Stefan G. Lechner

The Molecular and Cellular Identity of Peripheral Osmoreceptors

In mammals, the osmolality of the extracellular fluid (ECF) is highly stable despite radical changes in salt/water intake and excretion. Afferent systems are required to detect hypo- or hyperosmotic shifts in the ECF to trigger homeostatic control of osmolality. In humans, a pressor reflex is triggered by simply drinking water which may be mediated by peripheral osmoreceptors. Here, we identified afferent neurons in the thoracic dorsal root ganglia (DRG) of mice that innervate hepatic blood vessels and detect physiological hypo-osmotic shifts in blood osmolality. Hepatic sensory neurons are equipped with an inward current that faithfully transduces graded changes in osmolality within the physiological range (~15 mOsm). In mice lacking the osmotically activated ion channel, TRPV4, hepatic sensory neurons no longer exhibit osmosensitive inward currents and activation of peripheral osmoreceptors in vivo is abolished. We have thus identified a new population of sensory neurons that transduce ongoing changes in hepatic osmolality.

The Transcription Factor c-Maf Controls Touch Receptor Development and Function

Telling Sandpaper from Satin Pacinian corpuscles are mechano-receptors tuned to detect high-frequency, low-amplitude, signals. Found in human palm and fingertips, they are useful for discrimination of rough and smooth textures, a sensitivity seemingly amplified by the ridges of fingerprints. Wende et al. (p. 1373, published online 16 February) identified a mutation in humans that disrupts this sensitivity to texture, but leaves other facets of touch, such as tactile spatial acuity, intact. A mutation known to cause cataracts also disables a specialized mechanosensory receptor in mice and humans. The sense of touch relies on detection of mechanical stimuli by specialized mechanosensory neurons. The scarcity of molecular data has made it difficult to analyze development of mechanoreceptors and to define the basis of their diversity and function. We show that the transcription factor c-Maf/c-MAF is crucial for mechanosensory function in mice and humans. The development and function of several rapidly adapting mechanoreceptor types are disrupted in c-Maf mutant mice. In particular, Pacinian corpuscles, a type of mechanoreceptor specialized to detect high-frequency vibrations, are severely atrophied. In line with this, sensitivity to high-frequency vibration is reduced in humans carrying a dominant mutation in the c-MAF gene. Thus, our work identifies a key transcription factor specifying development and function of mechanoreceptors and their end organs.

The Molecular Basis of Acid Insensitivity in the African Naked Mole-Rat

Life in a high–carbon dioxide environment has eliminated acid-evoked pain in the naked mole-rat. Acid evokes pain by exciting nociceptors; the acid sensors are proton-gated ion channels that depolarize neurons. The naked mole-rat (Heterocephalus glaber) is exceptional in its acid insensitivity, but acid sensors (acid-sensing ion channels and the transient receptor potential vanilloid-1 ion channel) in naked mole-rat nociceptors are similar to those in other vertebrates. Acid inhibition of voltage-gated sodium currents is more profound in naked mole-rat nociceptors than in mouse nociceptors, however, which effectively prevents acid-induced action potential initiation. We describe a species-specific variant of the nociceptor sodium channel NaV1.7, which is potently blocked by protons and can account for acid insensitivity in this species. Thus, evolutionary pressure has selected for an NaV1.7 gene variant that tips the balance from proton-induced excitation to inhibition of action potential initiation to abolish acid nociception.

KCNQ4 K⁺ channels tune mechanoreceptors for normal touch sensation in mouse and man

Mutations inactivating the potassium channel KCNQ4 (Kv7.4) lead to deafness in humans and mice. In addition to its expression in mechanosensitive hair cells of the inner ear, KCNQ4 is found in the auditory pathway and in trigeminal nuclei that convey somatosensory information. We have now detected KCNQ4 in the peripheral nerve endings of cutaneous rapidly adapting hair follicle and Meissner corpuscle mechanoreceptors from mice and humans. Electrophysiological recordings from single afferents from Kcnq4−/− mice and mice carrying a KCNQ4 mutation found in DFNA2-type monogenic dominant human hearing loss showed elevated mechanosensitivity and altered frequency response of rapidly adapting, but not of slowly adapting nor of D-hair, mechanoreceptor neurons. Human subjects from independent DFNA2 pedigrees outperformed age-matched control subjects when tested for vibrotactile acuity at low frequencies. This work describes a gene mutation that modulates touch sensitivity in mice and humans and establishes KCNQ4 as a specific molecular marker for rapidly adapting Meissner and a subset of hair follicle afferents.

