The loss-of-function disease-mutation G301R in the Na(+)/K(+)-ATPase α(2) isoform decreases lesion volume and improves functional outcome after acute spinal cord injury in mice

BACKGROUND: The Na(+)/K(+)-ATPases are transmembrane ion pumps important for maintenance of ion gradients across the plasma membrane that serve to support multiple cellular functions, such as membrane potentials, regulation of cellular volume and pH, and co-transport of signaling transmitters in all animal cells. The α(2)Na(+)/K(+)-ATPase subunit isoform is predominantly expressed in astrocytes, which us the sharp Na(+)-gradient maintained by the sodium pump necessary for astroglial metabolism. Prolonged ischemia induces an elevation of [Na(+)](i), decreased ATP levels and intracellular pH owing to anaerobic metabolism and lactate accumulation. During ischemia, Na(+)/K(+)-ATPase-related functions will naturally increase the energy demand of the Na(+)/K(+)-ATPase ion pump. However, the role of the α(2)Na(+)/K(+)-ATPase in contusion injury to the spinal cord remains unknown. We used mice heterozygous mice for the loss-of-function disease-mutation G301R in the Atp1a2 gene (α(2)(+/G301R)) to study the effect of reduced α(2)Na(+)/K(+)-ATPase expression in a moderate contusion spinal cord injury (SCI) model. RESULTS: We found that α(2)(+/G301R) mice display significantly improved functional recovery and decreased lesion volume compared to littermate controls (α(2)(+/+)) 7 days after SCI. The protein level of the α(1) isoform was significantly increased, in contrast to the α(3) isoform that significantly decreased 3 days after SCI in both α(2)(+/G301R) and α(2)(+/+) mice. The level of the α(2) isoform was significantly decreased in α(2)(+/G301R) mice both under naïve conditions and 3 days after SCI compared to α(2)(+/+) mice. We found no differences in astroglial aquaporin 4 levels and no changes in the expression of chemokines (CCL2, CCL5 and CXCL1) and cytokines (TNF, IL-6, IL-1β, IL-10 and IL-5) between genotypes, just as no apparent differences were observed in location and activation of CD45 and F4/80 positive microglia and infiltrating leukocytes. CONCLUSION: Our proof of concept study demonstrates that reduced expression of the α(2) isoform in the spinal cord is protective following SCI. Importantly, the BMS and lesion volume were assessed at 7 days after SCI, and longer time points after SCI were not evaluated. However, the α(2) isoform is a potential possible target of therapeutic strategies for the treatment of SCI.