Macrophage-Derived Extracellular Succinate Licenses Neural Stem Cells to Suppress Chronic Neuroinflammation

摘要

class="head no_bottom_margin" id="sec1title">IntroductionAdvances in stem cell biology have raised hopes that diseases of the CNS may be ameliorated by non-hematopoietic stem cell medicines (). We have provided compelling evidence that the transplantation of somatic neural stem cells (NSCs) improves the clinico-pathological features of animal models of inflammatory CNS disorders. Beyond the structural replacement of injured CNS cells, our work has shown that transplanted NSCs engage in complex stem cell graft-to-host communication programs, overall leading to trophic support and modulation of adaptive and innate immune responses (, , , , ). Specifically, NSC transplants reduce the burden of inflammation at site of injury (, ), decrease the number of type 1 inflammatory mononuclear phagocytes (MPs) (), and promote the healing of the injured CNS via yet poorly characterized mechanisms.However, the clinical translation of experimental NSC therapies is still limited by the sources from which human NSCs (hNSCs) are derived (), the intrinsic immunogenicity of allogeneic hNSC lines (, ), and the stability of the so-called “intended clinical cell lot” (, ). Autologous and stably expandable directly induced NSCs (iNSCs) from patients’ dermal fibroblasts are emerging as a valid alternative to NSC therapies (, href="#bib28" rid="bib28" class=" bibr popnode">Meyer et al., 2015, href="#bib56" rid="bib56" class=" bibr popnode">Thier et al., 2012). The direct reprogramming into iNSCs avoids the laborious progression through a pluripotent state and subsequent differentiation into desired lineages described for induced pluripotent stem cell (iPSC) technology (href="#bib28" rid="bib28" class=" bibr popnode">Meyer et al., 2015, href="#bib56" rid="bib56" class=" bibr popnode">Thier et al., 2012). Therefore, making stably expandable iNSCs from somatic cells represents the most feasible way of obtaining autologous brain stem cells for downstream clinical applications (href="#bib62" rid="bib62" class=" bibr popnode">Wörsdörfer et al., 2013). However, the efficacy of directly reprogrammed iNSCs in treating inflammatory CNS disorders has not yet been tested.In progressive forms of multiple sclerosis (MS), chronic CNS inflammation is sustained by widespread activation of MPs that include both CNS resident microglia and monocyte-derived infiltrating macrophages (href="#bib22" rid="bib22" class=" bibr popnode">Mallucci et al., 2015). MPs are found in gray matter lesions, close to degenerating neurites and neuronal cell bodies (href="#bib37" rid="bib37" class=" bibr popnode">Peterson et al., 2001), and in white matter lesions, where the external rim of activated microglia is associated with chronic tissue damage (href="#bib6" rid="bib6" class=" bibr popnode">Bramow et al., 2010, href="#bib45" rid="bib45" class=" bibr popnode">Prineas et al., 2001). Areas of normal-appearing white matter are also characterized by MP accumulation, which leads to the formation of microglial nodules that drive disease pathology irrespective of concomitant T cell activation (href="#bib31" rid="bib31" class=" bibr popnode">Moll et al., 2011). The detrimental role of chronic MP-driven inflammation in progressive MS is also supported by evidence in animal disease models, where its overall burden correlates with impaired neuronal function (href="#bib38" rid="bib38" class=" bibr popnode">Planche et al., 2017), brain atrophy (href="#bib53" rid="bib53" class=" bibr popnode">Tambalo et al., 2015), and reduced regenerative responses (href="#bib15" rid="bib15" class=" bibr popnode">Jiang et al., 2014).Activation of MPs by pro-inflammatory stimuli causes a metabolic switch toward glycolysis and reduced oxidative phosphorylation (OXPHOS) (href="#bib16" rid="bib16" class=" bibr popnode">Kelly and O’Neill, 2015). Recent evidence suggests that, within this metabolic rewiring, type 1 inflammatory MPs accumulate succinate, with important pathophysiological implications (href="#bib54" rid="bib54" class=" bibr popnode">Tannahill et al., 2013). Intracellular succinate inhibits the activity of prolyl hydroxylases enzymes (PHDs), thereby stabilizing hypoxia responsive element (HIF)-1α and inducing the transcription of interleukin (IL)-1β (href="#bib54" rid="bib54" class=" bibr popnode">Tannahill et al., 2013). Furthermore, oxidation of succinate by succinate dehydrogenase (SDH) repurposes mitochondria from ATP synthesis to reactive oxygen species (ROS) production as additional pro-inflammatory signal (href="#bib30" rid="bib30" class=" bibr popnode">Mills et al., 2016). Type 1 inflammatory MPs also release succinate extracellularly and upregulate its cognate succinate receptor 1 (SUCNR1), a G-protein-coupled receptor (also known as GPR91), which functions as autocrine and paracrine sensor to enhance IL-1β production (href="#bib19" rid="bib19" class=" bibr popnode">Littlewood-Evans et al., 2016).As such, metabolism is emerging as an important therapeutic target to modulate the activation of both macrophages (href="#bib16" rid="bib16" class=" bibr popnode">Kelly and O’Neill, 2015) and microglia (href="#bib34" rid="bib34" class=" bibr popnode">Orihuela et al., 2016), and succinate-related pathways have key immune modulatory functions for acute and chronic inflammatory diseases (href="#bib50" rid="bib50" class=" bibr popnode">Ryu et al., 2003, href="#bib55" rid="bib55" class=" bibr popnode">Tannahill et al., 2015).Given the established immune modulatory properties of NSCs (href="#bib39" rid="bib39" class=" bibr popnode">Pluchino and Cossetti, 2013), we hypothesized that NSCs may exert their therapeutic effects in chronic neuroinflammation by modulating MP metabolism toward reduction of secondary CNS damage.In this work, we investigated the molecular mechanisms that underpin the capacity of somatic and directly induced NSCs to counteract the metabolic changes of type 1 inflammatory MPs both in vivo and in vitro. We show that transplanted iNSCs and NSCs are functionally equivalent in ameliorating chronic neuroinflammation in mice with experimental autoimmune encephalomyelitis (EAE). Transplanted iNSCs/NSCs switch in the activation profile of CNS-resident microglia and monocyte-derived infiltrating macrophages toward an anti-inflammatory phenotype, as well as reduce the levels of the immunometabolite succinate in the cerebrospinal fluid (CSF). iNSCs/NSCs also decrease extracellular succinate released by type 1 inflammatory MPs to reprogram their metabolism toward OXPHOS in vitro. Mechanistically, we show that succinate secreted by type 1 MPs elicits in iNSCs/NSCs a signaling cascade downstream SUCNR1, which enables their anti-inflammatory activity. This succinate-licensed anti-inflammatory function of iNSCs/NSCs is mediated by the secretion of prostaglandin (PG) E2, as well as by considerable scavenging of extracellular succinate. Loss of Sucnr1 function in NSCs leads to significantly reduced anti-inflammatory activities in vitro and in vivo after transplantation in EAE.Our study uncovers a succinate-SUCNR1 axis that clarifies how NSCs respond to inflammatory metabolic signals to inhibit the activation of type 1 MPs in chronic neuroinflammation.