Targeting TLR4-lipid rafts reverses hyper-excitability in pain-associated human dorsal root ganglia.

Cholesterol-rich lipid rafts are organizing platforms for many excitatory ion channels and receptors in neurons and for inflammatory receptors like toll-like receptor 4 (TLR4) in immune cells. Human dorsal root ganglion (hDRG) neurons that express nociceptor markers also express high levels of TLR4. Apolipoprotein A-I binding protein (AIBP) binds to TLR4 and promotes cholesterol depletion and lipid raft disruption in cells where TLR4 is expressed. The current study conducted with male and female hDRG demonstrates that TLR4-lipid rafts are expressed with higher density in pain- versus non-pain-associated tissues and localized to neurons expressing transient receptor potential vanilloid 1 (TRPV1) typically expressed by nociceptors. This increased density of TLR4-lipid rafts results in increased excitability of hDRG TRPV1 receptors. Additionally, whole cell recordings show that disruption of TLR4-rafts with AIBP inhibited pain-associated ectopic action potential generation and evoked neuronal excitation in hDRG neurons. AIBP treatment impacted several action potential dynamics specifically in cells with on-going ectopic action potentials rather than non-spontaneously active neurons. Finally, AIBP is shown to be localized with satellite cells in human DRG and its expression increases along with glutamine synthetase in pain-associated hDRG. Given that TLR4-raft expression in DRG neurons is largely confined to nociceptors, a TLR4-raft specific therapeutic such as AIBP may have potential as a novel pain therapeutic with a low-side effect profile.Significance statement Lipid rafts are critical regulators of numerous neurological processes and disorders and are shown here to increase in density in pain-associated human dorsal root ganglia. Selective disruption of lipid rafts in toll-like receptor 4-expressing nociceptors significantly reduced the excitability of these neurons and suppressed ectopic, pain-associated action potential discharges. The agent used to disrupt lipid rafts, apolipoprotein A-I binding protein, localizes to macrophages and satellite glial cells and engages an endogenous pain relief mechanism with an expected limited side effect profile. This work reveals a novel mechanism underlying the generation of chronic pain in patients and identifies a potential therapeutic strategy for its management.