Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes

Distinct firing properties among touch receptors are influenced by multiple, interworking anatomical structures. Our understanding of the functions and crosstalk of Merkel cells and their associated neurites—the end organs of slowly adapting type I (SAI) afferents—remains incomplete. Piezo2 mechanically activated channels are required both in Merkel cells and in sensory neurons for canonical SAI responses in rodents; however, a central unanswered question is how rapidly inactivating currents give rise to sustained action potential volleys in SAI afferents. The computational model herein synthesizes mechanotransduction currents originating from Merkel cells and neurites, in context of skin mechanics and neural dynamics. Its goal is to mimic distinct spike firing patterns from wildtype animals, as well as Atoh1 knockout animals that completely lack Merkel cells. The developed generator function includes a Merkel cell mechanism that represents its mechanotransduction currents and downstream voltage-activated conductances (slower decay of current) and a neurite mechanism that represents its mechanotransduction currents (faster decay of current). To mimic sustained firing in wildtype animals, a longer time constant was needed than the 200 ms observed for mechanically activated membrane depolarizations in rodent Merkel cells. One mechanism that suffices is to introduce an ultra-slowly inactivating current, with a time constant on the order of 1.7 s. This mechanism may drive the slow adaptation of the sustained response, for which the skin’s viscoelastic relaxation cannot account. Positioned within the sensory neuron, this source of current reconciles the physiology and anatomical characteristics of Atoh1 knockout animals

Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes

Distinct firing properties among touch receptors are influenced by multiple, interworking anatomical structures. Our understanding of the functions and crosstalk of Merkel cells and their associated neurites—the end organs of slowly adapting type I (SAI) afferents—remains incomplete. Piezo2 mechanically activated channels are required both in Merkel cells and in sensory neurons for canonical SAI responses in rodents; however, a central unanswered question is how rapidly inactivating currents give rise to sustained action potential volleys in SAI afferents. The computational model herein synthesizes mechanotransduction currents originating from Merkel cells and neurites, in context of skin mechanics and neural dynamics. Its goal is to mimic distinct spike firing patterns from wildtype animals, as well as Atoh1 knockout animals that completely lack Merkel cells. The developed generator function includes a Merkel cell mechanism that represents its mechanotransduction currents and downstream voltage-activated conductances (slower decay of current) and a neurite mechanism that represents its mechanotransduction currents (faster decay of current). To mimic sustained firing in wildtype animals, a longer time constant was needed than the 200 ms observed for mechanically activated membrane depolarizations in rodent Merkel cells. One mechanism that suffices is to introduce an ultra-slowly inactivating current, with a time constant on the order of 1.7 s. This mechanism may drive the slow adaptation of the sustained response, for which the skin’s viscoelastic relaxation cannot account. Positioned within the sensory neuron, this source of current reconciles the physiology and anatomical characteristics of Atoh1 knockout animals

The Functional Diversity of Mammalian Touch Receptors

Humans in the modern world can survive without the Aristotelian senses of vision, hearing, smell or taste, but no one is completely without the ability to sense touch. This sense is essential for everything from basic tasks like tool manipulation to the complex interactions that underlie social bonding, sexual reproduction and pleasure. Touch receptors are embedded in the skin, at the interface of our bodies and the world. A remarkable array of varied receptor types tile our skin to signal different features of the objects we touch and alert us to their shape and texture. An early investigator of the neurological basis of touch, Maximillian von Frey, proposed in 1895 that the morphological diversity of neural endings in the skin could represent functional specificity. It is indeed the evolution of diverse receptor structures that has endowed the sensory organ of our skin with remarkable somatosensory functions. Here I explore the evolution of mechanosensing, and discuss how diversity in form and organization of touch receptors, from the cellular to organismal level, can shape the function of touch reception

Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes

This folder contains data files and code to reproduce the work done for paper Computation predicts rapidly adapting mechanotransduction currents cannot account for tactile encoding in Merkel cell-neurite complexes by Gregory J. Gerling (*), Lingtian Wan, Benjamin U. Hoffman, Yuxiang Wang, and Ellen A. Lumpkin, accepted by PLOS Computational Biology in June 2018

