Layilin, a novel integral membrane protein, is a hyaluronan receptor

The actin cytoskeleton plays a significant role in changes of cell shape and motility, and interactions between the actin filaments and the cell membrane are crucial for a variety of cellular processes. Several adaptor proteins, including talin, maintain the cytoskeleton-membrane linkage by binding to integral membrane proteins and to the cytoskeleton. Layilin, a recently characterized transmembrane protein with homology to C-type lectins, is a membrane-binding site for talin in peripheral ruffles of spreading cells. To facilitate studies of layilin's function, we have generated a layilin-Fc fusion protein comprising the extracellular part of layilin joined to human immunoglobulin G heavy chain and used this chimera to identify layilin ligands. Here, we demonstrate that layilin-Fc fusion protein binds to hyaluronan immobilized to Sepharose. Microtiter plate-binding assays, coprecipitation experiments, and staining of sections predigested with different glycosaminoglycan-degrading enzymes and cell adhesion assays all revealed that layilin binds specifically to hyaluronan but not to other tested glycosaminoglycans. Layilin's ability to bind hyaluronan, a ubiquitous extracellular matrix component, reveals an interesting parallel between layilin and CD44, because both can bind to cytoskeleton-membrane linker proteins through their cytoplasmic domains and to hyaluronan through their extracellular domains. This parallelism suggests a role for layilin in cell adhesion and motility.

The cloning, genomic organization and expression of the focal contact protein paxillin in Drosophila

Paxillin is a focal adhesion scaffolding protein, which has been proposed to play a role in focal adhesion dynamics. We have isolated a cDNA clone of the Drosophila homologue of paxillin. Comparison of the Drosophila paxillin sequence with those of vertebrate paxillins shows strong conservation of the LIM domains and LD repeats. Using the Drosophila genomic sequence we have identified two partial curated transcripts and deduced the structure of the paxillin gene. No homologues of other members of the paxillin family such as HIC-5 or leupaxin are to be found in the Drosophila genome. Surprisingly paxillin mRNA is expressed in a restricted pattern during embryogenesis. In particular it is strongly expressed in cells and tissues undergoing cell shape changes or cell migration. Many of the sites of expression are also known to be sites of integrin function or FAK expression. The data support a role for paxillin as an adapter and/or signaling protein during developmental processes involving integrin-mediated adhesion.

Platelets adhere to and translocate on von Willebrand factor presented by endothelium in stimulated veins

With the use of intravital microscopy, a new type of platelet-endothelial interaction in mouse mesenteric venules at low shear (80-100 seconds(-1)) is described. Stimulation of these vessels with calcium ionophore A23187, a known secretagogue of Weibel-Palade bodies, induced immediate platelet adhesion (within 15 seconds) and translocation without the formation of aggregates. This stop-and-go process reached a maximum in approximately 1 minute, when approximately 25 000 platelets adhered/mm(2).s, and then adhesion progressively decreased. This adhesion process was dependent on von Willebrand factor (vWF) and independent of P-selectin. Immunohistologic analysis showed that the venules were not denuded with A23187 treatment, suggesting that platelets adhered to vWF secreted on the luminal face of the endothelial cells. Histamine treatment induced a similar adhesion phenomenon. Platelet adhesion was not abolished in beta3-deficient mice or when the platelets were treated with inhibitory antibodies to PECAM-1 or PSGL-1, indicating that these molecules are not required for platelet-endothelium interaction at low shear. The adhesion was mediated by platelet glycoprotein Ibalpha (GPIbalpha) because the adhesion of murine platelets expressing exclusively the human GPIbalpha could be prevented by a pretreatment with mocarhagin, a snake venom protease that cleaves human GPIbalpha. The results indicate that vWF released from Weibel-Palade bodies can dramatically increase the concentration of platelets along the vessel wall through an interaction with GPIbalpha. It is proposed that this process may rapidly recruit platelets to sites of injury or inflammation in veins.

