Notch signaling and its integration with other signaling mechanisms

An improved understanding of stem cell differentiation is critical for progress in regenerative medicine. It is an emerging view that a relatively small number of intracellular signaling mechanisms play particularly important roles in differentiation control. As one may expect, these pathways are highly evolutionarily conserved, used in many tissues and iteratively during differentiation of a particular tissue. The Notch signaling system is one pathway meeting these criteria. In many cases, Notch signaling keeps stem/progenitor cells undifferentiated, although it can in some cellular contexts be instructive for differentiation toward a particular fate. Here, we review our current understanding of how Notch controls cellular differentiation in various organs and how Notch integrates with other major signaling pathways, primarily focusing on Notch signaling in mammals. Given the importance of Notch in many stem cell fate decisions, the possibility of experimentally manipulating Notch signaling opens up new avenues to control stem cell differentiation.     

Notch信号通路在骨重建中的作用

Notch信号通路在胚胎发育、神经系统、血管系统、内分泌系统及肿瘤等领域具有广泛的影响。近年来的研究表明,Notch对于骨组织代谢尤其是骨重建有着重要的调控作用,而骨重建的调节紊乱和骨质疏松、骨关节炎等疾病的进展密切相关。Notch信号通路可以通过调控骨组织不同细胞的功能从而影响骨重建过程,但其在不同细胞中具体的参与方式仍然未知。综述近年来Notch信号通路在骨重建中的作用研究进展。     

小胶质细胞及其相关炎性信号通路

小胶质细胞作为中枢系统重要的免疫效应细胞，在中枢神经系统损伤及疾病处理中发挥着不可忽视的作用。本文从小胶质细胞的生物学特性及其相关的炎性信号通路（Notch信号通路、Toll样信号通路、AMPK信号通路、NF-κB信号通路）两大方面探讨，以期为临床抗炎药物的开发提供新思路。     

Coordination of insulin and Notch pathway activities by microRNA miR-305 mediates adaptive homeostasis in the intestinal stem cells of the Drosophila gut

Homeostasis of the intestine is maintained by dynamic regulation of a pool of intestinal stem cells. The balance between stem cell self-renewal and differentiation is regulated by the Notch and insulin signaling pathways. Dependence on the insulin pathway places the stem cell pool under nutritional control, allowing gut homeostasis to adapt to environmental conditions. Here we present evidence that miR-305 is required for adaptive homeostasis of the gut. miR-305 regulates the Notch and insulin pathways in the intestinal stem cells. Notably, miR-305 expression in the stem cells is itself under nutritional control via the insulin pathway. This link places regulation of Notch pathway activity under nutritional control. These findings provide a mechanism through which the insulin pathway controls the balance between stem cell self-renewal and differentiation that is required for adaptive homeostasis in the gut in response to changing environmental conditions.     

Human ligands of the Notch receptor

During development, the Notch signaling pathway is essential for the appropriate differentiation of many cell types in organisms across the phylogenetic scale, including humans. Notch signaling is also implicated in human diseases, including a leukemia and two hereditary syndromes known as Alagille and CADASIL. To generate tools for pursuing the role of the Notch pathway in human disease and development, we have cloned and analyzed the expression of three human homologues of the Notch ligands Delta and Serrate, human Jagged1 (HJ1), human Jagged2 (HJ2), and human Delta1 (H-Delta-1), and determined their chromosomal localizations. We have also raised antibodies to HJ1, and used these antibodies in conjunction with in situ hybridization to examine the expression of these ligands in normal and cancerous cervical tissue. We find that, as reported previously for Notch, the ligands are up-regulated in certain neoplastic tissues. This observation is consistent with the notion that Notch signaling is an important element in these pathogenic conditions, raising the possibility that modulation of Notch activity could be used to influence the fate of the cells and offering a conceivable therapeutic avenue.     

