Eve R. Schneider

Neuronal mechanism for acute mechanosensitivity in tactile-foraging waterfowl

Significance Like vision, audition, and olfaction, mechanosensation is a fundamental way in which animals interact with the environment, but it remains the least well understood at the cellular and molecular levels. Here, we explored evolutionary changes that contribute to the enhancement of mechanosensitivity in tactile-foraging ducks. We found that the somatosensory neurons that innervate the duck bill can detect physical force much more efficiently than analogous cells in other species, such as mice. Furthermore, ducks exhibit an increase in the number of neurons dedicated to this task in their sensory ganglia and a decrease in the number of neurons that detect temperature. Our findings provide an explanation for the acute mechanosensitivity of the duck bill at the level of somatosensory neurons. Relying almost exclusively on their acute sense of touch, tactile-foraging birds can feed in murky water, but the cellular mechanism is unknown. Mechanical stimuli activate specialized cutaneous end organs in the bill, innervated by trigeminal afferents. We report that trigeminal ganglia (TG) of domestic and wild tactile-foraging ducks exhibit numerical expansion of large-diameter mechanoreceptive neurons expressing the mechano-gated ion channel Piezo2. These features are not found in visually foraging birds. Moreover, in the duck, the expansion of mechanoreceptors occurs at the expense of thermosensors. Direct mechanical stimulation of duck TG neurons evokes high-amplitude depolarizing current with a low threshold of activation, high signal amplification gain, and slow kinetics of inactivation. Together, these factors contribute to efficient conversion of light mechanical stimuli into neuronal excitation. Our results reveal an evolutionary strategy to hone tactile perception in vertebrates at the level of primary afferents.

Novel mechanisms of PIEZO1 dysfunction in hereditary xerocytosis

Mutations in PIEZO1 are the primary cause of hereditary xerocytosis, a clinically heterogeneous, dominantly inherited disorder of erythrocyte dehydration. We used next-generation sequencing-based techniques to identify PIEZO1 mutations in individuals from 9 kindreds referred with suspected hereditary xerocytosis (HX) and/or undiagnosed congenital hemolytic anemia. Mutations were primarily found in the highly conserved, COOH-terminal pore-region domain. Several mutations were novel and demonstrated ethnic specificity. We characterized these mutations using genomic-, bioinformatic-, cell biology-, and physiology-based functional assays. For these studies, we created a novel, cell-based in vivo system for study of wild-type and variant PIEZO1 membrane protein expression, trafficking, and electrophysiology in a rigorous manner. Previous reports have indicated HX-associated PIEZO1 variants exhibit a partial gain-of-function phenotype with generation of mechanically activated currents that inactivate more slowly than wild type, indicating that increased cation permeability may lead to dehydration of PIEZO1-mutant HX erythrocytes. In addition to delayed channel inactivation, we found additional alterations in mutant PIEZO1 channel kinetics, differences in response to osmotic stress, and altered membrane protein trafficking, predicting variant alleles that worsen or ameliorate erythrocyte hydration. These results extend the genetic heterogeneity observed in HX and indicate that various pathophysiologic mechanisms contribute to the HX phenotype.

Low-cost functional plasticity of TRPV1 supports heat tolerance in squirrels and camels

Significance Thirteen-lined ground squirrels and Bactrian camels are capable of withstanding elevated environmental temperatures. In mammals, the polymodal transient receptor potential vanilloid 1 (TRPV1) ion channel responds to temperatures >40 °C and marks peripheral neurons responsible for detecting noxious heat. However, we find that both squirrels and camels express TRPV1 channels with dramatic decreases in thermosensitivity in the physiologically relevant range. To regain heat sensitivity, squirrel and camel TRPV1 require substitution of a single conserved amino acid. These data point to a common molecular mechanism used by camels and squirrels to adapt to high temperatures and reveal a remarkable functional plasticity of temperature activation of the TRPV1 channel. The ability to sense heat is crucial for survival. Increased heat tolerance may prove beneficial by conferring the ability to inhabit otherwise prohibitive ecological niches. This phenomenon is widespread and is found in both large and small animals. For example, ground squirrels and camels can tolerate temperatures more than 40 °C better than many other mammalian species, yet a molecular mechanism subserving this ability is unclear. Transient receptor potential vanilloid 1 (TRPV1) is a polymodal ion channel involved in the detection of noxious thermal and chemical stimuli by primary afferents of the somatosensory system. Here, we show that thirteen-lined ground squirrels (Ictidomys tridecemlineatus) and Bactrian camels (Camelus ferus) express TRPV1 orthologs with dramatically reduced temperature sensitivity. The loss of sensitivity is restricted to temperature and does not affect capsaicin or acid responses, thereby maintaining a role for TRPV1 as a detector of noxious chemical cues. We show that heat sensitivity can be reengineered in both TRPV1 orthologs by a single amino acid substitution in the N-terminal ankyrin-repeat domain. Conversely, reciprocal mutations suppress heat sensitivity of rat TRPV1, supporting functional conservation of the residues. Our studies suggest that squirrels and camels co-opt a common molecular strategy to adapt to hot environments by suppressing the efficiency of TRPV1-mediated heat detection at the level of somatosensory neurons. Such adaptation is possible because of the remarkable functional flexibility of the TRPV1 molecule, which can undergo profound tuning at the minimal cost of a single amino acid change.

