| A very large number of inherited and acquired degenerative conditions could negatively impact upon the central nervous system (CNS). Damage to neurons and their fibre tracts nearly can not be repaired. Because there is little or no intrinsic capacity for replacement of lost or dying neurons and minimal spontaneous repair of fibre pathways in most areas of the mammalian CNS. Traumatic or vascular injuries in brain and loss of neurons because of chronic degenerative disease, all result in long-lasting functional impairments. The repair of injured CNS is especially difficult after large lesions or in the chronic situation. It's now widely accepted that the limited regeneration of injured CNS is mostly due to the myelin inhibitor Nogo.nogo gene has three splice variants, Nogo-A,Nogo-B and Nogo-C. All three Nogo proteins have a same carboxyl terminus of 188 amino acids which contains two long hydrophobic domains and a short loop of 66 amino acids between the two hydrophobic domains, called Nogo-66. The Nogo-66 Receptor, NgR, which is widely expressed in many neurons, has been characterized as a receptor subunit that binds to the Nogo-66 site. As a GPI-linked cell surface protein, NgR lacks an intracellular domain and requires either p75 or TROY as co-receptors to mediate signal transduction across the cell membrane. Both are transmembrane proteins of the tumour necrosis factor receptor family. LINGO-1, another co-receptor, acts as an adaptor protein is also required for Nogo signaling. Interestingly, two other structurally unrelated myelin-associated growth inhibitors, MAG and OMgp, bind to NgR with high affinity as well. This means three myelin-associated inhibitory molecules, Nogo, MAG and OMgp, converge on a single receptor complex and share a common intracellular signaling pathway.However, the Nogo and NgR alone seems not to be able to explain the absence of regeneration in the CNS. Corticospinal tract neurons of NgR knock out mice do not regenerate after SCI. Moreover, although the Nogo-A-specific region does not bind to NgR, it inhibits neurite outgrowth and prevents spreading of different cell types. Addtionally, most neurons in the spinal cord express NgR not at all except motor neurons express it weakly. And NgR expression vary in the primary sensory neurons, although none express it strongly, and others not at all. If NgR is the principal neuronal receptor Nogo inhibits axonal regeneration in the CNS, the marked differences in NgR expression by different classes of neurons would appear to limit the range of neurons likely to be susceptible to the growth-inhibitory effects of Nogo-66, however, in fact it is not the truth. And whether NgR express in the Purkinje cells or it's distribution in the spinal cord is still uncertain. The neutralization of Nogo-66 domain activity brings limited axon regeneration after injury implicate that there are still unsolved mysteries about the axon growth inhibitory of Nogo, even two new Nogo-66 receptors, PirB and GPR50 were discovered separately in 2008 and 2009.In addition, there are few studies about Nogo and NgR expression in the visual pathway. Although the visual system is the preferred model in many researches to study neuroprotection and regenerative success or failure in the mature mammalian CNS. Such as the well-defined neuroanatomy in the retina, with seven major cell types and a circuitry that has been intensively investigated, intravitreal injections can be directly targeted in RGC, aspects of retinal function can be assessed using quantitative, non-invasive techniques, the optic nerve is a discrete, centrally derived white matter tract that is readily accessible within the orbit the extent of RGC survival and the amount of axonal regrowth after injury can readily be quantified, and because of the well-characterized anatomical and functional organisation of central visual pathways, it is possible to examine the connectivity and topographic order of regenerating RGC axons and assess the functional and behavioural consequences of neural plasticity and repair. Thus, to discover the expression of Nogo and NgR in the visual pathway can be well used not only study the regeneration of CNS after injury, but the interaction of Nogo and NgR.In the year 2008, after well studied many kinds of cells, Dr Hu of Yale University School of Medicine showed that another active site of Nogo-A, that different from the Nogo-66 domain, the N-terminal domain (Amino-Nogo) may exert the function via integrin receptor. And they also foundαv,α5, andα4 integrins are sensitive to Amino-Nogo. However, there are only one kind of neuron in this cells study, chick dorsal root ganglion neuron. Althoughαv andα5 integrins are widely expressed in the CNS,α5 integrin is absent in the chick retina andαv integrin is absent in the human retina. This challegened Amino-Nogo-integrin signaling pathway theory.The purposes of this study are: to discover the expression of Nogo and NgR in the visual pathway in order to provide theory basis to well study the regeneration of CNS after injury and the interaction of Nogo and NgR; to explore the signaling pathway of Amino-Nogo in the retina, offering a new target to therapy the CNS injury for the clinic.The study is divided into two parts: 1. The expression patterns and change of Nogo and NgR in the rat visual pathway before and after bilateral optic nerve transected. 2. By means of immunohistochemistry to study colocalization, immunoprecipitation to study the interaction, we exploring whether Amino-Nogo has integrin binding site in the retina.The main results and conclusions are as follows:1. Nogo and NgR are expressed in the cell body of retina, lateral geniculate body and visual cortex of neonatal rat. Our results provide proofs that Nogo-NgR system may take part in visual system plasticity, axons guiding. Because in the absence of NgR, Nogo can also regulate Ca2+ in the endoplasmatic reticulum (ER), which may affect cell migration, axon extension, and the growth cone direction. Thus, even Nogo and NgR are expressed in the same area, whether Nogo function exerting by NgR is still need further study.2. Nogo and NgR are expressed throughout the visual pathway, including retina, optic nerve, optic chiasm, optic tract, lateral geniculate body, and visual cortex of adult rat. After bilateral optic nerve transected, the expression of Nogo in the optic tract and lateral geniculate body increasing until 14 days later, but NgR in the same area and optic nerve does not change. Nogo and NgR can not be observed in the retina after injury. Although Nogo increased in the optic nerve as soon as injury, the change of it that decreased in the optic chiasm could only be detected 14 days later afer injury. However, NgR in the optic chiasm decreased at time of injury and 14 days later. Although Nogo increased in the visual cortex after injury, it is higher in the 7th day than 14th day. And NgR in the same area decreased more obviously in the 7th day than 14th day. Thus we concluded that the change expression pattern of Nogo and NgR in the optic nerve, optic tract, and lateral geniculate body that can be explained with Nogo-NgR system. However, in the rest area, the action mechanism of Nogo must be more complicated.3. The evidence from colocalization, immunoprecipitation studies shows that Amino-Nogo hasαv integrin binding site in the adult retina. Nogo limitation of axonal growth in the adult CNS appears to function may by two independent signaling pathways. The Nogo-66 domain enhanced by an adjacent binding site stimulates NgR, and the Amino-Nogo domain bind to integrin. The new signaling pathway hypothesis suggestion contribute additional sites to develop therapeutic approaches to stimulate axonal growth and behavioral recovery from adult neurological deficits, however, there is still need more detailed study before this hypothesis could be draw. |
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