Cerebral ischemia induces
neuroinflammation and microglial activation, in which activated microglia upregulate their proliferative activity and change their metabolic states. In activated microglia,
l-arginine is metabolized competitively by
inducible nitric oxide synthase (iNOS) and
arginase (Arg), which then synthesize NO or
polyamines, respectively. Our previous study demonstrated that
Sema4D deficiency inhibits iNOS expression and promotes proliferation of ionized
calcium-binding adaptor molecule 1 (Iba1)-positive (Iba1+) microglia in the ischemic cortex, although the underlying mechanisms were unclear. Using
middle cerebral artery occlusion, we tested the hypothesis that
Sema4D deficiency alters the balance of
l-arginine metabolism between iNOS and Arg, leading to an increase in the production of
polyamines, which are an essential factor for cell proliferation. In the peri-ischemic cortex, almost all iNOS+ and/or Arg1+ cells were Iba1+ microglia. In the peri-ischemic cortex of Sema4D-deficient (
Sema4D-/-) mice, the number of iNOS+ Arg1- Iba1+ microglia was smaller and that of iNOS- Arg1+ Iba1+ microglia was greater than those of wild-type (WT) mice. In addition,
urea and
polyamine levels in the ischemic cortex of
Sema4D-/- mice were higher than those of WT mice; furthermore, the presence of
Sema4D inhibited
polyamine production in primary microglia obtained from
Sema4D-/- mice. Finally, microglia cultured under
polyamine putrescine-supplemented conditions demonstrated increased proliferation rates over non-supplemented controls. These findings indicate that
Sema4D regulates microglial proliferation at least in part by regulating the competitive balance of
l-arginine metabolism.