Cell-specific deletion of C1qa identifies microglia as the dominant source of C1q in mouse brain

Maria I Fonseca,Melody J Fang,Michael X Hernandez,Shu-Hui Chu,Pooja Selvan,Grant R Macgregor,Andrea J Tenner,Lila Modarresi

doi:10.1186/s12974-017-0814-9

Maria I Fonseca, Melody J Fang + Show 6 more

Open Access

https://doi.org/10.1186/s12974-017-0814-9

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Abstract

BackgroundThe complement cascade not only provides protection from infection but can also mediate destructive inflammation. Complement is also involved in elimination of neuronal synapses which is essential for proper development, but can be detrimental during aging and disease. C1q, required for several of these complement-mediated activities, is present in the neuropil, microglia, and a subset of interneurons in the brain.MethodsTo identify the source(s) of C1q in the brain, the C1qa gene was selectively inactivated in the microglia or Thy-1+ neurons in both wild type mice and a mouse model of Alzheimer’s disease (AD), and C1q synthesis assessed by immunohistochemistry, QPCR, and western blot analysis.ResultsWhile C1q expression in the brain was unaffected after inactivation of C1qa in Thy-1+ neurons, the brains of C1qaFL/FL:Cx3cr1CreERT2 mice in which C1qa was ablated in microglia were devoid of C1q with the exception of limited C1q in subsets of interneurons. Surprisingly, this loss of C1q occurred even in the absence of tamoxifen by 1 month of age, demonstrating that Cre activity is tamoxifen-independent in microglia in Cx3cr1CreERT2/WganJ mice. C1q expression in C1qaFL/FL: Cx3cr1CreERT2/WganJ mice continued to decline and remained almost completely absent through aging and in AD model mice. No difference in C1q was detected in the liver or kidney from C1qaFL/FL: Cx3cr1CreERT2/WganJ mice relative to controls, and C1qaFL/FL: Cx3cr1CreERT2/WganJ mice had minimal, if any, reduction in plasma C1q.ConclusionsThus, microglia, but not neurons or peripheral sources, are the dominant source of C1q in the brain. While demonstrating that the Cx3cr1CreERT2/WganJ deleter cannot be used for adult-induced deletion of genes in microglia, the model described here enables further investigation of physiological roles of C1q in the brain and identification of therapeutic targets for the selective control of complement-mediated activities contributing to neurodegenerative disorders.

Highlights

The complement cascade provides protection from infection but can mediate destructive inflammation
Initial attempts to use the lacZ reporter detection in the C1q “reporter-first” gene-targeted mouse failed to provide a signal in the brain (Additional file 1), and the monoclonal anti-C1q antibody clone 27.1 was used to analyze C1q expression
Before investigating the source of synthesis of C1q protein in the brain, we first verified that expression of C1q in mice homozygous for a floxed allele of C1qa was unchanged compared to wild type (WT) using immunofluorescence (Fig. 1a, b, d, and e) and western blot analysis of whole brain extracts (Fig. 1g, h)

Summary

Introduction

The complement cascade provides protection from infection but can mediate destructive inflammation. The complement system provides rapid recognition and response to danger that threatens the host. The complement cascade is regulated by the local environment [1] and synergizes with other responses to infection or stress or both [2,3,4]. C1q, independent of C1r and C1s, can enhance clearance of apoptotic cells and cellular debris, downregulate proinflammatory cytokine expression by phagocytes in vitro [17,18,19], and protect neurons from nutrient stress and fibrillar amyloid-induced damage [20, 21] consistent with a role in maintaining homeostasis. Synthesis of C1q is increased in response to a variety of neuronal injuries (reviewed in [5])

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