Cellular and Molecular Mechanisms of Mammalian Touch-Dome Development

Touch sensation is initiated by diverse mechanosensory neurons that innervate distinct skin structures; however, little is known about how touch receptors are patterned during mammalian skin development. During the course of my PhD training, I analyzed embryonic and neonatal development of mouse touch domes, which contain Merkel cell-neurite complexes that encode pressure and object features. I found that developing touch domes share three key features with canonical sensory placodes: discrete patches of specialized epithelial, co-clustered mesenchymal cells capable of engaging in molecular crosstalk with the epithelium, and selective recruitment of sensory neurons. During embryogenesis, molecularly distinct patches of epithelial Merkel cells and keratinocytes clustered with a previously unsuspected population of BMP4-expressing dermal fibroblasts in nascent touch domes. Concurrently, two populations of sensory neurons preferentially targeted touch domes compared with other skin regions. Surprisingly, only one neuronal population persisted in mature touch domes. Overexpression of Noggin, a BMP antagonist, in epidermis at embryonic age 14.5 resulted in fewer touch domes, a loss of Merkel cells, and decreased innervation density in skin areas where touch domes are typically found. Thus, touch domes bear hallmarks of placode-derived sensory epithelia that require BMP signaling for proper specification

The Prediction of Nociceptive Neural Activity in Passive Tissues following Lumbar Spine Flexion

