Downregulation of Grem1 expression in the distal limb mesoderm is a necessary precondition for phalanx development.

BACKGROUND: The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth. RESULTS: As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges. Fate mapping confirms that subridge mesoderm cells contribute to the metatarsal and phalanges when subridge Grem1 is down-regulated. Grem1 overexpression specifically blocks chick phalanx development by inhibiting PFR activity. PFR activity and digit development are also disrupted following overexpression of a Gli3 repressor, which results in Grem1 expression in the distal limb and downregulation of Bmpr1b. CONCLUSIONS: Based on expression and fate mapping studies, we propose that downregulation of Grem1 in the distal limb marks the transition from metatarsal to phalanx development. This suggests that downregulation of Grem1 in the distal limb mesoderm is necessary for phalanx development. Grem1 downregulation allows for full PFR activity and phalanx progenitor cell commitment to digit fate.

Transcriptional regulatory control of mammalian nephron progenitors revealed by multi-factor cistromic analysis and genetic studies.

Nephron progenitor number determines nephron endowment; a reduced nephron count is linked to the onset of kidney disease. Several transcriptional regulators including Six2, Wt1, Osr1, Sall1, Eya1, Pax2, and Hox11 paralogues are required for specification and/or maintenance of nephron progenitors. However, little is known about the regulatory intersection of these players. Here, we have mapped nephron progenitor-specific transcriptional networks of Six2, Hoxd11, Osr1, and Wt1. We identified 373 multi-factor associated 'regulatory hotspots' around genes closely associated with progenitor programs. To examine their functional significance, we deleted 'hotspot' enhancer elements for Six2 and Wnt4. Removal of the distal enhancer for Six2 leads to a ~40% reduction in Six2 expression. When combined with a Six2 null allele, progeny display a premature depletion of nephron progenitors. Loss of the Wnt4 enhancer led to a significant reduction of Wnt4 expression in renal vesicles and a mildly hypoplastic kidney, a phenotype also enhanced in combination with a Wnt4 null mutation. To explore the regulatory landscape that supports proper target gene expression, we performed CTCF ChIP-seq to identify insulator-boundary regions. One such putative boundary lies between the Six2 and Six3 loci. Evidence for the functional significance of this boundary was obtained by deep sequencing of the radiation-induced Brachyrrhine (Br) mutant allele. We identified an inversion of the Six2/Six3 locus around the CTCF-bound boundary, removing Six2 from its distal enhancer regulation, but placed next to Six3 enhancer elements which support ectopic Six2 expression in the lens where Six3 is normally expressed. Six3 is now predicted to fall under control of the Six2 distal enhancer. Consistent with this view, we observed ectopic Six3 in nephron progenitors. 4C-seq supports the model for Six2 distal enhancer interactions in wild-type and Br/+ mouse kidneys. Together, these data expand our view of the regulatory genome and regulatory landscape underpinning mammalian nephrogenesis.

Quantifying cell-generated mechanical forces within living embryonic tissues.

Cell-generated mechanical forces play a critical role during tissue morphogenesis and organ formation in the embryo. Little is known about how these forces shape embryonic organs, mainly because it has not been possible to measure cellular forces within developing three-dimensional (3D) tissues in vivo. We present a method to quantify cell-generated mechanical stresses exerted locally within living embryonic tissues, using fluorescent, cell-sized oil microdroplets with defined mechanical properties and coated with adhesion receptor ligands. After a droplet is introduced between cells in a tissue, local stresses are determined from droplet shape deformations, measured using fluorescence microscopy and computerized image analysis. Using this method, we quantified the anisotropic stresses generated by mammary epithelial cells cultured within 3D aggregates, and we confirmed that these stresses (3.4 nN µm(-2)) are dependent on myosin II activity and are more than twofold larger than stresses generated by cells of embryonic tooth mesenchyme, either within cultured aggregates or in developing whole mouse mandibles.

Transcriptional repression of VEGF by ZNF24: mechanistic studies and vascular consequences in vivo.

