Lecture 1 Flashcards
Explain what autocrine, paracrine, endocrine, juxtacrine and synaptic signaling is
1. Autocrine Signaling
- Signal molecules act on the same cell that released them
- Short-range communication [1]
(eg cytokines, growth factors)
2. Paracrine Signaling
- Secretion of signal molecules locally in a tissue
- Short-range/localized communication to nearby cells [1]
(eg cytokines, growth factors)
3. Endocrine Signaling
- Signaling molecules (hormones) are secreted into the bloodstream
- Long-range communication to multiple target cells
- Hormones circulate in nano-molar concentrations
- Slower
(eg hormones)
4. Juxtacrine Signaling
- Contact-dependent signaling
- Signal molecules (often transmembrane proteins) on one cell interact with receptors on a target cell
- Forward and reverse signaling is common [4][5]
(eg adhesion molecules)
5. Synaptic Signaling
- Neurotransmitter-mediated signaling between neurons or neurons and target tissue cells
- Release of neurotransmitters in a synapse
- Specific target cell(s)
- Dependent on the cellular context and intracellular signaling molecules present [5]
- It affects the electrical properties of/ membrane potential of targets.
(eg NTs)
What is a first messenger?
A molecule (growth factor, cytokines, NTs, hormones, adhesion molecules) that reach the cell from the extracellular space and bind to their specific receptors and initiate/ change intracellular activity.
Ligand-gated ion channels
The receptor is also an ion channel
Upon ligand binding the ion channel opens
These type of receptors play an important role in the nervous system
– Regulation of membrane potential and
neuronal activity
Ion channels permeable to Ca2+ like NMDAR is especially involved in activation of signalling pathways
– Ca2+ leads to the activation of a cascade of
signaling molecules
– Synaptic plasticity and gene expression
changes
Intracellular receptors
• Well-known examples of first messengers binding to intracellular receptors are thyroid hormones and steroid hormones like the glucocorticoid corticol (stress hormone)
• These signalling molecules require carrier proteins to be transported in aqueous solutions (such as the bloodstream)
• These signalling molecules bind to receptors of the nuclear receptor family which acts as transcription factors when activated
– Ligand binding domain (LBD)
– DNA-binding domain (DBD)
• Receptors for these hormones exists in the CNS
Cell surface receptors
• Basic build up of cell surface receptors;
– Extracellular ligand (first messenger) binding site
– Transmembrane region
– Intracellular/cytoplasmic effector domain
• Upon ligand binding the receptor undergoes conformational change and the cytoplasmic (effector) domain mediates activation of different
intracellular signaling molecules forming a signal transduction pathway
• The response to the first messenger depends on the presence of specific
receptor as well as the cellular context, ie intracellular signalling molecules
present.
– Same messenger can be used in different situations / roles
G-protein coupled receptors (GPCRs)
• GPCR is the largest receptor family
• Most likely, all types of cells in the body express some GPCRs.
• Around 30-40% of all current drugs target GPCRs
• Seven transmembrane domain protein
• A very large number of first messengers can activate GPCRs
– Neurotransmittors (NTs)
– Photons
– Odorants
– Hormones
– Neuropeptides
– Endocannabinoids
• GPCRs signal using trimeric G-proteins
• Trimeric G-proteins consists of 3 subunits Gα, Gβ and Gγ
• Ligand binding to the GPCR induces a conformational change in the receptor
• When the receptor is activated the trimeric G-protein dissociate into Gα and Gβγ which can both mediate downstream signalling
• There are several different types of Gα
• Thus if a GPCR is activated many different signal transduction pathways could potentially be activated or inhibited
Enzyme linked receptors
• The receptor have an intracellular domain with enzymatic activity
• Receptor tyrosine-kinases (RTKs) or Tyrosine kinase receptors (TKRs)
– The receptor have an intracellular tyrosine kinase domain which mediates phosphorylation of tyrosine residues in target proteins
• Serine / Threonine kinase receptors
– The receptor have an intracellular serine/threonine kinase domain which
mediate phosphorylation of serine/threonine residues in target proteins
• Phosphorylation is a reversible modification which can regulate the function of many proteins
