FGF9 regulates early hypertrophic chondrocyte differentiation and skeletal vascularization in the developing stylopod

Gain-of-function mutations in fibroblast growth factor (FGF) receptors result in chondrodysplasia and craniosynostosis syndromes, highlighting the critical role for FGF signaling in skeletal development. Although the FGFRs involved in skeletal development have been well characterized, only a single FGF ligand, FGF18, has been identified that regulates skeletal development during embryogenesis. Here we identify Fgf9 as a second FGF ligand that is critical for skeletal development. We show that Fgf9 is expressed in the proximity of developing skeletal elements and that Fgf9-deficient mice exhibit rhizomelia (a disproportionate shortening of proximal skeletal elements), which is a prominent feature of patients with FGFR3-induced chondrodysplasia syndromes. Although Fgf9 is expressed in the apical ectodermal ridge in the limb bud, we demonstrate that the Fgf9-/- limb phenotype results from loss of FGF9 functions after formation of the mesenchymal condensation. In developing stylopod elements, FGF9 promotes chondrocyte hypertrophy at early stages and regulates vascularization of the growth plate and osteogenesis at later stages of skeletal development.     

Hepatocyte growth factor induces epithelial cell motility through transactivation of the epidermal growth factor receptor

Hepatocyte growth factor (HGF) is a potent inducer of motility in epithelial cells. Since we have previously found that activation of the epidermal growth factor receptor (EGFR) is an absolute prerequisite for induction of motility of corneal epithelial cells after wounding, we investigated whether induction of motility in response to HGF is also dependent on activation of the EGFR. We now report that HGF induces transactivation of the EGFR in an immortalized line of corneal epithelial cells, in human skin keratinocytes, and in Madin-Darby canine kidney cells. EGFR activation is unconditionally required for induction of motility in corneal epithelial cells, and for induction of a fully motile phenotype in Madin-Darby canine kidney cells. Activation of the EGFR occurs through amphiregulin and heparin-binding epidermal growth factor-like growth factor. Early after HGF stimulation, blocking EGFR activation does not inhibit extracellular-signal regulated kinase 1/2 (ERK1/2) activation by HGF, but the converse is seen after approximately 1 h, indicating the existence of EGFR-dependent and -independent routes of ERK1/2 activation. In summary, HGF induces transactivation of the EGFR in epithelial cells, and this is a prerequisite for induction of full motility.     

Epidermal growth factor and transforming growth factor-beta1 enhance HK-2 cell migration through a synergistic increase of matrix metalloproteinase and sustained activation of ERK signaling pathway

Epidermal growth factor (EGF) and transforming growth factor-beta1 (TGF-beta1), upregulated in renal diseases, have a combinational effect on epithelial-mesenchymal transformation (EMT) of renal proximal tubular cells. The aim of this study was to examine the mechanism regarding the combinational effect of EGF and TGF-beta1 on cell migration following EMT. The results demonstrated that EGF (10 ng/ml) and TGF-beta1 (3 ng/ml) synergistically increased cell migration, accompanied by an increase in matrix metalloproteinase-9 (MMP-9) gene expression, production and activity. Inhibition of MMP-9 production and activity by an MMP-2/MMP-9-specific inhibitor blocked the synergistic effect of EGF and TGF-beta1 on cell migration. The kinetic profile of extracellular signal-regulated kinase (ERK) signals demonstrated that ERK1/2 activation was rapidly and strongly induced by EGF but delayed and less marked by TGF-beta1 stimulation. In contrast, co-administration of EGF and TGF-beta1 caused an early pronounced and persistent ERK1/2 activation. Inhibition of the ERK1/2 activity by PD98059 abrogated the synergistic effect of EGF and TGF-beta1 on cell migration, MMP-9 production and activity, indicating that EGF and TGF-beta1 converged at the ERK signaling pathway to mediate cell migration. This study demonstrates that EGF and TGF-beta1 synergistically stimulate proximal tubular cell migration through the increased MMP-9 function and enhanced ERK1/2 activation.     

Laminin alpha5 is necessary for submandibular gland epithelial morphogenesis and influences FGFR expression through beta1 integrin signaling

Laminin alpha chains have unique spatiotemporal expression patterns during development and defining their function is necessary to understand the regulation of epithelial morphogenesis. We investigated the function of laminin alpha5 in mouse submandibular glands (SMGs). Lama5(-/-) SMGs have a striking phenotype: epithelial clefting is delayed, although proliferation occurs; there is decreased FGFR1b and FGFR2b, but no difference in Lama1 expression; later in development, epithelial cell organization and lumen formation are disrupted. In wild-type SMGs alpha5 and alpha1 are present in epithelial clefts but as branching begins alpha5 expression increases while alpha1 decreases. Lama5 siRNA decreased branching, p42 MAPK phosphorylation, and FGFR expression, and branching was rescued by FGF10. FGFR siRNA decreased Lama5 suggesting that FGFR signaling provides positive feedback for Lama5 expression. Anti-beta1 integrin antibodies decreased FGFR and Lama5 expression, suggesting that beta1 integrin signaling provides positive feedback for Lama5 and FGFR expression. Interestingly, the Itga3(-/-):Itga6(-/-) SMGs have a similar phenotype to Lama5(-/-). Our findings suggest that laminin alpha5 controls SMG epithelial morphogenesis through beta1 integrin signaling by regulating FGFR expression, which also reciprocally regulates the expression of Lama5. These data link changes in basement membrane composition during branching morphogenesis with FGFR expression and signaling.     

