ASIC1a-associated mechanical hypersensitivity in the GlaKO Fabry disease mouse model.

Pain in Fabry disease (FD) is generally accepted to result from neuronal damage in the peripheral nervous system as a consequence of excess lipid storage caused by alpha-galactosidase A (α-Gal A) deficiency. Signatures of pain arising from nerve injuries are generally associated with changes of number, location and phenotypes of immune cells within dorsal root ganglia (DRG). However, the neuroimmune processes in the DRG linked to accumulating glycosphingolipids in Fabry disease are insufficiently understood.Therefore, using indirect immune fluorescence microscopy, transmigration assays and FACS together with transcriptomic signatures associated with immune processes, we assessed age-dependent neuroimmune alterations in DRG obtained from mice with a global depletion of α-Gal A as a valid mouse model for FD. Macrophage numbers in the DRG of FD mice were unaltered, and BV-2 cells as a model for monocytic cells did not show augmented migratory reactions to glycosphingolipids exposure suggesting that these do not act as chemoattractants in FD. However, we found pronounced alterations of lysosomal signatures in sensory neurons and of macrophage morphology and phenotypes in FD DRG. Macrophages exhibited reduced morphological complexity indicated by a smaller number of ramifications and more rounded shape, which were age dependent and indicative of premature monocytic aging together with upregulated expression of markers CD68 and CD163.In our FD mouse model, the observed phenotypic changes in myeloid cell populations of the DRG suggest enhanced phagocytic and unaltered proliferative capacity of macrophages as compared to wildtype control mice. We suggest that macrophages may participate in FD pathogenesis and targeting macrophages at an early stage of FD may offer new treatment options other than enzyme replacement therapy.

Acid-sensing Ion Channels in Sensory Perception

Acid-sensing ion channels (ASICs) are cationic channels activated by extracellular acidosis that are expressed in both central and peripheral nervous systems. Although peripheral ASICs seem to be natural sensors of acidic pain (e.g., in inflammation, ischaemia, lesions or tumours), a direct demonstration is still lacking. We show that approximately 60% of rat cutaneous sensory neurons express ASIC3-like currents. Native as well as recombinant ASIC3 respond synergistically to three different inflammatory signals that are slight acidifications (approximately pH 7.0), hypertonicity and arachidonic acid (AA). Moderate pH, alone or in combination with hypertonicity and AA, increases nociceptors excitability and produces pain suppressed by the toxin APETx2, a specific blocker of ASIC3. Both APETx2 and the in vivo knockdown of ASIC3 with a specific siRNA also have potent analgesic effects against primary inflammation-induced hyperalgesia in rat. Peripheral ASIC3 channels are thus essential sensors of acidic pain and integrators of molecular signals produced during inflammation where they contribute to primary hyperalgesia.

Noninvasive and objective biomarkers for disease-associated pathology are critical for clinical trials. For Fabry disease, one important pathological change due to the deficiency of the lysosomal enzyme α-galactosidase A (α-GAL) caused is accumulation of globotriaosylceramide (Gb3) in dorsal root ganglion (DRG) neurons, which manifests as the overall DRG hypertrophy. Magnetic resonance imaging (MRI) has been successfully used to noninvasively measure DRG enlargement in Fabry patients, and DRG volumetric MRI can be a potential noninvasive biomarker for Gb3 accumulations in DRG neurons in clinical trials. To evaluate disease progression and treatment response in preclinical proof-of-concept studies, we developed an in vivo MRI method to measure DRG size in the G3Stg/GLA knockout mouse model of Fabry disease. Compared to the wild type mice, the DRG enlargement in the Fabry mice was observed as early as 8 weeks of age, and a single administration of the human α-GAL-encoding adeno-associated virus (AAVGLA) normalized the enlarged DRG to the age-matched wild type mice. The DRG normalization was observed within 4 weeks of gene therapy (12 weeks of age) and was sustained up to 24 weeks of age. Furthermore, behavioral testing and histological/immunohistochemistry analyses of the DRG tissues corroborated the MRI findings. Volumetric DRG MRI has the sensitivity to measure Gb3 pathology-induced DRG volume changes and treatment response in live mice and can be a translational imaging biomarker in clinical trials for Fabry disease.

