Vascular endothelial growth factor (VEGF) stimulates angiogenesis by activating VEGF receptor-2 (VEGFR-2). The role of its homolog, placental growth factor (PlGF), remains unknown. Both VEGF and PlGF bind to VEGF receptor-1 (VEGFR-1), but it is unknown whether VEGFR-1, which exists as a soluble or a membrane-bound type, is an inert decoy or a signaling receptor for PlGF during angiogenesis. Here, we report that embryonic angiogenesis in mice was not affected by deficiency of PlGF (Pgf-/-). VEGF-B, another ligand of VEGFR-1, did not rescue development in Pgf-/- mice. However, loss of PlGF impaired angiogenesis, plasma extravasation and collateral growth during ischemia, inflammation, wound healing and cancer. Transplantation of wild-type bone marrow rescued the impaired angiogenesis and collateral growth in Pgf-/- mice, indicating that PlGF might have contributed to vessel growth in the adult by mobilizing bone-marrow-derived cells. The synergism between PlGF and VEGF was specific, as PlGF deficiency impaired the response to VEGF, but not to bFGF or histamine. VEGFR-1 was activated by PlGF, given that anti-VEGFR-1 antibodies and a Src-kinase inhibitor blocked the endothelial response to PlGF or VEGF/PlGF. By upregulating PlGF and the signaling subtype of VEGFR-1, endothelial cells amplify their responsiveness to VEGF during the 'angiogenic switch' in many pathological disorders.

A human cDNA coding for a protein related to the vascular permeability factor (VPF) was isolated from a term placenta cDNA library; we therefore named its product placenta growth factor (PlGF). PlGF is a 149-amino-acid-long protein and is highly homologous (53% identity) to the platelet-derived growth factor-like region of human VPF. Computer analyses reveal a putative signal peptide and two probable N-glycosylation sites in the PlGF protein, one of which is also conserved in human VPF. By using N-glycosidase F, tunicamycin, and specific antibodies produced in both chicken and rabbit, we demonstrate that PlGF, derived from transfected COS-1 cells, is actually N-glycosylated and secreted into the medium. In addition, PlGF, like VPF, proves to be a dimeric protein. Finally, a conditioned medium from COS-1 cells containing PlGF is capable of stimulating specifically the growth of CPA, a line of endothelial cells, in vitro.

Kallmann's syndrome (clinically characterized by hypogonadotropic hypogonadism and inability to smell) is caused by a defect in the migration of olfactory neurons, and neurons producing hypothalamic gonadotropin-releasing hormone. A gene has now been isolated from the critical region on Xp22.3 to which the syndrome locus has been assigned: this gene escapes X inactivation, has a homologue on the Y chromosome, and shows an unusual pattern of conservation across species. The predicted protein has significant similarities with proteins involved in neural cell adhesion and axonal pathfinding, as well as with protein kinases and phosphatases, which suggests that this gene could have a specific role in neuronal migration.

Nodal proteins have crucial roles in mesendoderm formation and left-right patterning during vertebrate development. The molecular mechanisms of signal transduction by Nodal and related ligands, however, are not fully understood. In this paper, we present biochemical and functional evidence that the orphan type I serine/threonine kinase receptor ALK7 acts as a receptor for mouse Nodal and Xenopus Nodal-related 1 (Xnr1). Receptor reconstitution experiments indicate that ALK7 collaborates with ActRIIB to confer responsiveness to Xnr1 and Nodal. Both receptors can independently bind Xnr1. In addition, Cripto, an extracellular protein genetically implicated in Nodal signaling, can independently interact with both Xnr1 and ALK7, and its expression greatly enhances the ability of ALK7 and ActRIIB to respond to Nodal ligands. The Activin receptor ALK4 is also able to mediate Nodal signaling but only in the presence of Cripto, with which it can also interact directly. A constitutively activated form of ALK7 mimics the mesendoderm-inducing activity of Xnr1 in Xenopus embryos, whereas a dominant-negative ALK7 specifically blocks the activities of Nodal and Xnr1 but has little effect on other related ligands. In contrast, a dominant-negative ALK4 blocks all mesoderm-inducing ligands tested, including Nodal, Xnr1, Xnr2, Xnr4, and Activin. In agreement with a role in Nodal signaling, ALK7 mRNA is localized to the ectodermal and organizer regions of Xenopus gastrula embryos and is expressed during early stages of mouse embryonic development. Therefore, our results indicate that both ALK4 and ALK7 can mediate signal transduction by Nodal proteins, although ALK7 appears to be a receptor more specifically dedicated to Nodal signaling.

The location of the VEGF gene in the 6p21.3 region is a potential starting point for a linkage study. In addition, the isolation of YAC clones containing the VEGF gene will contribute to the construction of the physical map of this chromosomal region.