Wnt Signaling and the Control of Human Stem Cell Fate

Wnt signaling determines major developmental processes in the embryonic state and regulates maintenance, self-renewal and differentiation of adult mammalian tissue stem cells. Both β-catenin dependent and independent Wnt pathways exist, and both affect stem cell fate in developing and adult tissues. In this review, we debate the response to Wnt signal activation in embryonic stem cells and human, adult stem cells of mesenchymal, hematopoetic, intestinal, gastric, epidermal, mammary and neural lineages, and discuss the need for Wnt signaling in these cell types. Due to the vital actions of Wnt signaling in developmental and maintenance processes, deregulation of the pathway can culminate into a broad spectrum of developmental and genetic diseases, including cancer. The way in which Wnt signals can feed tumors and maintain cancer stem stells is discussed as well. Manipulation of Wnt signals both in vivo and in vitro thus carries potential for therapeutic approaches such as tissue engineering for regenerative medicine and anti-cancer treatment. Although many questions remain regarding the complete Wnt signal cell-type specific response and interplay of Wnt signaling with pathways such as BMP, Hedgehog and Notch, we hereby provide an overview of current knowledge on Wnt signaling and its control over human stem cell fate.     

Age-related changes in gene expression in tissues of the sea urchin Strongylocentrotus purpuratus

The life history of sea urchins is fundamentally different from that of traditional models of aging and therefore they provide the opportunity to gain new insight into this complex process. Sea urchins grow indeterminately, reproduce throughout their life span and some species exhibit negligible senescence. Using a microarray and qRT-PCR, age-related changes in gene expression were examined in three tissues (muscle, esophagus and nerve) of the sea urchin species Strongylocentrotus purpuratus. The results indicate age-related changes in gene expression involving many key cellular functions such as the ubiquitin-proteasome pathway, DNA metabolism, signaling pathways and apoptosis. Although there are tissue-specific differences in the gene expression profiles, there are some characteristics that are shared between tissues providing insight into potential mechanisms that promote lack of senescence in these animals. As an example, there is an increase in expression of genes encoding components of the Notch signaling pathway with age in all three tissues and a decrease in expression of the Wnt1 gene in both muscle and nerve. The interplay between the Notch and Wnt pathways may be one mechanism that ensures continued regeneration of tissues with advancing age contributing to the general lack of age-related decline in these animals.     

The PI3K/Akt pathway is required for LPS activation of microglial cells

Upregulation of inflammatory responses in the brain is associated with a number of neurodegenerative diseases. Microglia are activated in neurodegenerative diseases, producing pro-inflammatory mediators. Critically, lipopolysaccharide (LPS)-induced microglial activation causes dopaminergic neurodegeneration in vitro and in vivo. The signaling mechanisms triggered by LPS to stimulate the release of pro-inflammatory mediators in microglial cells are still incompletely understood. To further explore the mechanisms of LPS-mediated inflammatory response of microglial cells, we studied the role of phosphatidylinositol 3-kinase (PI3K)/Akt signal transduction pathways known to be activated by toll-like receptor-4 signaling through LPS. In the current study, we report that the activation profile of LPS-induced pAkt activation preceded those of LPS-induced NF-κB activation, suggesting a role for PI3K/Akt in the pathway activation of NF-κB-dependent inflammatory responses of activated microglia. These results, providing the first evidence that PI3K dependent signaling is involved in the inflammatory responses of microglial cells following LPS stimulation, may be useful in preventing inflammatory based neurodegenerative processes.     

Transient activation of Notch signaling in the injured adult brain

Brain injury induces various kinds of cellular responses that lead to tissue regeneration and repair. Recent studies have demonstrated that resident progenitors proliferate and then differentiate into mature neuronal cells. We show here that proliferating cells in the cryo-injured cerebral cortex transiently expressed Notch1 immunoreactivity in their cytoplasm. Since activated Notch signaling regulates cellular fate in the developing nervous system, similar regulation may exist in the injured adult brain. To monitor the Notch signaling pathway, we examined whether components of the signaling pathway were co-expressed in Notch1-positive cells. Presenilin-1, a membrane-spanning protease that is required for the release of the Notch intracellular domain, was detected in the Notch1-positive cells and Hes1, a target of the Notch intracellular domain, also co-localized with Notch1 three days after cryo-injury. These results suggest that transient activity of the Notch signaling pathway is involved in the regulation of proliferation and differentiation of progenitors in the injured brain.     

