Cell Communication_ Signaling I by hcj


									Cell Communication: Signaling I Dr. Jan Kitajewski (jkk9) Relevant reading: ECB2, Ch.16 p.533-60, 564-9 September 16, 2004 Transcriber: Chrisanne Botello (cb105) “Cell Communication”= one organ or cell talking to another Basic Concepts: I. Modes of Communication  Local vs. Long-Range Cell Communication

II. General Types/Classes of Signaling Receptors:   cell surface receptors intracellular receptors

III. Categories of Cell Surface Receptors:    ion channel-linked receptors G-protein-linked receptors Kinase-based (tyrosine) receptors (a.k.a. enzyme-linked receptors)

IV. Different Types of Signaling Pathways and Mechanisms of Signal Transduction:     Small, diffusible molecules Steroid hormones (acting on nuclear receptors) Peptide growth factors (acting on cell surface receptors) Cell-cell contact interactions (“contact-dependent”)

V. Output of Cell Communication:  Altered cell behavior, either through altered Gene Expression (slow), or directly via altered protein function (fast). (Fig. 16-23)

VI. Diverse Cell Responses are controlled by signaling pathways:       Proliferation Phagocytosis Apoptosis Secretion Exocytosis Migration

I. Modes of communication between animal cells: (Fig. 16-3) Local vs. Long Range: A. Endocrine secretion - specialized cell in one organ creates a signal that travels through the bloodstream and acts on its target at a distant site; usually transmitted by hormones. The signal can either interact with a cell surface receptor (peptide hormones) or make their way into the cell directly (steroid hormones). Remember: Endocrine = production in one tissue and target in another tissue (sometimes multiple targets). There are a variety of endocrine organs. 1. peptide hormones – includes peptide hormones, neuropeptides, and growth factors. Since they cannot cross the plasma membrane, they bind to cell surface receptors. 2. steroid hormones – includes the glucocorticoids, mineralocorticoids, and sex hormones such as testosterone, progesterone, and estrogen. They can cross the plasma membrane because they are made up of lipids. Thus, they first diffuse across the plasma membrane, then across the nuclear membrane to enter the nucleus, where they bind to receptors (transcription factors) that affect gene expression. B. Paracrine secretion – much more localized, such as within a tissue. Example: a cell producing a polypeptide growth factor interacts with a neighboring cell to send signals through a cell surface receptor. Some signals can also make their way into the bloodstream and can diffuse away from the site of production and signal to cells at a distance, or can be retained by the extracellular environment very near the release site to act nearby. Note: there are a variety of mechanisms to set the boundaries of where peptide growth factors are going to act. Many polypeptide growth factors are bound tightly to the extracellular material or to the surface and diffusion is therefore very limited. Thus, a signal molecule can still carry out a specialized cell function using paracrine communication.  Nitric oxide (NO) and carbon monoxide (CO) are examples of paracrine signaling molecules. C. Autocrine secretion – very local; cells respond to signals they secrete themselves. (i.e., a cell produces a ligand which stimulates its own receptors).  Prostaglandins act via an autocrine mechanism.

Specialized Cell Communication pathways: D. Neuronal - a synapse on a neuron releases a small molecule neurotransmitter that interacts with a receptor on a target cell. Similar to cell contact-dependent signaling, but through a small molecule. E. Contact-dependent signaling (specialized form of paracrine signaling) - a growth factor or ligand is attached to the cell: a membrane-bound ligand. You can even have a ligand with some receptor function, or can get bi-directional (reverse) signaling.
Example (Fig. 16-4): control of cell fate determination by a receptor system called “Notch,” whereby boundaries are set up that distinguish between one cell type and another. (Keep in mind that the fate of an animal cell depends on multiple extracellular signals, which are sensed by the cell.) Explanation: early in development, the neuronal system develops from an unspecified sheet of epithelial cells, and distinct cells then get chosen to become neurons from within this field of epithelial cells, which occurs through contact-dependent signaling. In this case, the ligand is a membrane-bound inhibitory signal called “Delta” and the receptor molecule is called “Notch,” whose activation prevents neuronal differentiation of these cells and allows for the epithelial phenotype to develop. On the other hand, Notch signal suppression (through reverse signaling) results in default pathway activation, leading to the development of a neuronal cell. Some experimental evidence: if you knockout the notch-gene in mice, you get excess neuronal cell production.


