The STAT3-Ser/Hes3 signaling axis is a specific type of intracellular signaling pathway that regulates several fundamental properties of cells.
Cells in tissues need to be able to sense and interpret changes in their environment. For example, cells must be able to detect when they are in physical contact with other cells in order to regulate their growth and avoid the generation of tumors (“carcinogenesis”). In order to do so, cells place receptor molecules on their surface, often with a section of the receptor exposed to the outside of the cell (extracellular environment), and a section inside the cell (intracellular environment). These molecules are exposed to the environment outside of the cell and, therefore, in position to sense it. They are called receptors because when these come into contact with particular molecules (termed ligands), then chemical changes are induced to the receptor. These changes typically involve alterations in the three-dimensional shape of the receptor. These 3D structure changes affect both the extracellular and intracellular parts (domains) of the receptor. As a result, interaction of a receptor with its specific ligand which is located outside of the cell causes changes to the receptor part which is inside the cell. A signal from the extracellular space, therefore, can affect the biochemical state inside the cell.
Following receptor activation by the ligand, several steps can sequentially ensue. For example, the 3D shape changes to the intracellular domain may render it recognizable to catalytic proteins (enzymes) that are located inside the cell and have physical access to it. These enzymes may then induce chemical changes to the intracellular domain of the activated receptor, including the addition of phosphate chemical groups to specific components of the receptor (phosphorylation), or the physical separation (cleavage) of the intracellular domain. Such modifications may enable the intracellular domain to act as an enzyme itself, meaning that it may now catalyze the modification of other proteins in the cell. Enzymes which catalyze phosphorylation modifications are termed kinases. These modified proteins may then also be activated and enabled to induce further modifications to other proteins, and so on. This sequence of catalytic modifications is termed a “signal transduction pathway” or “second messenger cascade”. It is a critical mechanism employed by cells to sense their environment and induce complex changes to their state. Such changes may include, as noted, chemical modifications to other molecules, as well as decisions concerning which genes are activated and which are not (transcriptional regulation).
There are many signal transduction pathways in a cell and each of these involves many different proteins. This provides many opportunities for different signal transduction pathways to intercept (cross-talk). As a result, a cell simultaneously processes and interprets many different signals, as would be expected since the extracellular environment contains many different ligands. Cross-talk also allows the cell to integrate these many signals as opposed to process them independently. For example, mutually opposing signals may be activated at the same time by different ligands, and the cell can interpret these signals as a whole. Signal transduction pathways are widely studied in biology as they provide mechanistic understanding of how a cell operates and takes critical decisions (e.g. to multiply, move, die, activate genes etc.). These pathways also provide many drug targets and are of great relevance to drug discovery efforts.
The notch/STAT3-Ser/Hes3 signaling axis is a recently identified signal transduction branch of the notch [1] signaling pathway, originally shown to regulate the number of neural stem cells in culture and in the living adult brain. [2] [3] Pharmacological activation of this pathway opposed the progression of neurodegenerative disease in rodent models. More recent efforts have implicated it in carcinogenesis and diabetes. The pathway can be activated by soluble ligands of the notch receptor which induce the sequential activation of intracellular kinases and the subsequent phosphorylation of STAT3 on the serine residue at amino acid position 727 (STAT3-Ser). This modification is followed by an increase in the levels of Hes3, a transcription factor belonging to the Hes/Hey family of genes (see HES1). [4] Hes3 has been used as a biomarker to identify putative endogenous stem cells in tissues. [5] The pathway is an example of non-canonical signaling as it represents a new branch of a previously established signaling pathway (notch). Several efforts are currently aimed at relating this pathway to other signaling pathways and to manipulate it in a therapeutic context.
In canonical notch signaling, ligand proteins bind to the extracellular domain of the notch receptor and induce the cleavage and release of the intracellular domain into the cytoplasm. This subsequently interacts with other proteins, enters the nucleus, and regulates gene expression. [1]
In 2006, a non-canonical branch of the notch signaling pathway was discovered. [2] Using cultures of mouse neural stem cells, notch activation was shown to lead to the phosphorylation of several kinases (PI3K, Akt, mTOR) and subsequent phosphorylation of the serine residue of STAT3 in the absence of any detectable phosphorylation of the tyrosine residue of STAT3, a modification that is widely studied in the context of cancer biology. [6] Following this event, Hes3 mRNA was elevated within 30 minutes. Subsequently, the consequences of this pathway were studied.