Anxiety-like behavior and altered hippocampal activity in a transgenic mouse model of Fabry disease.

Fabry disease (FD) is caused by α-galactosidase A (AGAL) enzyme deficiency, leading to globotriaosylceramide accumulation (Gb3) in several cell types. Pain is one of the pathophysiologically incompletely understood symptoms in FD patients. Previous data suggest an involvement of hypoxia and mitochondriopathy in FD pain development at dorsal root ganglion (DRG) level. Using immunofluorescence and quantitative real-time polymerase chain reaction (qRT PCR), we investigated patient-derived endothelial cells (EC) and DRG tissue of the GLA knockout (KO) mouse model of FD. We address the question of whether hypoxia and mitochondriopathy contribute to FD pain pathophysiology. In EC of FD patients (P1 with pain and, P2 without pain), we found dysregulated protein expression of hypoxia-inducible factors (HIF) 1a and HIF2 compared to the control EC (p < 0.01). The protein expression of the HIF downstream target vascular endothelial growth factor A (VEGFA, p < 0.01) was reduced and tube formation was hampered in the P1 EC compared to the healthy EC (p < 0.05). Tube formation ability was rescued by applying transforming growth factor beta (TGFβ) inhibitor SB-431542. Additionally, we found dysregulated mitochondrial fusion/fission characteristics in the P1 and P2 EC (p < 0.01) and depolarized mitochondrial membrane potential in P2 compared to control EC (p < 0.05). Complementary to human data, we found upregulated hypoxia-associated genes in the DRG of old GLA KO mice compared to WT DRG (p < 0.01). At protein level, nuclear HIF1a was higher in the DRG neurons of old GLA KO mice compared to WT mice (p < 0.01). Further, the HIF1a downstream target CA9 was upregulated in the DRG of old GLA KO mice compared to WT DRG (p < 0.01). Similar to human EC, we found a reduction in the vascular characteristics in GLA KO DRG compared to WT (p < 0.05). We demonstrate increased hypoxia, impaired vascular properties, and mitochondrial dysfunction in human FD EC and complementarily at the GLA KO mouse DRG level. Our data support the hypothesis that hypoxia and mitochondriopathy in FD EC and GLA KO DRG may contribute to FD pain development.

D-13 Thematic Poster - RPE, Pain and Fatigue Thursday, May 28, 2020, 1:30 PM - 3:30 PM Room: CC-2011 INTRODUCTION: Exercise training is an effective therapy for many pain-related conditions, and there is a difference in pain perception between athletes and unconditioned people. The mechanisms by which exercise modulates pain are poorly understood. Painful conditions can be associated with elevated levels of protons, metabolites and inflammatory factors, which can activate receptors and/or ion channels on nociceptive sensory neurons including acid sensing ion channels (ASICs) and transient receptor potential cation channel subfamily V member 1 (TRPV1). Additionally, strenuous exercise also causes the release of similar chemical signals, and ASICs within muscle afferents may mediate immediate exercise-induced pain (IEP) and fatigue, as well as reflex hemodynamic changes. We hypothesized that ASICs and TRPV1 have role in IEP and maximal exercise performance. METHOD: First, C57BL/6 mice were divided into sedentary (SED), low-intensity continuous training (LICT) and high-intensity interval training (HIIT) groups. Mice were trained on a treadmill every other day for 4 weeks. SED mice were placed on a non-moving treadmill for similar periods of time. After 4 weeks, exercise performance, ASICs and TRPV1 mRNA levels within lumbar dorsal root ganglion (DRG) were measured. In a separate group, we measured IEP at baseline and following exhaustive exercise before and after HIIT. In a third study, we compared the IEP and exercise performance in ASIC3-/- versus wild type (WT) mice. RESULTS: We found HIIT improved exercise performance compared to LICT and sedentary groups, diminished ASICs and TRPV1 mRNA levels in lumber DRG, and reduced IEP. We also found a negative relationship between mRNA levels of ASICs and TRPV1 and exercise performance (r = - 0.59, p < 0.001). In addition, ASIC3-/- showed a significant lower IEP compared to WT mice, while there was no difference in maximal exercise performance between groups. CONCLUSION: In summary, ASIC3 is required for IEP following exhaustive exercise, and exercise training downregulates ASICs and TRPV1 in muscle afferents and diminishes IEP. These findings suggest a possible role of ASICs in benefits of exercise training for many pain and fatigue conditions such as fibromyalgia and chronic fatigue syndrome conditions. Supported by Department of Veteran Affairs.

