Allied Health Symposium Notes

5th Annual Allied Health Symposium

  • Monday, April 21, Noon - 2 p.m.
  • Location: The Learning Collaboratory, University Center 115
  • Invited School Representatives: Towson University, UMSON, UMSOP, University of Maryland School of Medicine-Physical Therapy, UMBC-Sonography, Case Western Reserve

Administrative Announcement

  • This week's discussion: glucose transport, blood glucose levels, and diabetes.
  • Purpose:
    • Relate cell biology to daily life and public health
    • Improve data interpretation and critical thinking
  • No Wiley Plus homework due this Friday

Chapter 15: Cell Signaling and Signal Transduction

  • Communication between Cells
  • Intercellular to intracellular SIGNALING
  • Intracellular to intercellular
  • Resources:
    • the-amoeba-sisters-intro-to-cell-signaling
    • GPCR: https://www.youtube.com/watch?v=9Bq6qHJaSJs
    • Kinase: https://www.youtube.com/watch?v=xG2WOd_fWqo
    • RTK: https://www.youtube.com/watch?v=-osiUGKsu7o

Cell Response to Environment

  • Cells respond to the world around them: input(s) --> output (response, phenotype)
  • A combination of signals and the responsiveness of the cell dictates the cell's action.
  • Controlled molecularly.

Learning Objectives

  • Define autocrine, paracrine, and endocrine signaling.
  • Explain signal transduction and general features of a signaling pathway: receptors, effectors, etc.
  • Describe classes of molecules that signal through receptors.
  • Compare and contrast three generic signaling pathways and their involvement of ion channel, G-protein, and enzyme-coupled receptor.
  • Know how second messengers like cAMP are generated.
  • Describe pathways in which second messenger cAMP acts.
  • Explain how signaling can be amplified or turned off.
  • Know some important examples of cell signaling mediated by cAMP.
  • Explain kinase-mediated signaling pathways.
  • Describe several different effector molecules and their substrates.

Extracellular Signals and Receptors

  • Extracellular “signals” can take several forms
  • RECEPTORS: bind to signal molecules and initiate cellular responses to the signal.
  • Mostly small chemicals: peptides, nucleotides, amino acids, steroids, eicosanoids, lipids, odorants, pheromones, hormones, gases.
  • Non-chemicals: mechanical stimuli (touch, vibration, sound wave, heat/cold, or light).
  • Receptors are specific and react to individual signals.

Types of Extracellular Signaling

  • Autocrine: Cell responds to signals it releases itself.
  • Paracrine: Signaling molecules travel short distances through extracellular space.
  • Endocrine: Signaling molecules reach target cells through the bloodstream.

Generic Signal Transduction Cascade

  • Cell surface (plasma membrane) RECEPTORS give information to the cell upon binding to SIGNALS (LIGANDS) by:
    • Activating a cascade of signaling RELAY events signal output intracellular signaling molecules receptor
  • Players in the cascade are reversibly activated or inhibited and can be modified to alter their function.
  • Ion channels (effectors) and membrane potential (output) are also important.

Signaling Pathways

  • Consist of a series of proteins.
  • Each protein alters the conformation of the next protein (directly or indirectly).
  • Target proteins ultimately receive a message to alter cell activity.
  • Signal transduction: converts information carried by extracellular messenger molecules into cellular responses.
    • e.g., photoreceptor cells convert light signals into changes in membrane potentials.

General Signaling Pathways

  • Pathway 1: Cell surface receptors generate an intracellular second messenger through an effector enzyme.
    • Second messengers activate (or inactivate) specific proteins, including kinases.
  • Pathway 2: Surface receptors recruit proteins to their intracellular domains at the plasma membrane.
  • Pathway 3: Kinases and phosphatases

Kinases and Phosphatases

  • Protein conformation is usually altered by phosphorylation.
  • Kinases add phosphate groups, while phosphatases remove them.
  • Protein phosphorylation can change protein behavior by:
    • Activating or inactivating an enzyme
    • Increasing or decreasing protein-protein interactions
    • Changing the subcellular location of the protein
    • Triggering protein degradation

Amino Acids for Phosphorylation

  • Only 3 amino acids can be phosphorylated:
    • Serine
    • Threonine
    • Tyrosine
  • Phosphorylation: A phosphate group is added to the side chain (hydroxyl group) of the amino acids.

Cell Type Response

  • Different cell types can respond differently to the same signal input: acetylcholine
  • Outputs (phenotype) are molecularly controlled by receptors, second messengers, and effectors.

Signaling Diagrams

  • Boxes tend to represent proteins (or small molecules or genes); arrows/ blocks represent molecular actions
  • For example: activation by interaction, enzymatic action, expression
  • Double arrows may mean there are unknown steps

Pathways Can Be Modified

  • Discussion Question: If the protein represented by the orange is mutated and nonfunctional, what do you predict will be the effect on the output (the green node) of the pathway when the blue is activated?
    • A - No change
    • B - Increase
    • C - Decrease (Correct Answer)
    • D - I don't know

Receptor Classes

  • Response to a signal depends on the type of receptor to which it binds
  • RECEPTOR CLASSES:
    • G-protein coupled receptors (GPCR)
    • Enzyme coupled receptors (RTK)
    • Ligand-gated receptor (ion channels)
    • Contact-mediated receptor (integrins)
    • Intracellular receptors like steroid hormone receptors (nuclear receptors, NO)
  • cell surface/ transmembrane receptors

GPCRs in Sensory Perception

  • GPCRs are involved in sensory detection of light, odor, and select taste molecules.
    • Opsins in photoreceptors for light detection, such as rhodopsin in rods for black-and-white vision (PDE-cGMP pathway).
    • Odor receptors in olfactory sensory neurons. There are more than 400 types of these receptors in the human nose (AC3-cAMP pathway).
    • Taste receptors for bitter, sweet, or savory (umami) substances in taste receptor cells (PLC-IP3 pathway).
  • Without GPCR, we would be blind, have no smell, and taste impaired  Miserable!

