Expression of Foxi3 is regulated by ectodysplasin in skin appendage placodes.

BACKGROUND: Foxi3 is a member of the large forkhead box family of transcriptional regulators, which have a wide range of biological activities including manifold developmental processes. Heterozygous mutation in Foxi3 was identified in several hairless dog breeds characterized by sparse fur coat and missing teeth. A related phenotype called hypohidrotic ectodermal dysplasia (HED) is caused by mutations in the ectodysplasin (Eda) pathway genes. RESULTS: Expression of Foxi3 was strictly confined to the epithelium in developing ectodermal appendages in mouse embryos, but no expression was detected in the epidermis. Foxi3 was expressed in teeth and hair follicles throughout embryogenesis, but in mammary glands only during the earliest stages of development. Foxi3 expression was decreased and increased in Eda loss- and gain-of-function embryos, respectively, and was highly induced by Eda protein in embryonic skin explants. Also activin A treatment up-regulated Foxi3 mRNA levels in vitro. CONCLUSIONS: Eda and activin A were identified as upstream regulators of Foxi3. Foxi3 is a likely transcriptional target of Eda in ectodermal appendage placodes suggesting that HED phenotype may in part be produced by compromised Foxi3 activity. In addition to hair and teeth, Foxi3 may have a role in nail, eye, and mammary, sweat, and salivary gland development.

Missing-in-metastasis MIM/MTSS1 promotes actin assembly at intercellular junctions and is required for integrity of kidney epithelia.

MIM/MTSS1 is a tissue-specific regulator of plasma membrane dynamics, whose altered expression levels have been linked to cancer metastasis. MIM deforms phosphoinositide-rich membranes through its I-BAR domain and interacts with actin monomers through its WH2 domain. Recent work proposed that MIM also potentiates Sonic hedgehog (Shh)-induced gene expression. Here, we generated MIM mutant mice and found that full-length MIM protein is dispensable for embryonic development. However, MIM-deficient mice displayed a severe urinary concentration defect caused by compromised integrity of kidney epithelia intercellular junctions, which led to bone abnormalities and end-stage renal failure. In cultured kidney epithelial (MDCK) cells, MIM displayed dynamic localization to adherens junctions, where it promoted Arp2/3-mediated actin filament assembly. This activity was dependent on the ability of MIM to interact with both membranes and actin monomers. Furthermore, results from the mouse model and cell culture experiments suggest that full-length MIM is not crucial for Shh signaling, at least during embryogenesis. Collectively, these data demonstrate that MIM modulates interplay between the actin cytoskeleton and plasma membrane to promote the maintenance of intercellular contacts in kidney epithelia.

More than nervous: the emerging roles of plexins.

Plexins are the receptors for semaphorins, a large family of axon guidance cues. Accordingly, the role of plexins in the development of the nervous system was the first to be acknowledged. However, the expression of plexins is not restricted to neuronal cells, and recent research has been increasingly focused on the roles of plexin-semaphorin signalling outside of the nervous system. During embryogenesis, plexins regulate the development of many organs, including the cardiovascular system, skeleton and kidney. They have also been shown to be involved in immune system functions and tumour progression. Analyses of the plexin signalling in different tissues and cell types have provided new insight to the versatility of plexin interactions with semaphorins and other cell-surface receptors. In this review we try to summarise the current understanding of the roles of plexins in non-neural development and immunity.

Sema4C-Plexin B2 signalling modulates ureteric branching in developing kidney.

Semaphorins, originally identified as axon guidance molecules, have also been implicated in angiogenesis, function of the immune system and cancerous growth. Here we show that deletion of Plexin B2 (Plxnb2), a semaphorin receptor that is expressed both in the pretubular aggregates and the ureteric epithelium in the developing kidney, results in renal hypoplasia and occasional double ureters. The rate of cell proliferation in the ureteric epithelium and consequently the number of ureteric tips are reduced in the kidneys lacking Plexin B2 (Plxnb2-/-). Semaphorin 4C, a ligand for Plexin B2, stimulates branching of the ureteric epithelium in wild type and Plxnb2+/- kidney explants, but not in Plxnb2-/- explants. As shown by co-immunoprecipitation Plexin B2 interacts with the Ret receptor tyrosine kinase, the receptor of Glial-cell-line-derived neurotrophic factor (Gdnf), in embryonic kidneys. Isolated Plxnb2-/- ureteric buds fail to respond to Gdnf by branching, but this response is rescued by Fibroblast growth factor 7 and Follistatin as well as by the metanephric mesenchyme. The differentiation of the nephrogenic mesenchyme, its morphology and the rate of apoptosis in the Plxnb2-/- kidneys are normal. Plexin B2 is co-expressed with Plexin B1 (Plxnb1) in the kidney. The double homozygous Plxnb1-Plxnb2-deficient mice show high embryonic lethality prior to onset of nephrogenesis. The only double homozygous embryo surviving to E12 showed hypoplastic kidneys with ureteric branches and differentiating mesenchyme. Taken together, our results show that Sema4C-Plexin B2 signalling regulates ureteric branching, possibly through modulation of Gdnf signalling by interaction with Ret, and suggest non-redundant roles for Plexin B1 and Plexin B2 in kidney development.

