CDK1 and CDK2 regulate NICD1 turnover and the periodicity of the segmentation clock

Cellular and molecular control of vertebrate somitogenesis

Segmentation is a fundamental feature of the vertebrate body plan. This metameric organization is first implemented by somitogenesis in the early embryo, when paired epithelial blocks called somites are rhythmically formed to flank the neural tube. Recent advances in in vitro models have offered new opportunities to elucidate the mechanisms that underlie somitogenesis. Notably, models derived from human pluripotent stem cells introduced an efficient proxy for studying this process during human development. In this Review, we summarize the current understanding of somitogenesis gained from both in vivo studies and in vitro studies. We deconstruct the spatiotemporal dynamics of somitogenesis into four distinct modules: dynamic events in the presomitic mesoderm, segmental determination, somite anteroposterior polarity patterning, and epithelial morphogenesis. We first focus on the segmentation clock, as well as signalling and metabolic gradients along the tissue, before discussing the clock and wavefront and other models that account for segmental determination. We then detail the molecular and cellular mechanisms of anteroposterior polarity patterning and somite epithelialization.

Timing mechanisms: insights from comparative neural differentiation systems

Temporal control is central to deploy and coordinate genetic programs during development. At present, there is limited understanding of the molecular mechanisms that govern the duration and speed of developmental processes. Timing mechanisms may run in parallel and/or interact with each other to integrate temporal signals throughout the organism. In this piece, we consider findings on the extrinsic control of developmental tempo and discuss the intrinsic roles of cell cycle, metabolic rates, protein turnover, and post-transcriptional mechanisms in the regulation of tempo during neural development.

FBP1 inhibits NSCLC stemness by promoting ubiquitination of Notch1 intracellular domain and accelerating degradation

The existence of cancer stem cells is widely acknowledged as the underlying cause for the challenging curability and high relapse rates observed in various tumor types, including non-small cell lung cancer (NSCLC). Despite extensive research on numerous therapeutic targets for NSCLC treatment, the strategies to effectively combat NSCLC stemness and achieve a definitive cure are still not well defined. The primary objective of this study was to examine the underlying mechanism through which Fructose-1,6-bisphosphatase 1 (FBP1), a gluconeogenic enzyme, functions as a tumor suppressor to regulate the stemness of NSCLC. Herein, we showed that overexpression of FBP1 led to a decrease in the proportion of CD133-positive cells, weakened tumorigenicity, and decreased expression of stemness factors. FBP1 inhibited the activation of Notch signaling, while it had no impact on the transcription level of Notch 1 intracellular domain (NICD1). Instead, FBP1 interacted with NICD1 and the E3 ubiquitin ligase FBXW7 to facilitate the degradation of NICD1 through the ubiquitin-proteasome pathway, which is independent of the metabolic enzymatic activity of FBP1. The aforementioned studies suggest that targeting the FBP1-FBXW7-NICD1 axis holds promise as a therapeutic approach for addressing the challenges of NSCLC recurrence and drug resistance.

Uncovering NOTCH1 as a Promising Target in the Treatment of MLL-Rearranged Leukemia

MLL rearrangement (MLLr) is responsible for the development of acute leukemias with poor outcomes. Therefore, new therapeutic approaches are urgently needed. The NOTCH1 pathway plays a critical role in the pathogenesis of many cancers including acute leukemia. Using a CRISPR/Cas9 MLL-AF4/-AF9 translocation model, the newly developed NOTCH1 inhibitor CAD204520 with less toxic side effects allowed us to unravel the impact of NOTCH1 as a pathogenic driver and potential therapeutic target in MLLr leukemia. RNA sequencing (RNA-seq) and RT-qPCR of our MLLr model and MLLr cell lines showed the NOTCH1 pathway was overexpressed and activated. Strikingly, we confirmed this elevated expression level in leukemia patients. We also demonstrated that CAD204520 treatment of MLLr cells significantly reduces NOTCH1 and its target genes as well as NOTCH1 receptor expression. This was not observed with a comparable cytarabine treatment, indicating the specificity of the small molecule. Accordingly, treatment with CAD204520 resulted in dose-dependent reduced proliferation and viability, increased apoptosis, and the induction of cell cycle arrest via the downregulation of MLL and NOTCH1 target genes. In conclusion, our findings uncover the oncogenic relevance of the NOTCH1 pathway in MLLr leukemia. Its inhibition leads to specific anti-leukemic effects and paves the way for further evaluation in clinical settings.

