Cell Communication & Receptors & Signal Transduction

Signal Transduction

Molecular changes in or on a cell induced by a signaling molecules

Endogenous Ligand

Signaling molecule made by cells to activate receptors on/in cells.  Usually a protein, amino acid, nucleic acid, fatty acid, or biogenic amine.

Neurotransmitter

ligand that signals between neurons via a synapses.

Hormone

ligand transported through bloodstream (endocrine signaling)

Growth Factor

ligand that stimulates cells to grow and/or divide; usually peptides.

Cytokine or interleukin

ligands that signal between immune cells; peptides.

Chemokine

cytokine that attracts migration of immune cells.

Receptor

Protein that binds and reacts to ligand by changing conformation resulting in a change in cell function, thus initiating signal transduction.

Transcription Factor

Protein that binds DNA to induce or suppress transcription/ gene expression.

What type of receptor is Nuclear receptor (NRs)?

What are Nuclear receptor (NRs)?

Intracellular receptor: remains inside the cell

ligand binding induces gene transcription (aka ligand-activated transcription factor)

What type of receptor is Ligand-gated ion channels(LGICs)?

What are Ligand-gated ion channels(LGICs)?

Cell surface receptors: transmembrane receptor proteins with extracellular ligand-binding domains.

5 subunits open to allow ions to rapidly cross the membrane to hyperpolarize or depolarize the cell or increase [Ca²⁺]_i

What type of receptor is Tyrosine Kinases Receptor (TKRs)?

What are Tyrosine Kinases Receptor (TKRs)?

Cell surface receptors: transmembrane receptor proteins with extracellular ligand-binding domains

ligand binding induces activity of intracellular kinase enzymes

What type of receptor is G-protein-coupled/7-Transmembrane receptors (GPCRs or 7TMRs)?

What are G-protein-coupled/7-Transmembrane receptors (GPCRs or 7TMRs)?

Cell surface receptors: transmembrane receptor proteins with extracellular ligand-binding domains.

ligand binding activates intracellular G-proteins or beta-arrestins which activate or inhibit enzymes, transcription factors and/or ion channels

Cell Communication

Involves a cell producing a signaling molecules that bind and activate receptors in or on a cell to change the activity of the receiving cell.

• Various categories relate to distance ligand travels to reach receptors.

• Also goes from fast, short-duration to slower and longer-lasting effects.

What are the four types of cell communication that we talked about?

1. Surface contact

2. Synaptic

3. Paracrine & Autocrine

4. Endocrine

Surface Contact

Involves antigen presentation among immune cells: Tyrosine kinase receptors.

• Recent cancer drugs acts at these receptors

Also, occurs in interactions between other cell types to limit their growth.

Synaptic

Occurs between one neuron and one other; very fast, short-lived.

• Ligand-gated ion channel or G-protein-coupled receptors

Autocrine signaling

Cell that makes signaling molecule acted on by it.

• Limited mostly to signaling between immune cells or pre-synaptic neurons.

Paracrine signaling

Signaling molecules acts on cells nearby; intermediate speed & duration.

• Primarily involves local inflammatory signaling via GPCRs.

Endocrine signaling

Signaling molecule acts anywhere after traveling through blood; slow, lasting.

• Involves hormones involved in development and maintenance of homeostasis

Fill in the Blank - Surface Contact

Distance: ___________

Receptor: ___________

Speed: ___________

Role(s): ___________

Distance: Direct contact

Receptor: TKR

Speed: very fast

Role(s): mostly immune modulation

Fill in the Blank - Synaptic

Distance: ___________

Receptor: ___________

Speed: ___________

Role(s): ___________

Distance: across tiny cleft

Receptor: LGIC, GPCR

Speed: very fast

Role(s): “chemical synapse”; nervous system

Fill in the Blank - Paracrine & Autocrine

Distance: ___________

Receptor: ___________

Speed: ___________

Role(s): ___________

Distance: same cell or nearby cell

Receptor: TKR, GPCR

Speed: intermediate

Role(s): mostly immune modulation & inflammation

Fill in the Blank - Endocrine

Distance: ___________

Receptor: ___________

Speed: ___________

Role(s): ___________

Distance: close-very far via blood

Receptor: NR, TKR, GPCR

Speed: slow to very slow

Role(s): mostly in development or/and homeostasis

Ligand-Gates Ion Channel receptor (LGICs)

