Juha Partanen

Vascular endothelial growth factor C is required for sprouting of the first lymphatic vessels from embryonic veins

Lymphatic vessels are essential for immune surveillance, tissue fluid homeostasis and fat absorption. Defects in lymphatic vessel formation or function cause lymphedema. Here we show that the vascular endothelial growth factor C (VEGF-C) is required for the initial steps in lymphatic development. In Vegfc−/− mice, endothelial cells commit to the lymphatic lineage but do not sprout to form lymph vessels. Sprouting was rescued by VEGF-C and VEGF-D but not by VEGF, indicating VEGF receptor 3 specificity. The lack of lymphatic vessels resulted in prenatal death due to fluid accumulation in tissues, and Vegfc+/− mice developed cutaneous lymphatic hypoplasia and lymphedema. Our results indicate that VEGF-C is the paracrine factor essential for lymphangiogenesis, and show that both Vegfc alleles are required for normal lymphatic development.

The receptor tyrosine kinase TIE is required for integrity and survival of vascular endothelial cells.

Vascular endothelial cells are critical for the development and function of the mammalian circulatory system. We have analyzed the role of the endothelial cell‐specific receptor tyrosine kinase TIE in the mouse vasculature. Mouse embryos homozygous for a disrupted Tie allele developed severe edema, their microvasculature was ruptured and they died between days 13.5 and 14.5 of gestation. The major blood vessels of the homozygous embryos appeared normal. Cells lacking a functional Tie gene were unable to contribute to the adult kidney endothelium in chimeric animals, further demonstrating the intrinsic requirement for TIE in endothelial cells. We conclude that TIE is required during embryonic development for the integrity and survival of vascular endothelial cells, particularly in the regions undergoing angiogenic growth of capillaries. TIE is not essential, however, for vasculogenesis, the early differentiation of endothelial cells.

FGFR-4, a novel acidic fibroblast growth factor receptor with a distinct expression pattern.

We have previously identified two novel members of the fibroblast growth factor receptor (FGFR) gene family expressed in K562 erythroleukemia cells. Here we report cDNA cloning and analysis of one of these genes, named FGFR‐4. The deduced amino acid sequence of FGFR‐4 is 55% identical with both previously characterized FGFRs, flg and bek, and has the structural characteristics of a FGFR family member including three immunoglobulin‐like domains in its extracellular part. Antibodies raised against the carboxy terminus of FGFR‐4 detected 95 and 110 kd glycoproteins with a protein backbone of 88 kd in COS cells transfected with a FGFR‐4 cDNA expression vector. The FGFR‐4 protein expressed in COS cells could also be affinity‐labeled with radioiodinated acidic FGF. Furthermore, ligand binding experiments demonstrated that FGFR‐4 binds acidic FGF with high affinity but does not bind basic FGF. FGFR‐4 is expressed as a 3.0 kb mRNA in the adrenal, lung, kidney, liver, pancreas, intestine, striated muscle and spleen tissues of human fetuses. The expression pattern of FGFR‐4 is distinct from that of flg and bek and the yet additional member of the same gene family, FGFR‐3, which we have also cloned from the K562 leukemia cells. Our results suggest that FGFR‐4 along with other fibroblast growth factor receptors performs cell lineage and tissue‐specific functions.

The human p50csk tyrosine kinase phosphorylates p56lck at Tyr-505 and down regulates its catalytic activity.

Protein tyrosine kinases participate in the transduction and modulation of signals that regulate proliferation and differentiation of cells. Excessive or deregulated protein tyrosine kinase activity can cause malignant transformation. The catalytic activity of the T cell protein tyrosine kinase p56lck is normally suppressed by phosphorylation of a carboxyl‐terminal tyrosine, Tyr‐505, by another cellular protein tyrosine kinase. Here we characterize a human cytosolic 50 kDa protein tyrosine kinase, p50csk, which specifically phosphorylates Tyr‐505 of p56lck and a synthetic peptide containing this site. Phosphorylation of Tyr‐505 suppressed the catalytic activity of p56lck. We suggest that p50csk negatively regulates p56lck, and perhaps other cellular src family kinases.

FGFR1 Is Required for the Development of the Auditory Sensory Epithelium

The mammalian auditory sensory epithelium, the organ of Corti, comprises the hair cells and supporting cells that are pivotal for hearing function. The origin and development of their precursors are poorly understood. Here we show that loss-of-function mutations in mouse fibroblast growth factor receptor 1 (Fgfr1) cause a dose-dependent disruption of the organ of Corti. Full inactivation of Fgfr1 in the inner ear epithelium by Foxg1-Cre-mediated deletion leads to an 85% reduction in the number of auditory hair cells. The primary cause appears to be reduced precursor cell proliferation in the early cochlear duct. Thus, during development, FGFR1 is required for the generation of the precursor pool, which gives rise to the auditory sensory epithelium. Our data also suggest that FGFR1 might have a distinct later role in intercellular signaling within the differentiating auditory sensory epithelium.