The Efficacy of CB-103, a First-in-Class Transcriptional Notch Inhibitor, in Preclinical Models of Breast Cancer

BACKGROUND

The efficacy of CB-103 was evaluated in preclinical models of both ER+ and TNBC. Furthermore, the therapeutic efficacy of combining CB-103 with fulvestrant in ER+ BC and paclitaxel in TNBC was determined.

METHODS

CB-103 was screened in combination with a panel of anti-neoplastic drugs. We evaluated the anti-tumor activity of CB-103 with fulvestrant in ESR1-mutant (Y537S), endocrine-resistant BC xenografts. In the same model, we examined anti-CSC activity in mammosphere formation assays for CB-103 alone or in combination with fulvestrant or palbociclib. We also evaluated the effect of CB-103 plus paclitaxel on primary tumors and CSC in a GSI-resistant TNBC model HCC1187. Comparisons between groups were performed with a two-sided unpaired Students' t-test. A one-way or two-way ANOVA followed by Tukey's post-analysis was performed to analyze the in vivo efficacy study results.

THE RESULTS

CB-103 showed synergism with fulvestrant in ER+ cells and paclitaxel in TNBC cells. CB-103 combined with fulvestrant or paclitaxel potently inhibited mammosphere formation in both models. Combination of CB-103 and fulvestrant significantly reduced tumor volume in an ESR1-mutant, the endocrine-resistant BC model. In a GSI-resistant TNBC model, CB-103 plus paclitaxel significantly delayed tumor growth compared to paclitaxel alone.

CONCLUSION

our data indicate that CB-103 is an attractive candidate for clinical investigation in endocrine-resistant, recurrent breast cancers with biomarker-confirmed Notch activity in combination with SERDs and/or CDKis and in TNBCs with biomarker-confirmed Notch activity in combination with taxane-containing chemotherapy regimens.

Hydroxylation of the NOTCH1 intracellular domain regulates Notch signaling dynamics

Notch signaling plays a pivotal role in the development and, when dysregulated, it contributes to tumorigenesis. The amplitude and duration of the Notch response depend on the posttranslational modifications (PTMs) of the activated NOTCH receptor - the NOTCH intracellular domain (NICD). In normoxic conditions, the hydroxylase FIH (factor inhibiting HIF) catalyzes the hydroxylation of two asparagine residues of the NICD. Here, we investigate how Notch-dependent gene transcription is regulated by hypoxia in progenitor T cells. We show that the majority of Notch target genes are downregulated upon hypoxia. Using a hydroxyl-specific NOTCH1 antibody we demonstrate that FIH-mediated NICD1 hydroxylation is reduced upon hypoxia or treatment with the hydroxylase inhibitor dimethyloxalylglycine (DMOG). We find that a hydroxylation-resistant NICD1 mutant is functionally impaired and more ubiquitinated. Interestingly, we also observe that the NICD1-deubiquitinating enzyme USP10 is downregulated upon hypoxia. Moreover, the interaction between the hydroxylation-defective NICD1 mutant and USP10 is significantly reduced compared to the NICD1 wild-type counterpart. Together, our data suggest that FIH hydroxylates NICD1 in normoxic conditions, leading to the recruitment of USP10 and subsequent NICD1 deubiquitination and stabilization. In hypoxia, this regulatory loop is disrupted, causing a dampened Notch response.