In vivo roles of integrins during leukocyte development and traffic: insights from the analysis of mice chimeric for alpha 5, alpha v, and alpha 4 integrins

Mice chimeric for integrins alpha(5), alpha(V), or alpha(4) were used to dissect the in vivo roles of these adhesion receptors during leukocyte development and traffic. No major defects were observed in the development of lymphocytes, monocytes, or granulocytes or in the traffic of lymphocytes to different lymphoid organs in the absence of alpha(5) or alpha(V) integrins. However, in agreement with previous reports, the absence of alpha(4) integrins produced major defects in development of lymphoid and myeloid lineages and a specific defect in homing of lymphocytes to Peyer's patches. In contrast, the alpha(4) integrin subunit is not essential for localization of T lymphocytes into intraepithelial and lamina propria compartments in the gut, whereas one of the partners of alpha(4), the beta(7) chain, has been shown to be essential. However, alpha(4)-deficient T lymphocytes cannot migrate properly during the inflammatory response induced by thioglycolate injection into the peritoneum. Finally, in vitro proliferation and activation of lymphocytes deficient for alpha(5), alpha(V), or alpha(4) integrins upon stimulation with different stimuli were similar to those seen in controls. These results show that integrins play distinct roles during in vivo leukocyte development and traffic.

alpha3beta1 and alpha6beta4 integrin receptors for laminin-5 are not essential for epidermal morphogenesis and homeostasis during skin development

Continuous regeneration and homeostasis of the stratified epidermis requires coordinated regulation of cell proliferation, cell differentiation, and cell survival. Integrin-mediated cell adhesion to the extracellular matrix has important roles in regulating each of these processes. Integrins alpha3beta1 and alpha6beta4 are both receptors on epidermal keratinocytes for the basement membrane protein laminin-5, the major ligand for epidermal adhesion in mature skin. Ablation in mice of either alpha3beta1 or alpha6beta4, through null mutation of the gene encoding the alpha3, alpha6, or beta4 integrin subunit, results in epidermal blistering of varying severity. Our previous studies showed that, despite blistering, differentiation and stratification of the epidermis appeared essentially normal in mice that lacked either alpha3beta1 or alpha6beta4. However, these studies did not definitively address the specific developmental importance of each integrin, since they may have overlapping and/or compensatory functions. Given the individual importance of alpha3beta1 or alpha6beta4 in maintaining the dermo-epidermal junction in mature skin, we sought to determine the importance of these integrins for embryonic skin development and epidermal morphogenesis. In the current study, we analyzed skin development in mutant embryos that completely lack both integrins alpha3beta1 and alpha6beta4. Although alpha3beta1/alpha6beta4-deficient embryos displayed epidermal blistering by stage E15.5 of development, they also retained regions of extensive epidermal adhesion to the basement membrane through stage E16.5, indicating alternative adhesion mechanisms. Apoptosis was induced in detached epidermis of alpha3beta1/alpha6beta4-deficient embryos, exemplifying vividly the importance of epithelial attachment to the basement membrane for cell survival. However, apoptotic cells were completely absent from attached epidermis of alpha3beta1/alpha6beta4-deficient embryos, showing that epithelial adhesion that occurred independently of alpha3beta1 and alpha6beta4 also protected cells from apoptosis. Remarkably, in the absence of the known laminin-5 binding integrins (alpha3beta1, alpha6beta4, and alpha6beta1), keratinocytes retained the capacity to proliferate in the epidermis, and epidermal stratification and skin morphogenesis appeared normal prior to blister formation. These findings show that while alpha3beta1 and alpha6beta4 are both required for integrity of the dermo-epidermal junction, neither one is essential for epidermal morphogenesis during skin development.

Genomic analysis of metastasis reveals an essential role for RhoC

The most damaging change during cancer progression is the switch from a locally growing tumour to a metastatic killer. This switch is believed to involve numerous alterations that allow tumour cells to complete the complex series of events needed for metastasis. Relatively few genes have been implicated in these events. Here we use an in vivo selection scheme to select highly metastatic melanoma cells. By analysing these cells on DNA arrays, we define a pattern of gene expression that correlates with progression to a metastatic phenotype. In particular, we show enhanced expression of several genes involved in extracellular matrix assembly and of a second set of genes that regulate, either directly or indirectly, the actin-based cytoskeleton. One of these, the small GTPase RhoC, enhances metastasis when overexpressed, whereas a dominant-negative Rho inhibits metastasis. Analysis of the phenotype of cells expressing dominant-negative Rho or RhoC indicates that RhoC is important in tumour cell invasion. The genomic approach allows us to identify families of genes involved in a process, not just single genes, and can indicate which molecular and cellular events might be important in complex biological processes such as metastasis.

