SH2-B is required for both male and female reproduction

Analysis of gonadal transcriptome reveals core long non-coding RNA-mRNA regulatory network in sea cucumber Apostichopus japonicus

Apostichopus japonicus is a representative temperate sea cucumber species, that mainly inhabits in coastal zone of the continental shelf. With high nutritional value and important medical value, A. japonicus become an important commercial aquaculture species and produce significant economic value in recent years. A. japonicus has no sexual dimorphism that can be used to distinguish female and male individuals by external appearance and morphology. The phenotype sex can be only detected by dissecting and observing gonad tissue, thus the breeding efficiency could be greatly reduced. This limitation has hindered the advancement of selective breeding programs and sea cucumber industry. To investigate the genetic basis of reproductive biology in A. japonicus, advanced sequencing techniques, such as next- and third-generation sequencing, have been employed to explore the roles of non-coding RNAs and other genetic factors, offering new insights into sex determination mechanisms. To further gain a deeper understanding of the knowledge underlying lncRNAs in gonadal differentiation, we conducted a comparative transcriptome sequencing analysis of gonadal tissues from both sexes. In our research, a total of 3990 novel lncRNAs and 1441 differentially expressed lncRNAs were identified between female and male gonads. Additionally, a molecular regulatory network indicating lncRNA-mRNA interactions was constructed based on transcriptional profiles, which provide insights into the potential cis- and trans- target genes of lncRNAs. The gonadal transcriptome analysis identified a number of novel long non-coding RNAs involved in female and male reproduction process. Both cis- and trans-acting regulatory networks indicating lncRNA-mRNA interaction were constructed based on transcriptional profiles. These findings provide new insights into the lncRNA-mediated regulation of reproductive biology in marine invertebrates, indicating the crucial roles of long non-coding sequences in regulating expression profiles. Further, the GO and KEGG enrichment analyses of cis- and trans- targeted mRNA for differentially expressed lncRNA indicated that sexual reproduction (GO:0019953), germ cell development (GO:0007281), and negative regulation of hormone secretion (GO:0046888) are potentially involved in gonadal differentiation through the regulation of long non-coding sequences. Notably, besides the classical reproduction related signaling pathway like Gonadotropin-releasing hormone (GnRH) secretion (ko04929), several regulatory pathways, such as Epidermal growth factor receptor (ErbB) signaling pathway (ko04012), TGF-beta signaling pathway (ko04350), and neurotrophin signaling pathway (ko04722) were also enriched and potentially involved in sex differentiation and gonadal development.

Control of germinal center B cell survival and IgE production by an adaptor molecule containing PH and SH2 domains, Aps/Sh2b2

The germinal centers (GCs) are structure found within secondary lymphoid organs and are important for the antibody-producing response against foreign antigens. In GCs, antigen-specific B cells proliferate intensely, inducing immunoglobulin class switching. Recent studies have shown that GCs are also an important site for class switching to IgE, which is implicated in allergy. However, the mechanisms by which IgE production is regulated in GCs remain unclear. Here, we found impairment in IgE-specific production and a reduction of GC B cells after immunization in mice deficient in the Aps/Sh2b2 gene encoding the Lnk/Sh2b family adaptor protein Aps. GC B cells express higher levels of the Aps gene than non-GC B cells, and cell death of Aps-/- GC B cells is enhanced compared to wild-type GC B cells. An in vitro culture system with purified Aps-/- B cells induced the same level of IgE production and frequencies of IgE+ B cells as wild-type B cells. We found that Aps deficiency in B cells resulted in augmented depletion of IgE+ blasts by B cell receptor crosslinking with anti-CD79b antibodies compared to wild-type IgE+ cells. These results suggest that Aps regulates IgE production by controlling the survival of GC B cells and IgE+ plasma cells and may serve as a potential therapeutic target to control IgE production.

New perspective on SH2B1: An accelerator of cancer progression

SH2B1 is well-known as an adaptor protein, and deletion of SH2B1 results in severe obesity and both leptin and insulin resistance. Some studies have revealed that SH2B1 is involved in the progression of lung cancer, esophageal cancer, gastric cancer, oropharyngeal cancer, and so on. Biological function experiments have proven that SH2B1 can regulate cellular morphology, motility and adhesion by modifying the actin cytoskeletal reorganization, and it can promote cell mitogenesis, transformation, survival and differentiation via different signal pathways by enhancing the kinase activity of several receptor tyrosine kinases. In addition, SH2B1 is an obesity-related gene, and epidemiological surveys suggest a complex relationship between obesity and cancer. Therefore, what is the relationship between SH2B1 and cancer? Herein, we attempt to provide a mini overview of the roles of SH2B1 in cancer.

Potential Functions of IGFBP-2 for Ovarian Folliculogenesis and Steroidogenesis

Ovarian follicles, as transient structural and functional complexes with the oocyte and the associated cells, determine the female reproductive cycle and thus fertility. Ovarian function is subject to the strict control of hormones and growth factors and thus regulated by auto-, para-, and endocrine mechanisms but influenced also by endogenous factors. During the waves of follicular growth and development, one follicle (monoovulatory) or a limited number of them (polyovulatory) are selected under hypothalamic-gonadal control for maturation until ovulation, resulting in the fertile oocyte. Subordinate follicles inevitably enter different stages of atresia. A number of studies have observed species-specific alterations of IGFBP-2 levels during the phases of growth and development or selection and atresia of follicles. IGFBP-2 is thus probably involved in the process of follicle growth, differentiation, and degeneration. This may occur on the levels of IGF-dependent and -independent growth control but also due to the control of steroidogenesis, e.g., via induction of aromatase expression. In mice, IGFBP-2 delayed reproductive development most probably by IGF-independent mechanisms. Because reproductive development is closely linked to the control of life- or health-span and energy metabolism, we feel that the time is right now to resume research on the effects of IGFBP-2 in the ovarian follicular compartment.

SH2B1 is critical for the regulation of cardiac remodelling in response to pressure overload

AIMS

Src homology 2 (SH2) B adaptor protein 1 (SH2B1) is expressed in various tissues, including the heart. Previous studies have demonstrated that SH2B1 is involved in a variety of biological process, such as maintaining neuronal differentiation, regulating energy and glucose homeostasis, and promoting cell proliferation and motility. However, the role of SH2B1 in cardiac hypertrophy remains unclear. This study aimed at identifying the effects and the underlying mechanisms of SH2B1 in cardiac hypertrophy.

METHODS AND RESULTS

We performed gain- and loss-of-function studies using genetic approaches, and cardiac hypertrophy was evaluated through pathological, echocardiographic, haemodynamic, and molecular analyses. We found that SH2B1 expression was significantly increased in both failing human hearts and hypertrophic murine hearts. Mice overexpressing SH2B1 specifically in the heart displayed increased aortic banding (AB)-induced cardiac hypertrophy, fibrosis, ventricular dilation, and dysfunction compared with controls, whereas loss of SH2B1 produced the opposite phenotype. Consistently, similar results were observed in a global SH2B1-knockout rat model. Mechanistically, the pro-hypertrophic effects elicited by SH2B1 were associated with activation of the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signalling cascade. Furthermore, rescue experiments revealed that pharmacological inactivation of JAK2 rescued pressure overload-induced cardiac abnormalities in transgenic mice with cardiac-specific SH2B1 overexpression.

CONCLUSION

Taken together, our data demonstrate, for the first time, that SH2B1 is a key positive mediator of pathological cardiac hypertrophy, and that it primarily acts by regulating JAK2/STAT3 signalling.

