Notch ligand endocytosis generates mechanical pulling force dependent on dynamin, epsins, and actin

An activity-dependent proximity ligation platform for spatially resolved quantification of active enzymes in single cells

Integration of chemical probes into proteomic workflows enables the interrogation of protein activity, rather than abundance. Current methods limit the biological contexts that can be addressed due to sample homogenization, signal-averaging, and bias toward abundant proteins. Here we report a platform that integrates family-wide chemical probes with proximity-dependent oligonucleotide amplification and imaging to quantify enzyme activity in native contexts with high spatial resolution. Application of this method, activity-dependent proximity ligation (ADPL), to serine hydrolase and cysteine protease enzymes enables quantification of differential enzyme activity resulting from endogenous changes in localization and expression. In a competitive format, small-molecule target engagement with endogenous proteins in live cells can be quantified. Finally, retention of sample architecture enables interrogation of complex environments such as cellular co-culture and patient samples. ADPL should be amenable to diverse probe and protein families to detect active enzymes at scale and resolution out of reach with current methods.

The interrogation of enzyme activity involves the ensemble averaging of many cells, loss of spatial relationships and is often biased to abundant proteins. Here the authors develop activity-dependent proximity ligation to quantify enzyme activity at the cellular and sub-cellular level in relevant biological contexts.

NOTCH1 is a mechanosensor in adult arteries

Endothelial cells transduce mechanical forces from blood flow into intracellular signals required for vascular homeostasis. Here we show that endothelial NOTCH1 is responsive to shear stress, and is necessary for the maintenance of junctional integrity, cell elongation, and suppression of proliferation, phenotypes induced by laminar shear stress. NOTCH1 receptor localizes downstream of flow and canonical NOTCH signaling scales with the magnitude of fluid shear stress. Reduction of NOTCH1 destabilizes cellular junctions and triggers endothelial proliferation. NOTCH1 suppression results in changes in expression of genes involved in the regulation of intracellular calcium and proliferation, and preventing the increase of calcium signaling rescues the cell-cell junctional defects. Furthermore, loss of Notch1 in adult endothelium increases hypercholesterolemia-induced atherosclerosis in the descending aorta. We propose that NOTCH1 is atheroprotective and acts as a mechanosensor in adult arteries, where it integrates responses to laminar shear stress and regulates junctional integrity through modulation of calcium signaling.

Mammalian Numb protein antagonizes Notch by controlling postendocytic trafficking of the Notch ligand Delta-like 4

The biological antagonism between the signaling proteins Numb and Notch has been implicated in the regulation of many developmental processes, especially in asymmetric cell division. Mechanistic studies show that Numb inactivates Notch via endocytosis and proteasomal degradation that directly reduce Notch protein levels at the cell surface. However, some aspects of how Numb antagonizes Notch remain unclear. Here, we report a novel mechanism in which Numb acts as a Notch antagonist by controlling the intracellular destination and stability of the Notch ligand Delta-like 4 (Dll4) through a postendocytic-sorting process. We observed that Numb/Numblike knockdown increases the stability and cell-surface accumulation of Dll4. Further study indicated that Numb acts as a sorting switch to control the postendocytic trafficking of Dll4. Of note, the Numb/Numblike knockdown decreased Dll4 delivery to the lysosome, while increasing the recycling of Dll4 to the plasma membrane. Moreover, we demonstrate that this enrichment of Dll4 at the cell surface within Numb/Numblike knockdown cells could activate Notch signaling in neighboring cells. We also provide evidence that Numb negatively controls the Dll4 plasma membrane recycling through a well-documented recycling regulator protein AP1. In conclusion, our study has uncovered a molecular mechanism whereby Numb regulates the endocytic trafficking of the Notch ligand Dll4. Our findings provide a new perspective on how Numb counteracts Notch signaling and sheds additional critical insights into the antagonistic relationship between Numb and Notch signaling.

Do as I say, Not(ch) as I do: Lateral control of cell fate

Breaking symmetry in populations of uniform cells, to induce adoption of an alternative cell fate, is an essential developmental mechanism. Similarly, domain and boundary establishment are crucial steps to forming organs during development. Notch signaling is a pathway ideally suited to mediating precise patterning cues, as both receptors and ligands are membrane-bound and can thus act as a precise switch to toggle cell fates on or off. Fine-tuning of signaling by positive or negative feedback mechanisms dictate whether signaling results in lateral induction or lateral inhibition, respectively, allowing Notch to either induce entire regions of cell specification, or dictate binary fate choices. Furthermore, pathway activity is modulated by Fringe modification of receptors or ligands, co-expression of receptors with ligands, mode of ligand presentation, and cell surface area in contact. In this review, we describe how Notch signaling is fine-tuned to mediate lateral induction or lateral inhibition cues, and discuss examples from C.elegans, D. melanogaster and M. musculus. Identifying the cellular machinery dictating the choice between lateral induction and lateral inhibition highlights the versatility of the Notch signaling pathway in development.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

The Canonical Notch Signaling Pathway: Structural and Biochemical Insights into Shape, Sugar, and Force

The Notch signaling pathway relies on a proteolytic cascade to release its transcriptionally active intracellular domain, on force to unfold a protective domain and permit proteolysis, on extracellular domain glycosylation to tune the forces exerted by endocytosed ligands, and on a motley crew of nuclear proteins, chromatin modifiers, ubiquitin ligases, and a few kinases to regulate activity and half-life. Herein we provide a review of recent molecular insights into how Notch signals are triggered and how cell shape affects these events, and we use the new insights to illuminate a few perplexing observations.

