Brummer Mechanisms of cell communication

• transmitter and addressee must be clearly identifiable

• addressee must be ready to receive

• the signal must be sturdily, but still modulatable, its signal transmission must be fast

• signal must be disconnectible

• signal must be decodeable

Intercellular:

• [rapid] nervous system

• [slow] hormones

• cell-cell and cell-matrix adhesion

Intracellular:

• Ions

• metabolites

• phosphorylation cascades and other post-translational modifications

1. Contact dependent (membrane bound signal molecule)

2. paracrine (signal molecule secreted = local mediator)

3. synaptic (neurotransmitter released at synapse translating a electrical signal)

4. endocrine (hormone secreted and transported over bloodstream to target)

• combination of signals can trigger differnet responses in cell (survive, divide, differentiate, die)

• Same signal can trigger different response in different cell (acetylcholin triggering secretion in nerves, heart rate decrease in heart and contraction in muscle)

• can trigger fast (altered protein function) or slow response (altered protein synthesis [transcription/translation])

  
  

1. signal initiation in the cytoplasm yb membrane diffusable messengers

   • NO (nitric oxide) signalling (Blood preassure control —> NO causes rapid relaxation of smooth muscle cells)

   • steroid hormone receptors

2. signal transduction across the membrane

   • Ion channels (signal molecule binds —> opens ion channel)

   • G-protein coupled receptors (GPCRs)

   • receptor-tyrosine kinase (RTK) pathways

   • JAK/STAT (dimerazation of JAK, phosphorylation and cross linking of STAT) and NF kappa B [infalmation] (ubiquitination and degradation of inhibitor after phosphorylation by kinase) pathways

   • developemental pathways (WNT, Hh) or cell adhesion molecules (integrins, cadherins)

   
   

1. Steroids are hydrophobic molecules – entry by diffusion

2. Only cells expressing SR react (e.g. glucocorticoid receptor)

3. SR are ligand regulated transcription factors this means:

-they are sequestered away from DNA in absence of a ligand

-ligand binding induces a conformational change, triggering nuclear import (dissociation of inhibitory proteins) and DNA binding (interaction with Coactivators)

-transcription induces primary response proteins which turn on secondary (delayed) reponse proteins

—> Spatial Separation of signalling molecule localisation and action

—> Recurring theme!!! (Compare with NF-kB or JAK/STAT pathways)

G-protein coupled receptors: —> activation by conformational change

• signal molecule binding activates G-protein conformational change which allows enzyme to interact with G-protein (both membrane bound)

Enzyme coupled receptors: —> dimerization

• signaling molecule (can also be dimer) recruits two parts of a enzyme that is activated when it dimerizes, that again can activate an associated enzyme

—> Receptors are usually coupled to complex signalling chains or cascades, leading the signal from the plasmamembran to the nucleus

• high specificity

• modulation on many different levels

• better amplification of the signal

1. Diversification/branching

2. amplification / positive feedback

3. negative feedback

4. double negative feedback

• Scaffold proteins ensure rapid and specific transmission of signals

• pre organization of the intracellular signalling proteins

Enzymes, which transfer the g-Phosphat of ATP to the free hydroxylgroup of a serine-, threonine- or tyrosine residue of a substrate protein

Phosphorylation outcomes:

• priming (if tyrosine Y is phosphorylated serine S or threonin T kinase will follow)

• prevention (phosphorylation = change of chargelandscape that confuses proteins)

• conformational change

• change of portfolio (recruiting interaction partners)

Besides signalling by phosphrylation there is also signalling by GTP-binding

The binding of an extracellular signal molecule to its GPCR activates adenylyl cyclase via Gs and thereby increases cAMP concentration in the cytosol. This rise activates PKA, and the released catalytic subunits of PKA can then enter the nucleus, where they phosphorylate the transcription regulatory protein CREB. Once phosphorylated, CREB recruits the coactivator CBP, which stimulates gene transcription. In some cases, at least, the inactive CREB protein is bound to the cyclic AMP response element (CRE) in DNA before it is phosphorylated

RTK (receptor tyrosine kinases) domains are activated when dimerized (dimer signal moplecule bound) and then the kinase domain phosphorylates and transphosphorylates which triggers binding of signaling proteins to the phosphrylated tyrosines (SH2 domains)

—> accessability by conformational change

—> phosphorylation landscape (charge)

—> multiple requirements (binding of different sides)

RAS (Rat sarcoma) proteins are GTPases that play a crucial role in regulating cell growth and differentiation

Membrane bound part of the Ras - Raf - MEK - ERK pathway

Needs a GEF (Guanylyl Exchange Factor) to reach active GTP bound state and GAP (GTPase activating protein) to cut phosphate and reach inactive GDP bound state

1.Cells are constantly exposed to extra- and intracellular information, which both dictate cellular behaviour & fate

2.Every disease can be explained by aberrant signal transduction

3.Drugs interfere with physiological and pathological signals and thereby cause (un)desired effects!!!

4.Time and space (compartments) are both important aspects for understanding cellular signalling

5.Extracellular signals either pass (“diffuse”) through the (plasma)membrane or are received by receptors and converted into intracellular signals

6. Receptors either contain intrinsic enzymatic activities, e.g. RTKs, or are non-covalently bound to enzymes, e.g. GPCRs, antigen receptors

7. Spatial organisation of intracellular signals is achieved by adaptor, docking or scaffold proteins with specific interaction domains that “read” the activation state of their binding partner, e.g. pTyr-SH2 domain

8.Distinct signalling intensities are sensed by post-translational modifications and protein stability

9. Temporal control of (intra)cellular signals is mediated by positive, negative or double-negative feedbacks

Temporal control of (intra)cellular signals is mediated by positive, negative or double-negative feedbacks

Spatial organisation of intracellular signals is achieved by adaptor, docking or scaffold proteins with specific interaction domains that “read” the activation state of their binding partner, e.g. pTyr-SH2 domain

Distinct signalling intensities are sensed by post-translational modifications and protein stability, amplification (positive feedback)