– Create docking sites for specific downstream signaling proteins
– Alter catalytic ability of enzymes
Transmembrane nonenzyme receptors
• Membrane receptors that are not Ligand gated ion channels, GPCRs or
enzyme linked receptors
• In this course we will look at signaling from three types of transmembrane nonenzyme receptors
– Notch receptors
– Toll-like receptors
– Cytokine receptors
Adhesion molecules
• Originally thought of merely as a sort of glue, but now recognized that they act as signaling molecules and are properly described as receptors
• Ligand (first messenger) that interact with the adhesion molecule is generally insoluble and frequently adhesion molecules themselves
– On other cell
– In extracellular matrix (ECM)
• Juxtacrine signaling, contact dependent signalling
• Often bi-directional signaling
Adaptor and scaffold proteins
• Adaptor / scaffold proteins do not harbor enzymatic activity
• Work by providing docking sites for other signaling proteins
• Adaptor proteins are composed mainly of interaction domains and fit together other proteins to form functional units
• Scaffold proteins are multiadaptors and can fulfill the function of a platform enabling highly selective interactions
• Often subjected to regulatory modifications, eg phoshoprylation, that provide signal-directed docking sites for signaling proteins
• Adaptors/scaffold proteins play a important role by enhancing efficiency and specificity in signaling
– Adaptor protein Grb2 brings SOS to the activated RTK which results in activation of Ras-MAPK (Raf-Mek-Erk) pathway
– The KSR scaffold protein bind Raf, Mek and Erk and is important for specificity in this MAPK cascade
• Can also target signaling pathway to specific subcellular site
– The AKAP scaffold protein link downstream signaling effectors (PKA, PP2B, PKC) to the receptor
Protein-protein interaction domains
• Domains that bind to phophosphorylated amino acids, ie
phospotyrosine binding domains
– SH2
– PTB
• Domains that bind to specific amino acid (aa) sequences
– SH3 domain
– PDZ domain
• Domains that bind to the same domain in another protein
– PDZ
– Toll/IL-1 receptor (TIR) domain
– Death Domain (DD)
Membrane targeting domains
• Phospholipid binding domains
– Plecktrin (PH) domain:
• Affinity for phosphatidylinositol lipids (PIP2 and PIP3)
– C1 domain:
• Affinity for Diacylglycerol (DAG)
– C2 domain:
• Affinity for phospholipids
– Phosphatidylserine (PS) lipids
Signaling enzymes - Transducers
• Enzymatic activity of the transducer regulates the ability of the transducer to interact with downstream signaling molecules
• Transduce further signaling by interacting with down- stream signaling components
• GTPases (G proteins) are transducers
Trimeric G-proteins used by GPCRs
Monomeric G-proteins like Ras used by RTKs
• GTPases or GTP-binding proteins or G proteins
• Protein capable of hydrolysing GTP to GDP
• Have a G or GTPase domain; mediating GTP binding and GTPase activity
• Basic cycle of GTP binding and GTP hydrolysis switching them between active and inactive states
• Can interact with down-stream signalling components when in the active (GTP-bound) state
(Hetero)Trimeric G-proteins
• Used by GPCRs
• Trimeric G-proteins consists of 3 subunits Gα, Gβ and Gγ
• Gα is activate when bound to GTP and inactive when bound to GDP
• Ligand binding to the GPCR induces a conformational change in the receptor
• When the activated receptor interact with the trimeric G-protein, GDP is
exchanged to GTP in the Gα subunit and the heterotrimeric complex
dissociate into Gα-GTP and Gβγ (or Gα-GTP is re-oriented)
• Gα–GTP and Gβγ can individually interact with/signal to downstream effectors
• Signaling is terminated when the Gα subunit hydrolyses GTP to GDP and Gα reassociates with Gβγ.
Monomeric G proteins
• There are hundreds of monomeric G proteins, can be divided into 6 groups; Ras,Rho, Rab, Ran, Arf, Rem/Rad
• Monomeric GTPases are active when bound to GTP and inactive when bound to GDP
• Slow intrinsic hydrolysis activity
• Monomeric GTPase activity is controlled by guanine nucleotide exchange factors (GEF) and GTPase activating proteins (GAPs)
– GEFs activates the monomeric G-protein (Ras) by facilitating release of GDP and binding of GTP onto Ras
– GAPs stimulate the enzymatic activity of monomeric G proteins thus the
hydrolysis of GTP to GDP and inactivation of the monomeric G-protein (Ras or Rho) is enhanced
• The active GTP bound form can interact with downstream effectors
