Functional neurokinin and NMDA receptor activity in an animal naturally lacking substance P: the naked mole-rat

Focal Adhesion Kinase Inhibition Ameliorates Burn Injury-Induced Chronic Pain in Rats

Burn injury-induced pain (BIP) is a significant global health concern, affecting diverse populations including children, military veterans, and accident victims. Current pharmacotherapeutics for the management of BIP are associated with severe side effects including drug addiction, respiratory depression, sedation, and constipation posing significant barrier to their clinical utility. In the present study, we have investigated the potential role of focal adhesion kinase (p-FAK) for the very first time in BIP and elucidated the associated underlying mechanisms. Defactinib (DFT), a potent p-FAK inhibitor, administered at doses of 5, 10, and 20 mg/kg via intraperitoneal injection, demonstrates significant efficacy in reducing both evoked and spontaneous pain without causing addiction or other central nervous system toxicities. Burn injury triggers p-FAK-mediated phosphorylation of Erk1/2 and NR2B signaling in the DRG, resulting in heightened hypersensitivity through microglial activation, neuropeptide release, and elevated proinflammatory cytokines. Defactinib (DFT) counteracts these effects by reducing NR2B upregulation, lowering substance P levels, inhibiting microglial activation, and restoring IL-10 levels while leaving CGRP levels unchanged. These findings provide valuable insights into the pivotal role of p-FAK in regulating BIP and highlight the potential for developing novel therapeutics for burn injury-induced pain with minimal side effects.

Not just a cousin of the naked mole-rat: Damaraland mole-rats offer unique insights into biomedicine

Evolutionary medicine has been a fast-growing field of biological research in the past decade. One of the strengths of evolutionary medicine is to use non-traditional model organisms which often exhibit unusual characteristics shaped by natural selection. Studying these unusual traits could provide valuable insight to understand biomedical questions, since natural selection likely discovers solutions to those complex biological problems. Because of many unusual traits, the naked mole-rat (NMR) has attracted attention from different research areas such as aging, cancer, and hypoxia- and hypercapnia-related disorders. However, such uniqueness of NMR physiology may sometimes make the translational study to human research difficult. Damaraland mole-rat (DMR) shares multiple characteristics in common with NMR, but shows higher degree of similarity with human in some aspects of their physiology. Research on DMR could therefore offer alternative insights and might bridge the gap between experimental findings from NMR to human biomedical research. In this review, we discuss studies of DMR as an extension of the current set of model organisms to help better understand different aspects of human biology and disease. We hope to encourage researchers to consider studying DMR together with NMR. By studying these two similar but evolutionarily distinct species, we can harvest the power of convergent evolution and avoid the potential biased conclusions based on life-history of a single species.

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.

The naked mole-rat has a functional purinergic pain pathway despite having a non-functional peptidergic pain pathway

Naked mole-rats (Heterocephalus glaber) have adaptations within their pain pathway that are beneficial to survival in large colonies within poorly ventilated burrow systems, with lower O2 and higher CO2 ambient levels than ground-level environments. These adaptations ultimately lead to a partial disruption of the C-fiber pain pathway, which enables naked mole-rats to not feel pain from the acidosis associated with CO2 accumulation. One hallmark of this disruption is that naked mole-rats do not express neuropeptides, such as Substance P and calcitonin gene-related peptide in their cutaneous C-fibers, effectively making the peptidergic pain pathway hypofunctional. One C-fiber pathway that remains unstudied in the naked mole-rat is the non-peptidergic, purinergic pathway, despite this being a key pathway for inflammatory pain. The current study aimed to establish the functionality of the purinergic pathway in naked mole-rats and the effectiveness of cannabinoids in attenuating pain through this pathway. Cannabinoids can manage chronic inflammatory pain in both humans and mouse models, and studies suggest a major downstream role for the purinergic receptor, P2X3, in this treatment. Here we used Ca2+-imaging of cultured dorsal root ganglion neurons and in vivo behavioral testing to demonstrate that the P2X3 pathway is functional in naked mole-rats. Additionally, formalin-induced inflammatory pain was reduced by the cannabinoid receptor agonist, WIN55 (inflammatory, but not acute phase) and the P2X3 receptor antagonist A-317491 (acute and inflammatory phases). This study establishes that the purinergic C-fiber pathway is present and functional in naked mole-rats and that cannabinoid-mediated analgesia occurs in this species.

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.

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.