Bilateral Feedback in Oscillator Model Is Required to Explain the Coupling Dynamics of Hes1 with the Cell Cycle

Biological processes are governed by the expression of proteins, and for some proteins, their level of expression can fluctuate periodically over time (i.e., they oscillate). Many oscillatory proteins (e.g., cell cycle proteins and those from the HES family of transcription factors) are connected in complex ways, often within large networks. This complexity can be elucidated by developing intuitive mathematical models that describe the underlying critical aspects of the relationships between these processes. Here, we provide a mathematical explanation of a recently discovered biological phenomenon: the phasic position of the gene Hes1’s oscillatory expression at the beginning of the cell cycle of an individual human breast cancer stem cell can have a predictive value on how long that cell will take to complete a cell cycle. We use a two-component model of coupled oscillators to represent Hes1 and the cell cycle in the same cell with minimal assumptions. Inputting only the initial phase angles, we show that this model is capable of predicting the dynamic mitosis to mitosis behaviour of Hes1 and predicting cell cycle length patterns as found in real-world experimental data. Moreover, we discover that bidirectional coupling between Hes1 and the cell cycle is critical within the system for the data to be reproduced and that nonfixed asymmetry in the interactions between the oscillators is required. The phase dynamics we present here capture the complex interplay between Hes1 and the cell cycle, helping to explain nongenetic cell cycle variability, which has critical implications in cancer treatment contexts.

ATR activity controls stem cell quiescence via the cyclin F-SCF complex

A key property of adult stem cells is their ability to persist in a quiescent state for prolonged periods of time. The quiescent state is thought to contribute to stem cell resilience by limiting accumulation of DNA replication–associated mutations. Moreover, cellular stress response factors are thought to play a role in maintaining quiescence and stem cell integrity. We utilized muscle stem cells (MuSCs) as a model of quiescent stem cells and find that the replication stress response protein, ATR (Ataxia Telangiectasia and Rad3-Related), is abundant and active in quiescent but not activated MuSCs. Concurrently, MuSCs display punctate RPA (replication protein A) and R-loop foci, both key triggers for ATR activation. To discern the role of ATR in MuSCs, we generated MuSC-specific ATR conditional knockout (ATRcKO) mice. Surprisingly, ATR ablation results in increased MuSC quiescence exit. Phosphoproteomic analysis of ATRcKO MuSCs reveals enrichment of phosphorylated cyclin F, a key component of the Skp1–Cul1–F-box protein (SCF) ubiquitin ligase complex and regulator of key cell-cycle transition factors, such as the E2F family of transcription factors. Knocking down cyclin F or inhibiting the SCF complex results in E2F1 accumulation and in MuSCs exiting quiescence, similar to ATR-deficient MuSCs. The loss of ATR could be counteracted by inhibiting casein kinase 2 (CK2), the kinase responsible for phosphorylating cyclin F. We propose a model in which MuSCs express cell-cycle progression factors but ATR, in coordination with the cyclin F–SCF complex, represses premature stem cell quiescence exit via ubiquitin–proteasome degradation of these factors.

Notch controls the cell cycle to define leader versus follower identities during collective cell migration

Coordination of cell proliferation and migration is fundamental for life, and its dysregulation has catastrophic consequences, such as cancer. How cell cycle progression affects migration, and vice versa, remains largely unknown. We address these questions by combining in silico modelling and in vivo experimentation in the zebrafish trunk neural crest (TNC). TNC migrate collectively, forming chains with a leader cell directing the movement of trailing followers. We show that the acquisition of migratory identity is autonomously controlled by Notch signalling in TNC. High Notch activity defines leaders, while low Notch determines followers. Moreover, cell cycle progression is required for TNC migration and is regulated by Notch. Cells with low Notch activity stay longer in G1 and become followers, while leaders with high Notch activity quickly undergo G1/S transition and remain in S-phase longer. In conclusion, TNC migratory identities are defined through the interaction of Notch signalling and cell cycle progression.

Signalling dynamics in embryonic development

In multicellular organisms, cellular behaviour is tightly regulated to allow proper embryonic development and maintenance of adult tissue. A critical component in this control is the communication between cells via signalling pathways, as errors in intercellular communication can induce developmental defects or diseases such as cancer. It has become clear over the last years that signalling is not static but varies in activity over time. Feedback mechanisms present in every signalling pathway lead to diverse dynamic phenotypes, such as transient activation, signal ramping or oscillations, occurring in a cell type- and stage-dependent manner. In cells, such dynamics can exert various functions that allow organisms to develop in a robust and reproducible way. Here, we focus on Erk, Wnt and Notch signalling pathways, which are dynamic in several tissue types and organisms, including the periodic segmentation of vertebrate embryos, and are often dysregulated in cancer. We will discuss how biochemical processes influence their dynamics and how these impact on cellular behaviour within multicellular systems.

