226 Motor Neuronal Apoptosis and Neuromuscular Atrophy are Associated with Transmigration of Monocyte-derived Macrophages after Burn Injury

Patients with major burns are at high risk of systemic inflammation, which can lead to increased morbidity and in severe cases mortality. Burn-induced inflammation (BII) has been shown to trigger transmigration of monocyte-derived macrophages (MD-MΦ) to different organs (for e.g., liver), where they caused detrimental effects by releasing cytokines and inducing apoptosis. However, the effects of transmigration of MD-MΦ in the spinal cord and how these transmigrated MD-MΦ affect on motor neuron damage and neuromuscular atrophy after BII have not been studied yet. BII in wild type (WT) and YFP-expressing neurofilament transgenic (neurofilamentYFP) mice was inflicted by scald burns. Motor neuronal apoptosis in the spinal cord was identified by staining with NeuN antibody, Hoechst dye and TUNEL assay by immunohistochemistry (IHC). Transmigration of MD-MΦ and expression of YFP-neurons in the spinal cord were also assessed by FACS analysis. Nerve termini disintegration in rectus muscle of neurofilamentYFP mice was analyzed by fluorescent microscopy. Muscle atrophy in WT mice was determined by analyzing the changes in muscle mass at 21 days. Our results suggested that BII significantly increased transmigration of MD-MΦ and motor neuronal apoptosis in the spinal cord compared to sham mice at 21 days. The intensity of YFP-expressing neurons in BII mice was significantly reduced compared to sham mice at 21 days, as evaluated by FACS and IHC analyses. Additionally, the nerve termini in rectus muscle were disintegrated after BII as early as 3 days. Moreover, skeletal muscle mass was significantly decreased in burned mice compared to sham mice at 21 days. Overall, our findings suggest that BI induces transmigration of MD-MΦ, which is associated with motor neuronal apoptosis and neuromuscular atrophy. Inhibition of the transmigration of MD-MΦ into the spinal cord could be a useful therapeutic maneuver to reverse the motor neuronal damage and neuromuscular atrophy in burned patients.