L11 Long Range Control of Gene Expression: WNT Signaling PDF

L11 Long range control of gene expression: WNT signalling Learning outcomes: understand of the concept of regulating gene expression by the secreted family of signalling proteins: WNT discuss the details the molecular events that are required to transduce an external signal (i....

L11 Long range control of gene expression: WNT signalling Learning outcomes: understand of the concept of regulating gene expression by the secreted family of signalling proteins: WNT discuss the details the molecular events that are required to transduce an external signal (i.e. extracellular) into gene expression response in the nucleus will be reviewed be familiar with the concept and details of the internal regulation of this process will covered as well as examples of where this type of signalling is employed put these concepts will be put into a biological context by exploring the impact of mutations and their consequences in key steps of WNT signalling Understand of the concept of regulating gene expression by the secreted family of signalling proteins: WNT. WNT ligands are morphogens (act long-range, induce cell responses based on concentration, thus result in pattern formation). Examples of WNT action include: self-renewal of stem cells, proliferation, differentiation, tissue patterning. Well-studied tissues include neural, intestinal, epidermal. The Wnt signalling 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 signalling pathways use either nearby cell-cell communication (paracrine) or same-cell communication (autocrine).  There are 19 mammalian Wnt genes. In every pathway, activation occurs after a Wnt protein ligand binds to a frizzled (FZD) receptor, which activates the dishevelled protein inside the cell. There are 10 known human FZD receptors. Ultimately, activation of the Wnt pathway leads to regulation of gene expression. Wnt and its associated genes were one of the first examples of a gene involved in development and activated in cancer. In the canonical pathway, activation of the pathway prevents degradation of beta catenin. The destruction complex, consisting of proteins such as GSK3 and Axin, ubiquitinates beta catenin which targets it for degradation in the lysosome. When un-ubiquitinated, beta catenin enters the nucleus and displaced groucho bound to TCF. Beta catenin binds to TCF and activates it. TCF/LEF Factors switch from being in a repressor complex to an activator complex upon b-catenin binding Target genes of TCF include axin 2 and lef1, therefore creating a negative feedback loop for Wnt signalling. Discuss the details the molecular events that are required to transduce an external signal (i.e. extracellular) into gene expression response in the nucleus. Key events in WNT pathway (canonical) Wnt binds to frizzled (7 loop amino acids) and then to LRP in the membrane on extracellular domain. This binding activates dishevelled on the intracellular domain.  Dishevelled stabilises the destruction complex, meaning that it can no longer function to ubiquitinate beta catenin. Beta catenin is translocated to the nucleus, displaces groucho and initiates TCF/LEF transcription factor activity. Be familiar with the concept and details of the internal regulation of this process as well as examples of where this type of signalling is employed. Factors which regulate Wnt signalling There are several factors that determine whether or not WNT can bind to the membrane or have effects on gene expression. Control over WNT protein processing: WNT proteins modified by palmitoylation and glycosylation. Porcupine (porc) responsible for palmitoylation and possibly ER transport and secretion. You cannot secrete WNT without porcupine. Wntless/Wls/Evi are membrane proteins involved in WNT secretion There is no evidence that the function of genes like porcupine and Wls/Evi is restricted to particular cells, implying that once Wnt genes are expressed, their proteins will be made in any cell type. Wnt antagonists Wnt antagonists tend to bind directly to WNTs to modulate their pathway activity.  Frizzled related proteins (FRPs) resemble frizzled receptors but they are not localised to the membrane, so they reduce the concentration of WNT to bind to their real targets. WNT inhibitory factors (WIFs) bind to WNTs to stop them having their usual action. Dickkopf (Dkk) binds to and causes endocytosis of LRP so antagonises WNT binding. ZNFR3 and RNF43 are antagonists of WNT as they lead to WNT internalisation RSpondin is an agonist of WNT which inhibits ZNFR3 or RNF43. Wnt component degeneracy There are 19 WNTS and 10 FZDs which can interact with each other. Having many isoforms means that WNT signalling still happens even if one of the WNTs or receptors are mutated. Put these concepts into a biological context by exploring key functions of WNT and the impact of mutations and their consequences in key steps of WNT signalling. WNT role in cell cycle regulation WNT has roles in G1 and S phase progression. WNT inhibits GSK3, therefore removing its inhibitory effects on cell cycle components and genes e.g. cyclin d and e. APC and Dvl regulate the attachment of the mitotic spindle to the kinetochores, and together with Fzd and LRP6 modulate spindle orientation. GSK3, β‐catenin, and Axin2 are required at the centrosome to ensure a proper distribution of the  chromosomes during division. Wnt/GSK3 signalling promotes microtubule assembly by tau stabilization. Long range functions of WNT during homeostasis Cells upregulating the WNT pathway are located close to SOX2+ stem cells SOX2+ stem cells secrete paracrine WNT ligands that promote proliferation in neighbouring committed cells in the pituitary gland WNT signalling induces differentiation of stem cells. When there is Wls deletion in sox2, hypoplasia is observed. Paracrine signalling of WNT ligands from stem cells is conserved in stem cells of the adrenal medulla Key point: WNT signalling doesn't have the same effects in every cell WNT signalling in cancer WNT involved in cancer when mutated or overexpressed. Activation of the WNT pathway through b-catenin stabilisation during tooth development can lead to tumours composed of hundreds of small teeth This is not cancer, but is an indication that you’re more susceptible to cancer. Mutations in stem cells that secrete WNT signals next to tumours can cause tumours to reform. WNT signalling during development WNTs are posteriorising signals during embryonic development Ensuring the anterior forebrain does not acquire a posterior identity through perceiving long-range posteriorising signals Expression of factors that prevent WNT upregulation within the tissue. Antagonists secreted from underlying tissue Absence of TCF`3 which normally represses WNT targets results in no head being formed. WNT is required at all stages of mammary gland development for patterning, branching and proliferation