Naked mole-rat cortical neurons are resistant to acid-induced cell death

The Naked Mole-Rat as a Model for Healthy Aging

Naked mole-rats (NMRs, Heterocephalus glaber) are the longest-lived rodents with a maximum life span exceeding 37 years. They exhibit a delayed aging phenotype and resistance to age-related functional decline/diseases. Specifically, they do not display increased mortality with age, maintain several physiological functions until nearly the end of their lifetime, and rarely develop cancer and Alzheimer's disease. NMRs live in a hypoxic environment in underground colonies in East Africa and are highly tolerant of hypoxia. These unique characteristics of NMRs have attracted considerable interest from zoological and biomedical researchers. This review summarizes previous studies of the ecology, hypoxia tolerance, longevity/delayed aging, and cancer resistance of NMRs and discusses possible mechanisms contributing to their healthy aging. In addition, we discuss current issues and future perspectives to fully elucidate the mechanisms underlying delayed aging and resistance to age-related diseases in NMRs.

Unusual occurrence of domestication syndrome amongst African mole-rats: Is the naked mole-rat a domestic animal?

The Naked mole-rat (NMR) is becoming a prominent model organism due to its peculiar traits, such as eusociality, extreme longevity, cancer resistance, and reduced pain sensitivity. It belongs to the African mole-rats (AMR), a family of subterranean rodents that includes solitary, cooperative breeding and eusocial species. We identified and quantified the domestication syndrome (DS) across AMR, a set of morphological and behavioural traits significantly more common and pronounced amongst domesticated animals than in their wild counterparts. Surprisingly, the NMR shows apparent DS traits when compared to the solitary AMR. Animals can self-domesticate when a reduction of the fear response is naturally selected, such as in islands with no predators, or to improve the group’s harmony in cooperative breeding species. The DS may be caused by alterations in the physiology of the neural crest cells (NCC), a transient population of cells that generate a full range of tissues during development. The NCC contribute to organs responsible for transmitting the fear response and various other tissues, including craniofacial bones. Therefore, mutations affecting the NCC can manifest as behavioural and morphological alterations in many structures across the body, as seen in neurocristopathies. We observed that all social AMRs are chisel-tooth diggers, an adaption to hard soils that requires the flattening of the skull. We hypothesise that chisel-tooth digging could impose a selective pressure on the NCC that triggered the DS’s appearance, possibly facilitating the evolution of sociality. Finally, we discuss how DS traits are neutral or beneficial for the subterranean niche, strategies to test this hypothesis and report well-studied mutations in the NMR that are associated with the NCC physiology or with the control of the fear response. In conclusion, we argue that many of the NMR’s unconventional traits are compatible with the DS and provide a hypothesis about its origins. Our model proposes a novel avenue to enhance the understanding of the extraordinary biology of the NMR.

Carbon dioxide tolerability and toxicity in rat and man: A translational study

Background: Due the increasing need for storage of carbon dioxide (CO₂) more individuals are prone to be exposed to high concentrations of CO₂ accidentally released into atmosphere, with deleterious consequences.

Methods: We tested the effect of increasing CO₂ concentrations in humans (6–12%) and rats (10–50%) at varying inhalation times (10–60 min). In humans, a continuous positive airway pressure helmet was used to deliver the gas mixture to the participants. Unrestrained rats were exposed to CO₂ in a transparent chamber. In both species regular arterial blood gas samples were obtained. After the studies, the lungs of the animals were examined for macroscopic and microscopic abnormalities.

Results: In humans, CO₂ concentrations of 9% inhaled for >10 min, and higher concentrations inhaled for <10 min were poorly or not tolerated due to exhaustion, anxiety, dissociation or acidosis (pH < 7.2), despite intact oxygenation. In rats, concentrations of 30% and higher were associated with CO₂ narcosis, epilepsy, poor oxygenation and, at 50% CO₂, spontaneous death. Lung hemorrhage and edema were observed in the rats at inhaled concentrations of 30% and higher.

