From the low-density lipoprotein receptor-related protein 1 to neuropathic pain: a potentially novel target

Targeted Polymersomes Enable Enhanced Delivery to Peripheral Nerves Post-Injury

The gold standard therapy for peripheral nerve injuries involves surgical repair, which is invasive and leads to major variations in therapeutic outcomes. Because of this, smaller injuries often go untreated. However, alternative, noninvasive routes of administration are currently unviable due to the presence of the blood-nerve barrier (BNB), which prevents passage of small molecules from the blood into the endoneurium and the nerve. This paper demonstrates that ligands on the surface of nanoparticles, called polymersomes, can enable delivery to the nerve through noninvasive intramuscular injections. Polymersomes made from polyethylene glycol (PEG)-b-polylactic acid (PLA) were conjugated with either apolipoprotein E (ApoE) or rabies virus glycoprotein-based peptide RVG29 (RVG) and loaded with near-infrared dye, AlexaFluor647. ApoE was used to target receptors upregulated in post-injury inflammation, while RVG targets neural-specific receptors. Untagged, ApoE-tagged, and RVG-tagged polymersomes were injected at 100 mM either intranerve (IN) or intramuscular (IM) into Sprague-Dawley rats post sciatic nerve injury. The addition of the ApoE and RVG tags enabled increased AlexaFluor647 fluorescence in the injury site at 1 h post IN injection compared to the untagged polymersome control. However, only the RVG-tagged polymersomes increased the AlexaFluor647 fluorescence after IM injection. Ex vivo analysis of sciatic nerves demonstrated that ApoE-tagged polymersomes enabled the greatest retention of AlexaFluor647 regardless of the injection route. This led us to conclude that using ApoE to target inflammation enabled the greatest retention of polymersome-delivered payloads while using RVG to target neural cells more specifically enabled the penetration of polymersome-delivered payloads. Observations were confirmed by calculating the area under the curve pharmacokinetic parameters and the use of a two-compartment pharmacokinetic model.

Myelin debris phagocytosis in demyelinating disease

Demyelinating diseases are often caused by a variety of triggers, including immune responses, viral infections, malnutrition, hypoxia, or genetic factors, all of which result in the loss of myelin in the nervous system. The accumulation of myelin debris at the lesion site leads to neuroinflammation and inhibits remyelination; therefore, it is crucial to promptly remove the myelin debris. Initially, Fc and complement receptors on cellular surfaces were the primary clearance receptors responsible for removing myelin debris. However, subsequent studies have unveiled the involvement of additional receptors, including Mac-2, TAM receptors, and the low-density lipoprotein receptor-related protein 1, in facilitating the removal process. In addition to microglia and macrophages, which serve as the primary effector cells in the disease phase, a variety of other cell types such as astrocytes, Schwann cells, and vascular endothelial cells have been demonstrated to engage in the phagocytosis of myelin debris. Furthermore, we have concluded that oligodendrocyte precursor cells, as myelination precursor cells, also exhibit this phagocytic capability. Moreover, our research group has innovatively identified the low-density lipoprotein receptor as a potential phagocytic receptor for myelin debris. In this article, we discuss the functional processes of various phagocytes in demyelinating diseases. We also highlight the alterations in signaling pathways triggered by phagocytosis, and provide a comprehensive overview of the various phagocytic receptors involved. Such insights are invaluable for pinpointing potential therapeutic strategies for the treatment of demyelinating diseases by targeting phagocytosis.

Role of secretomes in cell-free therapeutic strategies in regenerative medicine

After an injury, peripheral nervous system neurons have the potential to rebuild their axons by generating a complicated activation response. Signals from the damaged axon are required for this genetic transition to occur. Schwann cells (SCs) near a damaged nerve's distal stump also play a role in the local modulation of axonal programs, not only via cell-to-cell contacts but also through secreted signals (the secretome). The secretome is made up of all the proteins that the cell produces, such as cytokines, growth factors, and extracellular vesicles. The released vesicles may carry signaling proteins as well as coding and regulatory RNAs, allowing for multilayer communication. The secretome of SCs is now well understood as being critical for both orchestrating Wallerian degeneration and maintaining axonal regeneration. As a consequence, secretome has emerged as a feasible tissue regeneration alternative to cell therapy. Separate SC secretome components have been used extensively in the lab to promote peripheral nerve regeneration after injury. However, in neurological therapies, the secretome generated by mesenchymal (MSC) or other derived stem cells has been the most often used. In fact, the advantages of cell treatment have been connected to the release of bioactive chemicals and extracellular vesicles, which make up MSCs' secretome.

