Tracking macrophages in diabetic neuropathy with two-color nanoemulsions for near-infrared fluorescent imaging and microscopy

Sustained pain and macrophage infiltration in a mouse muscle contusion model

INTRODUCTION/AIMS

Prior studies have emphasized the role of inflammation in the response to injury and muscle regeneration, but little emphasis has been placed on characterizing the relationship between innate inflammation, pain, and functional impairment. The aim of our study was to determine the contribution of innate immunity to prolonged pain following muscle contusion.

METHODS

We developed a closed-impact mouse model of muscle contusion and a macrophage-targeted near-infrared fluorescent nanoemulsion. Closed-impact contusions were delivered to the lower left limb. Pain sensitivity, gait dysfunction, and inflammation were assessed in the days and weeks post-contusion. Macrophage accumulation was imaged in vivo by injecting i.v. near-infrared nanoemulsion.

RESULTS

Despite hindpaw hypersensitivity persisting for several weeks, disruptions to gait and grip strength typically resolved within 10 days of injury. Using non-invasive imaging and immunohistochemistry, we show that macrophage density peaks in and around the affected muscle 3 day post-injury and quickly subsides. However, macrophage density in the ipsilateral sciatic nerve and dorsal root ganglia (DRG) increases more gradually and persists for at least 14 days.

DISCUSSION

In this study, we demonstrate pain sensitivity is influenced by the degree of lower muscle contusion, without significant changes to gait and grip strength. This may be due to modulation of pain signaling by macrophage proliferation in the sciatic nerve, upstream from the site of injury. Our work suggests chronic pain developing from muscle contusion is driven by macrophage-derived neuroinflammation in the peripheral nervous system.

In vitro Quality Assessments of Perfluorocarbon Nanoemulsions for Near-infrared Fluorescence Imaging of Inflammation in Preclinical Models

Tracking macrophages by non-invasive molecular imaging can provide useful insights into the immunobiology of inflammatory disorders in preclinical disease models. Perfluorocarbon nanoemulsions (PFC-NEs) have been well documented in their ability to be taken up by macrophages through phagocytosis and serve as 19F magnetic resonance imaging (MRI) tracers of inflammation in vivo and ex vivo. Incorporation of near-infrared fluorescent (NIRF) dyes in PFC-NEs can help monitor the spatiotemporal distribution of macrophages in vivo during inflammatory processes, using NIRF imaging as a complementary methodology to MRI. Here, we discuss in depth how both colloidal and fluorescence stabilities of the PFC-NEs are essential for successful and reliable macrophage tracking in vivo and for their detection in excised tissues ex vivo by NIRF imaging. Furthermore, PFC-NE quality assures NIRF imaging reproducibility and reliability across preclinical studies, providing insights into inflammation progression and therapeutic response. Previous studies focused on assessments of colloidal property changes in response to stress and during storage as a means of quality control. We recently focused on the joint evaluation of both colloidal and fluorescence properties and their relationship to NIRF imaging outcomes. In this protocol, we summarize the key assessments of the fluorescent dye-labeled nanoemulsions, which include long-term particle size distribution monitoring as the measure of colloidal stability and monitoring of the fluorescence signal. Due to its simplicity and reproducibility, our protocols are easy to adopt for researchers to assess the quality of PFC-NEs for in vivo NIRF imaging applications.

Folate-conjugated near-infrared fluorescent perfluorocarbon nanoemulsions as theranostics for activated macrophage COX-2 inhibition

Activated macrophages play a critical role in the orchestration of inflammation and inflammatory pain in several chronic diseases. We present here the first perfluorocarbon nanoemulsion (PFC NE) that is designed to preferentially target activated macrophages and can deliver up to three payloads (two fluorescent dyes and a COX-2 inhibitor). Folate receptors are overexpressed on activated macrophages. Therefore, we introduced a folate-PEG-cholesterol conjugate into the formulation. The incorporation of folate conjugate did not require changes in processing parameters and did not change the droplet size or fluorescent properties of the PFC NE. The uptake of folate-conjugated PFC NE was higher in activated macrophages than in resting macrophages. Flow cytometry showed that the uptake of folate-conjugated PFC NE occurred by both phagocytosis and receptor-mediated endocytosis. Furthermore, folate-conjugated PFC NE inhibited the release of proinflammatory cytokines (TNF-α and IL-6) more effectively than nonmodified PFC NE, while drug loading and COX-2 inhibition were comparable. The PFC NEs reported here were successfully produced on multiple scales, from 25 to 200 mL, and by using two distinct processors (microfluidizers: M110S and LM20). Therefore, folate-conjugated PFC NEs are viable anti-inflammatory theranostic nanosystems for macrophage drug delivery and imaging.