GPCRs

  • GPCRs are the largest superfamily of proteins and are highly conserved
    • Largest superfamily of proteins encoded by animal genomes (1000s).
    • Seven α-helical transmembrane domains—conserved structure!
    • Natural ligands that bind to GPCRs: hormones (both plant and animal), neurotransmitters, opium derivatives, chemical attractants, odorants, tastants, and photons).
  • GPCRs are heavily involved in cellular signaling and implicated in many diseases. They are major medicinal targets.

GPCR-mediated Signaling

  • GPCR-mediated signaling relies on heterotrimeric G proteins
    • C-term is intracellular
    • The 3rd intracellular loop interacts with heterotrimeric G-proteins
    • α and γ G-protein subunits are lipid-anchored
    • Down-stream effectors of activated Ga and βγ subunits include:
    • Enzymes: adenylate cyclase (AC), protein kinases, phospholipase C (PLC), etc
    • Ion channels: K^+ channels, Ca^{2+} channels, etc.

Activation of G-protein Subunits

  • Stimulation of GPCRs activates G-protein subunits
    • Ligand binding changes GPCR conformation and thereby activate the G-protein
    • Multiple G-proteins can be activated by a single receptor
    • Gα and Gβγ subunits signal
    • G_s – activate adenylyl cyclase
    • G_q – phospholipase C beta
    • G_i – inhibit adenylyl cyclase
    • G_{12/13} - less characterized – Rho GEFs
    • Gβγ (5 Gβ and 11 Gγ subunits)
  • G-protein signaling is complex and diverse.

G-proteins and Second Messengers

  • G-proteins can activate enzymes that produce second messengers
    • One of the best-characterized enzymes is adenylyl cyclase that produces cyclic AMP.
    • Gα switches itself off by hydrolyzing GTP (within seconds)
  • Signal is amplified in several steps, e.g., one ligand-bound receptor can activate many G proteins.
  • Phosphodiesterase stops or reduces signal by converting cAMP back to AMP

GPCR Signaling Cascade

  • Ligand → Receptor → G protein → Effector (adenylyl cyclase) → ATP converted to cAMP

Signal Amplification

  • Example: phototransduction
  • Light activates the GPCR rhodopsin
  • Signal is amplified in several steps, e.g., one ligand-bound receptor can activate many G proteins.

cAMP as a Second Messenger

  • cAMP is a second messenger that can rapidly diffuse through the cell
    • 5-HT – serotonin
    • Many cyclic AMP molecules can be made by an activated adenylyl cyclase, and each can activate a number of proteins (such as protein kinase A – PKA)
    • = AMPLIFICATION

Downstream Effectors of cAMP

  • cAMP Ligand-gated ion channels (e.g., cyclic nucleotide-gated channel)
  • PKA (multiple subtypes)
  • Membrane depolarization
  • Increase in intracellular Ca^{2+} level
  • Signal is amplified
  • PKA activity

Processes Affected by cAMP

  • A variety of processes can be affected by changes in [cAMP]
    • Some PKA molecules phosphorylate nuclear proteins.
    • Phosphorylated transcription factors regulate gene expression.
    • Phosphatases halt the reaction cascade.
    • cAMP is produced as long as the external stimulus is present (but may be at a different rate).

Blood Glucose Levels

  • Have you had lunch today?
  • How to maintain blood glucose level?
  • Glycogen: polysaccharide of glucose

Regulation of Blood Glucose

  • By hormones
  • The reactions that lead to glucose storage or mobilization.
  • Glycogen phosphorylase controls glucose mobilization – stimulated by glucagon and epinephrine.
  • Glycogen synthase controls glucose storage – activated by insulin.

Hormone Regulation

  • Different stimuli acting on the same target cell may induce the same response.
    • Glucagon and epinephrine bind to different receptors on the same cell.
    • Both hormones stimulate GLYCOGEN breakdown and inhibit its synthesis.
    • cAMP is activated by the G protein of both hormone receptors – responses are amplified by signal cascades
    • (Insulin acts in opposition through another pathway)
  • The reactions that lead to glucose storage or mobilization.

Glucose Mobilization

  • The reaction cascade occurs as the hormone binds to its receptor
    • Gas subunit activates adenylyl cyclase → cAMP formation
    • cAMP diffuses into the cytoplasm and binds a cAMP-dependent protein kinase A (PKA).
    • PKA-mediated phorsphorylation →
    • 1) inactivates glycogen synthase and 2) activates glycogen phosphorylase
  • The response by a liver cell to glucagon or epinephrine

Adaptation to Smell

  • Someone in the kitchen is baking a pie… cinnamon clove caramelized sugar apple
  • A little bit of each of the molecules has made its way to your nose, where they bind to GPCR odorant receptors and start a signal transduction cascade (through cAMP mediated channel opening, depolarization of the neuron, and an action potential that your brain knows means “apple pie”)
  • After a while, you do not smell the pie or the smell is reduced, Why?

Next Lecture

  • Modulation and termination of GPCR signaling