A research team led by Dr. Jonathan Kipnis from the University of Virginia School of Medicine used several Rett syndrome mouse models, Cre-lox strains and a green fluorescent protein (GFP) reporter strain to clarify the role of defective microglia in the pathology of Rett syndrome (Derecki et al. 2012). The findings may lead to new strategies for treating Rett syndrome.
Microglia are the resident macrophages in the brain and spinal cord, and defective microglia have been suspected of playing a role in Rett syndrome pathology. Because microglia are hematopoietically derived, the Kipnis team explored the possibility that bone marrow transplantation could arrest Rett syndrome in mouse models of the disease. So, they intravenously injected sub-lethally irradiated 28-day old Mecp2-null males (Mecp2y/-) of Rett syndrome mouse model B6.129P2(C)-Mecp2tm1.1Bird/J (003890) with syngeneic bone marrow from the "reporter" strain, C57BL/6-Tg(UBC-GFP)30Scha/J (004353). The Mecp2y/- mice lack a functional methyl-CpG binding protein 2 (Mecp2) gene, a transcription regulator on the X chromosome. More than 200 mutations in eight different hot spots within the human MECP2 locus have been associated with Rett syndrome. The reporter strain has C57BL/6J (000664) wild-type microglia and ubiquitously expresses GFP, permitting the engrafted cells derived from its bone marrow to be tracked.
The Kipnis team observed that, compared to controls, transplanted Mecp2y/- recipients live significantly longer (up to 48 weeks; most non-recipients die around 10 weeks). Their body, brain, and spleen sizes increase and approach those of normal mice, and their tremors, and apneas and breathing irregularities decrease. Mobility, gait, and overall appearance also improve. Disease is only slightly mitigated in Mecp2y/- males that receive bone transplants later (when 40-45 days old), demonstrating the need for early intervention. It is not mitigated at all if Mecp2-null mice are not first irradiated (to deplete host-derived microglia), even if they receive bone transplants when two days old. Together, these results indicated that bone marrow transplantation can arrest Rett syndrome in mice and might do so in humans.
The Kipnis team observed a high percentage of GFP+ microglia-like cells in the brain parenchyma of transplant recipient Mecp2y/- mice. To explore the mitigating effects of these cells, they transplanted wild-type bone marrow in another group of 28-day old Mecp2y/- mice. The heads of these mice were shielded from irradiation, preventing bone marrow-derived cells from engrafting into the brain parenchyma. The team found that Rett syndrome is not ameliorated in these mice, suggesting that the Mecp2-null microglia must be replaced with wild-type microglia to be therapeutic. The researchers then substantiated the mitigating effects of transplant-derived microglia-like cells molecularly. They transplanted marrow from donors expressing Mecp2 in myeloid cells only into Mecp2y/- recipients. The donors were produced by crossing B6.129P2-Lyz2tm1(cre)Ifo/J (004781) mice, which express Cre recombinase in microglia (and other myeloid cell lineages), with B6.129P2-Mecp2tm2Bird/J (006849) mice, in which the Mecp2 gene is non-functional due to the insertion of a loxP-flanked stop codon in intron 2. In the male progeny of this cross, Cre recombinase excises the stop codon, activating Mecp2 expression in myeloid cells to partially rescue the phenotype. Compared to that of untransplanted Mecp2y/- mice, the overall appearance and growth of the transplant recipients is improved, lifespan is significantly increased (up to 31 weeks), apnoeas and breathing irregularities are significantly reduced, and open-field activity is normal. Together, these results indicated that Mecp2-null microglia mediate Rett syndrome and that repopulation with wild-type microglia can arrest it.
The Kipnis team wondered how Mecp2-null microglia mediate Rett syndrome. Evidence from their research and that of others indicated that the immune responses and phagocytic activities of Mecp2-null microglia are defective. The team observed that, in the brain parenchyma of the transplant recipients mentioned earlier, only the bone marrow-derived myeloid cells in the brain parenchyma (derived from the B6J reporter strain) show evidence of phagocytic activity. And, if progeny of the B6.129P2-Lyz2tm1(cre)Ifo/J x B6.129P2-Mecp2tm2Bird/J cross mentioned above are treated with annexin V, a phagocytosis inhibitor, Rett syndrome is not ameliorated. These observations suggested that the inability of Mecp2-null microglia to clear the brain of naturally accumulating dead cells, shed membranes, and other debris further challenges a central nervous system already insulted by the loss of the MECP2 protein. The researchers emphasize that the defective phagocytic abilities of Mecp2-null microglia may not be the primary cause of Rett syndrome, but they sure seem to aggravate it.
In summary, the Kipnis team's results clarify the contribution of defective microglia to Rett syndrome pathology and suggest that bone marrow transplantation can alleviate it.