Characterization of apoptotic pathway associated with caspase-independent excision of DNA loop domains

Excision of chromatin loop domains and internucleosomal DNA fragmentation are widely considered as consecutive stages of chromatin disassembly during apoptosis. We report here on apoptosis induced by staurosporine in NB-2a neuroblastoma cells, which was accompanied by excision of chromatin loop domains, but proceeded without internucleosomal DNA cleavage. In contrast to apoptosis associated with internucleosomal DNA fragmentation, the apoptotic pathway associated with excision of chromatin loop domains was largely caspase independent. We identify here MAPK family member, p38/JNK, mitochondria, and topoisomerase II as the components of this caspase-independent apoptotic pathway. While caspase-independent excision of chromatin loop domains was a predominant mechanism of DNA disintegration in staurosporine-treated neuroblastoma, both caspase-dependent internucleosomal DNA fragmentation and caspase-independent excision of chromatin loop domains accompanied staurosporine-induced apoptosis of promyelocytic leukemia cells. Our results suggest that caspase-independent excision of chromatin loop domains represents a separate cell death pathway, which operates either in parallel or independently from caspase-dependent internucleosomal DNA fragmentation.     

Grb2 mediates negative regulation of stem cell factor receptor/c-Kit signaling by recruitment of Cbl

Aberrant activation of c-Kit is involved in a number of human diseases including cancers and leukemias. Certain receptor tyrosine kinases, such as epidermal growth factor receptor, have been shown to indirectly recruit Cbl through the adapter protein Grb2, leading to receptor ubiquitination and degradation. In order to study the role of Grb2 in c-Kit degradation, a series of mutations of the Grb2 binding sites in c-Kit were generated (Y703F, Y936F, and Y703F/Y936F). Since other signal transduction molecules are also known to bind Y703 and Y936, the more selective asparagine-to-alanine (N-to-A) mutants N705A, N938A, and N705A/N938A were generated. We could clearly demonstrate that binding of Grb2 was dependent on intact phosphorylation sites Y703 and Y936. Furthermore, we could demonstrate the presence of Cbl in a complex with Grb2 and c-Kit. Thus, Grb2 is able to indirectly recruit Cbl to c-Kit. In the N-to-A mutants, Cbl phosphorylation was strongly reduced, which correlated with reduced ubiquitination of c-Kit as well as decreased internalization and degradation of the receptor. Taken together, we have demonstrated that, in addition to its role in positive signaling via the Ras/Erk pathway, Grb2 mediates c-Kit degradation through recruitment of Cbl to c-Kit, leading to ubiquitination of c-Kit followed by internalization and degradation.     

Rab cascades and tethering factors in the endomembrane system

Rab GTPases are key proteins that determine organelle identity and operate at the center of fusion reactions. Like Ras, they act as switches that are connected to a diverse network of tethering factors, exchange factors and GTPase activating proteins. Recent studies suggest that Rabs are linked to each other via their effectors, thus coordinating protein transport in the endomembrane system. Within this review, we will focus on selected examples that highlight these issues.     

Plasmodium yoelii: the effect of second blood meal and anti-sporozoite antibodies on development and gene expression in the mosquito vector, Anopheles stephensi

The sporogonic development of the malaria parasite takes place in the mosquito and a wide range of factors modulates it. Among those, the contents of the blood meal can influence the parasite development directly or indirectly through the mosquito response to the infection. We have studied the effect of a second blood meal in previously infected mosquitoes and the effect of anti-sporozoite immune serum on parasite development and mosquito response to the infection. The prevalence and intensity of infection and gene expression of both Plasmodium yoelii and Anopheles stephensi was analyzed. We verified that a second blood meal and its immune status interfere with parasite development and with Plasmodium and mosquito gene expression.     

Cellular functions of NSF: not just SNAPs and SNAREs

N-ethylmaleimide sensitive factor (NSF) is an ATPases associated with various cellular activities protein (AAA), broadly required for intracellular membrane fusion. NSF functions as a SNAP receptor (SNARE) chaperone which binds, through soluble NSF attachment proteins (SNAPs), to SNARE complexes and utilizes the energy of ATP hydrolysis to disassemble them thus facilitating SNARE recycling. While this is a major function of NSF, it does seem to interact with other proteins, such as the AMPA receptor subunit, GluR2, and beta2-AR and is thought to affect their trafficking patterns. New data suggest that NSF may be regulated by transient post-translational modifications such as phosphorylation and nitrosylation. These new aspects of NSF function as well as its role in SNARE complex dynamics will be discussed.