VEGF is a key regulator of normal and pathologic angiogenesis. Although many trans-activating factors of VEGF have been described, the transcriptional repression of VEGF remains much less understood. We have previously reported the identification of a SCAN domain-containing C2H2 zinc finger protein, ZNF24, that represses the transcription of VEGF. In the present study, we identify the mechanism by which ZNF24 represses VEGF transcription. Using reporter gene and electrophoretic mobility shift assays, we identify an 11-bp fragment of the proximal VEGF promoter as the ZNF24-binding site that is essential for ZNF24-mediated repression. We demonstrate in 2 in vivo models the potent inhibitory effect of ZNF24 on the vasculature. Expression of human ZNF24 induced in vivo vascular defects consistent with those induced by VEGF knockdown using a transgenic zebrafish model. These defects could be rescued by VEGF overexpression. Overexpression of ZNF24 in human breast cancer cells also inhibited tumor angiogenesis in an in vivo tumor model. Analyses of human breast cancer tissues showed that ZNF24 and VEGF levels were inversely correlated in malignant compared with normal tissues. These data demonstrate that ZNF24 represses VEGF transcription through direct binding to an 11-bp fragment of the VEGF proximal promoter and that it functions as a negative regulator of tumor growth by inhibiting angiogenesis.

An antiangiogenic neurokinin-B/thromboxane A2 regulatory axis.

Establishment of angiogenic circuits that orchestrate blood vessel development and remodeling requires an exquisite balance between the activities of pro- and antiangiogenic factors. However, the logic that permits complex signal integration by vascular endothelium is poorly understood. We demonstrate that a "neuropeptide," neurokinin-B (NK-B), reversibly inhibits endothelial cell vascular network assembly and opposes angiogenesis in the chicken chorioallantoic membrane. Disruption of endogenous NK-B signaling promoted angiogenesis. Mechanistic analyses defined a multicomponent pathway in which NK-B signaling converges upon cellular processes essential for angiogenesis. NK-B-mediated ablation of Ca2+ oscillations and elevation of 3'-5' [corrected] cyclic adenosine monophosphate (cAMP) reduced cellular proliferation, migration, and vascular endothelial growth factor receptor expression and induced the antiangiogenic protein calreticulin. Whereas NK-B initiated certain responses, other activities required additional stimuli that increase cAMP. Although NK-B is a neurotransmitter/ neuromodulator and NK-B overexpression characterizes the pregnancy-associated disorder preeclampsia, NK-B had not been linked to vascular remodeling. These results establish a conserved mechanism in which NK-B instigates multiple activities that collectively oppose vascular remodeling.

Unique SMAD1/5/8 activity at the phalanx-forming region determines digit identity.

The zone of polarizing activity is the primary signaling center controlling anterior-posterior patterning of the amniote limb bud. The autopodial interdigits (IDs) are secondary signaling centers proposed to determine digit identity by acting on the cells of the digital ray. Here, we focus on events accompanying digital fate determination and define a region of the digital ray that expresses Sox9 and Bmpr1b and is phosphorylated-SMAD1/5/8 (p-SMAD1/5/8) positive. We name this region the phalanx-forming region (PFR), and show that the PFR cells arise from the distal subridge mesenchyme of digital ray. This phalanx-forming cell lineage is subsequently committed to the cartilage lineage; the fate of these cells is initially labile but becomes fixed as they are incorporated into the condensed cartilage of the digit primordium. Using an in vivo reporter assay, we establish that each digital PFR has a unique p-SMAD1/5/8 activity signature. In addition, we show that changes in this activity correlate with the identity of the digit that forms after experimental manipulation, supporting the idea that threshold signaling levels can lead to different developmental outcomes in a morphogenetic field. Our data define the molecular profile of the PFR, and we propose a model for understanding formation and variation of digits during autopodial development.

The development of archosaurian first-generation teeth in a chicken mutant.

Modern birds do not have teeth. Rather, they develop a specialized keratinized structure, called the rhamphotheca, that covers the mandible, maxillae, and premaxillae. Although recombination studies have shown that the avian epidermis can respond to tooth-inductive cues from mouse or lizard oral mesenchyme and participate in tooth formation, attempts to initiate tooth development de novo in birds have failed. Here, we describe the formation of teeth in the talpid2 chicken mutant, including the developmental processes and early molecular changes associated with the formation of teeth. Additionally, we show recapitulation of the early events seen in talpid2 after in vivo activation of beta-catenin in wild-type embryos. We compare the formation of teeth in the talpid2 mutant with that in the alligator and show the formation of decidedly archosaurian (crocodilian) first-generation teeth in an avian embryo. The formation of teeth in the mutant is coupled with alterations in the specification of the oral/aboral boundary of the jaw. We propose an epigenetic model of the developmental modification of dentition in avian evolution; in this model, changes in the relative position of a lateral signaling center over competent odontogenic mesenchyme led to loss of teeth in avians while maintaining tooth developmental potential.

