Signal Transduction

During evolution unicellular organisms developed…

1. Mechanisms to respond to physical and chemical signals

2. Mechanisms to interact with other cells.

Bacteria respond to…

Chemical signals secreted from other cells.

Quorum sensing

Cell to cell communication mechanism in bacteria that allow them to respond to chemical signals secreted from other cells.

Quorum sensing allow bacteria to…

Coordinate spore formation, antibiotic production, conjugation, and motility.

In yeasts…

Proteins were identified, including cell-surface receptors, G-proteins, and protein kinases.

Proteins in yeast…

Provide signal network

Yeasts have what similar to multicellular and homoeothermic higher animals?

Proteins and signal mechanisms

During evolution, signal systems in higher animals became…

Elaborate

Human genome has…

More than 1500 genes only encoding receptor proteins.

Insects, warms, and mammals utilize…

Essentially similar mechanisms of signal transduction.

A process of infomation transfer mediated by…

Extracellular/intracellular signals

Process initiates intracellular signaling process with…

Physiological change

Second messengers rapidly and transiently…

Increase concentration inside the cell

About … characterized by particular signaling components

200 types of human cells

Signaling pathways are chracterized by…

An enormous diversification

Practically all signaling proteins exist in…

A multiplicity of different isoforms.

Paul Ehrlich (1854-1915)

A father of modern pharmacology

Paul Enrlich introduced…

Concept of a “magic bullet” (drug)

A drug will NOT work…

Unless it is bound.

Paul Ehrlich introduced term…

Receptive substance or Receptor

Extracellular signal may be detected at…

10^-8 M

Receptors have…

Very high affinity

Affinity constant

K >= 10^8 liters/mole

Type of signaling

1. Contact-Dependent

2. Paracrine

3. Synaptic

4. Endocrine

Signals can operate at…

Short distance, long distance, or both

Contact-Dependent

Require cells to be in a direct membrane-membrane contact

Paracrine

Signals released and act upon neighboring cells

Synaptic

Signals release in the synaptic space

Endocrine

Endocrine cells release signals operating through the whole organism.

Cell respond to…

A specific combination of Signal Molecules

Cell of a typical multicellular organism is exposed to…

Hundreds of different signal molecules

Signal molecules may operate in…

Innumerable combinations

Cells respond in…

A specific fashion (cell specializartion) obtained through the evolution

Cell may respond to signals other than chemical molecules like…

1. Mechanical Forces

2. Osmolarity

3. Temperature

4. Light

Approximately … different families of receptors

25

Receptor families share…

One or more homologous domains

Plasma membrane receptors

Capable to detect and respond to

“Myriad” of chemical and physical stimuli

Divergent evolution resulted in…

Multiple receptor isoform

Receptor isoforms react to…

Different ligands

Mechanisms of best characterized receptors may…

Apply to the rest of the family

Major Classes of receptors

Voltage gated channels

Membrane depolarization/repolarization

Major Classes of receptors

Ligand-gated channels

Changes in membrane permeability

Major Classes of receptors

Seven-helix receptors: Trimeric G proteins

Diverse Responses

Major Classes of receptors

Diverse response

1. Two-component systems receptors/histidine kinase

2. Integrins: Nonreceptor Tyrosine Kinases

Major Classes of receptors

Receptor tyrosine kinase: RAS, MAP kinase, PLC, PI3

Alter gene expression

Major Classes of receptors

Gene Expression

1. Cytokine receptors: JAK Kinase, Stat Transcription Factors

2. Tyrosine Kinase Linked Receptors: Cytoplasmic Tyrosine Kinase

3. Receptor Serine/ Threonine kinase: SMAD Transcription Factors

4. Sphingomyelinase-Linked receptors: Ceremide-Activated kinases

5. Cytoplasemic Steroid Receptors → Active Transcription Factor

Major Classes of receptors

Membrane Guanylyl Cyclase Receptors: cGMP

Regulation of Kinases and Channels

Major Classes of receptors

Selectins

Cell adhesion

Major Classes of receptors

Cadherins

Contact Inhibition

Major Classes of receptors

Notch

Cell fate determination

Major Classes of receptors

Cytoplasmic Guanylyl Cyclase Receptors: cGMP → Kinase

cGMP-Gated Channels

Orignally suggested concept of drug (ligand) specificity

Drugs (ligands) bind…

ONLY to certain receptors

Orignally suggested concept of drug (ligand) specificity

Individual receptors recognize…

Only certain class of drugs (ligands)

No drug (ligand) is…

Completely speciifc in its action

One ligand activates…

Multiple signal pathways

Multiple ligands can…

Result in activation of the same pathway

One component of the pathways can…

Regulate components of different pathways

Types of resistance to ligands

1. Change in the receptors

2. Loss of receptors

3. Exhaustion of mediators

4. Increased metabolic degradation

5. Physiological adaptation

Types of Receptor regulation

1. Receptor upregulation

2. Receptor downregulation

In early theory, drug (ligands) bind…

ONLY to certain receptors

Receptor superfamilies and signal transduction

1. Steroid receptor signal

2. Transmembrane receptor proteins

3. Ion channel-linked receptors

4. G-protein-coupled receptors

Steroid receptor signal

Lipid-soluble signal tranduction

Steroid receptor signal locations

Cytoplasm or nucleus

Steroid hormone receptors act as…

Ligand-activated transcriptional factor because since hormones bind to the receptors, they can bind to DNA and regulate transcription (RNA production) for a specific gene

