Metabolic reprogramming of the inflammatory response in the nervous system: the crossover between inflammation and metabolism

Metabolism is a fundamental process by which biochemicals are broken down to produce energy (catabolism) or used to build macromolecules (anabolism). Metabolism has received renewed attention as a mechanism that generates molecules that modulate multiple cellular responses. This was first identified in cancer cells as the Warburg effect, but it is also present in immunocompetent cells. Studies have revealed a bidirectional influence of cellular metabolism and immune cell function, highlighting the significance of metabolic reprogramming in immune cell activation and effector functions. Metabolic processes such as glycolysis, oxidative phosphorylation, and fatty acid oxidation have been shown to undergo dynamic changes during immune cell response, facilitating the energetic and biosynthetic demands. This review aims to provide a better understanding of the metabolic reprogramming that occurs in different immune cells upon activation, with a special focus on central nervous system disorders. Understanding the metabolic changes of the immune response not only provides insights into the fundamental mechanisms that regulate immune cell function but also opens new approaches for therapeutic strategies aimed at manipulating the immune system.

IL-38 regulates intestinal stem cell homeostasis by inducing WNT signaling and beneficial IL-1β secretion

The IL-1 Family member IL-38 has been characterized primarily as an antiinflammatory cytokine in human and mouse models of systemic diseases. Here, we examined the role of IL-38 in the murine small intestine (SI). Immunostaining of SI revealed that IL-38 expression partially confines to intestinal stem cells. Cultures of intestinal organoids reveal IL-38 functions as a growth factor by increasing organoid size via inducing WNT3a. In contrast, organoids from IL-38-deficient mice develop more slowly. This reduction in size is likely due to the downregulation of intestinal stemness markers (i.e., Fzd5, Ephb2, and Olfm4) expression compared with wild-type organoids. The IL-38 binding to IL-1R6 and IL-1R9 is still a matter of debate. Therefore, to analyze the molecular mechanisms of IL-38 signaling, we also examined organoids from IL-1R9-deficient mice. Unexpectedly, these organoids, although significantly smaller than wild type, respond to IL-38, suggesting that IL-1R9 is not involved in IL-38 signaling in the stem cell crypt. Nevertheless, silencing of IL-1R6 disabled the organoid response to the growth property of IL-38, thus suggesting IL-1R6 as the main receptor used by IL-38 in the crypt compartment. In organoids from wild-type mice, IL-38 stimulation induced low concentrations of IL-1β which contribute to organoid growth. However, high concentrations of IL-1β have detrimental effects on the cultures that were prevented by treatment with recombinant IL-38. Overall, our data demonstrate an important regulatory function of IL-38 as a growth factor, and as an antiinflammatory molecule in the SI, maintaining homeostasis.

Pancreatic Ductal Adenocarcinoma Cells Regulate NLRP3 Activation to Generate a Tolerogenic Microenvironment

Defining feature of pancreatic ductal adenocarcinoma (PDAC) that participates in the high mortality rate and drug resistance is the immune-tolerant microenvironment which enables tumors to progress unabated by adaptive immunity. In this study, we report that PDAC cells release CSF-1 to induce nucleotide-binding domain, leucine-rich containing family, pyrin domain-containing-3 (NLRP3) activation in myeloid cells. Increased NLRP3 expression was found in the pancreas of patients with PDAC when compared with normal pancreas which correlated with the formation of the NLRP3 inflammasome. Using human primary cells and an orthotopic PDAC mouse model, we show that NLRP3 activation is responsible for the maturation and release of the inflammatory cytokine IL1β which selectively drives Th2-type inflammation via COX2/PGE2 induction. As a result of this inflammation, primary tumors were characterized by reduced cytotoxic CD8+ T-cell activation and increased tumor expansion. Genetic deletion and pharmacologic inhibition of NLRP3 enabled the development of Th1 immunity, increased intratumoral levels of IL2, CD8+ T cell–mediated tumor suppression, and ultimately limited tumor growth. In addition, we observed that NLRP3 inhibition in combination with gemcitabine significantly increased the efficacy of the chemotherapy. In conclusion, this study provides a mechanism by which tumor-mediated NLRP3 activation exploits a distinct adaptive immunity response that facilitates tumor escape and progression. Considering the ability to block NLRP3 activity with safe and small orally active molecules, this protein represents a new promising target to improve the limited therapeutic options in PDAC.

SIGNIFICANT

This study provides novel molecular insights on how PDAC cells exploit NLRP3 activation to suppress CD8 T-cell activation. From a translational perspective, we demonstrate that the combination of gemcitabine with the orally active NLRP3 inhibitor OLT1177 increases the efficacy of monotherapy.

