Dietary restriction started after spinal cord injury improves functional recovery

Abstract

Spinal cord injury typically results in limited functional recovery. Here we investigated whether therapeutic dietary restriction, a multi-faceted, safe, and clinically-feasible treatment, can improve outcome from cervical spinal cord injury. The well-established notion that dietary restriction increases longevity has kindled interest in its potential benefits in injury and disease. When followed for several months prior to insult, prophylactic dietary restriction triggers multiple molecular responses and improves outcome in animal models of stroke and myocardial infarction. However, the efficacy of the clinically-relevant treatment of post-injury dietary restriction is unknown. Here we report that “every-other-day fasting” (EODF), a form of dietary restriction, implemented after rat cervical spinal cord injury was neuroprotective, promoted plasticity, and improved behavioral recovery. Without causing weight loss, EODF improved gait-pattern, forelimb function during ladder-crossing, and vertical exploration. In agreement, EODF preserved neuronal integrity, dramatically reduced lesion volume by > 50%, and increased sprouting of corticospinal axons. As expected, blood β-hydroxybutyrate levels, a ketone known to be neuroprotective, were increased by 2–3 fold on the fasting days. In addition, we found increased ratios of full-length to truncated trkB (receptor for brain-derived neurotrophic factor) in the spinal cord by 2–6 folds at both 5 days (lesion site) and 3 weeks after injury (caudal to lesion site) which may further enhance neuroprotection and plasticity. Because EODF is a safe, non-invasive, and low-cost treatment, it could be readily translated into the clinical setting of spinal cord injury and possibly other insults.

Introduction

The ability of dietary restriction (DR) to increase longevity across many species, even if started in adulthood, is well established (Weindruch and Walford, 1982). This prompted further studies revealing biochemical and functional benefits in animal models of Parkinson's, Huntington's, and Alzheimer's disease, as well as of many non-neurologic diseases (Mattson et al., 2002). Moreover, prophylactic DR followed for several months prior to acute insults was found to be neuroprotective and promoted functional recovery in several models of neural injury including stroke (Bruce-Keller et al., 1999, Yu and Mattson, 1999, Anson et al., 2003, Maswood et al., 2004, Mattson, 2005). These beneficial effects of DR on a wide variety of systems are accompanied by multiple cellular responses, including a reduction of reactive oxygen species (ROS), oxidative damage, and lipid peroxidation (Sohal et al., 1994, Dubey et al., 1996, Sharma and Kaur, 2005). DR increases neuroprotective ketones (e.g. β-hydroxybutyrate), antioxidative and anti-apoptotic proteins, while decreasing the activation of apoptotic proteins (Yu and Mattson, 1999, Anson et al., 2003, Shelke and Leeuwenburgh, 2003, Sharma and Kaur, 2005). Additionally, DR, if followed for several months, increases brain-derived neurotrophic factor (BDNF) and its full length trkB receptor in the brain (Mattson, 2005). Many of these factors and pathways above are implemented in the secondary injury cascades occurring after spinal cord injury (Juurlink and Paterson, 1998, Beattie et al., 2000, Hall and Springer, 2004, Park et al., 2004, Fleming et al., 2006). Given this broad spectrum effect, we tested prophylactic DR for spinal cord injury in rats and observed the expected neuroprotective and behavioral benefits (submitted). However, prophylactic DR is of limited clinical value (except elective interventions) for acute incidents since few people will adopt dramatic dietary lifestyle changes despite the tremendous general health benefits. Therefore, in the present study, we investigated if a clinically translatable therapeutic implementation of DR started at time of a CNS injury would produce the similar positive outcomes after spinal cord injury.

There are two main forms of DR: a 20–40% daily reduction of caloric intake (calorie restriction — CR) and every-other-day fasting (EODF — a form of intermittent fasting). Direct comparison of prophylactic 40% CR and EODF demonstrated that EODF was more neuroprotective (Anson et al., 2003) justifying our choice of EODF as a treatment for spinal cord injury. Since most human spinal cord injuries are anatomically incomplete and occur in the cervical spinal cord (Ackery et al., 2004), we modeled a partial spinal cord injury at the cervical (C4) cord in rats. We report for the first time that EODF implemented after cervical spinal cord injury provided neuroprotection, anatomical plasticity and improved functional outcome.