Researchers reveal how the accumulation of extracellular matrix proteins and sugars prevents insulin from reaching neurons that regulate hunger, leading to impaired metabolism and increased risk of obesity.
In a recent study published in Natureresearchers reveal a novel mechanism by which hypothalamic inflammation drives fibrotic remodeling in perineuronal networks (PNNs), a specialized extracellular matrix (ECM) of the hypothalamic arcuate nucleus (ARC), to induce metabolic dysfunction.
Background
Elevated blood glucose levels trigger the beta cells of the pancreas to release more insulin. This hormone circulates in the ARC, which controls physiological processes. Loss of insulin sensitivity increases dietary intake, leading to fat accumulation and obesity. The ECM, a network of proteins and sugars, disrupts insulin access to ARC neurons that regulate hunger, contributing to obesity.
The ECM is a dynamic structure essential for tissue function. However, pathological changes can lead to increased fibrosis in the ECM in the form of perineural nets surrounding the Agouti protein-releasing (AgRP) neurons within the ARC of the hypothalamus. Fibrotic accumulation inhibits insulin activity. Studies report that insulin resistance can lead to metabolic diseases such as obesity and insulin-dependent diabetes.
About the study
The present study investigated the remodeling of the extracellular matrix of the ARC in metabolic diseases such as obesity.
The researchers fed mice either a normal diet or a high-fat, high-sugar (HFHS) diet. They used the HFHS diet to induce diabetes in mice. They monitored blood glucose and used mice to demonstrate stable blood glucose values for further experiments.
The researchers performed immunohistochemistry to study whether neuroinflammation induced network formation around hypothalamic AgRP neurons. Stereotaxic injections in mice induced and inhibited hypothalamic inflammation. Wisteria floribunda (WFA) lectin stains labeled perineurial nets in the ECM of HFHS-fed mice.
The researchers disrupted insulin receptors in AgRP neurons to understand the causal link between neurofibrosis and metabolic dysfunction. To determine whether obesity promotes insulin resistance, the researchers administered insulin-fluorescein isothiocyanate (FITC) and measured its entry and signaling in hypothalamic neurons. Metabolic measures included fasting glucose or plasma insulin and adiposity.
The researchers investigated whether inflammation in the hypothalamus enhanced the growth of perineurial nets and the accumulation of fibrin in the ECM of its neurons. To do this, they administered adeno-associated viruses (AAVs) that express receptors for tumor necrosis factor-alpha (TNF-α) and tumor growth factor-beta (TGF-β). TNF-α increases inflammation, while TGF-β is anti-inflammatory. The receptors would bind to their proteins, artificially increasing their expression. The researchers co-administered these AAVs with chondroitinase ABC (chABC), an enzyme that dissolves perineural networks.
Patch-clamp electrophysiology experiments assessed the interaction of insulin with the PNN in vitro. The researchers used genotyping in mouse samples to quantify gene expression in the ECM of the medial hypothalamus of lean and obese mice. To investigate its therapeutic potential, fluorosamine, a drug that blocks the synthesis of chondroitin sulfate, was administered intranasally and injected into the cerebrospinal fluid of mice for ten days.
Results
In obese mice, ECM accumulates in the form of perineural nets around ARC neurons. The perineural nets, once formed around these “starving neurons” facilitate their maturation. These mature neurons in the hypothalamic arcuate nucleus release AgRP, the neuropeptide that leads to fibrosis in the ECM. Obese rats, HFHS-fed mice, and genetically engineered models of metabolic disease showed increased formation of perineural networks around AgRP neurons.
Fibrinous nets reduced insulin levels in the arcuate nucleus and inhibited signaling activities from insulin receptors. Decreasing insulin function increased the firing of AgRP-releasing hunger neurons. However, insulin levels were restored after disruption of the perineural networks using enzymes. Dissociation also increased potassium ion concentrations. As a result, AgRP neuron firing was reduced. Restoration of insulin levels also led to improved glucose metabolism and reduced weight.
The fibrotic nets around the starved neurons reduced the expression of genes encoding insulin receptors in AgRP neurons. The finding showed that ECM remodeling with fibrosis around neurons impairs metabolic functions by increasing insulin resistance or decreasing insulin activity. However, the results in obese mice showed that the perineural nets did not affect the activities of leptin, a hormone that regulates body weight.
Obesity increases levels of inflammatory TNF-α but decreases expression of anti-inflammatory TGF-β. Increased body weight also decreases the expression of metalloproteinases. Metalloproteinases are enzymes that can digest the fibrous nets around starvation neurons. Suppression of inflammation in the hypothalamus using AAV improved metalloproteinase expression.
Fluorosamine restored insulin sensitivity and normal ECM formation in hypothalamic neurons. Subsequently, body weight decreased and metabolic function improved. The findings suggest that drugs that reduce hypothalamic inflammation and prevent fibrin accumulation in the ECM could improve metabolism by improving insulin and metalloproteinase functions.
Conclusion
Based on the findings, ECM remodeling in the hypothalamus may lead to metabolic disease. Drugs and enzymes that can disrupt the perineural networks that form around AgRP neurons could improve metabolism and reduce weight by improving insulin activity.
