Plant twitter: ligands under 140 amino acids enforcing stomatal patterning

Stomata are an essential land plant innovation whose patterning and density are under genetic and environmental control. Recently, several putative ligands have been discovered that influence stomatal density, and they all belong to the EPIDERMAL PATTERNING FACTOR-LIKE family of secreted cysteine-rich peptides. Two of these putative ligands, EPF1 and EPF2, are expressed exclusively in the stomatal lineage cells and negatively regulate stomatal density. A third, EPFL6 or CHALLAH, is also a negative regulator of density, but is expressed subepidermally in the hypocotyl. A fourth, EPFL9 or STOMAGEN, is expressed in the mesophyll tissues and is a positive regulator of density. Genetic evidence suggests that these ligands may compete for the same receptor complex. Proper stomatal patterning is likely to be an intricate process involving ligand competition, regional specificity, and communication between tissue layers. EPFL-family genes exist in the moss Physcomitrella patens, the lycophyte Selaginella moellendorffii, and rice, Oryza sativa, and their sequence analysis yields several genes some of which are related to EPF1, EPF2, EPFL6, and EPFL9. Presence of these EPFL family members in the basal land plants suggests an exciting hypothesis that the genetic components for stomatal patterning originated early in land plant evolution.     

Bone morphogenetic protein signaling in limb outgrowth and patterning

Bone morphogenetic proteins (BMPs) are multifunctional growth factors belonging to the transforming growth factor beta (TGFbeta) multigene family. Current evidence indicates that they may play different and even antagonistic roles at different stages of limb development. Refined studies of their function in these processes have been impeded in the mouse due to the early lethality of null mutants for several BMP ligands and their receptors. Recently, however, these questions have benefited from the very powerful Cre-loxP technology. In this review, I intend to summarize what has been learned from this conditional mutagenesis approach in the mouse limb, focusing on Bmp2, Bmp4 and Bmp7 while restricting my analysis to the initial phases of limb formation and patterning. Two major aspects are discussed, the role of BMPs in dorsal-ventral polarization of the limb bud, together with their relation to apical ectodermal ridge (AER) induction, and their role in controlling digit number and identity. Particular attention is paid to the methodology, its power and its limits.     

Roles of FGF receptors in mammalian development and congenital diseases

Four fibroblast growth factor receptors (FGFR1-4) constitute a family of transmembrane tyrosine kinases that serve as high affinity receptors for at least 22 FGF ligands. Gene targeting in mice has yielded valuable insights into the functions of this important gene family in multiple biological processes. These include mesoderm induction and patterning; cell growth, migration, and differentiation; organ formation and maintenance; neuronal differentiation and survival; wound healing; and malignant transformation. Furthermore, discoveries that mutations in three of the four receptors result in more than a dozen human congenital diseases highlight the importance of these genes in skeletal development. In this review, we will discuss recent progress on the roles of FGF receptors in mammalian development and congenital diseases, with an emphasis on signal transduction pathways.     

The boundary-expressed EPIDERMAL PATTERNING FACTOR-LIKE2 gene encoding a signaling peptide promotes cotyledon growth during Arabidopsis thaliana embryogenesis

The shoot organ boundaries have important roles in plant growth and morphogenesis. It has been reported that a gene encoding a cysteine-rich secreted peptide of the EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) family, EPFL2, is expressed in the boundary domain between the two cotyledon primordia of Arabidopsis thaliana embryo. However, its developmental functions remain unknown. This study aimed to analyze the role of EPFL2 during embryogenesis. We found that cotyledon growth was reduced in its loss-of-function mutants, and this phenotype was associated with the reduction of auxin response peaks at the tips of the primordia. The reduced cotyledon size of the mutant embryo recovered in germinating seedlings, indicating the presence of a factor that acted redundantly with EPFL2 to promote cotyledon growth in late embryogenesis. Our analysis suggests that the boundary domain between the cotyledon primordia acts as a signaling center that organizes auxin response peaks and promotes cotyledon growth.     