The GDNF target Vsnl1 marks the ureteric tip.

Glial cell line-derived neurotrophic factor (GDNF) is indispensable for ureteric budding and branching. If applied exogenously, GDNF promotes ectopic ureteric buds from the Wolffian duct. Although several downstream effectors of GDNF are known, the identification of early response genes is incomplete. Here, microarray screening detected several GDNF-regulated genes in the Wolffian duct, including Visinin like 1 (Vsnl1), which encodes a neuronal calcium-sensor protein. We observed renal Vsnl1 expression exclusively in the ureteric epithelium, but not in Gdnf-null kidneys. In the tissue culture of Gdnf-deficient kidney primordium, exogenous GDNF and alternative bud inducers (FGF7 and follistatin) restored Vsnl1 expression. Hence, Vsnl1 characterizes the tip of the ureteric bud epithelium regardless of the inducer. In the tips, Vsnl1 showed a mosaic expression pattern that was mutually exclusive with ß-catenin transcriptional activation. Vsnl1 was downregulated in both ß-catenin-stabilized and ß-catenin-deficient kidneys. Moreover, in a mouse collecting duct cell line, Vsnl1 compromised ß-catenin stability, suggesting a counteracting relationship between Vsnl1 and ß-catenin. In summary, Vsnl1 marks ureteric bud tips in embryonic kidneys, and its mosaic pattern demonstrates a heterogeneity of cell types that may be critical for normal ureteric branching.

Minor spliceosome components are predominantly localized in the nucleus.

Recently, it has been reported that there is a differential subcellular distribution of components of the minor U12-dependent and major U2-dependent spliceosome, and further that the minor spliceosome functions in the cytoplasm. To study the subcellular localization of the snRNA components of both the major and minor spliceosomes, we performed in situ hybridizations with mouse tissues and human cells. In both cases, all spliceosomal snRNAs were nearly exclusively detected in the nucleus, and the minor U11 and U12 snRNAs were further shown to colocalize with U4 and U2, respectively, in human cells. Additionally, we examined the distribution of several spliceosomal snRNAs and proteins in nuclear and cytoplasmic fractions isolated from human cells. These studies revealed an identical subcellular distribution of components of both the U12- and U2-dependent spliceosomes. Thus, our data, combined with several earlier publications, establish that, like the major spliceosome, components of the U12-dependent spliceosome are localized predominantly in the nucleus.

CIP2A increases self-renewal and is linked to Myc in neural progenitor cells.

The oncogenic transcription factor Myc has an established role in the regulation of stem cell self-renewal and differentiation. However, the regulation of Myc activity or expression in stem and progenitor cells is not thoroughly understood. We studied the expression and function of the Myc stabilizing protein and a newly found oncogene, cancerous inhibitor of protein phosphatase 2A (CIP2A) in mouse neural progenitor cells (NPCs). We found intensive CIP2A expression in the neurogenic areas of the developing E13 as well as of the adult mouse brain. Here we also show that retroviral overexpression of CIP2A increases and siRNA silencing of CIP2A decreases NPC self-renewal and proliferation. Differentiation of the NPCs correlates with diminished CIP2A expression although overexpression of CIP2A does not prevent differentiation of neurons and astrocytes. Lastly, we demonstrate that both Myc and CIP2A enhance each other's expression and siRNA against CIP2A in Myc-overexpressing NPCs significantly reduces the ability of Myc to increase self-renewal and proliferation thus indicating a functional connection between CIP2A and Myc in NPCs.

Conservation, expression, and knockdown of zebrafish plxnb2a and plxnb2b.

In mice lacking Plexin B2, a receptor of the axon guidance molecules Semaphorin 4C and Semaphorin 4D, the closure of the neural tube and structural organization of the cerebellum are severely impaired. We cloned two Plexin B2 orthologs, plxnb2a and plxnb2b, in zebrafish, which is a widely used model for the development of the vertebrate central nervous system (CNS). The predicted proteins, Plexin B2a and Plexin B2b, contain all the conserved and functional domains of the plexin B-subfamily. During embryonic development, plxnb2a is expressed, e.g., in pharyngeal arches while plxnb2b expression is more confined to neuronal structures like the cerebellum. However, both plxnb2a and plxnb2b are expressed at the midbrain-hindbrain boundary, in the otic vesicles, facial ganglia, and pectoral fins. Knockdown of both plxnb2a and plxnb2b simultaneously (>95% and 45%, respectively) resulted in normal CNS structure, axon guidance and swimming performance of the morphants.

beta-Catenin mediates adriamycin-induced albuminuria and podocyte injury in adult mouse kidneys.