Conclusion: This study provides essential insight into the occurrence of physiological changes in humans and fatalities in rats after acute exposure to high levels of CO₂. Humans tolerate 9% CO₂ and retain their ability to function coherently for up to 10 min. These data support reconsideration of the current CO₂ levels (<7.5%) that pose a risk to exposed individuals (<7.5%) as determined by governmental agencies to ≤9%.

Supermole-rat to the rescue: Does the naked mole-rat offer a panacea for all that ails us?

Over the previous several decades, many non-traditional research models have offered new avenues of exploration for biomedical research. The promise of these animals is primarily derived from adaptations to unique or challenging environments that share key factors with a disease or pathology of interest (e.g., hypoxemia or hypercarbia are clinically relevant and are also in vivo consequences of environmental hypoxia and hypercapnia, respectively). Animals adapted to such environments allow us to ask the question: how has nature solved a particular problem and what can we learn to inform novel translational research into the treatment of related diseases and pathologies? One of the most promising mammalian models that have garnered increasing attention from researchers and the public are naked mole-rats (NMRs). The NMR is a small and eusocial subterranean rodent species that live in a putatively hypoxic and hypercapnic burrow environment. Intriguingly, whereas most non-traditional biomedical models offer insight into one or only a few diseases related to a common physiological stress, NMRs in contrast have proven to be resistant to a very wide range of ailments, including aging, cancer, and hypoxia- and hypercapnia-related disorders, among many others. In the present commentary, we discuss progress made in understanding how NMRs overcome these challenges and speculate on the origins of their remarkable abilities.

Naked mole-rat brain mitochondria tolerate in vitro ischaemia

Naked mole-rats (NMRs; Heterocephalus glaber) are among the most hypoxia-tolerant mammals. There is evidence that the NMR brain tolerates in vitro hypoxia and NMR brain mitochondria exhibit functional plasticity following in vivo hypoxia; however, if and how these organelles tolerate ischaemia and how ischaemic stress impacts mitochondrial energetics and redox regulation is entirely unknown. We hypothesized that mitochondria fundamentally contribute to in vitro ischaemia resistance in the NMR brain. To test this, we treated NMR and CD-1 mouse cortical brain sheets with an in vitro ischaemic mimic and evaluated mitochondrial respiration capacity and redox regulation following 15 or 30 min of ischaemia or ischaemia/reperfusion (I/R). We found that, relative to mice, the NMR brain largely retains mitochondrial function and redox balance post-ischaemia and I/R. Specifically: (i) ischaemia reduced complex I and II-linked respiration ∼50-70% in mice, vs. ∼20-40% in NMR brain, (ii) NMR but not mouse brain maintained relatively steady respiration control ratios and robust mitochondrial membrane integrity, (iii) electron leakage post-ischaemia was lesser in NMR than mouse brain and NMR brain retained higher coupling efficiency, and (iv) free radical generation during and following ischaemia and I/R was lower from NMR brains than mice. Taken together, our results indicate that NMR brain mitochondria are more tolerant of ischaemia and I/R than mice and retain respiratory capacity while avoiding redox derangements. Overall, these findings support the hypothesis that hypoxia-tolerant NMR brain is also ischaemia-tolerant and suggest that NMRs may be a natural model of ischaemia tolerance in which to investigate evolutionarily derived solutions to ischaemic pathology. KEY POINTS: Ischaemia is highly deleterious to the mammalian brain and this damage is largely mediated by mitochondrial dysfunction. Naked mole-rats are among the most hypoxia-tolerant mammals and their brain tolerates ischaemia ex vivo, but the impact of ischaemia on mitochondrial function is unknown. Naked mole-rat but not mouse brain mitochondria retain respiratory capacity and membrane integrity following ischaemia or ischaemia/reperfusion. Differences in free radical management and respiratory pathway control between species may mediate this tolerance. These results help us understand how natural models of hypoxia tolerance also tolerate ischaemia in the brain.