Another important point: different responses can be induced by the same molecule. Example (Fig. 16-5): Acetylcholine (ACh) is a small neurotransmitter molecule that acts on similar classes of receptors, but induces a diverse array of responses in different cell types. In one case, ACh can decrease the rate of force contraction of a heart muscle cell, or in another case it can increase the rate of secretion after interacting with a receptor on a salivary gland cell, or in yet another case it can cause muscle cell contraction in skeletal muscle.

II. General Types/Classes of Signaling Receptors:  Cell surface receptors (for hydrophilic signal molecules) o Structure: usually dimeric or trimeric. Typically two or more subunits that must dimerize (or trimerize?) at the cell surface which allows for transmission of this signal into the cell. Example (movie in class): growth hormone – an unusual example, though, because it acts as a monomer with two distinct binding sites to the same receptor. The receptor itself has two identical subunits. The interfaces between each receptor subunit and the hormone are therefore completely different.

o Mechanism of action: signaling cascades and “molecular switches.” The latter are mechanisms that “turn on” or “turn off” the activated form of the receptor. (Fig.16-15)  Phosphorylation/dephosphorylation (covalent modification) - of hydroxyl groups on tyrosine, serine, and threonine amino acid residues of receptor protein; this changes the structure between active and inactive forms; requires ATP.   GTP binding - activation/GDP binding inactivation (noncovalent modification).

Intracellular receptors (for small, hydrophobic signal molecules) – see section IV, under “Diffusion”

III. Categories of Cell Surface Receptors: (see Fig. 16-14)  Ion channel-linked receptors o Helps transmit ions from one side of the cell to another (can be plasma membrane or other type of membrane) to send rapid signals. Signaling molecules therefore induce a change in the conformation of ion channels to help open up a pore. o Rule: the smaller the second messenger, the more rapid the signal (based on diffusion) that it can transmit. Example: Ca2+ channels that are opened by a neurotransmitter pulse.  G-protein-linked receptors o Trans-membrane receptor that interacts with a G-protein that then transmits a signal to an enzyme. A lot of action happens at the cell surface, and other effectors involved in the pathway are tethered to the intracellular side of plasma membrane, or are trans-membrane proteins themselves. Note: the GTP–α subunit is the active form of receptor, which acts upon its target, then switches to its GDP-bound inactive form and is deactivated.  Kinase-based (tyrosine) receptors (a.k.a. enzyme-linked receptors) o The receptor is the enzyme itself; most are kinases (especially tyrosine kinases) and therefore phosphorylate other proteins (intrinsic or split). This requires ATP as an energy source. The signal/ligand usually activates the receptor through dimerization (“intrinsic” kinase receptor), or can be

separated but still noncovalently associated with the kinase (“split” tyrosine kinase receptor). o Important features:  Ligand-binding domain  Transmembrane domain  Enzymatic/catalytic domain o Two families of kinases:  Tyrosine kinases  Serine-threonine kinases

IV. Different Types of Signaling Pathways and Mechanisms of Signal Transduction:    Small (diffusible) molecules (ex: nitric oxide) - move right into cell and affect signaling cascades. Steroid hormone nuclear receptors – signals go into cell and interact with intracellular receptors to affect gene transcription. Peptide growth factors (cell surface receptors) – signals use signal transduction pathways and cascades that impinge on transcription factors to affect gene transcription in the nucleus. Specialized form of signaling that depends on cell-to-cell contact - important during development for the formation of various body structures, as well as boundaries between different tissues.


Note: In general, a signaling pathway can target multiple “effector” proteins that cause multiple changes in the cell that are somewhat synchronized. The cascade, then, can have diverse targets (different proteins). There may also be signal amplification. Model: signal starts outside the cell  cell surface receptors  signal transmitted through intracellular signaling proteins (effector proteins), which are the ultimate mediators for transmission of the signal. Purpose: these intracellular effector proteins can alter cell functions via target proteins. Examples (Fig. 16-7): 1) altered metabolism involves changing activity of metabolic enzymes; 2) altered gene expression involves changing activity of transcription factors, and 3) altered cell shape involves changing the conformation/shape of cytoskeletal proteins.  Signal transmission across membrane:

