Investigations into the accumulation of hematopoietic cells in the spinal cord in a murine model of motor neuron disease

Transgenic mice over-expressing human mutant superoxide dismutase 1 (mSOD) develop motoneuron loss resembling amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease. Bone marrow (BM) chimeric mice created by myeloablating and transplanting mice with green fluorescent protein (GFP)-labelled BM were used to study the recruitment of BM-derived cells (BMDCs) into spinal cords of mSOD and control mice. Accumulation of GFP+ cells in mSOD spinal cord paralleled disease progression and significantly greater numbers of GFP+ cells were observed in mSOD spinal cord at symptomatic and disease end-stages compared to controls. GFP+ BMDCs expressed the macrophage markers CD11b and F4/80, which are also expressed by microglia. GFP+ BDMCs constituted 10-20% of total CD11b/F4/80 + cells within spinal cord, indicating expansion of microglia within mSOD spinal cord is primarily through proliferation of resident microglia. Analysis of morphology and proximity of BMDCs to blood vessels revealed that only a fraction of BMDCs acquire the stellate morphology and Iba1+ immunophenotype characteristic of parenchymal microglia, and the majority of BMDCs remained in close proximity to blood vessels. Mice transplanted with BM from donors expressing GFP in only CX3CR1+ cells demonstrated that this population of cells accumulates within control and mSOD spinal cord. To determine whether myelosuppressive regimens alternative to irradiation potentiate BMDC accumulation in mSOD and control spinal cord, mice were treated with the chemotherapeutic Busulfex (BU) and transplanted with GFP+ BM. GFP+ cells were observed in spinal cords of mSOD and control mice. Cytotoxic T-cells were also observed in control and mSOD spinal cord, suggesting the dose of BU used in this study has neurotoxic and neuroinflammatory effects. The differential accumulation of CX3CR1+ BM cells and cells derived from definitive hematopoiesis was analyzed by transplanting irradiated mSOD and control mice with the CX3CR1+/GFP fraction of BM cells along with red fluorescent protein (RFP) c-Kit+Lin-Sca1+ cells. Analysis of spinal cord at disease end-stage revealed CX3CR1+/GFP and RFP+ cells in the spinal cords of mSOD and controls, indicating that circulating cells from the CX3CR1+/GFP BM fraction and cells derived from definitive haematopoiesis are accumulate in spinal cord.