Structural plasticity of the adult cerebellar climbing fibres: an in vivo study on the role of the growth-associated protein 43

The growth-associated protein GAP-43 has a pivotal role in axonal growth and guidance during development. Towards the end of postnatal development its expression dramatically declines in most brain regions with few exceptions, such as cerebellar cortex and inferior olive (IO). When experimentally over-expressed in murine CNS in vivo, it is able to induce axonal sprouting, moreover it is up-regulated in conditions known to induce a structural remodelling of neuronal connectivity, such as injuries to the CNS and neurodegenerative disorders. However the reason why high expression is maintained through adulthood only in some regions in the adult brain has never been clarified. Taking advantage of lentiviral vectors and shRNAs to induce an efficient and stable gene silencing specifically in the IO, we chose to investigate GAP-43 role in axonal remodelling of the adult CFs, since they originate from one of the nuclei which retain GAP-43 high expression levels, they are conveniently arranged for morphological studies and they are endowed with profound plastic and regenerative potentials. Here we show in vivo, in adult rats, that silencing of GAP-43 causes an atrophy of olivocerebellar terminal axons and significant modification of its presynaptic boutons. These data suggest that GAP-43 plays a pivotal role in the maintenance of axonal structure of CF and of the organization of presynaptic plasma membrane in physiological conditions and it plays a complex role in CF lesion-induced sprouting. Finally, by means of two-photon microscopy and laser nanosurgery, we describe here for the first time in vivo a lesion-induced axonal sprouting in the mammalian CNS.