Spatiotemporal dynamics of axonal reinnervation in the cerebellar cortex

Neuronal damage caused by brain injury or neurodegenerative disorders often leads to a loss of axonal innervation in distal target areas. Damaged axons typically do not regenerate in the central nervous system, but surviving axons can sprout new collaterals to re-innervate the denervated target area. At this point, it is unclear how long axons are capable of sprouting new collaterals after damage and the extent to which a surviving axon can expand its innervation area. To observe the spatiotemporal dynamics of collateral sprouting in the intact brain, we performed longitudinal time-lapse imaging of isolated cerebellar climbing fiber (CF) axons in vivo. Sub-populations of CFs were damaged by injecting neurotoxin 3-acetylpyridine into the inferior olivary nucleus (the origin of CFs) of transgenic mice that express enhanced green fluorescent protein (EGFP) in CF terminals. Time-lapse in vivo two-photon imaging of surviving, isolated EGFP-positive CFs revealed two distinct modes of axonal outgrowth: lateral outgrowth to expand CF innervation territory and outgrowth in the sagittal plane to innervate PCs within a CF’s existing territory. Lateral outgrowth appears to have a limited time window of 4-6 weeks after IO damage, but sagittal outgrowth continues after this period ends. Our data suggests that lateral expansion of CF innervation territory is likely guided by long-range attractive cues released by denervated PCs for a limited time after injury. On the other hand, sagittal expansion may be guided by constitutively expressed short range cues such as cell adhesion molecules expressed in the PC plasma membrane. Lateral outgrowth may allow CFs to innervate PCs in new functional zones, while sagittal outgrowth allows for the refinement of synaptic connections with PCs in a previously innervated functional zone.