• 75 subunits

• Fastest / shortest-lasting

Nuclear Receptor (NRs)

• 48 types

• Slowest / longest-lasting

Tyrosine Kinase Receptors (TKRs)

• most numerous type of enzyme-linked receptor

• 91 types

• Intermediate speed & duration

G-Protein Coupled Receptors (GPCRs)

• 800 types

• Intermediate speed & duration

Ion Channels

• Multi-subunit complexes that open in response to different stimuli.

• Opening allow passage of ion types across a membrane down its gradient.

Ligand-gated ion channel

Mediate some neurotransmitter activity

• many psychoactive drugs

Voltage-gated ion channel

Mediate nerve neuron action potentials and muscle contraction

• drugs for hypertension or anesthesia

Mechanically-gated ion channel

Mediate sensation of touch and/or pain

• No drugs

Thermally-gated ion channel

Mediate sensation of hot, cold, and/or pain

• Menthol, capsaicin

Electrochemical membrane potential

• A membrane potential is established by an imbalance of charges and ion concentrations across a membrane; more net + outside and more net - inside

• It is energetically-favorable for an ion to move down its concentration gradient, but unfavorable for it to increase net negative charge inside cell.

Ion	Extracellular (mM)	Intracellular (mM)	channels
Na+	140	10	depolarize
Cl-	100	4	hyperpolarize
K+	5	140	hyperpolarize
(macro. anions)	0	65	N/A

When channels open, ions will move across a membrane down their concentration gradient

• ↓ net negative charge inside → depolarizes

• ↑ net negative charge inside → hyperpolarizes

• Opening Na⁺ channels allows Na⁺ to flow into the cell, which depolarizes the cell (by decreasing the net - charge inside)

• Opening K⁺ channels allows K⁺ to flow out of the cell, which hyperpolarizes the cell (by increasing the net - charge inside)

What does opening a Cl^- channel do? Would it flow into or out of the cell?

Opening the Cl^- channels allows Cl^- to flow out of the cell, which hyperpolarizes the cell (by increasing the net - charge inside)

Electrochemical membrane potential summary

• Decreasing – inside cell depolarizes

  • Opening Na⁺ channels

• Increasing – inside cell hyperpolarizes

  • Opening K⁺ or Cl^- channels

Synaptic transmission

• Post-synaptic receptors may cause depolarization or hyperpolarization of the post-synaptic neuron.

• Transmission is terminated by diffusion, degradation or re-uptake of  neurotransmitter.

Ligand-gated Ion channel receptors

These neurotransmitters excite or inhibit depolarization of neurons.

• Much synaptic/neuron signaling occurs via GPCRs

• Neurons receive inputs from 1000s of other neurons

• Each neuron integrate signals over times resulting in an Action Potentials or not.

Nuclear receptors

A.K.A ligand-activated transcription factos

Only receptor type not located in cell membrane; only found in animals

• Mediate long-term changes or homeostasis (endocrine systems)

What are the two types of nuclear signaling?