A mammalian mirtron miR-1224 promotes tube-formation of human primary endothelial cells by targeting anti-angiogenic factor epsin2

Angiogenesis, new vessel formation from pre-existing vessels, is a highly conserved event through vertebrates. However, the system for tuning angiogenesis by species-intrinsic factors is totally unknown. miR-1224 is a member of mammal-specific mirtrons, which were identified as non-canonical microRNAs. We found that the expression of miR-1224 was upregulated in capillary-like tube-forming human umbilical vein endothelial cells on Matrigel. Enforced expression of miR-1224 stimulated tube formation, whereas repression of endogenous miR-1224 inhibited formation. Enforced expression of miR-1224 enhanced VEGF signaling and repressed NOTCH signaling. The adaptor protein of clathrin-dependent endocytosis, epsin2, which has been shown to be a suppressor of angiogenesis, was a direct target of miR-1224. Knockdown of EPN2 stimulated tube formation, while overexpression of EPN2 repressed miR-1224-mediated stimulation. Our findings indicate that miR-1224 is a mammal specific modulator of angiogenesis.

Selective regulation of Notch ligands during angiogenesis is mediated by vimentin

Notch signaling is a key regulator of angiogenesis, in which sprouting is regulated by an equilibrium between inhibitory Dll4-Notch signaling and promoting Jagged-Notch signaling. Whereas Fringe proteins modify Notch receptors and strengthen their activation by Dll4 ligands, other mechanisms balancing Jagged and Dll4 signaling are yet to be described. The intermediate filament protein vimentin, which has been previously shown to affect vascular integrity and regenerative signaling, is here shown to regulate ligand-specific Notch signaling. Vimentin interacts with Jagged, impedes basal recycling endocytosis of ligands, but is required for efficient receptor ligand transendocytosis and Notch activation upon receptor binding. Analyses of Notch signal activation by using chimeric ligands with swapped intracellular domains (ICDs), demonstrated that the Jagged ICD binds to vimentin and contributes to signaling strength. Vimentin also suppresses expression of Fringe proteins, whereas depletion of vimentin enhances Fringe levels to promote Dll4 signaling. In line with these data, the vasculature in vimentin knockout (VimKO) embryos and placental tissue is underdeveloped with reduced branching. Disrupted angiogenesis in aortic rings from VimKO mice and in endothelial 3D sprouting assays can be rescued by reactivating Notch signaling by recombinant Jagged ligands. Taken together, we reveal a function of vimentin and demonstrate that vimentin regulates Notch ligand signaling activities during angiogenesis.

Delta-Notch signalling in segmentation

Modular body organization is found widely across multicellular organisms, and some of them form repetitive modular structures via the process of segmentation. It's vastly interesting to understand how these regularly repeated structures are robustly generated from the underlying noise in biomolecular interactions. Recent studies from arthropods reveal similarities in segmentation mechanisms with vertebrates, and raise the possibility that the three phylogenetic clades, annelids, arthropods and chordates, might share homology in this process from a bilaterian ancestor. Here, we discuss vertebrate segmentation with particular emphasis on the role of the Notch intercellular signalling pathway. We introduce vertebrate segmentation and Notch signalling, pointing out historical milestones, then describe existing models for the Notch pathway in the synchronization of noisy neighbouring oscillators, and a new role in the modulation of gene expression wave patterns. We ask what functions Notch signalling may have in arthropod segmentation and explore the relationship between Notch-mediated lateral inhibition and synchronization. Finally, we propose open questions and technical challenges to guide future investigations into Notch signalling in segmentation.

Stimulation of Notch Signaling in Mouse Osteoclast Precursors

Notch signaling is a key component of multiple physiological and pathological processes. The nature of Notch signaling, however, makes in vitro investigation of its varying and sometimes contradictory roles a challenge. As a component of direct cell-cell communication with both receptors and ligands bound to the plasma membrane, Notch signaling cannot be activated in vitro by simple addition of ligands to culture media, as is possible with many other signaling pathways. Instead, Notch ligands must be presented to cells in an immobilized state. Variations in methods of Notch signaling activation can lead to different outcomes in cultured cells. In osteoclast precursors, in particular, differences in methods of Notch stimulation and osteoclast precursor culture and differentiation have led to disagreement over whether Notch signaling is a positive or negative regulator of osteoclast differentiation. While closer comparisons of osteoclast differentiation under different Notch stimulation conditions in vitro and genetic models have largely resolved the controversy regarding Notch signaling and osteoclasts, standardized methods of continuous and temporary stimulation of Notch signaling in cultured cells could prevent such discrepancies in the future. This protocol describes two methods for stimulating Notch signaling specifically in cultured mouse osteoclast precursors, though these methods should be applicable to any adherent cell type with minor adjustments. The first method produces continuous stimulation of Notch signaling and involves immobilizing Notch ligand to a tissue culture surface prior to the seeding of cells. The second, which uses Notch ligand bound to agarose beads allows for temporary stimulation of Notch signaling in cells that are already adhered to a culture surface. This protocol also includes methods for detecting Notch activation in osteoclast precursors as well as representative transcriptional markers of Notch signaling activation.

Methods to investigate the structure and connectivity of the nervous system

Understanding the computations that take place in neural circuits requires identifying how neurons in those circuits are connected to one another. In addition, recent research indicates that aberrant neuronal wiring may be the cause of several neurodevelopmental disorders, further emphasizing the importance of identifying the wiring diagrams of brain circuits. To address this issue, several new approaches have been recently developed. In this review, we describe several methods that are currently available to investigate the structure and connectivity of the brain, and discuss their strengths and limitations.

Intercellular Tension Negatively Regulates Angiogenic Sprouting of Endothelial Tip Cells via Notch1-Dll4 Signaling

Mechanical force plays pivotal roles in vascular development during tissue growth and regeneration. Nevertheless, the process by which mechanical force controls the vascular architecture remains poorly understood. Using a systems bioengineering approach, we show that intercellular tension negatively regulates tip cell formation via Notch1-Dll4 signaling in mouse retinal angiogenesis in vivo, sprouting embryoid bodies, and human endothelial cell networks in vitro. Reducing the intercellular tension pharmacologically by a Rho-associated protein kinase inhibitor or physically by single cell photothermal ablation of the capillary networks promotes the expression of Dll4, enhances angiogenic sprouting of tip cells and increases the vascular density. Computational biomechanics, RNA interference, and single cell gene expression analysis reveal that a reduction of intercellular tension attenuates the inhibitory effect of Notch signaling on tip cell formation and induces angiogenic sprouting. Taken together, our results reveal a mechanoregulation scheme for the control of vascular architecture by modulating angiogenic tip cell formation via Notch1-Dll4 signaling.

