Intracellular Signaling Cascades

Each cell in a multicellular animal has been programmed to respond to specific _________ __________ produced by other cells or the environment. These signals act in various combinations to regulate the ____________ of the cell.

- Extracellular signals

- Behavior

What are the 8 steps in the communication by extracellular signals?

1. Synthesis

2. Release

3. Transport

4. Binding

5. Activation

6. Change

7. Response

8. Removal

1. Synthesis of _________________ chemical messenger in response to a _________.

- extracellular

- stimulus

2. Release of the chemical messenger by the ________________________ cell.

signaling/stimulated

What is another name for a chemical messenger?

A ligand

3. Transport of the chemical messenger to the _______________ cell.

target

The chemical messenger can diffuse through the plasma membrane as long as the chemical messenger is _________________.

hydrophobic

4. Binding of the chemical messenger by a specific __________________ protein.

receptor

5. Activation of an ______________ protein by the messenger-receptor complex (sometimes, the receptor is the _____________)

effector

effector

6. Change in cellular metabolism producing intracellular ___________________ _________________, function, or transcription triggered by the effector.

second messengers

7. Response through changes in protein function that could lead to changes in ________________ and _________________.

transcription

translation

8. Removal of the signal, which often __________________ the cellular response.

terminates

The chemical messengers in the nervous system are...

Neurotransmitters

What are some examples of neurotransmitters?

ACh, Epinephrine, Norepinephrine

The chemical messengers in the endocrine system are...

Hormones

What are some examples of hormones?

LH, Prol, Steroids, Insulin, Glucagon

The chemical messengers in the immune system are...

Cytokines

What are some examples of cytokines?

IL1-30, TGFb, CXCL

The chemical messengers in development and mature organs are...

Growth factors

What are some examples of growth factors?

FGF, EGF, PDGF, retinoids, eicosanoids

Chemical messengers are secreted by specific cells in response to a ____________.

stimulus

Chemical messengers ____________ or are _______________ through blood/other extracellular fluid to the target cell (endocrine, paracrine, autocrine)

diffuse

transported

A receptor in the target cell ____________________ binds the chemical messenger

specifically

Binding of a chemical messenger to receptor...

elicits a response

The signal ceases and is terminated by the degradation of...

ligands, receptors, and 2nd messenger

What are the 3 different types of extracellular signaling through secretion?

Endocrine signaling, Paracrine signaling, Autocrine signaling

What is endocrine signaling?

The chemical messenger (hormone) is secreted by a specific cell type (endocrine) gland, enters the blood, which carries the signal to the target cells distributed widely throughout the body.

What are examples of endocrine hormones?

Insulin, LH, Estrogen, Prolactin, Testosterone

What is paracrine signaling?

The chemical messenger secreted acts on nearby target cells. The secreted molecules are not allowed to diffuse too far; for this reason, they are often rapidly taken up by neighboring target cells, destroyed by extracellular enzymes.

What is an example of paracrine signaling?

- Neurotransmission.

- Ex. Acetylcholine neurotransmitter.

What is autocrine signaling

The chemical messenger acts on the cell from which it is secreted.

What are examples of autocrine signaling?

- Cellular differentiation during development, cancer cells often use autocrine signaling to overcome the normal controls on cell proliferation and survival.

- Ex. Wnt, BMP, FGF

Are all extracellular messengers secreted?

No

What are the two types of extracellular messengers that don't involve secretion?

- Contact-dependent signaling

- Gap junctions

What is contact-dependent signaling?

What are Gap junctions?

Cellular response to a particular extracellular signaling molecule depends on its binding to a specific receptor protein located on the _____________ of a target cell or in its _____________ or ________________.

- Surface (Cell surface receptors, transmembrane receptor)

- Cytosol or nucleus (Intracellular receptors).

Binding of a signaling molecule (ligand) to its receptor is _______________. The ligand "fits," a site on the receptor -- similar to "lock and key mechanism" of enzyme-substrate reaction

Selective

Binding of a ligand to its receptor causes a __________________ ________________ in the receptor that initiates a sequence of _________________ _________________ leading to a specific cellular response.

- Conformational change

- Intracellular reactions

What are the 3 reasons why signaling is specific?

- Typically, one receptor binds only one specific messenger

- Only certain cells (target cells) carry receptors for the messenger

- Each receptor initiates a characteristic signal transduction pathway

Do ligands metabolize to useful products via endosomes?

No

Are ligands an intermediate in any cellular activity?

No

Do ligands have any enzymatic properties?

No

What is the only function of the ligand?

Change the properties of the receptor

What is the pathway that leads to the release of glucose by glycogenolysis?

Glucagon >>> Glucagon receptors in liver >>> release of glucose by glycogenolysis

Can the pathway occur in muscles and intestines?

No

What is the reverse of the gycogenolysis pathway?

Too much sugar >>> secrete insulin >>> cells start taking up glucose and storing it

What is the difference between cell surface receptors and intracellular receptors in terms of ligand?

1. Cell surface receptor: 1.Ligands are usually polar (hydrophilic) molecules that cannot cross the plasma membrane

2. Intracellular receptors: Ligands are non-polar (hydrophobic) molecules which can cross the plasma membrane

Which type of receptor (Cell surface or Intracellular) has extracellular ligand binding domains (Multiple SH bonds)?

Cell surface receptors

Which type of cell receptor (Cell surface or Intracellular) has one or more membrane spanning domains?

Cell surface

Which type of cell receptor (Cell surface or Intracellular) are transcription factors and cofactors?

Intracellular receptors

What is the difference between cell surface receptors and intracellular receptors in terms of conformational changes?

