Cell Signaling Basics Independent Learning Module PDF

Independent  Learning  Module   Cell  Signaling  Basics    Context     Cellular  intercommunica.on  is  a  fundamental  property  of  life  in  mul.cellular   organisms.      Consider  that  the...

Independent  Learning  Module   Cell  Signaling  Basics    Context     Cellular  intercommunica.on  is  a  fundamental  property  of  life  in  mul.cellular   organisms.      Consider  that  the  human  body  is  made  up  of  a  great  diversity  of  cell   types  with  different  proper.es  and  behaviors,  all  of  which  must  be  .ghtly  and   mutually  regulated  in  order  for  life  to  be  possible.    Hence  the  capacity  for   sending  and/or  receiving  signals  is  an  essen.al  and  ubiquitous  feature  of  all   cells,  and  dysfunc.on  of  one  sort  or  another  in  cell  signaling  pathways  plays  a   role  in  most  diseases.    Moreover,  pharmacological  therapies  use  cell  signaling   pathways  to  alter  cell  behavior  and  treat  disease.     Already  in  the  Founda.ons  Block  you  have  encountered  many  physiological   processes  in  which  cell  signaling  is  central,  most  conspicuously  in  inflamma.on   and  immunity.    An  understanding  of  cell  signaling  will  also  be  cri.cal  in   upcoming  topics  within  the  Block  including  especially  pharmacodynamics,  cell   cycle,  cell  growth,  cell  differen.a.on,  and  early  development,  as  well  as  in   future  blocks.     This  ILM  will  provide  a  basic  overview  of  cell  signaling  pathways.    It  will  serve  as   background  for  later  topics  in  which  you  will  learn  specific  pathways  in  greater   detail.    Four  kinds  of  signaling   Extracellular signal Target sites Autocrine Receptor (receptors) are on Signaling the same cell Ligand on signaling cell Juxtacrine (or ECM protein) binds Signaling target cell receptors Target sites are on Paracrine Secretory cell adjacent cells; rapid Signaling breakdown of ligand helps prevent distant effects Endocrine Blood vessel Signaling Target sites are on distant cells Hormone secretion into blood by endocrine gland Modified  from  Kumar  et  al.,  Robbins  &  Cotran  Pathologic  Basis  of  Disease  8th  edn  2010        Four  kinds  of  signaling:     Examples  from  the  immune  system   Autocrine and Cytokines secreted by T helper cells upregulate T Paracrine helper cell activity as well as activity of adjacent cells Signaling Juxtacrine Interactions between T lymphocytes and antigen Signaling presenting cells Endocrine Elevated cortisol levels inhibit development of T cells in Signaling thymus and can cause stress-related immunodeficiency What other examples can you think of?  Some  signaling  proper.es  and   requirements   Cell-cell communication requires that one cell produces a signal, the ligand, and that another cell produces a receptor to which the ligand can bind. Specificity: A ligand, whether it is secreted or expressed on a cell's surface, may come in contact with many different kinds of cells, but only cells expressing the specific receptor for that ligand will be able to respond to the signal. The higher the specificity of ligand-receptor recognition, the greater will be the fidelity of the signal. Most signaling molecules act with very high affinity for their receptor. Signal transduction: Cell signaling requires that the ligand stimulus be converted into some sort of response or combination of responses by the target cell – eg., contraction, secretion, cell division, gene expression, protein synthesis, etc.  Some  signaling  proper.es  and   requirements  (cont.)   