Ropivacaine microsphere-loaded electroconductive nerve dressings for long-acting analgesia and functional recovery following diabetic peripheral nerve injury

In recent years, electroconductive hydrogels (ECHs) have shown great potential in promoting nerve regeneration and motor function recovery following diabetic peripheral nerve injury (PNI), attributed to their similar electrical and mechanical characteristics to innate nervous tissue. It is well-established that PNI causes motor deficits and pain, especially in diabetics. Current evidence suggests that ropivacaine (ROP) encapsulated in poly lactic-co-glycolic acid (PLGA) microspheres (MSs) yield a sustained analgesic effect. In this study, an ECH electroconductive network loaded with MS/ROP (ECH-MS/ROP) was designed as a promising therapeutic approach for diabetic PNI to exert lasting analgesia and functional recovery. This dual delivery system allowed ROP's slow and sequential release, achieving sustained analgesia as demonstrated by our in vivo experiments. Meanwhile, this system was designed like a lamellar dressing, with desirable adhesive and self-curling properties, convenient for treating injured nerve tissues via automatically wrapping tube-like structures, facilitating the process of implantation. Our in vitro assays verified that ECH-MS/ROP was able to enhance the adhesion and motility of Schwann cells. Besides, both in vitro and in vivo studies substantiated that ECH-MS/ROP stimulated myelinated axon regeneration through the MEK/ERK signaling pathway, thereby improving muscular denervation atrophy and facilitating functional recovery. Therefore, this study suggests that the ECH-MS/ROP dressing provides a promising strategy for treating diabetic PNI to facilitate nerve regeneration, functional recovery and pain relief.

Exosomes-loaded electroconductive nerve dressing for nerve regeneration and pain relief against diabetic peripheral nerve injury

Over the years, electroconductive hydrogels (ECHs) have been extensively applied for stimulating nerve regeneration and restoring locomotor function after peripheral nerve injury (PNI) with diabetes, given their favorable mechanical and electrical properties identical to endogenous nerve tissue. Nevertheless, PNI causes the loss of locomotor function and inflammatory pain, especially in diabetic patients. It has been established that bone marrow stem cells-derived exosomes (BMSCs-Exos) have analgesic, anti-inflammatory and tissue regeneration properties. Herein, we designed an ECH loaded with BMSCs-Exos (ECH-Exos) electroconductive nerve dressing to treat diabetic PNI to achieve functional recovery and pain relief. Given its potent adhesive and self-healing properties, this laminar dressing is convenient for the treatment of damaged nerve fibers by automatically wrapping around them to form a size-matched tube-like structure, avoiding the cumbersome implantation process. Our in vitro studies showed that ECH-Exos could facilitate the attachment and migration of Schwann cells. Meanwhile, Exos in this system could modulate M2 macrophage polarization via the NF-κB pathway, thereby attenuating inflammatory pain in diabetic PNI. Additionally, ECH-Exos enhanced myelinated axonal regeneration via the MEK/ERK pathway in vitro and in vivo, consequently ameliorating muscle denervation atrophy and further promoting functional restoration. Our findings suggest that the ECH-Exos system has huge prospects for nerve regeneration, functional restoration and pain relief in patients with diabetic PNI.

Fibroblast-derived PI16 sustains inflammatory pain via regulation of CD206+ myeloid cells

Originally identified in fibroblasts, Protease Inhibitor (PI)16 was recently shown to be crucial for the development of neuropathic pain via effects on blood-nerve barrier permeability and leukocyte infiltration, though its impact on inflammatory pain has not been established. Using the complete Freund's Adjuvant inflammatory pain model, we show that Pi16-/- mice are protected against sustained inflammatory pain. Accordingly, intrathecal delivery of a PI16 neutralizing antibody in wild-type mice prevented sustained CFA pain. In contrast to neuropathic pain models, we did not observe any changes in blood-nerve barrier permeability due to PI16 deletion. Instead, Pi16-/- mice display reduced macrophage density in the CFA-injected hindpaw. Furthermore, there was a significant bias toward CD206hi (anti-inflammatory) macrophages in the hindpaw and associated dorsal root ganglia. Following CFA, intrathecal depletion of CD206+ macrophages using mannosylated clodronate liposomes promoted sustained pain in Pi16-/- mice. Similarly, an IL-10 neutralizing antibody also promoted sustained CFA pain in the Pi16-/ when administered intrathecally. Collectively, our results point to fibroblast-derived PI16 mediating substantial differences in macrophage phenotype in the pain neuroaxis under conditions of inflammation. The co-expression of PI16 alongside fibroblast markers in human DRG raise the likelihood that a similar mechanism operates in human inflammatory pain states. Collectively, our findings may have implications for targeting fibroblast-immune cell crosstalk for the treatment of chronic pain.

