Control of Axon-Mediated Neuronal Degeneration
Neuronal cell death by apoptosis plays an important role in development and the establishment of neural circuits. However, if not properly regulated, neuronal apoptosis can lead to serious neurodegenerative disease. Helping to maintain the critical balance between survival and death are the neurotrophins, which promote neuronal survival through their interaction with members of the tropomyosin receptor kinase family of receptors. Neurotrophins can also signal via the p75 neurotrophin receptor (p75NTR) a member of the tumor necrosis factor receptor superfamily. Signaling via p75NTR can lead to axonal degeneration and apoptosis. Neurotrophins are produced by innervated target tissues and act locally on nerve axons. Thus, apoptosis resulting from neurotrophin signaling must involve retrograde transport of the signal through the axon to the cell nucleus. This led Vanderbilt Basic Sciences investigators Bruce Carter and Dylan Burnette, their laboratories, and collaborators in San Francisco, New York, Tel Aviv, and Santiago to investigate the mechanism of axonal retrograde degenerative signaling. They used rat sympathetic neurons cultured in microfluidic chambers that allowed separate treatments of the axons versus the cell bodies. They first confirmed that exposure of the cell axons (but not the cell bodies) to brain-derived neurotrophic factor (BDNF), an agonist of p75NTR, led to apoptosis. The cells were protected by treatment of the axons (but not the cell bodies) with an inhibitor of γ-secretase, consistent with prior data showing that α- and γ-secretase-mediated cleavage of p75NTR is required for apoptotic signaling. Secretase-dependent cleavage releases the biologically active intracellular domain (ICD) of p75NTR, and the investigators confirmed this release in BDFN-treated axons using GFP-labeled receptor. They further demonstrated that the labeled ICD associates with multi-vesicular bodies, which are known to be involved in retrograde axonal transport. Prior evidence had suggested a role for histone deacetylases (HDACs) in axonal transport, and the investigators used small molecule inhibitors and shRNA-mediated knockdown to show that HDAC1 is involved in BDNF-mediated apoptosis. The investigators went on to show that upon activation with BDNF, p75NTR is internalized by a mechanism that requires dynamin, but not HDAC1. This receptor internalization preceded secretase-mediated cleavage of the receptor to release the ICD. Using cells expressing the GFP-labeled receptor, the investigators demonstrated BDNF-promoted movement of the ICD towards the cell body, a process that was blocked by inhibition of HDAC1. Proteomics analysis revealed that a major target protein of HDAC1 in the neurons was p150Glued, a component of the dynactin activator complex that is required for retrograde axonal transport driven by the microtubule-associated motor protein dynein. Further work revealed that p150Gluedis acetylated at a highly conserved region that interacts with the dynein intermediate chain (DIC). Acetylation of p150Gluedat this site blocked its association with the DIC, whereas HDAC1-dependent removal of the acetyl group promoted the association. Together the data suggest that activation of p75NTR leads to receptor internalization followed by proteolytic release of the ICD that then carries apoptotic signals to the nucleus via dynein-dependent transport. HDAC1 is required to deacetylate p150Gluedso that it can associate with the dynactin activator complex in order to facilitate dynein function. Findings that the proposed mechanism may be involved in the neurodegeneration associated with amyotrophic lateral sclerosis suggests the potential importance of this discovery. The work is published in Developmental Cell [A. Pathak, et al. (2018) Dev. Cell, 46, 376].