Endocrine signaling
Signaling molecules (hormones) are secreted by specialized endocrine cells and carried through the circulation to target cells at distant body sites.
Paracrine signaling
Molecules released by one cell act on neighboring target cells Ex. Neurotransmitters
Autocrine signaling
Cells respind to signaling molecules that they themselces produce.
ex. T-lymphocytes respond to antigens by making a growth factor that drives their own proliferation, thereby amplifying the immune response
Intracellular receptors
receptors located inside the cell.
- respond to small hydrophobic molecules that can diffuse across the plasma membrane
- ex) steroid hormones, thyroid hormones, vitamin D3, retinoic acid
Direct cell-cell signaling
direct interaction of a cell with its neighbor (ex. via integrins and cadherins)
Signaling by secreted molecules
-three categories are based on distance over which signals are transmitted
Modes of cell signaling
- direct cell-cell signaling
2. signaling by secreted molecules
Steroid hormone types
(synthesized from cholesterol)
- Sex steroids
- Corticosteroids
- Ecdysone
- Brassinosteroids
List types of hormones
- steroid hormones
- thyroid hormone
- vitamin D3
- Retinoic acid and retinoids
Sex Steroids
- testosterone
- estrogen
- progesterone
(produced by gonads)
Corticosteroids
- Glucocorticoids-stimulate production of glucose
- Mineralocorticoids-act on the kidney to regulate salt and water balance.
(from the adrenal gland)
Ecdysone
an insect hormone that riggers metamorphosis of larvae to adults
Brassinosteroids
plant steroid hormones that control several processes including cell growh and differentiation
Thyroid hormone
synthesized from tyrosine in the thyroid gland; important in development and metabolism
Vitamin D2
regulate Ca2+ metabolism and bone growth
Retinoic acid and retinoids
synthesized from vitamin A; important in vertebrate development
Describe receptors of the glucocorticoid action pathway
Receptors for glucocorticoid are of the nuclear receptor superfamily, they are transcription factors that have domains for ligand binding
Describe Ligand binding regulation of glucocorticoid action
- ligand binding regulates glucocorticoid action by functioning as activators or repressors in genes
- Some nuclear receptors are inactive in the absence of hormone
- Glucocorticoid receptor is bound to Hsp90 chaperones in the absence of hormone
- Glucocorticoid binding displaces Hsp90 and leads to binding of regulatory DNA sequences
Gene regulation by the thyroid hormone receptor
- Without a hormone, thyroid hormone receptor is associated with a co-repressor complex and represses transcription of target genes.
- with a hormone, transcription is activated with a coactivator
Nitric Oxide characteristics
a) A paracrine signaling molecule
b) acts in the nervous, immune, and circulatory systems.
c) can cross plasma membrane
d) very unstable
e) synthesized by Arg+NOS (nitric oxide synthase)
Nitric Oxide functions
a) alters enzyme activities
b) Can indirectly induce muscle relaxation and blood vessel dilation
How does NO influence blood vessel dilation and muscle relaxation?
- initially stimulated by neurotransmitters acting on endothelial cells
- NO diffuses to smooth muscle cells and stimulates cGMP production
- NO binds to It’s target, Guanylyl cyclase; binding induces the synthesization of the second messenger cyclic GMP
- cGMP induces muscle relaxation and blood vessel dilation
Second Messenger
- a molecule that relays a signal from a receptor to a target inside the cell
ex) cGMP
ex) cAMP
Carbon Monoxide
(CO)
- a signaling molecule in the nervous system
- related to NO because it acts similarly as a neurotransmitter and mediator of blood vessel dilation
What are functions of neurotransmitters?
- carry signals between neurons
- carry signals between neurons to other target cells
- Diffuse across the synaptic cleft & bind to receptors on the target cell surface
What are characteristics of Neurotransmitters
- released as Action potential arrives at the end of a neuron
- Hydrophilic (so must bind to cell receptors bc they can’t cross PM)
Give 2 types of Neurotransmitter receptors and a brief description
- ligand gated ion channels (as neurotransmitter binds, channels open)
- G protein-coupled receptors
Give a few examples of neurotransmitters
Acetylcholine, epinephrine, serotonin, norepinephrine, dopamine….
