This Week in The Journal

Role for Cyclic Nucleotide-Gated Channels in Migraine

Christoforos Tsantoulas, Aidan Ng, Larissa Pinto, Anna P. Andreou, and Peter A. McNaughton

(see pages 7513–7529)

Migraine headaches are thought to stem from activation of trigeminal ganglion nociceptors that innervate the meninges. These neurons release calcitonin gene-related peptide (CGRP), which causes inflammation and might increase the likelihood of future headaches by altering nociceptive transmission. Notably, intravenous infusion of CGRP produces migraine-like attacks in people susceptible to migraines (migraineurs), and reducing CGRP levels or blocking CGRP receptors effectively reduces migraines in many patients.

CGRP receptors are G_s-coupled receptors that promote production of cAMP. Intriguingly, several drugs that often cause headaches also promote production of cAMP or cGMP, whereas most effective treatments for migraines reduce cAMP levels. This suggests that elevated levels of cyclic nucleotides contribute to migraine pain. Tsantoulas et al. provide evidence that they do so by promoting activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in trigeminal nociceptors, thus increasing nociceptor excitability.

Consistent with previous studies, injection of nitroglycerine, which induces migraines in ∼80% of migraineurs, produced hypersensitivity around the eyes—a migraine-related symptom—in rodents. This was accompanied by increased cGMP levels in the trigeminal nucleus, increased spontaneous firing of trigeminal nociceptive neurons, increased sensitivity of these neurons to electrical stimulation of the eyelid, and increased activation of neurons in the trigeminocervical complex (TCC), the target of trigeminal nociceptive neurons. The increased excitability of trigeminal nociceptors was attributable at least in part to a depolarizing shift in the voltage dependence and an increase in the speed of HCN channel activation. Consistent with this, nitroglycerine-induced increases in nociceptor and TCC activity, as well as periorbital hypersensitivity, were reversed or prevented by administering ivabradine, an HCN channel blocker that does not cross the blood–brain barrier, or by knocking out HCN2 selectively in peripheral nociceptors. Ivabradine administration and knocking out HCN2 also reduced hypersensitivity and blocked TCC activation in a mouse model of rebound headache resulting from overuse of headache medication.

These results suggest that nitroglycerine-dependent elevation of cGMP facilitates the opening of HCN2 channels in trigeminal nociceptors, thus increasing the excitability of these neurons and activating downstream pain pathways. Selective antagonists of HCN2 may therefore be a viable treatment for migraine headaches.

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Commissural axons normally turn and continue growing after crossing the floor plate (FP) of the spinal cord (top left). Axons fail to extend beyond the floor plate when Nedd4-1 (top right), Nedd4-2 (bottom left), or both (bottom right) are knocked out. See Gorla et al. for details.

Nedd4 Ubiquitin Ligases Tag Robo1 for Degradation

Madhavi Gorla, Karina Chaudhari, Maya Hale, Chloe Potter, and Greg J. Bashaw

(see pages 7547–7561)

During CNS development, growing axons are guided along their trajectories by combinations of attractive and repulsive cues. Axons with complex trajectories typically follow multiple sets of cues, changing their responsiveness to these cues as they pass different choice points along their way. For example, commissural axons in the mouse spinal cord are guided toward the ventral midline by netrin proteins secreted by cells in the floor plate; but after crossing, they are steered away from the midline by repellant slit proteins, which are also secreted by floor plate cells. Notably, commissural axons express the slit receptor Robo1 before they reach the ventral midline, but as they approach the floor plate, these receptors are targeted to late endosomes by Ndfip1 and Ndfip2. Gorla et al. now report that Ndfip proteins enable Robo1 to associate with Nedd4-1 and Nedd4-2, which ubiquitinate Robo1 receptors, thus tagging them for degradation.

Seven Nedd4-family proteins interacted with Ndfip1 and Ndfip2 when coexpressed in 293T cells, but only two of these—Nedd4-1 and Nedd4-2—increased ubiquitination and reduced expression of Robo1 in these cells. Both Nedd4-1 and Nedd4-2 formed complexes with Robo1 in 293T cells, but this occurred only if Ndfip proteins were also present. In addition, both Nedd4-1 and Nedd4-2 were detected in commissural axons in the developing spinal cord. But unlike Ndfip proteins, which are expressed most strongly in commissural axons before they cross the midline, Nedd4-1 and Nedd4-2 were expressed at similar levels before and after midline crossing. Importantly, an inhibitor of Nedd4 ligases increased the ability of slit proteins to induce growth cone collapse in cultures of commissural neurons. Moreover, knocking out Nedd4-1 or Nedd4-2 in the dorsal spinal cord reduced the number of axons that crossed the midline, and knocking out both proteins had an even greater effect.

These results suggest that Ndfip proteins reduce surface expression of Robo1 at least partly by promoting ubiquitination of the receptor by Nedd4-1 and Nedd4-2. The fact that commissural axons fail to cross the midline when Nedd4-1 and Nedd4-2 are knocked out suggests that Robo1 helps prevent premature crossing of commissural axons, and that it must be degraded to allow axons to cross.