Kory J. Lavine

Embryonic and Adult-Derived Resident Cardiac Macrophages Are Maintained through Distinct Mechanisms at Steady State and during Inflammation

Cardiac macrophages are crucial for tissue repair after cardiac injury but are not well characterized. Here we identify four populations of cardiac macrophages. At steady state, resident macrophages were primarily maintained through local proliferation. However, after macrophage depletion or during cardiac inflammation, Ly6c(hi) monocytes contributed to all four macrophage populations, whereas resident macrophages also expanded numerically through proliferation. Genetic fate mapping revealed that yolk-sac and fetal monocyte progenitors gave rise to the majority of cardiac macrophages, and the heart was among a minority of organs in which substantial numbers of yolk-sac macrophages persisted in adulthood. CCR2 expression and dependence distinguished cardiac macrophages of adult monocyte versus embryonic origin. Transcriptional and functional data revealed that monocyte-derived macrophages coordinate cardiac inflammation, while playing redundant but lesser roles in antigen sampling and efferocytosis. These data highlight the presence of multiple cardiac macrophage subsets, with different functions, origins, and strategies to regulate compartment size.

Origin and Functions of Tissue Macrophages

Macrophages are distributed in tissues throughout the body and contribute to both homeostasis and disease. Recently, it has become evident that most adult tissue macrophages originate during embryonic development and not from circulating monocytes. Each tissue has its own composition of embryonically derived and adult-derived macrophages, but it is unclear whether macrophages of distinct origins are functionally interchangeable or have unique roles at steady state. This new understanding also prompts reconsideration of the function of circulating monocytes. Classical Ly6c(hi) monocytes patrol the extravascular space in resting organs, and Ly6c(lo) nonclassical monocytes patrol the vasculature. Inflammation triggers monocytes to differentiate into macrophages, but whether resident and newly recruited macrophages possess similar functions during inflammation is unclear. Here, we define the tools used for identifying the complex origin of tissue macrophages and discuss the relative contributions of tissue niche versus ontological origin to the regulation of macrophage functions during steady state and inflammation.

Distinct macrophage lineages contribute to disparate patterns of cardiac recovery and remodeling in the neonatal and adult heart

Significance This study addresses a fundamentally important and widely debated issue in the field of inflammation, which is why inflammation can be simultaneously deleterious after injury and yet is essential for tissue repair. Recently, an important new paradigm has emerged in the macrophage field: Organs are replete with resident macrophages of embryonic origin, distinct from monocyte-derived macrophages. In this article, we use a new model of cardiac injury and show that distinct macrophage populations derived from embryonic and adult lineages are important determinants of tissue repair and inflammation, respectively. Our data suggest that therapeutics, which inhibit monocyte-derived macrophages and/or selectively harness the function of embryonic-derived macrophages, may serve as novel treatments for heart failure. The mechanistic basis for why inflammation is simultaneously both deleterious and essential for tissue repair is not fully understood. Recently, a new paradigm has emerged: Organs are replete with resident macrophages of embryonic origin distinct from monocyte-derived macrophages. This added complexity raises the question of whether distinct immune cells drive inflammatory and reparative activities after injury. Previous work has demonstrated that the neonatal heart has a remarkable capacity for tissue repair compared with the adult heart, offering an ideal context to examine these concepts. We hypothesized that unrecognized differences in macrophage composition is a key determinant of cardiac tissue repair. Using a genetic model of cardiomyocyte ablation, we demonstrated that neonatal mice expand a population of embryonic-derived resident cardiac macrophages, which generate minimal inflammation and promote cardiac recovery through cardiomyocyte proliferation and angiogenesis. During homeostasis, the adult heart contains embryonic-derived macrophages with similar properties. However, after injury, these cells were replaced by monocyte-derived macrophages that are proinflammatory and lacked reparative activities. Inhibition of monocyte recruitment to the adult heart preserved embryonic-derived macrophage subsets, reduced inflammation, and enhanced tissue repair. These findings indicate that embryonic-derived macrophages are key mediators of cardiac recovery and suggest that therapeutics targeting distinct macrophage lineages may serve as novel treatments for heart failure.

