Cell Communication_ Signaling I

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					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 de-
            activated.

      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 signal-
              receptor 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 7-
              pass 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
                           ii.   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 α-
           subunit, 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.

                        ii.        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

				
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