Notch signaling in reproduction

The Notch signaling pathway is conserved among mammalian species and controls proliferation, differentiation, and cell death in many organs throughout the body including the reproductive tract. Notch signaling plays critical roles in the development and function of both the male and female reproductive systems. Specifically, within the female reproductive tract, Notch signaling is hormone regulated and mediates key reproductive events important for ovarian and uterine function. In this review, we highlight the tissues that express Notch receptors, ligands, and downstream effectors and distinguish how these molecules regulate reproductive function in male and female mice, non-human primates, and humans. Finally, we describe some of the aberrations in Notch signaling in female reproductive pathologies and identify opportunities for future investigation.

Cell cycle arrest determines adult neural stem cell ontogeny by an embryonic Notch-nonoscillatory Hey1 module

Quiescent neural stem cells (NSCs) in the adult mouse brain are the source of neurogenesis that regulates innate and adaptive behaviors. Adult NSCs in the subventricular zone are derived from a subpopulation of embryonic neural stem-progenitor cells (NPCs) that is characterized by a slower cell cycle relative to the more abundant rapid cycling NPCs that build the brain. Yet, how slow cell cycle can cause the establishment of adult NSCs remains largely unknown. Here, we demonstrate that Notch and an effector Hey1 form a module that is upregulated by cell cycle arrest in slowly dividing NPCs. In contrast to the oscillatory expression of the Notch effectors Hes1 and Hes5 in fast cycling progenitors, Hey1 displays a non-oscillatory stationary expression pattern and contributes to the long-term maintenance of NSCs. These findings reveal a novel division of labor in Notch effectors where cell cycle rate biases effector selection and cell fate.

Adult neural stem cells are derived from an embryonic population of slowcycling progenitor cells, though how reduced cycling speed leads to establishment of the adult population has remained elusive. Here they show that non-oscillatory Notch-Hey signaling induced by slow-cycling contributes to long term maintenance of neural stem cells.

Differential phase register of Hes1 oscillations with mitoses underlies cell-cycle heterogeneity in ER+ breast cancer cells

Tumors exhibit heterogeneities that are not due to mutations, including cancer stem cells with different potencies. We show that the cancer stem-cell state predisposed to dormancy in vivo has a highly variable and long cell cycle. Using single-cell live imaging for the transcriptional repressor Hes1 (a key molecule in cancer), we show a type of circadian-like oscillatory expression of Hes1 in all cells in the population. The most potent cancer stem cells tend to divide around the trough of the Hes1 oscillatory wave, a feature predictive of a long cell cycle. A concept proposed here is that the position of cell division with respect to the Hes1 wave is predictive of its prospective cell-cycle length and cancer cellular substate.

Here, we study the dynamical expression of endogenously labeled Hes1, a transcriptional repressor implicated in controlling cell proliferation, to understand how cell-cycle length heterogeneity is generated in estrogen receptor (ER)⁺ breast cancer cells. We find that Hes1 shows oscillatory expression with ∼25 h periodicity and during each cell cycle has a variable peak in G1, a trough around G1–S transition, and a less variable second peak in G2/M. Compared to other subpopulations, the cell cycle in CD44^HighCD24^Low cancer stem cells is longest and most variable. Most cells divide around the peak of the Hes1 expression wave, but preceding mitoses in slow dividing CD44^HighCD24^Low cells appear phase-shifted, resulting in a late-onset Hes1 peak in G1. The position, duration, and shape of this peak, rather than the Hes1 expression levels, are good predictors of cell-cycle length. Diminishing Hes1 oscillations by enforcing sustained expression slows down the cell cycle, impairs proliferation, abolishes the dynamic expression of p21, and increases the percentage of CD44^HighCD24^Low cells. Reciprocally, blocking the cell cycle causes an elongation of Hes1 periodicity, suggesting a bidirectional interaction of the Hes1 oscillator and the cell cycle. We propose that Hes1 oscillations are functionally important for the efficient progression of the cell cycle and that the position of mitosis in relation to the Hes1 wave underlies cell-cycle length heterogeneity in cancer cell subpopulations.