FGF signaling through FGFR1 is required for olfactory bulb morphogenesis

During development, the embryonic telencephalon is patterned into different areas that give rise to distinct adult brain structures. Several secreted signaling molecules are expressed at putative signaling centers in the early telencephalon. In particular, Fgf8 is expressed at the anterior end of the telencephalon and is hypothesized to pattern it along the anteroposterior (AP) axis. Using a CRE/loxP genetic approach to disrupt genes in the telencephalon, we address the role of FGF signaling directly in vivo by abolishing expression of the FGF receptor Fgfr1. In the Fgfr1-deficient telencephalon, AP patterning is largely normal. However, morphological defects are observed at the anterior end of the telencephalon. Most notably, the olfactory bulbs do not form normally. Examination of the proliferation state of anterior telencephalic cells supports a model for olfactory bulb formation in which an FGF-dependent decrease in proliferation is required for initial bulb evagination. Together the results demonstrate an essential role for Fgfr1 in patterning and morphogenesis of the telencephalon.

The European dimension for the mouse genome mutagenesis program

The European Mouse Mutagenesis Consortium is the European initiative contributing to the international effort on functional annotation of the mouse genome. Its objectives are to establish and integrate mutagenesis platforms, gene expression resources, phenotyping units, storage and distribution centers and bioinformatics resources. The combined efforts will accelerate our understanding of gene function and of human health and disease.

FGFR1 is independently required in both developing mid- and hindbrain for sustained response to isthmic signals.

Fibroblast growth factors (FGFs) are signaling molecules of the isthmic organizer, which regulates development of the midbrain and cerebellum. Tissue‐specific inactivation of one of the FGF receptor (FGFR) genes, Fgfr1, in the midbrain and rhombomere 1 of the hindbrain of mouse embryos results in deletion of the inferior colliculi in the posterior midbrain and vermis of the cerebellum. Analyses of both midbrain–hindbrain and midbrain‐specific Fgfr1 mutants suggest that after establishment of the isthmic organizer, FGFR1 is needed for continued response to the isthmic signals, and that it has direct functions on both sides of the organizer. In addition, FGFR1 appears to modify cell adhesion properties critical for maintaining a coherent organizing center. This may be achieved by regulating expression of specific cell‐adhesion molecules at the midbrain–hindbrain border.

Developmental origin of smooth muscle cells in the descending aorta in mice

Aortic smooth muscle cells (SMCs) have been proposed to derive from lateral plate mesoderm. It has further been suggested that induction of SMC differentiation is confined to the ventral side of the aorta, and that SMCs later migrate to the dorsal side. In this study, we investigate the origin of SMCs in the descending aorta using recombination-based lineage tracing in mice. Hoxb6-cre transgenic mice were crossed with Rosa 26 reporter mice to track cells of lateral plate mesoderm origin. The contribution of lateral plate mesoderm to SMCs in the descending aorta was determined at different stages of development. SMC differentiation was induced in lateral plate mesoderm-derived cells on the ventral side of the aorta at embryonic day (E) 9.0-9.5, as indicated by expression of the SMC-specific reporter gene SM22α-lacZ. There was, however, no migration of SMCs from the ventral to the dorsal side of the vessel. Moreover, the lateral plate mesoderm-derived cells in the ventral wall of the aorta were replaced by somite-derived cells at E10.5, as indicated by reporter gene expression in Meox1-cre/Rosa 26 double transgenic mice. Examination of reporter gene expression in adult aortas from Hoxb6-cre/Rosa 26 and Meox1-cre/Rosa 26 double transgenic mice suggested that all SMCs in the adult descending aorta derive from the somites, whereas no contribution was recorded from lateral plate mesoderm.

Fibroblast Growth Factor Receptor 1 Is Required for the Proliferation of Hippocampal Progenitor Cells and for Hippocampal Growth in Mouse

Fibroblast growth factor receptor 1 (Fgfr1) is expressed at high levels by progenitor cells of the ventricular zone (VZ) within the hippocampal primordium. To investigate the role of Fgfr1 in these cells, in vivo Cre recombination of “floxed” Fgfr1 alleles was directed to cells of the radial glial lineage by using the human glial fibrillary acidic protein promoter. Radial glial-like cells of the hippocampal VZ are the progenitors of pyramidal neurons and granule cells of hippocampal dentate gyrus (DG). Mice carrying null Fgfr1 alleles (Fgfr1Δflox) in cells of this lineage showed a dramatic loss of Fgfr1 gene expression throughout the embryonic dorsal telencephalon. These Fgfr1Δflox mice exhibited a ∼30% decrease in dividing radial glial progenitor cells in the hippocampal VZ and DG in the late embryonic period, progressing to a ∼50-60% loss at birth, without any changes in cell survival. In addition, no FGF2-sensitive neural stem cells could be isolated from the Fgfr1Δflox hippocampal neuroepithelium, whereas epidermal growth factor-sensitive neural stem cells were not affected. The number of hippocampal pyramidal neurons and DG granule cells was ∼30-50% decreased from the perinatal period through adulthood, and the number of parvalbumin-containing interneurons was similarly decreased in both the DG and pyramidal cell fields. We conclude that Fgfr1 is necessary for hippocampal growth, because it promotes the proliferation of hippocampal progenitors and stem cells during development.