The TRPV channel, a book by Annika Malmberg et al. Part I

Dr. Malmberg is one of the former brain trust with Tony Yaksh at UCSD, then to UCSF, now to Elan Pharmaceuticals. We can genuinely say we hope they make a lot of money. They are the real deal.

We regret very much not being able to include illustrations from Annika Malmberg’s new book on the TRPV channel with Keith Bley, “Turning up the Heat on Pain”.

It is surprisingly readable for state of the art science, like much of Dr. Malmberg’s prior work. You can get it at Amazon,com and elsewhere.

Also known as the “heat pain” or capsaicin receptor, TRPV1 belongs to a superfamily of ion channels, known as TRP. TRP was first noted in fruit flies which react abnormally to light.

TRPV1 is made up of SIX domains. A number of other channels have a similar six domain structure, including a potassium channel. TRPV-1′s domains are largely made of ankyrin.

Ankyrin is a protein which forms a kind of skeleton for the cytoplasm of a cell (cytoskeletal protein). Ankyrin is made of 33 residues, each made of 24 copies of a repeating sequence of molecules. Thus, it is not surprising that this scaffolding made of essentially identical building blocks is more about structure than it is about minute chemical differences along the way. Calmodulin (CaM), reviewed in prior articles, is a pain protein which binds to TRPV1 at the first domain, or grouping which includes ankyrin.

The six transmembrane domains, which are linked together, have a water repelling (hydrophobic) area) between the fifth and sixth domain. This area penetrates the cell membrane in an extracellular loop, and here is where GLUTAMATE, a pain exciter, binds in multiple places. The loop also has overlapping residues of the amino acids serine and threonine. Keep in mind that structure and shape are very important, not just the chemical composition. In a house you are interested in bricks which allow space for windows, not the chemical composition of each individual brick.

TRP channels, at least in fruit flies, are multimers which gather to form even larger complexes. These complexes contain a G protein coupled receptor (see G protein, using search–G protein receptors are also called metabotropic) Rhodopsin; AND an effector (Phospholipase C or PLC), and two regulators (Protein Kinase C aka PKC, and Calmodulin). They also contain a scaffolding protein known as INAD. This makes a nice little package to help explain how PLC, PKC, and CaM, as discussed in prior articles may have a hand in pain, although the nexus is the TRPV1 channel.

TRPV1 apparently forms as a tetramer (four part complex). TRPV3 ix also a heat sensitive channel and produced by a gene right next to the one that makes TRPV1. However, this neighbor subunit does not seem to affect TRPV functionality either as to one individual channel nor the entire membrane set.

The next concept is very, very important in view of the fact that Central Pain patients feel, relatively speaking, like they have acid under the skin. This sensation in severe form is the unbearable burning dysesthesia, which is difficult to describe. Acids, also called by scientists “protons”, or “Hydrogen ions” (H+) act on the extracellular loop between domains 5 and 6 which bind glutamate. No wonder that CP patients feel as if they are being bathed in acid. They are.

For example, in a blister from any cause, blister fluid under the skin can reach a pH of 5.0, which of course helps cause the pain of a blister. Central Pain could be looked at as a blister over the entire body, (including the mouth and eyes when the causative lesion is high) and indeed the sensation of burning dysesthesia is most similar to acid just under the skin, yet it has a mix of other components, the compound of which makes the agony indescribable. Severe cases are clearly worse than severe sunburn, including the desire for skin not to touch clothing, and nearly everyone can relate to that.

For those unfamiliar with organic chemistry, TRPV1 has both an N or amino terminus and a C (or COOH) terminus. (see using SEARCH on what this means). Phosphatidyl inosital binds to the C terminus or end, while calmodulin binds to both C and N ends. The N terminus binding site for calmodulin is the first ankyrin domain repeat. Think of six sticks of dynamite across the cell memebrane with an extracellular loop between the fifth and sixth stick and you will have some idea of what TRPV-1 looks like.

As you know, phosphorylation is the ACTIVATION of a body chemical by adding a high energy phosphate bond (which acts like a battery). Unphosphorylated chemicals tend to be completely inert. Protein Kinase A, Protein Kinase C and Calmodulin dependent kinase II phosphorylate the residues of the amino acids serine and threonine in the extracellular loop between the fifth and sixth domains. This is also where protons (ACIDS) act on glutamate residues at the same loop. The acids are extracellular. We have written extensively about perineuronal acidosis in central pain, and the International Association for the Study of Pain has also published a paper discussing this, by one of our authors, who was the first to note it in central pain.

Thus, Malmberg, Bley, and the authors have done us a great favor to help us understand how all these known pain chemicals act to facilitate this little monster, TRPV1, which is the instrument of torture for CP patients. They also show that there are MANY possible locations and steps at which blocking may stop TRPV1 function. As you may know, resiniferatoxin destroys the TRPV-1 channel by overloading it.

Capsaicin strongly resembles three chemicals produced in the body, anandamide (see using SEARCH) and 12-hydroperoxyeicosatetranoic acid (also known as 12 HPETE), and N-arachidonoyldopamine (NADA). Arachidonic acid is part of the prostaglandin scheme and has its roots in eicosanoid chemistry. (see arachidonic using SEARCH at this site) You can read about eicosanoids and pain in more detail in the earlier article at this site. You may think of eicosanoid derived chemicals as acids but this is an oversimplification in chemistry, but not in the perception produced. Again, here are agents to be blocked in possible treatments. Fatty acids have also been discussed here, including fatty acid amylase.

Acids lower the threshold for channel activation and if the acid falls even further (to pH 6.0 and less) the acids can open the channel, ALL BY THEMSELVES. Relative behavior of acids in individuals is a step which could explain why some patients merely have pain at the level of a sunburn, while others are really suffering, with ghastly burning over large areas of the body. Relative acidity alone could explain such phenomena, but of course there are many other factors.

We will be revisiting Malmberg and Bley in future reviews here.