Endocrinology, lecture on Receptors and 2nd messenger systems
USD Department of Biology
Endocrinology
Summers
Hormones
Endocrine Glands
Receptors
Homeostasis
2nd Messengers
Nuclear Receptors
Genetic Regulation
Hormone Synthesis
Receptor Regulation
Hypothalamo-Hypophysial Communication
Tropic Hormones
Neurohypophysial Nonapeptides
Thyroid Axis
Steroids
Adrenal Axis
Adrenal Medulla
Osmotic-Pressure Balance
Reproductive Endocrinology
Prolactin
Somatic Axis
Growth Factors
Immune Factors
Ca++, PO4 Homeostasis
Pancreatic Hormones
GI Hormones
Guts 'n Brains
Brain Hormones
Neurosteroids
Neuropeptides
Endocrine Evolution
Figures for Endocrionology
text:Vertebrate Endocrinology5th Edition - David O. Norris:
Read pages 48-61 for this lecture
acronyms    end
V. Membrane Receptors ~(104+1/cell) & 2nd Messengers  			

  
	A. Extracellular region                       


		1. N-terminal portion, potentially glycosylated


		2. often many cysteines, with S-S bonds forming rigid
		   pockets ® hormone binding


			a. also link receptor chains (same or different receptors)


	B. Transmembrane segments


		1. helical structure (may be held together by cysteines)


			a. single or multiple membrane-spanning


				i. receptors with multiple-spanning segments may 
				   bind hormone(e.g. Epi) neither inside nor outside
				   the cell, but within the spanning segments


		2. ion channels


	C. Intracytoplasmic segments


		1. primarily involved in effector functions


		2. coupled to a G protein


			a. or may have intrinsic tyrosine protein kinase activity


		3. phosphorylation sites


	D. Second Messenger systems


		1. cAMP    


			a. receptor binding  catalyze GTP binding to Gs (transducer)
			   proteins (G refers to GTP binding)


				i. GTP dephosphorylated at the a subunit of Gs


					(1) a dissociates from b and g


						(a) a binds to AC


					(2) b and g associated with receptor


				ii. free intact Gs (a,b,g) may react with receptor


					(1) amplifying signal


					(2) b/g may inhibit receptor


				iii. inhibitory hormones, paracrines,
				     or neurotransmitters activate Gi


			b. a from Gs turn on activity of Adenylate Cyclase
			   to generates production of cAMP


				i. catalyzes cAMP from ATP


					(1) 4 phosphorylation sites on AC


						(a) AC made of 12 helices


						(b) amplifies signal


				ii. Gi inhibits AC


			c. 4 cAMP remove inhibition (2 regulatory subunits) from
			    Protein Kinase A (2)


			d. Protein Kinase A + ATP phosphorylates (activates - may
			    be inhibitory as with Gi)   a protein (enzyme)


				i. enzyme catalyzes cellular response


					(1) e.g. make glycogen, turn on active
					   transport (GH release), catabolize lipids


					(2) PKA may bind to transmitter receptor 
					   (e.g. GABA) to influence ion flow


				ii. PKA passes through nuclear pores,
				     bind to and phosphorylate CREB
				    (CREB = cAMP response element binding protein) 


					(1) phosphorylated CREB binds CBP (CREB binding protein)


						(a) together CREB and CBP bind to DNA
						    and stimulate transcription


							(i)  2x CREB bind CRE
								 (cAMP response element;
								 palindrome DNA segment) 

							(ii) CBP binds RNA polymerase II
								 which binds TATA box

					(2) activates immediate-early gene products
					    like c-fos and c-jun


			e. cause cAMP increase via Gs:  GHRH, GnRH, AVP, ACTH, CRF,
				FSH, LH, hCG, a-MSH, TSH,Glucagon, Gastrin,
				Secretin, VIP, PTH, Calcitonin, DA (D1 receptors),
				NE & Epi (b1 & b2), 5-HT, Histamine (H2), PGE 


				i. cAMP degraded to AMP by PDE (phosphodiesterase) 
				  following activation of A-kinase


			f. cause cAMP decrease via Gi: Somatostatin, Endorphins, Enkephalins, 
			   Bradykinin, P (in amphibians), DA (D2), NE (a2) 