Axon-derived PACSIN1 binds to the Schwann cell survival receptor, LRP1, and transactivates TrkC to promote gliatrophic activities

Schwann cells (SCs) undergo phenotypic transformation and then orchestrate nerve repair following PNS injury. The ligands and receptors that activate and sustain SC transformation remain incompletely understood. Proteins released by injured axons represent important candidates for activating the SC Repair Program. The low-density lipoprotein receptor-related protein-1 (LRP1) is acutely up-regulated in SCs in response to injury, activating c-Jun, and promoting SC survival. To identify novel LRP1 ligands released in PNS injury, we applied a discovery-based approach in which extracellular proteins in the injured nerve were captured using Fc-fusion proteins containing the ligand-binding motifs of LRP1 (CCR2 and CCR4). An intracellular neuron-specific protein, Protein Kinase C and Casein Kinase Substrate in Neurons (PACSIN1) was identified and validated as an LRP1 ligand. Recombinant PACSIN1 activated c-Jun and ERK1/2 in cultured SCs. Silencing Lrp1 or inhibiting the LRP1 cell-signaling co-receptor, the NMDA-R, blocked the effects of PACSIN1 on c-Jun and ERK1/2 phosphorylation. Intraneural injection of PACSIN1 into crush-injured sciatic nerves activated c-Jun in wild-type mice, but not in mice in which Lrp1 is conditionally deleted in SCs. Transcriptome profiling of SCs revealed that PACSIN1 mediates gene expression events consistent with transformation to the repair phenotype. PACSIN1 promoted SC migration and viability following the TNFα challenge. When Src family kinases were pharmacologically inhibited or the receptor tyrosine kinase, TrkC, was genetically silenced or pharmacologically inhibited, PACSIN1 failed to induce cell signaling and prevent SC death. Collectively, these studies demonstrate that PACSIN1 is a novel axon-derived LRP1 ligand that activates SC repair signaling by transactivating TrkC.

Systems genetics analysis reveals the common genetic basis for pain sensitivity and cognitive function

BACKGROUND

There is growing evidence of a strong correlation between pain sensitivity and cognitive function under both physiological and pathological conditions. However, the detailed mechanisms remain largely unknown. In the current study, we sought to explore candidate genes and common molecular mechanisms underlying pain sensitivity and cognitive function with a transcriptome-wide association study using recombinant inbred mice from the BXD family.

METHODS

The pain sensitivity determined by Hargreaves' paw withdrawal test and cognition-related phenotypes were systematically analyzed in 60 strains of BXD mice and correlated with hippocampus transcriptomes, followed by quantitative trait locus (QTL) mapping and systems genetics analysis.

RESULTS

The pain sensitivity showed significant variability across the BXD strains and co-varies with cognitive traits. Pain sensitivity correlated hippocampual genes showed a significant involvement in cognition-related pathways, including glutamatergic synapse, and PI3K-Akt signaling pathway. Moreover, QTL mapping identified a genomic region on chromosome 4, potentially regulating the variation of pain sensitivity. Integrative analysis of expression QTL mapping, correlation analysis, and Bayesian network modeling identified Ring finger protein 20 (Rnf20) as the best candidate. Further pathway analysis indicated that Rnf20 may regulate the expression of pain sensitivity and cognitive function through the PI3K-Akt signaling pathway, particularly through interactions with genes Ppp2r2b, Ppp2r5c, Col9a3, Met, Rps6, Tnc, and Kras.

CONCLUSIONS

Our study demonstrated that pain sensitivity is associated with genetic background and Rnf20-mediated PI3K-Akt signaling may involve in the regulation of pain sensitivity and cognitive functions.