SH2B1 regulation of energy balance, body weight, and glucose metabolism

The Src homology 2B (SH2B) family members (SH2B1, SH2B2 and SH2B3) are adaptor signaling proteins containing characteristic SH2 and PH domains. SH2B1 (also called SH2-B and PSM) and SH2B2 (also called APS) are able to form homo- or hetero-dimers via their N-terminal dimerization domains. Their C-terminal SH2 domains bind to tyrosyl phosphorylated proteins, including Janus kinase 2 (JAK2), TrkA, insulin receptors, insulin-like growth factor-1 receptors, insulin receptor substrate-1 (IRS1), and IRS2. SH2B1 enhances leptin signaling by both stimulating JAK2 activity and assembling a JAK2/IRS1/2 signaling complex. SH2B1 promotes insulin signaling by both enhancing insulin receptor catalytic activity and protecting against dephosphorylation of IRS proteins. Accordingly, genetic deletion of SH2B1 results in severe leptin resistance, insulin resistance, hyperphagia, obesity, and type 2 diabetes in mice. Neuron-specific overexpression of SH2B1β transgenes protects against diet-induced obesity and insulin resistance. SH2B1 in pancreatic β cells promotes β cell expansion and insulin secretion to counteract insulin resistance in obesity. Moreover, numerous SH2B1 mutations are genetically linked to leptin resistance, insulin resistance, obesity, and type 2 diabetes in humans. Unlike SH2B1, SH2B2 and SH2B3 are not required for the maintenance of normal energy and glucose homeostasis. The metabolic function of the SH2B family is conserved from insects to humans.

Regulation of insulin and type 1 insulin-like growth factor signaling and action by the Grb10/14 and SH2B1/B2 adaptor proteins

The effects of insulin and type 1 insulin-like growth factor (IGF-1) on metabolism, growth and survival are mediated by their association with specific receptor tyrosine kinases, which results in both receptor and substrate phosphorylation. Phosphotyrosine residues on receptors and substrates provide docking sites for signaling proteins containing SH2 (Src homology 2) domains, including molecular adaptors. This review focuses on the regulation of insulin/IGF-1 signaling and action by two adaptor families with a similar domain organization: the growth factor receptor-bound proteins Grb7/10/14 and the SH2B proteins. Both Grb10/14 and SH2B1/B2 associate with the activation loop of insulin/IGF-1 receptors through their SH2 domains, but association of Grb10/14 also involves their unique BPS domain. Consistent with Grb14 binding as a pseudosubstrate to the kinase active site, insulin/IGF-induced activation of receptors and downstream signaling pathways in cultured cells is inhibited by Grb10/14 adaptors, but is potentiated by SH2B1/B2 adaptors. Accordingly, Grb10 and Grb14 knockout mice show improved insulin/IGF sensitivity in vivo, and, for Grb10, overgrowth and increased skeketal muscle and pancreatic β-cell mass. Conversely, SH2B1-depleted mice display insulin and IGF-1 resistance, with peripheral depletion leading to reduced adiposity and neuronal depletion leading to obesity through associated leptin resistance. Grb10/14 and SH2B1 adaptors also modulate insulin/IGF-1 action by interacting with signaling components downstream of receptors and exert several tissue-specific effects. The identification of Grb10/14 and SH2B1 as physiological regulators of insulin signaling and action, together with observations that variants at their gene loci are associated with obesity and/or insulin resistance, highlight them as potential therapeutic targets for these conditions.

The Lnk adaptor protein: a key regulator of normal and pathological hematopoiesis

The development and function of blood cells are regulated by specific growth factors/cytokines and their receptors' signaling pathways. In this way, these factors influence cell survival, proliferation and differentiation of hematopoietic cells. Central to this positive and/or negative control are the adaptor proteins. Since their identification 10 years ago, members of the Lnk adaptor protein family have proved to be important activators and/or inhibitors in the hematopoietic, immune and vascular system. In particular, the generation of animal and cellular models for the Lnk and APS proteins has helped establish the physiological role of these molecules through the identification of their specific signaling pathways and the characterization of their binding partners. Moreover, the recent identification of mutations in the LNK gene in myeloproliferative disorders, as well as the correlation of a single nucleotide polymorphism on LNK with hematological, immune and vascular diseases have suggested its involvement in the pathophysiology of these malignancies. The latter findings have thus raised the possibility of addressing Lnk signaling for the treatment of certain human diseases. This review therefore describes the pathophysiological role of this adaptor protein in hematological malignancies and the potential benefits of Lnk therapeutic targeting.

Adapter protein SH2B1beta binds filamin A to regulate prolactin-dependent cytoskeletal reorganization and cell motility

Prolactin (PRL) regulates cytoskeletal rearrangement and cell motility. PRL-activated Janus tyrosine kinase 2 (JAK2) phosphorylates the p21-activated serine-threonine kinase (PAK)1 and the Src homology 2 (SH2) domain-containing adapter protein SH2B1β. SH2B1β is an actin-binding protein that cross-links actin filaments, whereas PAK1 regulates the actin cytoskeleton by different mechanisms, including direct phosphorylation of the actin-binding protein filamin A (FLNa). Here, we have used a FLNa-deficient human melanoma cell line (M2) and its derivative line (A7) that stably expresses FLNa to demonstrate that SH2B1β and FLNa are required for maximal PRL-dependent cell ruffling. We have found that in addition to two actin-binding domains, SH2B1β has a FLNa-binding domain (amino acids 200-260) that binds directly to repeats 17-23 of FLNa. The SH2B1β-FLNa interaction participates in PRL-dependent actin rearrangement. We also show that phosphorylation of the three tyrosines of PAK1 by JAK2, as well as the presence of FLNa, play a role in PRL-dependent cell ruffling. Finally, we show that the actin- and FLNa-binding-deficient mutant of SH2B1β (SH2B1β 3Δ) abolished PRL-dependent ruffling and PRL-dependent cell migration when expressed along with PAK1 Y3F (JAK2 tyrosyl-phosphorylation-deficient mutant). Together, these data provide insight into a novel mechanism of PRL-stimulated regulation of the actin cytoskeleton and cell motility via JAK2 signaling through FLNa, PAK1, and SH2B1β. We propose a model for PRL-dependent regulation of the actin cytoskeleton that integrates our findings with previous studies.

Phosphorylation controls a dual-function polybasic nuclear localization sequence in the adapter protein SH2B1β to regulate its cellular function and distribution

An intriguing question in cell biology is what targets proteins to, and regulates their translocation between, specific cellular locations. Here we report that the polybasic nuclear localization sequence (NLS) required for nuclear entry of the adapter protein and candidate human obesity gene product SH2B1β, also localizes SH2B1β to the plasma membrane (PM), most probably via electrostatic interactions. Binding of SH2B1β to the PM also requires its dimerization domain. Phosphorylation of serine residues near this polybasic region, potentially by protein kinase C, releases SH2B1β from the PM and enhances nuclear entry. Release of SH2B1β from the PM and/or nuclear entry appear to be required for SH2B1β enhancement of nerve growth factor (NGF)-induced expression of urokinase plasminogen activator receptor gene and neurite outgrowth of PC12 cells. Taken together, our results provide strong evidence that the polybasic NLS region of SH2B1 serves the dual function of localizing SH2B1 to both the nucleus and the PM, the latter most probably through electrostatic interactions that are enhanced by SH2B1β dimerization. Cycling between the different cellular compartments is a consequence of the phosphorylation and dephosphorylation of serine residues near the NLS and is important for physiological effects of SH2B1, including NGF-induced gene expression and neurite outgrowth.

Laboratory mouse models for the human genome-wide associations

The agnostic screening performed by genome-wide association studies (GWAS) has uncovered associations for previously unsuspected genes. Knowledge about the functional role of these genes is crucial and laboratory mouse models can provide such information. Here, we describe a systematic juxtaposition of human GWAS-discovered loci versus mouse models in order to appreciate the availability of mouse models data, to gain biological insights for the role of these genes and to explore the extent of concordance between these two lines of evidence. We perused publicly available data (NHGRI database for human associations and Mouse Genome Informatics database for mouse models) and employed two alternative approaches for cross-species comparisons, phenotype- and gene-centric. A total of 293 single gene-phenotype human associations (262 unique genes and 69 unique phenotypes) were evaluated. In the phenotype-centric approach, we identified all mouse models and related ortholog genes for the 51 human phenotypes with a comparable phenotype in mice. A total of 27 ortholog genes were found to be associated with the same phenotype in humans and mice, a concordance that was significantly larger than expected by chance (p<0.001). In the gene-centric approach, we were able to locate at least 1 knockout model for 60% of the 262 genes. The knockouts for 35% of these orthologs displayed pre- or post-natal lethality. For the remaining non-lethal orthologs, the same organ system was involved in mice and humans in 71% of the cases (p<0.001). Our project highlights the wealth of available information from mouse models for human GWAS, catalogues extensive information on plausible physiologic implications for many genes, provides hypothesis-generating findings for additional GWAS analyses and documents that the concordance between human and mouse genetic association is larger than expected by chance and can be informative.