Low back pain is a costly and debilitating disorder; however, most cases are categorized as being non-specific: low back pain without an identifiable origin or cause. Non-specific low back pain can be broadly considered and treated as either musculoskeletal disorders or pain disorders. In the musculoskeletal case, mechanics and loading history are believed to disrupt or damage tissues in the low back, which then generate nociceptive signals to be interpreted as pain. If the low back pain is a pain disorder, the disruption or damage is not with the tissues of the lower back, but rather the nervous system that transmits or interprets these nociceptive signals. Additionally, these subcategories of non-specific low back pain are not wholly independent since mechanical exposures can influence nervous system activity and vice versa. A specific outcome of this interconnectedness between mechanics and neural encoding is that a mechanical exposure can alter our ability to detect mechanical loads or mechanical sensitivity. One mechanical exposure that is linked to low back pain development and has been documented to alter neural activity is lumbar spine flexion. The purpose of this thesis was to determine the extent and mechanisms underlying how lumbar spine flexion can alter lower back mechanical sensitivity through a combination of viscoelastic creep and muscle activity, and to determine the implications those changes could have on the development of low back pain. The methods undertaken to achieve this thesis’ purpose were a combination of in-vivo human laboratory experiments, ex-vivo benchtop histology and mechanical testing, and in-silico modelling across four studies. Studies 1 and 2 quantified how mechanical sensitivity was altered over time in response to static and repetitive lumbar spine flexion respectively, Study 3 quantified the innervation properties of lumbar spine tissues, and Study 4 simulated mechanical exposures before and after lumbar spine flexion exposures to determine the nociceptive neural activity those exposures and conditions could generate. The first two studies employed a similar design and methodology measuring mechanical sensitivity and biomechanical variables before and up to 40 minutes after a 10-minute lumbar spine flexion exposure. For Study 1, the exposure was a static, seated, maximal lumbar spine flexion exposure and for Study 2, the exposure was a repetitive, standing, maximal lumbar spine flexion exposure. A custom motorized pressure algometer was constructed for these studies and used to track three measures of mechanical sensitivity—pressure-pain threshold, stimulus intensity, and stimulus unpleasantness—in the lower back and tibial shaft. Accelerometry was used in both studies to track the development and recovery from viscoelastic creep through lumbar spine flexion range of motion, and surface electromyography was used to determine flexion-relaxation (mean amplitude) in Study 1, and muscle fatigue (mean power frequency) in Study 2. Isometric joint strength and ratings of perceived exertion were also measured in Study 2. These data were fed into two main statistical processes: the first aimed to determine the time-course of mechanical sensitivity changes in the lower back relative to the tibial control site, and the second was to determine if any of the biomechanical variables (creep, muscle use, strength) or tibial mechanical sensitivities could predict lower back mechanical sensitivity changes. The static exposure generated a 10.3% creep response (4.4 ± 2.7°) in flexion range of motion that lasted for at least 40 minutes after the exposure. This exposure caused a transient increase in lower back stimulus unpleasantness but otherwise did not affect mechanical sensitivity nor did it affect flexion-relaxation. The strongest predictor of lower back mechanical sensitivity throughout the static exposure was the tibial surrogate; however, the magnitude of creep was also a significant predictor of changes in lower back pressure-pain thresholds. Despite being significant, these significant predictors could not explain the majority to the variance in mechanical sensitivity, and these changes appear more related to emotional affect than a physiological response. Study 1 concluded that a static lumbar spine flexion exposure that did not incorporate muscle activity did not alter nociceptive activity but could shape how nociceptive activity is experienced. The repetitive exposure generated a 5.0% creep response (2.7 ± 1.4°) in flexion range of motion dissipated within 5 minutes of the exposure ending. This exposure caused an immediate and transient decrease in lumbar spine extensor mean power frequency (5.1%) and lower back joint strength (9.8%) indicative of muscle fatigue, and a delayed 13.6% increase in lower back pressure-pain thresholds occurring 10 minutes after the exposure ended. Like Study 1, tibial mechanical sensitivities were the strongest predictor of lower back mechanical sensitivities, however interaction terms between these tibial surrogates and either creep magnitude or fatigue indicators (mean power frequency and strength) were also significant predictors. The delayed desensitization following this repetitive exposure was believed to arise from a combination of creep development and muscle use. The third study used lumbar spine tissues harvested from four cadaveric donors to determine the relative concentration of four neural membrane molecules (Protein Gene Product 9.5 (PGP9.5), Calcitonin Gene-Related Peptide, Bradykinin B1-Receptor, and Acid-Sensing Ion Channel 3 (ASIC3)) relevant to detecting mechanical stimuli in three tissues (dermal skin, superficial posterior annulus fibrosus, and the supraspinous-interspinous ligament complex) using Western Blotting. Only PGP9.5 and ASIC3 were found consistently in any of the three tissues. PGP9.5 had similar concentrations in skin and ligament, both of which were at least 12.8 times higher than in annular tissues. ASIC3 was most common in skin, followed by ligament, then annulus fibrosus, however the ratio of ASIC3:PGP9.5 was highest in annular tissue. The fourth study documents a model of nociceptive activity that predicts the likelihood that three exposures (pressure-pain threshold, flexion range of motion, and tissue failure) would generate nociceptive activity in the brainstem given a tissue (skin, annulus, or ligament), a viscoelastic state, ζ(t), and a muscle activity state, ϕ(t). The model simulated a single tissue-exposure combination for a sample of 100 mechanical sensitivities derived from the data in Studies 1 and 2. The model itself consisted of a Sensitivity Module that converted a tissue stress to an electrical current and a Neurological Module that used the electrical current to simulate the behaviour of a network of Hodgkin-Huxley neurons. The pressure-pain threshold exposure was used to validate the model and derive values for ζ(t) and ϕ(t), which were then applied to the other two exposures in annular and ligament tissues. While ζ(t), representing any effects related to creep following lumbar spine flexion, had minimal effects on nociceptive neural activity, ϕ(t), representing muscle activity-related effects of lumbar spine flexion, could inhibit nociceptive activity substantially. A major prediction from the model is that annulus fibrosus failure would be unlikely to generate any nociceptive activity in 12% of the population, and that characteristics of the exposure could increase that percentage to as many as 99.9% depending on the mode of failure. Flexion range of motion consistently generated no nociceptive activity in all tissues and conditions, and ligament failure consistently generated nociceptive activity regardless of other factors. While both viscoelastic creep and muscle activity related to lumbar spine flexion can influence mechanical sensitivity, the effects of muscle activity were more prominent, and could meaningfully influence the connection between tissue disruption and low back pain. These effects were most notable in exposures that have the potential to damage the annulus fibrosus