Persistence of platelet thrombus formation in arterioles of mice lacking both von Willebrand factor and fibrinogen

We used intravital microscopy to observe the formation of platelet plugs in ferric chloride-injured arterioles of live mice. With this model, we evaluated thrombus growth in mice lacking von Willebrand factor (vWF) and fibrinogen (Fg), the two key ligands known to mediate platelet adhesion and aggregation. In vWF(-/-) mice, despite the presence of arterial shear, delayed platelet adhesion occurred and stable thrombi formed. In many mice, a persisting high-shear channel never occluded. Abundant thrombi formed in Fg(-/-) mice, but they detached from the subendothelium, which ultimately caused downstream occlusion in all cases. Surprisingly, mice deficient in both vWF and Fg successfully formed thrombi with properties characteristic of both mutations, leading to vessel occlusion in the majority of vessels. Platelets of these doubly deficient mice specifically accumulated fibronectin in their alpha-granules, suggesting that fibronectin could be the ligand supporting the platelet aggregation.

Mouse keratinocytes immortalized with large T antigen acquire alpha3beta1 integrin-dependent secretion of MMP-9/gelatinase B

Remodeling of the extracellular matrix during tissue development, wound repair and tumor cell invasion depends on the coordinated regulation of cell adhesion receptors, matrix proteins and enzymes that proteolyse the extracellular matrix. Integrin alpha3beta1 is a major receptor on epidermal keratinocytes for laminin-5 in the cutaneous basement membrane and is required for normal basement membrane organization during skin development. alpha3beta1 is also expressed at high levels in the majority of adherent transformed cells and in most tumors, and it could have similar roles in extracellular matrix remodeling during tumorigenesis and cell invasion. In the present study, we show that alpha3beta1 expression is required in immortalized mouse keratinocytes (MK) for the production of the matrix metalloproteinase MMP-9/gelatinase B, an MMP that is coexpressed with alpha3beta1 in epithelial cell carcinomas and during wound healing, and contributes to the invasive potential of some tumor cells. MMP-9 was expressed in MK cells derived from wild-type mice, but not in MK cells derived from alpha3-null mice. Reconstitution of alpha3beta1 expression in alpha3-null MK cells through transfection with the alpha3 subunit restored MMP-9 secretion, indicating an alpha3beta1-dependent pathway for MMP-9 production. alpha3beta1-dependent expression of MMP-9 was associated with the immortalized phenotype, since nonimmortalized, primary keratinocytes required soluble growth factors, but not alpha3beta1, for efficient expression of MMP-9. Our results suggest that an alpha3beta1-independent pathway(s) for MMP-9 production is suppressed in keratinocytes immortalized with large T antigen, and that an alpha3beta1-dependent pathway is required for sustained production of MMP-9 in the absence of other pathways.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

Mice lacking beta3 integrins are osteosclerotic because of dysfunctional osteoclasts

Osteoclasts express the alphavbeta3 integrin, an adhesion receptor that has been implicated in bone resorption and that is therefore a potential therapeutic target. To assess the role of this heterodimer in skeletal development in vivo, we engineered mice in which the gene for the beta3 integrin subunit was deleted. Bone marrow macrophages derived from these mutants differentiate in vitro into numerous osteoclasts, thus establishing that alphavbeta3 is not necessary for osteoclast recruitment. Furthermore, the closely related integrin, alphavbeta5, does not substitute for alphavbeta3 during cytokine stimulation or authentic osteoclastogenesis. beta3 knockout mice, but not their heterozygous littermates, develop histologically and radiographically evident osteosclerosis with age. Despite their increased bone mass, beta3-null mice contain 3.5-fold more osteoclasts than do heterozygotes. These mutant osteoclasts are, however, dysfunctional, as evidenced by their reduced ability to resorb whale dentin in vitro and the significant hypocalcemia seen in the knockout mice. The resorptive defect in beta3-deficient osteoclasts may reflect absence of matrix-derived intracellular signals, since their cytoskeleton is distinctly abnormal and they fail to spread in vitro, to form actin rings ex vivo, or to form normal ruffled membranes in vivo. Thus, although it is not required for osteoclastogenesis, the integrin alphavbeta3 is essential for normal osteoclast function.

Expression of DFak56, a Drosophila homolog of vertebrate focal adhesion kinase, supports a role in cell migration in vivo

Focal adhesion kinase (FAK) is a highly conserved, cytoplasmic tyrosine kinase that has been implicated in promoting cell migration and transmission of antiapoptotic signals in vertebrate cells. In cultured cells, integrin engagement with the extracellular matrix promotes the recruitment of FAK to focal contacts and increases in its phosphotyrosine content and kinase activity, suggesting FAK is an intracellular mediator of integrin signaling. We have identified a Drosophila FAK homolog, DFak56, that is 33% identical to vertebrate FAK, with the highest degree of homology in domains critical for FAK function, including the kinase and focal adhesion targeting domains, and several protein-protein interaction motifs. Furthermore, when expressed in NIH 3T3 cells, DFak56 both localizes to focal contacts and displays the characteristic elevation of phosphotyrosine content in response to plating the cells on fibronectin. During embryogenesis, DFak56 is broadly expressed, and it becomes elevated in the gut and central nervous system at later stages. Consistent with a role in cell migration, we also observe that DFak56 is abundant in the border cells of developing egg chambers before the onset of, and during, their migration.