Expression of E2F-4 gene in colorectal adenocarcinoma and corresponding covering mucosa: an immunohistochemistry, image analysis, and immunoblot study.

E2F-4 is a transcription factor involved in the transition of the cell from the resting state (G0/G1) to the proliferative stage (S). It has been associated with the p107 and p130 members of the Rb-family and it is responsible for many important growth suppressive functions. E2F-1, one member of the E2F family, has a similar structure to E2F-4; however, both have different mechanisms of action in regulating cell-cycle progression. Although E2F-4 acts mainly as a repressor in the early part of the cell cycle, E2F-1 has the ability to function as both an oncogene and a tumor suppressor gene. In an attempt to identify the role of E2F-4 as a potential mediator of cell proliferation, differentiation, tumorigenesis, and apoptosis in colorectal mucosa comparing with that of E2F-1, the authors examine 20 patients with human colon cancer and their corresponding histologically healthy mucosa by using immunohistochemical methods, computerized quantitative image analysis, and immunoblot analysis. Immunohistochemical studies were performed with formalin-fixed, paraffin-embedded sections stained with a monoclonal antibody against the E2F-4 protein. Apoptosis levels were determined by in situ assay. Positivity was scored by a Computerized Image Analyzer to detect the relative amount of the protein. Immunoblot analysis was performed on protein extracts from snap-frozen tissues of the same specimens. The results show that the expression of E2F-4 was greater in the tumor cells than in their corresponding benign epithelium as determined by immunohistochemical staining and image analysis. This was confirmed by semiquantitative IB analysis of the E2F-4 protein. The labeling index (LI) of E2F-4 in the tumors was inversely proportional to the LI of apoptotic cells. Within these cases, 12 cases showed a very high E2F-4 LI corresponding to low apoptosis LI. Three cases with relatively lower levels of E2F-4 LI were characterized with high apoptotic rates. These data suggest that E2F-4 gene overexpression plays a role in the development of colorectal tumors and appears to play a role in suppressing apoptosis.

Filopodia: the cellular quills of hedgehog signaling?

Reporting in Nature, Sanders et al. (2013) implicate filopodial projections in Sonic hedgehog (Shh) patterning of the limb. Actin-based filopodia transport Shh from producing cells, while filopodia of responding cells bear Cdon and Boc: coreceptors in the Shh pathway. These findings suggest a new mechanism of ligand movement and transmission.

Novel zebrafish model reveals a critical role for MAPK in lymphangiogenesis.

PURPOSE: Lymphatic disorders are poorly understood with few animal models. We designed a novel assay to measure lymphatic development using transgenic zebrafish with fluorescently labeled endothelial cells. Two major branches of the vascular endothelial growth factor receptor (VEGFR) signaling pathway were examined: the MAPK and PI3K pathways. METHODS: Direct visualization of lymphatic development was performed in control embryos or under chemical inhibition. Treatment involved a 6-hour pulse of inhibitor at 3 days postfertilization. Fish were analyzed for the presence of the thoracic duct (TD) at 4 days postfertilization (n > 100 specimens). RESULTS: Thoracic duct formation was prevented using selective inhibitors against kinases (MAPK, PI3K/TOR, or VEGFR). These kinases were important for TD formation because the lymphatic vessel failed to form in most of treated animals. Remarkably, MAPK pathway inhibition most robustly reduced lymphangiogenesis, demonstrated by a lack of lymphatic endothelial cells. CONCLUSION: We conclude that MAPK pathway function downstream of the VEGFRs is crucial at the early stages of TD development. This study provides a novel animal model and a potential target pathway for further investigation. We suggest further examination of MAPK pathway deregulation as a potential mechanism underlying lymphatic disease in humans.

Chemical approaches to angiogenesis in development and regeneration.

