Synchrotron fluorescence imaging of individual mouse beta-cells reveals changes in zinc, calcium, and iron in a model of low-grade inflammation

Metal ion content of internal organs in the calorically restricted Wistar rat

BACKGROUND

Despite the agreed principle that access to food is a human right, undernourishment and metal ion deficiencies are public health problems worldwide, exacerbated in impoverished or war-affected areas. It is known that maternal malnutrition causes growth retardation and affects behavioral and cognitive development of the newborn. Here we ask whether severe caloric restriction leads per se to disrupted metal accumulation in different organs of the Wistar rat.

METHODS

Inductively coupled plasma optical emission spectroscopy was used to determine the concentration of multiple elements in the small and large intestine, heart, lung, liver, kidney, pancreas, spleen, brain, spinal cord, and three skeletal muscles from control and calorically restricted Wistar rats. The caloric restriction protocol was initiated from the mothers prior to mating and continued throughout gestation, lactation, and post-weaning up to sixty days of age.

RESULTS

Both sexes were analyzed but dimorphism was rare. The pancreas was the most affected organ presenting a higher concentration of all the elements analyzed. Copper concentration decreased in the kidney and increased in the liver. Each skeletal muscle responded to the treatment differentially: Extensor Digitorum Longus accumulated calcium and manganese, gastrocnemius decreased copper and manganese, whereas soleus decreased iron concentrations. Differences were also observed in the concentration of elements between organs independently of treatment: The soleus muscle presents a higher concentration of Zn compared to the other muscles and the rest of the organs. Notably, the spinal cord showed large accumulations of calcium and half the concentration of zinc compared to brain. X-ray fluorescence imaging suggests that the extra calcium is attributable to the presence of ossifications whereas the latter finding is attributable to the low abundance of zinc synapses in the spinal cord.

CONCLUSION

Severe caloric restriction did not lead to systemic metal deficiencies but caused instead specific metal responses in few organs.

Zinc and iron dynamics in human islet amyloid polypeptide-induced diabetes mouse model

Metal homeostasis is tightly regulated in cells and organisms, and its disturbance is frequently observed in some diseases such as neurodegenerative diseases and metabolic disorders. Previous studies suggest that zinc and iron are necessary for the normal functions of pancreatic β cells. However, the distribution of elements in normal conditions and the pathophysiological significance of dysregulated elements in the islet in diabetic conditions have remained unclear. In this study, to investigate the dynamics of elements in the pancreatic islets of a diabetic mouse model expressing human islet amyloid polypeptide (hIAPP): hIAPP transgenic (hIAPP-Tg) mice, we performed imaging analysis of elements using synchrotron scanning X-ray fluorescence microscopy and quantitative analysis of elements using inductively coupled plasma mass spectrometry. We found that in the islets, zinc significantly decreased in the early stage of diabetes, while iron gradually decreased concurrently with the increase in blood glucose levels of hIAPP-Tg mice. Notably, when zinc and/or iron were decreased in the islets of hIAPP-Tg mice, dysregulation of glucose-stimulated mitochondrial respiration was observed. Our findings may contribute to clarifying the roles of zinc and iron in islet functions under pathophysiological diabetic conditions.

The use of synchrotron X-ray fluorescent imaging to study distribution and content of elements in chemically fixed single cells: a case study using mouse pancreatic beta-cells

Synchrotron X-ray fluorescence microscopy (SXRF) presents a valuable opportunity to study the metallome of single cells because it simultaneously provides high-resolution subcellular distribution and quantitative cellular content of multiple elements. Different sample preparation techniques have been used to preserve cells for observations with SXRF, with a goal to maintain fidelity of the cellular metallome. In this case study, mouse pancreatic beta-cells have been preserved with optimized chemical fixation. We show that cell-to-cell variability is normal in the metallome of beta-cells due to heterogeneity and should be considered when interpreting SXRF data. In addition, we determined the impact of several immunofluorescence (IF) protocols on metal distribution and quantification in chemically fixed beta-cells and found that the metallome of beta-cells was not well preserved for quantitative analysis. However, zinc and iron qualitative analysis could be performed after IF with certain limitations. To help minimize metal loss using samples that require IF, we describe a novel IF protocol that can be used with chemically fixed cells after the completion of SXRF.

LDL Promotes Disorders in β-Cell Cholesterol Metabolism, Implications on Insulin Cellular Communication Mediated by EVs

Dyslipidemia is described as a hallmark of metabolic syndrome, promoting a stage of metabolic inflammation (metainflammation) that could lead to misbalances in energetic metabolism, contributing to insulin resistance, and modifying intracellular cholesterol pathways and the renin-angiotensin system (RAS) in pancreatic islets. Low-density lipoprotein (LDL) hypercholesterolemia could disrupt the tissue communication between Langerhans β-cells and hepatocytes, wherein extracellular vesicles (EVs) are secreted by β-cells, and exposition to LDL can impair these phenomena. β-cells activate compensatory mechanisms to maintain insulin and metabolic homeostasis; therefore, the work aimed to characterize the impact of LDL on β-cell cholesterol metabolism and the implication on insulin secretion, connected with the regulation of cellular communication mediated by EVs on hepatocytes. Our results suggest that β-cells can endocytose LDL, promoting an increase in de novo cholesterol synthesis targets. Notably, LDL treatment increased mRNA levels and insulin secretion; this hyperinsulinism condition was associated with the transcription factor PDX-1. However, a compensatory response that maintains basal levels of intracellular calcium was described, mediated by the overexpression of calcium targets PMCA1/4, SERCA2, and NCX1, together with the upregulation of the unfolded protein response (UPR) through the activation of IRE1 and PERK arms to maintain protein homeostasis. The LDL treatment induced metainflammation by IL-6, NF-κB, and COX-2 overexpression. Furthermore, LDL endocytosis triggered an imbalance of the RAS components. LDL treatment increased the intracellular levels of cholesterol on lipid droplets; the adaptive β-cell response was portrayed by the overexpression of cholesterol transporters ABCA1 and ABCG1. Therefore, lipotoxicity and hyperinsulinism induced by LDL were regulated by the natural compound auraptene, a geranyloxyn coumarin modulator of cholesterol-esterification by ACAT1 enzyme inhibition. EVs isolated from β-cells impaired insulin signaling via mTOR/p70S6Kα in hepatocytes, a phenomenon regulated by auraptene. Our results show that LDL overload plays a novel role in hyperinsulinism, mechanisms associated with a dysregulation of intracellular cholesterol, lipotoxicity, and the adaptive UPR, which may be regulated by coumarin-auraptene; these conditions explain the affectations that occur during the initial stages of insulin resistance.