Molecular mechanism of size control in development and human diseases

How multicellular organisms control their size is a fundamental question that fascinated generations of biologists. In the past 10 years, tremendous progress has been made toward our understanding of the molecular mechanism underlying size control. Original studies from Drosophila showed that in addition to extrinsic nutritional and hormonal cues, intrinsic mechanisms also play important roles in the control of organ size during development. Several novel signaling pathways such as insulin and Hippo-LATS signaling pathways have been identified that control organ size by regulating cell size and/or cell number through modulation of cell growth, cell division, and cell death. Later studies using mammalian cell and mouse models also demonstrated that the signaling pathways identified in flies are also conserved in mammals. Significantly, recent studies showed that dysregulation of size control plays important roles in the development of many human diseases such as cancer, diabetes, and hypertrophy.     

Wnt4 action in gonadal development and sex determination

Wnt4 is a growth factor involved in multiple developmental processes such as the formation of the kidney, adrenal, mammary gland, pituitary and the female reproductive system. During mammalian embryogenesis, Wnt4 is expressed in the gonads of both sexes before sex determination events take place and is subsequently down-regulated in the male gonad. Inactivation of the Wnt4 gene in mice has revealed that it is involved at several steps of female reproductive development. Wnt4 is implicated in Müllerian duct regression, the formation of sex-specific vasculature, the inhibition of steroidogenesis and in sex-specific cell migration events. A mouse model of sex-reversal has partially unravelled the molecular pathways in which Wnt4 operates during the development of the female reproductive system. However, the specific molecular mechanism of action of Wnt4 during gonadal development remains unknown. This and downstream signaling pathways involved in Wnt4 action during female gonad development are reviewed and models of Wnt4 action are proposed for Müllerian duct formation, sex-specific vasculature development, and sex determination events. Further identification of critical downstream effectors of the Wnt4 signaling pathway in mouse models and in patients with sex-reversal conditions could help in understanding sex-reversal pathologies in humans.     

Signaling pathways and preimplantation development of mammalian embryos

The mammalian preimplantation embryo is a critical and unique stage in embryonic development. This stage includes a series of crucial events: the transition from oocyte to embryo, the first cell divisions, and the establishment of cellular contacts. These events are regulated by multiple signal-transduction pathways. In this article we describe patterns of stage-specific expression in several signal-transduction pathways and try to give a profile of the signaling transduction network in preimplantation development of mammalian embryo.     

Quantitative phosphoproteomics of CXCL12 (SDF-1) signaling

CXCL12 (SDF-1) is a chemokine that binds to and signals through the seven transmembrane receptor CXCR4. The CXCL12/CXCR4 signaling axis has been implicated in both cancer metastases and human immunodeficiency virus type 1 (HIV-1) infection and a more complete understanding of CXCL12/CXCR4 signaling pathways may support efforts to develop therapeutics for these diseases. Mass spectrometry-based phosphoproteomics has emerged as an important tool in studying signaling networks in an unbiased fashion. We employed stable isotope labeling with amino acids in cell culture (SILAC) quantitative phosphoproteomics to examine the CXCL12/CXCR4 signaling axis in the human lymphoblastic CEM cell line. We quantified 4,074 unique SILAC pairs from 1,673 proteins and 89 phosphopeptides were deemed CXCL12-responsive in biological replicates. Several well established CXCL12-responsive phosphosites such as AKT (pS473) and ERK2 (pY204) were confirmed in our study. We also validated two novel CXCL12-responsive phosphosites, stathmin (pS16) and AKT1S1 (pT246) by Western blot. Pathway analysis and comparisons with other phosphoproteomic datasets revealed that genes from CXCL12-responsive phosphosites are enriched for cellular pathways such as T cell activation, epidermal growth factor and mammalian target of rapamycin (mTOR) signaling, pathways which have previously been linked to CXCL12/CXCR4 signaling. Several of the novel CXCL12-responsive phosphoproteins from our study have also been implicated with cellular migration and HIV-1 infection, thus providing an attractive list of potential targets for the development of cancer metastasis and HIV-1 therapeutics and for furthering our understanding of chemokine signaling regulation by reversible phosphorylation.     

Calcium-mediated mechanisms of cystic expansion

In this review, we will discuss several well-accepted signaling pathways toward calcium-mediated mechanisms of cystic expansion. The second messenger calcium ion has contributed to a vast diversity of signal transduction pathways. We will dissect calcium signaling as a possible mechanism that contributes to renal cyst formation. Because cytosolic calcium also regulates an array of signaling pathways, we will first discuss cilia-induced calcium fluxes, followed by Wnt signaling that has attributed to much-discussed planar cell polarity. We will then look at the relationship between cytosolic calcium and cAMP as one of the most important aspects of cyst progression. The signaling of cAMP on MAPK and mTOR will also be discussed. We infer that while cilia-induced calcium fluxes may be the initial signaling messenger for various cellular pathways, no single signaling mediator or pathway is implicated exclusively in the progression of the cystic expansion. This article is part of a Special Issue entitled: Polycystic Kidney Disease.     

