Which class of molecules could signal from outside to inside the cell without a channel or receptor?
Hydrophobic molecules: O2, CO2, N2, steroid hormones
Signaling via small molecules
- Independently of plasma membrane proteins
- Through plasma membrane channels
- Downstream of plasma membrane G-protein coupled receptors
Nitric Oxide
- made by the deamination of arginine by NO synthases
- Acts locally because of a 5-10 second half life
- Affects smooth muscle and other target cells
- nitroglycerine is used to treat heart pain: it is converted to NO and relaxes blood vessels, reducing workload on the heart
Small Molecule Transport
- Transported in extracellular fluids by carrier proteins
- dissociate from carriers upon cell entry
- in the cell they bind a member of the nuclear receptor superfamily
The Nuclear Receptor Superfamily
Contain binding sites for a small hydrophobic molecule and for DNA
- 48 identified in the human genome
- more than half only identified based on sequence analyses
- -> their ligands are unknown
- -> termed orphan nuclear receptors
What does ligand binding do?
It alters receptor conformation, and releases inhibitors, to promote DNA binding and downstream transcription
Complexity of Hormonal Transcriptional Responses: primary vs secondary
Primary (early): steroid binds to receptor, receptor-steroid-hormone complexes activate primary response genes –> induced synthesis of primary-response proteins
Secondary (delayed) response to steroid hormones: a primary-response protein shuts off primary-response genes –> a primary-response protein turns on secondary-response genes–> secondary response proteins are produced
Signaling via plasma membrane channels
Ion channels are a major class of signaling molecules: H+, Na+, HCO-3, K+, Ca2+, Cl-, Mg2+
Ion Channel functions
- electrical excitability of muscle cells
- electrical signaling in the nervous system
- leaf-closure responses in plants
- signal the single-celled paramecium to reverse its movement upon collision
- others (these channels are present in all animal cells)
Ion channel properties
- have narrow selective pores
- open/close rapidly
- up to 100 million ions can pass through an open channel/s
- transport is PASSIVE: based on electrochemical gradients across the plasma membrane and ion diffusion down these gradients
Ways Ion channels can be activated
Voltage-gated: plasma membrane is very polar; change in potential can cause it to open
ligand binding - induces conformational change
mechanically gated: ends are different; since one side **stretch can open this)
Voltage-gated sodium channels produce and respond to action potentials
- restricted outward leakage of potassium ions polarizes plasma membrane of excitable cells
- equal +ve and -ve on inside and outside of cell (normally do not see this)
- the ends are different on the membrane; so it is poalr again; however it is still almost equal…only becomes A BIT more polar
- the opening of sodium channels allows sodium ions to move in, depolarizing the membrane
- change induces a traveling wave of depolarization (AP)
Initiating An Action Potential
- A trigger opens sodium channels – depolarization
With strong depolarization, other sodium channels open, which depolarizes the membrane further, opening more channels (ex + feedback) - With time, mechanisms repolarize the initial membrane — thus wave is directional
At the presynaptic nerve terminal, the action potential triggers exocytosis of synaptic vesicles
In a resting chemical synapse: vesicles contain the neurotransmitters and transmitter gated ion channel on the post synaptic target cell remained closed
In an active chemical synapse: vesicle fuse with membrane and release the neurotransmitter which is then accepted by the transmitter-gated ion channel receptors
Neurons also activate other cell types by…
synaptic connections
Signaling via small molecules: downstream of plasma membrane G-protein coupled receptors
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G-protein coupled receptors
- 7 pass transmembrane proteins
- activated by proteins, small molecules and light
- more than 700 in humans (so they are very important)
The Core Players
- all G-protein coupled receptors signal into the cytoplasm via a membrane associated trimeric GTP-binding protein (alpha, beta, gamma)
Inactive GPCR
- GDP bound
- everything is still connected
Active GPCR
- extracellular signal molecule binds to the receptor
- exchange of GDP for GTP on the alpha subunit
- alpha subunit undergoes conformational change which alters conformation of other subunits, causing their release
- altered subunits bind downstream effectors
- the activity is turned off by a regulator of G protein signaling (RGS) which acts as a GTPase activating protein
- alpha, beta, gamma, or both of each could be released from the complex upon activation
G proteins can signal rapidly via cyclic AMP (cAMP
cAMP is synthesized from ATP by adenylyl cyclase and is destroyed by cAMP phosphodiesterase
The canonical cAMP pathway
The signal is transduced by increasing adenylyl cyclase activity above a constant background of phosphodiesterase activity
- accumulated cAMP activates PKA
- PKA phosphorylates CREB to activate transcription
A sense of smell with GPCRs and cAMP
- every single neuron has a different GPCR than another neuron
- 350 GPCR allow us to smell
- each receptor recognizes a different set of odourants and then produces cAMP-gated cation channels which induce an AP
Intracellular Ligand gated channel - each olfactory neuron expresses just one of these receptors
A smell is a compilation of different odurants - signals from different combinations of neurons allows us to distinguish 10 000 different smells
GPCR signaling via calcium
- signal molecule binds to receptor - activates GPCR
-activated Gq protein activates phospholipase C-B - phospholipase C-b signals IP3
-IP3 is a water soluble molecule that diffuses through the cytoplasm ( goes around in a 3D area) - DAG is a hydrophobic molecule that diffuses along the plasma membrane (goes around in a 2D surface)
- IP3 binds to open-gated Ca2+ release channel,
Ca2+ binds to PKAC which is bound to DAG, which signals to others….
