Peripheral pain is enhanced by insulin-like growth factor 1 and its receptors in a mouse model of type 2 diabetes mellitus

Follistatin drives neuropathic pain in mice through IGF1R signaling in nociceptive neurons

Neuropathic pain is a debilitating chronic condition that lacks effective treatment. The role of cytokine- and chemokine-mediated neuroinflammation in its pathogenesis has been well documented. Follistatin (FST) is a secreted protein known to antagonize the biological activity of cytokines in the transforming growth factor-β (TGF-β) superfamily. The involvement of FST in neuropathic pain and the underlying mechanism remain largely unknown. Here, we report that FST was up-regulated in A-fiber sensory neurons after spinal nerve ligation (SNL) in mice. Inhibition or deletion of FST alleviated neuropathic pain and reduced the nociceptive neuron hyperexcitability induced by SNL. Conversely, intrathecal or intraplantar injection of recombinant FST, or overexpression of FST in the dorsal root ganglion (DRG) neurons, induced pain hypersensitivity. Furthermore, exogenous FST increased neuronal excitability in nociceptive neurons. The biolayer interferometry (BLI) assay and coimmunoprecipitation (co-IP) demonstrated direct binding of FST to the insulin-like growth factor-1 receptor (IGF1R), and IGF1R inhibition reduced FST-induced activation of extracellular signal-regulated kinase (ERK) and protein kinase B (AKT), as well as neuronal hyperexcitability. Further co-IP analysis revealed that the N-terminal domain of FST exhibits the highest affinity for IGF1R, and blocking this interaction with a peptide derived from FST attenuated Nav1.7-mediated neuronal hyperexcitability and neuropathic pain after SNL. In addition, FST enhanced neuronal excitability in human DRG neurons through IGF1R. Collectively, our findings suggest that FST, released from A-fiber neurons, enhances Nav1.7-mediated hyperexcitability of nociceptive neurons by binding to IGF1R, making it a potential target for neuropathic pain treatment.

Opposite effects of acute and chronic IGF1 on rat dorsal root ganglion neuron excitability

Insulin-like growth factor-1 (IGF-1) is a polypeptide hormone with a ubiquitous distribution in numerous tissues and with various functions in both neuronal and non-neuronal cells. IGF-1 provides trophic support for many neurons of both the central and peripheral nervous systems. In the central nervous system (CNS), IGF-1R signaling regulates brain development, increases neuronal firing and modulates synaptic transmission. IGF-1 and IGF-IR are not only expressed in CNS neurons but also in sensory dorsal root ganglion (DRG) nociceptive neurons that convey pain signals. DRG nociceptive neurons express a variety of receptors and ion channels that are essential players of neuronal excitability, notably the ligand-gated cation channel TRPV1 and the voltage-gated M-type K+ channel, which, respectively, triggers and dampens sensory neuron excitability. Although many lines of evidence suggest that IGF-IR signaling contributes to pain sensitivity, its possible modulation of TRPV1 and M-type K+ channel remains largely unexplored. In this study, we examined the impact of IGF-1R signaling on DRG neuron excitability and its modulation of TRPV1 and M-type K+ channel activities in cultured rat DRG neurons. Acute application of IGF-1 to DRG neurons triggered hyper-excitability by inducing spontaneous firing or by increasing the frequency of spikes evoked by depolarizing current injection. These effects were prevented by the IGF-1R antagonist NVP-AEW541 and by the PI3Kinase blocker wortmannin. Surprisingly, acute exposure to IGF-1 profoundly inhibited both the TRPV1 current and the spike burst evoked by capsaicin. The Src kinase inhibitor PP2 potently depressed the capsaicin-evoked spike burst but did not alter the IGF-1 inhibition of the hyperexcitability triggered by capsaicin. Chronic IGF-1 treatment (24 h) reduced the spike firing evoked by depolarizing current injection and upregulated the M-current density. In contrast, chronic IGF-1 markedly increased the spike burst evoked by capsaicin. In all, our data suggest that IGF-1 exerts complex effects on DRG neuron excitability as revealed by its dual and opposite actions upon acute and chronic exposures.

