Hematopoietic prostaglandin D synthase is expressed in microglia in the developing postnatal mouse brain

Hematopoietic prostaglandin D synthase (HPGDS) is a PGD(2)-synthesizing enzyme and is expressed in antigen-presenting cells, mast cells, and other immunocompetent cells. We here report the HPGDS expression in microglia and the migration pathway of microglia in the developing mouse brain as detected by HPGDS immunohistochemistry. Expression of HPGDS mRNA peaked at postnatal day (PND) 10, decreased gradually thereafter, and reached a plateau at PND 20. The mRNAs for target molecules of PGD(2), i.e., DP receptor (DPR) and CRTH2 receptor, showed developmental profiles overlapped to that of HPGDS. Most of the HPGDS(+) cells at PND 10 had morphological characteristics of ameboid microglia and gave positive immunostaining with microglia-specific markers such as RCA-1, F4/80, or ER-MP12. These specific markers became less detectable later on, but HPGDS was still expressed even in resting microglia. Thus, HPGDS is a useful marker for investigation of microglial development. Spaciotemporal evaluation of microglial development and migration with HPGDS immunostaining revealed the following three major possible migration pathways of microglia in the postnatal brain: from the lateral ventricle via subventricular zones to brain parenchyma; from the leptomeninges around the cerebellopontine angle to the cerebellar white matter; and from the overlying leptomeninges to the hippocampus, basal forebrain, and brainstem.