RNA-Seq Reveals Sex Differences in Gene Expression during Peripheral Neuropathic Inflammation and in Pain Relief from a COX-2 Inhibiting Theranostic Nanoemulsion

Given decades of neuroinflammatory pain research focused only on males, there is an urgent need to better understand neuroinflammatory pain in females. This, paired with the fact that currently there is no long-term effective treatment for neuropathic pain furthers the need to evaluate how neuropathic pain develops in both sexes and how it can be relieved. Here we show that chronic constriction injury of the sciatic nerve caused comparable levels of mechanical allodynia in both sexes. Using a COX-2 inhibiting theranostic nanoemulsion with increased drug loading, both sexes achieved similar reduction in mechanical hypersensitivity. Given that both sexes have improved pain behavior, we specifically explored differential gene expression between sexes in the dorsal root ganglia (DRG) during pain and relief. Total RNA from the DRG revealed a sexually dimorphic expression for injury and relief caused by COX-2 inhibition. Of note, both males and females experience increased expression of activating transcription factor 3 (Atf3), however, only the female DRG shows decreased expression following drug treatment. Alternatively, S100A8 and S100A9 expression appear to play a sex specific role in relief in males. The sex differences in RNA expression reveal that comparable behavior does not necessitate the same gene expression.

Shortwave Infrared Fluorofluorophores for Multicolor In Vivo Imaging

Developing chemical tools to detect and influence biological processes is a cornerstone of chemical biology. Here we combine two tools which rely on orthogonality- perfluorocarbons and multiplexed shortwave infrared (SWIR) fluorescence imaging- to visualize nanoemulsions in real time in living mice. Drawing inspiration from fluorous and SWIR fluorophore development, we prepared two SWIR-emissive, fluorous-soluble chromenylium polymethine dyes. These are the most red-shifted fluorous fluorophores- "fluorofluorophores"-to date. After characterizing the dyes, their utility was demonstrated by tracking perfluorocarbon nanoemulsion biodistribution in vivo. Using an excitation-multiplexed approach to image two variables simultaneously, we gained insight into the importance of size and surfactant identity on biodistribution.

Sensory neuron dysfunction in orthotopic mouse models of colon cancer

Reports of neurological sequelae related to colon cancer are largely restricted to rare instances of paraneoplastic syndromes, due to autoimmune reactions. Systemic inflammation associated with tumor development influences sensory neuron function in other disease models, though the extent to which this occurs in colorectal cancer is unknown. We induced orthotopic colorectal cancer via orthotopic injection of two colorectal cancer cell lines (MC38 and CT26) in two different mouse strains (C57BL/6 and Balb/c, respectively). Behavioral tests of pain sensitivity and activity did not detect significant alterations in sensory sensitivity or diminished well-being throughout tumor development. However, immunohistochemistry revealed widespread reductions in intraepidermal nerve fiber density in the skin of tumor-bearing mice. Though loss of nerve fiber density was not associated with increased expression of cell injury markers in dorsal root ganglia, lumbar dorsal root ganglia neurons of tumor-bearing animals showed deficits in mitochondrial function. These neurons also had reduced cytosolic calcium levels in live-cell imaging and reduced spontaneous activity in multi-electrode array analysis. Bulk RNA sequencing of DRGs from tumor-bearing mice detected activation of gene expression pathways associated with elevated cytokine and chemokine signaling, including CXCL10. This is consistent with the detection of CXCL10 (and numerous other cytokines, chemokines and growth factors) in MC38 and CT26 cell-conditioned media, and the serum of tumor-bearing mice. Our study demonstrates in a pre-clinical setting that colon cancer is associated with latent sensory neuron dysfunction and implicates cytokine/chemokine signaling in this process. These findings may have implications for determining risk factors and treatment responsiveness related to neuropathy in colorectal cancer.