The Idiosyncratic Physiological Traits of the Naked Mole-Rat; a Resilient Animal Model of Aging, Longevity, and Healthspan

The subterranean-dwelling naked mole-rat (Heterocephalus glaber) is an extremophilic rodent, able to thrive in the harsh underground conditions of sub-Saharan Northeast Africa. This pelage-free mammal exhibits numerous unusual ecophysiological features including pronounced tolerance of thermolability, hypoxia, hypercapnia and noxious substances. As a mammal, the naked mole-rat provides a proof-of-concept that age-related changes in physiology are avoidable. At ages far beyond their expected lifespans given both their body size and/or the timing of early developmental milestones, naked mole-rats fail to exhibit meaningful changes in physiological health or demographic mortality. Lack of physiological deterioration with age is also evident in lean and fat mass, bone quality, and reproductive capacity. Rather, regardless of age, under basal conditions naked mole-rats appear to "idle on low" with their "shields up" as is manifested by low body temperature, metabolic rate, cardiac output and kidney concentrating ability, enabling better protection of organs and cellular function. When needed, they can nevertheless ramp up these functions, increasing cardiac output and metabolism 2-5 fold. Here we review many unusual aspects of their physiology and examine how these attributes facilitate both tolerance of the diverse suite of hostile conditions encountered in their natural milieu as well as contribute to their extraordinary longevity and resistance to common, age-related chronic diseases.

Eusociality and Senescence: Neuroprotection and Physiological Resilience to Aging in Insect and Mammalian Systems

Are eusociality and extraordinary aging polyphenisms evolutionarily coupled? The remarkable disparity in longevity between social insect queens and sterile workers-decades vs. months, respectively-has long been recognized. In mammals, the lifespan of eusocial naked mole rats is extremely long-roughly 10 times greater than that of mice. Is this robustness to senescence associated with social evolution and shared mechanisms of developmental timing, neuroprotection, antioxidant defenses, and neurophysiology? Focusing on brain senescence, we examine correlates and consequences of aging across two divergent eusocial clades and how they differ from solitary taxa. Chronological age and physiological indicators of neural deterioration, including DNA damage or cell death, appear to be decoupled in eusocial insects. In some species, brain cell death does not increase with worker age and DNA damage occurs at similar rates between queens and workers. In comparison, naked mole rats exhibit characteristics of neonatal mice such as protracted development that may offer protection from aging and environmental stressors. Antioxidant defenses appear to be regulated differently across taxa, suggesting independent adaptations to life history and environment. Eusocial insects and naked mole rats appear to have evolved different mechanisms that lead to similar senescence-resistant phenotypes. Careful selection of comparison taxa and further exploration of the role of metabolism in aging can reveal mechanisms that preserve brain functionality and physiological resilience in eusocial species.

Adult Neural Plasticity in Naked Mole-Rats: Implications of Fossoriality, Longevity and Sociality on the Brain's Capacity for Change

Naked mole-rats (Heterocephalus glaber) are small African rodents that have many unique behavioral and physiological adaptations well-suited for testing hypotheses about mammalian neural plasticity. In this chapter, we focus on three features of naked mole-rat biology and how they impact neural plasticity in this species: (1) their fossorial lifestyle, (2) their extreme longevity with a lack of demonstrable senescence, and (3) their unusual social structure. Critically, each of these features requires some degree of biological flexibility. First, their fossorial habitat situates them in an environment with characteristics to which the central nervous system is particularly sensitive (e.g., oxygen content, photoperiod, spatial complexity). Second, their long lifespan requires adaptations to combat senescence and declines in neural functioning. Finally, their extreme reproductive skew and sustained ability for release from reproductive suppression indicates remarkable neural sensitivity to the sociosexual environment that is distinct from chronological age. These three features of naked mole-rat life are not mutually exclusive, but they do each offer unique considerations for the possibilities, constraints, and mechanisms associated with adult neural plasticity.

Naked Mole-Rats: Resistant to Developing Cancer or Good at Avoiding It?