Cell adhesion: old and new questions

Metazoans clearly need cell adhesion to hold themselves together, but adhesion does much more than that. Adhesion receptors make transmembrane connections, linking extracellular matrix and adjacent cells to the intracellular cytoskeleton, and they also serve as signal transducers. In this article, I briefly summarize our present understanding of the molecular basis and biological consequences of cell adhesion and discuss how our current knowledge sheds light on questions of specificity of cell adhesion. I offer some thoughts and speculations about the evolution of cell-adhesion molecules and processes, consider their inter-relationships with other forms of cell-cell communication and discuss unresolved questions ripe for investigation as we enter the postgenomic era.

Overlapping and independent functions of fibronectin receptor integrins in early mesodermal development

Mouse embryos deficient in fibronectin (FN-null) die at E8.5 with mesodermal defects. Eight integrin heterodimers alpha3beta1, alpha4beta1, alpha5beta1, alpha8beta1, alphavbeta1, alphavbeta3, alphavbeta6, and alphaIIbbeta3 can bind to FN. However, embryos deficient in each of these integrins exhibit less severe defects than do FN-null embryos, raising questions as to which integrin(s) are the key FN receptors for these early FN-dependent processes. alpha5beta1 is believed to be the key receptor and alpha5-null embryos display mesodermal defects similar to, although less severe than, those of FN-null. Here we report that the alpha5-null mutation exhibits a more severe phenotype on a 129Sv (129) than on a C57BL/6 (B6) background, as does the FN-null mutation. While alpha5-null/B6 embryos develop normal headfolds, alpha5-null/129 embryos have headfold defects similar to those of FN-null. The differences between FN-null and alpha5-null embryos, however, cannot be attributed to genetic background. FN-null embryos never form somites, whereas in alpha5-null/129 embryos the somites do condense but fail to epithelialize. Second, we examined double mutants carrying all possible pairwise combinations of null mutations in alpha3, alpha4, and alpha5 integrin genes. There was no evidence for any synergy between paired mutations, suggesting that these integrin genes do not have overlapping functions during early embryonic development. Finally, we examined double-mutant embryos deficient in both alpha5 and alphav integrin genes. These double-mutant embryos have an amniotic defect similar to that of FN-null embryos, but die even earlier with a defect in gastrulation. These studies thus revealed a gradation in the severity of defects in the mutations alpha5(-/-); alphav(-/-) > FN(-/-) (129) > FN(-/-) (B6) > alpha5(-/-) (129) > alpha5(-/-) (B6), and in each step in this series there is a certain degree of phenotypic overlap, suggesting that the defects arising from these mutations may result from disruptions of the same embryonic process.

The Talin head domain binds to integrin beta subunit cytoplasmic tails and regulates integrin activation

The beta subunit cytoplasmic domains of integrin adhesion receptors are necessary for the connection of these receptors to the actin cytoskeleton. The cytoplasmic protein, talin, binds to beta integrin cytoplasmic tails and actin filaments, hence forming an integrin-cytoskeletal linkage. We used recombinant structural mimics of beta(1)A, beta(1)D and beta(3) integrin cytoplasmic tails to characterize integrin-binding sites within talin. Here we report that an integrin-binding site is localized within the N-terminal talin head domain. The binding of the talin head domain to integrin beta tails is specific in that it is abrogated by a single point mutation that disrupts integrin localization to talin-rich focal adhesions. Integrin-cytoskeletal interactions regulate integrin affinity for ligands (activation). Overexpression of a fragment of talin containing the head domain led to activation of integrin alpha(IIb)beta(3); activation was dependent on the presence of both the talin head domain and the integrin beta(3) cytoplasmic tail. The head domain of talin thus binds to integrins to form a link to the actin cytoskeleton and can thus regulate integrin function.