Various inputs into this pathway have been identified. Activators include ligands of a number of receptors. Because certain signal transduction pathways oppose the STAT3-Ser/Hes3 signaling axis, blockers (inhibitors) of these signal transduction pathways promote the STAT3-Ser/Hes3 signaling axis and, therefore, also act as activators:
The effects of a particular signal transduction pathway can be very different among distinct cell types. For example, the same signal transduction pathway may promote the survival of one cell type but the maturation of another. This depends both on the nature of a cell but also on its particular state which may change over the course of its lifetime. Identifying cell types where a signal transduction pathway is operational is a first step to uncovering potentially new properties of this pathway.
The STAT3-Ser/Hes3 signaling axis has been shown to operate on various cell types. So far, research has mostly focused on stem cells and cancerous tissue and, more recently, in the function of the endocrine pancreas:
An individual signal transduction pathway can regulate several proteins (e.g. kinases) as well as the activation of many genes. The consequences to the properties of the cell can be, therefore, very prominent. Identifying these properties (through theoretical predictions and experimentation) sheds light on the function of the pathway and provides possible new therapeutic targets.
Activation of the notch/STAT3-Ser/Hes3 signaling axis has significant consequences to several cell types; effects have been documented both in vitro and in vivo:
As stated above, the STAT3-Ser/Hes3 signaling axis regulates the number of neural stem cells (as well as other cell types) in culture. This prompted experiments to determine if the same pathway can also regulate the number of naturally resident (endogenous) neural stem cells in the adult rodent brain. If so, this would generate a new experimental approach to study the effects of increasing the number of endogenous neural stem cells (eNSCs). For example, would this lead to the replacement of lost cells by newly generated cells from eNSCs? Or, could this lead to the rescue of damaged neurons in models of neurodegenerative disease, since eNSCs are known to produce factors that can protect injured neurons? [21]
Various treatments that input into the STAT3-Ser/Hes3 signaling axis (Delta4, Angiopoietin 2, insulin, or a combined treatment consisting of all three factors and an inhibitor of JAK) induce the increase in numbers of endogenous neural stem cells as well as behavioral recovery in models of neurodegenerative disease. Several pieces of evidence suggest that in the adult brain, pharmacological activation of the STAT3-Ser/Hes3 signaling axis protects compromised neurons through increased neurotrophic support provided by activated neural stem cells / neural precursor cells, which can be identified by their expression of Hes3:
The emerging understanding of the role of eNSCs in the adult mammalian brain suggested the relevance of these cells to disease. To address this issue, experiments were performed where the activation of eNSCs was induced in models of disease. This allowed the study of the consequences of activating eNSCs in the diseased brain. Several lines of evidence implicate the STAT3-Ser/Hes3 signaling axis in various diseases:
In tissues, many different cell types interact with one another. In the brain, for example, neurons, astrocytes, and oligodendrocytes (specialized cells of the neural tissue, each with specific functions) interact with one another as well as with cells that comprise blood vessels. All these different cell types may interact with all others by the production of ligands that may activate receptors on the cell surface of other cell types. Understanding the way these different cell types interact with one another will allow to predict ways of activating eNSCs. For example, because eNSCs are found in close proximity with blood vessels, it has been hypothesized that signals (e.g., ligands) from cells comprising the blood vessel act on receptors found on the cell surface of eNSCs.
Endogenous neural stem cells are often in close physical proximity to blood vessels. Signals from blood vessels regulate their interaction with stem cells and contribute to the cytoarchitecture of the tissue. The STAT3-Ser/Hes3 signaling axis operating in Hes3+ cells is a convergence point for several of these signals (e.g. Delta4, Angiopoietin 2). Hes3, in turn, by regulating the expression of Shh and potentially other factors, can also exert an effect on blood vessels and other cells comprising their microenvironment.