Mechanism of NO2-induced migraine in rats: The exploration of the role of miR-653-3p/IGF1 axis

Migraine is a severely disabling primary neurological disorder. Although some studies have confirmed that nitrogen dioxide (NO2) pollution increases the risk of migraine, and our previous study demonstrated the role of the channel protein transient receptor potential cation channel subfamily V member 1 (TRPV1) in NO2-induced migraine, the underlying mechanisms have not been fully elucidated. This study aimed to explore the intrinsic toxicity mechanism of NO2-induced migraines using transcriptome sequencing. First, the differentially expressed genes in NO2-induced migraine, insulin-like growth factor 1 (IGF1) and miRNA miR-653-3p were identified using RNA and small RNA sequencing, and a protein interaction network was constructed using STRING to explore the possible mechanisms. Next, the targeting relationship between miR-653-3p and IGF1 was determined. NO2-induced migraine was verified by silencing miR-653-3p and IGF1, independently or in combination to regulate the protein kinase B (AKT)/TRPV1 signalling pathway through the miR-653-3p/IGF1 axis. These results indicate that the key molecular mechanism of NO2-induced migraine may be that the miR-653-3p/IGF1 axis regulates the AKT/TRPV1 signalling pathway to induce migraine. The findings of this study will further elucidate the neurotoxic mechanism of NO2-induced migraines and lay a new experimental foundation for implementing migraine-related preventive and therapeutic control measures.

Macrophages and microglia in inflammation and neuroinflammation underlying different pain states

Pain is a main symptom in inflammation, and inflammation induces pain via inflammatory mediators acting on nociceptive neurons. Macrophages and microglia are distinct cell types, representing immune cells and glial cells, respectively, but they share similar roles in pain regulation. Macrophages are key regulators of inflammation and pain. Macrophage polarization plays different roles in inducing and resolving pain. Notably, macrophage polarization and phagocytosis can be induced by specialized pro-resolution mediators (SPMs). SPMs also potently inhibit inflammatory and neuropathic pain via immunomodulation and neuromodulation. In this review, we discuss macrophage signaling involved in pain induction and resolution, as well as in maintaining physiological pain. Microglia are macrophage-like cells in the central nervous system (CNS) and drive neuroinflammation and pathological pain in various inflammatory and neurological disorders. Microglia-produced inflammatory cytokines can potently regulate excitatory and inhibitory synaptic transmission as neuromodulators. We also highlight sex differences in macrophage and microglial signaling in inflammatory and neuropathic pain. Thus, targeting macrophage and microglial signaling in distinct locations via pharmacological approaches, including immunotherapies, and non-pharmacological approaches will help to control chronic inflammation and chronic pain.

Analysis of matrisome expression patterns in murine and human dorsal root ganglia

The extracellular matrix (ECM) is a dynamic structure of molecules that can be divided into six different categories and are collectively called the matrisome. The ECM plays pivotal roles in physiological processes in many tissues, including the nervous system. Intriguingly, alterations in ECM molecules/pathways are associated with painful human conditions and murine pain models. Nevertheless, mechanistic insight into the interplay of normal or defective ECM and pain is largely lacking. The goal of this study was to integrate bulk, single-cell, and spatial RNA sequencing (RNAseq) datasets to investigate the expression and cellular origin of matrisome genes in male and female murine and human dorsal root ganglia (DRG). Bulk RNAseq showed that about 65% of all matrisome genes were expressed in both murine and human DRG, with proportionally more core matrisome genes (glycoproteins, collagens, and proteoglycans) expressed compared to matrisome-associated genes (ECM-affiliated genes, ECM regulators, and secreted factors). Single cell RNAseq on male murine DRG revealed the cellular origin of matrisome expression. Core matrisome genes, especially collagens, were expressed by fibroblasts whereas matrisome-associated genes were primarily expressed by neurons. Cell-cell communication network analysis with CellChat software predicted an important role for collagen signaling pathways in connecting vascular cell types and nociceptors in murine tissue, which we confirmed by analysis of spatial transcriptomic data from human DRG. RNAscope in situ hybridization and immunohistochemistry demonstrated expression of collagens in fibroblasts surrounding nociceptors in male and female human DRG. Finally, comparing human neuropathic pain samples with non-pain samples also showed differential expression of matrisome genes produced by both fibroblasts and by nociceptors. This study supports the idea that the DRG matrisome may contribute to neuronal signaling in both mouse and human, and that dysregulation of matrisome genes is associated with neuropathic pain.

