Sujata Rao

WNT7b mediates macrophage-induced programmed cell death in patterning of the vasculature

Macrophages have a critical role in inflammatory and immune responses through their ability to recognize and engulf apoptotic cells. Here we show that macrophages initiate a cell-death programme in target cells by activating the canonical WNT pathway. We show in mice that macrophage WNT7b is a short-range paracrine signal required for WNT-pathway responses and programmed cell death in the vascular endothelial cells of the temporary hyaloid vessels of the developing eye. These findings indicate that macrophages can use WNT ligands to influence cell-fate decisions—including cell death—in adjacent cells, and raise the possibility that they do so in many different cellular contexts.

Macrophage Wnt7b is critical for kidney repair and regeneration

Macrophages are required for tissue homeostasis through their role in regulation of the immune response and the resolution of injury. Here we show, using the kidney as a model, that the Wnt pathway ligand Wnt7b is produced by macrophages to stimulate repair and regeneration. When macrophages are inducibly ablated from the injured kidney, the canonical Wnt pathway response in kidney epithelial cells is reduced. Furthermore, when Wnt7b is somatically deleted in macrophages, repair of injury is greatly diminished. Finally, injection of the Wnt pathway regulator Dkk2 enhances the repair process and suggests a therapeutic option. Because Wnt7b is known to stimulate epithelial responses during kidney development, these findings suggest that macrophages are able to rapidly invade an injured tissue and reestablish a developmental program that is beneficial for repair and regeneration.

Regulation of angiogenesis by a non-canonical Wnt-Flt1 pathway in myeloid cells

Myeloid cells are a feature of most tissues. Here we show that during development, retinal myeloid cells (RMCs) produce Wnt ligands to regulate blood vessel branching. In the mouse retina, where angiogenesis occurs postnatally, somatic deletion in RMCs of the Wnt ligand transporter Wntless results in increased angiogenesis in the deeper layers. We also show that mutation of Wnt5a and Wnt11 results in increased angiogenesis and that these ligands elicit RMC responses via a non-canonical Wnt pathway. Using cultured myeloid-like cells and RMC somatic deletion of Flt1, we show that an effector of Wnt-dependent suppression of angiogenesis by RMCs is Flt1, a naturally occurring inhibitor of vascular endothelial growth factor (VEGF). These findings indicate that resident myeloid cells can use a non-canonical, Wnt–Flt1 pathway to suppress angiogenic branching.

Discovery and Characterization of a Small Molecule Inhibitor of the PDZ Domain of Dishevelled

Dishevelled (Dvl) is an essential protein in the Wnt signaling pathways; it uses its PDZ domain to transduce the Wnt signals from the membrane receptor Frizzled to downstream components. Here, we report identifying a drug-like small molecule compound through structure-based ligand screening and NMR spectroscopy and show the compound to interact at low micromolar affinity with the PDZ domain of Dvl. In a Xenopus testing system, the compound could permeate the cell membrane and block the Wnt signaling pathways. In addition, the compound inhibited Wnt signaling and reduced the levels of apoptosis in the hyaloid vessels of eye. Moreover, this compound also suppressed the growth of prostate cancer PC-3 cells. These biological effects suggest that by blocking the PDZ domain of Dvl, the compound identified in our studies effectively inhibits the Wnt signaling and thus provides a useful tool for studies dissecting the Wnt signaling pathways.

A direct and melanopsin-dependent fetal light response regulates mouse eye development

Vascular patterning is critical for organ function. In the eye, there is simultaneous regression of embryonic hyaloid vasculature (important to clear the optical path) and formation of the retinal vasculature (important for the high metabolic demands of retinal neurons). These events occur postnatally in the mouse. Here we have identified a light-response pathway that regulates both processes. We show that when mice are mutated in the gene (Opn4) for the atypical opsin melanopsin, or are dark-reared from late gestation, the hyaloid vessels are persistent at 8 days post-partum and the retinal vasculature overgrows. We provide evidence that these vascular anomalies are explained by a light-response pathway that suppresses retinal neuron number, limits hypoxia and, as a consequence, holds local expression of vascular endothelial growth factor (VEGFA) in check. We also show that the light response for this pathway occurs in late gestation at about embryonic day 16 and requires the photopigment in the fetus and not the mother. Measurements show that visceral cavity photon flux is probably sufficient to activate melanopsin-expressing retinal ganglion cells in the mouse fetus. These data thus show that light—the stimulus for function of the mature eye—is also critical in preparing the eye for vision by regulating retinal neuron number and initiating a series of events that ultimately pattern the ocular blood vessels.

Generation of mice with a conditional null allele for Wntless.

The Wnt‐signaling pathway is necessary in a variety of developmental processes and has been implicated in numerous pathologies. Wntless (Wls) binds to Wnt proteins and facilitates Wnt sorting and secretion. Conventional deletion of Wls results in early fetal lethality due to defects in body axis establishment. To gain insight into the function of Wls in later stages of development, we have generated a conditional null allele. Homozygous germline deletion of Wls confirmed prenatal lethality and failure of embryonic axis formation. Deletion of Wls using Wnt1‐cre phenocopied Wnt1 null abnormalities in the midbrain and hindbrain. In addition, conditional deletion of Wls in pancreatic precursor cells resulted in pancreatic hypoplasia similar to that previously observed after conditional β‐catenin deletion. This Wls conditional null allele will be valuable in detecting novel Wnt functions in development and disease. genesis 48:554–558, 2010.