Supervised learning based on tumor imaging and biopsy transcriptomics predicts response of hepatocellular carcinoma to transarterial chemoembolization

Although transarterial chemoembolization (TACE) is the most widely used treatment for intermediate-stage, unresectable hepatocellular carcinoma (HCC), it is only effective in a subset of patients. In this study, we combine clinical, radiological, and genomics data in supervised machine-learning models toward the development of a clinically applicable predictive classifier of response to TACE in HCC patients. Our study consists of a discovery cohort of 33 tumors through which we identify predictive biomarkers, which are confirmed in a validation cohort. We find that radiological assessment of tumor area and several transcriptomic signatures, primarily the expression of FAM111B and HPRT1, are most predictive of response to TACE. Logistic regression decision support models consisting of tumor area and RNA-seq gene expression estimates for FAM111B and HPRT1 yield a predictive accuracy of ∼90%. Reverse transcription droplet digital PCR (RT-ddPCR) confirms these genes in combination with tumor area as a predictive classifier for response to TACE.

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Tumor imaging and transcriptomics enables patient selection for good response to TACE

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PRETACE is a LR model based on tumor area and expression of FAM111B and HPRT1

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PRETACE predicts response to TACE with ∼90% accuracy

Biophysics of Notch Signaling

Notch signaling is a conserved system of communication between adjacent cells, influencing numerous cell fate decisions in the development of multicellular organisms. Aberrant signaling is also implicated in many human pathologies. At its core, Notch has a mechanotransduction module that decodes receptor-ligand engagement at the cell surface under force to permit proteolytic cleavage of the receptor, leading to the release of the Notch intracellular domain (NICD). NICD enters the nucleus and acts as a transcriptional effector to regulate expression of Notch-responsive genes. In this article, we review and integrate current understanding of the detailed molecular basis for Notch signal transduction, highlighting quantitative, structural, and dynamic features of this developmentally central signaling mechanism. We discuss the implications of this mechanistic understanding for the functionality of the signaling pathway in different molecular and cellular contexts.

NOTCH signaling pathway is required for bovine early embryonic development†

The NOTCH signaling pathway plays an important role in regulating various biological processes, including lineage specification and apoptosis. Multiple components of the NOTCH pathway have been identified in mammalian preimplantation embryos. However, the precise role of the NOTCH pathway in early embryonic development is poorly understood, especially in large animals. Here, we show that the expression of genes encoding key transcripts of the NOTCH pathway is dynamic throughout early embryonic development. We also confirm the presence of active NOTCH1 and RBPJ. By using pharmacological and RNA interference tools, we demonstrate that the NOTCH pathway is required for the proper development of bovine early embryos. This functional consequence could be partly attributed to the major transcriptional mediator, Recombination Signal Binding Protein For Immunoglobulin Kappa J Region (RBPJ), whose deficiency also compromised the embryo quality. Indeed, both NOTCH1 and RBPJ knockdown cause a significant increase of histone H3 serine 10 phosphorylation (pH3S10, a mitosis marker) positive blastomeres, suggesting a cell cycle arrest at mitosis. Importantly, RNA sequencing analyses reveal that either NOTCH1 or RBPJ depletion triggers a reduction in H1FOO that encodes the oocyte-specific linker histone H1 variant. Interestingly, depleting H1FOO results in detrimental effects on the developmental competence of early embryos, similar with NOTCH1 inhibition. Overall, our results reveal a crucial role for NOTCH pathway in regulating bovine preimplantation development, likely by controlling cell proliferation and maintaining H1FOO expression.