Schwann Cell Insulin-like Growth Factor Receptor Type-1 Mediates Metastatic Bone Cancer Pain in Mice

Insulin growth factor-1 (IGF-1), an osteoclast-dependent osteolysis biomarker, contributes to metastatic bone cancer pain (MBCP), but the underlying mechanism is poorly understood. In mice, the femur metastasis caused by intramammary inoculation of breast cancer cells resulted in IGF-1 increase in femur and sciatic nerve, and IGF-1-dependent stimulus/non-stimulus-evoked pain-like behaviors. Adeno-associated virus-based shRNA selective silencing of IGF-1 receptor (IGF-1R) in Schwann cells, but not in dorsal root ganglion (DRG) neurons, attenuated pain-like behaviors. Intraplantar IGF-1 evoked acute nociception and mechanical/cold allodynia, which were reduced by selective IGF-1R silencing in DRG neurons and Schwann cells, respectively. Schwann cell IGF-1R signaling promoted an endothelial nitric oxide synthase-mediated transient receptor potential ankyrin 1 (TRPA1) activation and release of reactive oxygen species that, via macrophage-colony stimulating factor-dependent endoneurial macrophage expansion, sustained pain-like behaviors. Osteoclast derived IGF-1 initiates a Schwann cell-dependent neuroinflammatory response that sustains a proalgesic pathway that provides new options for MBCP treatment.

Potential Role of Insulin-Like Growth Factors in Myofascial Pain Syndrome: A Narrative Review

ABSTRACT

Insulin-like growth factors have diverse functions in skeletal muscles by acting through multiple signaling pathways, including growth regulation and differentiation, anti-inflammation, and antioxidation. Insulin-like growth factors have anti-inflammatory effects and also play roles in nociceptive pathways, determining pain sensitivity, in addition to their protective role against ischemic injury in both the nervous system and skeletal muscle. In skeletal muscle, insulin-like growth factors maintain homeostasis, playing key roles in maintenance, accelerating muscle regeneration, and repair processes. As part of their maintenance role, increased levels of insulin-like growth factors may be required for the repair mechanisms after exercise. Although the role of insulin-like growth factors in myofascial pain syndrome is not completely understood, there is evidence from a recent study that insulin-like growth factor 2 levels in patients with myofascial pain syndrome are lower than those of healthy individuals and are associated with increased levels of inflammatory biomarkers. Importantly, higher insulin-like growth factor 2 levels are associated with increased pain severity in myofascial pain syndrome patients. This may suggest that too low or high insulin-like growth factor levels may contribute to musculoskeletal disorder process, whereas a midrange levels may optimize healing without contributing to pain hypersensitivity. Future studies are required to address the mechanisms of insulin-like growth factor 2 in myofascial pain syndrome and the optimal level as a therapeutic agent.

Molecular mechanisms of skin wound healing in non-diabetic and diabetic mice in excision and pressure experimental wounds

Experimental models for chronic skin lesions are excision and pressure ulcer, defined as “open” and “closed” lesions, respectively, only the latter characterized by tissue hypoxia. Moreover, systemic diseases, such as diabetes mellitus, affect wound repair. Thus, models for testing new therapies should be carefully selected according to the expected targets. In this study, we present an extensive and comparative histological, immunohistochemical, and molecular characterization of these two lesions in diabetic (db/db) and non-diabetic (C57BL/6 J) mice. In db/db mice, we found significant reduction in PGP9.5-IR innervation, reduction of capillary network, and reduced expression of NGF receptors. We found an increase in VEGF receptor Kdr expression, and the PI3K-Akt signaling pathway at the core of the altered molecular network. Db/db mice with pressure ulcers showed an impairment in the molecular regulation of hypoxia-related genes (Hif1a, Flt1, and Kdr), while extracellular matrix encoding genes (Itgb3, Timp1, Fn1, Col4a1) were upregulated by hyperglycemia and lesions. Overall, the molecular analysis suggests that db/db mice have a longer inflammatory phase of the wound repair process, delaying the progression toward the proliferation and remodeling phases.