Pharmacological targeting of the mammalian clock reveals a novel analgesic for osteoarthritis-induced pain

Environmental disruption of the circadian rhythm is linked with increased pain due to osteoarthritis (OA). We aimed to characterize the role of the clock gene in OA-induced pain more systemically using both genetic and pharmacological approaches. Genetically modified mice, (bmal1f/fNav1.8CreERT mice), generated by deleting the critical clock gene, bmal1, from Nav1.8 sensory neurons, were resistant to the development of mechanical hyperalgesia associated with OA induced by partial medial meniscectomy (PMM) of the knee. In wild-type mice, induction of OA by PMM surgery led to a substantial increase in BMAL1 expression in DRG neurons. Interestingly, pharmacological activation of the REV-ERB (a negative regulator of bmal1 transcription) with SR9009 resulted in reduction of BMAL1 expression, and a significant decrease in mechanical hyperalgesia associated with OA. Cartilage degeneration was also significantly reduced in mice treated with the REV-ERB agonist SR9009. Based on these data, we also assessed the effect of pharmacological activation of REV-ERB using a model of environmental circadian disruption with its associated mechanical hyperalgesia, and noted that SR9009 was an effective analgesic in this model as well. Our data clearly demonstrate that genetic disruption of the molecular clock, via deletion of bmal1 in the sensory neurons of the DRG, decreases pain in a model of OA. Furthermore, pharmacological activation of REV-ERB leading to suppression of BMAL1 expression may be an effective method for treating OA-related pain, as well as to reduce joint damage associated with this disease.

Capsaicin pretreatment reversed pulmonary arterial hypertension by alleviating inflammation via p38MAPK pathway

Purpose/Aim: Pulmonary arterial hypertension (PAH) is a lethal disease associated with pulmonary vascular remodeling as well as inflammation. As a kind of tachykinin secreted by nerves and inflammatory cells, substance P (SP) has been proved to be involved in the progression of PAH. Capsaicin can deplete substance P and provide benefits in PAH. However, the mechanism is still unclear. In this article, we aim to illustrate the possible mechanism involved in the process of capsaicin alleviating PAH.

MATERIALS AND METHODS

A single injection of monocrotaline (MCT) to male Sprague-Dawley (SD) rats was conducted to induce PAH. Capsaicin pretreatment was administered three days before MCT injection to deplete substance P. P38mitogen-activated protein kinase (p38MAPK) activator or inhibitor was given intraperitoneally after MCT injection. After 28 days, hemodynamic studies were carried out, and right ventricular systolic pressure (RVSP), right ventricular (RV)/left ventricle plus septum (LV+S), RV/body weight (BW), and lung weight (LW)/BW were recorded and calculated. In addition, the pulmonary vascular remodeling (pulmonary arterial medial wall thickness, area, perivascualr fibrosis), pro-inflammatory cytokines, the common signal pathways, such as peroxisome proliferator-activated receptor gamma (PPARγ), extracellular signal-regulated kinases (Erk), protein kinase B (Akt), and p38MAPK were also detected.

RESULTS

Capsaicin pretreatment reversed PAH, including decreasing RVSP, RV/(LV+S), RV/BW, and LW/BW, and alleviating inflammation. Phosphorylated-p38 (p-p38) MAPK was up-regulated, which was partially reversed by capsaicin pretreatment. Interestingly, expression of Akt, Erk, and PPARγ was not altered by capsaicin pretreatment. Inhibition of p38MAPK provided the same benefits with capsaicin pretreatment, whereas it failed to provide additional improvement in the presence of capsaicin. Besides, p38MAPK activator abolished the effects of capsaicin pretreatment on PAH, suggesting a key role of p38MPAK pathway in the effects of capsaicin reversing PAH.

CONCLUSIONS

Capsaicin pretreatment reversed PAH by alleviating inflammation via p38MAPK pathway.

Unraveling the message: insights into comparative genomics of the naked mole-rat

Animals have evolved to survive, and even thrive, in different environments. Genetic adaptations may have indirectly created phenotypes that also resulted in a longer lifespan. One example of this phenomenon is the preternaturally long-lived naked mole-rat. This strictly subterranean rodent tolerates hypoxia, hypercapnia, and soil-based toxins. Naked mole-rats also exhibit pronounced resistance to cancer and an attenuated decline of many physiological characteristics that often decline as mammals age. Elucidating mechanisms that give rise to their unique phenotypes will lead to better understanding of subterranean ecophysiology and biology of aging. Comparative genomics could be a useful tool in this regard. Since the publication of a naked mole-rat genome assembly in 2011, analyses of genomic and transcriptomic data have enabled a clearer understanding of mole-rat evolutionary history and suggested molecular pathways (e.g., NRF2-signaling activation and DNA damage repair mechanisms) that may explain the extraordinarily longevity and unique health traits of this species. However, careful scrutiny and re-analysis suggest that some identified features result from incorrect or imprecise annotation and assembly of the naked mole-rat genome: in addition, some of these conclusions (e.g., genes involved in cancer resistance and hairlessness) are rejected when the analysis includes additional, more closely related species. We describe how the combination of better study design, improved genomic sequencing techniques, and new bioinformatic and data analytical tools will improve comparative genomics and ultimately bridge the gap between traditional model and nonmodel organisms.

