C2006/F2402 '05 OUTLINE OF LECTURE #13

(c) 2005 Dr. Deborah Mowshowitz, Columbia University, New York, NY. Last update 03/02/2005 10:06 AM .

Handouts 13A & 13B. 13A = Two tables -- Effects of Epinephrine on Smooth Muscle, and Properties of Two Main types of Cell Surface Receptors. 13B = TK receptors and signaling. Not on web. You will also need 12A & 12B from last time.

    A. Review of cAMP pathway & regulation of glycogen metabolism. (Handouts 12A & B)

If you haven't done them yet, try problems 6-6 to 6-8.

    B. Reminder: The same 2nd messenger can generate different responses (in different cells). An Example: consider epinephrine response in skeletal muscle vs response in smooth muscle of lungs. (More details of responses in last lecture.)

1. Receptors are same. Epinephrine receptors in skeletal muscle and smooth muscle of lungs are the same -- activate same G protein -->  same second messenger (cAMP). 

2.  Response to same 2nd messenger is different. Response in skeletal muscle is breakdown of glycogen; response in smooth muscle  is change in Ca⁺⁺ (leading to relaxation of muscle).

3. How do you get different responses?  In both cases generate cAMP & stimulate PKA.

Target proteins available for PKA to phosphorylate are definitely different (in the two types of muscle), so you get a different response. 

Q to think about: In skeletal muscle, what enzyme/protein does PKA phosphorylate? (See handout 12-B.) In smooth muscle what enzyme/protein could PKA phosphorylate? (See table below.) Also try problem 6-10.

    C. The same hormone can generate different effects (in different cells) using different second messengers. 
(See Becker fig. 10-24 [10-23] & table below). An example -- effects of epinephrine (adrenaline) on smooth muscle.

1. The phenomenon: Epinephrine has different effects on different smooth muscles:

            On some smooth muscles --> contraction (helps divert blood from peripheral circulation to essential internal organs.)
            On other smooth muscles -->  relaxation (example as above -- allows lungs to breathe more deeply)

2. How Ca⁺⁺ fits in:

a. Ca⁺⁺ stimulates muscle contraction.

b. Epinephrine binds to receptors on some smooth muscles --> Ca⁺⁺ released from ER --> intracellular Ca⁺⁺ up --> stimulates contraction. 

c. Epinephrine binds to receptors on some smooth muscles --> Ca⁺⁺ pump activated ---> Ca⁺⁺ removed --> intracellular Ca⁺⁺ down --> relaxation! 

3. How does epinephrine work two ways on same type of tissue?

a. Two basic types of epinephrine  receptors -- called alpha and beta adrenergic receptors (adrenergic = for adrenaline). The two types are distinguished by their relative affinities for epinephrine and norepinephrine. (See note to table below.)

b. Some types of smooth muscle have mostly one type of receptors; some the other.

c. Two types activate different G proteins and generate different second messengers as on handout 12A.

(1). Beta receptors --> G protein type (G_s)--> cAMP response --> PKA --> phosphorylation of Ca⁺⁺ pump --> removal of cell Ca⁺⁺ --> relaxation

(2). Alpha₁  receptors --> different G protein (G_p) --> different second messenger (IP3) --> Ca⁺⁺ release from ER --> contraction

4. How 2 receptor types help to respond appropriately to stress (epinephrine).

a. Beta type receptors. Beta receptors are found in lung tissue in smooth muscle surrounding bronchioles. Stress (pop quiz, lion in street, etc.)--> epinephrine --> muscles relax --> bronchioles dilate -->  deeper breathing --> more oxygen --> energy to cope with stress. 

b. Alpha type receptors. Alpha receptors are found in smooth muscle surrounding blood vessels of peripheral circulation. Stress --> epinephrine --> muscles contract --> constrict peripheral circulation --> direct blood to essential organs for responding to stress (heart, lungs, skeletal muscle).

5. Medical Uses of all this.

Epinephrine can be used during an asthmatic attack to relax bronchi and ease breathing. Overuse of this type of broncho-dilator eases breathing temporarily but masks underlying problem (inflammation of lung tissue) and can have additional serious long term effects (from overstimulation of heart which also has beta receptors). Heart and lungs have slightly different types of beta receptors, so drugs have been developed that stimulate one and not the other (unlike epinephrine). 

6. Summary of epinephrine effects on smooth muscle (in lung vs peripheral circulation) -- see handout 13A

Note: There are more than two types of epinephrine receptors on smooth muscle cells, so epinephrine may affect smooth muscle in other tissues in other ways.  (There are subtypes of alpha and subtypes of beta.)

