High molecular weight hyaluronan reduces lipopolysaccharide mediated microglial activation

Toll-like receptor-4 (TLR4) signaling has been implicated in microglial activation and propagation of inflammation following spinal cord injury (SCI). As such, modulating microglial activation through TLR4 represents an attractive therapeutic approach to treat SCI. High molecular weight hyaluronan (HMW-HA), a polymer with multiple therapeutic uses, has been previously shown to modulate TLR4 activation in macrophages and has shown early promise as a therapeutic agent in SCI. However, the mechanism associated with HMW-HA has not been fully elucidated or tested in microglia, a similar cell type. In the current study, we sought to determine the effects of HMW-HA on TLR4 activation in microglia and to gain insights into the mechanism of action. Rat primary microglial cultures were exposed to lipopolysaccharides (LPS) and HMW-HA, and the extent and mechanisms of inflammation were studied. HMW-HA decreased LPS mediated IL-1β, IL-6, and Tumor necrosis factor-α gene expression and IL-6 and nitric oxide production. This decrease was associated with a reduction in ERK 1/2 and p38 phosphorylation, was dependent on the continued presence of HMW-HA, and activation of Akt and A20 protein expression was reduced by HMW-HA. Together, our results show that HMW-HA can reduce LPS-mediated inflammatory signaling in microglia. We suggest that HA possibly mediates its effects by blocking the induction of inflammatory signaling through an extracellular mechanism.