Male germ cell apoptosis: regulation and biology

Cellular apoptosis appears to be a constant feature in the adult testis and during early development. This is essential because mammalian spermatogenesis is a complex process that requires precise homeostasis of different cell types. This review discusses the latest information available on male germ cell apoptosis induced by hormones, toxins and temperature in the context of the type of apoptotic pathway either the intrinsic or the extrinsic that may be used under a variety of stimuli. The review also discusses the importance of mechanisms pertaining to cellular apoptosis during testicular development, which is independent of exogenous stimuli. Since instances of germ cell carcinoma have increased over the past few decades, the current status of research on apoptotic pathways in teratocarcinoma cells is included. One other important aspect that is covered in this review is microRNA-mediated control of germ cell apoptosis, a field of research that is going to see intense activity in near future. Since knockout models of various kinds have been used to study many aspects of germ cell development, a comprehensive summary of literature on knockout mice used in reproduction studies is also provided.

SH2B regulation of growth, metabolism, and longevity in both insects and mammals

SH2B1 is a key regulator of body weight in mammals. Here, we identified dSH2B as the Drosophila homolog of SH2B1. dSH2B bound to Chico and directly promoted insulin-like signaling. Disruption of dSH2B decreased insulin-like signaling and somatic growth in flies. dSH2B deficiency also increased hemolymph carbohydrate levels, whole-body lipid levels, life span, and resistance to starvation and oxidative stress. Systemic overexpression of dSH2B resulted in opposite phenotypes. dSH2B overexpression in fat body decreased lipid and glucose levels, whereas neuron-specific overexpression of dSH2B decreased oxidative resistance and life span. Genetic deletion of SH2B1 also resulted in growth retardation, obesity, and type 2 diabetes in mice; surprisingly, life span and oxidative resistance were reduced in SH2B1 null mice. These data suggest that dSH2B regulation of insulin-like signaling, growth, and metabolism is conserved in SH2B1, whereas dSH2B regulation of oxidative stress and longevity may be conserved in other SH2B family members.

Regulation of lifespan, metabolism, and stress responses by the Drosophila SH2B protein, Lnk

Drosophila Lnk is the single ancestral orthologue of a highly conserved family of structurally-related intracellular adaptor proteins, the SH2B proteins. As adaptors, they lack catalytic activity but contain several protein-protein interaction domains, thus playing a critical role in signal transduction from receptor tyrosine kinases to form protein networks. Physiological studies of SH2B function in mammals have produced conflicting data. However, a recent study in Drosophila has shown that Lnk is an important regulator of the insulin/insulin-like growth factor (IGF)-1 signaling (IIS) pathway during growth, functioning in parallel to the insulin receptor substrate, Chico. As this pathway also has an evolutionary conserved role in the determination of organism lifespan, we investigated whether Lnk is required for normal lifespan in Drosophila. Phenotypic analysis of mutants for Lnk revealed that loss of Lnk function results in increased lifespan and improved survival under conditions of oxidative stress and starvation. Starvation resistance was found to be associated with increased metabolic stores of carbohydrates and lipids indicative of impaired metabolism. Biochemical and genetic data suggest that Lnk functions in both the IIS and Ras/Mitogen activated protein Kinase (MapK) signaling pathways. Microarray studies support this model, showing transcriptional feedback onto genes in both pathways as well as indicating global changes in both lipid and carbohydrate metabolism. Finally, our data also suggest that Lnk itself may be a direct target of the IIS responsive transcription factor, dFoxo, and that dFoxo may repress Lnk expression. We therefore describe novel functions for a member of the SH2B protein family and provide the first evidence for potential mechanisms of SH2B regulation. Our findings suggest that IIS signaling in Drosophila may require the activity of a second intracellular adaptor, thereby yielding fundamental new insights into the functioning and role of the IIS pathway in ageing and metabolism.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Diet-induced obesity in male mice is associated with reduced fertility and potentiation of acrylamide-induced reproductive toxicity

The prevalence of human obesity and related chronic disorders such as diabetes, cardiovascular diseases, and cancer is rapidly increasing. Human studies have shown a direct relationship between obesity and infertility. The objective of the current work was to examine the effect of diet-induced obesity on male fertility and the effect of obesity on susceptibility to chemical-induced reproductive toxicity. From 5 to 30 wk of age, genetically intact male C57Bl/6J mice were fed a normal diet or one in which 60% of the kilocalories were from lard. Obese mice exhibited significant differences in the mRNA of several genes within the testes in comparison to lean males. Pparg was increased 2.2-fold, whereas Crem, Sh2b1, Dhh, Igf1, and Lepr were decreased 6.7, 1.4, 3.2, 1.6, and 7.2-fold, respectively. The fertility of male mice was compared through mating with control females. Acrylamide (AA)-induced reproductive toxicity was assessed in obese or lean males treated with water or 25 mg AA kg(-1) day(-1) via gavage for 5 days and then mated to control females. Percent body fat and weight were significantly increased in mice fed a high-fat vs. a normal diet. Obesity resulted in significant reduction in plugs and pregnancies of control females partnered with obese vs. lean males. Serum leptin and insulin levels were each approximately 5-fold higher in obese vs. age-matched lean mice. Sperm from obese males exhibited decreased motility and reduced hyperactivated progression vs. lean mice. Treatment with AA exacerbated male infertility of obese and lean mice; however, this effect was more pronounced in obese mice. Further, females partnered with AA-treated obese mice exhibited a further decrease in the percentage of live fetuses, whereas the percentage of resorptions increased. This work demonstrated that diet-induced obesity in mice caused a significant reduction in male fertility and exacerbated AA-induced reproductive toxicity and germ cell mutagenicity.

The Drosophila SH2B family adaptor Lnk acts in parallel to chico in the insulin signaling pathway

Insulin/insulin-like growth factor signaling (IIS) plays a pivotal role in the regulation of growth at the cellular and the organismal level during animal development. Flies with impaired IIS are developmentally delayed and small due to fewer and smaller cells. In the search for new growth-promoting genes, we identified mutations in the gene encoding Lnk, the single fly member of the SH2B family of adaptor molecules. Flies lacking lnk function are viable but severely reduced in size. Furthermore, lnk mutants display phenotypes reminiscent of reduced IIS, such as developmental delay, female sterility, and accumulation of lipids. Genetic epistasis analysis places lnk downstream of the insulin receptor (InR) and upstream of phosphoinositide 3-kinase (PI3K) in the IIS cascade, at the same level as chico (encoding the single fly insulin receptor substrate [IRS] homolog). Both chico and lnk mutant larvae display a similar reduction in IIS activity as judged by the localization of a PIP(3) reporter and the phosphorylation of protein kinase B (PKB). Furthermore, chico; lnk double mutants are synthetically lethal, suggesting that Chico and Lnk fulfill independent but partially redundant functions in the activation of PI3K upon InR stimulation.

A major lipid raft protein raftlin modulates T cell receptor signaling and enhances th17-mediated autoimmune responses

The membrane microdomains known as lipid rafts have been shown to act as platforms for the initiation of various receptor signals. Through proteomic analysis, we have identified a novel protein termed Raftlin (raft-linking protein) as a major protein in lipid rafts. To determine the physiological and immunological functions of Raftlin in mammals, we generated Raftlin-deficient mice, as well as Raftlin-transgenic (Tg) mice. Although Raftlin was originally identified in B cells, we observe no severe abnormalities in the B cells of these mice, presumably due to a high expression of Raftlin-homologue (Raftlin-2). T cells, in contrast, expressed a substantial amount of Raftlin but no Raftlin-2. In Raftlin-deficient mice, T cell-dependent Ab production was reduced, and experimental autoimmune encephalomyelitis, a Th17-dependent autoimmune disease model, was ameliorated. In Raftlin-Tg mice, in contrast, Ab production was enhanced and experimental autoimmune encephalomyelitis was more severe. Cytokine production, especially that of IL-17, was reduced in Raftlin-deficient T cells, while it was enhanced in Raftlin-Tg T cells. We found that these changes were associated with the strength of the TCR-mediated signals. Importantly, localization of Lck protein in the lipid rafts was enhanced by Raftlin overexpression and reduced by Raftlin deficiency. These data indicate that Raftlin modulates TCR signals and is necessary for the fine-tuning of T cell-mediated immune responses.