FGF9 and SHH regulate mesenchymal Vegfa expression and development of the pulmonary capillary network

The juxtaposition of a dense capillary network to lung epithelial cells is essential for air-blood gas exchange. Defective lung vascular development can result in bronchopulmonary dysplasia and alveolar capillary dysplasia. Although vascular endothelial growth factor A (Vegfa) is required for formation of the lung capillary network, little is known regarding the factors that regulate the density and location of the distal capillary plexus and the expression pattern of Vegfa. Here, we show that fibroblast growth factor 9 (FGF9) and sonic hedgehog (SHH) signaling to lung mesenchyme, but not to endothelial cells, are each necessary and together sufficient for distal capillary development. Furthermore, both gain- and loss-of-function of FGF9 regulates Vegfa expression in lung mesenchyme, and VEGF signaling is required for FGF9-mediated blood vessel formation. FGF9, however, can only partially rescue the reduction in capillary density found in the absence of SHH signaling, and SHH is unable to rescue the vascular phenotype found in Fgf9-/- lungs. Thus, both signaling systems regulate distinct aspects of vascular development in distal lung mesenchyme. These data suggest a molecular mechanism through which FGF9 and SHH signaling coordinately control the growth and patterning of the lung capillary plexus, and regulate the temporal and spatial expression of Vegfa.

Rescue of Cardiomyopathy in Peroxisome Proliferator-Activated Receptor-α Transgenic Mice by Deletion of Lipoprotein Lipase Identifies Sources of Cardiac Lipids and Peroxisome Proliferator-Activated Receptor-α Activators

Background— Emerging evidence in obesity and diabetes mellitus demonstrates that excessive myocardial fatty acid uptake and oxidation contribute to cardiac dysfunction. Transgenic mice with cardiac-specific overexpression of the fatty acid–activated nuclear receptor peroxisome proliferator-activated receptor-α (myosin heavy chain [MHC]-PPARα mice) exhibit phenotypic features of the diabetic heart, which are rescued by deletion of CD36, a fatty acid transporter, despite persistent activation of PPARα gene targets involved in fatty acid oxidation. Methods and Results— To further define the source of fatty acid that leads to cardiomyopathy associated with lipid excess, we crossed MHC-PPARα mice with mice deficient for cardiac lipoprotein lipase (hsLpLko). MHC-PPARα/hsLpLko mice exhibit improved cardiac function and reduced myocardial triglyceride content compared with MHC-PPARα mice. Surprisingly, in contrast to MHC-PPARα/CD36ko mice, the activity of the cardiac PPARα gene regulatory pathway is normalized in MHC-PPARα/hsLpLko mice, suggesting that PPARα ligand activity exists in the lipoprotein particle. Indeed, LpL mediated hydrolysis of very-low-density lipoprotein activated PPARα in cardiac myocytes in culture. The rescue of cardiac function in both models was associated with improved mitochondrial ultrastructure and reactivation of transcriptional regulators of mitochondrial function. Conclusions— MHC-PPARα mouse hearts acquire excess lipoprotein-derived lipids. LpL deficiency rescues myocyte triglyceride accumulation, mitochondrial gene regulatory derangements, and contractile function in MHC-PPARα mice. Finally, LpL serves as a source of activating ligand for PPARα in the cardiomyocyte.

Bone morphogenetic protein receptor 1A signaling is dispensable for hematopoietic development but essential for vessel and atrioventricular endocardial cushion formation

Bone morphogenetic protein 4 (BMP4) is crucial for the formation of FLK1-expressing (FLK1+) mesodermal cells. To further define the requirement for BMP signaling in the differentiation of blood, endothelial and smooth muscle cells from FLK1+ mesoderm, we inactivated Alk3 (Bmpr1a) in FLK1+ cells by crossing Alk3floxed/floxed and Flk1+/CreAlk3+/floxed mice. Alk3 conditional knockout (CKO) mice died between E10.5 and E11.5. Unexpectedly, Alk3 CKO embryos did not show any hematopoietic defects. However, Alk3 CKO embryos displayed multiple abnormalities in vascular development, including vessel remodeling and maturation, which contributed to severe abdominal hemorrhage. Alk3 CKO embryos also displayed defects in atrioventricular canal (AVC) endocardial cushion formation in the heart. Collectively, our studies indicate a crucial role for ALK3 in vessel remodeling, vessel integrity and endocardial cushion formation during the development of the circulation system.

