BDNF-dependent modulation of axonal transport is selectively impaired in ALS

Abstract

Axonal transport ensures long-range delivery of essential cargoes between proximal and distal compartments, and is needed for neuronal development, function, and survival. Deficits in axonal transport have been detected at pre-symptomatic stages in the SOD1G93A and TDP-43M337V mouse models of amyotrophic lateral sclerosis (ALS), suggesting that impairments in this critical process are fundamental for disease pathogenesis. Strikingly, in ALS, fast motor neurons (FMNs) degenerate first whereas slow motor neurons (SMNs) are more resistant, and this is a currently unexplained phenomenon. The main aim of this investigation was to determine the effects of brain-derived neurotrophic factor (BDNF) on in vivo axonal transport in different α-motor neuron (MN) subtypes in wild-type (WT) and SOD1G93A mice. We report that despite displaying similar basal transport speeds, stimulation of wild-type MNs with BDNF enhances in vivo trafficking of signalling endosomes specifically in FMNs. This BDNF-mediated enhancement of transport was also observed in primary ventral horn neuronal cultures. However, FMNs display selective impairment of axonal transport in vivo in symptomatic SOD1G93A mice, and are refractory to BDNF stimulation, a phenotype that was also observed in primary embryonic SOD1G93A neurons. Furthermore, symptomatic SOD1G93A mice display upregulation of the classical non-pro-survival truncated TrkB and p75NTR receptors in muscles, sciatic nerves, and Schwann cells. Altogether, these data indicate that cell- and non-cell autonomous BDNF signalling is impaired in SOD1G93A MNs, thus identifying a new key deficit in ALS.

Mutation of the ALS/FTD-associated RNA-binding protein FUS alters axonal cytoskeletal organisation

“…Therefore, these data together suggest that while both FUS(16R) and FUS(P525L) affect the developing axon, FUS(P525L) may perturb specific LPS-based processes that guide axonal development. It has recently been shown that fast motor neurons expressing an ALS-associated variant of SOD1 are refractory to BDNF-mediated increases in axonal transport (Tosolini et al, 2022), which reinforces this idea, as BDNF also regulates axonal branching (Cohen-Cory et al, 2010). The role of these in vivo pathways and their changed regulation in ALS/FTD is an exciting new area of future study.…”

Bimodal regulation of axonal transport by the GDNF-RET signalling axis in healthy and diseased motor neurons

“…3F-I). Interestingly, we have previously shown that stimulation with this concentration of BDNF enhances axonal transport in WT primary MNs [7]. This implies that the enhancement of signalling endosome transport upon GDNF treatment is likely to be independent of RET activation, suggesting GDNF may activate an alternative receptor in these cells.…”

“…Mice were assigned to control and treatment groups at random, nevertheless ensuring an even spread of treatments between different experimental days. A sample size of 6 or 7 mice per condition (27 in total) was selected based on results from previous in vivo transport experiments, where groups of 5-6 animals per condition were sufficient to identify small, yet significant changes in transport speeds [6,7,9] After injection, mice were then left to recover and kept under standard conditions with unlimited food and water supply. After 4+ h, mice were re-anaesthetised using isoflurane and the sciatic nerve of the right leg was exposed.…”

“…Human motor neuron (MN) axons can be over a metre long, necessitating efficient axonal transport. Accordingly, deficits in this process occur in many neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) [2,3], for which impairment in axonal transport is one of the earliest pathological outcomes [4][5][6][7].…”

“…Signalling endosomes are generated following the activation and internalisation of neurotrophin receptors, such as TrkB and p75 NTR , and predominantly undergo retrograde axonal transport [8]. Analysis of signalling endosome dynamics in the sciatic nerve of SOD1 G93A mice, an established animal model of ALS, revealed a significant slowing of retrograde transport compared to wild type (WT), both at pre-symptomatic and symptomatic disease stages [7,9]. Axonal transport deficits have also been observed in primary MN cultures from SOD1 G93A [5] and TDP-43 [10] embryos, cultured SOD1 G93A spinal cord explants [11], induced pluripotent stem cell (iPSC)-derived MNs harbouring ALS causative mutations in SOD1 [12], FUS [12] and TDP-43 [13], as well as in vivo, in transgenic TDP-43 M337V mice [6].…”