Partner und Internationale Organisationen
(Englisch)
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A.J.W. Furley, Sheffield (UK), C. Stuermer, Konstanz (D), Gennarini, Bari (I), A. Munoz, Madrid (E), P. Sonderegger, Zürich (CH)
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Abstract
(Englisch)
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This project set out to determine the potential role of immunoglobulin-like cell adhesion molecules (IgCAMs) in nerve regeneration. Specifically, we were interested in whether TAG-1/axonin or the related molecule F3 might be necessary for axon growth in vertebrates. This was based on our preliminary observations that TAG-1 expression was downregulated abruptly following optic nerve lesion in rats, which cannot regenerate their CNS axons. Our objectives were to confirm these observations and to compare the results in rodents with TAG-1 expression after optic nerve lesion in fish, where axon regeneration can occur, to try and understand how TAG-1 regulated in rodents and, if different, in fish and then to test whether upregulation of TAG-1 expression in mammalian neurons after regeneration could influence the ability of those neurons to regenerate. Our hopes were to manipulate TAG-1 expression both transgenically and using pharmacological reagents, particularly thyroid hormone. As the related molecule F3 is known to be inhibitory to neurite growth in certain circumstances, we also aimed to investigate its role in regeneration.
An early discovery of our investigations was that TAG-1 expression was indeed upregulated during optic nerve regeneration in fish, although its expression was not evident until 12-14 days after lesion, thus making it unlikely that its function is necessary for the initial stages of axon regeneration. However, we were able to make a number of striking observations. In particular, TAG-1 protein was upregulated only on nasal axons and only once these axons made contact with their ultimate target, the tectum. We were able to show that this pattern of expression also occurs during development. These data suggest that TAG-1 may well be involved in topographic mapping of retinal axons onto to their targets in the tectum, both in development and during regeneration. In an attempt to isolate the genes that control this expression, we also initiated a subtractive hybridisation screen to look for genes that are regulated as retinal axons reach their targets in normal development, and this yielded a number of genes with restricted expression. In related experiments we were able to show that, in contrast to TAG-1, F3 expression is maintained on regenerating mammalian retinal axons, as it is also in fish. We have just initiated experiments to determine whether F3 function is necessary for axon regeneration in the fish.
We have also achieved some of our goals to develop ways to manipulate the expression of IgCAMs. First, we have successfully identified regions of the regulatory regions of TAG-1 and F3 that direct expression in specific regions of the CNS, including the developing and adult retina. Current studies using transgenic mice carrying reporter genes driven by some of these promoters are aimed at determining whether any of these regulatory elements respond to axotomy. Moreover, we have used some of the defined regulatory regions of TAG-1 to misexpress F3 in the developing nervous system, which has resulted in a specific CNS phenotype. Analysis of this phenotype is revealing important new functions of IgCAMs in neural development. We have also discovered that expression of TAG-1 and the related IgCAM L1 can be manipulated by thyroid hormone, and that these molecules are markedly upregulated in hypothyroid rats. Given that hypothyroidism can result in mental retardation in humans, and that L1 mutations also lead to mental retardation, these observations have opened up a new avenue of enquiry into the molecular basis of these human disease states. Moreover it is clear that thyroid hormone may prove a useful agent in reactivating developmentally controlled genes during nerve regeneration.
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