It is widely accepted that cancer is driven by genetic mutations that confer uncontrolled cell proliferation and tumor formation. For tumors to take hold and grow, cancer cells evolve mechanisms to favorably shape their microenvironment and avoid being cleared by the immune system. Cancer is not unique to human, but rather affects nearly all multicellular organisms albeit to different degrees. The different degrees of cancer susceptibility across the animal kingdom could be attributed to several factors, which have been the subject of several studies in recent years. The naked mole-rat (NMR, Heterocephalus glaber), an exceptionally long-lived rodent, which, as discussed in detail in the next section, displays significant cancer resistance with only a small number of animals being reported to exhibit spontaneous neoplasms. The reason why studying cancer resistance in NMRs is of particular interest is that not only are they now an established laboratory species, but that NMRs are mammals and thus there is great potential for translating knowledge about their cancer resistance into preventing and/or treating cancer in humans and companion animals.

The Somatosensory World of the African Naked Mole-Rat

The naked mole-rat (Heterocephalus glaber) is famous for its longevity and unusual physiology. This eusocial species that lives in highly ordered and hierarchical colonies with a single breeding queen, also discovered secrets enabling somewhat pain-free living around 20 million years ago. Unlike most mammals, naked mole-rats do not feel the burn of chili pepper's active ingredient, capsaicin, nor the sting of acid. Indeed, by accumulating mutations in genes encoding proteins that are only now being exploited as targets for new pain therapies (the nerve growth factor receptor TrkA and voltage-gated sodium channel, Na_V1.7), this species mastered the art of analgesia before humans evolved. Recently, we have identified pain-insensitivity as a trait shared by several closely related African mole-rat species. In this chapter we will show how African mole-rats have evolved pain insensitivity as well as discussing what the proximate factors may have been that led to the evolution of pain-free traits.

African Naked Mole-Rats Demonstrate Extreme Tolerance to Hypoxia and Hypercapnia

Naked mole-rats are extremely tolerant to low concentrations of oxygen (hypoxia) and high concentrations of carbon dioxide (hypercapnia), which is consistent with the environment that they inhabit. Naked mole-rats combine subterranean living with living in very densely populated colonies where oxygen becomes depleted and carbon dioxide accumulates. In the laboratory, naked mole-rats fully recover from 5 h exposure to 5% O₂ and 5 h exposure to 80% CO₂, whereas both conditions are rapidly lethal to similarly sized laboratory mice. During anoxia (0% O₂) naked mole-rats enter a suspended animation-like state and switch from aerobic metabolism of glucose to anaerobic metabolism of fructose. Additional fascinating characteristics include that naked mole-rats show intrinsic brain tolerance to anoxia; a complete lack of hypoxia-induced and CO₂-induced pulmonary edema; and reduced aversion to high concentrations of CO₂ and acidic fumes. Here we outline a constellation of physiological and molecular adaptations that correlate with the naked mole-rat's hypoxic/hypercapnic tolerance and which offer potential targets for ameliorating pathological conditions in humans, such as the damage caused during cerebral ischemia.

Acid and inflammatory sensitisation of naked mole-rat colonic afferent nerves

Acid sensing in the gastrointestinal tract is required for gut homeostasis and the detection of tissue acidosis caused by ischaemia, inflammation and infection. In the colorectum, activation of colonic afferents by low pH contributes to visceral hypersensitivity and abdominal pain in human disease including during inflammatory bowel disease. The naked mole-rat (Heterocephalus glaber) shows no pain-related behaviour to subcutaneous acid injection and cutaneous afferents are insensitive to acid, an adaptation thought to be a consequence of the subterranean, likely hypercapnic, environment in which it lives. As such we sought to investigate naked mole-rat interoception within the gastrointestinal tract and how this differed from the mouse (Mus Musculus). Here, we show the presence of calcitonin gene-related peptide expressing extrinsic nerve fibres innervating both mesenteric blood vessels and the myenteric plexi of the smooth muscle layers of the naked mole-rat colorectum. Using ex vivo colonic-nerve electrophysiological recordings, we show differential sensitivity of naked mole-rat, compared to mouse, colonic afferents to acid and the prototypic inflammatory mediator bradykinin, but not direct mechanical stimuli. In naked mole-rat, but not mouse, we observed mechanical hypersensitivity to acid, whilst both species sensitised to bradykinin. Collectively, these findings suggest that naked mole-rat colonic afferents are capable of detecting acidic stimuli; however, their intracellular coupling to downstream molecular effectors of neuronal excitability and mechanotransduction likely differs between species.