Adapter protein SH2B1beta cross-links actin filaments and regulates actin cytoskeleton

The Src homology 2 (SH2) domain-containing adapter protein SH2B1beta plays a role in severe obesity, leptin and insulin resistance, and infertility. SH2B1beta was initially identified as a Janus tyrosine kinase 2 (JAK2) substrate, and it has been implicated in cell motility and regulation of the actin rearrangement in response to GH and platelet-derived growth factor. SH2B1beta is also required for maximal actin-based motility of Listeria. Here we have used a low-speed pelleting assay and electron microscopy to demonstrate that SH2B1beta has two actin-binding sites and that it cross-links actin filaments in vitro. Wild-type SH2B1beta localized to cell ruffles and along filopodia, but deletion of amino acids 150-200 (the first actin-binding site) led to mislocalization of the protein to filopodia tip complexes where it colocalized with vasodilator-stimulated phosphoprotein (VASP). Based on studies performed in VASP-deficient MVD7(-/-) cells, with or without green fluorescent protein-VASP reconstitution, we concluded that the proper intracellular localization of native SH2B1beta required the presence of the first SH2B1beta actin-binding site and VASP. Finally, we found that both SH2B1beta actin-binding domains were required for maximal GH- and prolactin-induced cell ruffling. Together, these results suggest that SH2B1beta functions as an adapter protein that cross-links actin filaments, leading to modulation of cellular responses in response to JAK2 activation.

[Sh2b3/Lnk family adaptor proteins in the regulation of lymphohematopoiesis]

Sh2b3/Lnk consisting of an N-terminal proline-rich region, PH-, SH2-domains and a tyrosine phosphorylation site, forms an intracellular adaptor protein family conserved from drosophila to mammals, together with Sh2b1/SH2-B and Sh2b2/APS (adaptor protein with PH and SH2 domains). Lnk negatively regulates lymphopoiesis and early hematopoiesis. The lnk-deficiency results in enhanced production of B cells, and expansion as well as enhanced function of hematopoietic stem cells (HSCs), demonstrating negative regulatory functions of Sh2b3/Lnk in cytokine signaling. Our recent studies also revealed that Sh2b3/Lnk functions in responses controlled by cell adhesion and in crosstalk between integrin- and cytokine-mediated signaling. Importantly, recent genome-wide association studies of the autoimmune type 1 diabetes or celiac disease identified risk variants in the SH2B3/LNK region, indicating possible unrevealed functions mediated by this adaptor molecule. This review summarizes roles of Sh2b3/Lnk in the regulation of B-lymphopoiesis and HSCs expansion and function, and briefly introduces our approach for modulating HSCs function by targeting Sh2b3/Lnk-mediated pathways.

Lnk adaptor protein down-regulates specific Kit-induced signaling pathways in primary mast cells

Stem cell factor (SCF) plays critical roles in proliferation, survival, migration, and function of hematopoietic progenitor and mast cells through binding to Kit receptor. Previous studies have implicated the adaptor protein Lnk as an important negative regulator of SCF signaling. However, the molecular mechanism underlying this regulation is unclear. Here, we showed that the Src homology 2 domain (SH2) of Lnk binds directly and preferentially to phosphorylated tyrosine 567 in Kit juxtamembrane domain. Using Lnk−/− bone marrow mast cells (BMMCs) transduced with different Lnk proteins, we demonstrated that Lnk down-regulates SCF-induced proliferation with attenuation of mitogen-activated protein kinase (MAPK) and c-jun N-terminal kinase signaling. Furthermore, we showed that Lnk−/− BMMCs displayed increased SCF-dependent migration compared with wild-type cells, revealing a novel Lnk-mediated inhibitory function. This correlated with enhanced Rac and p38 MAPK activation. Finally, we found that Lnk domains and carboxy-terminal tyrosine contribute differently to inhibition of in vitro expansion of hematopoietic progenitors. Altogether, our results demonstrate that Lnk, through its binding to Kit tyrosine 567, negatively modulates specific SCF-dependent signaling pathways involved in the proliferation and migration of primary hematopoietic cells.

PSM/SH2B1 splice variants: critical role in src catalytic activation and the resulting STAT3s-mediated mitogenic response

A role of PSM/SH2B1 had been shown in mitogenesis and extending to phenotypic cell transformation, however, the underlying molecular mechanism remained to be established. Here, four alternative PSM splice variants and individual functional protein domains were compared for their role in the regulation of Src activity. We found that elevated cellular levels of PSM variants resulted in phenotypic cell transformation and potentiated cell proliferation and survival in response to serum withdrawal. PSM variant activity presented a consistent signature pattern for any tested response of highest activity observed for gamma, followed by delta, alpha, and beta with decreasing activity. PSM-potentiated cell proliferation was sensitive to Src inhibitor herbimycin and PSM and Src were found in the same immune complex. PSM variants were substrates of the Src Tyr kinase and potentiated Src catalytic activity by increasing the V(max) and decreasing the K(m) for ATP with the signature pattern of variant activity. Dominant-negative PSM peptide mimetics including the SH2 or PH domains inhibited Src catalytic activity as well as Src-mediated phenotypic cell transformation. Activation of major Src substrate STAT3 was similarly potentiated by the PSM variants in a Src-dependent fashion or inhibited by PSM domain-specific peptide mimetics. Expression of a dominant-negative STAT3 mutant blocked PSM variant-mediated phenotypic cell transformation. Our results implicate an essential role of the PSM variants in the activation of the Src kinase and the resulting mitogenic response--extending to phenotypic cell transformation and involving the established Src substrate STAT3.

Essential role of PSM/SH2-B variants in insulin receptor catalytic activation and the resulting cellular responses

The positive regulatory role of PSM/SH2-B downstream of various mitogenic receptor tyrosine kinases or gene disruption experiments in mice support a role of PSM in the regulation of insulin action. Here, four alternative PSM splice variants and individual functional domains were compared for their role in the regulation of specific metabolic insulin responses. We found that individual PSM variants in 3T3-L1 adipocytes potentiated insulin-mediated glucose and amino acid transport, glycogenesis, lipogenesis, and key components in the metabolic insulin response including p70 S6 kinase, glycogen synthase, glycogen synthase kinase 3 (GSK3), Akt, Cbl, and IRS-1. Highest activity was consistently observed for PSM alpha, followed by beta, delta, and gamma with decreasing activity. In contrast, dominant-negative peptide mimetics of the PSM Pro-rich, pleckstrin homology (PH), or src homology 2 (SH2) domains inhibited any tested insulin response. Potentiation of the insulin response originated at the insulin receptor (IR) kinase level by PSM variant-specific regulation of the Km (ATP) whereas the Vmax remained unaffected. IR catalytic activation was inhibited by peptide mimetics of the PSM SH2 or dimerization domain (DD). Either peptide should disrupt the complex of a PSM dimer linked to IR via SH2 domains as proposed for PSM activation of tyrosine kinase JAK2. Either peptide abolished downstream insulin responses indistinguishable from PSM siRNA knockdown. Our results implicate an essential role of the PSM variants in the activation of the IR kinase and the resulting metabolic insulin response. PSM variants act as internal IR ligands that in addition to potentiating the insulin response stimulate IR catalytic activation even in the absence of insulin.