Peripheral sensitisation of nociceptors via G-proteindependent potentiation of mechanotransduction currents

Mechanical stimuli impinging on the skin are converted into electrical signals by mechanically gated ion channels located at the peripheral nerve endings of dorsal root ganglion (DRG) neurons. Under inflammatory conditions sensory neurons are commonly sensitised to mechanical stimuli; a putative mechanism that may contribute to such sensitisation of sensory neurons is enhanced responsiveness of mechanotransduction ion channels. Here we show that the algogens UTP and ATP potentiate mechanosensitive RA currents in peptidergic nociceptive DRG neurons and reduce thresholds for mechanically induced action potential firing in these neurones. Pharmacological characterisation suggests that this effect is mediated by the Gq‐coupled P2Y2 nucleotide receptor. Moreover, using the in vitro skin nerve technique, we show that UTP also increases action potential firing rates in response to mechanical stimuli in a subpopulation of skin C‐fibre nociceptors. Together our findings suggest that UTP sensitises a subpopulation of cutaneous C‐fibre nociceptors via a previously undescribed G‐protein‐dependent potentiation of mechanically activated RA‐type currents.

A Genetic Basis for Mechanosensory Traits in Humans

Hearing and touch are genetically related, and people with excellent hearing are more likely to have a fine sense of touch and vice versa.

Nerve Growth Factor and Nociception: From Experimental Embryology to New Analgesic Therapy

Nerve growth factor (NGF) is central to the development and functional regulation of sensory neurons that signal the first events that lead to pain. These sensory neurons, called nociceptors, require NGF in the early embryo to survive and also for their functional maturation. The long road from the discovery of NGF and its roles during development to the realization that NGF plays a major role in the pathophysiology of inflammatory pain will be reviewed. In particular, we will discuss the various signaling events initiated by NGF that lead to long-lasting thermal and mechanical hyperalgesia in animals and in man. It has been realized relatively recently that humanized function blocking antibodies directed against NGF show remarkably analgesic potency in human clinical trials for painful conditions as varied as osteoarthritis, lower back pain, and interstitial cystitis. Thus, anti-NGF medication has the potential to make a major impact on day-to-day chronic pain treatment in the near future. It is therefore all the more important to understand the precise pathways and mechanisms that are controlled by NGF to both initiate and sustain mechanical and thermal hyperalgesia. Recent work suggests that NGF-dependent regulation of the mechanosensory properties of sensory neurons that signal mechanical pain may open new mechanistic avenues to refine and exploit relevant molecular targets for novel analgesics.

PIEZO2 is required for mechanotransduction in human stem cell-derived touch receptors

Human sensory neurons are inaccessible for functional examination, and thus little is known about the mechanisms mediating touch sensation in humans. Here we demonstrate that the mechanosensitivity of human embryonic stem (hES) cell–derived touch receptors depends on PIEZO2. To recapitulate sensory neuron development in vitro, we established a multistep differentiation protocol and generated sensory neurons via the intermediate production of neural crest cells derived from hES cells or human induced pluripotent stem (hiPS) cells. The generated neurons express a distinct set of touch receptor–specific genes and convert mechanical stimuli into electrical signals, their most salient characteristic in vivo. Strikingly, mechanosensitivity is lost after CRISPR/Cas9-mediated PIEZO2 gene deletion. Our work establishes a model system that resembles human touch receptors, which may facilitate mechanistic analysis of other sensory subtypes and provide insight into developmental programs underlying sensory neuron diversity.

Gαq/11 signaling tonically modulates nociceptor function and contributes to activity-dependent sensitization

