Extracellular matrix and integrin signaling in lens development and cataract

During development of the vertebrate lens there are dynamic interactions between the extracellular matrix (ECM) of the lens capsule and lens cells. Disruption of the ECM causes perturbation of lens development and cataract. Similarly, changes in cell signaling can result in abnormal ECM and cataract. Integrins are key mediators of ECM signals and recent studies have documented distinct repertoires of integrin expression during lens development, and in anterior subcapsular cataract (ASC) and posterior caspsule opacification (PCO). Increasingly, studies are being directed to investigating the signaling pathways that integrins modulate and have identified Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK) as downstream kinases that mediate proliferation, differentiation and morphological changes in the lens during development and cataract formation.     

Ras signaling is essential for lens cell proliferation and lens growth during development

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.     

Cross-talk between signaling pathways in murine embryonic palate cells: Effect of TGFβ and cAMP on EGF-induced DNA synthesis

Summary Signaling pathways utilized by EGF, cAMP, and TGFβ have been demonstrated to play critical roles in normal palate development. Stimulation of these pathways has been shown in palate cells and numerous other systems to affect cell growth. Because proper regulation of cell growth is critical to palate development, we speculate that fine regulation of palatal cell growth may be accomplished through crosstalk between these signaling pathways. We therefore set out to determine the effects of cAMP and TGFβ on EGF-induced cell proliferation in murine embryonic palate cells. We found that both TGFβ and cAMP inhibited the proliferative response of cells to treatment with EGF, whereas H89, a serine/threonine protein kinase inhibitor with selectivity towards cAMP-dependent protein kinase, increased the cells’ proliferative response to EGF. Genestein, a selective inhibitor of tyrosine kinases, at high doses abrogated the cells’ proliferative response to EGF, confirming that EGF’s ability to induce cell proliferation is critically dependent upon tyrosine kinase activity. Lower doses of genestein, however, actually enhanced cellular response to EGF. The data suggest that both the TGFβ- and cAMP-mediated signaling pathways may be involved in modulation of the effects of EGF on palate cell growth in vivo.     

Sprouty is a negative regulator of transforming growth factor β-induced epithelial-to-mesenchymal transition and cataract

Fibrosis affects an extensive range of organs and is increasingly acknowledged as a major component of many chronic disorders. It is now well accepted that the elevated expression of certain inflammatory cell-derived cytokines, especially transforming growth factor β (TGFβ), is involved in the epithelial-to-mesenchymal transition (EMT) leading to the pathogenesis of a diverse range of fibrotic diseases. In lens, aberrant TGFβ signaling has been shown to induce EMT leading to cataract formation. Sproutys (Sprys) are negative feedback regulators of receptor tyrosine kinase (RTK)-signaling pathways in many vertebrate systems, and in this study we showed that they are important in the murine lens for promoting the lens epithelial cell phenotype. Conditional deletion of Spry1 and Spry2 specifically from the lens leads to an aberrant increase in RTK-mediated extracellular signal-regulated kinase 1/2 phosphorylation and, surprisingly, elevated TGFβ-related signaling in lens epithelial cells, leading to an EMT and subsequent cataract formation. Conversely, increased Spry overexpression in lens cells can suppress not only TGFβ-induced signaling, but also the accompanying EMT and cataract formation. On the basis of these findings, we propose that a better understanding of the relationship between Spry and TGFβ signaling will not only elucidate the etiology of lens pathology, but will also lead to the development of treatments for other fibrotic-related diseases associated with TGFβ-induced EMT.     

