2016 Group Project 1: Difference between revisions

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===What does PCP pathway do?===
===What does PCP pathway do?===
Observation of vertebrate embryos shows that a set of PCP genes have the roles of regulating convergent extension (CE). This involves the extension of the anterior–posterior (A–P) body axis, and during this time, the mediolateral (ML) axis is narrowed. This has been observed in frog (Xenopus laevis) and zebrafish (Danio rerio) embryos. The PCP pathway has many roles in mammalian development, including neural tube closure to determine the left–right (L–R) asymmetry, thus, it is vital for normal vertebrae development. Thus, if CE is affected or the process experiences any faults, neural tube closure will not occur, leading to the occurrence of spina bifida. The closure process of the neural tube occurs at different locations of the A-P axis, thus, there are two open ends of the neural tube, the head (cranial) and tail (caudal) ends, they are also called the anterior and posterior neuropores, respectively. These neuropores will eventually close that the neural tube is completely sealed. If the anterior neuropore fails to close, then anencephaly will occur. If the posterior neuropore fails to close, then spina bifida will occur. Failure to close of the entire neural tube results in craniorachischisis. Another major role of the PCP pathways is the formation of the sensory hair cells, located in the cochlea in the inner ear. There are three outer rows of hair cells, and one inner row. When the PCP pathway occurs normally, then the hairs are faced in the same direction, when there is an error in the PCP pathway, then the hairs are facing in random directions. Through technological development, some of the PCP proteins can be inactivated to determine what their true effect. There are two proteins involved in gastrulation, WnT5b and Wnt11, it allows the formation of L-R asymmetry during gastrulation, it regulates the direction of the nodal flow, which results in eliminating the bilateral symmetry. The absence of this protein results in random nodal flow, causing mutations. PCP is also required for the extension of limbs along the proximal-distal (P-D) axis. It is known that the PCP pathway is not regulated at the transcription level as none of the PCP proteins are transcription factors. But rather, it is regulated by asymmetrical protein localisation at the protein level. However, it is unknown whether the proteins in the PCP pathway act to initiate PCP, or whether they acts only as permissive signals. There are several genes associated with the PCP pathway, and each of the genes has a particular role. For example, the WnT5a gene is involved in sensory hair orientation, so that all hairs are facing the same direction. This gene also assists in neural tube closure, and if this process occurs efficiently, then birth defects will not occur. The WnT5b gene is involved in convergent extension during gastrulation. WnT9b is also involved in convergent extension, but also polarises the division of kidney epithelial cells. WnT11 is involved in convergent extension and regulates the extension of muscle fibres, as well as orienting the muscle cells.


===What does Calcium ion pathway do?===
===What does Calcium ion pathway do?===

Revision as of 22:55, 19 October 2016

2016 Student Projects 
Signalling: 1 Wnt | 2 Notch | 3 FGF Receptor | 4 Hedgehog | 5 T-box | 6 TGF-Beta
2016 Group Project Topic - Signaling in Development

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Introduction

There are 19 wnt genes and 12 Frizzled receptors found in vertebrates which are responsible for regulating many cellular activities such as proliferation and cell migration. Failure in signalling transduction or overactivation may lead to serious birth defects or post-natal diseases. Although the wnt signalling pathways are relatively highly conserved across species providing many useful experimental models for research study, its involvement of massive cellular proteins and complicated crosstalk with wnt and other cell signalling pathways have made it difficult to reveal its mechanism and function clearly.[1]

Based on the current research articles, our group has managed to briefly introduce the three main wnt signalling pathways, describing their molecular basis and functions in embryological development, and list a few animal models commonly used for research. Abnormalities caused by dysfunction of wnt signalling pathway will also be mentioned.