Peptide signaling molecules
- peptide hormones
- neuropeptides
- polypeptide growth factors
Peptide hormones
- Pituitary gland hormones
a) growth hormone
b) follicle stimulating hormone
c) Prolactin - Insulin
- Glucagon
Neuropeptides
- secreted by some neurons
- 2 types: Enkephalins & Endorphins
What do Enkephalins and Endorphins have in common?
a) both are neuropeptides
b) both act as neurotransmitters or neurohormones
c) natural analgesic that decreases pain response
c) bind to the same receptor on brain cells as morphine
Polypeptide growth factors
- NGF
- EGF
- PDGF
- cytokines
- membrane anchored growth factors
NGF
Nerve growth factors
-member of neurotropin family that regulates development and survival of neurons
EGF
Epidermal growth factors
- stimulate cell proliferation
- commonly studied
PDGF
- stored in blood platelets
- released during blood clotting at site of wound
- stimulates proliferation of fibroblasts and contributes to regrowth of damaged tissue
Cytokine
- a polypeptide growth factor
- regulate development and differentiation of blood cells
- regulate activities of lymphocytes during immune response
Membrane Anchored Growth Factors
- remain associated with PM
- Function as signaling molecules in direct cell-cell interactions
Intracellular signal transduction
surface receptors–>intracellular enzymes–>intracellular targets—>(alteration of gene expression)
Surface receptors
a) regulate intracellular enzymes
b) where most ligands responsible for cell-cell signaling bind to
Intracellular enzymes
a) stimulated by signals from surface receptors
b) transmit signals from the receptor to a series of additional intracellular targets
Intracellular targets
a) targets of signaling pathways frequently include transcription factors so that as ligands bind to receptors a chain of intracellular reactions is initiated and ultimatley reaches the nucleus & alters gene expression
G protein-coupled receptors
a) largest family of cell surface receptors
b) a type of neurotransmitter receptor
c) signals are transmitted via guanine nucleotide binding protein (G-protein)
d) receptors have seven membrane spanning alpha helices
Give steps for hormonal activation of adenylyl cyclase
- ligand binds to a receptor and induces conformational change
- change allows cystolic domain to activate G protein on the inner face of the plasma membrane
- the activated g-protein stimulates the exchange of GTP to GDP and causes the alpha and beta-gamma complex to dissociate from the receptor
- activated alpha subunit carries signal to intracellular target to synthesize cAMP by using ATP
- G-proteins are intermediates in adenylyl cyclase activation which synthesizes cAMP
G protein structure
- made up of heterotrimeric G proteins: alpha, beta, gamma
- in the inactive state alpha binds to GDP in a complex with beta and gamma.
- alpha binds to guanine nucleotides to regulate G protein activity
How is cAMP synthesized and degraded?
- Synthesized from ATP by adenylyl cyclase
- Degraded to AMP by cAMP Phosphodiesterase
How are effects of cAMP mediated?
-by cAMP dependent protein kinase=protein kinase A
What is the structure of the inactive form of protein kinase A?
- inactive form:
- 2 regulatory subunits
- 2 catalytic subunits
How is protein kinase activated? What happens to products?
- cAMP binds to the regulatory subunits which causes dissociation of kinase (catalytic subunits)
- free catalytic subunits can phosphorylate serine on target proteins
Pathway of regulation of glycogen metabolism by epinephrine
- Epinephrine binds to epinephrine receptor which is bound to a G-protein complex
- binding stimulates adeylyl cyclase to increase cAMP concentration via ATP
- The increase in cAMP concentration leads to increase in binding of cAMP to protein kinase A
- 2 catalytic sites are released from Protein kinase A and catalytic sites are phosphorylated by the breakdown of ATP, this triggers activation of the Enzyme phosphorylase kinase
- Phosphorylase kinase activation subsequently activates glycogen phosphorylase (which catalyzes glycogen breakdown)
- Glycogen synthase is inactivated and glycogen synthesis is blocked.
In animal cells, an increase in cAMP activates____? How?
