartilage differentiation. We have identified the presence of aggrecan in two other highly differentiated systems, brain and notochord, with clearly distinct expression patterns. In chick cartilage, aggrecan starts to be expressed at embryonic day 5 in limb rudiments, continues through the entire period of chondrocyte development, and remains a biochemical marker of the cartilage phenotype thereafter. In brain, aggrecan has a very low level of expression beginning at day 7, increases up to day 13, markedly decreases after day 16, and is not expressed posthatching. This pattern coincides with migration and establishment of neuronal nuclei in the chick telencephalon and has been proposed to be a component of the migration arrest mechanism. In very primitive embryos, aggrecan is detected as early as stage 16 in the notochord, long before chondrogenesis occurs, is then expressed up to day 5 and decreases thereafter. The expression of aggrecan occurs during the time of active neural crest migration and through the onset of sclerotomal differentiation, and correlates with the notochords' ability to inhibit neural crest cell migration. Animal models defective in aggrecan biosynthesis have been invaluable in delineating these functions. In addition we have characterized these proteoglycans by chemical, biosynthetic, and molecular analyses. Although significant post-translation differences distinguish the cell-specific aggrecan species, their core proteins are the products of a single gene. Our findings of the expression of the same gene (aggrecan) in multiple ontogenously unrelated differentiating tissue systems and at different times over the developmental life of an organism provide an elegant model system to study the regulation and interplay in expression of that gene, as well as the effect of alterations in that single gene simultaneously in several developing programs.

A diverse set of proteoglycans is expressed in the developing and adult brain. This is in stark contrast to the fact that most extracellular matrix components, including fibronectin, laminin, and collagens, are not expressed in adult brain parenchyma. This suggests that proteoglycans may play a major functional role in cell-cell and cell-matrix interactions in the brain. Brevican is a member of the aggrecan/versican family of proteoglycans, containing a hyaluronic acid-binding domain in its N-terminus and a lectin-like domain in its C-terminus. Brevican has the smallest core protein among this family and is one of the most abundant chondroitin sulfate proteoglycans in the adult brain. Expression of brevican is highly specific in the brain and increases as the brain develops. These observations suggest that brevican may play a role in maintaining the extracellular environment of mature brain as a major constituent of the adult brain extracellular matrix.

The survival of developing motor neurons has long been known to depend on contact with target muscle. This observation caused an intensive search for motor neuron trophic factors. During that search, a surprisingly large number of factors, including neurotrophins, glia-derived neurotrophic factor, fibroblast growth factors, and ciliary neurotrophic factor (CNTF) were found to promote motor neuron survival in vitro. The present review article examines in detail the evidence concerning the potential motor neuron trophic role of CNTF in vivo. The main conclusion of the article is that CNTF likely functions as a maintenance and repair factor for adult motor neurons and is less likely to play a significant developmental role. In addition, the article reviews the literature concerning the use of CNTF for treating motor neuron diseases and possible side effects of such treatment.

A cardinal event in the development of all brain structures is the time at which progenitor cells leave the cell cycle and begin to differentiate. We examined cell genesis in the retina of the macaque monkey (Macaca mulatta) by labeling dividing cells with radioactive thymidine ([3H]TdR) and following their fate at terminal division by virtue of their remaining radiolabeled after a long survival period. A number of distinct patterns of cell genesis were observed. The two tissues generated by the optic vesicle, the retinal pigment epithelium and neuroretina, share closely coincident temporal and spatial patterns of cell genesis, indicating that this process may be controlled by a common mechanism. Although overlapping to varying degrees, a clear sequence of genesis was revealed between specific cell types within the neuroretina: ganglion cells are generated first, followed by horizontal cells, cone photoreceptors, amacrine cells, Müller cells, bipolar cells, and, finally, rod photoreceptors. Retinal ganglion cells of differing soma diameter are born at different times-the smallest cells are generated early, the largest late, suggesting a further refined sequence of the functional classes of monkey retinal ganglion cells (first P gamma, then P beta, last P alpha). In addition, at sites where a homogeneous population of cells are crowded and stacked on top of each other (the foveola and perifovea for cones and ganglion cells, respectively) there is a vitreal-to-scleral intralaminar pattern of [3H]TdR labeled cell placement, which reflects both time of genesis and pattern of movement during foveation. These gradients suggest several scenarios for cell fate specification in the retina, many of which might not be obvious in mammals that develop more quickly and have less specialized retinal structure. Thus, data from the highly specialized and slowly developing macaque retina can help to understand visual development in humans and indicate useful avenues for future experimental studies in other species.

The retinotectal projection in chick is a well studied model system for axon guidance. The 'stripe assay' provides a unique tool for investigating underlying molecules and mechanisms of axon guidance by non-diffusible substrate bound molecules in vitro. By combining this assay with a modified 'Campenot chamber', we have now investigated the role of several second messenger systems in this type of axon guidance by confronting growing axons with various drugs that are known to influence intracellular signaling. We have shown that extracellular, and most probably intracellular Ca++ is not required for this type of axon guidance, which also rules out the need for Ca++-dependent adhesion molecules like cadherins. While at least calmodulin and protein kinase C seem to be involved in axon elongation, inhibiting their function did not alter the growth cones' choice. Inhibition of other kinases, G-proteins and signaling components also failed to influence this guidance. These results may indicate that parallel signaling pathways take part in the molecular mechanism of this type of axon guidance.

The brain must balance the need for synaptic precision with the ability to generate and change connectivity patterns in response to environmental stimuli. GAP-43 is a phosphoprotein associated with the cytosolic surface of the membrane, and is one of the most abundant among the small subset of total cellular proteins transported to the growth cone. It appears to play an unusual role amplifying signals from the microenvironment. One of the several ways to perform this task involves interaction of GAP-43 with the G protein transduction cascade. In mice rendered GAP-43 null by homologous recombination, some nerves manifest aberrant growth at decision points, such as the optic chiasm. Thus, GAP-43 may work via modulation of signaling cascades, rather than autonomously causing growth, and could serve to keep plasticity within constraints needed to generate and maintain accurate synaptic wiring.