Turn It Down a Notch

In the developing vertebrate embryo, segmentation initiates through the formation of repeated segments, or somites, on either side of the posterior neural tube along the anterior to posterior axis. The periodicity of somitogenesis is regulated by a molecular oscillator, the segmentation clock, driving cyclic gene expression in the unsegmented paraxial mesoderm, from which somites derive. Three signaling pathways underlie the molecular mechanism of the oscillator: Wnt, FGF, and Notch. In particular, Notch has been demonstrated to be an essential piece in the intricate somitogenesis regulation puzzle. Notch is required to synchronize oscillations between neighboring cells, and is moreover necessary for somite formation and clock gene oscillations. Following ligand activation, the Notch receptor is cleaved to liberate the active intracellular domain (NICD) and during somitogenesis NICD itself is produced and degraded in a cyclical manner, requiring tightly regulated, and coordinated turnover. It was recently shown that the pace of the segmentation clock is exquisitely sensitive to levels/stability of NICD. In this review, we focus on what is known about the mechanisms regulating NICD turnover, crucial to the activity of the pathway in all developmental contexts. To date, the regulation of NICD stability has been attributed to phosphorylation of the PEST domain which serves to recruit the SCF/Sel10/FBXW7 E3 ubiquitin ligase complex involved in NICD turnover. We will describe the pathophysiological relevance of NICD-FBXW7 interaction, whose defects have been linked to leukemia and a variety of solid cancers.

Plasma Membrane Sheets for Studies of B Cell Antigen Internalization from Immune Synapses

Surrogate planar and membrane systems have been employed to study the architecture of immune synapses; however, they often do not recapitulate trans-synaptic extraction and endocytosis of ligands by the immune cells. Transendocytosis (or trogocytosis) of antigen from immune synapses is particularly critical for antigen processing and presentation by B cells. Here we describe a protocol for preparation of plasma membrane sheets (PMSs), which are flexible and fluid membrane substrates that support robust B cell antigen extraction. We show how to attach B cell antigens to the PMSs and how to investigate antigen extraction and endocytosis by fluorescent microscopy and computational image analysis. These techniques should be broadly applicable to studies of transendocytosis in a variety of cellular systems.

Targeting High Dynamin-2 (DNM2) Expression by Restoring Ikaros Function in Acute Lymphoblastic Leukemia

Dynamin-2 (DNM2) is a GTPase essential for intracellular vesicle formation and trafficking, cytokinesis and receptor endocytosis. Mutations in DNM2 are common in early T-cell precursor acute lymphoblastic leukemia. However, DNM2 expression in other types of ALL are not reported. We studied DNM2 mRNA level in adults with B- and T-cell ALL. We found DNM2 is more highly expressed compared with normals in both forms of ALL. High DNM2 expression is associated with some clinical and laboratory features, inferior outcomes and with leukaemia cell proliferation. We also found Ikaros directly binds the DNM2 promoter and suppresses DNM2 expression. Consequently IKZF1 deletion is associated with high DNM2 expression. Conversely, casein kinase-2 (CK2)-inhibitor increases Ikaros function thereby inhibiting DNM2 expression. Inhibiting DNM2 suppresses proliferation of leukemia cells and synergizes with CK2 inhibition. Our data indicate high DNM2 expression is associated with Ikaros dysregulation and may be important in the development of B-ALL.

Growth factor and co-receptor release by structural regulation of substrate metalloprotease accessibility

Release of cytokines, growth factors and other life-essential molecules from precursors by a-disintegrin-and-metalloproteases (ADAMs) is regulated with high substrate-specificity. We hypothesized that this is achieved by cleavage-regulatory intracellular-domain (ICD)-modifications of the precursors. We show here that cleavage-stimuli-induced specific ICD-modifications cause structural substrate changes that enhance ectodomain sensitivity of neuregulin-1 (NRG1; epidermal-growth-factor) or CD44 (receptor-tyrosine-kinase (RTK) co-receptor) to chymotrypsin/trypsin or soluble ADAM. This inside-out signal transfer required substrate homodimerization and was prevented by cleavage-inhibitory ICD-mutations. In chimeras, regulation could be conferred to a foreign ectodomain, suggesting a common higher-order structure. We predict that substrate-specific protease-accessibility-regulation controls release of numerous ADAM substrates.

A new mechanism for spatial pattern formation via lateral and protrusion-mediated lateral signalling

Tissue organization and patterning are critical during development when genetically identical cells take on different fates. Lateral signalling plays an important role in this process by helping to generate self-organized spatial patterns in an otherwise uniform collection of cells. Recent data suggest that lateral signalling can be mediated both by junctional contacts between neighbouring cells and via cellular protrusions that allow non-neighbouring cells to interact with one another at a distance. However, it remains unclear precisely how signalling mediated by these distinct types of cell-cell contact can physically contribute to the generation of complex patterns without the assistance of diffusible morphogens or pre-patterns. To explore this question, in this work we develop a model of lateral signalling based on a single receptor/ligand pair as exemplified by Notch and Delta. We show that allowing the signalling kinetics to differ at junctional versus protrusion-mediated contacts, an assumption inspired by recent data which show that the cleavage of Notch in several systems requires both Delta binding and the application of mechanical force, permits individual cells to act to promote both lateral activation and lateral inhibition. Strikingly, under this model, in which Delta can sequester Notch, a variety of patterns resembling those typical of reaction-diffusion systems is observed, together with more unusual patterns that arise when we consider changes in signalling kinetics, and in the length and distribution of protrusions. Importantly, these patterns are self-organizing-so that local interactions drive tissue-scale patterning. Together, these data show that protrusions can, in principle, generate different types of patterns in addition to contributing to long-range signalling and to pattern refinement.

