Developmental regulation of group I metabotropic glutamate receptors in the premature brain and their protective role in a rodent model of periventricular leukomalacia

Abstract

Cerebral white matter injury in premature infants, known as periventricular leukomalacia (PVL), is common after hypoxia-ischemia (HI). While ionotropic glutamate receptors (iGluRs) can mediate immature white matter injury, we have previously shown that excitotoxic injury to premyelinating oligodendrocytes (preOLs) in vitro can be attenuated by group I metabotropic glutamate receptor (mGluR) agonists. Thus, we evaluated mGluR expression in developing white matter in rat and human brain, and tested the protective efficacy of a central nervous system (CNS)-penetrating mGluR agonist on injury to developing oligodendrocytes (OLs) in vivo. Group I mGluRs (mGluR1 and mGluR5) were strongly expressed on OLs in neonatal rodent cerebral white matter throughout normal development, with highest expression early in development on preOLs. Specifically at P6, mGluR1 and mGLuR5 were most highly expressed on GalC-positive OLs compared to neurons, axons, astrocytes and microglia. Systemic administration of (1S,3R) 1-aminocyclopentane-trans-1,3,-dicarboxylic acid (ACPD) significantly attenuated the loss of myelin basic protein in the white matter following HI in P6 rats. Assessment of postmortem human tissue showed both mGluR1 and mGluR5 localized on immature OLs in white matter throughout development, with mGluR5 highest in the preterm period. These data indicate group I mGluRs are highly expressed on OLs during the peak period of vulnerability to HI and modulation of mGluRs is protective in a rodent model of PVL. Group I mGluRs may represent important therapeutic targets for protection from HI-mediated white matter injury.

Fig. 1.. Developmental expression of mGluR1 in rodent callosal white matter.

At postnatal day 6 (P6), mGluR1 (E, green) is highly expressed on GalC-positive OL cell bodies and processes (red) in callosal white matter (B-F). mGluR1 (green) is also strongly co-expressed on GalC-positive OL cell bodies and processes (red) from P3 (A-C) through P14 (G-I).

Fig. 2.. Developmental expression of mGluR5 in rodent callosal white matter.

At postnatal day 6 (P6), mGluR5 (E, green) is highly expressed on the cell bodies of GalC-positive OLs (red) in callosal white matter (D-F). In contrast to the consistent GluR1 expression pattern throughout development (Fig. 1), the peak of mGluR5 (green) on GalC-positive OLs (red) is between P3 (C) and P6 (F). By P14 (H), mGLuR5-GalC co-expression is reduced and restricted to patches of OL processes throughout the corpus callosum and is similar to that observed in older animals.

Fig. 3.. Administration of ACPD attenuates white matter loss after HI in postnatal day 6 (P6) Long-Evans rats.

HI results in the significant loss of MBP in the hemisphere ipsilateral to carotid ligation (A), compared to MBP expression in the uninjured, contralateral hemisphere (B). Systemic administration of ACPD for 48 h significantly attenuates callosal and periventricular white matter loss 96 h following HI when administered at 5 mg kg⁻¹ (C,D) or 2o mg kg⁻¹ (E,F). Treatment with ACPD preserves myelin processes radiating into the cortex and pericallosal process bundles at each dose examined (C,E) compared to vehicle-treated controls, and significantly decreases overall MBP injury scores (G).

Fig. 4.. Developmental expression of mGluR1 on human OLs.

Expression of mGluR1 (green) on O4-positive preOLs (red) in parietal-occipital white matter is consistent throughout gestation. O4-mGluR1 co-expression (yellow) is high through 19 post-conceptional weeks (PCW, A-C), 29 PCW (D-F) and 38 PCW (G-I).

Fig. 5.. Developmental expression of mGluR5 on human OLs.

A biphasic expression pattern is observed with mGluR5 (green) on human O4-positive OLs (red) whereby co-expression (yellow) is low at 19 post-conceptional weeks (PCW, A-C) and 38 PCW (G-I). The peak of mGluR5 expression on O4-positive preOLs is observed at 29 PCW.

Fig. 6.. Developmental expression of mGluR5 in human subcortical white matter.

Western blot analyses reveal that mGluR5 protein is strongly expressed in human subcortical, parietal-occipital white matter from 20 to 38 PCW, compared to a marked postnatal decrease (76 PCW-7 years) and very low levels sustained through adulthood.