Comprehensive analysis of CLE polypeptide signaling gene expression and overexpression activity in Arabidopsis

Intercellular signaling is essential for the coordination of growth and development in higher plants. Although hundreds of putative receptors have been identified in Arabidopsis (Arabidopsis thaliana), only a few families of extracellular signaling molecules have been discovered, and their biological roles are largely unknown. To expand our insight into the developmental processes potentially regulated by ligand-mediated signal transduction pathways, we undertook a systematic expression analysis of the members of the Arabidopsis CLAVATA3/ESR-RELATED (CLE) small signaling polypeptide family. Using reporter constructs, we show that the CLE genes have distinct and specific patterns of promoter activity. We find that each Arabidopsis tissue expresses at least one CLE gene, indicating that CLE-mediated signaling pathways are likely to play roles in many biological processes during the plant life cycle. Some CLE genes that are closely related in sequence have dissimilar expression profiles, yet in many tissues multiple CLE genes have overlapping patterns of promoter-driven reporter activity. This observation, plus the general absence of detectable morphological phenotypes in cle null mutants, suggest that a high degree of functional redundancy exists among CLE gene family members. Our work establishes a community resource of CLE-related biological materials and provides a platform for understanding and ultimately manipulating many different plant signaling systems.     

Distinct developmental programs require different levels of Bmp signaling during mouse retinal development

The Bmp family of secreted signaling molecules is implicated in multiple aspects of embryonic development. However, the cell-type-specific requirements for this signaling pathway are often obscure in the context of complex embryonic tissue interactions. To define the cell-autonomous requirements for Bmp signaling, we have used a Cre-loxP strategy to delete Bmp receptor function specifically within the developing mouse retina. Disruption of a Bmp type I receptor gene, Bmpr1a, leads to no detectable eye abnormality. Further reduction of Bmp receptor activity by removing one functional copy of another Bmp type I receptor gene, Bmpr1b, in the retina-specific Bmpr1a mutant background, results in abnormal retinal dorsoventral patterning. Double mutants completely lacking both of these genes exhibit severe eye defects characterized by reduced growth of embryonic retina and failure of retinal neurogenesis. These studies provide direct genetic evidence that Bmpr1a and Bmpr1b play redundant roles during retinal development, and that different threshold levels of Bmp signaling regulate distinct developmental programs such as patterning, growth and differentiation of the retina.     

The ephrins and Eph receptors in angiogenesis.

Eph receptors are a unique family of receptor tyrosine kinases that play critical roles in embryonic patterning, neuronal targeting, vascular development and adult neovascularization. Engagement of Eph receptors by ephrin ligands mediates critical steps of angiogenesis, including juxtacrine cell-cell contacts, cell adhesion to extracellular matrix, and cell migration. Recent evidence from in vitro angiogenesis assays and analysis of mice deficient for one or more members of the Eph family establishes the role of Eph signaling in sprouting angiogenesis and blood vessel remodeling during vascular development. Furthermore, elevated expression of Eph receptors and ephrin ligands is associated with tumors and associated tumor vasculature, suggesting that Eph receptors and their ephrin ligands also play critical roles in tumor angiogenesis and tumor growth. This review will focus on the relevance of Eph receptor signaling in embryonic and adult neovascularization, and possible contributions to tumor growth and metastasis.     

Eph受体家族及其配体的信号转导途径及功能

Eph受体是已知最大的酪氨酸蛋白激酶受体家族,Eph受体和其膜附着型配体（ephrin）在发育过程中呈现不同的表达模式,近来研究证明,Eph受体和其配体在包括神经网络形成,神经管和轴旁中胚层的成型（patterning）,细胞迁移导向和轴突路径导引,血管形成等许多的发育过程中起重要作用.Eph受体及其配体也与肿瘤发生有关,因此深入分析这些分子尤其在肿瘤细胞生长中的功能而应用于治疗具有重要的临床意义.     