High-throughput screening in niche-based assay identifies compounds to target preleukemic stem cells

Current chemotherapies for T cell acute lymphoblastic leukemia (T-ALL) efficiently reduce tumor mass. Nonetheless, disease relapse attributed to survival of preleukemic stem cells (pre-LSCs) is associated with poor prognosis. Herein, we provide direct evidence that pre-LSCs are much less chemosensitive to existing chemotherapy drugs than leukemic blasts because of a distinctive lower proliferative state. Improving therapies for T-ALL requires the development of strategies to target pre-LSCs that are absolutely dependent on their microenvironment. Therefore, we designed a robust protocol for high-throughput screening of compounds that target primary pre-LSCs maintained in a niche-like environment, on stromal cells that were engineered for optimal NOTCH1 activation. The multiparametric readout takes into account the intrinsic complexity of primary cells in order to specifically monitor pre-LSCs, which were induced here by the SCL/TAL1 and LMO1 oncogenes. We screened a targeted library of compounds and determined that the estrogen derivative 2-methoxyestradiol (2-ME2) disrupted both cell-autonomous and non-cell-autonomous pathways. Specifically, 2-ME2 abrogated pre-LSC viability and self-renewal activity in vivo by inhibiting translation of MYC, a downstream effector of NOTCH1, and preventing SCL/TAL1 activity. In contrast, normal hematopoietic stem/progenitor cells remained functional. These results illustrate how recapitulating tissue-like properties of primary cells in high-throughput screening is a promising avenue for innovation in cancer chemotherapy.

Notch, lipids, and endothelial cells

PURPOSE OF REVIEW

Notch signaling is an evolutionary conserved pathway critical for cardiovascular development and angiogenesis. More recently, the contribution of Notch signaling to the homeostasis of the adult vasculature has emerged as an important novel paradigm, but much remains to be understood.

RECENT FINDINGS

Recent findings shed light on the impact of Notch in vascular and immune responses to microenvironmental signals as well as on the onset of atherosclerosis. In the past year, studies in human and mice explored the role of Notch in the maintenance of a nonactivated endothelium. Novel pieces of evidence suggest that this pathway is sensitive to environmental factors, including inflammatory mediators and diet-derived by-products.

SUMMARY

An emerging theme is the ability of Notch to respond to changes in the microenvironment, including glucose and lipid metabolites. In turn, alterations in Notch enable an important link between metabolism and transcriptional changes, thus this receptor appears to function as a metabolic sensor with direct implications to gene expression.

Monitoring cell-cell contacts in vivo in transgenic animals

We used a synthetic genetic system based on ligand-induced intramembrane proteolysis to monitor cell-cell contacts in animals. Upon ligand-receptor interaction in sites of cell-cell contact, the transmembrane domain of an engineered receptor is cleaved by intramembrane proteolysis and releases a protein fragment that regulates transcription in the interacting partners. We demonstrate that the system can be used to regulate gene expression between interacting cells, both in vitro and in vivo, in transgenic Drosophila We show that the system allows for detection of interactions between neurons and glia in the Drosophila nervous system. In addition, we observed that when the ligand is expressed in subsets of neurons with a restricted localization in the brain it leads to activation of transcription in a selected set of glial cells that interact with those neurons. This system will be useful to monitor cell-cell interactions in animals, and can be used to genetically manipulate cells that interact with one another.

Cellular stress induces TRB3/USP9x-dependent Notch activation in cancer

Expression of the Notch ligand JAG1 and Notch pathway activation promote poor prognosis, basal-like breast cancer. We have recently shown that the pseudokinase Tribbles homolog 3 (TRB3) regulates JAG1 expression in this malignancy. TRB3 is a stress and metabolic sensor, and here we show that nutrient deprivation or endoplasmic reticulum stress markedly upregulate TRB3, which serves as a scaffold for the deubiquitinase USP9x. USP9x in turn stimulates JAG1 activity through two mechanisms: (1) through TRB3 deubiquitination and stabilization, and (2) through deubiquitination and activation of Mind Bomb 1, an E3 ligase required for JAG1 ubiquitination-mediated endocytosis and Notch activation. These USP9x activities are confined to the signal-sending cell of a cell pair undergoing Notch signaling. We demonstrate that USP9x is required for TRB3 upregulation and Notch activation in response to cellular stress in basal-like breast cancer cells. These data suggest that TRB3 functions as a sensor of tumor microenvironmental stress and together with USP9x induces the cell survival and tumor-promoting activities of Notch. These findings identify a novel mechanism by which cancer cells survive in their hostile environment and provide potential therapeutic targets in breast cancer.

Notch signalling in context

The highly conserved Notch signalling pathway functions in many different developmental and homeostatic processes, which raises the question of how this pathway can achieve such diverse outcomes. With a direct route from the membrane to the nucleus, the Notch pathway has fewer opportunities for regulation than do many other signalling pathways, yet it generates exquisitely patterned structures, including sensory hair cells and branched arterial networks. More confusingly, its activity promotes tissue growth and cancers in some circumstances but cell death and tumour suppression in others. Many different regulatory mechanisms help to shape the activity of the Notch pathway, generating functional outputs that are appropriate for each context. These mechanisms include the receptor-ligand landscape, the tissue topology, the nuclear environment and the connectivity of the regulatory networks.

Defining Single Molecular Forces Required for Notch Activation Using Nano Yoyo

Notch signaling, involved in development and tissue homeostasis, is activated at the cell-cell interface through ligand-receptor interactions. Previous studies have implicated mechanical forces in the activation of Notch receptor upon binding to its ligand. Here we aimed to determine the single molecular force required for Notch activation by developing a novel low tension gauge tether (LTGT). LTGT utilizes the low unbinding force between single-stranded DNA (ssDNA) and Escherichia coli ssDNA binding protein (SSB) (∼4 pN dissociation force at 500 nm/s pulling rate). The ssDNA wraps around SSB and, upon application of force, unspools from SSB, much like the unspooling of a yoyo. One end of this nano yoyo is attached to the surface though SSB, while the other end presents a ligand. A Notch receptor, upon binding to its ligand, is believed to undergo force-induced conformational changes required for activating downstream signaling. If the required force for such activation is larger than 4 pN, ssDNA will unspool from SSB, and downstream signaling will not be activated. Using these LTGTs, in combination with the previously reported TGTs that rupture double-stranded DNA at defined forces, we demonstrate that Notch activation requires forces between 4 and 12 pN, assuming an in vivo loading rate of 60 pN/s. Taken together, our study provides a direct link between single-molecular forces and Notch activation.