Phosphorylation (and de-phosphorylation): Addition or removal of phosphate groups (by kinases and phosphatases respectively) to molecules participating in signaling pathways is a major means of effecting signal transfer from one step to another within a pathway. Amplification: In general signaling pathways include multiple steps, and at each step there is an increase in the strength of the signal. For example, a receptor activated by bound ligand can activate multiple molecules and each of these can in turn activate multiple downstream molecules, etc. In this way a very small initial stimulus can result in large downstream effects. Regulation: Signaling pathways are dynamic and reversible. For example, where phosphorylating activates a downstream signal, dephosphorylation will de-activate it. The ultimate behavior of a cell from moment to moment depends on up- and down-regulation by competing signals from many interacting signaling pathways.  Four  main  types  of  signaling  pathway   Regardless of whether signaling is autocrine, juxtacrine, paracrine, endocrine or some combination of these routes, signaling occurs via four main signaling pathways within target cells. These are classified based on the type of receptor they utilize: 1. Ligand-gated ion channels: this pathway involves transmembrane receptors that, when bound by the appropriate ligand, change conformation in such a way that a channel opens up across the membrane and allows ions to enter or leave the cell. 2. G-protein-coupled receptors are transmembrane proteins whose cytoplasmic domain, when activated by ligand-binding of the extracellular side of the receptor, binds a GTP-binding protein (called a G protein). The G protein dissociates into active components that in turn activate an effector molecule, eg adenylate cyclase. The effector molecule recruits 'second messengers', eg cyclic AMP, that go on to activate other molecules that stimulate specific cell behaviors. 3. Kinase-linked receptors are transmembrane proteins that are phosphorylated by a cytoplasmic kinase that interacts with the ligand-bound receptor, or that self-activate by becoming phosphorylated when the receptor is bound by ligand. Phosphorylation of the receptor initiates a cascade of kinase-mediated phosphorylation events that in turn regulate gene expression by the cell. 4. Intracellular receptors: Lipid-soluble signaling molecules can diffuse across the plasma membrane into the cell. They act by binding their receptors in the cytoplasm, or more commonly, the nucleus. The ligand-bound receptor then functions to alter gene expression. The  following  slides  will  illustrate  these  pathways…    1.  Ligand-­‐gated  Ion  Channel     ligand   ions   Plasma   membrane   1.  Ligand-­‐receptor  binding   opens  ion  channel   receptor   2.  Ions  enter  the  cell  causing   hyperpolariza.on  or   depolariza.on   Example:  When  the  neurotransmi_er,  acetylcholine,   3.  Cellular  effects   binds  to  its  receptor  on  skeletal  muscle  cells,  the   receptor  channel  opens  and  allows  Na+  ions  to  enter   the  cell.    The  resul.ng  depolariza.on  causes  Ca++   release  from  sarcoplasmic  re.culum,  and  this  causes   muscle  contrac.on.         More  about  ligand-­‐gated  ion  channels  in  the  Nervous   System  Block.    2.  G  protein-­‐coupled  receptors   inac.ve  G  protein   2.  Ac.vated  G   ligand   inac.ve   1.  Ligand-­‐receptor  binding   protein  ac.vates   effector   effector  enzyme   enzyme   causes  binding  to  and   plasma   ac.va.on  of  G  protein   membrane   receptor   3.  Ac.ve  effector  generates  second   messengers,  and  depending  on  the   specific  pathway,    these  cause…   Many  pep.de  hormones  act  via  G  protein-­‐coupled   receptors,  including  Follicle  S.mula.ng  Hormone,   Luteinizing  Hormone,  Thyroid  S.mula.ng  Hormone,   Adrenocor.cotrophic  Hormone,  etc.    –  more  about  these  in   4.        Ca++            Phosphoryla.on            other   DMH  and  Life  Cycle  Blocks.     NB:  G  protein-­‐coupled  receptors  will  be  discussed  in  more   detail  in  the  Pharmacodynamics  lectures  in  Founda.ons,   5.  Cellular  effects   because  most  drugs  act  via  G  protein  coupled  receptors!     