Suppression of tumorigenesis in LUAD by LRP1B through regulation of the IL-6-JAK-STAT3 pathway

Lung adenocarcinoma (LUAD) is the most common type of lung cancer. LRP1B was initially identified as a cancer suppressor in several cancers. However, the potential biological phenotypes and molecular mechanisms of LRP1B in LUAD have not been fully investigated. In our study, we showed that the expression of LRP1B in LUAD tissues was lower than that in normal tissues. Knockdown of LRP1B markedly enhanced malignancy of LUAD cells. Genomic analysis indicated that the population expressing low-levels of LRP1B had higher genomic instability, which accounted for a larger proportion of aneuploidy and inflammation subtyping. Enrichment analysis of bulk and cell-line transcriptomic data both showed that the low expression of LRP1B could induce the activation of IL-6-JAK-STAT3, chemokine, cytokine, and other inflammation signaling pathways. Moreover, our findings revealed that knockdown LRP1B enhanced the secretion of IL-6 and IL-8, as confirmed by ELISA assays. Further validation using PCR and WB confirmed that downregulation of LRP1B mRNA significantly upregulated the activity of the IL-6-JAK-STAT3 pathway. Collectively, this study highlights LRP1B as a tumor suppressor gene and reveals that LRP1B knockdown promotes malignant progression in LUAD by inducing inflammation through the IL-6-JAK-STAT3 pathway.

High affinity of β-amyloid proteins to cerebral capillaries: implications in chronic lead exposure-induced neurotoxicity in rats

Lead (Pb) is a known environmental risk factor in the etiology of Alzheimer's disease (AD). The existing reports suggest that Pb exposure increases beta-amyloid (Aβ) levels in brain tissues and cerebrospinal fluid (CSF) and facilitates the formation of amyloid plaques, which is a pathological hallmark for AD. Pb exposure has long been associated with cerebral vasculature injury. Yet it remained unclear if Pb exposure caused excessive Ab buildup in cerebral vasculature, which may damage the blood-brain barrier and cause abnormal Ab accumulation. This study was designed to investigate the impact of chronic Pb exposure on Aβ accumulation in cerebral capillary and the expression of low-density lipoprotein receptor protein-1 (LRP1), a critical Aβ transporter, in brain capillary and parenchyma. Sprague-Dawley rats received daily oral gavage at doses of 0, 14 (low-dose), and 27 (high-dose) mg Pb/kg as Pb acetate, 5 d/wk, for 4 or 8 wks. At the end of Pb exposure, a solution containing Aβ40 was infused into the brain via the cannulated internal carotid artery. Data by ELISA showed a strikingly high affinity of Ab to cerebral vasculature, which was approximately 7-14 times higher than that to the parenchymal fractions collected from control brains. Pb exposure further aggravated the Aβ accumulation in cerebral vasculature in a dose-dependent manner. Western blot analyses revealed that Pb exposure decreased LRP1 expression in cortical capillaries and hippocampal parenchyma. Immunohistochemistry (IHC) studies further revealed a disrupted distribution of LRP1 alongside hippocampal vasculature accompanied with a decreased expression in hippocampal neurons by Pb exposure. Taken together, the current study demonstrated that the cerebral vasculature naturally possessed a high affinity to Aβ present in circulating blood. Pb exposure significantly increased Aβ accumulation in cerebral vasculature; such an increased Aβ accumulation was due partly to the diminished expression of LRP1 in response to Pb in tested brain regions. Perceivably, Pb-facilitated Ab aggravation in cerebral vasculature may contribute to Pb-associated amyloid alterations.