Stem cells and injectable hydrogels: Synergistic therapeutics in myocardial repair

One of the major problems in the treatment of cardiovascular diseases is the inability of myocardium to self-regenerate. Current therapies are unable to restore the heart's function after myocardial infarction. Myocardial tissue engineering is potentially a key approach to regenerate damaged heart muscle. Myocardial patches are applied surgically, whereas injectable hydrogels provide effective minimally invasive approaches to recover functional myocardium. These hydrogels are easily administered and can be either cell free or loaded with bioactive agents and/or cardiac stem cells, which may apply paracrine effects. The aim of this review is to investigate the advantages and disadvantages of injectable stem cell-laden hydrogels and highlight their potential applications for myocardium repair.

Constructing modular and universal single molecule tension sensor using protein G to study mechano-sensitive receptors

Recently a variety of molecular force sensors have been developed to study cellular forces acting through single mechano-sensitive receptors. A common strategy adopted is to attach ligand molecules on a surface through engineered molecular tethers which report cell-exerted tension on receptor-ligand bonds. This approach generally requires chemical conjugation of the ligand to the force reporting tether which can be time-consuming and labor-intensive. Moreover, ligand-tether conjugation can severely reduce the activity of protein ligands. To address this problem, we developed a Protein G (ProG)-based force sensor in which force-reporting tethers are conjugated to ProG instead of ligands. A recombinant ligand fused with IgG-Fc is conveniently assembled with the force sensor through ProG:Fc binding, therefore avoiding ligand conjugation and purification processes. Using this approach, we determined that molecular tension on E-cadherin is lower than dsDNA unzipping force (nominal value: 12 pN) during initial cadherin-mediated cell adhesion, followed by an escalation to forces higher than 43 pN (nominal value). This approach is highly modular and potentially universal as we demonstrate using two additional receptor-ligand interactions, P-selectin & PSGL-1 and Notch & DLL1.

Regulation of Notch Signaling Through Intracellular Transport

The highly conserved Notch-signaling pathway performs a central role in cell differentiation, survival, and proliferation. A major mechanism by which cells modulate signaling is by controlling the intracellular transport itinerary of Notch. Indeed, Notch removal from the cell surface and its targeting to the lysosome for degradation is one way in which Notch activity is downregulated since it limits receptor exposure to ligand. In contrast, Notch-signaling capacity is maintained through repeated rounds of receptor recycling and redelivery of Notch to the cell surface from endosomal stores. This review discusses the molecular mechanisms by which Notch transit through the endosome is controlled and how various intracellular sorting decisions are thought to impact signaling activity.

Context-Dependent Sensitivity to Mutations Disrupting the Structural Integrity of Individual EGF Repeats in the Mouse Notch Ligand DLL1

The highly conserved Notch-signaling pathway mediates cell-to-cell communication and is pivotal for multiple developmental processes and tissue homeostasis in adult organisms. Notch receptors and their ligands are transmembrane proteins with multiple epidermal-growth-factor-like (EGF) repeats in their extracellular domains. In vitro the EGF repeats of mammalian ligands that are essential for Notch activation have been defined. However, in vivo the significance of the structural integrity of each EGF repeat in the ligand ectodomain for ligand function is still unclear. Here, we analyzed the mouse Notch ligand DLL1. We expressed DLL1 proteins with mutations disrupting disulfide bridges in each individual EGF repeat from single-copy transgenes in the HPRT locus of embryonic stem cells. In Notch transactivation assays all mutations impinged on DLL1 function and affected both NOTCH1 and NOTCH2 receptors similarly. An allelic series in mice that carried the same point mutations in endogenous Dll1, generated using a mini-gene strategy, showed that early developmental processes depending on DLL1-mediated NOTCH activation were differently sensitive to mutation of individual EGF repeats in DLL1. Notably, some mutations affected only somite patterning and resulted in vertebral column defects resembling spondylocostal dysostosis. In conclusion, the structural integrity of each individual EGF repeat in the extracellular domain of DLL1 is necessary for full DLL1 activity, and certain mutations in Dll1 might contribute to spondylocostal dysostosis in humans.

Notch-Mediated Cell Adhesion

Notch family members are generally recognized as signaling molecules that control various cellular responses in metazoan organisms. Early fly studies and our mammalian studies demonstrated that Notch family members are also cell adhesion molecules; however, information on the physiological roles of this function and its origin is limited. In this review, we discuss the potential present and ancestral roles of Notch-mediated cell adhesion in order to explore its origin and the initial roles of Notch family members dating back to metazoan evolution. We hypothesize that Notch family members may have initially emerged as cell adhesion molecules in order to mediate multicellularity in the last common ancestor of metazoan organisms.

Quantitative Analysis of Delta-like 1 Membrane Dynamics Elucidates the Role of Contact Geometry on Notch Signaling

Notch signaling is ubiquitously used to coordinate differentiation between adjacent cells across metazoans. Whereas Notch pathway components have been studied extensively, the effect of membrane distribution and dynamics of Notch receptors and ligands remains poorly understood. It is also unclear how cellular morphology affects these distributions and, ultimately, the signaling between cells. Here, we combine live-cell imaging and mathematical modeling to address these questions. We use a FRAP-TIRF assay to measure the diffusion and endocytosis rates of Delta-like 1 (Dll1) in mammalian cells. We find large cell-to-cell variability in the diffusion coefficients of Dll1 measured in single cells within the same population. Using a simple reaction-diffusion model, we show how membrane dynamics and cell morphology affect cell-cell signaling. We find that differences in the diffusion coefficients, as observed experimentally, can dramatically affect signaling between cells. Together, these results elucidate how membrane dynamics and cellular geometry can affect cell-cell signaling.

