Characterisation of the axonal transport dynamics of tetanus toxin in health and disease
Tetanus toxin is internalised at the neuromuscular junction into vesicular carriers undergoing fast retrograde transport to the spinal cord. We determined the pH regulation of this compartment in living motor neurons using a chimera of the tetanus toxin binding fragment (TeNT He) and a pH-sensitive variant of the green fluorescent protein (ratiometric pHluorin). We have demonstrated that moving retrograde carriers display a narrow range of neutral pHs, which is kept constant during transport. Stationary TeNT Hc-positive organelles instead exhibit a wide spectrum of pHs, ranging from acidic to neutral. This distinct pH regulation is due to a differential targeting of the vacuolar (H*) ATPase (vATPase), which is not present on moving TeNT He compartments. Accordingly, inhibition of the vATPase does not affect axonal retrograde transport of TeNT He. However, a functional vATPase is required for early steps of TeNT He trafficking following endocytosis, and it is localised to axonal vesicles containing TeNT He- Altogether, these findings indicate that the vATPase plays a specific role in early sorting events directing TeNT He to axonal carriers, but not in their subsequent progression along the retrograde transport route. This novel regulatory role for vATPase in a sorting event linked to retrograde transport adds to the numerous functions of this protein complex. Hydrogen peroxide inhibits the vATPase localised to synaptic vesicles, which leads to a disturbed trafficking of neurotransmitters. Increased levels of toxic oxygen radicals have been detected in mice overexpressing human mutant Cu/Zn superoxide dismutase (SOD 1093A) found in patients with familial amyotrophic lateral sclerosis (ALS). These mice develop motor neuron degeneration and muscle paralysis as observed in ALS patients. Evidence suggests that defects in axonal transport play an important role in neurodegeneration. We show that retrograde axonal transport defects are already present in motoneurons of S0D1G93A mice during embryonic development. Surprisingly, crossing S0D1G93A mice with mice, which have a single point mutation in the dynein motor complex (Loa) delays disease progression and significantly increases life span of Loa/SODlG93A mice. Moreover, we observed a complete recovery in axonal transport of these mice, which may be responsible for amelioration of the symptoms. We propose that impaired axonal transport is a prime cause of neuronal death in neurodegenerative disorders such as ALS.