Notch2 is required for the proliferation of cardiac neural crest-derived smooth muscle cells.

Mutations in Notch receptors and their ligands have been identified as the cause of human congenital heart diseases, indicating the importance of the Notch signaling pathway during heart development. In our study, we use Cre-Lox technology to inactivate Notch2 in several cardiac cell lineages to determine the functional requirements for Notch2 during mammalian heart development. Inactivation of Notch2 in cardiac neural crest cells resulted in abnormally narrow aortas and pulmonary arteries due to a decrease in smooth muscle tissue. The reduction in smooth muscle tissue was not due to cell migration defects but instead was found to be caused by less proliferation in smooth muscle cells during mid to late gestation. Our findings demonstrate that Notch2 is required cell autonomously for proper formation of the heart outflow tract and provides insights into the role of Notch2 in vascular smooth muscle development and the cardiovascular defects associated with Alagille syndrome.     

Anti-inflammatory properties of the PI3K pathway are mediated by IL-10/DUSP regulation

Resolution of inflammation is an important hallmark in the course of infectious diseases. Dysregulated inflammatory responses may have detrimental consequences for the affected organism. Therefore, tight regulation of inflammation is indispensable. Among numerous modulatory signaling pathways, the PI3K/PTEN signaling pathway has been proposed recently to be involved in the regulation of innate immune reactions. Here, we attempted to elucidate molecular mechanisms that contribute to the modulatory properties of the PI3K signaling pathway in inflammation. PTEN-deficient macrophages, which harbor constitutively active PI3Ks, were analyzed in response to gram-negative bacteria and PAMPs such as LPS. PTEN-deficient cells showed reduced inflammatory cytokine production, which was accompanied by reduced MAPK signaling activation in early- as well as late-phase activation. Simultaneously, we found increased levels of the MKP DUSP1, as well as the anti-inflammatory cytokine IL-10. Our data suggest that differential DUSP1 regulation coupled with enhanced IL-10 production contributes to the anti-inflammatory properties of the PI3K pathway.     

The role of notch 1 activation in cardiosphere derived cell differentiation

Cardiosphere derived cells (CDC) are present in the human heart and include heterogeneous cell populations of cardiac progenitor cells, multipotent progenitors that play critical roles in the physiological and pathological turnover of heart tissue. Little is known about the molecular pathways that control the differentiation of CDC. In this study, we examined the role of Notch 1/J kappa-recombining binding protein (RBPJ) signaling, a critical cell-fate decision pathway, in CDC differentiation. We isolated CDC from mouse cardiospheres and analyzed the differentiation of transduced cells expressing the Notch1 intracellular domain (N1-ICD), the active form of Notch1, using a terminal differentiation marker polymerase chain reaction (PCR) array. We found that Notch1 primarily supported the differentiation of CDC into smooth muscle cells (SMC), as demonstrated by the upreguation of key SMC proteins, including smooth muscle myosin heavy chain (Myh11) and SM22α (Tagln), in N1-ICD expressing CDC. Conversely, genetic ablation of RBPJ in CDC diminished the expression of SMC differentiation markers, confirming that SMC differentiation CDC is dependent on RBPJ. Finally, in vivo experiments demonstrate enhanced numbers of smooth muscle actin-expressing implanted cells after an injection of N1-ICD-expressing CDC into ischemic myocardium (44±8/high power field (hpf) vs. 11±4/high power field (hpf), n=7 sections, P<0.05). Taken together, these results provide strong evidence that Notch1 promotes SMC differentiation of CDC through an RBPJ-dependent signaling pathway in vitro, which may have important implications for progenitor cell-mediated angiogenesis.     