Endocardial Notch Signaling in Cardiac Development and Disease

The Notch signaling pathway is an ancient and highly conserved signaling pathway that controls cell fate specification and tissue patterning in the embryo and in the adult. Region-specific endocardial Notch activity regulates heart morphogenesis through the interaction with multiple myocardial-, epicardial-, and neural crest-derived signals. Mutations in NOTCH signaling elements cause congenital heart disease in humans and mice, demonstrating its essential role in cardiac development. Studies in model systems have provided mechanistic understanding of Notch function in cardiac development, congenital heart disease, and heart regeneration. Notch patterns the embryonic endocardium into prospective territories for valve and chamber formation, and later regulates the signaling processes leading to outflow tract and valve morphogenesis and ventricular trabeculae compaction. Alterations in NOTCH signaling in the endocardium result in congenital structural malformations that can lead to disease in the neonate and adult heart.

Notch1 endocytosis is induced by ligand and is required for signal transduction

The Notch signalling pathway is widely utilised during embryogenesis in situations where cell-cell interactions are important for cell fate specification and differentiation. DSL ligand endocytosis into the ligand-expressing cell is an important aspect of Notch signalling because it is thought to supply the force needed to separate the Notch heterodimer to initiate signal transduction. A functional role for receptor endocytosis during Notch signal transduction is more controversial. Here we have used live-cell imaging to examine trafficking of the Notch1 receptor in response to ligand binding. Contact with cells expressing ligands induced internalisation and intracellular trafficking of Notch1. Notch1 endocytosis was accompanied by transendocytosis of ligand into the Notch1-expressing signal-receiving cell. Ligand caused Notch1 endocytosis into SARA-positive endosomes in a manner dependent on clathrin and dynamin function. Moreover, inhibition of endocytosis in the receptor-expressing cell impaired ligand-induced Notch1 signalling. Our findings resolve conflicting observations from mammalian and Drosophila studies by demonstrating that ligand-dependent activation of Notch1 signalling requires receptor endocytosis. Endocytosis of Notch1 may provide a force on the ligand:receptor complex that is important for potent signal transduction.

Mutational analysis of the Notch2 negative regulatory region identifies key structural elements for mechanical stability

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Mutations within heterodimerization (HD) domain of Notch2 reduce mechanical stability.

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Structural changes are observed within the LNRA:B linker region/LNRB and αC helix.

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The LNRC:HD domain interaction is also reduced in stability.

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Changes in mechanical stability versus chemical stability are highlighted.

The Notch signalling pathway is fundamental to cell differentiation in developing and self-renewing tissues. Notch is activated upon ligand-induced conformational change of the Notch negative regulatory region (NRR), unmasking a key proteolytic site (S2) and facilitating downstream events. The favoured model requires endocytosis of a tightly bound ligand to transmit force to the NRR region, sufficient to cause a structural change that exposes the S2 site. We have previously shown, using atomic force microscopy and molecular dynamics simulations, that application of force to the N-terminus of the Notch2 NRR facilitates metalloprotease cleavage at an early stage in the unfolding process. Here, mutations are made within the heterodimerization (HD) domain of the NRR that are known to cause constitutive activation of Notch1 whilst having no effect on the chemical stability of Notch2. Comparison of the mechanical stability and simulated forced unfolding of recombinant Notch2 NRR proteins demonstrates a reduced stability following mutation and identifies two critical structural elements of the NRR in its response to force – the linker region between Lin12-Notch repeats LNRA and LNRB and the α3 helix within the HD domain – both of which mask the S2 cleavage site prior to Notch activation. In two mutated proteins, the LNRC:HD domain interaction is also reduced in stability. The observed changes to mechanical stability following these HD domain mutations highlight key regions of the Notch2 NRR that are important for mechanical, but not chemical, stability. This research could also help determine the fundamental differences in the NRRs of Notch1 and Notch2.

Lighting Up the Force: Investigating Mechanisms of Mechanotransduction Using Fluorescent Tension Probes

The ability of cells to sense the physical nature of their surroundings is critical to the survival of multicellular organisms. Cellular response to physical cues from adjacent cells and the extracellular matrix leads to a dynamic cycle in which cells respond by remodeling their local microenvironment, fine-tuning cell stiffness, polarity, and shape. Mechanical regulation is important in cellular development, normal morphogenesis, and wound healing. The mechanisms by which these finely balanced mechanotransduction events occur, however, are not well understood. In large part, this is due to the limited availability of tools to study molecular mechanotransduction events in live cells. Several classes of molecular tension probes have been recently developed which are rapidly transforming the study of mechanotransduction. Molecular tension probes are primarily based on fluorescence resonance energy transfer (FRET) and report on piconewton scale tension events in live cells. In this minireview, we describe the two main classes of tension probes, genetically encoded tension sensors and immobilized tension sensors, and discuss the advantages and limitations of each type. We discuss future opportunities to address major biological questions and outline the challenges facing the next generation of molecular tension probes.

CMV-induced embryonic mouse organ of corti dysplasia: Network architecture of dysfunctional lateral inhibition

BACKGROUND

Congenital cytomegalovirus infection is the major nongenetic cause of sensorineural hearing loss at birth and beyond. Among other pathologies, there is a striking dysplasia/hyperplasia of organ of Corti hair and supporting cells.

METHODS

Using an in vitro embryonic mouse model of cytomegalovirus-induced cochlear teratogenesis that mimics the known human pathology, and functional signaling network modeling, we tested the hypothesis that cytomegalovirus disrupts the highly ordered organ of Corti hair and supporting cells pattern by dysregulating Notch and Fgfr3, their cognate ligands and downstream effectors.

RESULTS

Several novel emergent properties of the critical lateral inhibition subnetwork became apparent. The subnetwork has classic small-world properties such as short paths between most gene pairs, few long-distance links, and considerable clustering. Concomitantly, the calculated probability that our specific gene expression dataset is from dysplastic organs of Corti is highly significant (p < 1 × 10(-12) ). Furthermore, we determined that the subnetwork has a highly heterogeneous scale-free topology in which the highly linked genes (hubs), Notch and Fgfr3, play a central role in mediating interactions among the less linked genes.