Local Inflammatory Mediators Involved in Neuropathic Pain

Polyneuropathy (PNP) is a term to describe diseases of the peripheral nervous system, 50% of which present with neuropathic pain. In some types of PNP, pain is restricted to the skin distally in the leg, suggesting a local regulatory process leading to pain. In this study, we proposed a pro-inflammatory pathway mediated by NF-κB that might be involved in the development of pain in patients with painful PNP. To test this hypothesis, we have collected nerve and skin samples from patients with different etiologies and levels of pain. We performed RT-qPCR to analyze the gene expression of the proposed inflammatory pathway components in sural nerve and in distal and proximal skin samples. In sural nerve, we showed a correlation of TLR4 and TNFα to neuropathic pain, and an upregulation of TNFα in patients with severe pain. Patients with an inflammatory PNP also presented a lower expression of TRPV1 and SIRT1. In distal skin, we found a reduced expression of TLR4 and miR-146-5p, in comparison to proximal skin. Our findings thus support our hypothesis of local inflammatory processes involved in pain in PNP, and further show disturbed anti-inflammatory pathways involving TRPV1 and SIRT1 in inflammatory PNP.

Roles of constitutively secreted extracellular chaperones in neuronal cell repair and regeneration

Protein quality control involves many processes that jointly act to regulate the expression, localization, turnover, and degradation of proteins, and has been highlighted in recent studies as critical to the differentiation of stem cells during regeneration. The roles of constitutively secreted extracellular chaperones in neuronal injury and disease are poorly understood. Extracellular chaperones are multifunctional proteins expressed by many cell types, including those of the nervous system, known to facilitate protein quality control processes. These molecules exert pleiotropic effects and have been implicated as playing important protective roles in a variety of stress conditions, including tissue damage, infections, and local tissue inflammation. This article aims to provide a critical review of what is currently known about the functions of extracellular chaperones in neuronal repair and regeneration and highlight future directions for this important research area. We review what is known of four constitutively secreted extracellular chaperones directly implicated in processes of neuronal damage and repair, including transthyretin, clusterin, α2-macroglobulin, and neuroserpin, and propose that investigation into the effects of these and other extracellular chaperones on neuronal repair and regeneration has the potential to yield valuable new therapies.

How the Competition for Cysteine May Promote Infection of SARS-CoV-2 by Triggering Oxidative Stress

SARS-CoV-2 induces a broad range of clinical manifestations. Besides the main receptor, ACE2, other putative receptors and co-receptors have been described and could become genuinely relevant to explain the different tropism manifested by new variants. In this study, we propose a biochemical model envisaging the competition for cysteine as a key mechanism promoting the infection and the selection of host receptors. The SARS-CoV-2 infection produces ROS and triggers a massive biosynthesis of proteins rich in cysteine; if this amino acid becomes limiting, glutathione levels are depleted and cannot control oxidative stress. Hence, infection succeeds. A receptor should be recognized as a marker of suitable intracellular conditions, namely the full availability of amino acids except for low cysteine. First, we carried out a comparative investigation of SARS-CoV-2 proteins and human ACE2. Then, using hierarchical cluster protein analysis, we searched for similarities between all human proteins and spike produced by the latest variant, Omicron BA.1. We found 32 human proteins very close to spike in terms of amino acid content. Most of these potential SARS-CoV-2 receptors have less cysteine than spike. We suggest that these proteins could signal an intracellular shortage of cysteine, predicting a burst of oxidative stress when used as viral entry mediators.

LRP1-Mediated Endocytosis May Be the Main Reason for the Difference in Cytotoxicity of Curcin and Curcin C on U2OS Osteosarcoma Cells

Curcin and Curcin C, both of the ribosome-inactivating proteins of Jatropha curcas, have apparent inhibitory effects on the proliferation of osteosarcoma cell line U20S. However, the inhibitory effect of the latter is 13-fold higher than that of Curcin. The mechanism responsible for the difference has not been studied. This work aimed to understand and verify whether there are differences in entry efficiency and pathway between them using specific endocytosis inhibitors, gene silencing, and labeling techniques such as fluorescein isothiocyanate (FITC) labeling. The study found that the internalization efficiency of Curcin C was twice that of Curcin for U2OS cells. More than one entering pathway was adopted by both of them. Curcin C can enter U2OS cells through clathrin-dependent endocytosis and macropinocytosis, but clathrin-dependent endocytosis was not an option for Curcin. The low-density lipoprotein receptor-related protein 1 (LRP1) was found to mediate clathrin-dependent endocytosis of Curcin C. After LRP1 silencing, there was no significant difference in the 50% inhibitory concentration (IC₅₀) and endocytosis efficiency between Curcin and Curcin C on U2OS cells. These results indicate that LRP1-mediated endocytosis is specific to Curcin C, thus leading to higher U2OS endocytosis efficiency and cytotoxicity than Curcin.