- activates transcription of genes that have a regulatory sequence called cAMP response element (CRE)
- the free catalytic unit (obtained from cAMP binding to protein kinase A) goes into nucleus and phosphorylates the transcription factor CREB (CRE binding protein)
What happens as CREB is phosphorylated in animal cells?
-leads to recruitment of coactivators and expression of cAMP-inducable genes
How is the protein phosphorylation by protein kinase A and protein phosphatase 1 regulated?
-protein phosphorylation is rapidly reversed by protein phosphatases which dephosphorylate the target protein
How does cAMP lead to initiation of a nerve impulse?
-it opens Na+ channels in the PM and leads to initiation of a nerve impulse
synthesization and degredation of cGMP
- cGMP is formed from GTP by guanylyl cyclases
- degraded to GMP by a phosphodiesterase
Give an application for cAMP and cGMP as second messengers
cAMP: is a second messenger in the olfactory pathway
cGMP: is a second messenger in the visual signals pathway: it converts visual signals to nerve impulses in vertebrate eye
Rhodopsin
a photoreceptor in retinal rod cells
Give the pathway corresponding to the role of cGMP in photoreception
- light is absorbed by 11-cis-retinal which isomerizes to all-trans-retinol and activates rhodopsin
- Rhodopsin activates the G-protein transducin
- Transducin stimulates cGMP phosphodiesterase which leads to decreased levels of cGMP.
- cGMP levels are translated to nerve impulses by a direct effect on ion channels by cGMP
What are tyrosine kinases?What is there function?
- cell surface receptors
- directly linked to intracellular enzymes
- Function to phosphorylate substrates on tyrosin residues
Describe structure of tyrosine kinases
- all have a N-terminal on the extracellular ligand domain (cysteine-rich)
- one transmembrane alpha helix
- there is a cytosolic c-terminal domain with protein-tyrosine kinase activity
What types of receptors are included in receptor tyrosine kinases
- includes receptors for most polypeptide growth factors
- human genome encodes 58 receptor tyrosine kinases including: EGF, NGF, PDGF, Insulin, other growth factors
Describe pathway that proceeds when growth factors bind to receptor tyrosine kinases
- binding of ligand (GF) to the extracellular domain activates cytosolic kinase domain
- results in phosphorylation of both the receptors & intracellular target proteins that propogate the signal
- ligand induced receptor dimerizes and results in receptor autophosphorylation: 2 polypeptide chains cross-phosphate each other
Roles of autophosphorylation
- Phosphorylation of tyrosine in the catalytic domain increases protein kinase activity
- Phosphorylation of tyrosine outside the catalytic domain creates binding sites for other proteins that transmit signals downstream from the activated receptors
Downstream signaling molecules
- have domains that bind to specific phosphotyrosine-containing peptides of the activated receptors
ex) SH2 domain
Nonreceptor tyrosine kinase
stimulates intracellular tyrosine kinases with which they are noncovalently associated
Cytokine receptor superfamily
- receptors for tyrosine kinases, but cystolic domains don’t have catalytic activity
- includes receptors for most cytokines and some polypeptide hormones
Steps to provide downstream signaling molecules with SH2 domain.
- ligand binding to cytokine receptors induces dimerization of receptors
- cross phosphorylation of associated nonreceptor tyrosine kinases occurs
- activated kinases then phosphorylate the receptor
- this provides Phosphotyrosine-binding sites for recruitment of downstream signaling molecules with SH2 domains
MAP kinase pathway
MAP kinases
a) mitogen-activated protein kinases
b) serine/threonine kinases
- belong to ERK family
ERK family
-extracellular signal-regulated kinase family
Ras proteins
- oncogenic proteins of viruses that cause sarcomas in rats
- guanine nucleotide-binding proteins that alternate between inactive GDP bound and active GTP bound
Guanine nucleotide exchange factors
- GEFs
- stimulate exchange of GDP for GTP and form the active Ras-GTP complex
GAPs
-GTPase-activating proteins
stimulate termination of Ras–GTP activity by GTP hydrolysis
Mutations of ras genes in cancers
- inhibit GTP hydrolysis
- Ras proteins remain continuosly in the active GTP bound form and drive the proliferation of cancer cells
LIst steps in pathway that activates Raf starting from receptor tyrosine kinase
- autophosphorylation of receptor tyrosine kinase leads to binding of Ras GEFs
- (GEFs) stimulate exchange of GDP for GTP and form the active Ras-GTP complex)
- Activation of Ras leads to activation of Raf protein serine/threonine kinase
Raf protein
- serine/threonine kinase
- phosphorylates and activates a second protein kinase (MEK then ERK)
ERK function
- phosphorylates a variety of target proteins
- some activated ERK goes to the nucleus and regulates transcription factrors by phosphorylation
MEK function
-phosphorylates ERK
SRE
- Serum response element (SRE)
- recognized by transcription factors including serum response factor (SRF) and Elk-1
- mediates rapid transcription of immediate-early genes
Secondary response genes
-expression by induction of SRE whch leads to altered expression of a battery of other downstream genes (2ary response genes)
MAP kinase pathways of yeasts and mammalian cells
- each cascade consists of 3 protein kinases:
1. terminal MAP kinase
2. two upstream kinases (analogous to Raf and MEK)
Mammalian MAP kinases
ERK, JNK, p38 kinases
Specificity of MAP kinase signaling.
