How are ion channels regulated in neurons?

Accessory proteins

Acid-sensing ion channels are know to bind to a whole host of molecules including PICK1, PSD-95, Lin7b, and many others. An interesting family of proteins that bind to and regulate ASICs are the SPFH family of integral membrane proteins (Stomatin, Stomatin-like proteins 1-3). These proteins regulate ASIC channels in an isoform specific manner, largely reducing the total ASIC current. The mechanism of this reduction in current is an open question that we are attempting to understand.

We have recently shown using FRET and electrophysiology that Stomatin binds to ASIC3, but not ASICs 1 or 2, and reduces the currents more than 100 fold. Moreoever, we were able to show that Stomatin interacts with ASIC3 in two regions of the channel. The first is the distal C-terminus and this binding site is required for complex formation. The second is the first transmembrane domain. TM1 is not necessary for Stomatin binding but is absolutely needed for the regulatory effect.

We now want to understand how interaction with TM1 of the channel is able to exert such a strong inhibitory effect on ASIC3. Could the hairpin of Stomatin block the channel? Or does interaction with Stomatin lock ASIC3 in the closed or desensitized state? Can we measure this?

In addition, we hope to uncover the physiological role of this complex. Its odd to have a protein bind to and completely inhibit an ion channel. It suggests the possibility that this complex is dynamically regulated. There are a number of possibilities for how this might occur that we are pursuing.

 

 

2020 Klipp et al. Originally published in JGP. https://rupress.org/jgp/article/152/3/e201912471/133684/Insights-into-the-molecular-mechanisms-underlying

 

Lipids

In response to injury, damaged tissue undergoes a complex immune reaction called inflammation. In addition to aiding in recovery, the extracellular signals associated with inflammation alter the electrical properties of nociceptive nerve terminals by directly exciting the nerve terminals (ATP, protons, 5-HT) or by sensitizing the nerve (bradykinin, prostaglandins, cytokines) through activation of membrane receptors. The mechanisms of nerve sensitization are thought to involve indirect effects on ion channels at the nerve terminals through activation of membrane receptors.

One emerging mechanism involves changes in the lipid composition of the plasma membrane. In many cases this involves the stimulation of receptors that initiate a signaling cascade that alters lipids in the membrane. One of the most well studies lipid familes, phosphoinositides, have been demonstrated to regulate a large number of ion channels. Preliminary work suggests that ASICs may not be regulated by this family of lipids. However, it is also known that inflammation can lead to activation of the enzyme Phospholipase A2 which cleaves fatty acids into their constituent single tail lipids. This leads to the release of polyunsaturated fatty acids (PUFAs) like arachidonic acid (aa) and docosahexanoic acid (DHA).

PUFAs have been shown to potentiate ASICs but the mechanism of potentiation or the role this potentiation plays in cellular excitability are completely unknown.