BACKGROUND: Glomerular slit diaphragm (SD) represents a modified adherens junction composed of molecules belonging to both immunoglobulin and cadherin superfamilies. Cadherins associate with the cytosolic scaffolding protein beta-catenin, but the precise role of beta-catenin in mature or injured podocytes is not known. METHODS: The conditional podocyte-specific beta-catenin-deficient mouse line was generated using the doxycycline-inducible Cre-loxP system. Expression of the beta-catenin-deficient gene was turned off at the age of 8 weeks by doxycycline treatment and the kidney phenotype was analysed. In addition, beta-catenin-deficient and control mice were treated with adriamycin (ADR) and analysed for albuminuria and morphological alterations. RESULTS: Deletion of beta-catenin in mature podocytes did not change the morphology of podocytes nor did it lead to albuminuria. However, lack of beta-catenin attenuated albuminuria after ADR treatment. Electron microscopic examination showed increased podocyte foot process effacement associated with SD abnormalities in ADR-treated control mice compared to beta-catenin-deficient mice. CONCLUSIONS: These results show that beta-catenin in podocytes is dispensable for adult mice, but appears to be important in modulating the SD during ADR-induced perturbation of the filtration barrier.

Lymphatic endothelium stimulates melanoma metastasis and invasion via MMP14-dependent Notch3 and ß1-integrin activation.

Lymphatic invasion and lymph node metastasis correlate with poor clinical outcome in melanoma. However, the mechanisms of lymphatic dissemination in distant metastasis remain incompletely understood. We show here that exposure of expansively growing human WM852 melanoma cells, but not singly invasive Bowes cells, to lymphatic endothelial cells (LEC) in 3D co-culture facilitates melanoma distant organ metastasis in mice. To dissect the underlying molecular mechanisms, we established LEC co-cultures with different melanoma cells originating from primary tumors or metastases. Notably, the expansively growing metastatic melanoma cells adopted an invasively sprouting phenotype in 3D matrix that was dependent on MMP14, Notch3 and ß1-integrin. Unexpectedly, MMP14 was necessary for LEC-induced Notch3 induction and coincident ß1-integrin activation. Moreover, MMP14 and Notch3 were required for LEC-mediated metastasis of zebrafish xenografts. This study uncovers a unique mechanism whereby LEC contact promotes melanoma metastasis by inducing a reversible switch from 3D growth to invasively sprouting cell phenotype.

The expression of plexins during mouse embryogenesis.

Plexins are large transmembrane proteins that are receptors for semaphorins, either alone or in a complex with neuropilin-1 or -2. Nine different mouse plexins have been found: Plexin-A1-4, -B1-3, -C1 and -D1. The expression and function of plexins in non-neuronal tissues has been poorly characterized, although Plexin-A1 has been shown to have a role during lung and cardiac morphogenesis. We have done an extensive non-radioactive in situ hybridisation survey of Plxna1-a4, Plxnb1 -b3 and Plxnc1 in E14 mouse embryo. At E14, Plxnb3 expression could not be detected by in situ hybridisation. All other plexins studied are widely expressed both in neuronal and non-neuronal tissues. We have also followed the expression patterns of plexins during the development of the kidney, tooth and testis. Plxnb1 and Plxnb2 are expressed in the immature glomeruli and mesenchyme of the developing kidney. In the tooth bud, Plxna1 and Plxnb1 are expressed in the oral epithelium, enamel knot and in both the inner and outer enamel epithelium, whereas the expression of Plxnb2 is more restricted to the inner enamel epithelium. In the testis, Plxna1, Plxnb1 and Plxnc1 are expressed in the developing sex chords. This study shows that during development, plexins are expressed in specific and distinct patterns also in non-neuronal tissues.

Biological effects of the dual phenotypic Janus mutation of ret cosegregating with both multiple endocrine neoplasia type 2 and Hirschsprung's disease.

Gain-of-function mutations of ret receptor tyrosine kinase, the signaling receptor for glial cell line-derived neurotrophic factor, cause sporadic thyroid and adrenal malignancies as well as endocrine cancer syndromes, such as multiple endocrine neoplasia types 2A and 2B (MEN 2A and MEN 2B) and familial medullary thyroid carcinoma. Loss-of-function mutations of ret cause Hirschsprung's disease (HSCR) or colonic aganglionosis. In 20-30% of families with a mutation at residues 609, 611, 618, or 620 of RET, MEN 2A and familial medullary thyroid carcinoma cosegregate with HSCR. These mutations constitutively activate RET due to aberrant disulfide homodimerization and diminish the level of RET at the plasma membrane. It is not known how these mutations simultaneously lead to both gain- and loss-of-function RET-associated diseases. We provide an explanation for the dual phenotypic Janus mutation at Cys620 of RET. In Madin-Darby canine kidney (MDCK) cells, the Janus mutation impairs the glial cell line-derived neurotrophic factor-induced effects of RET on cell migration, differentiation, and survival but simultaneously promotes rapid cell proliferation.