Activation of Extracellular Signal-Regulated Protein Kinase in Sensory Neurons After Noxious Gastric Distention and Its Involvement in Acute Visceral Pain in Rats

Objective: Skeletal muscle (SM) pain and fatigue are common in Fabry disease (FD), and strongly impact the patient's quality of life. Still, the SM in FD is poorly investigated. Although energetic alterations are reported in cells from FD patients, they have never been related to fatigue and pain. Given the pivotal relevance of energetics for SM health, we undertook the investigation of the SM. Design and method: We consistently observed a reduced tolerance to aerobic activity and lactate accumulation in FD-humanized mouse model and patients. Accordingly, in sedentary mouse FD SM we detected an increase in fast/glycolytic-fibers, mirrored by an upregulation of glycolytic enzymes and glucose-transporters Results: In fibroblasts derived from FD patients, we confirmed a high glycolytic-rate and accordingly, metabolomic/lipidomic-analysis revealed that lipids are underutilized as energetic fuel in FD-patients. In the quest for a tentative mechanism, we explored analogies with genetic myopathies, that show altered expression of energetic metabolism. Specifically, we explored HIF-1 upregulation and found it increased in FD mice and patients. Consistent with our previous screening of miRNAs associated with metabolic stress in FD, we observed a significant upregulation of miR-17 in FD patients and mice. Of note, miR-17 has been associated with cellular metabolic remodeling and HIF-1 up-regulation in different contexts. In our experimental setting, a specific antagomir targeting miR-17 was able to inhibit HIF-1 accumulation and revert metabolic-remodeling in FD-cells. Conclusions: Our findings unveil an anaerobic glycolytic-switch under normoxia, known as Warburg Effect, which is induced by miR-17-mediated-upregulation of HIF-1. The miR-17/HIF-1 pathway can be a new therapeutic target in FD, and Exercise-testing and blood lactate may become a new diagnostic and monitoring-tool in FD.

Mammalian dorsal root ganglia (DRG) contain a diverse collection of sensory neuronal subtypes specialized to detect different sensory stimuli. Most of these cells respond to mechanical stimulation of some sort, be they proprioceptors detecting body movements, receptors for gentle touch or nociceptors activated by painful levels of pressure, and yet the proteins that detect mechanical stimuli remain unknown. In this issue of The EMBO Journal, Gary Lewin and colleagues give insights into the developmental acquisition of mechanosensitivity by different classes of sensory neurons and in doing so, they offer a new approach for determining the molecular basis of this sensory modality.

Acid-sensing ion channels (ASICs) are key receptors for extracellular protons. These neuronal nonvoltage-gated Na(+) channels are involved in learning, the expression of fear, neurodegeneration after ischemia, and pain sensation. We have applied a systematic approach to identify potential pH sensors in ASIC1a and to elucidate the mechanisms by which pH variations govern ASIC gating. We first calculated the pK(a) value of all extracellular His, Glu, and Asp residues using a Poisson-Boltzmann continuum approach, based on the ASIC three-dimensional structure, to identify candidate pH-sensing residues. The role of these residues was then assessed by site-directed mutagenesis and chemical modification, combined with functional analysis. The localization of putative pH-sensing residues suggests that pH changes control ASIC gating by protonation/deprotonation of many residues per subunit in different channel domains. Analysis of the function of residues in the palm domain close to the central vertical axis of the channel allowed for prediction of conformational changes of this region during gating. Our study provides a basis for the intrinsic ASIC pH dependence and describes an approach that can also be applied to the investigation of the mechanisms of the pH dependence of other proteins.

