Favorable effects of MMP-9 knockdown in murine herpes simplex encephalitis using small interfering RNA.

Background And Purpose: The prognosis of herpes simplex encephalitis (HSE) remains poor despite available antiviral treatment. Matrix metalloproteinase-9 (MMP-9) is currently considered to play a major role in promoting cerebrovascular complications which contribute to the high mortality and morbidity of HSE. We hypothesize that temporally knockdown MMP-9 expression in early phase of HSE might be an effective treatment strategy.

Methods: The animal models of herpes simplex encephalitis were established by intracerebrally inoculated herpes simplex virus type 1 (HSV-1) in mice. Mice were inoculated intracerebrally with MMP-9 targeting siRNA (MMP-9 siRNA). MMP-9 expression was assessed by RT-PCR and western blot analysis at 3 and 7 days after HSV-1 infected. The blood-brain barrier (BBB) permeability was quantitated by Evans blue dye extravasations and brain water content. Immunohistochemistry method was adopted to analyse the expression of AQP4 protein. Quantitative real-time PCR analysis was used to detect cytokines expression. Neurological score was quantified using an established neurological scale at 7 days after HSE.

Results: Using synthetic small interfering RNA, we found a single intracerebral injection of siRNA targeting murine MMP-9 mRNA (MMP-9 siRNA) silenced MMP-9 expression and reduced it to normal level at day 7 post-infection. The improvement in neurological function and increased cumulative survival reflected the functional consequence of this therapy. MMP-9 knockdown mice also displayed less uptake of Evans blue and reduced brain water content compared with control siRNA-treated group. Also the HSV-1-induced upregulation of proinflammatory cytokines was significantly diminished in MMP-9 siRNA-treated mice. In addition, aquaporin-4 expression in perivascular decreased in MMP-9 siRNA-treated mice and might contribute to the protection of blood-brain barrier.

Discussion: This compelling evidence suggests that MMP-9 is a key pathogenic factor within HSE, and local injection of synthetic siRNA in the brain could knock down MMP-9 expression in acute phase of HSE, reduce brain edema and improves mice neurological function and increase cumulative survival.