What is wnt ligands what is Fz receptor what is G-protein

History

Molecular Basis of Wnt signalling pathway

Wnt ligands are glycoproteins with lipid modifications. They are secreted into the extracellular space and interact with the Frizzled (Fz) receptors on the surface of the effector cells.[2] The Fz receptors are a family of seven-transmembrane spanning proteins and are coupled with G-proteins at their intracellular domain.[3] After binding with a Fz receptor, the signal is transducted downstream either through G-proteins or through other proteins also coupled with the Fz receptor such as Dishevelled (Dsh/ Dvl).[2]

Canonical Pathway

The Wnt family of secreted glycoproteins play an important role in the role of embryonic development and adult homeostasis. One method they do so is by regulating the level of transcriptional co-activator β-catenin. β-catenin can accumulate in the cytoplasm and eventually be translocated into the nucleus to form a complex with TCF (Transcription Factor) to activate transcription of Wnt target genes which are essential for development.The canonical Wnt pathway (Wnt/ β-catenin pathway) is the Wnt pathway that causes β-catenin to stay in the cytoplasm rather than be degraded as it is otherwise done through ubiquitination induced by destruction complexes. The conservation of canonical Wnt pathway is a key factor of stem cell pluripotency and cell fate decision during embryo genesis. (R) Consequently, this developmental cascade involves signals from other pathways according to the cell type and tissue it is affecting.

When the Wnt glycoprotein is secreted and binds to Frizzled receptors, a larger cell surface complex is formed in combination with LRP5/6. ZNRF3 and RNF43 are proteins that have the ability to ubiquitnate Frizzled receptors. However when R-spondins are bound to LGR5/6, Frizzled receptors are unable to be ubiquinated thus highlighting the function of R-spondins to increase sensitivity of cells to the Wnt ligand.

In the absence of Wnt

  1. Destruction complexes targeting for ubiquitination of β-catenin
    In the absence of Wnt proteins, the signalling pool of β-catenin remains at low levels as it is continuously degraded. β-catenin is targeted for ubiquitination by the Actin destruction complex which is made up of: scaffolding protein Axin, tumour suppressor adenomatosis polyposiscoli gene product (APC), protein phosphatase 2A (PP2A), glycogen synthase kinase 3 (GSK3) and casein kinase 1 (CK1). CK1 ad GSK3 have the specific roles of phosphorylating the amino terminal region of β-catenin which leads it be recognised by β-transducin repeat containing protein (βTrCP), an E3 ubiquitin ligase subunit. This pathway also known as the βTrCP/SKp pathway ultimately leads β-catenin to subsequent ubiquitination and proteasomal degradation.
  2. Destruction complex
    The actual phosphorylation of β-catenin occurs in a multiprotein complex which is made up of axin, denomatous polyposis coli (APC) and diversin. [4]
  3. APC function adn APC-Axin cross regulation

In the presence of Wnt

  1. Activation of Wnt receptors
    Wnt signaling requires both Fz and LRP6 (or LRP5) likely through a Wnt-induced Fz-LRP6 complex. Wnt-induced LRP6 phosphorylation is a key event in receptor activaiton. LRP6, LRP5 and Arrow each have five reiterated PPPSPxS motifs (P, proline; S, serine or threonine, x, a variable residue), which are essential for LRP6 function nd are each transferrable to . a heterologous receptor to result in constitutive B-catenin signalling. These dually phosphorylated PPPSPxS motifs are docking sites for the Axin complex.
  2. Inhibition of β-catenin phosphorylation
    This signalling pathway is crucial for deciding the fate of cells during early embryogenesis.Once the stabilised β-catenin enters the cell nucleus it acts as a transcriptional coactivator for transcription of Wnt-target genes. The primary family of transcription factor which β-catenin associates with is the TCF/LEF family. Activation through β-catenin is mediated with compounds such as histone acetyl transferase CBP, the chromatin-remodeling SWI/SNF complex and Bcl9 bound to pygopus (Pyg).
  3. β-catenin and TCF/LEF nuclear function

<pubmed>19279717 </pubmed>

PCP Pathway

The non-canonical, β-catenin independent pathway is also known as the Planar Cell Polarity pathway (PCP). A lot of research has been conducted on the behaviour of cells, as well as its interaction and apoptosis, however, there is a lot that remains unknown on the polarity of the cells. The polarity of some cells is knowns, for example, epithelial cells display apical-basal polarity, while neurons are classified as either receiving (dendrites) or transmitting (axons) cell signals. Mesenchymal cells also have a polarity that allows movement in different directions. PCP has several roles in the human body, many occurring during embryonic development. The roles of the pathway include the elongation of craniofacial processes, guidance of axons, organogenesis of the heart, lungs, kidneys, and eyes. The pathway also acts to regulate the polarity of cells as well as the movement of dorsal mesodermal cells at the time of convergent extension and throughout the neural tube closure process. There are two modules in place to explain the initiation and establishment of the PCP mechanism, these modules are known as the global and local modules, these are mainly based on studies performed on Drosophila.