Temperature Sensitivity of Two-Pore (K2P) Potassium Channels

At normal body temperature, the two-pore potassium channels TREK-1 (K2P2.1/KCNK2), TREK-2 (K2P10.1/KCNK10), and TRAAK (K2P4.1/KCNK2) regulate cellular excitability by providing voltage-independent leak of potassium. Heat dramatically potentiates K2P channel activity and further affects excitation. This review focuses on the current understanding of the physiological role of heat-activated K2P current, and discusses the molecular mechanism of temperature gating in TREK-1, TREK-2, and TRAAK.

Evolutionary Specialization of Tactile Perception in Vertebrates

Evolution has endowed vertebrates with the remarkable tactile ability to explore the world through the perception of physical force. Yet the sense of touch remains one of the least well understood senses at the cellular and molecular level. Vertebrates specializing in tactile perception can highlight general principles of mechanotransduction. Here, we review cellular and molecular adaptations that underlie the sense of touch in typical and acutely mechanosensitive vertebrates.

Molecular basis of tactile specialization in the duck bill

Significance Tactile-specialist birds of the Anatidae family possess unique mechanosensory abilities with which they efficiently select edible matter in muddy water without visual or olfactory cues. Mechanical stimuli are transmitted by trigeminal mechanoreceptors innervating the bill, a highly specialized tactile organ. We show mechanosensory specialization in ducks involves the formation of functional rapidly adapting mechanoreceptors prior to hatching. Unlike in visually foraging chicken, most trigeminal neurons in ducks are touch receptors, which develop following a unique pattern of neurotrophic factor receptor expression and produce robust mechano-current via the Piezo2 channel with novel properties. Our results uncover possible evolutionary adaptations contributing to potentiation of mechanoreception in an organ-specific manner and reveal the molecular identity of a neuronal mechanotransducer with prolonged inactivation kinetics. Tactile-foraging ducks are specialist birds known for their touch-dependent feeding behavior. They use dabbling, straining, and filtering to find edible matter in murky water, relying on the sense of touch in their bill. Here, we present the molecular characterization of embryonic duck bill, which we show contains a high density of mechanosensory corpuscles innervated by functional rapidly adapting trigeminal afferents. In contrast to chicken, a visually foraging bird, the majority of duck trigeminal neurons are mechanoreceptors that express the Piezo2 ion channel and produce slowly inactivating mechano-current before hatching. Furthermore, duck neurons have a significantly reduced mechano-activation threshold and elevated mechano-current amplitude. Cloning and electrophysiological characterization of duck Piezo2 in a heterologous expression system shows that duck Piezo2 is functionally similar to the mouse ortholog but with prolonged inactivation kinetics, particularly at positive potentials. Knockdown of Piezo2 in duck trigeminal neurons attenuates mechano current with intermediate and slow inactivation kinetics. This suggests that Piezo2 is capable of contributing to a larger range of mechano-activated currents in duck trigeminal ganglia than in mouse trigeminal ganglia. Our results provide insights into the molecular basis of mechanotransduction in a tactile-specialist vertebrate.