Neuropilins: structure, function and role in disease

NRPs (neuropilins) are co-receptors for class 3 semaphorins, polypeptides with key roles in axonal guidance, and for members of the VEGF (vascular endothelial growth factor) family of angiogenic cytokines. They lack a defined signalling role, but are thought to mediate functional responses as a result of complex formation with other receptors, such as plexins in the case of semaphorins and VEGF receptors (e.g. VEGFR2). Mutant mouse studies show that NRP1 is essential for neuronal and cardiovascular development, whereas NRP2 has a more restricted role in neuronal patterning and lymphangiogenesis, but recent findings indicate that NRPs may have additional biological roles in other physiological and disease-related settings. In particular, NRPs are highly expressed in diverse tumour cell lines and human neoplasms and have been implicated in tumour growth and vascularization in vivo. However, despite the wealth of information regarding the probable biological roles of these molecules, many aspects of the regulation of cellular function via NRPs remain uncertain, and little is known concerning the molecular mechanisms through which NRPs mediate the functions of their various ligands in different cell types.     

Eph receptors and ephrins

The Eph receptors are the largest known family of receptor tyrosine kinases. The Eph receptors and their membrane-attached ligands, ephrins, show diverse expression patterns during development. Recent studies have demonstrated that Eph receptors and ephrins play important roles in many developmental processes, including neuronal network formation, the patterning of the neural tube and the paraxial mesoderm, the guidance of cell migration, and vascular formation. In the nervous system, Eph receptors and ephrins have been shown to act as positional labels to establish topographic projections. They also play a key role in pathway finding by axons and neural crest cells. The crucial roles of Eph receptors and ephrins during development suggest involvement of these genes in congenital disorders affecting the nervous system and other tissues. It has also been suggested that Eph receptors and ephrins may be involved in carcinogenesis. It is therefore of clinical importance to further analyse the function of these molecules, as manipulation of their function may have therapeutic applications.     

Nuclear pore complex integrity requires Lnp1, a regulator of cortical endoplasmic reticulum

The nuclear envelope (NE) and endoplasmic reticulum (ER) are components of the same contiguous membrane system and yet have distinct cellular functions. Mounting evidence suggests roles for some ER proteins in the NE for proper nuclear pore complex (NPC) structure and function. In this study, we identify a NE role in Saccharomyces cerevisiae for Lnp1 and Sey1, proteins required for proper cortical ER formation. Both lnp1Δ and sey1Δ mutants exhibit synthetic genetic interactions with mutants in genes encoding key NPC structural components. Both Lnp1 and Sey1 physically associate with other ER components that have established NPC roles, including Rtn1, Yop1, Pom33, and Per33. Of interest, lnp1Δ rtn1Δ mutants but not rtn1Δ sey1Δ mutants exhibit defects in NPC distribution. Furthermore, the essential NPC assembly factor Ndc1 has altered interactions in the absence of Sey1. Lnp1 dimerizes in vitro via its C-terminal zinc finger motif, a property that is required for proper ER structure but not NPC integrity. These findings suggest that Lnp1''s role in NPC integrity is separable from functions in the ER and is linked to Ndc1 and Rtn1 interactions.     

酪氨酸激酶受体Eph亚族的研究进展

酪氨酸激酶受体（RTK）参与细胞生长、分化、胚胎发育及细胞内信号传递等过程,具有相当重要的生理功能.目前已发现50多种RTK基因分属于14种亚族,Eph亚族是其中最大的家族,由14个基因组成,一些基因主要在脑的发育中表达,另一些则在各种组织中广泛表达.最近该亚族胞外配体的发现为深入研究其生理功能打下基础.综述了Eph亚族成员的来源、表达及其配体的研究概况.     

ALK7, a receptor for nodal, is dispensable for embryogenesis and left-right patterning in the mouse

Mesendoderm formation and left-right patterning during vertebrate development depend upon selected members of the transforming growth factor beta superfamily, particularly Nodal and Nodal-related ligands. Two type I serine/threonine kinase receptors have been identified for Nodal, ALK4 and ALK7. Mouse embryos lacking ALK4 fail to produce mesendoderm and die shortly after gastrulation, resembling the phenotype of Nodal knockout mice. Whether ALK4 contributes to left-right patterning is still unknown. Here we report the generation and initial characterization of mice lacking ALK7. Homozygous mutant mice were born at the expected frequency and remained viable and fertile. Viability at weaning was not different from that of the wild type in ALK7(-/-); Nodal(+/-) and ALK7(-/-); ALK4(+/-) compound mutants. ALK7 and ALK4 were highly expressed in interdigital regions of the developing limb bud. However, ALK7 mutant mice displayed no skeletal abnormalities or limb malformations. None of the left-right patterning abnormalities and organogenesis defects identified in mice carrying mutations in Nodal or in genes encoding ActRIIA and ActRIIB coreceptors, including heart malformations, pulmonary isomerism, right-sided gut, and spleen hypoplasia, were observed in mice lacking ALK7. Finally, the histological organization of the cerebellum, cortex, and hippocampus, all sites of significant ALK7 expression in the rodent brain, appeared normal in ALK7 mutant mice. We conclude that ALK7 is not an essential mediator of Nodal signaling during mesendoderm formation and left-right patterning in the mouse but may instead mediate other activities of Nodal and related ligands in the development or function of particular tissues and organs.     