    D. Details for DAG/IP3/Ca⁺⁺ pathway. See handout 12A bottom & Becker figs. 10-8 & 10-9 or Purves 15.11 (15.13)

1. How IP3 and DAG are generated.

a. Activated G protein binds to phospholipase C (PLC)

b. PLC cleaves PIP2 in membrane --> PIP3 (soluble, in cytoplasm; also known as InsP3) and DAG ( remains in membrane). For structure see handout 12A and Becker fig. 10-8.

c. Other inositol derivatives are involved in signaling -- this is a current hot subject of investigation.

2. Role of IP3/Ca⁺⁺

a. IP3 opens Ca⁺⁺ channels in the ER, raising Ca⁺⁺ in cytoplasm. (Becker fig. 10-10 for IP3 effect; fig 10-11 for overall Ca⁺⁺ regulation.)

b. Ca⁺⁺ acts alone or binds to calmodulin; complex (calmodulin-Ca⁺⁺) or Ca⁺⁺ alone alters activity of many proteins. (See Becker fig. 10-12  for pictures of calmodulin.)

c.  Ca⁺⁺ affects many processes -- can trigger exocytosis (& secretion) or muscle contraction & big changes in Ca⁺⁺ levels are involved in egg fertilization (see Becker fig.10-13 or Purves  15.12 (15.14) for some nice pictures). 

d. Examples of use of IP3/Ca⁺⁺ signaling: 

(1). IP3 --> --> --> synthesis of NO (nitric oxide; acts as diffusible signal) --> dilation of blood vessels; Viagra prolongs the response to NO. If you are interested, see Becker fig. 10-16 [10-15]  or Purves 15.13 (15.15) and accompanying text. (Note: the two texts differ in some of the early steps in the pathway leading to activation of NO synthase. Different signals may activate NO synthase through different pathways.)

(2). In some smooth muscle tissue, epinephrine -->receptor -->  G prot --> --> IP3 -->Ca⁺⁺ release --> contraction.

3. Role of DAG

a. DAG (in membrane) activates protein kinase C (= PKC, not to be confused with PLC).

b. "PKC" is a family of related enzymes involved in many different processes. If you are interested, see Becker or advanced texts for details. Some PKC's require Ca⁺⁺.

Try problems 6-5, 6-9 & 6-16.

II. How do Tyrosine Kinase Linked Receptors Work?

    A.  Overview: What are the 3 basic types of receptors?

1.  Steroid Receptors -- Internal receptors for lipid soluble hormones (steroids & TH)

2. GPLRs (or GPCRs) -- Cell surface receptors linked to G proteins (also called G Protein Coupled Receptors or  GPCRs)

3. RTKs -- Cell surface receptors that are (or are linked to) tyrosine kinases

    B. How do the Two main types of Cell Surface Receptors Compare? -- See handout 13A for summary of basic features of  TK or TK linked receptors and comparison to G protein linked receptors.  See Purves 15.6 (15.7) for structure & mechanism or Becker fig. 10-17 [10-16]  for structure and Becker 10-18 [10-17] for mechanism. (In even older editions of Becker, signaling is in chap. 23).

    C. Summary of Properties of Receptor TKs (See handout 12A).

1. Receptor is usually a single pass protein 

2. Ligand binding usually leads to dimerization of receptors (Purves 15.6 (15.7) or Becker 10-17 [10-16].)

3 Dimerization activates a TK in cytoplasmic domain of receptor or in separate TK (that binds to receptor).

4. Active kinase auto-phosphorylates itself. TK adds phosphates (from ATP) to its own tyrosines -- each subunit phosphorylates the others.

5. Other proteins are activated by binding to phosphorylated TK's. (Becker 10-18 [10-17])

a. Some proteins can be activated by binding directly to TK 

b. Some proteins are activated by binding indirectly -- these proteins bind to "adapter proteins" that are bound to the TK. How it happens: 

(1). Adapter proteins bind directly to the TK 

(2). Additional proteins bind to the adapters 

(3). Binding to adapters --> activation of the additional "recruited" proteins.

c. Proteins that bind directly to TK have certain types of domains -- usually called SH2 binding domains. 

d. Recruitment. Note that the target protein to be activated comes to or is "recruited by" the TK; this is the opposite of the situation with most 2nd messengers where the messenger diffuses throughout the cytoplasm and "seeks out" the target protein to be activated. The recruitment method may be important in localizing the response to a particular part of the cell.