Hormones include…

Steroids, cortisol, progesterone, estradiol, testosterone, and retinoic acid

Steroid Hormone process

1. crosses the membrane

2. binds with intracellular receptor

3. transport into the nucleus

4. hormone affects hormone response element

Two major mechanisms of Lipid-Soluble signal tranduction

1. Nuclear Initiated Steroid Signaling NISS (Classical)

2. Membrane-Initiated Steroid Signaling (MISS)

Nuclear Initiated Steroid Signaling NISS (classical)

1. Steroid ligands

2. Ligand diffuses through the membrane

3. Binds to soluble receptor in cytoplasm or nucleus

4. Activate nuclear DNA-ligand binding domain

5. Gene Activation depends upon cell type and a nature of a ligand

NISS

1. Steroid ligands

• Corticosteroids, mineralocorticoids, sex steroids

• Vitamin A, Vitamin D, retinoid, and thyroid hormones

NISS

3. Binds to soluble receptor in cytoplasm or nucelus

• Type I receptors: corticostroid, mineralocorticoids, sex steroids)

• Type II receptors: Vitamin A, vitamin D, retinoid, and thyroid hormones

Ligand binding domain

• Hormone Response Element

NISS

4. Activate nuclear DNA- ligand binding domain (HRE)

• HRE acts as

Ligand activated transcription factor

NISS

4. Activate nuclear DNA- ligand binding domain (HRE)

… receptor

Gene-active

NISS

4. Activate nuclear DNA- ligand binding domain (HRE)

Action time

minutes - hours - days

NISS

4. Activate nuclear DNA- ligand binding domain (HRE)

Effect may last for

hours - days

NISS

4. Activate nuclear DNA- ligand binding domain (HRE)

Persistent effect is due to

Slow turnover of enzymes and proteins

Membrane-Initiated Steroid Signaling (MISS)

1. Membrane receptors are located

On the outer membrane surface in caveola

Membrane-Initiated Steroid Signaling (MISS)

2. Rapid effect due to

Action upon steroid receptors on plasma membrane

Membrane-Initiated Steroid Signaling (MISS)

3. Induces biological effects

Faster than the classical NISS pathway

Membrane-Initiated Steroid Signaling (MISS)

4. Membrane receptors have

The same protein structure as intracellular steroid receptors

Steroid signaling pathway affects the major parts of the cell

• Cytoplasm

• Nucleus

• Membrane

Rapid biological responses can involve

• Ion channels

• Transcription

• Translation

• Provides a general biological response to the steroid hormone

Transmembrane receptor proteins

Kinase-linked receptors

II. Transmembrane Rceptor Protein (Kinase-llinked receptors)

1. Ligands

Insulin, and pharmacological agents

II. Transmembrane Rceptor Protein (Kinase-llinked receptors)

2. Common architecture of the receptor

Large extracellular binding domain

II. Transmembrane Rceptor Protein (Kinase-llinked receptors)

3. Cytoplasmic enzyme domain

Tyrosine kinase, serine kinase, or guanylyl cyclase

II. Transmembrane Rceptor Protein (Kinase-llinked receptors)

4. Involved in

Cell growth and differentiation, indirectly control gene transcription

Insulin receptors as an example

1. Specific receptor

Transmembrane glycoprotein

Insulin receptors as an example

2. Receptor consists of

2 alpha and 2 beta subunits (400,000 MW)

Insulin receptors as an example

3. Alpha and beta subunits are linked by

Disulfide bonds (S-S)

Insulin receptors as an example

4. Alpha subunits

Entirely extracellular (ligand-binding site)

Insulin receptors as an example

5. Beta subunits

Transmembrane proteins with tyrosine-kinase activity

Insulin action upon the receptors

1. Insulin binds with

Extracellular ligand-binding site

• Dimerization of the receptor occurs

Insulin action upon the receptors

2. Beta subunits act upon themselves (autophosphorylation)

Beta subunits enhance the action of the kinases on other targets

Insulin action upon the receptors

3. Typical targets

Synthesis of glycogen, lipids, proteins, and enzymes

Insulin action upon the receptors

4. Translocation of glucose transporters (GLUT-4) also takes place

GLUT-4 is recruited from golgi apparatus to plasma membrane

Insulin action upon the receptors

5. Glucose facilitated diffusion through

Glucose transporter

Insulin-receptor complex after some time internalize in vesicles

Loss of insulin receptors occur

(Receptor down regulation)

This phenomenon may explain resistance to insulin

Ion channel-linked receptors

Ionotropic

Ionotropic

Directly control ion flow across the cell membrane when a chemical binds, causing the receptor protein to change shape and open an integral ion channel for rapid electrical signaling