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RESEARCH PRODUCT

Semaphorin 6A Improves Functional Recovery in Conjunction with Motor Training after Cerebral Ischemia

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Stroke is a major health problem in industrialized societies. Despite numerous attempts at developing acute stroke therapies aimed at minimizing acute infarct development, the only approved therapy so far is recombinant tissue plasminogen activator (rtPA). In recent years, the attention of the stroke community has therefore also put increased emphasis on understanding processes of post-stroke recovery, and their potential exploitability for therapeutic purposes. The brain has a remarkable ability to adapt to changes after stroke. Mechanisms that contribute to this plasticity are re-mapping and expansion of cortical areas to neighboring regions of functional motor cortex areas after injury [2]–[4], and reorganization of ipsilateral cortical regions distant from the injury [5]. The correlates of these plastic changes are changes in neuronal networks, mediated by the generation of new neurons (neurogenesis) [6], [7], changes in dendritic and synaptic morphology [8], and changes in long-distance connectivity, requiring axonal outgrowth and pathfinding. On a search for molecular determinants of those processes we have conducted a gene expression study in the photothrombotic model and searched for differentially expressed genes in the ipsi- and contralateral homotopic cortex [1]. One of the regulated genes appeared highly interesting in the context of post-stroke re-organization of neuronal networks after injury and axonal pathfinding to define new connections. This was a member of the semaphorin family of axonal pathfinding genes, Semaphorin 6A (Sema6A). This gene was induced ipsilaterally starting 48 h post ischemia, and increased in expression further at 21 d after ischemia (by quantitative PCR the regulation was determined as approximately 2-fold at 48 h, and 3-fold after 21 d in the ipsilateral cortex) [1]. At 21 d there was also significant upregulation detectable in the contralateral homotopic cortex (app. 2.5-fold). This induction on the mRNA level could be confirmed on the protein level by immunohistochemistry: At 48 h and 21 d, there was strong periinfarct expression of Semaphorin VIa in neurons, while induction on the contralateral homotopic cortex was only detected at 21 d. This regulation pattern suggested involvement in long-term plasticity processes in the periinfarct and homotopic contralateral cortex. Semaphorins are involved in many processes in development, including cell migration, axon guidance, or dendritogenesis [9]. Transmembrane semaphorins of class 6 are important in vivo mediators of axon guidance and cell migration in different parts of the brain. Sema6A is characterized by an extracellular semaphorin domain, a transmembrane domain, and a long cytoplasmic domain [10]. There is evidence that these transmembrane semaphorins and their invertebrate orthologues can also function as receptors, also due to the presence of the long cytoplasmic tail harbouring an Evl domain. Sema6A can repel sympathetic and dorsal root ganglion axons in vitro [11] suggesting that it fulfills the functions of an axonal guidance signal. Sema6A was identified in a gene-trap approach to find genes involved in axonal pathfinding [12]. The most striking phenotype of the Sema6A knock-out that was observed in this study was a defect in thalamocortical projections in homozygous k.o. animals [12]. In the cerebellum, Sema6A is involved in migration of granule cells [13]. Here, the receptor PlxA2 seems to be the responsible counterpart of Sema6A. Recently, a striking defect in the building of the corticospinal tract (CST) has been described in Sema6A mutants [14]. This function seems to require the PlxnA4. Here we have studied the effects of enhancing the expression of Sema6A in the recovery phase after cerebral cortical ischemia by AAV2-mediated gene transfer to the cortex.