The sialidase neuraminidase (NA) cleaves terminal sialic acid from glycoproteins and glycolipids. Among its various locations, it is present in the envelope/membrane of some bacteria/viruses (e.g. influenza virus), where it is involved in infectiveness and dispersion. The injection of NA within the brain lateral ventricle represents a model of acute sterile
inflammation. The relevance of the toll-like receptors TLR2 and TLR4 (particularly those in
microglial cells) in such process was investigated using mouse strains deficient in these
receptors. In septofimbria and hypothalamus, IBA1-positive and IL-1β-positive cell counts
increased after NA injection in wild type (WT) mice. In TLR4-/- mice such increases were largely abolished, while only slightly affected in TLR2-/- mice. Similarly, the NA-induced
expression of IL-1β, TNFα and IL-6 (evaluated by qPCR) was completely blocked in TLR4-/-
mice, and only partially reduced in TLR2-/- mice. Microglia was isolated from the three mouse
strains and exposed to NA or to specific TLR2 and TLR4 agonists (Pam3CSK4 and LPS
respectively) in vitro. NA induced a cytokine response (IL-1β, TNFα and IL-6) in WT
microglia, but was unable to do so in TLR4-/- microglia; TLR2 deficiency partially affected
the NA-induced microglia response. To investigate if such response of microglial cells to NA
was dependent on the sialidase activity of the enzyme, WT microglia was exposed in vitro to
NA previously inactivated with heat, or inhibited with two different sialidase inhibitors
(oseltamivir phosphate and N-acetyl-2,3-dehydro-2-deoxyneuraminic acid). In all cases, NA-
induced microglia activation was dependent on the intact sialidase activity of NA. Therefore,
we conclude that NA is able to directly activate microglial cells, mostly through TLR4
receptor and due to its sialidase activity. Accordingly, the inflammatory reaction induced by
NA in vivo is partially dependent on TLR2, while TLR4 plays a crucial role.
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