Rapid molecular evolution of pain insensitivity in multiple African rodents.

Noxious substances, called algogens, cause pain and are used as defensive weapons by plants and stinging insects. We identified four previously unknown instances of algogen-insensitivity by screening eight African rodent species related to the naked mole-rat with the painful substances capsaicin, acid (hydrogen chloride, pH 3.5), and allyl isothiocyanate (AITC). Using RNA sequencing, we traced the emergence of sequence variants in transduction channels, like transient receptor potential channel TRPA1 and voltage-gated sodium channel Nav1.7, that accompany algogen insensitivity. In addition, the AITC-insensitive highveld mole-rat exhibited overexpression of the leak channel NALCN (sodium leak channel, nonselective), ablating AITC detection by nociceptors. These molecular changes likely rendered highveld mole-rats immune to the stings of the Natal droptail ant. Our study reveals how evolution can be used as a discovery tool to find molecular mechanisms that shut down pain.

Publisher Correction: Lipidome determinants of maximal lifespan in mammals.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Fructose-driven glycolysis supports anoxia resistance in the naked mole-rat.

The African naked mole-rat's (Heterocephalus glaber) social and subterranean lifestyle generates a hypoxic niche. Under experimental conditions, naked mole-rats tolerate hours of extreme hypoxia and survive 18 minutes of total oxygen deprivation (anoxia) without apparent injury. During anoxia, the naked mole-rat switches to anaerobic metabolism fueled by fructose, which is actively accumulated and metabolized to lactate in the brain. Global expression of the GLUT5 fructose transporter and high levels of ketohexokinase were identified as molecular signatures of fructose metabolism. Fructose-driven glycolytic respiration in naked mole-rat tissues avoids feedback inhibition of glycolysis via phosphofructokinase, supporting viability. The metabolic rewiring of glycolysis can circumvent the normally lethal effects of oxygen deprivation, a mechanism that could be harnessed to minimize hypoxic damage in human disease.

Lipidome determinants of maximal lifespan in mammals.

Maximal lifespan of mammalian species, even if closely related, may differ more than 10-fold, however the nature of the mechanisms that determine this variability is unresolved. Here, we assess the relationship between maximal lifespan duration and concentrations of more than 20,000 lipid compounds, measured in 669 tissue samples from 6 tissues of 35 species representing three mammalian clades: primates, rodents and bats. We identify lipids associated with species' longevity across the three clades, uncoupled from other parameters, such as basal metabolic rate, body size, or body temperature. These lipids clustered in specific lipid classes and pathways, and enzymes linked to them display signatures of greater stabilizing selection in long-living species, and cluster in functional groups related to signaling and protein-modification processes. These findings point towards the existence of defined molecular mechanisms underlying variation in maximal lifespan among mammals.

Hypofunctional TrkA Accounts for the Absence of Pain Sensitization in the African Naked Mole-Rat.

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.

Measurement of Vibration Detection Threshold and Tactile Spatial Acuity in Human Subjects.

Tests that allow the precise determination of psychophysical thresholds for vibration and grating orientation provide valuable information about mechanosensory function that are relevant for clinical diagnosis as well as for basic research. Here, we describe two psychophysical tests designed to determine the vibration detection threshold (automated system) and tactile spatial acuity (handheld device). Both procedures implement a two-interval forced-choice and a transformed-rule up and down experimental paradigm. These tests have been used to obtain mechanosensory profiles for individuals from distinct human cohorts such as twins or people with sensorineural deafness.

Structural Changes and Lack of HCN1 Channels in the Binaural Auditory Brainstem of the Naked Mole-Rat (Heterocephalus glaber).

Naked mole-rats (Heterocephalus glaber) live in large eu-social, underground colonies in narrow burrows and are exposed to a large repertoire of communication signals but negligible binaural sound localization cues, such as interaural time and intensity differences. We therefore asked whether monaural and binaural auditory brainstem nuclei in the naked mole-rat are differentially adjusted to this acoustic environment. Using antibody stainings against excitatory and inhibitory presynaptic structures, namely the vesicular glutamate transporter VGluT1 and the glycine transporter GlyT2 we identified all major auditory brainstem nuclei except the superior paraolivary nucleus in these animals. Naked mole-rats possess a well structured medial superior olive, with a similar synaptic arrangement to interaural-time-difference encoding animals. The neighboring lateral superior olive, which analyzes interaural intensity differences, is large and elongated, whereas the medial nucleus of the trapezoid body, which provides the contralateral inhibitory input to these binaural nuclei, is reduced in size. In contrast, the cochlear nucleus, the nuclei of the lateral lemniscus and the inferior colliculus are not considerably different when compared to other rodent species. Most interestingly, binaural auditory brainstem nuclei lack the membrane-bound hyperpolarization-activated channel HCN1, a voltage-gated ion channel that greatly contributes to the fast integration times in binaural nuclei of the superior olivary complex in other species. This suggests substantially lengthened membrane time constants and thus prolonged temporal integration of inputs in binaural auditory brainstem neurons and might be linked to the severely degenerated sound localization abilities in these animals.

ASICs and mammalian mechanoreceptor function.