Macrophages define dermal lymphatic vessel calibre during development by regulating lymphatic endothelial cell proliferation

Macrophages have been suggested to stimulate neo-lymphangiogenesis in settings of inflammation via two potential mechanisms: (1) acting as a source of lymphatic endothelial progenitor cells via the ability to transdifferentiate into lymphatic endothelial cells and be incorporated into growing lymphatic vessels; and (2) providing a crucial source of pro-lymphangiogenic growth factors and proteases. We set out to establish whether cells of the myeloid lineage are important for development of the lymphatic vasculature through either of these mechanisms. Here, we provide lineage tracing evidence to demonstrate that lymphatic endothelial cells arise independently of the myeloid lineage during both embryogenesis and tumour-stimulated lymphangiogenesis in the mouse, thus excluding macrophages as a source of lymphatic endothelial progenitor cells in these settings. In addition, we demonstrate that the dermal lymphatic vasculature of PU.1–/– and Csf1r–/– macrophage-deficient mouse embryos is hyperplastic owing to elevated lymphatic endothelial cell proliferation, suggesting that cells of the myeloid lineage provide signals that act to restrain lymphatic vessel calibre in the skin during development. In contrast to what has been demonstrated in settings of inflammation, macrophages do not comprise the principal source of pro-lymphangiogenic growth factors, including VEGFC and VEGFD, in the embryonic dermal microenvironment, illustrating that the sources of patterning and proliferative signals driving embryonic and disease-stimulated lymphangiogenesis are likely to be distinct.

Wntless functions in mature osteoblasts to regulate bone mass

Recent genome-wide association studies of individuals of Asian and European descent have found that SNPs located within the genomic region (1p31.3) encoding the Wntless (Wls)/Gpr177 protein are associated significantly with reduced bone mineral density. Wls/Gpr177 is a newly identified chaperone protein that specifically escorts Wnt ligands for secretion. Given the strong functional association between the Wnt signaling pathways and bone development and homeostasis, we generated osteoblast-specific Wls-deficient (Ocn-Cre;Wls-flox) mice. Homozygous conditional knockout animals were born at a normal Mendelian frequency. Whole-body dual-energy X-ray absorptiometry scanning revealed that bone-mass accrual was significantly inhibited in homozygotes as early as 20 d of age. These homozygotes had spontaneous fractures and a high frequency of premature lethality at around 2 mo of age. Microcomputed tomography analysis and histomorphometric data revealed a dramatic reduction of both trabecular and cortical bone mass in homozygous mutants. Bone formation in homozygotes was severely impaired, but no obvious phenotypic change was observed in mice heterozygous for the conditional deletion. In vitro studies showed that Wls-deficient osteoblasts had a defect in differentiation and mineralization, with significant reductions in the expression of key osteoblast differentiation regulators. In summary, these results reveal a surprising and crucial role of osteoblast-secreted Wnt ligands in bone-mass accrual.

Obligatory participation of macrophages in an angiopoietin 2-mediated cell death switch

Macrophages have a critical function in the recognition and engulfment of dead cells. In some settings, macrophages also actively signal programmed cell death. Here we show that during developmentally scheduled vascular regression, resident macrophages are an obligatory participant in a signaling switch that favors death over survival. This switch occurs when the signaling ligand angiopoietin 2 has the dual effect of suppressing survival signaling in vascular endothelial cells (VECs) and stimulating Wnt ligand production by macrophages. In response to the Wnt ligand, VECs enter the cell cycle and in the absence of survival signals, die from G1 phase of the cell cycle. We propose that this mechanism represents an adaptation to ensure that the macrophage and its disposal capability are on hand when cell death occurs.

Macrophage Wnt-Calcineurin-Flt1 signaling regulates mouse wound angiogenesis and repair.

The treatment of festering wounds is one of the most important aspects of medical care. Macrophages are important components of wound repair, both in fending off infection and in coordinating tissue repair. Here we show that macrophages use a Wnt-Calcineurin-Flt1 signaling pathway to suppress wound vasculature and delay repair. Conditional mutants deficient in both Wntless/GPR177, the secretory transporter of Wnt ligands, and CNB1, the essential component of the nuclear factor of activated T cells dephosporylation complex, displayed enhanced angiogenesis and accelerated repair. Furthermore, in myeloid-like cells, we show that noncanonical Wnt activates Flt1, a naturally occurring inhibitor of vascular endothelial growth factor-A-mediated angiogenesis, but only when calcineurin function is intact. Then, as expected, conditional deletion of Flt1 in macrophages resulted in enhanced wound angiogenesis and repair. These results are consistent with the published link between enhanced angiogenesis and enhanced repair, and establish novel therapeutic approaches for treatment of wounds.