Yochai Wolf

A new type of microglia gene targeting shows TAK1 to be pivotal in CNS autoimmune inflammation

Microglia are brain macrophages and, as such, key immune-competent cells that can respond to environmental changes. Understanding the mechanisms of microglia-specific responses during pathologies is hence vital for reducing disease burden. The definition of microglial functions has so far been hampered by the lack of genetic in vivo approaches that allow discrimination of microglia from closely related peripheral macrophage populations in the body. Here we introduce a mouse experimental system that specifically targets microglia to examine the role of a mitogen-associated protein kinase kinase kinase (MAP3K), transforming growth factor (TGF)-β-activated kinase 1 (TAK1), during autoimmune inflammation. Conditional depletion of TAK1 in microglia only, not in neuroectodermal cells, suppressed disease, significantly reduced CNS inflammation and diminished axonal and myelin damage by cell-autonomous inhibition of the NF-κB, JNK and ERK1/2 pathways. Thus, we found TAK1 to be pivotal in CNS autoimmunity, and we present a tool for future investigations of microglial function in the CNS.

Progressive replacement of embryo-derived cardiac macrophages with age

Molawi et al. examine the origin and cellular dynamics of macrophages in the heart during postnatal development. Cardiac macrophages derived from CX3CR1+ embryonic progenitors persist into adulthood, but the contribution of these cells to resident macrophages declines after birth with diminished self-renewal as the mice age. Over time, the heart is progressively reconstituted with bone marrow–derived macrophages, even in the absence of inflammation.

Genetic Cell Ablation Reveals Clusters of Local Self-Renewing Microglia in the Mammalian Central Nervous System

During early embryogenesis, microglia arise from yolk sac progenitors that populate the developing central nervous system (CNS), but how the tissue-resident macrophages are maintained throughout the organisms lifespan still remains unclear. Here, we describe a system that allows specific, conditional ablation of microglia in adult mice. We found that the microglial compartment was reconstituted within 1 week of depletion. Microglia repopulation relied on CNS-resident cells, independent from bone-marrow-derived precursors. During repopulation, microglia formed clusters of highly proliferative cells that migrated apart once steady state was achieved. Proliferating microglia expressed high amounts of the interleukin-1 receptor (IL-1R), and treatment with an IL-1R antagonist during the repopulation phase impaired microglia proliferation. Hence, microglia have the potential for efficient self-renewal without the contribution of peripheral myeloid cells, and IL-1R signaling participates in this restorative proliferation process.

Microglia, seen from the CX3CR1 angle

Microglial cells in brain and spinal cord are characterized by high expression of the chemokine receptor CX3CR1. Expression of the sole CX3CR1 ligand, the membrane-tethered and sheddable chemokine CX3CL1/fractalkine, is restricted in the brain parenchyma to selected neurons. Here we summarize our current understanding of the physiological role of CX3CR1 for microglia function and the CX3C axis in microglial/neuronal crosstalk in homeostasis and under challenge. Moreover, we will discuss the efforts of our laboratory and others to exploit CX3CR1 promoter activity for the visualization and genetic manipulation of microglia to probe their functional contributions in the central nerve system (CNS) context.

Microglia: unique and common features with other tissue macrophages.

Microglia are highly specialized tissue macrophages of the brain with dedicated functions in neuronal development, homeostasis and recovery from pathology Despite their unique localization in the central nervous system (CNS), microglia are ontogenetically and functionally related to their peripheral counterparts of the mononuclear phagocytic system in the body, namely tissue macrophages and circulating myeloid cells. Recent developments provided new insights into the myeloid system in the body with microglia emerging as intriguing unique archetypes. Similar to other tissue macrophages, microglia develop early during embryogenesis from immature yolk sac progenitors. But in contrast to most of their tissue relatives microglia persist throughout the entire life of the organism without any significant input from circulating blood cells due to their longevity and their capacity of self-renewal. Notably, microglia share some features with short-lived blood monocytes to limit CNS tissue damage in pathologies, but only bone marrow-derived cells display the ability to become permanently integrated in the parenchyma. This emphasizes the therapeutic potential of bone marrow-derived microglia-like cells. Further understanding of both fate and function of microglia during CNS pathologies and considering their uniqueness among other tissue macrophages will be pivotal for potential manipulation of immune cell function in the CNS, thereby reducing disease burden. Here, we discuss new aspects of myeloid cell biology in general with special emphasis on the brain-resident macrophages and microglia.

Brown-adipose-tissue macrophages control tissue innervation and homeostatic energy expenditure

Tissue macrophages provide immunological defense and contribute to the establishment and maintenance of tissue homeostasis. Here we used constitutive and inducible mutagenesis to delete the nuclear transcription regulator Mecp2 in macrophages. Mice that lacked the gene encoding Mecp2, which is associated with Rett syndrome, in macrophages did not show signs of neurodevelopmental disorder but displayed spontaneous obesity, which was linked to impaired function of brown adipose tissue (BAT). Specifically, mutagenesis of a BAT-resident Cx3Cr1+ macrophage subpopulation compromised homeostatic thermogenesis but not acute, cold-induced thermogenesis. Mechanistically, malfunction of BAT in pre-obese mice with mutant macrophages was associated with diminished sympathetic innervation and local titers of norepinephrine, which resulted in lower expression of thermogenic factors by adipocytes. Mutant macrophages overexpressed the signaling receptor and ligand PlexinA4, which might contribute to the phenotype by repulsion of sympathetic axons expressing the transmembrane semaphorin Sema6A. Collectively, we report a previously unappreciated homeostatic role for macrophages in the control of tissue innervation. Disruption of this circuit in BAT resulted in metabolic imbalance.