TOC summary The functional role of Gq/11 G proteins in nociceptors not only spans pathological pain, but, surprisingly, also includes tonic modulation of nociception. ABSTRACT Peripheral injury or inflammation leads to a release of mediators capable of binding to a variety of ion channels and receptors. Among these are the 7‐transmembrane receptors (G protein‐coupled receptors) coupling to Gs, Gi/o, G12/13, or Gq/11 G proteins. Each of the G protein‐coupled receptor pathways is involved in nociceptive modulation and pain processing, but the relative contribution of individual signaling pathways in vivo has not yet been worked out. The Gq/G11 signaling branch is of particular interest because it leads to the activation of phospholipase C‐β, protein kinase C, the release of calcium from intracellular stores, and it modulates extracellular regulated kinases. To investigate the contribution of the entire Gq/11‐signaling pathway in nociceptors towards regulation of pain, we generated double‐deficient mice lacking Gq/11 selectively in nociceptors using a conditional gene‐targeting approach. We observed that nociceptor‐specific loss of Gq and G11 results in reduced pain hypersensitivity following paw inflammation or spared nerve injury. Surprisingly, our behavioral and electrophysiological experiments also indicated defects in basal mechanical sensitivity in Gq/11 mutant mice, suggesting a novel function for Gq/11 in tonic modulation of acute nociception. Patch‐clamp recordings revealed changes in voltage‐dependent tetrodotoxin‐resistant and tetrodotoxin‐sensitive sodium channels in nociceptors upon a loss of Gq/11, whereas potassium currents remained unchanged. Our results indicate that the functional role of the Gq/G11 branch of G‐protein signaling in nociceptors in vivo not only spans sensitization mechanisms in pathological pain states, but is also operational in tonic modulation of basal nociception and acute pain.

Peripheral and spinal circuits involved in mechanical allodynia.

Sensory information from nociceptors and touch receptors, such as Aβ-fi ber mechanoreceptors and C-fi ber low-threshold mechanoreceptors (C-LTMRs), is normally relayed and processed by separate neural circuits in the spinal cord.19 After nerve injury or infl ammation, however, touch-related information is also relayed to nociceptive circuits in the superfi cial dorsal horn, which results in touch-evoked pain.16 Here, we depict the spinal circuits that facilitate this modality crosstalk and form the cellular basis of mechanical allodynia. A role of Aβ fi bers in allodynia was originally proposed by human studies showing that compression block of Aβ fi bers abolishes touch-evoked pain.3,7 Subsequent studies confi rmed that after nerve injury, neurokinin-1 receptor (NK1R) expressing projection neurons in lamina I receive excitatory input from Aβ fi bers via a preexisting polysynaptic connection that includes somatostatin (SOM) expressing interneurons, which also receive input from nociceptors.2,6,17 This connection is normally inhibited by dynorphin-/ GAD67-expressing GABAergic interneurons in lamina II,4,6 the activity of which is controlled by polysynaptic input from Aβ fi bers and by monoand poly-synaptic inputs from Aδand C-fi bers nociceptors. Another circuit links Aβ-fi bers with lamina I NK1R− projection neurons through PKCγ+/SOM+ interneurons, central cells and SOM+ interneurons in outer lamina II. Information fl ow through this connection is controlled by feed-forward inhibition mediated by glycinergic and dynorphin-expressing interneurons.6,13 The Aβ-fi ber subtypes that provide input to these circuits are unknown. Studies in humans14 and mice with impaired glutamate release from central synapses of C-LTMRs18 suggested that these afferents, which normally signal pleasant touch,10 might also signal mechanical allodynia. This hypothesis was, however, challenged by others who could not fi nd differences in allodynia in mice lacking C-LTMRs11 and who were unable to induce tactile allodynia in patients lacking Aβ fi bers.9 A possible explanation for this controversy is that C-LTMRs co-release the protein TAFA4, which counterbalances excitatory actions of glutamate.5 Thus, C-LTMR–specifi c loss of glutamatergic neurotransmission only,18 would result in a different phenotype than does the complete loss of C-LTMRs.11 C-fi ber low-threshold mechanoreceptors project to lamina IIi and provide polysynaptic input to lamina I spinoparabrachial neurons,1 most of which express NK1R. They do not project to PKCγ+ neurons in the same lamina, which receive input from Aβ fi bers.15 Putative C-LTMRs, as identifi ed by means of conduction velocity and fi ber diameter, also provide excitatory drive onto GABAergic islet cells in lamina II, which regulate information fl ow from C-fi ber nociceptors via central and vertical cells to NK1R+ projection neurons.12 Existence of this C-LTMR–driven pain-inhibiting connection is consistent with recent fi ndings suggesting an analgesic effect of C-LTMR signaling,5 which is reduced during tactile allodynia.9 Whether there is a direct crosstalk between C-LTMR– and Aβ-fi ber–processing circuits or whether these 2 pathways only converge at the level of lamina I projection neurons is still unclear. However, given the evidence for a role of both fi ber types in mechanical allodynia and considering the columnar organization of the spinal projections of Aβ fi bers and C-LTMRs from the same skin area,8 a functional connection between these circuits seems highly likely.