Lecture 21-24: Cell Signaling PDF

Introduction- Cell Signaling. Ch 16 Cells must respond adequately to external stimuli to survive. Cells respond to stimuli via cell signaling. Cell signaling is involved in the regulation of cell growth and division. Cell signaling affects virtually every aspect of cell struc...

Introduction- Cell Signaling. Ch 16 Cells must respond adequately to external stimuli to survive. Cells respond to stimuli via cell signaling. Cell signaling is involved in the regulation of cell growth and division. Cell signaling affects virtually every aspect of cell structure and function. Some signal molecules enter cells; others bind to cell-surface receptors. Introduction Each Cell Is Programmed to Respond to Specific Combinations of Extracellular Signals Loading… Figure 16.1 Modes of cell–cell signaling (A) direct cell–cell contacts (B) the action of secreted signaling molecules. Endocrine: hormones are carried through the circulatory system to act on distant target cells. Paracrine: a molecule released from one cell acts locally to affect nearby target cells. Autocrine: a cell produces a signaling molecule to which it also responds. Extracellular Signals Can Act Over Short or Long Distances Loading… Extracellular Signal Molecules Bind to Specific Receptors  In general, cells can only respond to a signal if they possess the correct receptor Types of Receptors Receptor types include: – Steroid hormone receptors (nuclear receptor superfamily) – G-protein coupled receptors (GPCRs) – Receptor protein-tyrosine kinases (RTKs) – Non-Receptor protein-tyrosine kinases – Ligand gated channels – Specific receptors such as B-and T-cell receptors Figure 16.2 Structure of steroid hormones, thyroid hormone, vitamin D3, and retinoic acid Cross the PM. Can you tell from the structure? Bind to members of the nuclear receptor superfamily. nuclear receptor superfamily members are transcription factors with domains for ligand binding, DNA binding, transcriptional activation Figure 16.3 Glucocorticoid action (ex cortisol) Glucocorticoids diffuse across the PM. Binds to the glucocorticoid receptor, displaces Hsp90. Receptor dimers form Receptors translocate to the nucleus, bind DNA, and associate with coactivators with histone acetyltransferase (HAT) activity Transcription of target genes occurs Figure 16.4 Gene regulation by the thyroid hormone receptor Thyroid hormone receptor binds DNA in the presence or absence of hormone. In the absence of hormone, the receptor associates with corepressors with histone deacetylase (HDAC) activity. Loading… In the presence of hormone, the receptor associates with coactivators with histone acetyltransferase (HAT) activity. Figure 16.5 Other signaling molecules Small signaling molecules Peptides (EGF, insulin, endorphins, growth factors, others ) EGF Most signals bind to cell-surface receptors  2 Types of signaling pathways: General properties Most signaling pathways Are not linear, but branched and interconnected Produce second Cytoplasmic messengers domain converted to recruiting station for Small molecules that signaling proteins Activate or inactivate specific proteins Common Elements of Cell Signaling Systems: kinase cascades are common to signal transduction Signaling pathways consist of a series of proteins. – Each protein in a pathway alters the conformation of the next protein. – Protein conformation is commonly altered by phosphorylation. – Kinases add phosphate groups while phosphatases remove them. – Target proteins ultimately receive a message to alter cell activity. – This overall process is called signal transduction. Common Elements of Cell Signaling Systems: What does phosphorylation do? Protein phosphorylation can change protein behavior in different ways. – It can activate or inactivate an enzyme. – It can increase or decrease protein-protein interactions. – It can change the subcellular location of the protein. – It can trigger protein degradation. – Phosphorylation patterns differ between cell types Besides kinases/phosphatases, GTP-binding proteins are also common to signaling pathways Note check 1. Proteins in the nuclear receptor superfamily are transcription factors that contain domains for ligand binding, DNA binding, and transcriptional activation. Sketch a figure that shows how the glucocorticoid receptor regulates gene expression, indicating those domains. 2. Among the modes of cell-to-cell signaling, what is unique about steroid signaling? 