Protection of melatonin against acidosis-induced neuronal injuries

Acidosis, a common feature of cerebral ischaemia and hypoxia, plays a key role in these pathological processes by aggravating the ischaemic and hypoxic injuries. To explore the mechanisms, in this research, we cultured primary neurons in an acidic environment (potential of hydrogen [pH]6.2, 24 hours) to mimic the acidosis. By proteomic analysis, 69 differentially expressed proteins in the acidic neurons were found, mainly related to stress and cell death, synaptic plasticity and gene transcription. And, the acidotic neurons developed obvious alterations including increased neuronal death, reduced dendritic length and complexity, reduced synaptic proteins, tau hyperphosphorylation, endoplasmic reticulum (ER) stress activation, abnormal lysosome‐related signals, imbalanced oxidative stress/anti‐oxidative stress and decreased Golgi matrix proteins. Then, melatonin (1 × 10⁻⁴ mol/L) was used to pre‐treat the cultured primary neurons before acidic treatment (pH6.2). The results showed that melatonin partially reversed the acidosis‐induced neuronal death, abnormal dendritic complexity, reductions of synaptic proteins, tau hyperphosphorylation and imbalance of kinase/phosphatase. In addition, acidosis related the activations of glycogen synthase kinase‐3β and nuclear factor‐κB signals, ER stress and Golgi stress, and the abnormal autophagy‐lysosome signals were completely reversed by melatonin. These data indicate that melatonin is beneficial for neurons against acidosis‐induced injuries.

Independent evolution of pain insensitivity in African mole-rats: origins and mechanisms

The naked mole-rat (Heterocephalus glaber) is famous for its longevity and unusual physiology. This eusocial species that lives in highly ordered and hierarchical colonies with a single breeding queen, also discovered secrets enabling somewhat pain-free living around 20 million years ago. Unlike most mammals, naked mole-rats do not feel the burn of chili pepper's active ingredient, capsaicin, nor the sting of acid. Indeed, by accumulating mutations in genes encoding proteins that are only now being exploited as targets for new pain therapies (the nerve growth factor receptor TrkA and voltage-gated sodium channel, NaV1.7), this species mastered the art of analgesia before humans evolved. Recently, we have identified pain insensitivity as a trait shared by several closely related African mole-rat species. One of these African mole-rats, the Highveld mole-rat (Cryptomys hottentotus pretoriae), is uniquely completely impervious and pain free when confronted with electrophilic compounds that activate the TRPA1 ion channel. The Highveld mole-rat has evolved a biophysical mechanism to shut down the activation of sensory neurons that drive pain. In this review, we will show how mole-rats have evolved pain insensitivity as well as discussing what the proximate factors may have been that led to the evolution of pain-free traits.

Nest Carbon Dioxide Masks GABA-Dependent Seizure Susceptibility in the Naked Mole-Rat

African naked mole-rats were likely the first mammals to evolve eusociality, and thus required adaptations to conserve energy and tolerate the low oxygen (O₂) and high carbon dioxide (CO₂) of a densely populated fossorial nest. As hypercapnia is known to suppress neuronal activity, we studied whether naked mole-rats might demonstrate energy savings in GABAergic inhibition. Using whole-colony behavioral monitoring of captive naked mole-rats, we found a durable nest, characterized by high CO₂ levels, where all colony members spent the majority of their time. Analysis of the naked mole-rat genome revealed, uniquely among mammals, a histidine point variation in the neuronal potassium-chloride cotransporter 2 (KCC2). A histidine missense substitution mutation at this locus in the human ortholog of KCC2, found previously in patients with febrile seizures and epilepsy, has been demonstrated to diminish neuronal Cl^- extrusion capacity, and thus impairs GABAergic inhibition. Seizures were observed, without pharmacological intervention, in adult naked mole-rats exposed to a simulated hyperthermic surface environment, causing systemic hypocapnic alkalosis. Consistent with the diminished function of KCC2, adult naked mole-rats demonstrate a reduced efficacy of inhibition that manifests as triggering of seizures at room temperature by the GABA_A receptor (GABA_AR) positive allosteric modulator diazepam. These seizures are blocked in the presence of nest-like levels of CO₂ and likely to be mediated through GABA_AR activity, based on in vitro recordings. Thus, altered GABAergic inhibition adds to a growing list of adaptations in the naked mole-rat and provides a plausible proximate mechanism for nesting behavior, where a return to the colony nest restores GABA-mediated inhibition.