Wnt signaling during cochlear development

Wnt signaling is a hallmark of all embryonic development with multiple roles at multiple developmental time points. Wnt signaling is also important in the development of several organs, one of which is the inner ear, where it participates in otic specification, the formation of vestibular structures, and the development of the cochlea. In particular, we focus on Wnt signaling in the auditory organ, the cochlea. Attempting to dissect the multiple Wnt signaling pathways in the mammalian cochlea is a challenging task due to limited expression data, particularly at proliferating stages. To offer predictions about Wnt activity, we compare cochlear development with that of other biological systems such as Xenopus retina, brain, cancer cells and osteoblasts. Wnts are likely to regulate development through crosstalk with other signaling pathways, particularly Notch and FGF, leading to changes in the expression of Sox2 and proneural (pro-hair cell) genes. In this review we have consolidated the known signaling pathways in the cochlea with known developmental roles of Wnts from other systems to generate a potential timeline of cochlear development.     

Wg signaling in Drosophila heart development as a pioneering model

The heart is one of the first functional embryonic organs occurring during development. The fundamental developmental processes and genes involved in cardiogenesis are conserved between the invertebrates and vertebrates. In the past fifteen years, one of signaling pathways that has been best characterized in heart development in both invertebrates and vertebrates is the Wg/Wnt signaling pathways. Since our discovery of the Wg signaling required for the early heart development in Drosophila, the past fifteen years have witnessed tremendous progress in the understanding of specific Wnt signaling pathways in vertebrate cardiogenesis. This review will summarize the current state of knowledge of Wg signaling transduction in Drosophila heart development, which will benefit our understanding of vertebrate cardiogenesis and human congenital malformations.     

Morphological and molecular changes associated with Pitchfork during mouse palate development

Calcium signaling is involved in a multitude of physiological and pathophysiological mechanisms. Over the last decade, it has been increasingly recognized as an important factor in epileptogenesis, and it is becoming obvious that the excess synchronization of neurons that is characteristic for seizures can be linked to various calcium signaling pathways. These include immediate effects on membrane excitability by calcium influx through ion channels as well as delayed mechanisms that act through G-protein coupled pathways. Calcium signaling is able to cause hyperexcitability either by direct modulation of neuronal activity or indirectly through calcium-dependent gliotransmission. Furthermore, feedback mechanisms between mitochondrial calcium signaling and reactive oxygen species are able to cause neuronal cell death and seizures. Unravelling the complexity of calcium signaling in epileptogenesis is a daunting task, but it includes the promise to uncover formerly unknown targets for the development of new antiepileptic drugs.     

The mammalian target of rapamycin-p70 ribosomal S6 kinase but not phosphatidylinositol 3-kinase-Akt signaling is responsible for fibroblast growth factor-9-induced cell proliferation

Fibroblast growth factor-9 (FGF9) is a potent mitogen that stimulates normal and cancer cell proliferation though the signaling mechanism is not fully understood. In this study, we aimed to unravel the signaling cascades mediate FGF9 actions in human uterine endometrial stromal cell. Our results demonstrate that the mitogenic effect of FGF9 is transduced via two parallel but additive signaling pathways involving mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase. Activation of mTOR by FGF9 induces p70 ribosomal S6 kinase (S6K1) phosphorylation, cyclin expression, and cell proliferation, which are independent of phosphatidylinositol 3-kinase and Akt. Coimmunoprecipitation analysis demonstrates that mTOR physically associates with S6K1 upon FGF9 treatment, whereas ablation of mTOR activity using RNA interference or pharmacological inhibitor blocks S6K1 phosphorylation and cell proliferation induced by FGF9. Further study demonstrates that activation of mTOR is regulated by a phospholipase Cgamma-controlled calcium signaling pathway. These studies provide evidence to demonstrate, for the first time, that a novel signaling cascade involving phospholipase Cgamma, calcium, mTOR, and S6K1 is activated by FGF9 in a receptor-specific manner.     

Crosstalks of GSK3 signaling with the mTOR network and effects on targeted therapy of cancer

The introduction of therapeutics targeting specific tumor-promoting oncogenic or non-oncogenic signaling pathways has revolutionized cancer treatment. Mechanistic (previously mammalian) target of rapamycin (mTOR), a highly conserved Ser/Thr kinase, is a central hub of the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR network, one of the most frequently deregulated signaling pathways in cancer, that makes it an attractive target for therapy. Numerous mTOR inhibitors have progressed to clinical trials and two of them have been officially approved as anticancer therapeutics. However, mTOR-targeting drugs have met with a very limited success in cancer patients. Frequently, the primary impediment to a successful targeted therapy in cancer is drug-resistance, either from the very beginning of the therapy (innate resistance) or after an initial response and upon repeated drug treatment (evasive or acquired resistance). Drug-resistance leads to treatment failure and relapse/progression of the disease. Resistance to mTOR inhibitors depends, among other reasons, on activation/deactivation of several signaling pathways, included those regulated by glycogen synthase kinase-3 (GSK3), a protein that targets a vast number of substrates in its repertoire, thereby orchestrating many processes that include cell proliferation and survival, metabolism, differentiation, and stemness. A detailed knowledge of the rewiring of signaling pathways triggered by exposure to mTOR inhibitors is critical to our understanding of the consequences such perturbations cause in tumors, including the emergence of drug-resistant cells.Here, we provide the reader with an updated overview of intricate circuitries that connect mTOR and GSK3 and we relate them to the efficacy (or lack of efficacy) of mTOR inhibitors in cancer cells.     