		2. cGMP is similar: Receptor-KHD-mGC ®+ GTP --® cGMP ®+ PKG
				KHD = kinase-homology regulatory domain		
			a. sGC binds NO ®+ GTP --® cGMP
            
			b. increases cGMP: ACh, Insulin, GHRIH, ANP/ANF, BNP, CNP, NO, CO

				i. cGMP ®+ ion channel conductance, glycogenolysis, apoptosis, vasodilation
                
				ii. cAMP and cGMP may have antagonizing effects
                
					1) but cGMP requires cAMP 

			c. decreases cGMP: light ® rhodopsin(7tm)conf D ®+ Gt ®+ cGMP-PDE ®- cGMP

		3. Inositol Phosphate and diacylglycerol system

							     gene = GNAQ, GNA11...
			a. receptor ® Gq/P/11/14/15 protein ® PLC or PDE ---
								                            ¯
			b. PIP2  (phosphatidylinositol bisphosphate) ------® IP3  +  DG


			c. DG (diacylglycerol)® Kinase C

				i. DG is also converted by DAGL (diacylglycerol lipase) into 2-AG

					1) 2-AG (2-arachidonoyl-glycerol) is an endocannabinoid

			d. IP3 (inositol trisphosphate) releases calcium from endoplasmic reticulum/
			   membrane Ca++ channels


			e. Ca++ binds to calmodulin ® activates Ca++/calmodulin-Kinase


			f. cause increase in inositol phosphate system: TRH, GnRH, AVP (V1), 
			OT, CCK, Ang II, VIP, substance P, Histamine (H1), Epi, NE (a1),
			5-HT (5-HT2), ACh, PGF2a, NGF, EGF


		4. free Ca++ activates Ca++/calmodulin-Kinase


			a. calcium ion channel activated by depolarization of the membrane


				i. or by a hormone or 2nd messenger (IP3)


					(1) via PDE


			b. PDE ® activates Guanylate cyclase ® cGMP ® G-Kinase


				i. Ca++ may also stimulate cGMP


		5. JaK/STAT


			a. tyrosine kinase associated with the receptor


				i. 2 Janus Kinase or JaK tyrosine kinase
				    molecules bound to receptor


					(1) JaK family: JaK1, JaK2, JaK3, TyK2


			b. hormone binding dimerizes receptor


				i. and phosphorylates JaK


			c. Jak phoshorylates the receptor


			d. phosphorylated-R binds
			    STAT (Signal Transducers and Activation of Transcription) proteins


				i. JaK phosphorylates STAT ® STAT dimerizes


			e. STAT dimer translocates to the nucleus and binds to DNA


			f. activate JaK/STAT:  EPO, GH, IL3, PRL 
			

		6. MAP Kinase


			a. hormone binding phosphorylates receptor: tyrosine PK


			b. phosphorylated receptor activates
			GRB2    (Growth factor Receptor Bound protein)


			c. GRB2  activates Sos (conformational change)


				i. Sos is the protein product of Sos gene


			d. Sos binding Ras  allows GTP phosphorylation of Ras


				i. Ras is a G protein from the Ras gene


					(1) GAP protein stimulates Ras GTPase


			e. Ras activates Raf1
			(conformational change and maybe association with membrane)


			f. Raf1 is a kinase and phosphorylates MAP KK = MEK


				i. MAP KK/MEK is a dual specificity Kinase


					(1) may also be activated by PKC or Gbg


			g. MEK  phosphorylates MAP Kinase     (Mitogen Activation Proteins)


			h. MAP K/ErK phosporylates transcription factors ® translocate to nucleus


			i. activate MAP K: insulin, EGF, NGF, PDGF
			
			
	E. Membrane-bound Steroid Receptors
	
	
		1. G-protein mediated receptors have been found in brain
		    and ovary for E, B, and P
		   
		
			a. birds, amphibians & fish
			
			
		2. ligand-gated ion channels have a specific binding site
		
		
			a. steroids modify channel opening and ion movement
			
			
			b. GABAA, NMDA (Glu), Gly-R have been demonstrated
			    to have steroid site
			   
			
				i. family of receptors/channels/cell connectors
				
				
					(1) above plus nicotinic (ACh), 5-HT3, AMPA (Glu),
					     KA (Glu), voltage-gated ion channels, gap-junctions
						
						
						(a) all are probably steroid sensitive

VI. Nuclear Receptors