APOE3, but not APOE4, bone marrow transplantation mitigates behavioral and pathological changes in a mouse model of Alzheimer disease.

Apolipoprotein E4 (APOE4) genotype is the strongest genetic risk factor for late-onset Alzheimer disease and confers a proinflammatory, neurotoxic phenotype to microglia. Here, we tested the hypothesis that bone marrow cell APOE genotype modulates pathological progression in experimental Alzheimer disease. We performed bone marrow transplants (BMT) from green fluorescent protein-expressing human APOE3/3 or APOE4/4 donor mice into lethally irradiated 5-month-old APPswe/PS1ΔE9 mice. Eight months later, APOE4/4 BMT-recipient APPswe/PS1ΔE9 mice had significantly impaired spatial working memory and increased detergent-soluble and plaque Aß compared with APOE3/3 BMT-recipient APPswe/PS1ΔE9 mice. BMT-derived microglia engraftment was significantly reduced in APOE4/4 recipients, who also had correspondingly less cerebral apoE. Gene expression analysis in cerebral cortex of APOE3/3 BMT recipients showed reduced expression of tumor necrosis factor-α and macrophage migration inhibitory factor (both neurotoxic cytokines) and elevated immunomodulatory IL-10 expression in APOE3/3 recipients compared with those that received APOE4/4 bone marrow. This was not due to detectable APOE-specific differences in expression of microglial major histocompatibility complex class II, C-C chemokine receptor (CCR) type 1, CCR2, CX3C chemokine receptor 1 (CX3CR1), or C5a anaphylatoxin chemotactic receptor (C5aR). Together, these findings suggest that BMT-derived APOE3-expressing cells are superior to those that express APOE4 in their ability to mitigate the behavioral and neuropathological changes in experimental Alzheimer disease.

Perivascular, but not parenchymal, cerebral engraftment of donor cells after non-myeloablative bone marrow transplantation.

Myeloablative (MyA) bone marrow transplantation (BMT) results in robust engraftment of BMT-derived cells in the central nervous system (CNS) and is neuroprotective in diverse experimental models of neurodegenerative diseases of the brain and retina. However, MyA irradiation is associated with significant morbidity and mortality and does not represent a viable therapeutic option for the elderly. Non-myeloablative (NMyA) BMT is less toxic, but it is not known if the therapeutic efficacy observed with MyA BMT is preserved. As a first step to address this important gap in knowledge, we evaluated and compared engraftment characteristics of BMT-derived monocytes/microglia using several clinically relevant NMyA pretransplant conditioning regimens in C57BL/6 mice. These included chemotherapy (fludarabine and cyclophosphamide) with or without 2 Gy irradiation, and 5.5 Gy irradiation alone. Each regimen was followed by transplantation of whole bone marrow from green fluorescent protein-expressing wild type (wt) mice. While stable hematopoietic engraftment occurred, to varying degrees, in all NMyA regimens, only 5.5 Gy irradiation resulted in significant engraftment of BMT-derived cells in the brain, where these cells were exclusively localized to perivascular, leptomeningeal, and related anatomic regions. Engraftment in retina under 5.5 Gy NMyA conditions was significantly reduced compared to MyA, but robust engraftment was identified in the optic nerve. Advancing the therapeutic applications of BMT to neurodegenerative diseases will require identification of the barrier mechanisms that MyA, but not NMyA, BMT is able to overcome.

Ablation of the microglial protein DOCK2 reduces amyloid burden in a mouse model of Alzheimer's disease.

Alzheimer's disease (AD) neuropathology is characterized by innate immune activation primarily through prostaglandin E2 (PGE2) signaling. Dedicator of cytokinesis 2 (DOCK2) is a guanyl nucleotide exchange factor expressed exclusively in microglia in the brain and is regulated by PGE2 receptor EP2. DOCK2 modulates microglia cytokine secretion, phagocytosis, and paracrine neurotoxicity. EP2 ablation in experimental AD results in reduced oxidative damage and amyloid beta (Aß) burden. This discovery led us to hypothesize that genetic ablation of DOCK2 would replicate the anti-Aß effects of loss of EP2 in experimental AD. To test this hypothesis, we crossed mice that lacked DOCK2 (DOCK2-/-), were hemizygous for DOCK2 (DOCK2+/-), or that expressed two DOCK2 genes (DOCK2+/+) with APPswe-PS1Δe9 mice (a model of AD). While we found no DOCK2-dependent differences in cortex or in hippocampal microglia density or morphology in APPswe-PS1Δe9 mice, cerebral cortical and hippocampal Aß plaque area and size were significantly reduced in 10-month-old APPswe-PS1Δe9/DOCK2-/- mice compared with APPswe-PS1Δe9/DOCK2+/+ controls. DOCK2 hemizygous APPswe-PS1Δe9 mice had intermediate Aß plaque levels. Interestingly, soluble Aß42 was not significantly different among the three genotypes, suggesting the effects were mediated specifically in fibrillar Aß. In combination with earlier cell culture results, our in vivo results presented here suggest DOCK2 contributes to Aß plaque burden via regulation of microglial innate immune function and may represent a novel therapeutic target for AD.