SH2B1 enhances leptin signaling by both Janus kinase 2 Tyr813 phosphorylation-dependent and -independent mechanisms

Leptin controls body weight by activating its long form receptor (LEPRb). LEPRb binds to Janus kinase 2 (JAK2), a cytoplasmic tyrosine kinase that mediates leptin signaling. We previously reported that genetic deletion of SH2B1 (previously known as SH2-B), a JAK2-binding protein, results in severe leptin-resistant and obese phenotypes, indicating that SH2B1 is a key endogenous positive regulator of leptin sensitivity. Here we show that SH2B1 regulates leptin signaling by multiple mechanisms. In the absence of leptin, SH2B1 constitutively bound, via its non-SH2 domain region(s), to non-tyrosyl-phosphorylated JAK2, and inhibited JAK2. Leptin stimulated JAK2 phosphorylation on Tyr(813), which subsequently bound to the SH2 domain of SH2B1. Binding of the SH2 domain of SH2B1 to phospho-Tyr(813) in JAK2 enhanced leptin induction of JAK2 activity. JAK2 was required for leptin-stimulated phosphorylation of insulin receptor substrate 1 (IRS1), an upstream activator of the phosphatidylinositol 3-kinase pathway. Overexpression of SH2B1 enhanced both JAK2- and JAK2(Y813F)-mediated tyrosine phosphorylation of IRS1 in response to leptin, even though SH2B1 did not enhance JAK2(Y813F) activation. Leptin promoted the interaction of SH2B1 with IRS1. These data suggest that constitutive SH2B1-JAK2 interaction, mediated by the non-SH2 domain region(s) of SH2B1 and the non-Tyr(813) region(s) in JAK2, increases the local concentration of SH2B1 close to JAK2 and inhibits JAK2 activity. Leptin-stimulated SH2B1-JAK2 interaction, mediated by the SH2 domain of SH2B1 and phospho-Tyr(813) in JAK2, promotes JAK2 activation, thus globally enhancing leptin signaling. SH2B1-IRS1 interaction facilitates IRS1 phosphorylation by recruiting IRS1 to JAK2 and/or by protecting IRS1 from dephosphorylation, thus specifically enhancing leptin stimulation of the phosphatidylinositol 3-kinase pathway.

SH2B1beta adaptor is a key enhancer of RET tyrosine kinase signaling

The RET gene encodes two main isoforms of a receptor tyrosine kinase (RTK) implicated in various human diseases. Activating germ-line point mutations are responsible for multiple endocrine neoplasia type 2-associated medullary thyroid carcinomas, inactivating germ-line mutations for Hirschsprung's disease, while somatic rearrangements (RET/PTCs) are specific to papillary thyroid carcinomas. SH2B1beta, a member of the SH2B adaptors family, and binding partner for several RTKs, has been recently described to interact with proto-RET. Here, we show that both RET isoforms and its oncogenic derivatives bind to SH2B1beta through the SRC homology 2 (SH2) domain and a kinase activity-dependent mechanism. As a result, RET phosphorylates SH2B1beta, which in turn enhances its autophosphorylation, kinase activity, and downstream signaling. RET tyrosine residues 905 and 981 are important determinants for functional binding of the adaptor, as removal of both autophosphorylation sites displaces its recruitment. Binding of SH2B1beta appears to protect RET from dephosphorylation by protein tyrosine phosphatases, and might represent a likely mechanism contributing to its upregulation. Thus, overexpression of SH2B1beta, by enhancing phosphorylation/activation of RET transducers, potentiates the cellular differentiation and the neoplastic transformation thereby induced, and counteracts the action of RET inhibitors. Overall, our results identify SH2B1beta as a key enhancer of RET physiologic and pathologic activities.

Identification of SH2B2beta as an inhibitor for SH2B1- and SH2B2alpha-promoted Janus kinase-2 activation and insulin signaling

The SH2B family has three members (SH2B1, SH2B2, and SH2B3) that contain conserved dimerization (DD), pleckstrin homology, and SH2 domains. The DD domain mediates the formation of homo- and heterodimers between members of the SH2B family. The SH2 domain of SH2B1 (previously named SH2-B) or SH2B2 (previously named APS) binds to phosphorylated tyrosines in a variety of tyrosine kinases, including Janus kinase-2 (JAK2) and the insulin receptor, thereby promoting the activation of JAK2 or the insulin receptor, respectively. JAK2 binds to various members of the cytokine receptor family, including receptors for GH and leptin, to mediate cytokine responses. In mice, SH2B1 regulates energy and glucose homeostasis by enhancing leptin and insulin sensitivity. In this work, we identify SH2B2beta as a new isoform of SH2B2 (designated as SH2B2alpha) derived from the SH2B2 gene by alternative mRNA splicing. SH2B2beta has a DD and pleckstrin homology domain but lacks a SH2 domain. SH2B2beta bound to both SH2B1 and SH2B2alpha, as demonstrated by both the interaction of glutathione S-transferase-SH2B2beta fusion protein with SH2B1 or SH2B2alpha in vitro and coimmunoprecipitation of SH2B2beta with SH2B1 or SH2B2alpha in intact cells. SH2B2beta markedly attenuated the ability of SH2B1 to promote JAK2 activation and subsequent tyrosine phosphorylation of insulin receptor substrate-1 by JAK2. SH2B2beta also significantly inhibited SH2B1- or SH2B2alpha-promoted insulin signaling, including insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1. These data suggest that SH2B2beta is an endogenous inhibitor of SH2B1 and/or SH2B2alpha, negatively regulating insulin signaling and/or JAK2-mediated cellular responses.

PSM/SH2-B distributes selected mitogenic receptor signals to distinct components in the PI3-kinase and MAP kinase signaling pathways

The Pro-rich, PH, and SH2 domain containing mitogenic signaling adapter PSM/SH2-B has been implicated as a cellular partner of various mitogenic receptor tyrosine kinases and related signaling mechanisms. Here, we report in a direct comparison of three peptide hormones, that PSM participates in the assembly of distinct mitogenic signaling complexes in response to insulin or IGF-I when compared to PDGF in cultured normal fibroblasts. The complex formed in response to insulin or IGF-I involves the respective peptide hormone receptor and presumably the established components leading to MAP kinase activation. However, our data suggest an alternative link from the PDGF receptor via PSM directly to MEK1/2 and consequently also to p44/42 activation, possibly through a scaffold protein. At least two PSM domains participate, the SH2 domain anticipated to link PSM to the respective receptor and the Pro-rich region in an association with an unidentified downstream component resulting in direct MEK1/2 and p44/42 regulation. The PDGF receptor signaling complex formed in response to PDGF involves PI 3-kinase in addition to the same components and interactions as described for insulin or IGF-I. PSM associates with PI 3-kinase via p85 and in addition the PSM PH domain participates in the regulation of PI 3-kinase activity, presumably through membrane interaction. In contrast, the PSM Pro-rich region appears to participate only in the MAP kinase signal. Both pathways contribute to the mitogenic response as shown by cell proliferation, survival, and focus formation. PSM regulates p38 MAP kinase activity in a pathway unrelated to the mitogenic response.

Adaptor protein SH2-B linking receptor-tyrosine kinase and Akt promotes adipocyte differentiation by regulating peroxisome proliferator-activated receptor gamma messenger ribonucleic acid levels

Adipocyte differentiation is regulated by insulin and IGF-I, which transmit signals by activating their receptor tyrosine kinase. SH2-B is an adaptor protein containing pleckstrin homology and Src homology 2 (SH2) domains that have been implicated in insulin and IGF-I receptor signaling. In this study, we found a strong link between SH2-B levels and adipogenesis. The fat mass and expression of adipogenic genes including peroxisome proliferator-activated receptor gamma (PPARgamma) were reduced in white adipose tissue of SH2-B-/- mice. Reduced adipocyte differentiation of SH2-B-deficient mouse embryonic fibroblasts (MEFs) was observed in response to insulin and dexamethasone, whereas retroviral SH2-B overexpression enhanced differentiation of 3T3-L1 preadipocytes to adipocytes. SH2-B overexpression enhanced mRNA level of PPARgamma in 3T3-L1 cells, whereas PPARgamma levels were reduced in SH2-B-deficient MEFs in response to insulin. SH2-B-mediated up-regulation of PPARgamma mRNA was blocked by a phosphatidylinositol 3-kinase inhibitor, but not by a MAPK kinase inhibitor. Insulin-induced Akt activation and the phosphorylation of forkhead transcription factor (FKHR/Foxo1), a negative regulator of PPARgamma transcription, were up-regulated by SH2-B overexpression, but reduced in SH2-B-deficient MEFs. These data indicate that SH2-B is a key regulator of adipogenesis both in vivo and in vitro by regulating the insulin/IGF-I receptor-Akt-Foxo1-PPARgamma pathway.

