Fc Gamma Receptor (FcγR) Expression and Signaling in the Developing Rat Brain

Maternal antibodies specific for antigens in the developing brain are implicated as risk factors for neurodevelopmental disorders, but how these antibodies interfere with neurodevelopment remain speculative. It has been postulated that immunoglobulin G-immune complexes (IgG-IC) activate Fc gamma receptors (FcγR) on non-immune cells in the brain, thereby modulating intracellular signaling and/or internalizing function-blocking antibodies specific for intracellular antigens. However, testing this hypothesis has been hindered by the paucity of data regarding FcγR in the developing brain. Thus, we first investigated FcγR expression in the brain of neonatal male and female rats using quantitative PCR analyses. FcgrIa, FcgrIIa, FcgrIIb, FcgrIIIa and Fcgrt transcripts were detectable in the cortex, hippocampus and cerebellum at postnatal days 1 and 7. These transcripts were also present in primary hippocampal and cortical cell cultures, where their expression was upregulated by IFNγ. In order to confirm protein abundance of FcγRIa, FcγRIIb and FcγRIIIa in cultured hippocampal and cortical neurons and astrocytes on the single cell and tissue level we used immunocytochemistry, western blotting, proteotype analysis, and flow cytometry. The data shows that a subpopulation of these cells co-express the excitatory FcγRIa and the inhibitory FcγRIIb. Functional analysis shows that exposure of hippocampal and cortical cell cultures to IgG-IC increased intracellular calcium and Erk phosphorylation, and triggered FcγR-mediated internalization of IgG. Collectively, these data demonstrate that developing neurons and astrocytes express signaling competent FcγR. which could establish a molecular mode of action of maternal antibodies could influence vulnerability to neurodevelopmental disorders via direct interactions with FcγR on non-immune cells in the developing brain. These findings support the hypothesis that maternal antibodies influence vulnerability to neurodevelopmental disorders via direct interactions with FcγR on non-immune cells in the developing brain.

针对发育中大脑抗原的母源性抗体被认定为神经发育障碍的风险因素，但其干扰神经发育的具体机制仍属推测范畴。有假说提出，免疫球蛋白G免疫复合物（IgG-IC）可激活脑内非免疫细胞表面的Fcγ受体（FcγR），进而调控细胞内信号通路，或内化针对细胞内抗原的功能阻断型抗体。然而，由于发育中大脑FcγR相关数据匮乏，该假说的验证工作长期受阻。
为此，本研究首先通过定量聚合酶链反应分析，检测了新生雌雄大鼠脑内的FcγR表达情况。在出生后第1天与第7天的皮层、海马体与小脑中，均可检测到FcgrIa、FcgrIIa、FcgrIIb、FcgrIIIa及Fcgrt的转录本。上述转录本同样存在于原代海马与皮层细胞培养体系中，且其表达可被干扰素γ（IFNγ）上调。
为在单细胞与组织水平验证培养的海马神经元、皮层神经元与星形胶质细胞中FcγRIa、FcγRIIb及FcγRIIIa的蛋白丰度，本研究采用了免疫细胞化学染色、蛋白质免疫印迹、蛋白质组型分析及流式细胞术技术。
研究数据显示，此类细胞的一个亚群可共表达兴奋性FcγRIa与抑制性FcγRIIb。功能分析结果表明，将原代海马及皮层细胞培养体系暴露于IgG-IC后，可升高细胞内钙离子水平并诱导细胞外调节蛋白激酶（Erk）磷酸化，同时触发FcγR介导的IgG内化过程。
综上，本研究数据证实，发育中的神经元与星形胶质细胞可表达具备信号转导能力的FcγR，明确了母源性抗体可通过与发育中脑内非免疫细胞表面的FcγR直接相互作用，影响个体对神经发育障碍易感性的潜在分子作用机制。本研究结果亦支持前述假说：母源性抗体可通过与发育中脑内非免疫细胞表面的FcγR直接相互作用，影响个体对神经发育障碍的易感性。