TMEM150C/Tentonin3 Is a Regulator of Mechano-gated Ion Channels

SUMMARY Neuronal mechano-sensitivity relies on mechanogated ion channels, but pathways regulating their activity remain poorly understood. TMEM150C was proposed to mediate mechano-activated current in proprioceptive neurons. Here, we studied functional interaction of TMEM150C with mechano-gated ion channels from different classes (Piezo2, Piezo1, and the potassium channel TREK-1) using two independent methods of mechanical stimulation. We found that TMEM150C significantly prolongs the duration of the mechano-current produced by all three channels, decreases apparent activation threshold in Piezo2, and induces persistent current in Piezo1. We also show that TMEM150C is co-expressed with Piezo2 in trigeminal neurons, expanding its role beyond proprioceptors. Finally, we cloned TMEM150C from the trigeminal neurons of the tactile-foraging domestic duck and showed that it functions similarly to the mouse ortholog, demonstrating evolutionary conservation among vertebrates. Our studies reveal TMEM150C as a general regulator of mechano-gated ion channels from different classes.

Investigating the Role of Nav1.5 in Somatosensory Mechanosensation

Low-threshold mechanoreceptors (LTMR) are a diverse subpopulation of primary afferent neurons responsible for sensing the variety of mechanical stimuli experienced on a day-to-day basis. However, molecular details of touch sensation in LTMRs remain elusive. To investigate the neurons and molecules underlying touch sensation, we study the mechanosensation in a novel model system - trigeminal neurons of tactile foraging ducks (Anas peking). Ducks rely on the abundance of mechanoreceptors in the glabrous skin of their bill in order to identify food in the absence of olfactory or visual cues. To understand the molecular basis of mechanosensation in trigeminal LTMRs, we compared the transcriptome of duck trigeminal (TG) and dorsal root ganglia (DRG) (Schneider et al., PNAS 2014). The analysis revealed elevated expression of the mechanosensitive voltage-gated sodium channel Nav1.5 (SCN5a) in duck TG compared to DRG. Nav1.5 is known for its contribution to the cardiac action potential, but its role in the sense of touch is unexplored. Considering the high level of Nav1.5 expression in duck TG by RNA-seq and the mechanosensitivity of Nav1.5, we hypothesize a role of Nav1.5 in transduction or transmission of touch. Our work demonstrates a wide distribution of the SCN5a transcript in duck TG neurons, providing further rationale for a role of Nav1.5 in touch sensation. Here, we performed biophysical characterization of duck Nav1.5 in vitro and in trigeminal neurons. Our data reveals key functional differences between the human and duck orthologs of Nav1.5 and suggest a novel role for this channel in the sense of touch.

Sensing Force by Trigeminal Neurons of Acutely Mechanosensitive Birds

Mechanosensation is a fundamental way animals interact with the environment, but it remains the least well understood at cellular and molecular level. Somatosensory ganglia of the standard laboratory species house a highly diverse population of neurons, where low-threshold mechanoreceptors - the neurons that innervate light touch receptors in the skin - represent only a small fraction. This heterogeneity significantly impedes progress in understanding functional roles of somatosensory neurons in light touch perception. Here, we explored functional specialization of somatosensory ganglia from animals which have taken the sense of touch to the extreme - tactile foraging ducks. These animals have acutely mechanosensitive bill innervated by trigeminal (TG) neurons, and as such provide an opportunity to study general principles of mechanotransduction from an unconventional standpoint. We found that, in contrast to species without tactile specialization, the majority (85%) of duck TG neurons are large-diameter myelinated mechanoreceptors expressing the mechano-gated ion channel Piezo2. Electrophysiological analyses showed that mechanosensitivity of duck TG neurons has been optimized in three ways. Compared to mouse cells, duck neurons exhibit (i) lowered threshold of mechano-activation, (ii) elevated signal amplification gain, and (iii) prolonged kinetics of inactivation, all of which increase the amount of depolarizing charge entering the cell upon mechanical stimulation. Thus, duck TG neurons have augmented intrinsic ability to convert mechanical force into excitatory ionic current, which explains the acute mechanosensory properties of the duck bill. Our studies emphasize a key role of the intrinsic mechanosensory ability of somatosensory neurons in touch physiology, reveal an evolutionary strategy utilized by vertebrates to hone tactile perception, and suggest a novel model system to study the sense of touch at the cellular and molecular level.Schneider ER, Gracheva EO, Bagriantsev SN et al, PNAS 2014 (e-pub Sept 22).