The p75 neurotrophin receptor is essential for neuronal cell survival and improvement of functional recovery after spinal cord injury

The mechanisms initiating post-spinal cord injury (SCI) apoptotic cell death remain incompletely understood. The p75 neurotrophin receptor (p75NTR) has been shown to exert both pro-survival and pro-apoptotic effects on neural cells in vitro. While a previous study had shown that there is decreased oligodendrocyte apoptosis distal to a clean partial transection injury of the cord in mice with nonfunctional p75NTR, most human spinal cord injuries do not involve partial transections but are rather due to compression/contusion injuries with significant perilesional ischemia. Therefore, we sought to examine the role of the p75NTR in a clinically relevant clip compression model of SCI in p75 null mice with an exon III mutation. Mice with a functional p75NTR had increased caspase-9 activation at 3 days after SCI in comparison to the functionally deficient p75NTR mice. However, at 7 days following SCI there was no difference in the activation of the effector caspases (caspase-3 and caspase-6) at the spinal cord lesion. Moreover, at 7 days after injury, there was increased terminal deoxynucleotidyl transferase-mediated dUTP nick-end (TUNEL) positive cell death at the injury site in the functionally deficient p75NTR mice. Using double labeling with TUNEL and cell specific markers we showed that the absence of p75NTR function increased the extent of neuronal but not oligodendroglial cell death at the injury site. This selective loss of neuronal cells after SCI was confirmed with a decrease in levels of microtubule-associated protein 2 in the p75 null mice. Furthermore, the wild-type animals had dramatically improved survival and enhanced locomotor recovery at 8 weeks after SCI when compared with the p75NTR null mice. Also at 8 weeks, there were significantly more neurons present at the injury site of wild-type mice when compared with p75 null mice. We conclude that the p75NTR receptor is integral to neuronal cell survival and endogenous reparative mechanisms after compressive/contusive SCI.