The N-terminal globular domain of Eph receptors is sufficient for ligand binding and receptor signaling.

The Eph family of receptor protein-tyrosine kinases (RTKs) have recently been implicated in patterning and wiring events in the developing nervous system. Eph receptors are unique among other RTKs in that they fall into two large subclasses that show distinct ligand specificities and for the fact that they themselves might function as ''ligands'', thereby activating bidirectional signaling. To gain insight into the mechanisms of ligand-receptor interaction, we have mapped the ligand binding domain in Eph receptors. By using a series of deletion and domain substitution mutants, we now report that an N-terminal globular domain of the Nuk/Cek5 receptor is the ligand binding domain of the transmembrane ligand Lerk2. Using focus formation assays, we show that the Cek5 globular domain is sufficient to confer Lerk2-dependent transforming activity on the Cek9 orphan receptor. Extending our binding studies to other members of both subclasses of receptors, it became apparent that the same domain is used for binding of both transmembrane and glycosylphosphatidyl-anchored ligands. Our studies have determined the first structural elements involved in ligand-receptor interaction and will allow more fine-tuned genetic experiments to elucidate the mechanism of action of these important guidance molecules.     

C. elegans LIN-18 is a Ryk ortholog and functions in parallel to LIN-17/Frizzled in Wnt signaling

Wnt proteins are intercellular signals that regulate various aspects of animal development. In Caenorhabditis elegans, mutations in lin-17, a Frizzled-class Wnt receptor, and in lin-18 affect cell fate patterning in the P7.p vulval lineage. We found that lin-18 encodes a member of the Ryk/Derailed family of tyrosine kinase-related receptors, recently found to function as Wnt receptors. Members of this family have nonactive kinase domains. The LIN-18 kinase domain is dispensable for LIN-18 function, while the Wnt binding WIF domain is required. We also found that Wnt proteins LIN-44, MOM-2, and CWN-2 redundantly regulate P7.p patterning. Genetic interactions indicate that LIN-17 and LIN-18 function independently of each other in parallel pathways, and different ligands display different receptor specificities. Thus, two independent Wnt signaling pathways, one employing a Ryk receptor and the other a Frizzled receptor, function in parallel to regulate cell fate patterning in the C. elegans vulva.     

ErbB and HB-EGF signaling in heart development and function

The epidermal growth factor (EGF)-ErbB signaling network is composed of multiple ligands of the EGF family and four tyrosine kinase receptors of the ErbB family. In higher vertebrates, these four receptors bind a multitude of ligands. Ligand binding induces the formation of various homo- and heterodimers of ErbB, potentially providing for a high degree of signal diversity. ErbB receptors and their ligands are expressed in a variety of tissues throughout development. Recent advances in gene targeting strategies in mice have revealed that the EGF-ErbB signaling network has fundamental roles in development, proliferation, differentiation, and homeostasis in mammals. The heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors that binds to and activates the EGF receptor (EGFR/ErbB1) and ErbB4. Recent studies using several mutant mice lacking HB-EGF expression have revealed that HB-EGF has a critical role in normal heart function and in normal cardiac valve formation in conjunction with ErbB receptors. HB-EGF signaling through ErbB2 is essential for the maintenance of homeostasis in the adult heart, whereas HB-EGF signaling through EGFR is required during cardiac valve development. In this review, we introduce and discuss the role of ErbB receptors in heart function and development, focusing on the physiological function of HB-EGF in these processes.