Regenerative medicine for neuropathic pain: physiology, ultrasound and therapies with a focus on alpha-2-macroglobulin

The currently available drugs to treat neuropathic pain do not provide adequate pain management. As such, other treatments including stem cells, platelet-rich plasma and plasma-derived molecules such as alpha-2 macroglobulin (A2M) are being explored because they show promising potential for neuropathic pain. The various mechanisms and immunomodulatory effects could be a desirable approach in targeting neuropathic pain. This review indicates that A2M can be highly efficacious due to its conformational change during activation and specificity of action on various cytokines. Its ability to reduce neuropathic pain can further the future of neuropathic intervention. However, there is a lack of robust clinical studies and thus further research is needed to verify and expand the understanding of its therapeutic effects.

Proteomic Investigation in Plasma from Women with Fibromyalgia in Response to a 15-wk Resistance Exercise Intervention

PURPOSE

Fibromyalgia (FM) is a complex pain condition, and exercise is considered the first option of treatment. Few studies have examined the effect of exercise on molecular mechanisms in FM. The aim of this study was to analyze the plasma proteome in women with FM and healthy controls (CON) before and after 15 wk of resistance exercise. This study further investigated whether clinical and exercises-related outcomes correlated with identified plasma proteins in FM.

METHODS

Plasma samples from 40 FM/25 CON (baseline) and 21 FM/24 CON (postexercise) were analyzed using shotgun proteomics. Clinical/background data were retrieved through questionnaires. Exercise-related variables and pressure pain thresholds were assessed using standardized instruments. Multivariate statistics were applied to analyze the proteomic profile at baseline and postexercise, and correlation with clinical/exercise-related data.

RESULTS

Fifteen weeks of resistance exercises improved clinical symptoms and muscle strength, and affected circulating proteins related to immunity, stress, mRNA stability, metabolic processes, and muscle structure development in FM. Pressure pain threshold was related to a specific protein profile, with proteins involved in metabolic and immune response. Subgroups of FM based on plasma proteins, FM duration, and improved muscle strength were identified.

CONCLUSIONS

Exercise seems to affect circulating proteins, clinical characteristics, and muscle strength in FM. This study contributes to better understanding of systemic protein changes in FM compared with CON and how resistance exercise affects such changes.

Development of a Model for Predicting the Effectiveness of Pulsed Radiofrequency on Zoster-Associated Pain

Introduction

Zoster-associated pain (ZAP), which may cause anxiety, depression, and sleep disorders and reduce quality of life, is often refractory to current standard treatments. Studies have shown that pulsed radiofrequency (PRF) can alleviate ZAP and reduce the incidence of postherpetic neuralgia (PHN). This study aimed to explore the clinical characteristics associated with PRF responsiveness, develop a model for identifying risk factors of inadequate PRF management, and help clinicians make better decisions.

Methods

Patients who underwent PRF for ZAP between January 2017 and October 2020 in our hospital were included in this study. Patients were evaluated using the numerical rating scale (NRS), Insomnia Severity Index, Patient Health Questionnaire-9, and 36-Item Short Form Health Survey (SF-36) before and 3 months after the procedure. Patient demographic data and blood test results were also collected. We defined the effectiveness of PRF for ZAP as relief of > 50% in NRS scores compared to pre-PRF. Least absolute shrinkage and selection operator (LASSO) regression analyses were subsequently performed to identify factors related to the therapeutic effect of PRF in patients with ZAP. The performance of the prediction model was assessed by the area under the receiver operating characteristic curve (AUC).