Transcriptome analysis of the response provided by Lasiopodomys mandarinus to severe hypoxia includes enhancing DNA repair and damage prevention

Background

Severe hypoxia induces a series of stress responses in mammals; however, subterranean rodents have evolved several adaptation mechanisms of energy metabolisms and O₂ utilization for hypoxia. Mammalian brains show extreme aerobic metabolism. Following hypoxia exposure, mammals usually experience irreversible brain damage and can even develop serious diseases, such as hypoxic ischemic encephalopathy and brain edema. To investigate mechanisms underlying the responses of subterranean rodents to severe hypoxia, we performed a cross-species brain transcriptomic analysis using RNA sequencing and identified differentially expressed genes (DEGs) between the subterranean rodent Lasiopodomys mandarinus and its closely related aboveground species L. brandtii under severe hypoxia (5.0% O₂, 6 h) and normoxia (20.9% O₂, 6 h).

Results

We obtained 361 million clean reads, including 69,611 unigenes in L. mandarinus and 69,360 in L. brandtii. We identified 359 and 515 DEGs by comparing the hypoxic and normoxia groups of L. mandarinus and L. brandtii, respectively. Gene Ontology (GO) analysis showed that upregulated DEGs in both species displayed similar terms in response to severe hypoxia; the main difference is that GO terms of L. brandtii were enriched in the immune system. However, in the downregulated DEGs, GO terms of L. mandarinus were enriched in cell proliferation and protein transport and those of L. brandtii were enriched in nuclease and hydrolase activities, particularly in terms of developmental functions. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that upregulated DEGs in L. mandarinus were associated with DNA repair and damage prevention as well as angiogenesis and metastasis inhibition, whereas downregulated DEGs were associated with neuronal synaptic transmission and tumor-associated metabolic pathways. In L. brandtii, upregulated KEGG pathways were enriched in the immune, endocrine, and cardiovascular systems and particularly in cancer-related pathways, whereas downregulated DEGs were associated with environmental information processing and misregulation in cancers.

Conclusions

L. mandarinus has evolved hypoxia adaptation by enhancing DNA repair, damage prevention, and augmenting sensing, whereas L. brandtii showed a higher risk of tumorigenesis and promoted innate immunity toward severe hypoxia. These results reveal the hypoxic mechanisms of L. mandarinus to severe hypoxia, which may provide research clues for hypoxic diseases.

In silico screening of GMQ-like compounds reveals guanabenz and sephin1 as new allosteric modulators of acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) are voltage-independent cation channels that detect decreases in extracellular pH. Dysregulation of ASICs underpins a number of pathologies. Of particular interest is ASIC3, which is recognised as a key sensor of acid-induced pain and is important in the establishment of pain arising from inflammatory conditions, such as rheumatoid arthritis. Thus, the identification of new ASIC3 modulators and the mechanistic understanding of how these compounds modulate ASIC3 could be important for the development of new strategies to counteract the detrimental effects of dysregulated ASIC3 activity in inflammation. Here, we report the identification of novel ASIC3 modulators based on the ASIC3 agonist, 2-guanidine-4-methylquinazoline (GMQ). Through a GMQ-guided in silico screening of Food and Drug administration (FDA)-approved drugs, 5 compounds were selected and tested for their modulation of rat ASIC3 (rASIC3) using whole-cell patch-clamp electrophysiology. Of the chosen drugs, guanabenz (GBZ), an α₂-adrenoceptor agonist, produced similar effects to GMQ on rASIC3, activating the channel at physiological pH (pH 7.4) and potentiating its response to mild acidic (pH 7) stimuli. Sephin1, a GBZ derivative that lacks α₂-adrenoceptor activity, has been proposed to act as a selective inhibitor of a regulatory subunit of the stress-induced protein phosphatase 1 (PPP1R15A) with promising therapeutic potential for the treatment of multiple sclerosis. However, we found that like GBZ, sephin1 activates rASIC3 at pH 7.4 and potentiates its response to acidic stimulation (pH 7), i.e. sephin1 is a novel modulator of rASIC3. Furthermore, docking experiments showed that, like GMQ, GBZ and sephin1 likely interact with the nonproton ligand sensor domain of rASIC3. Overall, these data demonstrate the utility of computational analysis for identifying novel ASIC3 modulators, which can be validated with electrophysiological analysis and may lead to the development of better compounds for targeting ASIC3 in the treatment of inflammatory conditions.