Targeting Notch in oncology: the path forward

Notch signalling is involved in many aspects of cancer biology, including angiogenesis, tumour immunity and the maintenance of cancer stem-like cells. In addition, Notch can function as an oncogene and a tumour suppressor in different cancers and in different cell populations within the same tumour. Despite promising preclinical results and early-phase clinical trials, the goal of developing safe, effective, tumour-selective Notch-targeting agents for clinical use remains elusive. However, our continually improving understanding of Notch signalling in specific cancers, individual cancer cases and different cell populations, as well as crosstalk between pathways, is aiding the discovery and development of novel investigational Notch-targeted therapeutics.

Notch Pathway: A Journey from Notching Phenotypes to Cancer Immunotherapy

Notch is a key evolutionary conserved pathway, which has fascinated and engaged the work of investigators in an uncountable number of biological fields, from development of metazoans to immunotherapy for cancer. The study of Notch has greatly contributed to the understanding of cancer biology and a substantial effort has been spent in designing Notch-targeting therapies. Due to its broad involvement in cancer, targeting Notch would allow to virtually modulate any aspect of the disease. However, this means that Notch-based therapies must be highly specific to avoid off-target effects. This review will present the newest mechanistic and therapeutic advances in the Notch field and discuss the promises and challenges of this constantly evolving field.

The cellular and molecular landscape of hypothalamic patterning and differentiation from embryonic to late postnatal development

The hypothalamus is a central regulator of many innate behaviors essential for survival, but the molecular mechanisms controlling hypothalamic patterning and cell fate specification are poorly understood. To identify genes that control hypothalamic development, we have used single-cell RNA sequencing (scRNA-Seq) to profile mouse hypothalamic gene expression across 12 developmental time points between embryonic day 10 and postnatal day 45. This identified genes that delineated clear developmental trajectories for all major hypothalamic cell types, and readily distinguished major regional subdivisions of the developing hypothalamus. By using our developmental dataset, we were able to rapidly annotate previously unidentified clusters from existing scRNA-Seq datasets collected during development and to identify the developmental origins of major neuronal populations of the ventromedial hypothalamus. We further show that our approach can rapidly and comprehensively characterize mutants that have altered hypothalamic patterning, identifying Nkx2.1 as a negative regulator of prethalamic identity. These data serve as a resource for further studies of hypothalamic development, physiology, and dysfunction.

Waiting on the Fringe: cell autonomy and signaling delays in segmentation clocks

The rhythmic and sequential segmentation of the vertebrate body axis into somites during embryogenesis is governed by a multicellular, oscillatory patterning system called the segmentation clock. Despite many overt similarities between vertebrates, differences in genetic and dynamic regulation have been reported, raising intriguing questions about the evolution and conservation of this fundamental patterning process. Recent studies have brought insights into two important and related issues: (1) whether individual cells of segmentation clocks are autonomous oscillators or require cell-cell communication for their rhythm; and (2) the role of delays in the cell-cell communication that synchronizes the population of genetic oscillators. Although molecular details differ between species, conservation may exist at the level of the dynamics, hinting at rules for evolutionary trajectories in the system.

APEC infection affects cytokine-cytokine receptor interaction and cell cycle pathways in chicken trachea

Avian pathogenic Escherichia coli (APEC) can lead to extraintestinal disease in avian species via respiratory tract infection. However, the regulatory mechanism of APEC on the pathogenicity of chicken trachea epithelium remains unknown. In this study, we examined pathological changes in chicken trachea at different infection times (4, 8, 12 and 24 h). The RNA sequencing of APEC infection group and the PBS group (negative control) of chicken trachea epithelium were analysed. Our studies revealed that the oedema, heterophil infiltration and hyperaemia appeared at 8 and 12 h post APEC infection. And the hyperaemia phenomenon and heterophilic granulocyte infiltration disappeared at 24 h post infection. Then RNA sequencing showed many genes were dynamically expressed in the APEC infection group. At 4, 8 and 12 h post infection, the mRNA of differentially expressed genes were enriched by cytokine-cytokine receptor interaction and the toll-like receptor signalling pathway. The cell cycle pathway was enriched at 24 h post infection. Altogether, these findings suggest that APEC infection induces pathological change in the chicken trachea, the mRNA of differentially expressed genes participating in inflammation and hyperplasia signalling pathways. Which not only provide more evidence for regulatory mechanism of APEC on the pathogenicity of chicken trachea epithelium, but also facilitate the effective management of APEC infections in poultry through trachea.