6. What ligands use TK receptors? Many growth factors (such as EGF) and other paracrines, autocrines and juxtacrines act through TK receptors. Insulin, but not most other endocrines, acts through TK receptors. (See Purves 15.6 (15.7))

7. Activation of TK receptors often leads to changes in TF's and transcription. (Note this not the usual case with G-linked receptors -- their end target is usually not a TF.)

Try problems 6-1 & 6-3

    D. Important examples of proteins activated by binding to receptor TK's (directly or through adapters)

1. PLC.  Ligand binding to TK-linked receptor can activate a type of PLC --> IP3 --> trigger cascade using IP3. This means that the IP3 pathway can be activated by both types of receptors -- GPCRs and TKs.  The cAMP pathway (as far as we know) can only be activated by GPCRs.  

2. ras -- a famous (infamous?) protein. See Purves fig. 15.9 (15.11) or Becker 10-18 [10-17] and handout 13B (more details next time).

a. What is ras? It's a small G protein . 

(1). Ras is a G protein -- it has GTP bound (active) and GDP bound (inactive) forms. 

(2). What makes it "small?" It's not trimeric -- it has no subunits

(3). Ras (& other small G proteins) require other proteins to allow activation (binding of GTP) and to control rate of inactivation (GTP hydrolysis).

b. Why does anyone care about ras?  Large % (about 30%) of tumors have messed up ras (= ras stuck in the activated, GTP bound, form). Therefore we know ras has important role(s) in growth control.

c. How is ras activated? Activation of receptor TK -->binding of adapter proteins --> binding of ras to adapter proteins & activation of  ras --  conversion of ras-GDP to ras-GTP.

d. What does activated ras do?

(1). Ras works by activating the series (cascade) of protein kinases generally called MAP kinases. End result = activation of final kinase in cascade = MAP kinase or mitogen-activated protein kinase. See Purves 15.9 (15.11) or Becker 17-39 [17-38].

(2). Many growth factors use ras signaling to trigger the G1 to S transition as follows:

GF --> binds to TK receptor --> activated ras (see above) --> MAP kinase cascade --> activated TF's --> transcription of genes for cyclin --> mRNA for cyclin  (& CDK if cell was in G0) --> cyclin buildup --> activation of CDK (start kinase) --> cell can pass start checkpoint & enter S. (More details on this next time.)

(3). 'Ras' is really a family of proteins whose members serve many different important functions in addition to that described above.

3. The same signal (same activated TK receptor) may trigger more than one signaling pathway. For example, EGF can trigger both the IP3 pathway and the ras pathway. (Becker fig. 10-18 [10-17]).

    E. A human example of TK receptor signaling:  FGF (Fibroblast growth factor) and FGF Receptor.  See Becker figs. 10-19 & 10-20 [10-18 & 10-19]. Important as an example of a "dominant negative."

1. Signaling system uses a TK receptor  

2. FGF & FGFR needed for proper development as described in Becker chap.10. Failure of signal transmission --> developmental abnormalities. See Becker fig.10-20 [10-19].

3. Why is achondroplasia (type of dwarfism), due to a defective FGFR, dominant? See Becker fig. 10-19 [10-18], and box on handout 13B.

a. The general principle: TK receptors must dimerize in order to act. If 1/2 the receptors are abnormal, most of the dimers that form do not work.

b. The consequences: "lack of function" mutations in receptors are often dominant. (For an example see Becker figs. 10-19 & 10-20 [10-18 & 10-19]. Dimers form, but they are inactive.

c. This case: In a heterozygote for achondroplasia, 1/2 the FGF receptors are defective. In the defective receptors, the cytoplasmic domain of the protein is missing. Dimers form, but most dimers are never activated -- the two monomers can not phosphorylate each other. Therefore the signaling (required to form bone) is badly disrupted.

4. Dominant Negatives.

a. Definition: An abnormal allele like the one that causes achondroplasia is called a "dominant negative mutation." A dominant negative allele (or mutation) makes an inactive protein that disrupts function even in the presence of a normal allele (and normal protein). Note: This is not the usual case --  most negative or "lack of function" alleles (or mutations) are recessive.

b. Significance: Existence of a dominant negative mutation implies that the gene involved codes for a protein that must polymerize in order to act.

    F. Signaling Pathways all interrelate

1. Different 2nd messengers can influence the same enzyme/pathway

2. Each signaling system can affect the others -- For example, Ca⁺⁺ levels can affect kinases/phosphatases and phosphorylations can affect Ca⁺⁺ pumps (& therefore Ca⁺⁺ levels). See an advanced text if you are interested in the details.

To sum up signaling, try problems 6-15, 6-17 & 6-18.

     B. Overview of how mutations mess up G1/S switch signaling

Try problem 15-9.