Effects of G-CSF on Monocytes and Neurons: in vitro and in vivo studies in a Mouse Model of Alzheimer\u27s Disease

G-CSF is routinely used to treat neutropenia/leukopenia or to increase hematopoietic stem cell generation in bone marrow donors. G-CSF and its receptor, G-CSFR, are produced by various cell types both in the peripheral circulation and within brain. As a consequence, exogenous administration of G-CSF results in a broad spectrum of effects involving hematopoietic, immune and central nervous systems. G-CSF administration in a mouse model of Alzheimer\u27s disease (AD) has revealed both cognitive benefits and disease modifying effects: a) decreased Aβ plaque burden, b) increased microgliosis, c) increased neurogenesis and d) improved performance in radial arm water maze (RAWM). In clinical studies, G-CSF plasma levels were found to be lower in patients with early AD in comparison to healthy age matched controls. A course of G-CSF administration in humans is known to increase levels of circulating hematopoietic stem cells (CD34 cells), monocytes and neutrophils in patients with neutropenia and when administered to patients with AD, there is also a similar increase in absolute monocyte count, CD34 cells and total neutrophils. The extent to which the beneficial effects of G-CSF in AD depend on monocyte infiltration into CNS, compared to direct neurotrophic actions of G-CSF on the CNS, is not known. The overall goal of this study was to investigate and understand the effects of G-CSF in an AD mouse model, but more specifically to distinguish the actions of G-CSF that affect the peripheral monocyte population from the direct actions on CNS. The first approach was to examine in vitro effects of G-CSF within a monocytic cell line (THP-1) and a neuronal cell line (SH-SY5Y). The second approach was to study effects of G-CSF on infiltration of bone marrow-derived cells into the brain by utilizing a chimeric GFP+ APP/PS1 AD mouse model. The third approach was to assess the effects of G-CSF on hippocampal neurogenesis in both a wild-type and AD mouse model. Comparison of the monocytic and neuronal cell lines showed a) G-CSF interacts with its cognate receptor with different binding kinetics and with a greater affinity for the monocyte G-CSFR, b) the number of G-CSF receptors in neurons is greater than in monocytes, and c) the anti-apoptotic response in neurons occurs at lower concentrations of G-CSF than in monocytes. Various concentrations of G-CSF increased proliferation of both the monocytic and neuronal cell line in vitro. G-CSF did not improve migratory properties of the monocytic cell line, either adhesiveness or migration through a membrane. In vivo G-CSF treatment (250μg/kg s.c. qod for 2 ½ weeks) in both the AD chimeric and non-chimeric AD mice resulted in increased microgliosis and decreased amyloid plaque burden in the hippocampus. In the chimeric AD mice, G-CSF treatment did not increase infiltration of GFP+ bone marrow derived cells (BMDC) into brain parenchyma and did not increase adhesion to microvasculature. In the non-chimeric AD mice there was improvement of neurogenesis to non-transgenic levels after G-CSF treatment and an increase in synaptogenesis in the CA1 region of the hippocampus. The effects of G-CSF on the endogeneous microglial population are most likely responsible for the increase in microgliosis, as no significant increase of BMDC infiltration into the brain parenchyma was found in vivo. The enhanced proliferation and improved viability of the neuronal cell line after G-CSF treatment may explain the improvement in neurogenesis and significant increase in synaptogenesis seen in the AD mouse model. The actions of G-CSF on neural stem/progenitor cells to stimulate hippocampal neurogenesis and to enhance resident microglial capacity to decrease amyloid burden are the most likely mechanisms responsible for the behavioral improvement seen in the AD mouse model