Nociceptors arising from the dorsal root ganglia (DRG) express acid-sensing ion channel-1 (ASIC1) subtypes to mediate the perception of inflammatory and neuropathic pain, and as such, these receptors are attractive targets for the development of analgesics for these painful conditions. Nevertheless, given that the human and rodent DRG differ considerably in subtype proportions of ASIC1 and that the pharmacological properties of rodent ASIC1 subtypes and their human homologues are distinct, ASIC1 inhibitors that demonstrate analgesic properties in rodents may not necessarily be effective in preventing pain in humans. In this study, we show that human embryonic kidney (HEK) 293 cells, which are routinely used as a cellular vehicle for the heterologous expression and pharmacological characterization of receptors and ion channels, natively transcribe the human homologues of ASIC1a and ASIC1b at similar proportions to those found in the human DRG. Importantly, HEK 293 ASIC1 is sensitive to inhibition by amiloride, ethylisopropyl amiloride, and the snake toxin mambalgin-1, but insensitive to inhibition by the ASIC1a inhibitor psalmotoxin-1 when applied at a physiological conditioning pH. Given that the human DRG transcribes the same set of ASIC1 subtypes as HEK 293 cells, our data support the notion that mambalgin-1 may be effective against acid pain sensation in humans. Moreover, our data suggest that the HEK 293 cell line may be a suitable tool for pharmacological screening and characterization of heteromeric human ASIC1.

Upregulation of ASIC1a channels in an in vitro model of Fabry disease

The human lysosomal enzymes alpha-galactosidase (alpha-GAL, EC 3.2.1.22) and alpha-N-acetylgalactosaminidase (alpha-NAGAL, EC 3.2.1.49) share 46% amino acid sequence identity and have similar folds. The active sites of the two enzymes share 11 of 13 amino acids, differing only where they interact with the 2-position of the substrates. Using a rational protein engineering approach, we interconverted the enzymatic specificity of alpha- GAL and alpha-NAGAL. The engineered alpha-GAL (which we call alpha-GAL(SA)) retains the antigenicity of alpha-GAL but has acquired the enzymatic specificity of alpha-NAGAL. Conversely, the engineered alpha-NAGAL (which we call alpha-NAGAL(EL)) retains the antigenicity of alpha-NAGAL but has acquired the enzymatic specificity of the alpha-GAL enzyme. Comparison of the crystal structures of the designed enzyme alpha-GAL(SA) to the wild-type enzymes shows that active sites of alpha-GAL(SA) and alpha-NAGAL superimpose well, indicating success of the rational design. The designed enzymes might be useful as non-immunogenic alternatives in enzyme replacement therapy for treatment of lysosomal storage disorders such as Fabry disease.

Background: Hindpaw injection of formalin in rodents is used to assess acute persistent pain. The response to formalin is biphasic. The initial response (first minutes) is thought to be linked to inflammatory, peripheral mechanisms, while the latter (around 30 min after the injection), is linked to central mechanisms. This model is useful to analyze the effect of drugs at one or both phases, and the involvement of ion channels in the response. Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in pain conditions. Recently, psalmotoxin-1 (Pctx-1), a toxin that inhibits ASIC1a-constituted channels, and antisense ASIC1a-RNA, intrathecal administered in mice were shown to affect both phases of the test. Methods: The mouse formalin test was performed on C57/BL6 7- to 9-week-old mice. Behavioral tests were conducted and tissue was extracted to detect proteins (ASIC1 and pERK) and ASIC1-mRNA and mir485-5p levels. Results: The injection of formalin was accompanied by an increase in ASIC1 levels. This was detected at the contralateral anterior cingulate cortex (ACC) compared to the ipsilateral side, and both sides of the ACC of vehicle-injected animals. At the spinal cord and dorsal root ganglia, ASIC1 levels followed a gradient stronger at lumbar (L) 3 and decreased towards L5. Gender differences were detected at the ACC; with female mice showing higher ASIC1a levels at the ACC. No significant changes in ASIC1-mRNA levels were detected. Evidence suggests ASIC1 upregulation depends on regulatory microRNAs. Conclusion: This work highlights the important role of ASIC1 in pain and the potential role of pharmacological therapies aimed at this channel.