Polysialyltransferase overexpression in Schwann cells mediates different effects during peripheral nerve regeneration.

The polysialic acid (PSA) moiety of the neural cell adhesion molecule (NCAM) has been shown to support dynamic changes underlying peripheral nerve regeneration. Using transgenic mice expressing polysialyltransferase ST8SiaIV under control of a glial-specific (proteolipid protein, PLP) promoter (PLP-ST8SiaIV-transgenic mice), we tested the hypothesis that permanent synthesis of PSA in Schwann cells impairs functional recovery of lesioned peripheral nerves. After sciatic nerve crush, histomorphometric analyses demonstrated impaired remyelination of regenerated axons at the lesion site and in target tissue of PLP-ST8SiaIV-transgenic mice, though the number and size of regenerating unmyelinated axons were not changed. This was accompanied by slower mechanosensory recovery in PLP-ST8SiaIV-transgenic mice. However, the proportion of successfully mono-(re)innervated motor endplates in the foot pad muscle was significantly increased in PLP-ST8SiaIV-transgenic mice when compared with wild-type littermates, suggesting that long-term increase in PSA levels in regenerating nerves may favor selective motor target reinnervation. The combined negative and positive effects of a continuous polysialyltransferase overexpression observed during peripheral nerve regeneration suggest that an optimized time- and differentiation-dependent control of polysialyltransferase expression in Schwann cells may further improve recovery after peripheral nerves injury.

Level and localization of polysialic acid is critical for early peripheral nerve regeneration.

PolySia, the most striking post-translational modification of the neural cell adhesion molecule, is down-regulated during postnatal development. After peripheral nerve lesion, polySia is located on neuronal and glial cells normally not synthesizing polySia. However, structural consequences of reduced polySia content for peripheral nerve regeneration have not yet been clear. Furthermore, the contribution of sialyltransferases ST8SiaII and ST8SiaIV for the up-regulation of polySia has not been studied so far. In order to investigate the impact of polySia on regeneration processes of myelinated axons, we examined mouse mutants retaining only one functional sialyltransferase allele. In the absence of ST8SiaII, quantification of myelinated axons revealed a significant decrease in number and size of regenerated fibers without impairment of remyelination. In contrast, St8SiaIV deficiency resulted in increased fiber outgrowth and axonal maturation. Western blot analysis demonstrated that both ST8SiaII and St8SiaIV direct up-regulation of polySia. Cell-specific induction of polySia in myelinating Schwann cells and on regenerated axons in the presence of ST8SiaIV, but not ST8SiaII, indicates that not only the amount of polySia but also its cellular localization has a high impact on the regeneration progress of peripheral nerves.

Fibroblast growth factor (FGF)-2 and FGF receptor 3 are required for the development of the substantia nigra, and FGF-2 plays a crucial role for the rescue of dopaminergic neurons after 6-hydroxydopamine lesion.

Basic fibroblast growth factor (FGF-2) is involved in the development and maintenance of the nervous system. Exogenous administration of FGF-2 increased dopaminergic (DA) graft survival in different animal models of Parkinson's disease. To study the physiological function of the endogenous FGF-2 system, we analyzed the nigrostriatal system of mice lacking FGF-2, mice overexpressing FGF-2, and FGF-receptor-3 (FGFR3)-deficient mice both after development and after 6-hydroxydopamine lesion. FGFR3-deficient mice (+/-) displayed a reduced number of DA neurons compared with the respective wild type. Whereas absence of FGF-2 led to significantly increased numbers of DA neurons, enhanced amount of the growth factor in mice overexpressing FGF-2 resulted in less tyrosine hydroxylase expression and a reduced DA cell density. The volumes of the substantia nigra were enlarged in both FGF-2(-/-) and in FGF-2 transgenic mice, suggesting an important role of FGF-2 for the establishment of the proper number of DA neurons and a normal sized substantia nigra during development. In a second set of experiments, the putative relevance of endogenous FGF-2 after neurotoxin application was investigated regarding the number of rescued DA neurons after partial 6-OHDA lesion. Interestingly, the results after lesion were directly opposed to the results after development: significantly less DA neurons survived in FGF-2(-/-) mice compared with wild-type mice. Together, the results indicate that FGFR3 is crucially involved in regulating the number of DA neurons. The lack of FGF-2 seems to be (over)compensated during development, but, after lesion, compensation mechanisms fail. The transgenic mice showed that endogenous FGF-2 protects DA neurons from 6-OHDA neurotoxicity.

Mice lacking basic fibroblast growth factor showed faster sensory recovery.

Neurotrophic factors have been shown to stimulate and support peripheral nerve repair. One of these factors is basic fibroblast growth factor (FGF-2), which is up-regulated after peripheral nerve injury and influences early sciatic nerve regeneration by regulating Schwann cell proliferation. Our previous study on FGF-2 deficient mice indicated that FGF-2 is important for axonal maturation and remyelination one week after sciatic nerve crush (Jungnickel, J., Claus, P., Gransalke, K., Timmer, M. and Grothe, C., 2004. Targeted disruption of the FGF-2 gene affects the response to peripheral nerve injury. Mol. Cell. Neurosci. 25, 444-452). However, the functional impact of these effects on sensory and motor fibers was not clear. After performing pinch test, walking track analysis and rotarod, we found faster recovery of mechanosensory but not of motor function in mutant mice. To elucidate the role of FGF-2 on structural recovery, we analyzed FGF-2 deficient mice and wild-type littermates 2 and 4 weeks after sciatic nerve crush. Two weeks after peripheral nerve injury, regenerating fibers of mutant mice showed both significantly increased axon and myelin size, but no difference in the number of myelinated and unmyelinated fibers. Molecular analysis indicated that the expression level of myelin protein zero was significantly enhanced in lesioned nerves in the absence of FGF-2. These results suggest that loss of FGF-2 could positively influence restoration of mechanosensory function by accelerating structural recovery transiently.