CONCLUSION

This phenomenon has important biologic and therapeutic implications.

Intramyocardial Delivery of Notch Ligand-Containing Hydrogels Improves Cardiac Function and Angiogenesis Following Infarction

Myocardial infarction (MI) is the leading cause of death worldwide. Notch1 signaling plays a critical role in cardiac development, in survival, cardiogenic lineage commitment, differentiation of cardiac stem/progenitor cells, and in regenerative responses following myocardial injury. The objective of this study was the evaluation of the therapeutic effect of delivering the Notch ligand-containing hydrogels in a rat model of MI. Self-assembling peptide (SAP) hydrogels were functionalized with a peptide mimic of the Notch1 ligand Jagged1 (RJ). In rats subjected to experimental MI, delivery of RJ-containing hydrogel to the infarcted heart resulted in improvement in cardiac function back to sham-operated levels. A significant decrease in fibrosis and an increase in the endothelial vessel area and Ki67 expression were also observed in rats treated with the RJ hydrogels compared to untreated rats or rats treated with unmodified or scrambled peptide hydrogels. This study demonstrates the functional benefit of Notch1-activating peptide delivered in SAP hydrogels for cardiac repair.

Mechanical Allostery: Evidence for a Force Requirement in the Proteolytic Activation of Notch

Ligands stimulate Notch receptors by inducing regulated intramembrane proteolysis (RIP) to produce a transcriptional effector. Notch activation requires unmasking of a metalloprotease cleavage site remote from the site of ligand binding, raising the question of how proteolytic sensitivity is achieved. Here, we show that application of physiologically relevant forces to the Notch1 regulatory switch results in sensitivity to metalloprotease cleavage, and bound ligands induce Notch signal transduction in cells only in the presence of applied mechanical force. Synthetic receptor-ligand systems that remove the native ligand-receptor interaction also activate Notch by inducing proteolysis of the regulatory switch. Together, these studies show that mechanical force exerted by signal-sending cells is required for ligand-induced Notch activation and establish that force-induced proteolysis can act as a mechanism of cellular mechanotransduction.

Notch signaling as a novel regulator of metabolism

Evolutionarily unprepared for modern high-calorie diets and sedentary lifestyles, humans are now unprecedentedly susceptible to metabolic disorders such as obesity, type 2 diabetes (T2D), nonalcoholic fatty liver, and cardiovascular disease. These metabolic conditions are intertwined, together known as metabolic syndrome, and compromise human life quality as well as lives. Notch signaling, a fundamental signal transduction pathway critical for cell-cell communication and development, has recently been recognized as a key player in metabolism. This review summarizes the emerging roles of Notch signaling in regulating the metabolism of various cell and tissue types, with emphasis on the underlying molecular mechanisms and the potential of targeting this signal axis to treat metabolic diseases.

Sequential Elution Interactome Analysis of the Mind Bomb 1 Ubiquitin Ligase Reveals a Novel Role in Dendritic Spine Outgrowth

The mind bomb 1 (Mib1) ubiquitin ligase is essential for controlling metazoan development by Notch signaling and possibly the Wnt pathway. It is also expressed in postmitotic neurons and regulates neuronal morphogenesis and synaptic activity by mechanisms that are largely unknown. We sought to comprehensively characterize the Mib1 interactome and study its potential function in neuron development utilizing a novel sequential elution strategy for affinity purification, in which Mib1 binding proteins were eluted under different stringency and then quantified by the isobaric labeling method. The strategy identified the Mib1 interactome with both deep coverage and the ability to distinguish high-affinity partners from low-affinity partners. A total of 817 proteins were identified during the Mib1 affinity purification, including 56 high-affinity partners and 335 low-affinity partners, whereas the remaining 426 proteins are likely copurified contaminants or extremely weak binding proteins. The analysis detected all previously known Mib1-interacting proteins and revealed a large number of novel components involved in Notch and Wnt pathways, endocytosis and vesicle transport, the ubiquitin-proteasome system, cellular morphogenesis, and synaptic activities. Immunofluorescence studies further showed colocalization of Mib1 with five selected proteins: the Usp9x (FAM) deubiquitinating enzyme, alpha-, beta-, and delta-catenins, and CDKL5. Mutations of CDKL5 are associated with early infantile epileptic encephalopathy-2 (EIEE2), a severe form of mental retardation. We found that the expression of Mib1 down-regulated the protein level of CDKL5 by ubiquitination, and antagonized CDKL5 function during the formation of dendritic spines. Thus, the sequential elution strategy enables biochemical characterization of protein interactomes; and Mib1 analysis provides a comprehensive interactome for investigating its role in signaling networks and neuronal development.

Inside-out Regulation of Ectodomain Cleavage of Cluster-of-Differentiation-44 (CD44) and of Neuregulin-1 Requires Substrate Dimerization

Ectodomain shedding of transmembrane precursor proteins generates numerous life-essential molecules, such as epidermal growth factor receptor ligands. This cleavage not only releases the regulatory growth factor, but it is also the required first step for the subsequent processing by γ-secretase and the release of gene regulatory intracellular fragments. Signaling within the cell modifies the cytoplasmic tails of substrates, a step important in starting the specific and regulated cleavage of a large number of studied substrates. Ectodomain cleavage occurs, however, on the outside of the plasma membrane and is carried out by membrane-bound metalloproteases. How the intracellular domain modification communicates with the ectodomain of the substrate to allow for cleavage to occur is unknown. Here, we show that homodimerization of a cluster-of-differentiation-44 or of pro-neuregulin-1 monomers represents an essential pre-condition for their regulated ectodomain cleavage. Both substrates are associated with their respective metalloproteases under both basal or cleavage-stimulated conditions. These interactions only turn productive by specific intracellular signal-induced intracellular domain modifications of the substrates, which in turn regulate metalloprotease access to the substrates' ectodomain and cleavage. We propose that substrate intracellular domain modification induces a relative rotation or other positional change of the dimerization partners that allow metalloprotease cleavage in the extracellular space. Our findings fill an important gap in understanding substrate-specific inside-out signal transfer along cleaved transmembrane proteins and suggest that substrate dimerization (homo- or possibly heterodimerization) might represent a general principle in ectodomain shedding.