Macrophages Facilitate Electrical Conduction in the Heart

Organ-specific functions of tissue-resident macrophages in the steady-state heart are unknown. Here, we show that cardiac macrophages facilitate electrical conduction through the distal atrioventricular node, where conducting cells densely intersperse with elongated macrophages expressing connexin 43. When coupled to spontaneously beating cardiomyocytes via connexin-43-containing gap junctions, cardiac macrophages have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes. Conversely, macrophages render the resting membrane potential of cardiomyocytes more positive and, according to computational modeling, accelerate their repolarization. Photostimulation of channelrhodopsin-2-expressing macrophages improves atrioventricular conduction, whereas conditional deletion of connexin 43 in macrophages and congenital lack of macrophages delay atrioventricular conduction. In the Cd11bDTR mouse, macrophage ablation induces progressive atrioventricular block. These observations implicate macrophages in normal and aberrant cardiac conduction.

Hedgehog signaling is critical for maintenance of the adult coronary vasculature in mice

Hedgehog (HH) signaling has emerged as a critical pathway involved in the pathogenesis of a variety of tumors. As a result, HH antagonists are currently being evaluated as potential anticancer therapeutics. Conversely, activation of HH signaling in the adult heart may be beneficial, as HH agonists have been shown to increase coronary vessel density and improve coronary function after myocardial infarction. To investigate a potential homeostatic role for HH signaling in the adult heart, we ablated endogenous HH signaling in murine myocardial and perivascular smooth muscle cells. HH signaling was required for proangiogenic gene expression and maintenance of the adult coronary vasculature in mice. In the absence of HH signaling, loss of coronary blood vessels led to tissue hypoxia, cardiomyocyte cell death, heart failure, and subsequent lethality. We further showed that HH signaling specifically controlled the survival of small coronary arteries and capillaries. Together, these data demonstrate that HH signaling is essential for cardiac function at the level of the coronary vasculature and caution against the use of HH antagonists in patients with prior or ongoing heart disease.

Reciprocal epithelial-mesenchymal FGF signaling is required for cecal development

Fibroblast growth factor (FGF) signaling mediates reciprocal mesenchymal-epithelial cell interactions in the developing mouse lung and limb. In the gastrointestinal (GI) tract, FGF10 is expressed in the cecal mesenchyme and signals to an epithelial splice form of FGF receptor (FGFR) 2 to regulate epithelial budding. Here, we identify FGF9 as a reciprocal epithelial-mesenchymal signal required for cecal morphogenesis. Fgf9 null (Fgf9-/-) mouse embryos have agenesis of the embryonic cecum, lacking both mesenchymal expansion and an epithelial bud. In the cecal region of Fgf9-/- embryos, mesenchymal expression of Fgf10 and Bmp4 is notably absent, whereas the expression of epithelial markers, such as sonic hedgehog, is not affected. Using epithelial and whole explant cultures, we show that FGF9 signals to mesenchymal FGFRs and that FGF10 signals to epithelial FGFRs. Taken together, these data show that an epithelial FGF9 signal is necessary for the expansion of cecal mesenchyme and the expression of mesenchymal genes that are required for epithelial budding. Thus, these data add to our understanding of FGF-mediated reciprocal epithelial-mesenchymal signaling.

Hedgehog signaling to distinct cell types differentially regulates coronary artery and vein development

Vascular development begins with formation of a primary capillary plexus that is later remodeled to give rise to the definitive vasculature. Although the mechanism by which arterial and venous fates are acquired is well understood, little is known about when during vascular development arterial and venous vessels emerge and how their growth is regulated. Previously, we have demonstrated that a hedgehog (HH)/vascular endothelial growth factor (VEGF) and angiopoeitin 2 (ANG2) signaling pathway is essential for the development of the coronary vasculature. Here, we use conditional gene targeting to identify the cell types that receive HH signaling and mediate coronary vascular development. We show that HH signaling to the cardiomyoblast is required for the development of coronary veins, while HH signaling to the perivascular cell (PVC) is necessary for coronary arterial growth. Moreover, the cardiomyoblast and PVC appear to be the exclusive cell types that receive HH signals, as ablation of HH signaling in both cell types leads to an arrest in coronary development. Finally, we present evidence suggesting that coronary arteries and veins may be derived from distinct lineages.