[Background] Cytokines secreted by the skeletal muscle are collectively called myokines. The muscle-released myokines have physiological effects at distant organs other than muscle, including skin, liver, kidney, and brain. Myostatin (MSTN), a myokine solely expressed in muscle, is a glycoprotein belonging to the TGF-β superfamily. MSTN functions as a negative regulator of muscle growth and, therefore, is upregulated during cancer-related muscle wasting (MW). Bovine and mouse strains with defective/abnormal MSTN genes have been observed to have a 2- to 3-fold increase in muscle mass compared to wildtype (WT) animals. MSTN deficiency has been reported to improve survival in critical illness of sepsis with mitigated damage to kidney, liver and the heart and prevented MW. Major burn injury (BI) is characterized by systemic inflammation with proinflammatory cytokine release which can cause distant damage to organs (e.g., respiratory and circulatory systems, nerves, and skeletal muscles including MW). We have documented that BI is associated with motor neuron loss together with neuroinflammation, evidenced as proinflammatory cytokine release. Myokine effects on the central nervous system is unknown. In the present study using MSTN knockout (KO) and WT mice, we tested the hypotheses that MSTN deficiency has beneficial effects on both spinal neuronal inflammation and motor neuron loss after major BI compared to wild type mice with BI and intact MSTN.

[Material and Methods] Weight-matched male MSTN KO and WT mice were randomly divided into sham-burn [Sham] or burn injury [BI] groups and third-degree 35% total body surface area (major BI) was administered under anesthesia. Fourteen days after BI, the mice were euthanized, and the spinal cord was harvested. Expression of inflammatory cytokines (IL-1β and TNF-α) was assessed by qPCR. Microglia and motor neurons in lumbar spinal cord sections were immunohistochemically stained against Iba1 and ChAT, respectively.

[Results] IL1-β mRNA expression in WT-BI was significantly increased compared to WT-Sham (3.87 ± 0.78-fold, p< 0. 01) but BI caused no significant changes in cytokine levels in the spinal cords of MSTN KO mice (1.88 ± 0.90-fold). Similarly, TNF-α expression in WT-BI was significantly increased compared to WT-Sham (2.77 ± 0.46-fold, p< 0. 01) but cytokine levels were unchanged after BI to MSTN KO (1.37 ± 0.49-fold) [Fig. 1]. In WT group, number of microglia labeled against Iba1 were upregulated after BI, consistent with cytokine-related neuroinflammation. In contrast, minimal changes in microglia numbers were observed in MSTN KO [Fig. 2]. Number of ChAT-labeled motor neurons was significantly reduced in WT mice with BI compared to Sham (WT-Sham: 30.8 ± 4.16 cells/unilateral ventral spinal area (uvspa), WT-BI: 17.5 ± 2.72 cells/uvspa, p< 0. 01). On the other hand, MSTN KO showed no decrease in motor neurons even after BI (MSTN KO-Sham: 36.0 ± 1.22 cells/uvspa, MSTN KO-BI: 37.0 ± 2.51 cells/uvspa) [Fig. 3].

[Conclusion] Deletion of MSTN resulted in inhibition of BI-induced release of cytokines, L-1β and TNF-α, after BI in the spinal cord. MSTN knockout also reduced microglial activation after BI. Furthermore, it also led to a suppression of motor neuron loss in spinal cord. In conclusion, MSTN absence was protective on spinal cord neuroinflammation and motor neuron loss after severe burn injury. Myostatin has distant positive effects on the CNS.