1. Type I in cytosol when inactive; activated by steroids & homodimerize

   a. sexual development/function/fertility (estradiol, progesterone, & testosterone)

   b. adaption to stress (cortisol)

   c. electrolyte levels (aldosterone)

2. Type II bind DNA even when inactive, activated by non-steroids & heterodimerize.

   a. metabolism (thyroid hormone, PPAR)

   b. Ca²⁺ levels/bone remodeling (VitD)

   c. some activated by xenobiotics

Enzyme-Linked Receptors

• Tyrosine Kinases Receptor

  • 91

  • numerous drugs

• Serine/Threonine Kinase Receptors

  • 13; 6 active + partners 

  • few drugs; like TKRs, but attach Pi to ser or thr

• Tyrosine Phosphatase Receptors (RTPs)

  • 20

  • no drugs

• Guanylyl cyclase-coupled Receptors

  • 6

  • Natriuretic factor receptors

  • 1 drug

Kinases

• Very important regulators of cell function/protein activity.

• Phosphorylation may active or inactive substrate

What are Tyrosine Kinase Receptors(TKRs)?

Protein ligands, may stimulate growth

• Most cancers derived from mutations in growth factor signaling

Where are TKRs located? What are the 4 canonical pathways?

In cell membrane with extracellular ligand-binding domain, and intracellular kinase domain.

• Drug replace/supplement hormones or block growth (mostly treat cancer)

Canonical Pathways:

1. Ras-MAPK

2. Protein Kinase B (PKB)/Akt

3. Phospholipase C (PLC)

4. STAT

What are some examples of ligands for GPCRs?

• photons

• neuropeptides

• monoamines

• scent molecules

• a protease

The G-protein cycle steps

1. Inactive form of G-protein has GDP bound to G_a. 

2. G-protein activated by GPCR (usually via agonist binding) to release GDP and bind GTP.

3. G_a -GTP uncouples from GPCR and G_bg, and each complex acts on effector(s).

4. Intrinsic GTPase of G_a cleaves GTP to GDP + PO₄^3- (inactivates G_a)

5. then G_bg can couple to G_a (inactivates G_bg)

6. Heterotrimerthen can couple GPCR again.

G_a protein effectors

G_a (~20 types) Effector :

• a_s  stimulates AC (Adenylyl Cyclase) ATP à cAMP à PKA

• a_i/o   inhibit AC

• a_q  stimulates phospholipase C (PLC) à ↑ [Ca²⁺]_I

• a_12/13  stimulate RhoGEF à RhoA (small G-proteins) à ROCK & kinases

G_bg protein effectors

• G_b (6 types) form tight heterodimers with G_g (12 types) AC

  • PLC, TKRs, too

    • ↑ GIRK (G-protein-gated Inwardly-Rectifying K⁺ channel = K_ir) activity

    • ↓ voltage-gated Ca²⁺ channel (Ca_V2) activity

PI3-kinase (à PKB/Akt), TKRs, too

Src (à MAPK/ERK), TKRs, too

Bruton’s Tyrosine Kinase (BTK)

GRK (G-protein coupled Receptor Kinase)

Phosducin (Phd) [binds/sequesters G_bgs]

cAMP Signaling

1. Ga_s & some G_bg stimulate AC

2. Ga_i & some G_bg inhibits AC

3. Ga_t & Ga_gust stimulate PDE: cAMP à AMP

Ca²⁺ channel signaling

Activation of Ga_o and some G_bg combinations decreases the opening of Ca²⁺ channels

K⁺ channel signaling

Activation of Ga_z or other G_bg combinations increases the opening of GIRK (G-protein-gated inwardly-rectifying potassium) channels.

Phospholipase C (PLC) signaling

Activation of Ga_q or some G_bg combinations stimulate phospholipase C, which cleaves PIP₂ to IP₃ and DAG, increasing [Ca²⁺] in the cytosol and PKC activity (and other Ca²⁺-dependent processes)

GRKs and arrestins

• beta-arrestin1 and beta-arrestin2 might interact with all GPCRs

• GRKs phosphorylate GPCRs to create beta-arrestin binding site

  3 possible effects: 1) block G-protein signaling, 2) couple GPCRs to clathrin (internalization/desensitization of GPCR), and/or 3) activate signaling pathways