Foxo1 selectively regulates static mechanical pain by interacting with Nav1.7

ABSTRACT

Mechanical allodynia is a debilitating condition for millions of patients with chronic pain. Mechanical allodynia can manifest in distinct forms, including brush-evoked dynamic and filament-evoked static allodynia. In the nervous system, the forkhead protein Foxo1 plays a critical role in neuronal structures and functions. However, the role of Foxo1 in the somatosensory signal remains unclear. Here, we found that Foxo1 selectively regulated static mechanical pain. Foxo1 knockdown decreased sensitivity to static mechanical stimuli in normal rats and attenuated static mechanical allodynia in rat models for neuropathic, inflammatory, and chemotherapy pain. Conversely, Foxo1 overexpression selectively enhanced sensitivity to static mechanical stimuli and provoked static mechanical allodynia. Furthermore, Foxo1 interacted with voltage-gated sodium Nav1.7 channels and increased the Nav1.7 current density by accelerating activation rather than by changing the expression of Nav1.7 in dorsal root ganglia neurons. In addition, the serum level of Foxo1 was found to be increased in chronic pain patients and to be positively correlated with the severity of chronic pain. Altogether, our findings suggest that serum Foxo1 level could be used as a biological marker for prediction and diagnosis of chronic pain. Moreover, selective blockade of Foxo1/Nav1.7 interaction may offer a new therapeutic approach in patients with mechanical pain.

Effects of Topical Application of CHF6467, a Mutated Form of Human Nerve Growth Factor, on Skin Wound Healing in Diabetic Mice

Nerve growth factor (NGF) is the protein responsible for the development and maintenance of sensory skin innervation. Given the role of appropriate innervation in skin healing, NGF has been indicated as a possible prohealing treatment in pathologic conditions characterized by nerve-ending loss, such as chronic ulcers in diabetes; however, its use as a therapeutic agent is limited by its hyperalgesic effect. We tested the effect of topical application of the nonalgogenic NGF derivative hNGFP61S/R100E in two models of skin ulcer induced in dbdb diabetic mice, investigating healing time, skin histology, reinnervation, and angiogenesis using morphologic and molecular approaches. We showed that the topical administration of CHF6467, a recombinant human NGF in which an amino acid substitution (R100E) abolished the hyperalgesic effect usually associated with NGF, accelerated skin repair in experimental wounds (full-excision and pressure-ulcer) induced in diabetic mice (dbdb). CHF6467-induced acceleration of wound healing was accompanied by increased re-epithelization, reinnervation, and revascularization as assessed by histology, immunohistochemistry, and image analysis. Bioinformatic analysis of differentially expressed genes and signaling pathways in the wound tissues showed that protein kinase B-mammalian target of rapamycin was the most regulated pathway. In spite of the transdermal absorption leading to measurable, dose-dependent increases in CHF6467 plasma levels, no systemic thermal or local mechanical hyperalgesia was observed in treated mice. When tested in vitro in human cell lines, CHF6467 stimulated keratinocyte and fibroblast proliferation and tube formation by endothelial cells. Collectively, these results support a possible use of CHF6467 as a prohealing agent in skin lesions in diabetes. SIGNIFICANCE STATEMENT: Topical application of CHF6467 accelerates reinnervation, neoangiogenesis, and wound healing in diabetic mice in both full-thickness skin-excision and pressure-ulcer models through the protein kinase B/mammalian target of rapamycin pathway and does not induce hyperalgesia.

The Role of Insulin-Like Growth Factors and Insulin-Like Growth Factor-Binding Proteins in the Nervous System

The insulin-like growth factors (IGF-I and IGF-II) and their receptors are widely expressed in nervous tissue from early embryonic life. They also cross the blood brain barriers by active transport, and their regulation as endocrine factors therefore differs from other tissues. In brain, IGFs have paracrine and autocrine actions that are modulated by IGF-binding proteins and interact with other growth factor signalling pathways. The IGF system has roles in nervous system development and maintenance. There is substantial evidence for a specific role for this system in some neurodegenerative diseases, and neuroprotective actions make this system an attractive target for new therapeutic approaches. In developing new therapies, interaction with IGF-binding proteins and other growth factor signalling pathways should be considered. This evidence is reviewed, gaps in knowledge are highlighted, and recommendations are made for future research.