Pharmacologic inhibition of NLRP3 reduces the levels of α-synuclein and protects dopaminergic neurons in a model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is characterized by a progressive degeneration of dopaminergic neurons, which leads to irreversible loss of peripheral motor functions. Death of dopaminergic neurons induces an inflammatory response in microglial cells, which further exacerbates neuronal loss. Reducing inflammation is expected to ameliorate neuronal loss and arrest motor dysfunctions. Because of the contribution of the NLRP3 inflammasome to the inflammatory response in PD, we targeted NLRP3 using the specific inhibitor OLT1177^®.

METHODS

We evaluated the effectiveness of OLT1177^® in reducing the inflammatory response in an MPTP neurotoxic model of PD. Using a combination of in vitro and in vivo studies, we analyzed the effects of NLRP3 inhibition on pro-inflammatory markers in the brain, α-synuclein aggregation, and dopaminergic neuron survival. We also determined the effects of OLT1177^® on locomotor deficits associated with MPTP and brain penetrance.

RESULTS

Treatment with OLT1177^® prevented the loss of motor function, reduced the levels of α-synuclein, modulated pro-inflammatory markers in the nigrostriatal areas of the brain, and protected dopaminergic neurons from degeneration in the MPTP model of PD. We also demonstrated that OLT1177^® crosses the blood-brain barrier and reaches therapeutic concentrations in the brain.

CONCLUSIONS

These data suggest that targeting the NLRP3 inflammasome by OLT1177^® may be a safe and novel therapeutic approach to arrest neuroinflammation and protect against neurological deficits of Parkinson's disease in humans.

Interleukin-4 and interleukin-13 induce different metabolic profiles in microglia and macrophages that relate with divergent outcomes after spinal cord injury

Background: Microglia and macrophages adopt a pro-inflammatory phenotype after spinal cord injury (SCI), what is thought to contribute to secondary tissue degeneration. We previously reported that this is due, in part, to the low levels of anti-inflammatory cytokines, such as IL-4. Since IL-13 and IL-4 share receptors and both cytokines drive microglia and macrophages towards an anti-inflammatory phenotype in vitro, here we studied whether administration of IL-13 and IL-4 after SCI leads to beneficial effects. Methods: We injected mice with recombinant IL-13 or IL-4 at 48 h after SCI and assessed their effects on microglia and macrophage phenotype and functional outcomes. We also performed RNA sequencing analysis of macrophages and microglia sorted from the injured spinal cords of mice treated with IL-13 or IL-4 and evaluated the metabolic state of these cells by using Seahorse technology. Results: We observed that IL-13 induced the expression of anti-inflammatory markers in microglia and macrophages after SCI but, in contrast to IL-4, it failed to mediate functional recovery. We found that these two cytokines induced different gene signatures in microglia and macrophages after SCI and that IL-4, in contrast to IL-13, shifted microglia and macrophage metabolism from glycolytic to oxidative phosphorylation. These findings were further confirmed by measuring the metabolic profile of these cells. Importantly, we also revealed that macrophages stimulated with IL-4 or IL-13 are not deleterious to neurons, but they become cytotoxic when oxidative metabolism is blocked. This suggests that the metabolic shift, from glycolysis to oxidative phosphorylation, is required to minimize the cytotoxic responses of microglia and macrophages. Conclusions: These results reveal that the metabolic fitness of microglia and macrophages after SCI contributes to secondary damage and that strategies aimed at boosting oxidative phosphorylation might be a novel approach to minimize the deleterious actions of microglia and macrophages in neurotrauma.

Inhibition of the NLRP3 inflammasome by OLT1177 induces functional protection and myelin preservation after spinal cord injury

Spinal cord injury (SCI) leads to irreversible functional deficits due to the disruption of axons and the death of neurons and glial cells. The inflammatory response that occurs in the injured spinal cord results in tissue degeneration; thus, targeting inflammation after acute SCI is expected to ameliorate histopathological evidence indicative of damage and, consequently, reduce functional disabilities. Interleukin 1 beta (IL-1β) and interleukin 18 (IL-18) are pro-inflammatory cytokines members of the IL-1 family that initiate and propagate inflammation. Here, we report that protein levels of IL-1β and IL-18 were increased in spinal cord parenchyma after SCI, but with different expression profiles. Whereas levels of IL-1β were rapidly increased reaching peak levels at 12 h after the injury, levels of IL-18 did not increase until 7 days after the injury. Since activation of the NLRP3 inflammasome is required for the processing and release of IL-1β and IL-18, we intraperitoneally administered OLT1177, a selective inhibitor of the NLRP3 inflammasome, to reduce the contribution of these cytokines to SCI. At a dose of 200 mg/kg, OLT1177 protected against neurological deficits and histological evidence of damage. OLT1177 also reduced the levels of IL-1β in the spinal cord after contusion injury and diminished the accumulation of neutrophils and macrophages at later time points. These data suggest that targeting the NLRP3 inflammasome with OLT1177 could be a novel therapeutic strategy to arrest neuroinflammation and reduce functional impairments after acute SCI in humans.