Note  also  that  many  of  the  neurotransmi_ers  that  you  will   encounter  in  the  Nervous  System  block  act  via  G-­‐proteins   (i.e.,  dopamine,  NE,  serotonin,  ACH,  etc.)    3a.  Kinase-­‐linked  receptors  -­‐  intrinsic   Unphosphorylated   downstream  kinase     1.  Ligand-­‐receptor  binding  causes   ligand   receptor  phosphoryla.on  by   intrinsic  kinase     plasma     membrane   phosphates   Receptor:   2.  Kinase  cascade:   cytoplasmic   Phosphorylated  receptor  in   domain  has   turn  phosphorylates  other   tyrosine  kinase   kinases  which  in  turn   domain   phosphorylate,  and  so  on…   Note  that  signaling  via  kinase-­‐linked  receptors  may  lead  to  mul.ple   3.  Gene  transcrip.on   signal  transduc.on  pathways.  A  dominant  pathway  involves   ac.va.on  of  RAS  then  RAF  then  MAP-­‐kinase  which  induces  cyclin-­‐D   expression  and  cell  prolifera.on.   4.  Protein  synthesis     Many  growth  factors  -­‐  including  Epidermal  Growth  Factor,   Fibroblast  Growth  Factor,    Platelet  Derived  Growth  Factor  -­‐  all  act   5.  Cellular  effects   via  this  type  of  receptor,  and  defects  in  kinase-­‐linked  receptor   pathways  are  associated  with  many  cancers.    Hence,  more  detail   about  this  pathway  later  in  Founda.ons  when  we  discuss  cell  cycle   and  neoplasia.      3b.  Kinase-­‐associated  receptors     1.  Binding  of  ligand  to  extracellular  side  of   Unphosphorylated   receptor  induces  a  conforma.onal  change   downstream  kinase     that  allows  binding  of  associated  tyrosine   kinase  to  cytoplasmic  side  of  receptor   Inac.ve       ligand   tyrosine   2.  Bound  kinase  phosphorylates  itself  and   kinase   receptor   plasma   membrane   receptor   3.  Phosphorylated  receptor-­‐ kinase  complex  ac.vates   transcrip.on  factor,  which   A  major  pathway  ac.vated  by  kinase-­‐associated  receptors  is   then  translocates  into  the   the  JAK-­‐STAT  pathway,  and  it  is  u.lized  by  many  different   nucleus   cytokines  including  IL-­‐2  -­‐3,  -­‐4,  -­‐5  and  -­‐6,  as  well  as  by   4.  Gene  transcrip.on   erythropoie.n  and  growth  hormone.     Ligand  binding  allows  binding  to  the  receptor  and  ac.va.on   5.  Protein  synthesis   of  JAK  (Janus  kinase),  which  then  phosphorylates  STAT   (Signal  Transducers  and  Ac.vators  of  Transcrip.on),  which   then  translocates  to  the  nucleus  and  ini.ates  transcrip.on.     6.  Cellular  effects    4.  Intracellular  receptors     Lipophilic  ligand   Plasma   membrane   1.  Ligand  diffuses  through   plasma  membrane  and   binds  to  receptor  in   cytoplasm  or  (usually)   Receptor  in   nucleus   cytoplasm  or   nucleus   2.  Receptor  with  bound  ligand   ac.vates  gene  transcrip.on   Steroid  hormones  including  estrogen,   progesterone,  testosterone,  cor.costeroids,  as   well  as  thyroid  hormone,  prostaglandins  and   3.  Protein  synthesis   Vitamin  D  act  via  intracellular  receptors.     These  are  all  lipid-­‐soluble  molecules  that  can   4.  Cellular  effects   diffuse  through  the  plasma  membrane.    Their   receptors  are  ligand-­‐dependent  transcrip.on   factors  that  bind  directly  to  DNA  .   Summary  of  Signaling  Pathways   Pharmacology, Rang et al., 5th ed. 2003.  References   1. Boron  WF,  and  EL  Boulpaep,  Medical  Physiology:  A  Cellular  and  Molecular   Approach  2nd  edi.on.  Philadelphia,  Pa:  Elsevier  Saunders,  2009.   2. Kumar  V,  AK  Abbas,  N  Fausto,  Robbins  and  Cotran  Pathologic  Basis  of   Disease  8th  ed.  Philadelphia:  Elsevier  Saunders,  2009.     h_p://www.mdconsult.com/books/about.do?about=true&eid=4-­‐u1.0-­‐ B978-­‐1-­‐4377-­‐0792-­‐2..X5001-­‐9-­‐-­‐ TOP&isbn=978-­‐1-­‐4377-­‐0792-­‐2&uniqId=418828351-­‐2     3. Lodish,  H,  A  Berk,  SL  Zipursky,  P  Matsudaira,  D  Bal.more  and  J  Darnell   Molecular  Cell  Biology,  4th  ed.  New  York:  WH  Freeman,  2000.     h_p://www.ncbi.nlm.nih.gov/books/NBK21475/     The  End