1. Cell surface receptor: Binding of ligand causes a conformational change that is communicated to the intracellular domain

2. Intracellular receptor: Binding of ligand causes a conformational change that allows its binding to DNA

What is the difference between cell surface receptors and intracellular receptors in terms of the rate of changes in transcription and translation?

1. Cell surface receptors: Intracellular domain initiates a characteristic signal transduction pathway which can either cause a rapid change in the cell or slower changes through altering transcription and translation

2. Intracellular receptors: Usually induce slower changes through changes in transcription and translation

Major Receptor signaling pathways in Eukaryotic Cells

Which receptor signaling pathway is the fastest?

Ion channels (ms)

Which receptor signaling pathway is the slowest?

Nuclear receptors

Binding of ligands to many cell-surface receptors activates ________________ _____________ which stimulate a short-lived increase (or decrease) in the concentration of the intracellular signaling molecules termed ____________ _______________.

- downstream effectors

- second messengers

Second messengers either ____________ or ______________ from storage. Act as intracellular ______________________.

- synthesized or released

- activators/ligand

What are the (4) general characteristics of second messengers?

- Low amounts in resting state

- Synthesis is tightly regulated

- Destruction is tightly regulated

- Mode of act through other proteins/effectors

What are examples of second messengers?

- Cyclic nucleotides (cAMP & cGMP)

- Calcium

- Lipid derivatives: Inositol triphosphate (IP3), derived from plasma phospholipid & Diacylglycerol (DAG), derived from plasma phospholipid

_________________________ are secreted from neurons in response to an electrical stimulus, called the _____________ ________________.

- Neurotransmitters

- Action potential

What is an action potential?

A voltage difference across the plasma membrane caused by changes in Na+ and K+ gradients that is propagated along a nerve

The neurotransmitters diffuse across a _____________ to another excitable cell where they elicit a response.

Synapse

_______________ is the neurotransmitter at neuromuscular junctions, where it transmits a signal from a motor nerve to a muscle fiber that elicits contraction.

Acetylcholine (Ach)

ACh is sequestered in vesicles clustered near an active zone in the ______________ _______________.

presynaptic membrane

What happens when the action potential reaches the presynaptic membrane?

The presynaptic membrane has voltage-gated Ca2+ channels that open when the action potential reaches them, resulting in an influx of Calcium.

The Ca2+ triggers ____________ of the vesicles with the plasma membrane, and ACh is released into the ______________ _________.

- fusion

- synaptic cleft

The release of neurotransmitters into the synaptic cleft is an example of what type of cellular signaling?

Paracrine

What constitutes the largest family of cell-surface receptors?

G Protein-Coupled Receptors (GPCRs)

How many GPCRs family members are there?

~800 family members

Do all GPCRs have a similar structure?

Yes

Do all GPCRs have similar signaling molecules?

No, there is chemical and functional diversity of the signal molecules that bind to them

How many polypeptide chains are there in a GPCR?

Single polypeptide chain

How many times does the GPCR polypeptide chain thread back and forth the membrane?

Seven times

What are other names for the GPCRs?

- "Seven transmembrane"

- "Heptahelical"

- "Serpentine" receptors

GPCR receptors that bind protein ligands have a distinct and __________ extracellular domain.

large

Receptors that have small ligands have __________ extracellular domains.

small

What is an example of a small ligand?

Adrenaline (epinephrine)

In the GPCR, the small ligands bind ________ within the plane of the membrane to a site that is formed by amino acids from several _______________________ segments.

- deep

- transmembrane

Activation and inactivation of GPCRs and G proteins

What do GPCRs activate?

Heterotrimeric GTP-binding proteins or G proteins

How many protein subunits do G proteins have?

Three protein subunits: α, β, and γ

Where are G proteins anchored?

Inner leaflet of the plasma membrane

In the unstimulated state, the receptor and G-proteins may or may not ___________; the alpha subunit has _______ bound and the G proteins are ____________.

- Associate

- GDP

- Inactive

What happens when extracellular signaling molecules bind to the GPCRs?

The receptor undergoes a conformational change, which in turn alters the conformation of the G protein.

The conformational change causes the α subunit to release its bound ___________, allowing ___________ to bind in its place, resulting in a _____________ ______________ in the α subunit.

- GDP

- GTP

- Conformational change

The conformational change of the α subunit causes the trimer to ____________ from the receptor and into two activated components -- an _____ subunit and a ______ complex - each then interacts with different downstream signaling proteins.

- dissociate

- α subunit

- βγ complex

What are the 7 steps in desensitization, inactivation, and degradation of GPCRs?

After α subunit activates its target protein (effector protein - either enzymes or ion channels), it shuts itself off by hydrolyzing its bound __________ to __________.

GTP to GDP

The inactivation of the α subunit leads to the dissociation from the target protein and reassociates with a ___ _______________ to re-form an inactive G protein

- βγ complex

Binding to the target protein or regulators of G Protein Signaling (RGS) usually stimulates the __________ ______________ of the α subunit.

GTPase activity

Depending on the Gα subunit, GPCRs can activate diverse signaling pathways

What were Gα proteins originally classified by?

Their ability to stimulate or inhibit the enzyme Adenylate cyclase, which converts ATP to cyclic AMP (cAMP)

How many classes are Gα proteins divided into?

6 classes: Gαs, Gαi, Golf, G0, Gq and G11/12

What is the Gα subunit that stimulates the enzyme?

Gαs

What is the Gα subunit that inhibits the enzyme?

Gαi

Which Gα subunit results in an increase in IP3 and DAG?

Gαq

Which Gα subunit results in an increase in cAMP

Gαs

Which Gα subunit results in a decrease in cAMP

Gαi

What are some examples of different Gα subunits activating different effector-signaling pathways?