  • The local module explains the association between the PCP proteins across the cell membrane. It also explains how cells interact with each other, and thus create a positive feedback mechanism that regulates the PCP proteins and polarisation in cells.
  • The global module explains the process by which the direction of PCP is associated with tissue axes. It is also thought that this module also regulates the Hippo pathway, whose role involves the regulation of organ size.

There is another model called the Frizzled gradient model, where a Frizzled activity gradient is formed due to a gradient of unknown signals. The signal is then utilised by the cells and compared to neighbouring cells, so that the cell can have a lower Frizzled activity in comparison to its neighbours.

The PCP pathway does not act alone, but is aided by a co-receptor, where it is though that one of the following plays a role in this pathway; use NRH1, Ryk, PTK7 or ROR2. Initially, the pathway initiates through the binding of Wnt to Fz and the co-receptor. Dishevelled proteins are attracted by the receptor involved. This protein utilises two sites on its structure, PDZ and DIX domains, which forms a linkage to the Dishevelled-associated activator of morphogenesis 1 (DAAM1). The G-protein Rho is activated by DAAM1, by exchanging the base protein, guanine. Rho, in turn, activates the Rho-associated kinase (ROCK), a cytoskeleton regulator. At the same time, the dishevelled protein binds with Rac1 and allows the binding of profilin to actin. The Rac1 initiates the role of JNL, causing actin polymerisation to occur. Also, the binding of profilin and actin causes gastrulation and a restructuring of the cytoskeleton.

Calcium Ion Pathway

The Wnt/calcium ion signalling pathway is activated by the wnt-5a family wnt ligands, including wnt-4, wnt-5a and wnt-11. The activation of this wnt pathway increases the activity of several calcium ion sensitive proteins such as CaMKII, PKC and calcineurin (CaCn/Cn).

As another pathway belongs to the wnt/non-canonical signalling pathway (the other one is the PCP pathway), beta-catenin is not involved in the signalling transduction either. Instead, the intracellular beta-catenin concentration is downregulated by the activity of wnt/calcium ion pathway. Therefore, the canonical pathway and the calcium ion pathway commonly antagonists each other and possesses opposite functions.

The calcium ion pathway is also different from the PCP pathway, however, not much. Both pathways share some common wnt ligands such as wnt-5a and wnt-11. It is note worthy that wnt-5a may also activate the canonical pathway under some conditions. Another difference between the PCP pathway and the calcium ion pathway is that different downstream effectors are involved. In PCP pathway, the main effectors are Rho GTPase and Jun-N-terminal kinase (JNK). However, cross talks between the two pathways are common especially when the common wnt ligand is used to induce the cells.[5]

The general mechanism of wnt/calcium ion signalling is described below:

  1. Binding with Fz receptor
    Wnt ligands firstly interact and physically bind to the extracellular portion of the transmembrane Fz receptors (e.g. Fz-7), which are G-protein-coupled receptors. This interaction allows the formation of a functional complex assembled by Dishevelled (Dvl/Dsh), β and γ subunits of trimeric G-protein and other proteins such as beta-Arrastin 2 (Arrb2). [6]
  2. G-protein activity
    The G-protein beta/gamma dimer formed then activates the enzyme phospholipase C (PLC) which cleaves phosphatidylinositol-4,5-bisphosphate (PIP2), which is a membrane protein, into inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 then migrates away from the cell membrane and binds to the IP3 receptor on the surface of endoplasmic reticulum (ER). The IP3 receptor functions as a calcium ion channel which activation causes release of calcium ion from ER into the cytoplasm and therefore, increases the intracellular calcium ion concentration.[7]
  3. Result of increased calcium ion concentration
    As mentioned before, increased calcium concentration leads to activation of several calcium sensitive proteins (CaMKII, CaCN and PKC). Their effects will then be discussed separately.
    • CaMKII
      CaMKII activity is modulated by calcium/calmodulin. Binding of calcium/calmodulin with the CaMKII induces a conformational change of CaMKII so that the catalytic site is now exposed towards the outside. [8]Further investigation has revealed TAK-1/NLK of being a downstream target of CaMKII following CaMKII activation. Both TAK-1 and NLK proteins were believed to belong to the mitogen-activated protein kinase (MAPK) pathway, which has been reported to be able to down regulate effect of canonical wnt pathway. As a result, it seems that wnt calcium ion pathway activates the TAK-1/NLK MAPK pathway through CaMKII and disrupts the binding of beta-catenin with TCF leading to downregulation of canonical pathway.[9] CaMKII may also activate other transcription factors such as CREB, ATF-1 and ELK-1 leading to other activities in the cell.[5]
    • CaCN
      CaCN is a protein regulated by calcium/calmodulin, Fe3+ and Zn2+ ions together[10]. Its activation may also lead to activation of ELK-1, indicating the potential cross talk between CaCN activity and CaMKII effects. More importantly, it may also activate nuclear factors of activated T-cells (NF-AT).[11] NF-AT may further activates AP1 or GATA transcription factors.[12] NF-AT itself may be exported out of the nuclear by glycogen synthase kinase 3beta (GSK3beta), jun N-terminal kinase (JNK) or p38.[13] Taken together, NF-AT is a common node for many different pathways and may interpret distinct signals making it the more important downstream effector of CaCN signalling.
    • PKC
      Protein Kinase C (PKC) proteins are a family of catalytic proteins which are able to phosphorylate and activate downstream effector proteins. They can be classified into two types, the classical PKCs and the novel PKCs. Although both types are activated by DAG, the activation of classical PKCs requires the presence of calcium ions.[14] As a result, all classical PKCs and PKC delta, which belongs to the novel PKCs, can be activated by wnt/calcium ion signalling pathway.[15] Recently, Arrb2, which forms the signalling complex with Dishevelled and β and γ subunits of trimeric G-protein, is also involved in activation and translocation of PKC alpha to the cellular membrane, indicating that location might also be important for the signalling downstream of PKC activation.[6]


Overall, it seems that wnt/calcium ion pathway is mainly responsible for inhibition of TCF/beta-catenin signalling through CaMKII, determination of ventral fate through CaCN and tissue separation through PKC. However, the complicated cross talks between wnt/calcium ion pathway components, between wnt pathways and between wnt and other pathways make the in depth research on wnt signalling a rather difficult work.

Wnt signalling in Embryonic Development

What does canonical pathway do?

  • Wnt family of signalling proteins plays various roles in fetus development of embryogenesis.
  • Wnt signals are pleiotropic (when a gene has effects on two or more seemingly unrelated phenotypic traits). The signals have effects on mitogenic stimulation, cell fate specification and differentiation.
  • The canonical Wnt signalling pathway determines much of cell fate during embryogenesis.
  • A lot of information about the role of Wnt canonical pathway in fetus development was found through study of the Xenopus system. (Xenopus is a type of highly aquatic frog that has been commonly studied as a model organism)

Spermann-Mangold Organizer

  • Once fertilization has taken place, in order for the signalling centre known as the Spemann-Mangold Organizer to be formed at the dorsal side of the embryo, a dorsalizing factor is moved to the future dorsal side by the process of cortical rotation.
  • Dsh protein and other elements of the Wnt pathway that lead to stabilization of B-catenin reach the future dorsal side of early Xenopus embryo via cortical rotation.
  • Until recently it remained unclear whether a Wnt ligand caused the formation of this dorsal organizer or whether the Wnt signalling cascade was activated intracellularly without the need for a ligand. However recent studies have proved that Wnt11 in fact is the Wnt ligand responsible for this action in early dorsal axis formation.
  • In early development of Xenopus, B-catenin/TCF complex has also been found to promote transcription of Twin and Siamosis which encode homeodomain transcription factors. Both Twin and Siamosis are crticial for the expression of organizer specific genes.
  • Therefore from the data gathered, it can be concluded that the canonical pathway is required for dorsal axis formation during early development.