Regulation of PCTAIRE1 protein stability by AKT1, LKB1 and BRCA1

PCTAIRE1, also known as CDK16, is a cyclin-dependent kinase that is regulated by cyclin Y. It is a member of the serine-threonine family of kinases and its functions have primarily been implicated in cellular processes like vesicular transport, neuronal growth and development, myogenesis, spermatogenesis and cell proliferation. However, as extensive studies on PCTAIRE1 have not yet been conducted, the signaling pathways for this kinase involved in governing many cellular processes are yet to be elucidated in detail. Here, we report the association of PCTAIRE1 with important cellular proteins involved in major cell signaling pathways, especially cell proliferation. In particular, here we show that PCTAIRE1 interacts with AKT1, a key player of the PI3K signaling pathway that is responsible for promoting cell survival and proliferation. Our studies show that PCTAIRE1 is a substrate of AKT1 that gets stabilized by it. Further, we show that PCTAIRE1 also interacts with and is degraded by LKB1, a kinase that is known to suppress cellular proliferation and also regulate cellular energy metabolism. Moreover, our results show that PCTAIRE1 is also degraded by BRCA1, a well-known tumor suppressor. Together, our studies highlight the regulation of PCTAIRE1 by key players of the major cell signaling pathways involved in regulating cell proliferation, and therefore, provide crucial links that could be explored further to elucidate the mechanistic role of PCTAIRE1 in cell proliferation and tumorigenesis.     

Hedgehogs as Negative Regulators of the Cell Cycle

During the development of multicellular animals, cell proliferation must be precisely controlled, as deregulated proliferation can lead to overgrowth and cancer. In addition, proliferation must be tightly integrated with pattern formation and differentiation to generate the required number of cells in the right organs, and at the right time. All major signaling pathways employed during embryogenesis have been implicated in cell cycle regulation, indicating that no single pathway has been dedicated to this task. Also, the precise role of a particular signaling pathway in regulating proliferation is highly dependent on the cellular context, and may have opposite effects on cell-cycle progression in different cells and tissues. The Hedgehog (Hh) family of signaling proteins is known to control both differentiation and proliferation during development. So far, studies addressing the effect of Hh signaling on proliferation have shown it to have a stimulatory effect on cell-cycle progression. Here we review several recent studies indicating that Hh signaling can also have the opposite effect, directing cell-cycle exit in a number of cell types in vertebrate and in invertebrate embryos.     

肾脏发育中信号通路的调控作用

肾脏发育是一个复杂的过程,需要在输尿管芽细胞和基质细胞相互诱导下,引起细胞生长、增殖、分化,从而产生肾单位及各种管状结构,最终发育为成熟的肾脏。在肾脏发育过程中,GDNF/Ret、Wnt、BMP等一系列信号通路参与了发育的调控过程。这些信号通路在肾脏发育的不同阶段或不同位置发挥着重要的调控作用,并且通路之间存在相互的调控,从而形成了一个复杂而精细的调控网络,保证了肾脏的正常发育。文章概括了肾脏发育的过程,总结了肾脏发育过程中相关信号通路对肾脏发育的调控作用以及信号通路之间的相互调控。     

A role for Wnt/planar cell polarity signaling during lens fiber cell differentiation?

Wnt signaling through frizzled (Fz) receptors plays key roles in just about every developmental system that has been studied. Several Wnt-Fz signaling pathways have been identified including the Wnt/planar cell polarity (PCP) pathway. PCP signaling is crucial for many developmental processes that require major cytoskeletal rearrangements. Downstream of Fz, PCP signaling is thought to involve the GTPases, Rho, Rac and Cdc42 and regulation of the JNK cascade. Here we report on the localization of these GTPases and JNK in the lens and assess their involvement in the cytoskeletal reorganisation that is a key element of FGF-induced lens fiber cell differentiation.     

FGFs/FGFRs系统对骨调节的研究进展

成纤维细胞生长因子家族(fibroblast growth factors,FGFs)及其受体FGFRs系统影响骨骼发育和形成过程,FGF与细胞表面FGFR结合,激活信号通路调控多种细胞生长、分化和凋亡。骨是FGF的重要靶器官,研究表明FGFs/FGFRs系统对骨组织成骨细胞、破骨细胞、软骨细胞的增殖和分化起重要调控作用,本文就FGFs/FGFRs系统对骨组织调节研究进展进行综述。     

Wnt与MAPK信号通路在肿瘤发生中的串话

Wnt和MAPK信号通路在生物进化过程中高度保守,参与调控胚胎发育和细胞增殖、分化及凋亡等。Wnt和MAPK信号通路调控失常可导致胚胎发育异常和肿瘤形成。近年来发现这两条信号通路在肿瘤发生发展中存在着大量串话（crosstalk）,彼此之间相互调节,共同发挥促癌或抑癌作用,因此,更好地了解两条通路是如何在肿瘤形成中发生交叉对话对于将来肿瘤治疗非常有价值。     

