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The transcription factor EGR2 is the molecular linchpin connecting STAT6 activation to the stable epigenomic program of alternative macrophage polarization

ABSTRACT: Macrophages polarize into functionally distinct subtypes while responding to microenvironmental cues. The identity of proximal transcription factors (TFs) downstream of the polarization signals are known, but their activity is typically transient, failing to explain the long-term, stable epigenomic programs developed. Here, we mapped the early and late epigenomic changes of interleukin-4 (IL-4)-induced alternative macrophage polarization. We identified the TF, Early Growth Response 2 (EGR2), bridging the early-transient and late-stable gene expression program of polarization. EGR2 is a direct target of IL-4-activated STAT6, having broad action indispensable for 77% of the induced gene signature of alternative polarization, including its autoregulation and a robust, downstream TF cascade involving PPARG. Mechanistically, EGR2 binding results in chromatin opening and the recruitment of chromatin remodelers and RNA-polymerase II. Egr2 induction is evolutionarily conserved during alternative polarization of mouse and human macrophages. In the context of tissue resident macrophages, Egr2 expression is most prominent in the lung of a variety of species. Thus, EGR2 is an example of an essential and evolutionarily conserved broad acting factor, linking transient polarization signals to stable epigenomic and transcriptional changes in macrophages.

ORGANISM(S): Mus musculus

PROVIDER: GSE151015 | GEO | 2020/09/15

REPOSITORIES: GEO

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Project description:Background: Macrophage polarization programs, commonly referred to as âclassicalâ and âalternativeâ activation, are widely considered as distinct states that are exclusive of one another, and are associated with different functions such as inflammation and wound healing, respectively. In a number of disease contexts, such as traumatic brain injury (TBI), macrophage polarization influences the extent of pathogenesis, and efforts are underway to eliminate pathogenic subsets. However, previous studies have not distinguished whether the simultaneous presence of both classical and alternative activation signatures represents the admixture of differentially polarized macrophages, or if they have adopted a unique state characterized by components of both classical and alternative activation. Results: We analyzed the polarization of individual macrophages responding to TBI using single-cell RNA sequencing. Analysis of signature polarization genes revealed diverse activation states, including M(IL4), M(IL10), and M(LPS, IFNÎ³). However, the expression of a given polarization marker was no more likely than at random to predict simultaneous expression or repression of markers of another polarization program within the same cell, suggesting a lack of exclusivity in macrophage polarization states in vivo in TBI. Also unexpectedly, individual TBI macrophages simultaneously expressed high levels of signature polarization genes across two or three different polarization states, and in several distinct and seemingly incompatible combinations. Conclusions: Single-cell gene expression profiling demonstrated that monocytic macrophages in TBI are not comprised of distinctly polarized subsets, but are uniquely and broadly activated. TBI macrophage activation in vivo is deeply complex, with individual cells concurrently adopting both inflammatory and reparative features. These data provide physiologically relevant evidence that the early macrophage response to TBI is comprised of novel activation states that are discordant with the current paradigm of macrophage polarizationâa key consideration for therapeutic modulation. Monocyte derived macrophages were isolated from the ipsilateral hemisphere of mouse brains one day following traumatic brain injury elicited by control cortical impact in C57BL/6 adult male mice. Single-cells were isolated and processed for RNA sequencing using a Fluidigm C1 integrated fluidic circuit chip. 45 biological replicates were analyzed.

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The transcription factor EGR2 is the molecular linchpin connecting STAT6 activation to the stable epigenomic program of alternative macrophage polarization

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