Janus kinase (JAK) is a family of intracellular, non-receptor tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. They were initially named "just another kinase" 1 and 2, but were ultimately published as "Janus kinase". The name is taken from the two-faced Roman god of beginnings, endings and duality, Janus, because the JAKs possess two near-identical phosphate-transferring domains. One domain exhibits the kinase activity, while the other negatively regulates the kinase activity of the first.
A neurotransmitter receptor is a membrane receptor protein that is activated by a neurotransmitter. Chemicals on the outside of the cell, such as a neurotransmitter, can bump into the cell's membrane, in which there are receptors. If a neurotransmitter bumps into its corresponding receptor, they will bind and can trigger other events to occur inside the cell. Therefore, a membrane receptor is part of the molecular machinery that allows cells to communicate with one another. A neurotransmitter receptor is a class of receptors that specifically binds with neurotransmitters as opposed to other molecules.
In cellular biology, paracrine signaling is a form of cell signaling, a type of cellular communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance, as opposed to cell signaling by endocrine factors, hormones which travel considerably longer distances via the circulatory system; juxtacrine interactions; and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.
The Notch signaling pathway is a highly conserved cell signaling system present in most animals. Mammals possess four different notch receptors, referred to as NOTCH1, NOTCH2, NOTCH3, and NOTCH4. The notch receptor is a single-pass transmembrane receptor protein. It is a hetero-oligomer composed of a large extracellular portion, which associates in a calcium-dependent, non-covalent interaction with a smaller piece of the notch protein composed of a short extracellular region, a single transmembrane-pass, and a small intracellular region.
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling pathways use either nearby cell-cell communication (paracrine) or same-cell communication (autocrine). They are highly evolutionarily conserved in animals, which means they are similar across animal species from fruit flies to humans.
A biochemical cascade, also known as a signaling cascade or signaling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus. This stimulus, known as a first messenger, acts on a receptor that is transduced to the cell interior through second messengers which amplify the signal and transfer it to effector molecules, causing the cell to respond to the initial stimulus. Most biochemical cascades are series of events, in which one event triggers the next, in a linear fashion. At each step of the signaling cascade, various controlling factors are involved to regulate cellular actions, in order to respond effectively to cues about their changing internal and external environments.
Biological crosstalk refers to instances in which one or more components of one signal transduction pathway affects another. This can be achieved through a number of ways with the most common form being crosstalk between proteins of signaling cascades. In these signal transduction pathways, there are often shared components that can interact with either pathway. A more complex instance of crosstalk can be observed with transmembrane crosstalk between the extracellular matrix (ECM) and the cytoskeleton.
In biology, cell signaling or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellular life in prokaryotes and eukaryotes. Signals that originate from outside a cell can be physical agents like mechanical pressure, voltage, temperature, light, or chemical signals. Cell signaling can occur over short or long distances, and as a result can be classified as autocrine, juxtacrine, intracrine, paracrine, or endocrine. Signaling molecules can be synthesized from various biosynthetic pathways and released through passive or active transports, or even from cell damage.
The Hedgehog signaling pathway is a signaling pathway that transmits information to embryonic cells required for proper cell differentiation. Different parts of the embryo have different concentrations of hedgehog signaling proteins. The pathway also has roles in the adult. Diseases associated with the malfunction of this pathway include cancer.
Neural stem cells (NSCs) are self-renewing, multipotent cells that firstly generate the radial glial progenitor cells that generate the neurons and glia of the nervous system of all animals during embryonic development. Some neural progenitor stem cells persist in highly restricted regions in the adult vertebrate brain and continue to produce neurons throughout life. Differences in the size of the central nervous system are among the most important distinctions between the species and thus mutations in the genes that regulate the size of the neural stem cell compartment are among the most important drivers of vertebrate evolution.
The p75 neurotrophin receptor (p75NTR) was first identified in 1973 as the low-affinity nerve growth factor receptor (LNGFR) before discovery that p75NTR bound other neurotrophins equally well as nerve growth factor. p75NTR is a neurotrophic factor receptor. Neurotrophic factor receptors bind Neurotrophins including Nerve growth factor, Neurotrophin-3, Brain-derived neurotrophic factor, and Neurotrophin-4. All neurotrophins bind to p75NTR. This also includes the immature pro-neurotrophin forms. Neurotrophic factor receptors, including p75NTR, are responsible for ensuring a proper density to target ratio of developing neurons, refining broader maps in development into precise connections. p75NTR is involved in pathways that promote neuronal survival and neuronal death.