Genistein: A Potent Anti-Breast Cancer Agent

Genistein is an isoflavonoid present in high quantities in soybeans. Possessing a wide range of bioactives, it is being studied extensively for its tumoricidal effects. Investigations into mechanisms of the anti-cancer activity have revealed many pathways including induction of cell proliferation, suppression of tyrosine kinases, regulation of Hedgehog-Gli1 signaling, modulation of epigenetic activities, seizing of cell cycle and Akt and MEK signaling pathways, among others via which the cancer cell proliferation can be controlled. Notwithstanding, the observed activities have been time- and dose-dependent. In addition, genistein has also shown varying results in women depending on the physiological parameters, such as the early or post-menopausal states.     

Twists and turns in the development and maintenance of the mammalian small intestine epithelium

Experimental studies during the last decade have revealed a number of signaling pathways that are critical for the development and maintenance of the intestinal epithelium and that demonstrate the molecular basis for a variety of diseases. The Notch-Delta, Wnt, Hedge Hog, TGF-beta, and other signaling pathways have been shown to form and steadily maintain the crypt-villus system, generating the proper quantities of highly-specialized cells, and ultimately defining the architectural shape of the system. Based on the characterized phenotypes and functional defects of mice resulting from various targeted knockouts, and overexpression and misexpressions of genes, a picture is emerging of the sequence of gene expression events from within the epithelium, and in the underlying mesenchyme that contribute to the regulation of cell differentiation and proliferation. This review focuses on the contributions of multiple signaling pathways to intestinal epithelial proliferation, differentiation, and structural organization, as well as the possible opportunities for cross-talk between pathways. The Notch pathway's potential ability to maintain and regulate the intestinal epithelial stem cell is discussed, in addition to its role as the primary mediator of lineage specification. Recent research that has shed light on the function of Wnt signaling and epithelial-mesenchymal cross-talk during embryonic and postnatal development is examined, along with data on the interplay of heparan sulfate proteoglycans in the signaling process.     

Sef is a negative regulator of fiber cell differentiation in the ocular lens

Growth factor signaling, mediated via receptor tyrosine kinases (RTKs), needs to be tightly regulated in many developmental systems to ensure a physiologically appropriate biological outcome. At one level this regulation may involve spatially and temporally ordered patterns of expression of specific RTK signaling antagonists, such as Sef (similar expression to fgfs). Growth factors, notably FGFs, play important roles in development of the vertebrate ocular lens. FGF induces lens cell proliferation and differentiation at progressively higher concentrations and there is compelling evidence that a gradient of FGF signaling in the eye determines lens polarity and growth patterns. We have recently identified the presence of Sef in the lens, with strongest expression in the epithelial cells. Given the important role for FGFs in lens developmental biology, we employed transgenic mouse strategies to determine if Sef could be involved in regulating lens cell behaviour. Over-expressing Sef specifically in the lens of transgenic mice led to impaired lens and eye development that resulted in microphthalmia. Sef inhibited primary lens fiber cell elongation and differentiation, as well as increased apoptosis, consistent with a block in FGFR-mediated signaling during lens morphogenesis. These results are consistent with growth factor antagonists, such as Sef, being important negative regulators of growth factor signaling. Moreover, the lens provides a useful paradigm as to how opposing gradients of a growth factor and its antagonist could work together to determine and stabilise tissue patterning during development and growth.     