The PI3K/Akt pathway is required for LPS activation of microglial cells

Upregulation of inflammatory responses in the brain is associated with a number of neurodegenerative diseases. Microglia are activated in neurodegenerative diseases, producing pro-inflammatory mediators. Critically, lipopolysaccharide (LPS)-induced microglial activation causes dopaminergic neurodegeneration in vitro and in vivo. The signaling mechanisms triggered by LPS to stimulate the release of pro-inflammatory mediators in microglial cells are still incompletely understood. To further explore the mechanisms of LPS-mediated inflammatory response of microglial cells, we studied the role of phosphatidylinositol 3-kinase (PI3K)/Akt signal transduction pathways known to be activated by toll-like receptor-4 signaling through LPS. In the current study, we report that the activation profile of LPS-induced pAkt activation preceded those of LPS-induced NF-κB activation, suggesting a role for PI3K/Akt in the pathway activation of NF-κB-dependent inflammatory responses of activated microglia. These results, providing the first evidence that PI3K dependent signaling is involved in the inflammatory responses of microglial cells following LPS stimulation, may be useful in preventing inflammatory based neurodegenerative processes.     

Deregulation of the Notch pathway as a common road in viral carcinogenesis

The Notch pathway is a conserved signaling pathway and a form of direct cell‐cell communication related to many biological processes during development and adulthood. Deregulation of the Notch pathway is involved in many diseases, including cancer. Almost 20% of all cancer cases have an infectious etiology, with viruses responsible for at least 1.5 million new cancer cases per year. Seven groups of viruses have been classified as oncogenic: hepatitis B and C viruses (HBV and HCV respectively), Epstein‐Barr virus (EBV), Kaposi sarcoma‐associated herpesvirus (KSHV), human T lymphotropic virus (HTLV‐1), human papillomavirus (HPV), and Merkel cell polyomavirus (MCPyV). These viruses share the ability to manipulate a variety of cell pathways that are critical in proliferation and differentiation, leading to malignant transformation. Viral proteins interact directly or indirectly with different members of the Notch pathway, altering their normal function. This review focuses exclusively on the direct interactions of viral oncoproteins with Notch elements, providing a deeper understanding of the dual behavior of the Notch pathway as activator or suppressor of neoplasia in virus‐related cancers.     

微重力对骨髓间充质干细胞成骨分化的影响

骨髓间充质干细胞（BMSCs）是一种多能成体于细胞,是组织工程重要的种子细胞来源之一.微重力对BMSCs成骨分化具有抑制作用,可使骨量减少和骨微结构改变,从而导致骨质疏松症.这一过程受到多条信号通路的调控,如MAPK信号通路、Notch信号通路和Wnt/β-catenin信号通路等,它们协同调节微重力下BMSCs向成骨细胞方向的分化.研究微重力对BMSCs成骨分化的影响,可以阐明骨质流失机理,为相关疾病的治疗提供新的靶点,促进我国太空宇航事业的发展.     

Off the beaten pathway: the complex cross talk between Notch and NF-kappaB

The canonical Notch pathway that has been well characterized over the past 25 years is relatively simple compared to the plethora of recently published data suggesting non-canonical signaling mechanisms and cross talk with other pathways. The manner in which other pathways cross talk with Notch signaling appears to be extraordinarily complex and, not surprisingly, context-dependent. While the physiological relevance of many of these interactions remains to be established, there is little doubt that Notch signaling is integrated with numerous other pathways in ways that appear increasingly complex. Among the most intricate cross talks described for Notch is its interaction with the NF-kappaB pathway, another major cell fate regulatory network involved in development, immunity, and cancer. Numerous reports over the last 11 years have described multiple cross talk mechanisms between Notch and NF-kappaB in diverse experimental models. This article will provide a brief overview of the published evidence for Notch-NF-kappaB cross talk, focusing on vertebrate systems.     

The role of T regulatory cells in asthma

As a chronic inflammatory disease, much of the research related to asthma has been focused on proinflammatory mechanisms. Recently, advances have been made in defining mechanisms that control inflammation and induce immune tolerance to specific antigens. Subsets of CD4(+) cells known as T regulatory cells play an important role in directing these processes, and recent experiments have begun to define crucial molecular and signaling pathways. There is a growing body of evidence describing the function of T regulatory cells in the development, disease activity, and treatment responses related to asthma and other atopic diseases. Collectively, this new information suggests that a greater understanding of these pathways might lead to new therapeutic targets for asthma and other diseases of chronic airway inflammation.