Neuronal SH2B1 is essential for controlling energy and glucose homeostasis

SH2B1 (previously named SH2-B), a cytoplasmic adaptor protein, binds via its Src homology 2 (SH2) domain to a variety of protein tyrosine kinases, including JAK2 and the insulin receptor. SH2B1-deficient mice are obese and diabetic. Here we demonstrated that multiple isoforms of SH2B1 (alpha, beta, gamma, and/or delta) were expressed in numerous tissues, including the brain, hypothalamus, liver, muscle, adipose tissue, heart, and pancreas. Rat SH2B1beta was specifically expressed in neural tissue in SH2B1-transgenic (SH2B1(Tg)) mice. SH2B1(Tg) mice were crossed with SH2B1-knockout (SH2B1(KO)) mice to generate SH2B1(TgKO) mice expressing SH2B1 only in neural tissue but not in other tissues. Systemic deletion of the SH2B1 gene resulted in metabolic disorders in SH2B1(KO) mice, including hyperlipidemia, leptin resistance, hyperphagia, obesity, hyperglycemia, insulin resistance, and glucose intolerance. Neuron-specific restoration of SH2B1beta not only corrected the metabolic disorders in SH2B1(TgKO) mice, but also improved JAK2-mediated leptin signaling and leptin regulation of orexigenic neuropeptide expression in the hypothalamus. Moreover, neuron-specific overexpression of SH2B1 dose-dependently protected against high-fat diet-induced leptin resistance and obesity. These observations suggest that neuronal SH2B1 regulates energy balance, body weight, peripheral insulin sensitivity, and glucose homeostasis at least in part by enhancing hypothalamic leptin sensitivity.

SH2B1 (SH2-B) and JAK2: a multifunctional adaptor protein and kinase made for each other

Src homology 2 (SH2) B adaptor protein 1 (SH2B1; originally named SH2-B) is a member of a family of adaptor proteins that influences a variety of signaling pathways mediated by Janus kinase (JAK) and receptor tyrosine kinases. Although SH2B1 performs classical adaptor functions, such as recruitment of specific proteins to activated receptors, it also demonstrates a unique ability to enhance the kinase activity of the cytokine receptor-associated tyrosine kinase JAK2, as well as that of several receptor tyrosine kinases. SH2B1 is also among a small number of adaptor proteins shown to undergo nucleocytoplasmic shuttling, although its exact role within the nucleus is not yet clear. Deletion of the SH2B1 gene results in severe obesity and both leptin and insulin resistance, as well as infertility, which might be a consequence of resistance to insulin-like growth factor I. Thus, knockout mice support a role for SH2B1 as a positive regulator of JAK2 signaling pathways initiated by leptin, as well as of pathways initiated by insulin and, potentially, by insulin-like growth factor I.

Binding of SH2-B family members within a potential negative regulatory region maintains JAK2 in an active state

The tyrosine kinase Janus kinase 2 (JAK2) transduces signaling for the majority of known cytokine receptor family members and is constitutively activated in some cancers. Here we examine the mechanisms by which the adapter proteins SH2-Bbeta and APS regulate the activity of JAK2. We show that like SH2-Bbeta, APS binds JAK2 at multiple sites and that binding to phosphotyrosine 813 is essential for APS to increase active JAK2 and to be phosphorylated by JAK2. Binding of APS to a phosphotyrosine 813-independent site inhibits JAK2. Both APS and SH2-Bbeta increase JAK2 activity independent of their N-terminal dimerization domains. SH2-Bbeta-induced increases in JAK2 dimerization require only the SH2 domain and only one SH2-Bbeta to be bound to a JAK2 dimer. JAK2 mutations and truncations revealed that amino acids 809 to 811 in JAK2 are a critical component of a larger regulatory region within JAK2, most likely including amino acids within the JAK homology 1 (JH1) and JH2 domains and possibly the FERM domain. Together, our data suggest that SH2-Bbeta and APS do not activate JAK2 as a consequence of their own dimerization, recruitment of an activator of JAK2, or direct competition with a JAK2 inhibitor for binding to JAK2. Rather, they most likely induce or stabilize an active conformation of JAK2.

The human and mouse complement of SH2 domain proteins-establishing the boundaries of phosphotyrosine signaling

SH2 domains are interaction modules uniquely dedicated to the recognition of phosphotyrosine sites and are embedded in proteins that couple protein-tyrosine kinases to intracellular signaling pathways. Here, we report a comprehensive bioinformatics, structural, and functional view of the human and mouse complement of SH2 domain proteins. This information delimits the set of SH2-containing effectors available for PTK signaling and will facilitate the systems-level analysis of pTyr-dependent protein-protein interactions and PTK-mediated signal transduction. The domain-based architecture of SH2-containing proteins is of more general relevance for understanding the large family of protein interaction domains and the modular organization of the majority of human proteins.

Structural basis for phosphotyrosine recognition by the Src homology-2 domains of the adapter proteins SH2-B and APS

SH2-B, APS, and Lnk constitute a family of adapter proteins that modulate signaling by protein tyrosine kinases. These adapters contain an N-terminal dimerization region, a pleckstrin homology domain, and a C-terminal Src homology-2 (SH2) domain. SH2-B is recruited via its SH2 domain to various protein tyrosine kinases, including Janus kinase-2 (Jak2) and the insulin receptor. Here, we present the crystal structure at 2.35 A resolution of the SH2 domain of SH2-B in complex with a phosphopeptide representing the SH2-B recruitment site in Jak2 (pTyr813). The structure reveals a canonical SH2 domain-phosphopeptide binding mode, but with specific recognition of a glutamate at the +1 position relative to phosphotyrosine, in addition to recognition of a hydrophobic residue at the +3 position. Biochemical studies of SH2-B and APS demonstrate that, although the SH2 domains of these two adapter proteins share 79% sequence identity, the SH2-B SH2 domain binds preferentially to Jak2, whereas the APS SH2 domain has higher affinity for the insulin receptor. This differential specificity is attributable to the difference in the oligomeric states of the two SH2 domains: monomeric for SH2-B and dimeric for APS.

The neuropeptide neuromedin U activates eosinophils and is involved in allergen-induced eosinophilia

Neuromedin U (NMU) is a neuropeptide expressed not only in the central nervous system but also in various organs, including the gastrointestinal tract and lungs. NMU interacts with two G protein-coupled receptors, NMU-R1 and NMU-R2. Although NMU-R2 is expressed in a specific region of the brain, NMU-R1 is expressed in various peripheral tissues, including immune and hematopoietic cells. Our recent study demonstrated an important role of NMU in mast cell-mediated inflammation. In this study, we showed that airway eosinophilia was reduced in NMU-deficient mice in an allergen-induced asthma model. There were no differences in the antigen-induced Th2 responses between wild-type and NMU knockout mice. NMU-R1 was highly expressed in the eosinophil cell line, and NMU directly induced Ca(2+) mobilization and extracellular/signal-regulated kinase phosphorylation. NMU also induced cell adhesion to components of the extracellular matrix (fibronectin and collagen type I), and chemotaxis in vitro. Furthermore, NMU-R1 was also expressed in human peripheral blood eosinophils, and NMU induced cell adhesion in a dose-dependent manner. These data indicate that NMU promotes eosinophil infiltration into inflammatory sites by directly activating eosinophils. Our study suggests that NMU receptor antagonists could be novel targets for pharmacological inhibition of allergic inflammatory diseases, including asthma.

