Myeloid cell neuropilin 1 ameliorates high-fat diet-induced insulin resistance via suppression of Nlrp3 inflammasome

Abstract

The history of neuropilin 1 (Nrp1) research is checkered with many unexpected and exciting findings. Nrp1 functions as a co-receptor for class 3 semaphorins, and several canonical growth factors, including vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF). It has been implicated in the development of central nervous system, angiogenesis, and migration. Accumulating evidence demonstrates that Nrp1 is also highly expressed in immune cells, including macrophages and dendritic cells. Until now, the functions of Nrp1 within these cells remained poorly studied and elusive. Here, we provide exciting insights on a novel role for myeloid cell Nrp1 in the mitigation of dietary insulin resistance through inhibiting Nlrp3 inflammasome.

Globally, obesity has become a critical public health problem, and is closely associated with insulin resistance ^[1]. In a classic sequence of cellular events, macrophages infiltrate insulin sensitive tissues, instigate inflammation, and lead to insulin resistance ^[2]. Therefore, tissue macrophages represent a major contributor of metabolic diseases, and macrophage regulation may indeed present opportunities at protecting against these conditions.

Although Nrp1 was originally identified in neuronal and endothelial cells, it was later discovered to be highly expressed in macrophages ^[3]. Subsequently, it was demonstrated that Sema3A/Nrp1 signaling was critical to the entry of macrophages into hypoxic tumor areas, where recruited tumor-associated macrophages promoted angiogenesis, impaired anti-tumor immunity and resulted in tumor growth ^[4]. Pro-tumorigenic effect of macrophage Nrp1 has been recently confirmed in another tumor model, Gliomas ^[5]. However, the roles of macrophage Nrp1 in obesity-associated insulin resistance have been poorly understood. In our recently published article, we provided first evidence of a direct link between Nrp1 and obesity. HFD-fed obese mice exhibited decreased Nrp1 expression in macrophages rather than other detected cells ^[6]. In addition, by generating myeloid cell-specific Nrp1 deficient (Nrp1^myel-KO) mice, we demonstrated that Nrp1 deficiency exacerbated insulin resistance upon HFD relative to wild-type, control mice group ^[6]. Collectively, these data indicate that macrophage Nrp1 negatively regulates obesity-induced insulin resistance.

With respect to how Nrp1 ablation exacerbates insulin resistance in vivo, a number of important findings are noteworthy. Firstly, we observed that systemic inflammation significantly increased in HFD-fed Nrp1^myel-KO mice ^[6]. Secondly, Nrp1^myel-KO mice displayed aberrant activation of Nlrp3 inflammasome, as evidenced by elevated levels of Nlrp3, cleaved caspase 1 p20, pro-IL-1β, IL-1β p17, or IL-18 ^[6]. Finally, Nlrp3 deficiency rescued the phenotype shown in HFD-fed Nrp1^myel-KO mice ^[6]. In agreement with our Vandanmagsar et al. reported that Nlrp3 inflammasomes sense and become activated against endogenous danger signals during obesity, which results in systemic chronic low-grade inflammation and dampened insulin signaling ^[7]. addition, Nlrp3-ASC inflammasome activation impairs pancreatic function and promotes the development of T2DM ^[8].

Mechanistically, Nrp1 deletion promoted macrophage Nlrp3 inflammasome priming and activation ^[6]. We further observed highly activated NF-κB pathway in Nrp1^−/− bone marrow-derived macrophages (BMDMs) compared with WT BMDMs ^[6]. However, canonical MAPK signaling targets, such as ERK and p38, showed no obvious difference between WT and Nrp1^−/− BMDMs, suggesting specific regulation of NF-κB signaling by Nrp1 ^[6]. Luminescence assay confirmed increased activity of NF-κB p65 transcription factor in Nrp1^−/− BMDMs compared with WT BMDMs ^[6]. In addition, Nrp1 deletion correlated with increased ASC cytoplasmic foci formation and Nlrp3-ASC inflammasome assembly ^[6]. Taken together, we propose the working model that HFD-induced Nrp1 attenuation leads to aberrant Nlrp3 priming through activating NF-κB signaling, and directly instigates Nlrp3 inflammasome activation through promoting Nlrp3-ASC complex assembly, which accentuate insulin resistance (Figure 1). Further studies are required to delineate how Nrp1 controls NF-κB pathway. In addition, the mechanistic nature and regulation by which Nrp1 promotes ASC foci formation and Nlrp3-ASC assembly warrants further investigations.

Figure 1. Schematic model for Nrp1 attenuating obesity-associated insulin resistance.

High-fat diet downregulates Nrp1 in macrophages, which activates Nlrp3 inflammasome via NF-κB-mediated priming and Nlrp3-ASC inflammasome assembly. Consequently, increased IL-1β and IL-18 suppress AKT phosphorylation, and induce insulin resistance.

In conclusion, we have uncovered a novel and essential role for myeloid cell Nrp1 in modulating obesity-associated insulin resistance. These findings may support potential target intervention strategies in metabolic and inflammatory disease.