Research on blood vessel formation has provided a great deal of information regarding both normal and pathological forms of angiogenesis during development and in different disease states. Central to these studies is the role of the vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs). VEGF stimulation promotes the division, survival, and migration of endothelial cells, and is necessary for the formation of blood and lymphatic vessels. The conserved functions of the VEGF ligands and receptors from fish to mammals have allowed a near-seamless translation of a cellular and molecular mechanism of vascular assembly between vertebrate models. An added advantage to the conserved gene function is the ability to apply chemical approaches to modulate zebrafish angiogenesis. In this chapter we will discuss current and potential future uses of chemical strategies in the zebrafish model to further our understanding of angiogenesis, lymphangiogenesis, and regenerative biology.

Regulation of Gremlin expression in the posterior limb bud.

Proper outgrowth of the limb bud requires a positive feedback loop between Sonic hedgehog (Shh) in the zone of polarizing activity (ZPA) and Fgfs in the overlying apical ectodermal ridge. The Bmp antagonist Gremlin is expressed in a domain anterior to the ZPA and is thought to act as a signaling intermediate between Shh and Fgf. It is currently unclear whether Shh acts directly or indirectly to initiate and maintain Gremlin. In this study, we confirm that Bmp activity is necessary and sufficient for induction of Gremlin. Beads soaked in the Bmp antagonist Noggin downregulate Gremlin, while beads soaked in Bmp2 cause its upregulation. Furthermore, Bmp2 is also capable of upregulating Gremlin in oligozeugodactyly mutant limbs that lack Shh activity, demonstrating that Gremlin expression does not depend on the combined exposure to both these factors. In spite of the ability of Bmp2 to induce Gremlin, beads soaked in high concentrations of Bmp2 downregulate Gremlin around the bead without apparent induction of cell death, whereas another target gene Msx2 is upregulated around the bead. Consistent with this concentration-dependent effect, we find that low concentrations of Bmp2 upregulate Gremlin while high concentrations of Bmp2 downregulate Gremlin in limb mesenchyme cultures. These data implicate Bmp activity as a required intermediate in the Shh-Fgf4 signaling loop. Though we show that Bmp activity is sufficient to upregulate Gremlin, Gremlin expression is excluded from a posterior domain of the limb, and expansion of this domain as limb outgrowth proceeds is important in terminating the Shh-Fgf4 signaling loop. We find that the posterior limb is refractory to Gremlin induction in response to Bmp2, suggesting that termination of the Shh-Fgf4 signaling loop results from inability of Bmp activity to induce Gremlin in the posterior. In contrast, in the oligozeugodactyly limb, we find that beads soaked in Bmp2 can induce Gremlin in the posterior, demonstrating that Shh activity is required for exclusion of Gremlin in the posterior. Finally, by blocking Shh activity with cyclopamine, we find evidence that continued Shh activity is also required to maintain refractoriness to Gremlin expression in response to Bmp activity.

The chick oligozeugodactyly (ozd) mutant lacks sonic hedgehog function in the limb.

We have analyzed a new limb mutant in the chicken that we name oligozeugodactyly (ozd). The limbs of this mutant have a longitudinal postaxial defect, lacking the posterior element in the zeugopod (ulna/fibula) and all digits except digit 1 in the leg. Classical recombination experiments show that the limb mesoderm is the defective tissue layer in ozd limb buds. Molecular analysis revealed that the ozd limbs develop in the absence of Shh expression, while all other organs express Shh and develop normally. Neither Ptc1 nor Gli1 are detectable in mutant limb buds. However, Bmp2 and dHAND are expressed in the posterior wing and leg bud mesoderm, although at lower levels than in normal embryos. Activation of Hoxd11-13 occurs normally in ozd limbs but progressively declines with time. Phase III of expression is more affected than phase II, and expression is more severely affected in the more 5' genes. Interestingly, re-expression of Hoxd13 occurs at late stages in the distal mesoderm of ozd leg buds, correlating with formation of digit 1. Fgf8 and Fgf4 expression are initiated normally in the mutant AER but their expression is progressively downregulated in the anterior AER. Recombinant Shh protein or ZPA grafts restore normal pattern to ozd limbs; however, retinoic acid fails to induce Shh in ozd limb mesoderm. We conclude that Shh function is required for limb development distal to the elbow/knee joints, similar to the Shh(-/-) mouse. Accordingly we classify the limb skeletal elements as Shh dependent or independent, with the ulna/fibula and digits other than digit 1 in the leg being Shh dependent. Finally we propose that the ozd mutation is most likely a defect in a regulatory element that controls limb-specific expression of Shh.