Anterior head formation and neural patterning

  • The canonical Wnt pathway also regulates anterior head formation and neural patterning
  • A number of Wnt inhibitors (such as Cerberus, WIF, Dickkopf and Frzb) are expressed in and secreted from the Spemann-Mangold Organizer to control the formation of the anterior of the embryo and also promote anterior head formation.
  • The inhibitory proteins physically bind Wnt and prevent Wnt/Fx complez from being formed. This leads to to low levels of nuclear B-catenein protein in the anterior region and a higher level within posterior region of the gastrula embryo.
  • This gradient of Wnt signal along anteroposterior axis has been found to be essential for the formation of anterior head structure and neuroectodermal pattering.
  • However canonical Wnt signalling also plays a role in controlling of posterior patterning and tail formation as well as various organ systems such as the heart, lungs, kidney, skin and bone.
  • Finally, Wnt signalling also has recently been found to have a role in stem cell renewal
  • Thus is can be concluded that the canonical Wnt pathway impacts formation of all organ systems during embryogenesis, whether that be in direct or indirect ways.

<pubmed>15473860 </pubmed>

What does PCP pathway do?

Observation of vertebrate embryos shows that a set of PCP genes have the roles of regulating convergent extension (CE). This involves the extension of the anterior–posterior (A–P) body axis, and during this time, the mediolateral (ML) axis is narrowed. This has been observed in frog (Xenopus laevis) and zebrafish (Danio rerio) embryos. The PCP pathway has many roles in mammalian development, including neural tube closure to determine the left–right (L–R) asymmetry, thus, it is vital for normal vertebrae development. Thus, if CE is affected or the process experiences any faults, neural tube closure will not occur, leading to the occurrence of spina bifida. The closure process of the neural tube occurs at different locations of the A-P axis, thus, there are two open ends of the neural tube, the head (cranial) and tail (caudal) ends, they are also called the anterior and posterior neuropores, respectively. These neuropores will eventually close that the neural tube is completely sealed. If the anterior neuropore fails to close, then anencephaly will occur. If the posterior neuropore fails to close, then spina bifida will occur. Failure to close of the entire neural tube results in craniorachischisis. Another major role of the PCP pathways is the formation of the sensory hair cells, located in the cochlea in the inner ear. There are three outer rows of hair cells, and one inner row. When the PCP pathway occurs normally, then the hairs are faced in the same direction, when there is an error in the PCP pathway, then the hairs are facing in random directions. Through technological development, some of the PCP proteins can be inactivated to determine what their true effect. There are two proteins involved in gastrulation, WnT5b and Wnt11, it allows the formation of L-R asymmetry during gastrulation, it regulates the direction of the nodal flow, which results in eliminating the bilateral symmetry. The absence of this protein results in random nodal flow, causing mutations. PCP is also required for the extension of limbs along the proximal-distal (P-D) axis. It is known that the PCP pathway is not regulated at the transcription level as none of the PCP proteins are transcription factors. But rather, it is regulated by asymmetrical protein localisation at the protein level. However, it is unknown whether the proteins in the PCP pathway act to initiate PCP, or whether they acts only as permissive signals. There are several genes associated with the PCP pathway, and each of the genes has a particular role. For example, the WnT5a gene is involved in sensory hair orientation, so that all hairs are facing the same direction. This gene also assists in neural tube closure, and if this process occurs efficiently, then birth defects will not occur. The WnT5b gene is involved in convergent extension during gastrulation. WnT9b is also involved in convergent extension, but also polarises the division of kidney epithelial cells. WnT11 is involved in convergent extension and regulates the extension of muscle fibres, as well as orienting the muscle cells.

What does Calcium ion pathway do?