Results

The effectiveness of PRF in patients with ZAP was 69.6% (total 313 patients) after 3 months. LASSO regression analysis extracted the seven most powerful features in the developed prediction model: sex, stage of herpes zoster (HZ), pregabalin dose, bodily pain indicators of SF-36, lymphocyte count, and low-density lipoprotein cholesterol (LDLC) and complement C4 in peripheral blood. Model = 1.586 + 0.148 × lymphocyte + (−0.001) × bodily pain indicators of SF-36 + (−0.001) × pregabalin dose + 0.028 × LDLC + 0.001 × C4 + (−0.508) × sex + (−0.128) × stage of HZ. We generated the ROC curve for the prediction model, and the final AUC was 0.701. The sensitivity, specificity, and overall accuracy of the model were 90%, 33%, and 73%, respectively.

Conclusions

Seven factors were significantly associated with poor PRF outcome: male sex, advanced stage of HZ, higher pregabalin dose, higher bodily pain indicators of SF-36, and lower lymphocyte count, LDLC, and complement C4 in the peripheral blood. PRF should be applied to patients with ZAP as early as possible to achieve satisfactory outcomes.

α1-Antitrypsin derived SP16 peptide demonstrates efficacy in rodent models of acute and neuropathic pain

SP16 is an innovative peptide derived from the carboxyl-terminus of α1-Antitrypsin (AAT), corresponding to residues 364-380, and contains recognition sequences for the low-density lipoprotein receptor-related protein-1 (LRP1). LRP1 is an endocytic and cell-signaling receptor that regulates inflammation. Deletion of Lrp1 in Schwann cells increases neuropathic pain; however, the role of LRP1 activation in nociceptive and neuropathic pain regulation remains unknown. Herein, we show that SP16 is bioactive in sensory neurons in vitro. Neurite length and regenerative gene expression were increased by SP16. In PC12 cells, SP16 activated Akt and ERK1/2 cell-signaling in an LRP1-dependent manner. When formalin was injected into mouse hind paws, to model inflammatory pain, SP16 dose-dependently attenuated nociceptive pain behaviors in the early and late phases. In a second model of acute pain using capsaicin, SP16 significantly reduced paw licking in both male and female mice (p < .01) similarly to enzymatically inactive tissue plasminogen activator, a known LRP1 interactor. SP16 also prevented development of tactile allodynia after partial nerve ligation and this response was sustained for nine days (p < .01). Immunoblot analysis of the injured nerve revealed decreased CD11b (p < .01) and Toll-like receptor-4 (p < .005). In injured dorsal root ganglia SP16 reduced CD11b⁺ cells (p < .05) and GFAP (p < .005), indicating that inflammatory cell recruitment and satellite cell activation were inhibited. In conclusion, administration of SP16 blocked pain-related responses in three distinct pain models, suggesting efficacy against acute nociceptive, inflammatory, and neuropathic pain. SP16 also attenuated innate immunity in the PNS. These studies identify SP16 as a potentially effective treatment for pain.

Neutralization of Hv1/HVCN1 With Antibody Enhances Microglia/Macrophages Myelin Clearance by Promoting Their Migration in the Brain

Microglia dynamically monitor the microenvironment of the central nervous system (CNS) by constantly extending and retracting their processes in physiological conditions, and microglia/macrophages rapidly migrate into lesion sites in response to injuries or diseases in the CNS. Consequently, their migration ability is fundamentally important for their proper functioning. However, the mechanisms underlying their migration have not been fully understood. We wonder whether the voltage-gated proton channel HVCN1 in microglia/macrophages in the brain plays a role in their migration. We show in this study that in physiological conditions, microglia and bone marrow derived macrophage (BMDM) express HVCN1 with the highest level among glial cells, and upregulation of HVCN1 in microglia/macrophages is presented in multiple injuries and diseases of the CNS, reflecting the overactivation of HVCN1. In parallel, myelin debris accumulation occurs in both the focal lesion and the site where neurodegeneration takes place. Importantly, both genetic deletion of the HVCN1 gene in cells in vitro and neutralization of HVCN1 with antibody in the brain in vivo promotes migration of microglia/macrophages. Furthermore, neutralization of HVCN1 with antibody in the brain in vivo promotes myelin debris clearance by microglia/macrophages. This study uncovers a new role of HVCN1 in microglia/macrophages, coupling the proton channel HVCN1 to the migration of microglia/macrophages for the first time.