As a crucial cell signaling pathway in metazoan development, the Notch signaling pathway controls cell fate decisions in many different tissues in multicellular organisms. The Notch signaling pathway is composed of the Notch receptor, the Notch ligand (DSL protein), the CSL (CBF-1, Suppressor of hairless, a combination of Lag) DNA-binding protein, other effectors, and the regulatory molecule of Notch. In 1917, Morgan discovered the Notch gene in mutant Drosophila, which was named because a partial loss of function in the gene that caused a nick on the edge of the fruit fly. Mammals have four different Notch receptors (Notchl-4) and five Notch ligands (Delta-like 1, 3, 4, Jagged 1 and Jagged 2). The receptor subsequently trafﬁcs to the cell surface to interact with its ligands. And ligands are type I transmembrane proteins with a large extracellular domain and a relatively short intracellular domain. The ligand binds to the receptor at the interface between the two signaling cells. This leads to the release of the intracellular portion of the receptor that translocates into the nucleus, and interacts with a transcription factor and coactivators to activate transcription. The Notch extracellular domain undergoes its ﬁrst proteolytic cleavage (S1) in the Golgi complex by furin-like proteases. This processing is thought to contribute to the net signaling activity by facilitating exocytosis of Notch. Upon ligand–receptor interactions, the ligand–receptor complex becomes endocytosed into the signal sending cell (trans-endocytosis) creating a “pulling force” that leads to a conformational change that promotes receptor activation. This leads to the second (S2) cleavage of the Notch receptor. The second cleavage releases the intracellular domain of Notch, which translocates to the nucleus. In the signal-receiving cell, γ-secretase (also involved in Alzheimer’s disease) releases the NICD from the TM (S3 cleavage), which allows for nuclear translocation where it associates with the CSL family of transcriptional regulators and forms part of a Notch target gene–activating complex. So it can activate the target genes of the basichelix-loop-helix (bHLH) transcriptional repressor family and exert biological effect.
Fig. 1 Notch signaling pathway and agents in clinical development
Cross-talk between Notch and other signaling pathways
The Notch signaling pathway regulates cellular identity, proliferation, differentiation and apoptosis by means of cell–cell interactions. It also plays a crucial role in a wide array of developmental processes, including the regulation of neurogenesis, myogenesis, angiogenesis, hematopoiesis and epithelial to mesenchymal transition, and tissue homeostasis. Notch signaling has recently been shown to interact with several of the other key signaling mechanisms. Notch displays functional interactions with other signaling pathways in sequential cell fate assignations, such as Wnt signaling. And Notch interacts with the Ras/MAP kinase signaling pathway, although the outcome of this interaction appears to vary depending on the cellular context. Ras activates Notch in Ras-transformed human cultured cells, and Notch is required to maintain a neoplastic phenotype caused by Ras signaling. Interplay between Notch and TGF-β / BMP signaling has been identified. It was observed that the cell cycle-inhibitory effects of TGF-β in cultured cells could be overridden by Notch ICD. A direct interaction between Notch ICD and SMAD3 (intracellular transducer of TGF- β signaling) was demonstrated where activation of TGF-β leads to an increase in expression of the Hes1 gene. Similar cross-talk occurs also between Notch and BMP.
Fig. 2 Simpliﬁed scheme of Notch interactions with other signaling pathways in cancer
Fine-Tuning & Relationship with Disease and Development
Notch signaling regulates many aspects of metazoan development and tissue renewal. Accordingly, loss or the misregulation of Notch signaling underlies a wide range of human disorders. Recent human genetic and genomic studies have led to the discovery that Notch plays a role in numerous diseases, from developmental syndromes to adult-onset diseases such as congenital disorders, stroke, and especially cancer.
Notch signaling has been extensively characterized as a regulator of cell-fate decisions in a variety of organisms and tissues. In most cases, Notch signaling acts as a “gatekeeper against differentiation” .It can block differentiation towards a primary differentiation fate in a cell and instead directs the cell to a second, alternative differentiation program or forces the cell to remain in an undifferentiated state. Therefore, dysregulated Notch signaling causes several diseases with underlying developmental defects. Mutations in Notch signaling pathway members cause developmental phenotypes that affect the liver, skeleton, heart, eye, face, kidney, and vasculature. Notch associated disorders involves aberrant cellular differentiation or tissue development, including T cell acute lymphoblastic leukemia (T-ALL), Alagille syndrome, spondylocostal dysostosis, tetralogy of Fallot, CADASIL syndrome, and aortic valve disease. Drugs that are being developed based on these structural studies will not only be useful in the clinics but also in labs to study different biological and pathological events that are regulated by Notch signaling. Small molecules against the γ-secretase complex and monoclonal antibodies against Notch receptors and ligands are currently available and are useful to pharmacologically manipulate Notch activity.
It is conceivable that appropriate manipulation of Notch signaling may become a useful tool in addressing a variety of human aging conditions as well as tissue regeneration. Notch signaling is involved in the speciﬁcation, proliferation, and migration of endothelial tip and stalk cells including the coordination of multiple aspects of endothelial behavior during vessel patterning. A decline in capillary density and blood flow with age is a major cause of mortality and morbidity. Understanding why this occurs is key to future gains in human health. NAD precursors reverse aspects of aging, in part, by activating sirtuin deacylases (SIRT1–SIRT7) that mediate the beneﬁts of exercise and dietary restriction (DR). Endothelial SIRT1 regulates proangiogenic signals secreted from myocytes and improves muscle health. Treatment of mice with NAD precursor nicotinamide mononucleotide improves vascular and increases endurance in aging mice by regulating the Notch signaling pathway. Recent ﬁndings have implications for improving blood ﬂow to organs and tissues, increasing human performance, and reestablishing a virtuous cycle of mobility in the elderly.
In sum, Notch signaling can be thought of as a “double-edged sword” that needs to be constantly monitored and carefully controlled for proper activity. Continuous efforts to uncover novel genes and proteins that regulate and fine-tune Notch signaling are important as they may provide specific targets for drugs. In the past decades of years, many studies brought about a revolution in a large number of fields, including developmental and stem cell biology, neuroscience, and cancer biology. The pathway inhibitors used include antibodies against NOTCH receptors and DLL ligands, GSIs, and small peptides that inhibit formation of the transcriptional complex. Two major classes of Notch inhibitors are currently in early clinical development: γ-secretase inhibitors (GSIs) and monoclonal antibodies (mAbs) against Notch receptors or ligands.
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