  1. Body Axis
    • Promoting involution during xenopus gastrulation
      Binding of wnt-11 with Fz-7 receptor activates canonical, PCP and calcium ion signalling pathways in the experimental xenopus embryos. Signalling of the calcium ion pathway is done through the G-protein coupled with Fz-7 and PKC-alpha. The cells making up the blastocoel roof (BCR) and the anterior mesoderm are possessing the same cadherin molecules. Moreover, there is no physical barrier between those two cell populations. Knocking down of Fz-7 results in failure in activating the wnt/calcium ion signalling pathway and fusion of two cell layers, which then leads to severe gastrulation defects. Moreover, mesodermal cells with Fz-7 activated remains separated with the endodermal BCR, while cells without Fz-7 activation eventually sink down to the BCR and fuse with the cells there. Therefore, it seems that Fz-7 activation is also related with mesodermal cell fate determination.[16]
    • Promoting Convergent extension (CE)
      Research literatures have revealed that wnt-11 can also promote convergent extension in xenopus embryos. This signalling transduction is also conducted through a functional complex formed with Dishevelled (Dvl), beta-arrestin 2 (Arrb2) and beta and gamma-subunits of the G-protein coupled with Fz-7 receptor. This complex can then activate CaMKII and PKC-alpha. Arrb2 may also induce the translocation of PKC-alpha to the cellular membrane. Cdc42 protein, which can modify the actin skeleton of the cells, has also been shown to be a downstream target of PKC. Taken together, it seems that the result of the wnt-11 signalling is to promote the migration of cells which is critical for the CE movement in xenopus during gastrulation.[6]
  2. Neural Crest Formation and ESC differentiation into osteogenic lineage
    In vitro experiments with mouse embryonic stem cells have revealed that wnt-5a signalling through the calcium ion signalling pathway might be related to the formation of cranial neural crest and determination of the cell fate. Although wnt-5a is able to activate the canonical and/or calcium ion signalling pathways, it is shown that nuclear beta-catenin stabilization was decreased following wnt-5a induction. Beta-catenin has generally been considered as an essential factor for stem cell differentiation to the osteogenic lineage. However, based on the data presented, early beta-catenin induction (day 5-7) actually down regulates the differentiation. While wnt-5a induction, which elevate the intracellular calcium ion concentration, upregulates the expression of several osteogenic markers and increases calcification at this early stage. The condition becomes opposite later. From day 7-9, further wnt-5a induction results in decreased degree of differentiation while wnt-3a (canonical pathway ligand) induction is able to antagonist the calcium ion pathway and promotes differentiation. It seems that wnt/calcium ion signalling pathway is more likely to be related with the embryonic stem cell fate decision (towards a osteogenic lineage) at the early stage rather than guiding the whole differentiation process. How wnt-5a signalling is related with neural crest formation was not mentioned in the literature, however, the mechanism can be expected to be similar to the mechanism taking place in xenopus gastrulation, since both CaMKII and PKC are activated in the experimental cells. [17]
  3. Muscle development
    Experimental results have suggested that wnt-5a (stage 18) was expressed earlier than wnt-11 (late stage 22) during the development of limbs in the chicken model. It seems that wnt/canonical pathway, which can be activated by wnt-3a affects the number of the terminal muscle fibers, whereas wnt-5a and wnt-11 promotes the cell fate of those muscle fibers. Moreover, it is believed that wnt-3a and wnt-11 act in an opposite manner with wnt-3a decreasing the number of fast muscle fibers and increasing the number of slow muscle fibers and wnt-11 decreasing and increasing the number of slow and fast fibers respectively. Both in vivo and in vitro results have favoured this conclusion. However, the degree of influence in in vivo experiment is more moderate than that of the in vitro experiment, suggesting that epigenetic influences may also play a role in determining the muscle cell fate.[18]
  4. Vascular development
    It is reported that wnt-5a is able to inhibit the proliferation of human hematopoietic stem cells (HSC) probably acting through BCL-2 and expression of Cdknib gene. Moreover, the canonical wnt signalling pathway is also inhibited following wnt-5a induction. It seems that on one hand, wnt-3a activates the canonical signalling pathway and promotes the proliferation of human HSCs, and on the other hand, wnt-5a may antagonist the effect of wnt-3a and keep HSCs at the quiescent phase. A hypothesis has also been proposed mentioning that wnt-5a not only silences the proliferation, but also induces self-renewal of the HSCs at the same time, leading to a larger proliferative capacity. [19]

Experimental Models

Mainly talking about advantages and disadvantages of each models, like can this model mimic the true condition in Human? cost, ethical issues,

Drosophila

Human

Mouse

Xenopus

Diseases related with wnt dysfunction

  1. Wnt calcium ion signalling pathway dysfunction may lead to cardiac hypertrophy. Experimental results have revealed that in experimental mouse model which possesses a phenotype of cardiac hypertrophy, wnt calcium ion signalling pathway is activated and further induces the activation of CaMKII. As a result, the cellular concentration of Histone deacetylase 4 (HDAC4) is signaficantly decreased. HDAC4 functions as a suppressor of myosin enhancer factor 2 (MEF2), and its down-regulation results in increased MEF2 activity which then leads to cardiac hypertrophy.[20]

Glossary

Quiz/ summary table/ summary graph

Topic: WnT Signaling Pathway in skin of fetus

Things to do/ Reference

Friday 9/9/16: 1. Identify all the components of each pathway. (Can possibly look into the similarities vs differences/ interactions between the pathways. 2. Research into how each pathway contributes to fetal devleopment in different body. (Can look into which fetal part we can focus on e.g. skin) 3. Look for picture/diagrams/graphs etc.