Adaptations to a subterranean environment and longevity revealed by the analysis of mole rat genomes

Subterranean mammals spend their lives in dark, unventilated environments that are rich in carbon dioxide and ammonia and low in oxygen. Many of these animals are also long-lived and exhibit reduced aging-associated diseases, such as neurodegenerative disorders and cancer. We sequenced the genome of the Damaraland mole rat (DMR, Fukomys damarensis) and improved the genome assembly of the naked mole rat (NMR, Heterocephalus glaber). Comparative genome analyses, along with the transcriptomes of related subterranean rodents, revealed candidate molecular adaptations for subterranean life and longevity, including a divergent insulin peptide, expression of oxygen-carrying globins in the brain, prevention of high CO2-induced pain perception, and enhanced ammonia detoxification. Juxtaposition of the genomes of DMR and other more conventional animals with the genome of NMR revealed several truly exceptional NMR features: unusual thermogenesis, an aberrant melatonin system, pain insensitivity, and unique processing of 28S rRNA. Together, these genomes and transcriptomes extend our understanding of subterranean adaptations, stress resistance, and longevity.

Specific paucity of unmyelinated C-fibers in cutaneous peripheral nerves of the African naked-mole rat: comparative analysis using six species of Bathyergidae

In mammalian peripheral nerves, unmyelinated C-fibers usually outnumber myelinated A-fibers. By using transmission electron microscopy, we recently showed that the saphenous nerve of the naked mole-rat (Heterocephalus glaber) has a C-fiber deficit manifested as a substantially lower C:A-fiber ratio compared with other mammals. Here we determined the uniqueness of this C-fiber deficit by performing a quantitative anatomical analysis of several peripheral nerves in five further members of the Bathyergidae mole-rat family: silvery (Heliophobius argenteocinereus), giant (Fukomys mechowii), Damaraland (Fukomys damarensis), Mashona (Fukomys darlingi), and Natal (Cryptomys hottentotus natalensis) mole-rats. In the largely cutaneous saphenous and sural nerves, the naked mole-rat had the lowest C:A-fiber ratio (∼1.5:1 compared with ∼3:1), whereas, in nerves innervating both skin and muscle (common peroneal and tibial) or just muscle (lateral/medial gastrocnemius), this pattern was mostly absent. We asked whether lack of hair follicles alone accounts for the C-fiber paucity by using as a model a mouse that loses virtually all its hair as a consequence of conditional deletion of the β-catenin gene in the skin. These β-catenin loss-of function mice (β-cat LOF mice) displayed only a mild decrease in C:A-fiber ratio compared with wild-type mice (4.42 compared with 3.81). We suggest that the selective cutaneous C-fiber deficit in the cutaneous nerves of naked mole-rats is unlikely to be due primarily to lack of skin hair follicles. Possible mechanisms contributing to this unique peripheral nerve anatomy are discussed.

Blunted behavioral and c Fos responses to acidic fumes in the African naked mole-rat

Acidosis in the skin triggers activation of pain pathways and behaviors indicative of pain in vertebrates. The exception is the naked mole-rat, the only known vertebrate to show physiological and behavioral insensitivity to acid pain in the skin. The goal of the present study was to determine behavioral and physiological responses of this species to airborne acidic fumes, which would be expected to affect the trigeminal pain pathway in other species. Behaviorally, naked mole-rats did not avoid fumes from moderately high concentrations of acetic acid (10 and 20%), and c Fos labeling showed no increase in activity in the trigeminal nuclei and nucleus tractus solitarius. In contrast, these concentrations triggered behavioral aversion and increased Fos activity in other laboratory rodents. For a very high concentration of acetic acid (50%), naked mole-rats showed significant avoidance behavior and increased Fos labeling in the nucleus tractus solitarius caudal region, which receives vagal chemosensory information. However, there was no increase in trigeminal labeling, and in fact, activity significantly decreased. This pattern is opposite of that associated with another irritant, ammonia fumes, which elicited an increase in trigeminal but not nucleus tractus solitarius Fos labeling, and no behavioral avoidance. Behavioral avoidance of acidic fumes, but no increased labeling in the trigeminal pain nucleus is consistent with the notion of adaptations to blunt acid pain, which would be advantageous for naked mole-rats as they normally live under chronically high levels of acidosis-inducing CO₂.

