Transient receptor potential (TRP) channels in sensory neurons serve as first responders to painful stimuli, and when overactive, can drive pathological pain. Two recent studies make headway in understanding the developmental expression and functional modulation of two family members, TRPV1 and TRPA1. The findings hint at new roles for TRP channels, and reveal unappreciated complexity of channel function in nociceptive sensory neurons.
One study, led by Brian Davis, University of Pittsburgh School of Medicine in Pennsylvania indicates that the tissue that a nociceptor innervates strongly influences TRPV1 and TRPA1 expression, and determines which inflammatory signals modulate channel activity. Meanwhile, Allan Basbaum and colleagues at the University of California, San Francisco, investigated developmental regulation of TRPV1 over the lifespan of mice, and their results support the idea that TRPV1 is a fundamental marker of nociceptor function. Findings from both studies offer reminders that nociceptors resist easy categorization. The Davis group’s paper was published July 20, and the Basbaum group’s, July 13, both in the Journal of Neuroscience.
In the study from the Davis lab, first author Sacha Malin and colleagues used calcium imaging of dorsal root ganglion (DRG) neurons innervating muscle, viscera (colon), or skin to evaluate functional TRP channel expression in the various tissues. There were considerable differences: Based on responses to the TRPV1 agonist capsaicin and the TRPA1 agonist mustard oil, afferents innervating colon and muscle tissue were twice as likely to express TRPV1 and TRPA1 compared to neurons from skin. There also appeared to be tissue specificity in the response to inflammation. TRPV1, TRPA1, and the growth factors artemin and NGF all increased their expression in models of skin and colon inflammation, but the magnitude and time course of the change differed between the two tissues.
The Davis group went on to show that the ability of exogenous growth factors to potentiate TRP channel activity in isolated sensory neurons varied according to the origin of the neurons. For example, TRPV1 responses were twice as likely to be potentiated by artemin in afferents from skin compared to muscle. When it came to TRPA1 responses, skin neurons showed no potentiation by GDNF, whereas the growth factor did boost response in 40 percent of muscle and colon neurons. The differences in responses suggest that neurons express distinct sets of growth factor receptors depending on their target tissues.
Thus, not all nociceptors that contain TRPV1 or TRPA1, or both, are alike. As researchers look more deeply at channel properties in their various settings, Davis expects they may find fundamental differences in channel function. “Sensory neurons going to different organs use the same channels, receptors, and other proteins differently depending on context. That’s really important to understand, and it really colors how we understand what the molecules do,” Davis told PRF.
Case in point: TRPV1 is known largely as a temperature sensor, but when it is deep below the skin, could it be doing something else? Davis says he is now investigating how TRP channel-containing afferents not only respond to, but can actually initiate, inflammatory conditions in the pancreas. In a recent study, he and his collaborators reported that TRPV1 and TRPA1 expression and activity increased in a mouse model of pancreatitis, and channel antagonists reduced inflammation and pain behaviors (Schwartz et al., 2011).
Nociceptors grow up
The Basbaum lab, in their new study, delved into TRPV1 distribution across nociceptor subtypes and developmental stages. To achieve sensitive measurements of TRPV1 expression, Daniel Cavanaugh in the lab and his colleagues used two strains of knock-in mice. In one, Cre recombinase is expressed under the TRPV1 promoter. When this mouse is crossed with a lacZ or enhanced yellow fluorescent protein (EYFP) reporter line, a picture of the developmental pattern of TRPV1 expression, from the embryo on, is revealed. In the other strain, placental alkaline phosphatase (PLAP) and nuclear lacZ are expressed under control of the TRPV1 promoter and reflect only contemporaneous TRPV1 expression (Cavanaugh et al., 2011).
In adult animals, TRPV1 is present predominantly in one class of nociceptors, the peptidergic C fibers, which are marked by their expression of the neuropeptides CGRP and substance P. (Nonpeptidergic neurons, on the other hand, are defined by binding of isolectin B4, IB4.) Looking at different stages of development using the PLAP-nlacZ strain, the Basbaum group saw that TRPV1 is present in a large and diverse swath of sensory neurons early on, and its expression peaks in late embryogenesis; only later is its expression restricted mainly to peptidergic neurons. Importantly, the Cre mice showed broad expression of reporter proteins, which revealed that TRPV1 levels decrease during development not because TRPV1-containing neurons die, but because TRPV1 expression is downregulated in nonpeptidergic neurons.
The developmental regulation of TRPV1 expression is consistent with Basbaum’s contention that that the presence or absence of TRPV1 is a fundamental marker of nociceptor function. That concept is fueled by earlier studies showing that ablation of TRPV1-containing neurons in mice selectively wiped out the animals’ ability to respond to noxious heat (Cavanaugh et al., 2009; Mishra and Hoon, 2010). “TRPV1 expression really assigns a functional property to the afferents,” Basbaum told PRF. In fact, he says, TRPV1 expression seems to be more indicative of nociceptor function than the traditional peptidergic/nonpeptidergic division—which, in adult mice, TRPV1 expression approximates, but doesn’t precisely match.
Notably, in rats, TRPV1 expression bulldozes the peptidergic/nonpeptidergic boundary, showing extensive expression in both classes of nociceptors (Tominaga et al., 1998). Thus, building a full and nuanced view of nociceptor function may be a delicate operation, and translating findings beyond mouse models may be tricky, too.
Also not yet known is what sculpts TRPV1 expression, and what happens if the developmental trajectory is disrupted. For example, there is evidence that injury near the time of birth can cause persistent changes in nociceptor circuits and pain sensitivity (Ruda et al., 2000). Based on their new findings, the Basbaum group proposes that disrupting normal downregulation of TRPV1 could be one underlying mechanism.
Basbaum also notes that many of the changes in neuronal protein expression that normally happen during development are revisited after nerve injury or inflammation. “To what extent are they recapitulating a developmental process?” he asked. “Can we learn from the embryological process what is happening after injury?” If developmental programs are aberrantly restarted after injury, the hope is that researchers might find ways to steer them back in the right direction.