Let's get some more information up by next Thursday, 15/9/16

Friday 16/9/16: Good resource: Omim.org (Contains alot of information about different genes and every signalling pathways. How to reference?)

The signalling pathways:

Canonical Pathway - Gloria

    • The Wnt family of secreted glycolipoproteins play an important role in the role of embryonic development and adult homeostasis.
    • They do so via the transcription of β-catenin, which accumulates in the cytoplasm and eventually gets translocated into the nucleus to act as a transcriptional coactivator of transcription factors that belong to the TCF/LEF family.
    • The canonical Wnt pathway of (Wnt/ β-catenin pathway) is the Wnt pathway that causes β-catenin to stay in the cytoplasm rather than be degraded through ubiquitination which is induced by destruction complexes. The destruction complexes which sends β-catenin to proteasome for digestion are proteins such as: Axin, adenomatosis, polyposiscoli (APC), protein phosphatase 2A (PP2A), glycogen synthase kinase 3 (GSK3) and casein kinase 1α (CK1α).
    • Once the stabilised β-catenin enters the cell nucleus it acts as a transcriptional coactivator for transcription of Wnt-target genes. The primary family of transcription factor which β-catenin associates with is the TCF/LEF family. Activation through β-catenin is mediated with compounds such as histone acetyl transferase CBP, the chromatin-remodeling SWI/SNF complex and Bcl9 bound to pygopus (Pyg).
    • This signalling pathway is crucial for deciding the fate of cells during early embryogenesis.


<pubmed>19279717 </pubmed>

Canonical Pathway: How it works - Gloria
  • Wnt family of signalling proteins plays various roles in fetus development of embryogenesis.
  • Wnt signals are pleiotropic (when a gene has effects on two or more seemingly unrelated phenotypic traits). The signals have effects on mitogenic stimulation, cell fate specification and differentiation.
  • In the canonical pathway, Wnt ligands bind to frizzle receptors that are at the cell surface.
  • Due to this activation of Wnt signalling, B-catenin, which is normally degraded within the cell, starts to accumulate in the cell and ultimately the nucleus.
  • At normal levels of B-catenin, the protein binds at the intracellular side of the membrane with cadherins to promote cell adhesion and also controls cell shape through actin microfilament cytoskeletal network.
  • However at elevated level of this protein, activation of transcription occurs alongside co-transcription factors such as action of Lefs/Tcfs.

<pubmed>15473860 </pubmed>

Canonical Pathway & Embryogenesis - Gloria

Non-Canonical Pathway - Caroline

Pathways
  • The Wnt signal pathway consists of proteins that assist cell communication by passing the signal to the cell surface receptors.
  • The general process of the pathway involves binding the protein to a Frizzled family receptor, this then passes the signal to the disheveled protein located on the interior of the cell
  • The non-canonical pathway can also be referred to as the beta-catenin independent pathway, due to the absence of β-Catenin.
  • The non-canonical pathway can be divided into two pathways, one of them is known as the Planar Cell Polarity pathway or the PCP pathway, and the other is known as the Wnt Calcium pathway
Role
  • The non-canonical Wnt pathway regulates cell polarity and movements of dorsal mesodermal cells during convergent extension and later during neural tube closure.
  • Studies suggest that the non-canonical pathway has an impact on the expression of early cardiac genes. The non-canonical pathway affects the histone deacetylase (HDAC) activity, which in turn is affected by CaMKII, which is necessary for the expression of the cardiac genes. Thus, any discrepancy in the non-canonical pathway would result in a discrepancy in the normal cardiac development
PCP Pathway
  • The PCP pathway was discovered through genetic studies in Drosophila. It was found that mutations in Wnt signaling resulted in a randomised orientation of epithelial structures.
  • During vertebrate gastrulation, the mesodermal and ectodermal cells undergo convergent extension. Polarized cells will thus intercalate along the mediolateral axis, resulting in mediolateral narrowing (convergent) and anteroposterior elongation (extension).
  • The PCP signaling pathway has another role involving the cell-contact mediated neural crest cell guidance
  • PTK7 is a protein coding gene and has been linked to the non-canonical Wnt PCP pathway, this allows and encourages movement of the cells and tissues
Studies
  • A study shows the importance of the non-canonical pathway in the migration of the epithelial cells throughout the embryo to allow further embryonic development
  • Another study has been performed on the embryos of Xenopus laevis (the clawed frog), to determine what the non-canonical Wnt signaling pathway does. Due to the size of the embryo, it could be easily manipulated to provide the answers needed.