Hypoxia associated NMDA receptor 2 subunit composition: developmental comparison between the hypoxia-tolerant subterranean mole-rat, Spalax, and the hypoxia-sensitive rat

Vertebrate brains are sensitive to oxygen depletion, which may lead to cell death. Hypoxia sensitivity originates from the high intrinsic rate of ATP consumption of brain tissue, accompanied by the release of glutamate, leading to the opening of ionotropic glutamate receptors, such as N-methyl-D-aspartate (NMDA) receptors (NMDARs). The relative expression levels of the four NMDAR-2 (NR2) subunits change during mammalian development with higher levels of units NR2B and NR2D observed during early development and correlated with hypoxic tolerance during embryonic and neonatal stages of development. Higher levels of NR2D are also abundant in brains of hypoxia tolerant species such as the crucian carp. The subterranean mole-rat, Spalax spends its life underground in sealed burrows and has developed a wide range of adaptations to this special niche including hypoxia-tolerance. In this study, we compared the in vivo mRNA expression of NR2 subunits in the brains of embryonic, neonatal and adult Spalax and rat. Our results demonstrate that under normoxic conditions, mRNA levels of NR2D are higher in Spalax than in rat at all developmental stages studied and are similar to levels in neonatal rat and in other hypoxia/anoxia tolerant species. Furthermore, under hypoxia Spalax NR2D mRNA levels increase while no response was observed in rat. Similarly, hypoxia induces an increase in mRNA levels of Spalax NR2A, claimed to promote neuronal survival. We suggest that indeed the proportional combinations of NMDAR-2 subunits contribute to the ability of the Spalax brain to cope with hypoxic environments.

Blunted neuronal calcium response to hypoxia in naked mole-rat hippocampus

Naked mole-rats are highly social and strictly subterranean rodents that live in large communal colonies in sealed and chronically oxygen-depleted burrows. Brain slices from naked mole-rats show extreme tolerance to hypoxia compared to slices from other mammals, as indicated by maintenance of synaptic transmission under more hypoxic conditions and three fold longer latency to anoxic depolarization. A key factor in determining whether or not the cellular response to hypoxia is reversible or leads to cell death may be the elevation of intracellular calcium concentration. In the present study, we used fluorescent imaging techniques to measure relative intracellular calcium changes in CA1 pyramidal cells of hippocampal slices during hypoxia. We found that calcium accumulation during hypoxia was significantly and substantially attenuated in slices from naked mole-rats compared to slices from laboratory mice. This was the case for both neonatal (postnatal day 6) and older (postnatal day 20) age groups. Furthermore, while both species demonstrated more calcium accumulation at older ages, the older naked mole-rats showed a smaller calcium accumulation response than even the younger mice. A blunted intracellular calcium response to hypoxia may contribute to the extreme hypoxia tolerance of naked mole-rat neurons. The results are discussed in terms of a general hypothesis that a very prolonged or arrested developmental process may allow adult naked mole-rat brain to retain the hypoxia tolerance normally only seen in neonatal mammals.

An antinociceptive role for substance P in acid-induced chronic muscle pain

Release of substance P (SP) from nociceptive nerve fibers and activation of its receptor neurokinin 1 (NK1) are important effectors in the transmission of pain signals. Nonetheless, the role of SP in muscle pain remains unknown. Here we show that a single i.m. acid injection in mice lacking SP signaling by deletion of the tachykinin precursor 1 (Tac1) gene or coadministration of NK1 receptor antagonists produces long-lasting hyperalgesia rather than the transient hyperalgesia seen in control animals. The inhibitory effect of SP was found exclusively in neurons expressing acid-sensing ion channel 3, where SP enhances M-channel-like potassium currents through the NK1 receptor in a G protein-independent but tyrosine kinase-dependent manner. Furthermore, the SP signaling could alter action potential thresholds and modulate the expression of TTX-resistant sodium currents in medium-sized muscle nociceptors. Thus, i.m. SP mediates an unconventional NK1 receptor signal pathway to inhibit acid activation in muscle nociceptors, resulting in an unexpected antinociceptive effect against chronic mechanical hyperalgesia, here induced by repeated i.m. acid injection.