Induced-Pluripotent-Stem-Cell-Derived Primitive Macrophages Provide a Platform for Modeling Tissue-Resident Macrophage Differentiation and Function

Summary Tissue macrophages arise during embryogenesis from yolk‐sac (YS) progenitors that give rise to primitive YS macrophages. Until recently, it has been impossible to isolate or derive sufficient numbers of YS‐derived macrophages for further study, but data now suggest that induced pluripotent stem cells (iPSCs) can be driven to undergo a process reminiscent of YS‐hematopoiesis in vitro. We asked whether iPSC‐derived primitive macrophages (iMacs) can terminally differentiate into specialized macrophages with the help of growth factors and organ‐specific cues. Co‐culturing human or murine iMacs with iPSC‐derived neurons promoted differentiation into microglia‐like cells in vitro. Furthermore, murine iMacs differentiated in vivo into microglia after injection into the brain and into functional alveolar macrophages after engraftment in the lung. Finally, iPSCs from a patient with familial Mediterranean fever differentiated into iMacs with pro‐inflammatory characteristics, mimicking the disease phenotype. Altogether, iMacs constitute a source of tissue‐resident macrophage precursors that can be used for biological, pathophysiological, and therapeutic studies. Graphical Abstract Figure. No Caption available. HighlightsHuman and mouse iPSCs can recapitulate YS hematopoiesisHuman and mouse iPSCs can differentiate into YS macrophage‐like cells (iMacs)iMacs can further differentiate to tissue macrophage‐like cells with organ‐specific cuesiMacs derived from an FMF patient’s iPSCs exhibit disease‐specific characteristics &NA; Yolk‐sac (YS) embryonic macrophages contribute to tissue‐resident macrophages but remain difficult to study because of their stage‐dependent limited availability. Takata et al. demonstrate that iPSCs can generate YS macrophage‐like cells (iMacs) that differentiate into functional tissue‐resident macrophage‐like cells upon receiving organ‐specific cues, thus providing a platform for modeling tissue‐resident macrophages.

Microglia contribute to circuit defects in Mecp2 null mice independent of microglia-specific loss of Mecp2 expression

Microglia, the resident CNS macrophages, have been implicated in the pathogenesis of Rett Syndrome (RTT), an X-linked neurodevelopmental disorder. However, the mechanism by which microglia contribute to the disorder is unclear and recent data suggest that microglia do not play a causative role. Here, we use the retinogeniculate system to determine if and how microglia contribute to pathogenesis in a RTT mouse model, the Mecp2 null mouse (Mecp2tm1.1Bird/y). We demonstrate that microglia contribute to pathogenesis by excessively engulfing, thereby eliminating, presynaptic inputs at end stages of disease (≥P56 Mecp2 null mice) concomitant with synapse loss. Furthermore, loss or gain of Mecp2 expression specifically in microglia (Cx3cr1CreER;Mecp2fl/yor Cx3cr1CreER; Mecp2LSL/y) had little effect on excessive engulfment, synapse loss, or phenotypic abnormalities. Taken together, our data suggest that microglia contribute to end stages of disease by dismantling neural circuits rendered vulnerable by loss of Mecp2 in other CNS cell types. DOI: http://dx.doi.org/10.7554/eLife.15224.001

Activation of the alternative NFκB pathway improves disease symptoms in a model of Sjogren's syndrome.

The purpose of our study was to understand if Toll-like receptor 9 (TLR9) activation could contribute to the control of inflammation in Sjogrens syndrome. To this end, we manipulated TLR9 signaling in non-obese diabetic (NOD) and TLR9−/− mice using agonistic CpG oligonucleotide aptamers, TLR9 inhibitors, and the in-house oligonucleotide BL-7040. We then measured salivation, inflammatory response markers, and expression of proteins downstream to NF-κB activation pathways. Finally, we labeled proteins of interest in salivary gland biopsies from Sjogrens syndrome patients, compared to Sicca syndrome controls. We show that in NOD mice BL-7040 activates TLR9 to induce an alternative NF-κB activation mode resulting in increased salivation, elevated anti-inflammatory response in salivary glands, and reduced peripheral AChE activity. These effects were more prominent and also suppressible by TLR9 inhibitors in NOD mice, but TLR9−/− mice were resistant to the salivation-promoting effects of CpG oligonucleotides and BL-7040. Last, salivary glands from Sjogrens disease patients showed increased inflammatory and decreased anti-inflammatory biomarkers, in addition to decreased levels of alternative NF-κB pathway proteins. In summary, we have demonstrated that activation of TLR9 by BL-7040 leads to non-canonical activation of NF-κB, promoting salivary functioning and down-regulating inflammation. We propose that BL-7040 could be beneficial in treating Sjogrens syndrome and may be applicable to additional autoimmune syndromes.

Adaptive Immune Regulation of Mammary Postnatal Organogenesis

Postnatal organogenesis occurs in an immune competent environment and is tightly controlled by interplay between positive and negative regulators. Innate immune cells have beneficial roles in postnatal tissue remodeling, but roles for the adaptive immune system are currently unexplored. Here we show that adaptive immune responses participate in the normal postnatal development of a non-lymphoid epithelial tissue. Since the mammary gland (MG) is the only organ developing predominantly after birth, we utilized it as a powerful system to study adaptive immune regulation of organogenesis. We found that antigen-mediated interactions between mammary antigen-presenting cells and interferon-γ (IFNγ)-producing CD4+ T helper 1 cells participate in MG postnatal organogenesis as negative regulators, locally orchestrating epithelial rearrangement. IFNγ then affects luminal lineage differentiation. This function of adaptive immune responses, regulating normal development, changes the paradigm for studying players of postnatal organogenesis and provides insights into immune surveillance and cancer transformation.