Lysophospholipids in development: Miles apart and edging in

Sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) are endogenous bioactive lipids that participate in the regulation of mammalian cell proliferation, apoptosis, migration, and angiogenesis. These processes are each critical for successful embryogenesis, raising the possibility that lysophospholipid signaling may contribute to normal animal development. In fact, recent studies in developmental model systems have established that S1P and LPA are necessary for diverse developmental programs including those required for morphogenesis of vertebrate reproductive, cardiovascular and central and peripheral nervous systems (PNS), as well as the establishment of maternal-fetal circulation and the immune system. Genetic, morphological, and biochemical characterization of developmental model systems offer powerful approaches to elucidating the molecular mechanisms of lysophospholipid signaling and its contributions to animal development and postnatal physiology. In this review, the routes of S1P and LPA metabolism and our current understanding of lysophospholipid-mediated signal transduction in mammalian cells will be summarized. The evidence implicating lysophospholipid signaling in the development of specific vertebrate systems will then be reviewed, with an emphasis on signals mediated through G protein-coupled receptors of the Edg family. Lastly, recent insights derived from the study of simple metazoan models and implications regarding lysophospholipid signaling in organisms in which Edg receptors are not conserved will be explored.     

Targeting the mTOR signaling network in cancer

The mammalian target of rapamycin (mTOR) is an unconventional protein kinase that is centrally involved in the control of cancer cell metabolism, growth and proliferation. The mTOR pathway has attracted broad scientific and clinical interest, particularly in light of the ongoing clinical cancer trials with mTOR inhibitors. The mixed clinical results to date reflect the complexity of both cancer as a disease target, and the mTOR signaling network, which contains two functionally distinct mTOR complexes, parallel regulatory pathways, and feedback loops that contribute to the variable cellular responses to the current inhibitors. In this review, we discuss the regulatory pathways that govern mTOR activity, and highlight clinical results obtained with the first generation of mTOR inhibitors to reach the oncology clinics.     

经典WNT信号通路与哺乳动物肺器官发育

肺器官发育是上皮和问充质相互作用的过程,由多条信号通路共同调控。已知经典WNT信号通路对细胞的增殖、凋亡和分化起着重要的调控作用,在小鼠等模式生物上研究发现,它也参与了哺乳动物肺器官发育的调控过程。综述近年来经典WNT信号通路成员在哺乳动物肺器官发育过程中的表达变化、作用功能及表达异常可能诱发的肺部疾病,以期为研究经典WNT信号通路调控人类肺器官发育的分子机制及相关肺部疾病的诊治奠定基础。     

Calcium influx and signaling in yeast stimulated by intracellular sphingosine 1-phosphate accumulation

In mammalian cells, intracellular sphingosine 1-phosphate (S1P) can stimulate calcium release from intracellular organelles, resulting in the activation of downstream signaling pathways. The budding yeast Saccharomyces cerevisiae expresses enzymes that can synthesize and degrade S1P and related molecules, but their possible role in calcium signaling has not yet been tested. Here we examine the effects of S1P accumulation on calcium signaling using a variety of yeast mutants. Treatment of yeast cells with exogenous sphingosine stimulated Ca(2+) accumulation through two distinct pathways. The first pathway required the Cch1p and Mid1p subunits of a Ca(2+) influx channel, depended upon the function of sphingosine kinases (Lcb4p and Lcb5p), and was inhibited by the functions of S1P lyase (Dpl1p) and the S1P phosphatase (Lcb3p). The biologically inactive stereoisomer of sphingosine did not activate this Ca(2+) influx pathway, suggesting that the active S1P isomer specifically stimulates a calcium-signaling mechanism in yeast. The second Ca(2+) influx pathway stimulated by the addition of sphingosine was not stereospecific, was not dependent on the sphingosine kinases, occurred only at higher doses of added sphingosine, and therefore was likely to be nonspecific. Mutants lacking both S1P lyase and phosphatase (dpl1 lcb3 double mutants) exhibited constitutively high Ca(2+) accumulation and signaling in the absence of added sphingosine, and these effects were dependent on the sphingosine kinases. These results show that endogenous S1P-related molecules can also trigger Ca(2+) accumulation and signaling. Several stimuli previously shown to evoke calcium signaling in wild-type cells were examined in lcb4 lcb5 double mutants. All of the stimuli produced calcium signals independent of sphingosine kinase activity, suggesting that phosphorylated sphingoid bases might serve as messengers of calcium signaling in yeast during an unknown cellular response.