Physiology 3140A Lecture Notes - Lecture 17: Fluorescent Tag, Confocal Microscopy, Pepsinae
2 May 2018
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Cell Physiology Lecture 17
GPCR 1
- Can tag receptors with fluorescent proteins (e.g. GFP)
o When a fluorescent tag is added onto a receptor, you can image the cell with a confocal
microscope and see where the receptor is in the cell
- In a resting cell, many receptors are seen at the cell surface
Activation of GPCR signaling
Receptors
- 7 transmembrane alpha helical structure
- G protein-coupled receptors (GPCRs) form LARGEST family of integral membrane receptor proteins
in humans just greater than 2% of genome
- A large portion of the human genome encodes GPCRs
o Estimated that more than 650 GPCRs
o ~ 300 (almost half) encode olfactory receptors
▪ Things that we can smell (small molecules) serve as ligands for GPCR
o ~ 250 receptors have been ascribed endogenous ligands
▪ Adrenaline, dopamine, serotonin, hormones, neurotransmitters
o ~ 100 orphan receptors with no know function
▪ These are parts of the genome that encode GPCRs (know this based on the sequence
has the same structural features as GPCRs)
▪ We know they are receptors and they can be tagged with GFP
▪ Do not know what the ligands are
▪ There are still a large number of GPCR for which we do not know the function,
ligand, or what drugs can be used to modulate the function
o ~ 1/2 of all prescribed drugs target GPCRs either directly or indirectly
▪ Drugs are developed around the endogenous ligand to alter biological effects
Ligands
- Enormous diversity in ligands that activate G protein-coupled receptors (GPCRs)
- GPCRs respond to ligands as diverse as light, smell, taste, hormones, neurotransmitters, ions,
nucleotides, small peptides, large glycoproteins, fatty acids, amino acids, and pheromones
o There are small and large molecules, proteins, fatty acids, etc.
- The same ligand can activate multiple different receptors in the same family
- Ligands work as FAMILIES
o There is not a single adrenaline receptor or a single Ach receptor
o The numbers are always changing because there are orphan receptors for which we do not
know the function for
o New receptors and new functions are constantly being discovered
- Receptors are in FAMILIES there can be multiple receptors for an individual ligand
o Each of the receptors can have a different outcome
- Examples
o 9 distinct receptors are activated by adrenaline
o 5 receptors are activated by acetylcholine
o 7 receptors are activated by glutamate
find more resources at oneclass.com
find more resources at oneclass.com
o 15 receptors are activated by serotonin
Diversity of physiological responses stimulated by GPCRs (short list)
- Receptor can be connected to different intracellular signaling molecules and pathways and they can
lead to different outputs
- Many GPCRs regulate fat breakdown
- ACTH:
o ACTH binds to its families of receptors in the adrenal cortex and leads to cortisol release
o In fat tissues, ACTH is involved in triglyercide breakdown (modulation of energy sources in
the cell and metabolism)
o ACTH leads to different responses in different tissues
- Can have the same ligand and same family of receptors that are present in different tissues
- TSH: found in the thyroid gland and fat
- Adrenaline:
o Involved in glycogen breakdown in muscle cells
o Involved in regulating heart rate and force of contraction in the heart
- There are tissue specific responses and receptor specific responses
o Different functions with the same ligand
- Responses are governed by the signaling pathway that the receptor is coupled to
- The same receptor can have multiple functions
o E.g. adrenaline: can bind to beta 2 receptor in muscle for glycogen breakdown
▪ Beta 2 receptor in heart can be involved in the regulation of heart function
- The same receptor can have different intracellular signaling pathways because the end target that
they signal to is linked to a different biological function
Examples: top selling drugs targeting GPCRs
- ~1/2 drugs that are prescribed are directed to GPCRs
- GPCRs are used as targets to develop drugs against
- E.g. histamine receptors:
o In the gut, histamine receptors are involved in ulcer development and inflammation
▪ Drugs: zantac and pepsid are antagonists of histamine receptors and treatments for
ulcers
o In the respiratory system, histamine receptors are involved in allergies
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find more resources at oneclass.com
▪ Drug: Claritin and allegra are used for bronchial constrictions
o The receptor can have different biological functions and the drugs are targeted towards a
variety of histamine receptors
- E.g. 5-HT receptors:
o Related to many brain-related things (CNS)
o Ranges from psychosis to schizophrenia to migraines
o Drugs have different outcomes and effects on different 5-HT receptors
- Different receptors are expressed differentially in different tissues even though the same ligand
binds to them
o Different drugs are developed to address specific receptor subtypes within specific tissues to
fix a specific physical condition
- There are subsets of receptors that have different tissue distributions
o Same ligand can bind to different subsets of receptors and can mediate different biological
outcomes
o Drugs can be targeted towards the SUBTYPE OF THE RECEPTOR
- Drug companies look for the selectivity of the drug they want to develop for a subtype of receptor
within a particular tissue
GPCR structure and topology
Domain functions
- 7 transmembrane domains (7 alpha helices)
o Attached on the inside and outside generating:
▪ 3 intracellular loops
▪ 3 extracellular loops
- Extracellular amino-terminal
- Intracellular and carboxy-terminal
- Two domains where ligands can bind: amino terminal, transmembrane domain
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Can tag receptors with fluorescent proteins (e. g. gfp: when a fluorescent tag is added onto a receptor, you can image the cell with a confocal microscope and see where the receptor is in the cell. In a resting cell, many receptors are seen at the cell surface. G protein-coupled receptors (gpcrs) form largest family of integral membrane receptor proteins in humans just greater than 2% of genome. Enormous diversity in ligands that activate g protein-coupled receptors (gpcrs) Gpcrs respond to ligands as diverse as light, smell, taste, hormones, neurotransmitters, ions, nucleotides, small peptides, large glycoproteins, fatty acids, amino acids, and pheromones: there are small and large molecules, proteins, fatty acids, etc. The same ligand can activate multiple different receptors in the same family. Receptors are in families there can be multiple receptors for an individual ligand: each of the receptors can have a different outcome.