Identification of SH2-B as a key regulator of leptin sensitivity, energy balance, and body weight in mice

Leptin regulates energy balance and body weight by activating its receptor LEPRb and multiple downstream signaling pathways, including the STAT3 and the IRS2/PI 3-kinase pathways, in the hypothalamus. Leptin stimulates activation of LEPRb-associated JAK2, which initiates cell signaling. Here we identified SH2-B, a JAK2-interacting protein, as a key regulator of leptin sensitivity, energy balance, and body weight. SH2-B homozygous null mice were severely hyperphagic and obese and developed a metabolic syndrome characterized by hyperleptinemia, hyperinsulinemia, hyperlipidemia, hepatic steatosis, and hyperglycemia. The expression of hypothalamic orexigenic NPY and AgRP was increased in SH2-B(-/-) mice. Leptin-stimulated activation of hypothalamic JAK2 and phosphorylation of hypothalamic STAT3 and IRS2 were significantly impaired in SH2-B(-/-) mice. Moreover, overexpression of SH2-B counteracted PTP1B-mediated inhibition of leptin signaling in cultured cells. Our data suggest that SH2-B is an endogenous enhancer of leptin sensitivity and required for maintaining normal energy metabolism and body weight in mice.

The neuropeptide neuromedin U promotes inflammation by direct activation of mast cells

Neuromedin U (NMU) is a neuropeptide that is expressed in the gastrointestinal tract and central nervous system. NMU interacts with two G protein–coupled receptors, NMU-R1 and NMU-R2. Whereas NMU-R2 localizes predominantly to nerve cells, NMU-R1 is expressed in peripheral tissues including lymphocytes and monocytes, suggesting a role of NMU in immunoregulation. However, the functions of NMU in peripheral tissues have not been clarified. In this study, using NMU-deficient mice, we first demonstrated that NMU plays an important role in mast cell-mediated inflammation. Complete Freund's adjuvant-induced mast cell degranulation as well as edema and neutrophil infiltration, which occurred weakly in mast cell–deficient WBB6F₁-W/W^v mice, did not occur in NMU-deficient mice. Moreover, intraplantar injection of NMU into paws induced early inflammatory responses such as mast cell degranulation, vasodilation, and plasma extravasation in WT mice but not in WBB6F₁-W/W^v mice. NMU-R1 was highly expressed in primary mast cells, and NMU induced Ca²⁺ mobilization and degranulation in peritoneal mast cells. These data indicate that NMU promotes mast cell–mediated inflammation; therefore, NMU receptor antagonists could be a novel target for pharmacological inhibition of mast cell–mediated inflammatory diseases.

APS, an adaptor molecule containing PH and SH2 domains, has a negative regulatory role in B cell proliferation

Adaptor molecule containing PH and SH2 domains (APS) is an intracellular adaptor protein that forms part of an adaptor family along with Lnk and SH2-B. APS transcripts are expressed in various tissues including brain, kidney, and muscle, as well as in splenic B cells but not in T cells. We investigated the functions of APS in B cell development and activation by generating APS-transgenic (APS-Tg) mice that overexpressed APS in lymphocytes. The number of B-1 cells in the peritoneal cavity was reduced in APS-Tg mice, as were B-2 cells in the spleen. B cell development in the bone marrow was partially impaired at the transition stage from proliferating large pre-B to small pre-B cells. B cell proliferation induced by B cell receptor (BCR) crosslinking but not by other B cell mitogens was also impaired in APS-Tg mice. APS co-localized with BCR complexes and filamentous actin in activated APS-Tg B cells. Thus, APS appears to play novel negative regulatory roles in BCR signaling, actin reorganization pathways, and control of compartment sizes of B-lineage cells.

Scaffold attachment factor B1 functions in development, growth, and reproduction

Scaffold attachment factor B1 (SAFB1) is a multifunctional protein that can bind both DNA and RNA and is involved in RNA processing and stress response. In addition, SAFB1 contains a transcriptional repression domain and can bind certain hormone receptors and repress their activity. To assess the role of SAFB1 in vivo, we generated SAFB1 mutant mice through targeted deletion in embryonic stem cells. While viable homozygous mutant (SAFB1-/-) mice were obtained, genotypic distribution indicated that homozygous deficiency resulted in both prenatal and neonatal lethality. Mice lacking SAFB1 exhibited dwarfism, as a result of in utero growth retardation, and had low serum insulin-like growth factor 1 (IGF1) levels. In agreement with the previous characterization of SAFB1 as a corepressor for hormone receptors, we found that SAFB1-/- mice displayed dramatic defects in the development and function of the reproductive system. Male SAFB1 null mice were infertile, apparently because of low circulating levels of testosterone. SAFB1-/- testes were small and showed progressive degeneration of the germinal epithelium, increased apoptosis of germ cells, and Leydig cell hyperplasia. SAFB-/- female mice were subfertile and showed progressive infertility, in part because of defects in oviductal transport and reduced numbers of follicles. Immortalized SAFB1-/- mouse embryonic fibroblasts showed cell-intrinsic defects including increased transcriptional estrogen receptor alpha activity and enhanced responsiveness to IGF1. Together, these in vivo findings establish a critical role for SAFB1 in development, growth regulation, and reproduction.

Kinase activation through dimerization by human SH2-B

The isoforms of SH2-B, APS, and Lnk form a family of signaling proteins that have been described as activators, mediators, or inhibitors of cytokine and growth factor signaling. We now show that the three alternatively spliced isoforms of human SH2-B readily homodimerize in yeast two-hybrid and cellular transfections assays, and this is mediated specifically by a unique domain in its amino terminus. Consistent with previous reports, we further show that the SH2 domains of SH2-B and APS bind JAK2 at Tyr813. These findings suggested a model in which two molecules of SH2-B or APS homodimerize with their SH2 domains bound to two JAK2 molecules, creating heterotetrameric JAK2-(SH2-B)2-JAK2 or JAK2-(APS)2-JAK2 complexes. We further show that APS and SH2-B isoforms heterodimerize. At lower levels of SH2-B or APS expression, dimerization approximates two JAK2 molecules to induce transactivation. At higher relative concentrations of SH2-B or APS, kinase activation is blocked. SH2-B or APS homodimerization and SH2-B/APS heterodimerization thus provide direct mechanisms for activating and inhibiting JAK2 and other kinases from the inside of the cell and for potentiating or attenuating cytokine and growth factor receptor signaling when ligands are present.

Spred-1 negatively regulates allergen-induced airway eosinophilia and hyperresponsiveness

T helper 2 cytokines, including interleukin (IL)-4, IL-5, and IL-13, play a critical role in allergic asthma. These cytokines transmit signals through the Janus kinase/signal transducer and activator of transcription (STAT) and the Ras–extracellular signal-regulated kinase (ERK) signaling pathways. Although the suppressor of cytokine signaling (SOCS) family proteins have been shown to regulate the STAT pathway, the mechanism regulating the ERK pathway has not been clarified. The Sprouty-related Ena/VASP homology 1–domain-containing protein (Spred)-1 has recently been identified as a negative regulator of growth factor–mediated, Ras-dependent ERK activation. Here, using Spred-1–deficient mice, we demonstrated that Spred-1 negatively regulates allergen-induced airway eosinophilia and hyperresponsiveness, without affecting helper T cell differentiation. Biochemical assays indicate that Spred-1 suppresses IL-5–dependent cell proliferation and ERK activation. These data indicate that Spred-1 negatively controls eosinophil numbers and functions by modulating IL-5 signaling in allergic asthma.

Disruption of the SH2-B gene causes age-dependent insulin resistance and glucose intolerance

Insulin regulates glucose homeostasis by binding and activating the insulin receptor, and defects in insulin responses (insulin resistance) induce type 2 diabetes. SH2-B, an Src homology 2 (SH2) and pleckstrin homology domain-containing adaptor protein, binds via its SH2 domain to insulin receptor in response to insulin; however, its physiological role remains unclear. Here we show that SH2-B was expressed in the liver, skeletal muscle, and fat. Systemic deletion of SH2-B impaired insulin receptor activation and signaling in the liver, skeletal muscle, and fat, including tyrosine phosphorylation of insulin receptor substrate 1 (IRS1) and IRS2 and activation of the phosphatidylinositol 3-kinase/Akt and the Erk1/2 pathways. Consequently, SH2-B-/- knockout mice developed age-dependent hyperinsulinemia, hyperglycemia, and glucose intolerance. Moreover, SH2-B directly enhanced autophosphorylation of insulin receptor and tyrosine phosphorylation of IRS1 and IRS2 in an SH2 domain-dependent manner in cultured cells. Our data suggest that SH2-B is a physiological enhancer of insulin receptor activation and is required for maintaining normal insulin sensitivity and glucose homeostasis during aging.

