The cell is not a bag of water

We have over 800 G protein coupled receptors (GPCRs).  We have not found 800 distinct intracellular messengers (such cyclic adenosine monophosphate — aka CAMP).  A single cell can express up to 100 GPCRS — Mol. Pharm. vol. 88 pp. 181 – 187 ’15.  Some of them raise CAMP levels, others decrease it.  CAMP is supposed to diffuse freely within the cell.  If so, different GPCRs which change cellular CAMP levels to the same extent they should produce identical effects. But they don’t.

One example — Isuprel stimulation of beta adrenergic GPCRs increases cardiac contractile force and activates glycogen metabolism.  Prostaglandin E1 (PGE1) GPCR causes the same CAMP increase without this effect.

A recent fascinating paper may explain why [ Cell vol. 185 pp. 1130 – 1142 ’22 ]  The authors had previously done work showing that under basal conditions CAMP is mostly bound to a protein (regulatory protein kinase A subunit — aka PKA RIalpha ) leading to very low concentrations of free CAMP.

So free diffusion occurs only if CAMP levels are elevated well above the number of binding sites for it.

As usual, to get new interesting results, new technology had to be used.  A biosensor for CAMP based on Forster Resonance Energy Transfer aka FRET —  https://en.wikipedia.org/wiki/Förster_resonance_energy_transfer, was added to two different GPCRs — one for the Glucagon Like Peptide 1 (GLP1) and the other for the beta2 adrenergic receptor.

Even better, they fused the biosensor to the GPCRs using rulerlike  spacers each 300 Angstroms (30 nanoMeters) long.  So they could measure CAMP levels at 30 and 60 nanoMeters from the GPCR.  Levels were highest close to the receptor, but even at 30 and 60 nanoMeters away they were higher than the levels in the cytoplasm away from the cell membrane.  So this is pretty good evidence for what the authors call RAINs (Receptor Associated Independent camp Domains — God they love acronyms don’t they?).

Similar localized responses were seen with the beta2 adrenergic receptors, suggesting that RAINs might be a general phenomenon of GPCRs — but a lot more work is needed.

Even more interesting was the fact that there was no crosstalk between the RAINS of GLP1R and the beta2 adrenergic receptor.  Stimulation of one GPCR changed only the RAIN associated with it and didn’t travel to other RAINs

So the cell with its GPCRs resembles a neuron with its synapses on dendritic spines, where processing at each synapse remains fairly local before the neuron cell body integrates all of them.  It’s like Las Vegas — what happens at GPCR1 (synapse1) stays in GPCR1 (synapse1).  Well not quite, but you get the idea.

The neuropharmacological brilliance of the meningococcus

The meningococcus can kill you within 12 hours after the spots appear — https://en.wikipedia.org/wiki/Waterhouse–Friderichsen_syndrome.  Who would have thought that it would be teaching us neuropharmacology.   But it is —  showing us how to make a new class of drugs, that no one has ever thought of.

One of the most important ways that the outside of a cell tells the inside what’s going on and what to do is the GPCR (acronym for G Protein Coupled Receptor).  Our 20,000 protein coding genome contains 826 of them. 108 G-protein-coupled receptors (GPCRs) are the targets of 475 Food and Drug Administration (FDA)-approved drugs (slightly over 1/3).   GPCRs are embedded in the outer membrane of the cell, with the protein going back and forth through the membrane 7 times (transmembrane segment 1 to 7 (TM1 – TM7). As the GPCR sits there usually the 7 TMs cluster together, and signaling molecules such as norepinephrine, dopamine, serotonin etc. etc. bind to the center of the cluster.   This is where the 475 drugs try to modify things.

Not so the meningococcus. It binds to the beta2 adrenergic receptor on the surface of brain endothelial cells lining cerebral blood vessels, turning on a signaling cascade which eventually promotes opening junctions of the brain endothelial cells with each other, so the bug can get into the brain.  All sorts of drugs are used to affect beta2 adrenergic receptors, in particular drugs for asthma which activate the receptor causing lung smooth muscle to relax.  All of them are small molecules which bind within the 7 TM cluster.

According to Nature Commun. vol 10 pp. 4752 –> ’19, the little hairs (pili) on the outside of the organism bind to sugars attached to the extracellular surface of the receptor, pulling on it activating the receptor.

This a completely new mechanism to alter GPCR function (which, after all,  is what our drugs are trying to do).  This means that we potentially have a whole new class of drugs, and 826 juicy targets to explore them with.

Here is one clinical experience I had with the meningococcus.  A middle aged man presented with headache, stiff neck and fever.  Normally spinal fluid is as clear as water.  This man’s was cloudy, a very bad sign as it usually means pus (lots of white blood cells).  I started the standard antibiotic (at the time)  for bacterial meningitis — because you don’t wait for the culture to come back which back then took two days.  The lab report showed no white cells, which I thought was screwy, so I went down to the lab to look for myself — there weren’t any.  The cloudiness was due to a huge number of meningococcal bacteria.  I though he was a goner, but amazingly he survived and went home. Unfortunately his immune system was quite abnormal, and the meningitis was the initial presentation of multiple myeloma.