Alpha4 integrins regulate the proliferation/differentiation balance of multilineage hematopoietic progenitors in vivo

We investigated roles of alpha4 integrins during hematopoiesis using mutant and chimeric mice. Yolk sac erythropoiesis and migration of hematopoietic progenitors to fetal liver, spleen, and bone marrow can occur without alpha4 integrins. Although terminal differentiation of these progenitors is possible without alpha4 integrins, these receptors are essential to maintain normal hematopoiesis in fetal liver, spleen, and bone marrow microenvironments. Moreover, alpha4-deficient erythroid progenitors and pre-B cells neither transmigrate beneath the stroma nor expand-properly in vitro. In contrast, alpha4-null cells migrate and differentiate efficiently into T lymphocytes within the thymus. In summary, alpha4 integrins are essential for normal development of all hematopoietic lineages in fetal liver, bone marrow, and spleen, likely by regulating the proliferation/differentiation balance of hematopoietic progenitors.

Id1 and Id3 are required for neurogenesis, angiogenesis and vascularization of tumour xenografts

Id proteins may control cell differentiation by interfering with DNA binding of transcription factors. Here we show that targeted disruption of the dominant negative helix-loop-helix proteins Id1 and Id3 in mice results in premature withdrawal of neuroblasts from the cell cycle and expression of neural-specific differentiation markers. The Id1-Id3 double knockout mice also display vascular malformations in the forebrain and an absence of branching and sprouting of blood vessels into the neuroectoderm. As angiogenesis both in the brain and in tumours requires invasion of avascular tissue by endothelial cells, we examined the Id knockout mice for their ability to support the growth of tumour xenografts. Three different tumours failed to grow and/or metastasize in Id1+/- Id3-/- mice, and any tumour growth present showed poor vascularization and extensive necrosis. Thus, the Id genes are required to maintain the timing of neuronal differentiation in the embryo and invasiveness of the vasculature. Because the Id genes are expressed at very low levels in adults, they make attractive new targets for anti-angiogenic drug design.

Multiple, targeted deficiencies in selectins reveal a predominant role for P-selectin in leukocyte recruitment

We extend our previous analyses of mice deficient in selectins by describing the generation and comparative phenotype of mice lacking one, two, or three selectins after sequential ablation of the murine genes encoding P-, E-, and L-selectins. All mice deficient in selectins are viable and fertile as homozygotes. However, mice missing both P- and E-selectins (PE(-/-)), and mice missing all three selectins (ELP(-/-)) develop mucocutaneous infections that eventually lead to death. Mice deficient in multiple selectins display varying degrees of leukocytosis, resulting in part from alterations in leukocyte rolling and recruitment. PE(-/-) mice, ELP(-/-) mice, and mice missing both P- and L-selectins (PL(-/-)) show drastic reductions in leukocyte rolling and in extravasation of neutrophils in thioglycollate-induced peritonitis. In a separate inflammatory model (ragweed-induced peritoneal eosinophilia), we demonstrate P-selectin to be both necessary and sufficient for the recruitment of eosinophils. The phenotype of mice missing both E- and L-selectins (EL(-/-)) is less severe than those seen in the other double knockouts. Comparisons among the double knockouts suggest that P-selectin normally cooperates with both E- and L-selectins. Our results indicate a preeminent role for P-selectin in regulating leukocyte behavior in mice. Data from the ELP(-/-) mice indicate, however, that all three selectins are important to leukocyte homeostasis and efficient neutrophil recruitment.

Insights and questions arising from studies of a mouse model of Glanzmann thrombasthenia

In summary, studies of beta 3-deficient mice demonstrate that they provide an excellent model of the human bleeding disorder, Glanzmann thrombasthenia (GT), and replicate essentially all features of the human disease. Furthermore, these mice reveal additional defects consequent upon the absence of integrins alpha v beta 3 and alpha IIB beta 3 that had not become evident from studies of the human patients. These include defects in placental development and in bone resorption. In contrast, despite published evidence suggesting a role for alpha v beta 3 in angiogenesis, beta 3-null mice have not yet shown any defects in this process. The availability of these mice will allow extensive further studies of GT pathogenesis and therapy and of the roles of beta 3 integrins in different forms of angiogenesis, including the pathologically important angiogenic processes in the retina and in response to tumors. Furthermore, they open the way to studies of a variety of other processes in which roles for beta 3 integrins have been suggested.

