Signaling of stem cell factor/c-kit receptor in the regulation of microglia

Abstract

Signaling between sum cell factor (SCF) produced by stroma cells and its receptor, encoded by c-kit, expressed by stem cells or progenitor cells, is required for the normal development of hematopoietic, melanogenic, and gametogenic cell lineages. SCF and c-kit are also expressed in the nervous system, but their role in the nervous system is unknown. Using immunocytochemistry, I found that SCF was produced mainly by neurons, whereas the c-kit receptor was expressed by some neurons that often make synapses with the SCF-producing neurons and by glial cells, suggesting that SCF/c-kit receptor plays a role in neuron-neuron interaction and in a paracrine interaction between the SCF-producing neurons and the c-kit-expressing glia. Microglia, isolated from neopallial cell cultures, expressed c-kit mRNA, as shown by Northern blot analysis, and protein, shown by Western blot and immunocytochemistry. The expression of c-kit receptor in microglia in culture was found to be subject to regulation by cytokines such as SCF, colony-stimulating factor-1 (CSF-1) and interferon γ (IFN-γ). Microglia in culture also expressed SCF, as demonstrated by Northern blot and immunocytochemistry. Stimulation of microglia by IFN-γ or CSF-1 upregulated SCF expression. Thus, an autocrine action through SCF and its c-kit receptor may occur in microglia, especially when microglia are stimulated by IFN-γ or CSF-1. Microglia in culture require CSF-1 for survival. Withdrawal of CSF-1 from culture medium causes microglial cell death, whereas I found that microglia were partially rescued by the addition of SCF. Moreover, at a high dosage, SCF maintained microglia in a bipolar or tripolar shape, a relatively quiescent state. Addition of SCF to microglia in culture increased the expression of mRNAs of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and ciliary neurotrophic factor (CNTF), whereas it did not affect mRNA expression of inflammatory cytokines, interleukin 1 (IL-1), tumor necrosis factor α (TNFα), and IL-6. Using a neuron-microglia co-culture system, I demonstrated that SCF-treated microglia supported cerebral cortical neuron survival and neurite outgrowth. These observations suggest that SCF maintains microglia in a quiescent state and that microglia in this state are trophic to neurons. Stimulation of microglia in culture by CSF-1 results in microglial proliferation. I found that SCF suppressed CSF-1-induced microglial proliferation in a dose-dependent manner. Stimulation of microglia by lipopolysaccharide (LPS) or IFN-γ induced significant mRNA expression of the inflammatory cytokines IL-1, TNFα, and IL-6. Addition of SCF together with IFN-γ suppressed the induction of IL-1 and TNFα mRNAs, but addition of SCF to microglia cultures increased the production of lipocortin-1 (LC-1), an anti-inflammatory cytokine. Hence, SCF in cultures of microglia suppresses the production of inflammatory cytokines and at the same time increases the production of anti-infammmatory cytokine.