Bone morphogenetic protein 4 is a protein that in humans is encoded by BMP4 gene. BMP4 is found on chromosome 14q22-q23.
The transforming growth factor beta (TGFB) signaling pathway is involved in many cellular processes in both the adult organism and the developing embryo including cell growth, cell differentiation, cell migration, apoptosis, cellular homeostasis and other cellular functions. The TGFB signaling pathways are conserved. In spite of the wide range of cellular processes that the TGFβ signaling pathway regulates, the process is relatively simple. TGFβ superfamily ligands bind to a type II receptor, which recruits and phosphorylates a type I receptor. The type I receptor then phosphorylates receptor-regulated SMADs (R-SMADs) which can now bind the coSMAD SMAD4. R-SMAD/coSMAD complexes accumulate in the nucleus where they act as transcription factors and participate in the regulation of target gene expression.
Stem-cell niche refers to a microenvironment, within the specific anatomic location where stem cells are found, which interacts with stem cells to regulate cell fate. The word 'niche' can be in reference to the in vivo or in vitro stem-cell microenvironment. During embryonic development, various niche factors act on embryonic stem cells to alter gene expression, and induce their proliferation or differentiation for the development of the fetus. Within the human body, stem-cell niches maintain adult stem cells in a quiescent state, but after tissue injury, the surrounding micro-environment actively signals to stem cells to promote either self-renewal or differentiation to form new tissues. Several factors are important to regulate stem-cell characteristics within the niche: cell–cell interactions between stem cells, as well as interactions between stem cells and neighbouring differentiated cells, interactions between stem cells and adhesion molecules, extracellular matrix components, the oxygen tension, growth factors, cytokines, and the physicochemical nature of the environment including the pH, ionic strength and metabolites, like ATP, are also important. The stem cells and niche may induce each other during development and reciprocally signal to maintain each other during adulthood.
Ephrins are a family of proteins that serve as the ligands of the Eph receptor. Eph receptors in turn compose the largest known subfamily of receptor protein-tyrosine kinases (RTKs).
Proteinase-activated receptor 1 (PAR1) also known as protease-activated receptor 1 or coagulation factor II (thrombin) receptor is a protein that in humans is encoded by the F2R gene. PAR1 is a G protein-coupled receptor and one of four protease-activated receptors involved in the regulation of thrombotic response. Highly expressed in platelets and endothelial cells, PAR1 plays a key role in mediating the interplay between coagulation and inflammation, which is important in the pathogenesis of inflammatory and fibrotic lung diseases. It is also involved both in disruption and maintenance of endothelial barrier integrity, through interaction with either thrombin or activated protein C, respectively.
Cell surface receptors are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving extracellular molecules. They are specialized integral membrane proteins that allow communication between the cell and the extracellular space. The extracellular molecules may be hormones, neurotransmitters, cytokines, growth factors, cell adhesion molecules, or nutrients; they react with the receptor to induce changes in the metabolism and activity of a cell. In the process of signal transduction, ligand binding affects a cascading chemical change through the cell membrane.
Gliogenesis is the generation of non-neuronal glia populations derived from multipotent neural stem cells.
Endogenous regeneration in the brain is the ability of cells to engage in the repair and regeneration process. While the brain has a limited capacity for regeneration, endogenous neural stem cells, as well as numerous pro-regenerative molecules, can participate in replacing and repairing damaged or diseased neurons and glial cells. Another benefit that can be achieved by using endogenous regeneration could be avoiding an immune response from the host.
In cellular biology, dependence receptors are proteins that mediate programmed cell death by monitoring the absence of certain trophic factors that otherwise serve as ligands (interactors) for the dependence receptors. A trophic ligand is a molecule whose protein binding stimulates cell growth, differentiation, and/or survival. Cells depend for their survival on stimulation that is mediated by various receptors and sensors, and integrated via signaling within the cell and between cells. The withdrawal of such trophic support leads to a form of cellular suicide.