It is well established that some members of the Deg/ENaC super family of amiloride sensitive ion channels can participate directly in the transduction of mechanical stimuli by sensory neurons in invertebrates. A large body of work has also implicated the acid sensing ion channels family (ASIC1-4) as participants in regulating mechanoreceptor sensitivity in vertebrates. In this review we provide an overview of the physiological and genetic evidence for involvement of ASICs in mechanosensory function. On balance, the available evidence favors the idea that these channels have an important regulatory role in mechanosensory function. It is striking how diverse the consequences of Asic gene deletion are on mechanosensory function with both gain and loss of function effects being observed depending on sensory neuron type. We conclude that other, as yet unknown, molecular partners of ASIC proteins may be decisive in determining their precise physiological role in mechanosensory neurons. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Water-induced finger wrinkles do not affect touch acuity or dexterity in handling wet objects.

Human non-hairy (glabrous) skin of the fingers, palms and soles wrinkles after prolonged exposure to water. Wrinkling is a sympathetic nervous system-dependent process but little is known about the physiology and potential functions of water-induced skin wrinkling. Here we investigated the idea that wrinkling might improve handling of wet objects by measuring the performance of a large cohort of human subjects (n = 40) in a manual dexterity task. We also tested the idea that skin wrinkling has an impact on tactile acuity or vibrotactile sensation using two independent sensory tasks. We found that skin wrinkling did not improve dexterity in handling wet objects nor did it affect any aspect of touch sensitivity measured. Thus water-induced wrinkling appears to have no significant impact on tactile driven performance or dexterity in handling wet or dry objects.

A stomatin dimer modulates the activity of acid-sensing ion channels.

Stomatin proteins oligomerize at membranes and have been implicated in ion channel regulation and membrane trafficking. To obtain mechanistic insights into their function, we determined three crystal structures of the conserved stomatin domain of mouse stomatin that assembles into a banana-shaped dimer. We show that dimerization is crucial for the repression of acid-sensing ion channel 3 (ASIC3) activity. A hydrophobic pocket at the inside of the concave surface is open in the presence of an internal peptide ligand and closes in the absence of this ligand, and we demonstrate a function of this pocket in the inhibition of ASIC3 activity. In one crystal form, stomatin assembles via two conserved surfaces into a cylindrical oligomer, and these oligomerization surfaces are also essential for the inhibition of ASIC3-mediated currents. The assembly mode of stomatin uncovered in this study might serve as a model to understand oligomerization processes of related membrane-remodelling proteins, such as flotillin and prohibitin.

Cholesterol-depletion corrects APP and BACE1 misstrafficking in NPC1-deficient cells.

Cholesterol accumulation in Niemann-Pick type C disease (NPC) causes increased levels of the amyloid-precursor-protein C-terminal fragments (APP-CTFs) and intracellular amyloid-ß peptide (Aß), the two central molecules in Alzheimer's disease (AD) pathogenesis. We previously reported that cholesterol accumulation in NPC-cells leads to cholesterol-dependent increased APP processing by ß-secretase (BACE1) and decreased APP expression at the cell surface (Malnar et al. Biochim Biophys Acta. 1802 (2010) 682-691.). We hypothesized that increased formation of APP-CTFs and Aß in NPC disease is due to cholesterol-mediated altered endocytic trafficking of APP and/or BACE1. Here, we show that APP endocytosis is prerequisite for enhanced Aß levels in NPC-cells. Moreover, we observed that NPC cells show cholesterol dependent sequestration and colocalization of APP and BACE1 within enlarged early/recycling endosomes which can lead to increased ß-secretase processing of APP. We demonstrated that increased endocytic localization of APP in NPC-cells is likely due to both its increased internalization and its decreased recycling to the cell surface. Our findings suggest that increased cholesterol levels, such as in NPC disease and sporadic AD, may be the upstream effector that drives amyloidogenic APP processing characteristic for Alzheimer's disease by altering endocytic trafficking of APP and BACE1.

The molecular basis of acid insensitivity in the African 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 Na(V)1.7, which is potently blocked by protons and can account for acid insensitivity in this species. Thus, evolutionary pressure has selected for an Na(V)1.7 gene variant that tips the balance from proton-induced excitation to inhibition of action potential initiation to abolish acid nociception.

Niemann-Pick type C cells show cholesterol dependent decrease of APP expression at the cell surface and its increased processing through the beta-secretase pathway.

The link between cholesterol and Alzheimer's disease has recently been revealed in Niemann-Pick type C disease. We found that NPC1(-/-) cells show decreased expression of APP at the cell surface and increased processing of APP through the beta-secretase pathway resulting in increased C99, sAPPbeta and intracellular Abeta40 levels. This effect is dependent on increased cholesterol levels, since cholesterol depletion reversed cell surface APP expression and lowered Abeta/C99 levels in NPC1(-)(/)(-) cells to the levels observed in wt cells. Finding that overexpression of C99, a direct gamma-secretase substrate, does not lead to increased intracellular Abeta levels in NPC1(-)(/)(-) cells vs. CHOwt suggests that the effect on intracellular Abeta upon cholesterol accumulation in NPC1(-)(/)(-) cells is not due to increased APP cleavage by gamma-secretase. Our results indicate that cholesterol may modulate APP processing indirectly by modulating APP expression at the cell surface and thus its cleavage by beta-secretase.