Rab32 connects ER stress to mitochondrial defects in multiple sclerosis.

Endoplasmic reticulum (ER) stress is a hallmark of neurodegenerative diseases such as multiple sclerosis (MS). However, this physiological mechanism has multiple manifestations that range from impaired clearance of unfolded proteins to altered mitochondrial dynamics and apoptosis. While connections between the triggering of the unfolded protein response (UPR) and downstream mitochondrial dysfunction are poorly understood, the membranous contacts between the ER and mitochondria, called the mitochondria-associated membrane (MAM), could provide a functional link between these two mechanisms. Therefore, we investigated whether the guanosine triphosphatase (GTPase) Rab32, a known regulator of the MAM, mitochondrial dynamics, and apoptosis, could be associated with ER stress as well as mitochondrial dysfunction.This article is freely available via Open Access. Click on the Additional Link above to access the full-text via the publisher's site

Attenuated Induction of the Unfolded Protein Response in Adult Human Primary Astrocytes in Response to Recurrent Low Glucose.

AIMS/HYPOTHESIS: Recurrent hypoglycaemia (RH) is a major side-effect of intensive insulin therapy for people with diabetes. Changes in hypoglycaemia sensing by the brain contribute to the development of impaired counterregulatory responses to and awareness of hypoglycaemia. Little is known about the intrinsic changes in human astrocytes in response to acute and recurrent low glucose (RLG) exposure. METHODS: Human primary astrocytes (HPA) were exposed to zero, one, three or four bouts of low glucose (0.1 mmol/l) for three hours per day for four days to mimic RH. On the fourth day, DNA and RNA were collected. Differential gene expression and ontology analyses were performed using DESeq2 and GOseq, respectively. DNA methylation was assessed using the Infinium MethylationEPIC BeadChip platform. RESULTS: 24 differentially expressed genes (DEGs) were detected (after correction for multiple comparisons). One bout of low glucose exposure had the largest effect on gene expression. Pathway analyses revealed that endoplasmic-reticulum (ER) stress-related genes such as HSPA5, XBP1, and MANF, involved in the unfolded protein response (UPR), were all significantly increased following low glucose (LG) exposure, which was diminished following RLG. There was little correlation between differentially methylated positions and changes in gene expression yet the number of bouts of LG exposure produced distinct methylation signatures. CONCLUSIONS/INTERPRETATION: These data suggest that exposure of human astrocytes to transient LG triggers activation of genes involved in the UPR linked to endoplasmic reticulum (ER) stress. Following RLG, the activation of UPR related genes was diminished, suggesting attenuated ER stress. This may be a consequence of a successful metabolic adaptation, as previously reported, that better preserves intracellular energy levels and a reduced necessity for the UPR.The article is available via Open Access. Click on the 'Additional link' above to access the full-text.Published version, accepted version, submitted versio

A Role for <i>SPARC</i> in the Moderation of Human Insulin Secretion

<div><p>Aims/Hypothesis</p><p>We have previously shown the implication of the multifunctional protein SPARC (Secreted protein acidic and rich in cysteine)/osteonectin in insulin resistance but potential effects on beta-cell function have not been assessed. We therefore aimed to characterise the effect of SPARC on beta-cell function and features of diabetes.</p><p>Methods</p><p>We measured <i>SPARC</i> expression by qRT-PCR in human primary pancreatic islets, adipose tissue, liver and muscle. We then examined the relation of <i>SPARC</i> with glucose stimulated insulin secretion (GSIS) in primary human islets and the effect of <i>SPARC</i> overexpression on GSIS in beta cell lines.</p><p>Results</p><p><i>SPARC</i> was expressed at measurable levels in human islets, adipose tissue, liver and skeletal muscle, and demonstrated reduced expression in primary islets from subjects with diabetes compared with controls (p< = 0.05). SPARC levels were positively correlated with GSIS in islets from control donors (p< = 0.01). Overexpression of <i>SPARC</i> in cultured beta-cells resulted in a 2.4-fold increase in insulin secretion in high glucose conditions (p< = 0.01).</p><p>Conclusions</p><p>Our data suggest that levels of <i>SPARC</i> are reduced in islets from donors with diabetes and that it has a role in insulin secretion, an effect which appears independent of SPARC’s modulation of obesity-induced insulin resistance in adipose tissue.</p></div

The association of <i>SPARC</i> expression on glucose stimulated insulin secretion in primary human islets.

<p>The figures on the Y axis refer to insulin stimulation index (SI) following a 28 mM glucose challenge. The expression level of <i>SPARC</i> is given on the X axis. Expression levels were calculated relative to the endogenous controls <i>GUSB, B2M</i> and <i>PP1A</i> and normalised to the mean level of <i>SPARC</i> across the cohort.</p

Glucose stimulated insulin secretion and basal insulin according to <i>SPARC</i> expression in primary human islet cultures.

<p>Associations between Glucose-stimulated insulin secretion and basal insulin levels were assessed by linear regression analysis, adjusted for BMI. Significant associations are indicated in bold italic type.</p

Changes in the insulin stimulation index in <i>SPARC</i>-transfected rat beta cells under different glucose conditions.

<p>The figure shows the stimulation index (SI) of transfected rat beta cells (INS-1) in response to 2.8 mM, 5.6 mM and 16.7 mM glucose or IBMX/forskolin. Results from cells transfected with empty vector are given in light grey, and those from <i>SPARC</i>-transfected cells are given in dark grey. Error bars represent standard error of measurements. Data are combined results from the two transfected cell lines. Statistical significance is indicated by stars.</p

Characteristics of donors and samples for human primary islet collection.

<p>M = Male, F = female, N = number of samples tested. BMI = Body Mass Index. HbA1c = Glycosylated haemoglobin.</p