Neurokinin-1 receptor activation is sufficient to restore the hypercapnic ventilatory response in the Substance P-deficient naked mole-rat

Naked mole-rats (NMRs) live in large colonies within densely populated underground burrows. Their collective respiration generates significant metabolic carbon dioxide (CO₂) that diffuses slowly out of the burrow network, creating a hypercapnic environment. Currently, the physiological mechanisms that underlie the ability of NMRs to tolerate environmental hypercapnia are largely unknown. To address this, we used whole-body plethysmography and respirometry to elucidate the hypercapnic ventilatory and metabolic responses of awake, freely behaving NMRs to 0%-10% CO₂. We found that NMRs have a blunted hypercapnic ventilatory response (HCVR): ventilation increased only in 10% CO₂. Conversely, metabolism was unaffected by hypercapnia. NMRs are insensitive to cutaneous acid-based pain caused by modified substance P (SP)-mediated peripheral neurotransmission, and SP is also an important neuromodulator of ventilation. Therefore, we re-evaluated physiological responses to hypercapnia in NMRs after an intraperitoneal injection of exogenous substance P (2 mg/kg) or a long-lived isoform of substance P {[pGlu5-MePhe8-MeGly9]SP(5-11), DiMe-C7; 40-400 μg/kg}. We found that both drugs restored hypercapnia sensitivity and unmasked an HCVR in animals breathing 2%-10% CO₂. Taken together, our findings indicate that NMRs are remarkably tolerant of hypercapnic environments and have a blunted HCVR; however, the signaling network architecture required for a "normal" HCVR is retained but endogenously inactive. This muting of chemosensitivity likely suits the ecophysiology of this species, which presumably experiences hypercapnia regularly in their underground niche.

Identification of TMEM206 proteins as pore of PAORAC/ASOR acid-sensitive chloride channels

Acid-sensing ion channels have important functions in physiology and pathology, but the molecular composition of acid-activated chloride channels had remained unclear. We now used a genome-wide siRNA screen to molecularly identify the widely expressed acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR. ASOR is formed by TMEM206 proteins which display two transmembrane domains (TMs) and are expressed at the plasma membrane. Ion permeation-changing mutations along the length of TM2 and at the end of TM1 suggest that these segments line ASOR’s pore. While not belonging to a gene family, TMEM206 has orthologs in probably all vertebrates. Currents from evolutionarily distant orthologs share activation by protons, a feature essential for ASOR’s role in acid-induced cell death. TMEM206 defines a novel class of ion channels. Its identification will help to understand its physiological roles and the diverse ways by which anion-selective pores can be formed.

Two central pattern generators from the crab, Cancer borealis, respond robustly and differentially to extreme extracellular pH

The activity of neuronal circuits depends on the properties of the constituent neurons and their underlying synaptic and intrinsic currents. We describe the effects of extreme changes in extracellular pH – from pH 5.5 to 10.4 – on two central pattern generating networks, the stomatogastric and cardiac ganglia of the crab, Cancer borealis. Given that the physiological properties of ion channels are known to be sensitive to pH within the range tested, it is surprising that these rhythms generally remained robust from pH 6.1 to pH 8.8. The pH sensitivity of these rhythms was highly variable between animals and, unexpectedly, between ganglia. Animal-to-animal variability was likely a consequence of similar network performance arising from variable sets of underlying conductances. Together, these results illustrate the potential difficulty in generalizing the effects of environmental perturbation across circuits, even within the same animal.