哺乳动物Hippo信号通路:肿瘤治疗的新标靶

Hippo信号通路是首次在果蝇中发现具有调节细胞增殖与凋亡作用的信号通路。最近发现果蝇Hippo信号通路的组成、分子作用机制和生物学功能在进化过程中高度保守。Hippo信号通路在胚胎发育中对细胞的生长分化、组织器官形成以及成体干细胞的维持和自稳态的保持等方面具有重要作用。同时,Hippo信号通路与Wnt信号通路、Notch信号通路等相互作用、密切联系,在肿瘤的发生、发展过程中也起到关键作用。文章综述了哺乳动物Hippo信号通路的作用机理、与其他信号通路和蛋白质因子的相互联系及与肿瘤的关系,对于肿瘤的诊断、预防和治疗具有一定的参考价值。     

Apoptosis in lens development and pathology

The ocular lens is a distinct system to study cell death for the following reasons. First, during animal development, the ocular lens is crafted into its unique shape. The crafting processes include cell proliferation, cell migration, and apoptosis. Moreover, the lens epithelial cells differentiate into lens fiber cells through a process, which utilizes the same regulators as those in apoptosis at multiple signaling steps. In addition, introduction of exogenous wild-type or mutant genes or knock-out of the endogenous genes leads to apoptosis of the lens epithelial cells followed by absence of the ocular lens or formation of abnormal lens. Finally, both in vitro and in vivo studies have shown that treatment of adult lens with stress factors induces apoptosis of lens epithelial cells, which is followed by cataractogenesis. The present review summarizes the current knowledge on apoptosis in the ocular lens with emphasis on its role in lens development and pathology.     

Activated Ras induces lens epithelial cell hyperplasia but not premature differentiation

Growth factor signaling is implicated in the regulation of lens cell proliferation and differentiation during development. Activation of growth factor receptor tyrosine kinases is known to activate Ras proteins, small GTP-binding proteins that function as part of the signal transduction machinery. In the present study, we examined which classical Ras genes are expressed in lens cells during normal development and whether expression of an activated version of Ras is sufficient to induce either lens cell proliferation or fiber cell differentiation in transgenic mice. In situ hybridization showed H-Ras, K-Ras and N-Ras are ubiquitously expressed in all cells of the embryonic (E13.5) eye, with N-Ras showing the highest level of expression. The expression level of N-Ras decreases during later stages of embryonic development, and is nearly undetected in postnatal day 21 lenses. To generate transgenic mice, a constitutively active H-Ras mutant was linked to a chimeric regulatory element containing the mouse alphaA-crystallin promoter fused to the chick delta1-crystallin lens enhancer element. In the lenses of the transgenic mice, the transgene was expressed in both lens epithelial and fiber cells. Expression of activated Ras was sufficient to stimulate lens cell proliferation but not differentiation, implying that alternative or additional signal transduction pathways are required to induce fiber cell differentiation.     

Interaction between Connexin50 and Mitogen-activated Protein Kinase Signaling in Lens Homeostasis

Both connexins and signal transduction pathways have been independently shown to play critical roles in lens homeostasis, but little is known about potential cooperation between these two intercellular communication systems. To investigate whether growth factor signaling and gap junctional communication interact during the development of lens homeostasis, we examined the effect of mitogen-activated protein kinase (MAPK) signaling on coupling mediated by specific lens connexins by using a combination of in vitro and in vivo assays. Activation of MAPK signaling pathways significantly increased coupling provided by Cx50, but not Cx46, in paired Xenopus laevis oocytes in vitro, as well as between freshly isolated lens cells in vivo. Constitutively active MAPK signaling caused macrophthalmia, cataract, glucose accumulation, vacuole formation in differentiating fibers, and lens rupture in vivo. The specific removal or replacement of Cx50, but not Cx46, ameliorated all five pathological conditions in transgenic mice. These results indicate that MAPK signaling specifically modulates coupling mediated by Cx50 and that gap junctional communication and signal transduction pathways may interact in osmotic regulation during postnatal fiber development.     

An essential role for FGF receptor signaling in lens development

Since the days of Hans Spemann, the ocular lens has served as one of the most important developmental systems for elucidating fundamental processes of induction and differentiation. More recently, studies in the lens have contributed significantly to our understanding of cell cycle regulation and apoptosis. Over 20 years of accumulated evidence using several different vertebrate species has suggested that fibroblast growth factors (FGFs) and/or fibroblast growth factor receptors (FGFRs) play a key role in lens development. FGFR signaling has been implicated in lens induction, lens cell proliferation and survival, lens fiber differentiation and lens regeneration. Here we will review and discuss historical and recent evidence suggesting that (FGFR) signaling plays a vital and universal role in multiple aspects of lens development.