Jag1 Modulates an Oscillatory Dll1-Notch-Hes1 Signaling Module to Coordinate Growth and Fate of Pancreatic Progenitors

Notch signaling controls proliferation of multipotent pancreatic progenitor cells (MPCs) and their segregation into bipotent progenitors (BPs) and unipotent pro-acinar cells (PACs). Here, we showed that fast ultradian oscillations of the ligand Dll1 and the transcriptional effector Hes1 were crucial for MPC expansion, and changes in Hes1 oscillation parameters were associated with selective adoption of BP or PAC fate. Conversely, Jag1, a uniformly expressed ligand, restrained MPC growth. However, when its expression later segregated to PACs, Jag1 became critical for the specification of all but the most proximal BPs, and BPs were entirely lost in Jag1; Dll1 double mutants. Anatomically, ductal morphogenesis and organ architecture are minimally perturbed in Jag1 mutants until later stages, when ductal remodeling fails, and signs of acinar-to-ductal metaplasia appear. Our study thus uncovers that oscillating Notch activity in the developing pancreas, modulated by Jag1, is required to coordinate MPC growth and fate.

Decoding the PTM-switchboard of Notch

The developmentally indispensable Notch pathway exhibits a high grade of pleiotropism in its biological output. Emerging evidence supports the notion of post-translational modifications (PTMs) as a modus operandi controlling dynamic fine-tuning of Notch activity. Although, the intricacy of Notch post-translational regulation, as well as how these modifications lead to multiples of divergent Notch phenotypes is still largely unknown, numerous studies show a correlation between the site of modification and the output. These include glycosylation of the extracellular domain of Notch modulating ligand binding, and phosphorylation of the PEST domain controlling half-life of the intracellular domain of Notch. Furthermore, several reports show that multiple PTMs can act in concert, or compete for the same sites to drive opposite outputs. However, further investigation of the complex PTM crosstalk is required for a complete understanding of the PTM-mediated Notch switchboard. In this review, we aim to provide a consistent and up-to-date summary of the currently known PTMs acting on the Notch signaling pathway, their functions in different contexts, as well as explore their implications in physiology and disease. Furthermore, we give an overview of the present state of PTM research methodology, and allude to a future with PTM-targeted Notch therapeutics.

A tale of two clocks: phosphorylation of NICD by CDKs links cell cycle and segmentation clock

The Notch signaling pathway is tightly controlled via post-transcriptional regulatory mechanisms that promote or terminate pathway activity. In this issue, Carrieri et al [1] show that phosphorylation of the Notch intracellular domain (NICD) by cyclin-dependent kinases (CDKs) suppresses Notch activity by promoting NICD turnover. These findings link Notch pathway activity to the cell cycle, and the authors propose connections between this regulation and the segmentation clock that times embryonic somitogenesis.

CDK1 and CDK2 regulate NICD1 turnover and the periodicity of the segmentation clock

All vertebrates share a segmented body axis. Segments form from the rostral end of the presomitic mesoderm (PSM) with a periodicity that is regulated by the segmentation clock. The segmentation clock is a molecular oscillator that exhibits dynamic clock gene expression across the PSM with a periodicity that matches somite formation. Notch signalling is crucial to this process. Altering Notch intracellular domain (NICD) stability affects both the clock period and somite size. However, the mechanism by which NICD stability is regulated in this context is unclear. We identified a highly conserved site crucial for NICD recognition by the SCF E3 ligase, which targets NICD for degradation. We demonstrate both CDK1 and CDK2 can phosphorylate NICD in the domain where this crucial residue lies and that NICD levels vary in a cell cycle-dependent manner. Inhibiting CDK1 or CDK2 activity increases NICD levels both in vitro and in vivo, leading to a delay of clock gene oscillations and an increase in somite size.