GIve example
- maintained partly by physical association on scaffold proteins
example: KSR scaffold protein organizes ERK and its upstream activators Raf and MEK into a single cassette
PIP2
- Phospholipid Phosphatidylinositol 4,5-bisphosphate
- derived from membrane
PI 3 kinase
-phosphorylates PIP2 to yield the 2nd messenger PIP3
PIP3
phosphatidylinositol 3,4,5-triphosphate
Akt
- a serine/threonine kinase
- activated by phosphorylation, PDK1, and phosphorylation by mTORC2
PI 3-kinase/Akt pathway
- PI 3-kinase is recruited to activated receptor tyrosine kinases via its SH2 domain
- PIP3 is formed by phosphorylation of PIP2 by PI 3-kinase
- PIP3 targets Akt via its pleckstrin homology (PH domain)
- Akt is phosphorylated and activated by another protein kinase (PDK1)
- Activation of Akt also resuires phosphorylation by MTORC2
mTORC2
- phosphorylates and activates Akt
- protein kinase
Phospholipase C/Calcium pathway
- phospholiplase c-gamma (PLC-gamma) binds to activated receptor tyrosine kinases via its SH2 domain
- tyrosine phosphorylation increases PLC gamma activity and stimulates hydrolysis of PIP2
- DAG and IP3 are stimulated downstream
Hydrolysis of phospholipase C produces what two second messengers?
- DAG: diacylglycerol
2. inositol 1,4,5-triphosphate (IP3)
DAG
after produced by hydrolysis of phospholipase C, it remains associated with the PM and activates serine/threonine kinases of the protein kinase C family
IP3
After produced by hydrolysis of phospholipase C, IP3 binds to receptors that are ligand gated Ca2+ channels in the ER.
-as channels open, Ca2+ can move out of the ER
Calmodulin
- a major Ca2+ binding protein
- mediates effects of Ca2+
- is activated when Ca2+ concentration increases
Ca2+/calmodulin complex
-binds to target proteins, including protein kinases
Myosin light chain kinase
-signals actin-myosin contraction by phosphorylating one of the myosin light chains
CaM kinase family, give 2 examples of application
- activated by Ca2+/calmodulin
- phosphorylate metabolic enzymes, ion channels, transcription factors
ex. one form regulates synthesis and release of neurotransmitters
ex. can regulate gene expression by phosphorylating transcription factors
CREB
-phosphorylated by CaM kinase and also by protein kinase A
Regulation of intracellular Ca2+ in electrically excitable cells
- Ca2+ is increased by uptake of extracellular Ca2+ by regulated channels in the PM
- in electrically excitable cells of nerve and muscle, voltage-gated Ca2+ channels are opened by membrane depolarization which results in an increase in intracellular Ca2+ signals
Ryanodine receptors
- channels that open in the ER membrane to release Ca2+ into cytosol due to the increase in intracellular Ca2+ by membrane depolarization signals
- R receptors in muscle cells in the SR may also be opened directly in response to membrane depolatization
Ca2+
versitile second messenger that controls a wide treange of celluar processes including the trigger of release of neurotransmitters