Molecular Mechanoneurobiology: An Emerging Angle to Explore Neural Synaptic Functions

Neural synapses are intercellular asymmetrical junctions that transmit biochemical and biophysical information between a neuron and a target cell. They are very tight, dynamic, and well organized by many synaptic adhesion molecules, signaling receptors, ion channels, and their associated cytoskeleton that bear forces. Mechanical forces have been an emerging factor in regulating axon guidance and growth, synapse formation and plasticity in physiological and pathological brain activity. Therefore, mechanical forces are undoubtedly exerted on those synaptic molecules and modulate their functions. Here we review current progress on how mechanical forces regulate receptor-ligand interactions, protein conformations, ion channels activation, and cytoskeleton dynamics and discuss how these regulations potentially affect synapse formation, stabilization, and plasticity.

Notch1-Dll4 signalling and mechanical force regulate leader cell formation during collective cell migration

At the onset of collective cell migration, a subset of cells within an initially homogenous population acquires a distinct “leader” phenotype with characteristic morphology and motility. However, the factors driving leader cell formation as well as the mechanisms regulating leader cell density during the migration process remain to be determined. Here, we use single cell gene expression analysis and computational modeling to show that leader cell identity is dynamically regulated by Dll4 signaling through both Notch1 and cellular stress in a migrating epithelium. Time-lapse microscopy reveals that Dll4 is induced in leader cells after the creation of the cell-free region and leader cells are regulated via Notch1-Dll4 lateral inhibition. Furthermore, mechanical stress inhibits Dll4 expression and leader cell formation in the monolayer. Collectively, our findings suggest that a reduction of mechanical force near the boundary promotes Notch1-Dll4 signaling to dynamically regulate the density of leader cells during collective cell migration.

Asymmetric cell division in the Drosophila bristle lineage: from the polarization of sensory organ precursor cells to Notch-mediated binary fate decision

Asymmetric cell division (ACD) is a simple and evolutionary conserved process whereby a mother divides to generate two daughter cells with distinct developmental potentials. This process can generate cell fate diversity during development. Fate asymmetry may result from the unequal segregation of molecules and/or organelles between the two daughter cells. Here, I will review how fate asymmetry is regulated in the sensory bristle lineage in Drosophila and focus on the molecular mechanisms underlying ACD of the sensory organ precursor cells (SOPs). WIREs Dev Biol 2015, 4:299–309. doi: 10.1002/wdev.175

For further resources related to this article, please visit theWIREs website.

Conflict of interest: The author has declared no conflicts of interest for this article.

Confusing signals: recent progress in CTLA-4 biology

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Cell extrinsic and cell intrinsic mechanisms of action of CTLA-4 are unclear.

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Data suggest that the extracellular domain of CTLA4 elicits regulatory function.

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The function of CTLA-4 tail may lie in regulating localization rather than signaling.

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Membrane levels of CTLA-4 directly impact access of CD28 to shared ligands.

The mechanism of action of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) remains surprisingly unclear. Regulatory T (Treg) cells can use CTLA-4 to elicit suppression; however, CTLA-4 also operates in conventional T cells, reputedly by triggering inhibitory signals. Recently, interactions mediated via the CTLA-4 cytoplasmic domain have been shown to preferentially affect Treg cells, yet other evidence suggests that the extracellular domain of CTLA-4 is sufficient to elicit suppression. Here, we discuss these paradoxical findings in the context of CTLA-4-mediated ligand regulation. We propose that the function of CTLA-4 cytoplasmic domain is not to transmit inhibitory signals but to precisely control the turnover, cellular location, and membrane delivery of CTLA-4 to facilitate its central function: regulating the access of CD28 to their shared ligands.

Left ventricular noncompaction: a distinct cardiomyopathy or a trait shared by different cardiac diseases?

Whether left ventricular noncompaction (LVNC) is a distinct cardiomyopathy or a morphologic trait shared by different cardiomyopathies remains controversial. Current guidelines from professional organizations recommend different strategies for diagnosing and treating patients with LVNC. This state-of-the-art review discusses new insights into the basic mechanisms leading to LVNC, its clinical manifestations, treatment modalities, anatomy and pathology, embryology, genetics, epidemiology, and imaging. Three markers currently define LVNC: prominent left ventricular trabeculae, deep intertrabecular recesses, and a thin compacted layer. Although new genetic data from mice and humans supports LVNC as a distinct cardiomyopathy, evidence for LVNC as a shared morphological trait is not ruled out. Criteria supporting LVNC as a shared morphological trait may depend on consensus guidelines from the multiple professional organizations. Enhanced imaging and increased use of genetics are both predicted to significantly impact our overall understanding of the basic mechanisms causing LVNC and its optimal management.

Inhibitors of endocytosis prevent Wnt/Wingless signalling by reducing the level of basal β-catenin/Armadillo

A key step in the canonical Wnt signalling pathway is the inhibition of GSK3β, which results in the accumulation of nuclear β-catenin (also known as CTNNB1), and hence regulation of target genes. Evidence suggests that endocytosis is required for signalling, yet its role and the molecular understanding remains unclear. A recent and controversial model suggests that endocytosis contributes to Wnt signalling by causing the sequestration of the ligand-receptor complex, including LRP6 and GSK3 to multivesicular bodies (MVBs), thus preventing GSK3β from accessing β-catenin. Here, we use specific inhibitors (Dynasore and Dyngo-4a) to confirm the essential role of endocytosis in Wnt/Wingless signalling in human and Drosophila cells. However, we find no evidence that, in Drosophila cells or wing imaginal discs, LRP6/Arrow traffics to MVBs or that MVBs are required for Wnt/Wingless signalling. Moreover, we show that activation of signalling through chemical blockade of GSK3β is prevented by endocytosis inhibitors, suggesting that endocytosis impacts on Wnt/Wingless signalling downstream of the ligand-receptor complex. We propose that, through an unknown mechanism, endocytosis boosts the resting pool of β-catenin upon which GSK3β normally acts.