o Ligand-receptor: via cell surface receptors that span the membrane (receptor tyrosine kinases, G-protein coupled receptors, or cell surface receptors for some types of lipid hormones - not mentioned in lecture, so probably not significant for our current purposes). o Diffusion: directly across membrane, as mentioned above. This is the most common route for steroid hormones whose receptors are located intracellularly (either in the cytoplasm or the nucleus, or can start in the cytoplasm and then move into the nucleus).  Mechanism of transmission from cytosol to nucleus: o Primary Transduction:  via tyrosine kinases: Largest family of enzyme-linked receptors. Act by phosphorylating their substrate protein on tyrosine residues. Often act by first dimerizing, then cross-phosphorylating. After signalreceptor binding, various relay steps take place, in which modulation by other factors can occur. In this case, phosphorylation by a cascade of kinases occurs, which leads to signal amplification. This signal amplification can thus lead to a divergence of multiple targets. (See Fig. 16-8) Hence, one molecule → one receptor → many target proteins affected.

o Coupled Transduction:  via G protein - coupled receptors: These use a different mechanism in which a signal/ligand interacts with the extracellular domain of a 7pass membrane spanning receptor known as a G-protein, whose action is coupled to effector molecules inside the cell via the intracellular domain (*see “effector pathway” below*). Note: There are no intrinsic catalytic domains in G-protein coupled receptors. They must bind to, and activate G-proteins via their cytoplasmic domains to couple their signal transduction elements. 1. There are two types of G-proteins: a. Heterotrimeric G-proteins i. Inactive (GDP-α): α,β,γ (α,γ have covalently attached lipid tails that are anchored in plasma membrane G-protein complexes dissociate when activated


o Signal molecules bind receptor with high affinity o Trimeric G-protein associates with intracellular face of receptor through αsubunit, which changes conformation in order to change the affinity of the binding pocket away from GDP to favor GTP. (Exhanges GDP for GTP). o “Molecular switch”: activation/inactivation (Figs. 16-17, 16-18)  Activation: 1. alteration of α-subunit of G-protein allows it to exchange its GDP for GTP. 2. G-protein breaks up into two active components – an α-subunit and a β, - complex (which can regulate the activity of target proteins in plasma membrane. 3. target protein activated by α-GTP subunit  Inactivation: 4. hydrolysis of GTP by α-subunit inactivates this subunit and causes it to dissociate from the target protein. Note: may be assisted in turning off activity by “arrestins.” 5. inactive α-subunit reassembles with β,γ - complex to re-form an inactive G-protein. b.Small, monomeric Ras-like G-proteins  inactive: GDP-bound; mediated by GTPase (cleaving) activating proteins.  Active: GTP bound; o mediated by protein tyrosine-kinase receptors, which autophosphorylate. o guanine nucleotide exchange factor (GEF) stimulates release of bound GDP and its exchange for GTP. The Ras-like G-protein is now in the activated state. o cascade of serine/threonine protein kinases (example: MAP- kinase pathway, * see next lecture !*)


Effector pathway: The target proteins for G-protein subunits are either ion channels or membrane-bound enzymes. i. Ion channel regulation (Fig. 16-19) - immediate change in the state and behavior of the cell. Example: ligand: ACh (acetylcholine); receptor: G-protein-linked receptor on heart muscle cells; channel (that’s regulated): an activated β,γ - complex binds to, and opens K+ channel. Membrane-bound enzyme activation – more complex interaction; causes production of additional intracellular signaling molecules (second messengers – see next lecture*). Example: adenylyl cyclase (the enzyme) → cAMP (the second messenger); phospholipase C (the enzyme) → IP3 and DAG (the second messengers). Important point: signal amplification occurs !


o Diffusion:  For hydrophobic molecules that can cross the membrane and bind to intracellular receptors (example: cortisol, a steroid hormone). The binding of a signal with an intracellular receptor protein induces a conformational change that activates it. In the case of cortisol, the activated receptor-cortisol complex then moves into the nucleus and binds to the regulatory region of the target gene to activate transcription. This regulatory region includes promoters and specific sequences that will activate gene expression and produce a distinct protein. Thus, the receptor acts as a transcriptional regulator with the help of transcription factors (TATA-binding proteins, enhancers, etc.).


Activation of gene transcription: o Through intracellular second-messengers such as cAMP, IP3, and Ca2+. o See next lecture (Signaling II) !!

V. Output of Cell Communication:   Altered cell behavior, either through altered gene expression (slow), or directly via altered protein function (fast). (Fig. 16-23) Diverse cell responses are controlled by signaling pathways including o Proliferation, phagocytosis, apoptosis, secretion, exocytosis, and migration

To top