3. What are some categories of proteins that are common to signal transduction pathways? Section 16.2 G Proteins and Cyclic AMP Types of Receptors Receptor types include: – Steroid hormone receptors (nuclear receptor superfamily) – G-protein coupled receptors (GPCRs) – Receptor protein-tyrosine kinases (RTKs) – Non-receptor protein-tyrosine kinases – Ligand gated channels – Specific receptors such as B-and T-cell receptors Figure 17.7 Structure of a G protein-coupled receptor (GPCR) Largest family of cell-surface receptors The odorant receptor protein family The amino acids that are conserved in other odorant receptor clones are shown in white and the more variable amino acids in black. FIGURE 16.8 Hormonal activation of adenylyl cyclase Binding of hormone promotes the interaction of the receptor with a G protein (heterotrimeric). The activated G protein α subunit dissociates from the GPCR and stimulates adenylyl cyclase (AC). AC catalyzes the conversion of ATP to cAMP. Figure 16.9 Regulation of G proteins Details of GPCR activation and deactivation Figure 16.10 Synthesis and degradation of cAMP- a second messenger Many GPCR’s activate AC to produce cAMP signaling *CAMP activates protein kinase A (PKA) FIGURE 16.11 Regulation of glycogen metabolism by epinephrine An example of GPCR mediated signaling. Fight or flight response: release glucose into blood stream Epinephrine binding leads to G protein-mediated activation of adenylyl cyclase. cAMP activates protein kinase A, which consists of two regulatory (R) and two catalytic (C) subunits in its inactive form. Binding of cAMP to the regulatory subunits leads to dissociation of the catalytic subunits, which are then enzymatically active. Protein kinase A phosphorylates and activates phosphorylase kinase Phosphorylase kinase activates glycogen phosphorylase, which catalyzes the breakdown of glycogen to glucose-1-phosphate. What is glycogen? Signal amplification occurs during signal transduction. Video 16.3 Figure 17.12 Cyclic AMP-inducible gene expression Another role for PKA… The free catalytic subunit of protein kinase A translocates to the nucleus Phosphorylates the transcription factor CREB (CRE-binding protein) Leads to recruitment of coactivators and expression of cAMP-inducible genes. Figure 17.13 Regulation of protein phosphorylation by protein kinase A and protein phosphatase 1 Protein phosphorylation is reversed by dephosphorylation (via phosphatases) Note check Explain the role of adenylate cyclase in signaling. Describe the steps in the activation/deactivation of heterotrimeric G-proteins (16.9) Types of Receptors Receptor types include: – Steroid hormone receptors (nuclear receptor superfamily) – G-protein coupled receptors (GPCRs) – Receptor protein-tyrosine kinases (RTKs) – – Loading… Non-Receptor protein-tyrosine kinases Ligand gated channels – Specific receptors such as B-and T-cell receptors  2 Types of signaling pathways: General properties Most signaling pathways Are not linear, but branched and interconnected Produce second Cytoplasmic messengers domain converted to recruiting station for Small molecules that signaling proteins Activate or inactivate specific proteins 16.3 Receptor Tyrosine Kinases and non-receptor tyrosine kinases Figure 16.14 Receptor tyrosine kinase activation The receptors for most growth factors are tyrosine kinases. Growth factor (signal) binding induces receptor dimerization, which results in receptor autophosphorylation as the two polypeptide chains cross-phosphorylate one another. Figure 16.15 Association of downstream signaling molecules with receptor tyrosine kinases SH2 domains of signalling proteins bind to specific phosphotyrosine-containing sequences of the activated receptors. Figure 16.16 Activation of nonreceptor tyrosine kinases Confusing name: these are receptors similar to RTK’s, but kinase activity is not intrinsic to the receptor, but in an associated protein kinase. Ligand binding induces dimerization of receptor with its associated (nonreceptor) tyrosine kinases Dimerization leads to the activation of the associated nonreceptor tyrosine kinases via cross-phosphorylation. The activated kinases then phosphorylate tyrosine residues of the receptor, creating phosphotyrosine-binding sites for downstream signaling molecules. Figure 16.17 The JAK/STAT pathway- signalling in blood cells and immune cells An example… The STAT proteins are transcription factors with SH2 domains In unstimulated cells, STAT proteins are inactive in the cytosol. Stimulation of cytokine receptors leads to the binding of STAT proteins to phosphotyrosine- binding sites on the receptor, where they are phosphorylated by the receptor-associated JAK tyrosine kinases. The phosphorylated STAT proteins then dimerize and translocate to the nucleus, where they activate the transcription of target genes. Figure 16.18 Integrin signaling via FAK – a non-receptor tyrosine kinase Integrins link cells to ECM. Also serve as receptors to activate signaling pathways Can control movement and other aspects of cell behaviour in response to cell- matrix interactions. Binding of integrins to the extracellular matrix leads to integrin clustering and activation of the nonreceptor tyrosine kinase FAK by autophosphorylation. Src binds to the FAK autophosphorylation site and phosphorylates FAK on additional tyrosine residues Those serve as binding sites for additional downstream signaling molecules. Note check 1. Explain this figure to your classmate. 2. Differentiate between RTK and non-RTK