Fibronectin regulates assembly of actin filaments and focal contacts in cultured cells via the heparin-binding site in repeat III13

Fibroblasts, when plated on the extracellular matrix protein fibronectin (FN), rapidly spread and form an organized actin cytoskeleton. This process is known to involve both the central alpha5beta1 integrin-binding and the C-terminal heparin-binding regions of FN. We found that within the heparin-binding region, the information necessary for inducing organization of stress fibers and focal contacts was located in a 29-amino acid segment of FN type III module 13 (III13). We did not find a cytoskeleton-organizing role for repeat III14, which had previously been implicated in this process. Within III13, the same five basic amino acids known to be most important for heparin binding were also necessary for actin organization. A substrate of III13 alone was only weakly adhesive but strongly induced formation of filopodia and lamellipodia. Stress fiber formation required a combination of III13 and III7-11 (which contains the integrin alpha5beta1 recognition site), either as a single fusion protein or as separate polypeptides, and the relative amounts of the two binding sites appeared to determine whether stress fibers or filopodia and lamellipodia were the predominant actin structures formed. We propose that a balance of signals from III13 and from integrins regulates the type of actin structures assembled by the cell.

Infection-stimulated infraosseus inflammation and bone destruction is increased in P-/E-selectin knockout mice

Infections of the dental pulp commonly result in infraosseus inflammation and bone destruction. However, the role of phagocytic leucocytes in the pathogenesis of pulpal infections has been uncertain. In this work we used P/E-/- selectin-deficient mice, which lack rolling adhesion of leucocytes to endothelium and mimic the human syndrome, leucocyte adhesion deficiency II (LAD-II), to test the hypothesis that phagocytic leucocytes protect against pulpal infection and subsequent periapical infraosseus bone resorption. P/E-/- mice and P/E+/+ wild-type controls were subjected to surgical pulp exposure, and both groups were infected with a mixture of pulpal pathogens including Prevotella intermedia, Fusobacterium nucleatum, Peptostreptococcus micros and Streptococcus intermedius. Animals were killed after 20 days, and the extent of infraosseus bone destruction was quantified by histomorphometry. In two separate experiments, P/E-/- mice had significantly greater bone resorption than P/E+/+ controls. The increased bone destruction correlated with a twofold decrease in polymorphonuclear (PMN) infiltration into periapical inflammatory tissues of P/E-/- mice. P/E-/- mice had higher tissue levels of the bone resorptive cytokine, interleukin (IL)-1alpha. Tissue levels of IL-2, IL-4, IL-10, tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) were all higher in P/E-/- mice, but the increases were not statistically significant. Only IL-12 was higher in P/E+/+ mice, possibly reflecting a greater number of infiltrating monocytes in wild-type mice. These findings demonstrate that phagocytic leucocytes are protective in this model, and suggest that elevated expression of inflammatory cytokines is responsible for the observed bone destruction.

Integrins in vascular development

Many growth factors and their protein kinase receptors play a role in regulating vascular development. In addition, cell adhesion molecules, such as integrins and their ligands in the extracellular matrix, play important roles in the adhesion, migration, proliferation, survival and differentiation of the cells that form the vasculature. Some integrins are known to be regulated by angiogenic growth factors and studies with inhibitors of integrin functions and using strains of mice lacking specific integrins clearly implicate some of these molecules in vasculogenesis and angiogenesis. However, the data are incomplete and sometimes discordant and it is unclear how angiogenic growth factors and integrin-mediated adhesive events cooperate in the diverse cell biological processes involved in forming the vasculature. Consideration of the results suggests working hypotheses and raises questions for future research directions.

Mouse primordial germ cells lacking beta1 integrins enter the germline but fail to migrate normally to the gonads

Primordial germ cells are the founder cells of the gametes. They are set aside at the initial stages of gastrulation in mammals, become embedded in the hind-gut endoderm, then actively migrate to the sites of gonad formation. The molecular basis of this migration is poorly understood. Here we sought to determine if members of the integrin family of cell surface receptors are required for primordial germ cell migration, as integrins have been implicated in the migration of several other motile cell types. We have established a line of mice which express green fluorescent protein in germline cells that has enabled us to efficiently purify primordial germ cells at different stages by flow cytometry. We have catalogued the spectrum of integrin subunit expression by primordial germ cells during and after migration, using flow cytometry, immunocytochemistry and RT-PCR. Through analysis of integrin beta1(−/−)-->wild-type chimeras, we show that embryonic cells lacking beta1 integrins can enter the germline. However, integrin beta1(−/−) primordial germ cells do not colonize the gonad efficiently. Embryos with targeted deletion of integrin subunit alpha3, alpha6, or alphaV show no major defects in primordial germ cell migration. These results demonstrate a role for beta1-containing integrins in the development of the germline, although an equivalent role for * integrin subunit(s) has yet to be established.