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

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 C1qa FL/FL :Cx3cr1 CreERT2 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 Cx3cr1 CreERT2/WganJ mice. C1q expression in C1qa FL/FL : Cx3cr1 CreERT2/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 C1qa FL/FL : Cx3cr1 CreERT2/WganJ mice relative to controls, and C1qa FL/FL : Cx3cr1 CreERT2/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 Cx3cr1 CreERT2/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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-017-0814-9) contains supplementary material, which is available to authorized users.

Comparative analysis of CreER transgenic mice for the study of brain macrophages: A case study

“…4B–D). This is in line with an earlier report on Cx 3 cr1 CreER mice that harbored a different ‘floxed’ allele and also our recent more detailed RiboTag analysis of these animals . In the CreER transgenic approach that was originally introduced by the Chambon group , a Cre / estrogen receptor (ER) fusion protein is kept latent in the cytoplasm through binding to HSP90 .…”

STOP floxing around: Specificity and leakiness of inducible Cre/loxP systems

“…Indeed, a similar recent study provided similar conclusions [22]. Furthermore, CreER leakiness has previously been observed in both fluorescent reporter and conditional knock-out studies [23,24], suggesting that this is highly likely a conserved limitation of the CreER system. The authors also exclude that the tamoxifen-independent recombination was associated to the strong synthetic 'CAG' promoter present in some reporter strains.…”

C1q: A fresh look upon an old molecule

“…In contrast to the predominant synthesis of C1q in the adult brain by microglia (Fonseca et al, 2017), here C1q was produced by the neurons in a TGFβ and activity dependent fashion (Bialas and Stevens, 2013). Subsequently, these investigators and others have further uncovered that excessive complement-mediated synapse elimination is associated with neuronal degeneration (Williams et al, 2016) and with cognitive loss in diverse mouse models of aging and neurological disorders (Shi et al, 2015; Hong et al, 2016; Lui et al, 2016; Vasek et al, 2016).…”

“…In addition, while plasma containing circulating C1q as part of C1 complex (Ziccardi and Tschopp, 1982) may represent a possible source of tissue-bound C1q in cases of breached vasculature, it is increasing evident that C1q can be synthesized and secreted locally by various cell types, including macrophages, dendritic cells, fibroblasts, and mast cells that are ubiquitously distributed throughout the body (Ghebrehiwet et al, 2012). In addition, other cells selectively localized in specific tissues and organs such as microglial cells, glomerular and tubular cells, osteoclasts, and trophoblasts contribute to local production of C1q (Fonseca et al, 2017; Xavier et al, 2017). Although C1q is likely to be secreted in limited amount at the extravascular sites, recent evidence suggests that locally synthesized C1q is involved in the regulation of multiple cellular functions in addition to the cellular control of innate and adaptive immunity, as described above.…”