A phenylalanine zipper mediates APS dimerization

The APS, SH2-B and LNK proteins are adapters that activate and modulate receptor tyrosine kinase and JAK/STAT signaling. We now show that a conserved N-terminal domain mediates APS homodimerization. We determined the crystal structure of the dimerization domain at a resolution of 1.7 A using bromide ion MAD phasing. Each molecule contributes two helices to a compact four-helix bundle having a bisecting-U topology. Its most conspicuous feature is a stack of interdigitated phenylalanine side chains at the domain core. These residues create a new motif we refer to as a 'phenylalanine zipper,' which is critical to dimerization. A newly developed bridging yeast tri-hybrid assay showed that APS dimerizes JAK2, insulin receptor and IGF1 receptor kinases using its SH2 and dimerization domains. Dimerization via the phenylalanine zipper domain provides a mechanism for activating and modulating tyrosine kinase activity even in the absence of extracellular ligands.

Roles of a conserved family of adaptor proteins, Lnk, SH2-B, and APS, for mast cell development, growth, and functions: APS-deficiency causes augmented degranulation and reduced actin assembly

Lnk, SH2-B, and APS form a conserved adaptor protein family. All of those proteins are expressed in mast cells and their possible functions in signaling through c-Kit or FcRI have been speculated. To investigate roles of Lnk, SH2-B or APS in mast cells, we established IL-3-dependent mast cells from Ink-/-, SH2-B-/-, and APS -/- mice. IL-3-dependent growth of those cells was comparable. Proliferation or adhesion mediated by c-Kit as well as degranulation induced by cross-linking FcRI were normal in the absence of Lnk or SH2-B. In contrast, APS-deficient mast cells showed augmented degranulation after cross-linking FcRI compared to wild-type cells, while c-Kit-mediated proliferation and adhesion were kept unaffected. APS-deficient mast cells showed reduced actin assembly at steady state, although their various intracellular responses induced by cross-linking FcRI were indistinguishable compared to wild-type cells. Our results suggest potential roles of APS in controlling actin cytoskeleton and magnitude of degranulation in mast cells.

Increased numbers of B-1 cells and enhanced responses against TI-2 antigen in mice lacking APS, an adaptor molecule containing PH and SH2 domains

APS (adaptor molecule containing PH and SH2 domains) is an intracellular adaptor protein that forms an adaptor family along with Lnk and SH2-B. While experiments using cultured cell lines have demonstrated that APS is phosphorylated in response to various stimuli, its in vivo functions remain unclear. We attempted to determine the physiological roles of APS by generating APS-deficient (APS(-/-)) mice. APS(-/-) mice were viable and fertile and showed no abnormalities or growth retardation. Immunologically, APS(-/-) mice showed normal development and distribution of lymphocytes and myeloid cells, except for increased numbers of B-1 cells in the peritoneal cavity. APS(-/-) mice exhibited an enhanced humoral immune response against trinitrophenol-Ficoll, a thymus-independent type 2 antigen, while APS(-/-) B-2 cells exhibited normal proliferative responses and tyrosine phosphorylation of intracellular proteins upon B-cell receptor (BCR) cross-linking. APS colocalized with filamentous actin (F-actin) accumulated during the capping of BCRs in APS-transgenic B cells. After BCR stimulation, F-actin contents were lower in APS(-/-) B-1 cells than in wild-type B-1 cells. Our results indicate that APS might have a novel regulatory role in actin reorganization and control of B-1 cell compartment size.

Spred-2 suppresses aorta-gonad-mesonephros hematopoiesis by inhibiting MAP kinase activation

In midgestation mouse embryos, the aorta-gonad-mesonephros (AGM) region generates hematopoietic stem cells and definitive hematopoiesis is regulated by cell–cell interaction and signaling molecules. We showed that a Ras/mitogen-activated protein (MAP) kinase signaling-specific inhibitor and a dominant negative mutant Ras blocked the production of CD45⁺ hematopoietic cells in embryonic day 11.5 AGM culture, indicating an essential role for the MAP kinase pathway in AGM hematopoiesis. Overexpression of the Ras/MAP kinase pathway regulator, Spred-2, in the AGM culture significantly reduced the number of CD45⁺ cells. In contrast, production of CD45⁺ cells from the AGM region of Spred-2–null mice was up-regulated as compared with wild-type littermates. Furthermore, Spred-2–deficient mice exhibited elevated hematopoietic colony formation from vascular endothelial-cadherin⁺ cells. These data indicate that Spred-2 functions as a negative regulator of AGM hematopoiesis by inhibiting hematopoietic cytokine signaling.

Regulation of hematopoietic development in the aorta-gonad-mesonephros region mediated by Lnk adaptor protein

Development of hematopoietic cells in the aorta-gonad-mesonephros (AGM) region in the midgestation mouse embryo involves a multistep process, sequentially changing from endothelial cell-like cells, including hemangioblasts, into hematopoietic stem cells, progenitors, and/or lineage-committed cells. An adaptor molecule, Lnk, is known to negatively control the production of pro- and pre-B cells and hematopoietic progenitor cells in adult bone marrow. Here we show a role of Lnk in hematopoietic development in the AGM region. Lnk was predominantly expressed in the endothelial cells lining the dorsal aorta at embryonic day 11.5 (E11.5). Overexpression of Lnk in the primary culture of the AGM region at E11.5 suppressed the emergence of CD45+ hematopoietic cells. Point mutation in the SH2 domain of Lnk, which abolishes the binding capability of Lnk to c-Kit upon stimulation with stem cell factor (SCF), led to loss of Lnk-dependent inhibition of hematopoietic cell development in AGM cultures, suggesting Lnk-mediated inhibition of the SCF/c-Kit signaling pathway. In cultured AGM cells from Lnk homozygous mutant mouse embryos, the number of emerged CD45+ cells was 2.5-fold larger than that from heterozygous littermates. Furthermore, aorta cells of E11.5 Lnk homozygous mutant mice also showed enhanced hematopoietic colony-forming activity. Thus, Lnk is a negative regulator of hematopoiesis in the AGM region.

Increased insulin sensitivity and hypoinsulinemia in APS knockout mice

A tyrosine kinase adaptor protein containing pleckstrin homology and SH2 domains (APS) is rapidly and strongly tyrosine phosphorylated by insulin receptor kinase upon insulin stimulation. The function of APS in insulin signaling has heretofore remained unknown. APS-deficient (APS(-/-)) mice were used to investigate its function in vivo. The blood glucose-lowering effect of insulin, as assessed by the intraperitoneal insulin tolerance test, was increased in APS(-/-) mice. Plasma insulin levels during fasting and in the intraperitoneal glucose tolerance test were lower in APS(-/-) mice. APS(-/-) mice showed an increase in the whole-body glucose infusion rate as assessed by the hyperinsulinemic-euglycemic clamp test. These findings indicated that APS(-/-) mice exhibited increased sensitivity to insulin. However, overexpression of wild-type or dominant-negative APS in 3T3L1 adipocytes did not affect insulin receptor numbers, phosphorylations of insulin receptor, insulin receptor substrate-1, or Akt and mitogen-activated protein kinase. The glucose uptake and GLUT4 translocation were not affected by insulin stimulation in these cells. Nevertheless, the insulin-stimulated glucose transport in isolated adipocytes of APS(-/-) mice was increased over that of APS(+/+) mice. APS(-/-) mice also showed increased serum levels of leptin and adiponectin, which might explain the increased insulin sensitivity of adipocytes.