PMID 20576942 <pubmed>19279717</pubmed> <pubmed>27101101</pubmed> <pubmed>26680417</pubmed> <pubmed>26555387 </pubmed> <pubmed> 26499793</pubmed> <pubmed>17045694</pubmed>

<pubmed> 26035863</pubmed>

<pubmed> 25732825</pubmed>

<pubmed> 25428587</pubmed>

<pubmed> 25448697 </pubmed>

<pubmed> 25540130</pubmed>

<pubmed> 25410658</pubmed>

<pubmed> 25266145</pubmed>

WnT-Calcium Ion Pathway - Tony

  • PMID 21903638
    • WNT5 ligands were specifically studied since calcium ions are secondary signaling molecules of wnt5. During gastrulation, wnt/beta-catenin pathway is switched on to promote migration of cells, while wnt/calcium pathway is later turned on to stop its effect. Wnt signalling pathway is especially important in guiding the movement of embryonic cells, and wnt/calcium pathway is especially active during gastrulation process.
    • Drosophila and zebrafish models are frequently used to study wnt singalling pathway, due to the fact that wnt signaling is quite conserved across species.
    • Wnt/calcium pathway has also been know to be able to alter the signalling transduction of wnt ligand through other receptors.

How does it work in a foetus (skin formation) - Arsalan

Wnt signalling is an important component of skin formation in the very early stages of foetus development. Once the embryo undergoes gastrulation, cells of the ectoderm will form differentiate to form the epidermis and the mesoderm will form the dermis.

WnT signalling strongly influences the decision of the ectoderm layer specifying as nervous system or skin epithelium. WnT signalling inhibits the ectoderm’s responsiveness to the fibroblast growth factors. This inhibition is essential for the ectodermal cells to be able to express bone morphogenetic proteins that result in blocking neural induction and directing the cells to differentiate into keratin expressing cells that form the epidermis.

In terms of the mesoderm WnT signals are important for the process of somitogenesis, in which the mesoderm segments into somites that eventually form the dermis. WnT signals will also instruct the lateral plate mesoderm and somite derived cells to create mesenchymal cells which are essential cells within the dermis.

<pubmed>PMC3552514</pubmed>

===What can go wrong?=== z341763

Cells need to communicate with each other so they can act in a coordinated manner in response to the environment. Communication occurs through signalling pathways which, stimulate, inhibit and coordinate the behaviour of cells to grow or divide during the appropriate times. This can hence become a very pedantic and intricate process and little changes can interrupt the entire system. When things go wrong in signalling, cancer can happen.

In most normal cells the Wnt pathway is inactive and the beta catenin is destroyed and gene transcription is inhibited. In tumour cells, the Wnt pathway may be activated despite the absence of a Wnt signal. This is a result of a mutation of a gene that carries code for a protein complex and hence disintegrates the protein complex. The beta catenin is now no longer tagged for destruction and its cellular level continues to increase. This is very similar to a normal cell where the pathway was actually activated by the Wnt Signal. Beta catenin reaches the nucleus and activates the TCF and LEF transcription factors which activates an RNA polymerase and transcription of several genes begin. This however is inappropriate and uncontrolled and hence leads to cancer. One of the most common mutations is the mutation of the APC protein and is directly linked with the development of colon cancer. People who inherit defective APC alleles’ develop large numbers of polyps which become malignant adenocarcinoma(insertfootnote). People who have mutant APC have nearly a 100% chance of developing colon cancer by the age of 40 years.

Reference(im not sure how to reference things that arent from pubmed) http://www.cell.com/cell/fulltext/S0092-8674(06)01344-4 Wnt/β-Catenin Signaling in Development and Disease Clevers, Hans Cell , Volume 127 , Issue 3 , 469 - 480

References

PMID 21903638

Glossary

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