Interleukin 6 reduces allopregnanolone synthesis in the brain and contributes to age-related cognitive decline in mice

Cognitive decline with age is a harmful process that can reduce quality of life. Multiple factors have been established to contribute to cognitive decline, but the overall etiology remains unknown. Here, we hypothesized that cognitive dysfunction is mediated, in part, by increased levels of inflammatory cytokines that alter allopregnanolone (AlloP) levels, an important neurosteroid in the brain. We assessed the levels and regulation of AlloP and the effects of AlloP supplementation on cognitive function in 4-month-old and 24-month-old male C57BL/6 mice. With age, the expression of enzymes involved in the AlloP synthetic pathway was decreased and corticosterone (CORT) synthesis increased. Supplementation of AlloP improved cognitive function. Interestingly, interleukin 6 (IL-6) infusion in young animals significantly reduced the production of AlloP compared with controls. It is notable that inhibition of IL-6 with its natural inhibitor, soluble membrane glycoprotein 130, significantly improved spatial memory in aged mice. These findings were supported by in vitro experiments in primary murine astrocyte cultures, indicating that IL-6 decreases production of AlloP and increases CORT levels. Our results indicate that age-related increases in IL-6 levels reduce progesterone substrate availability, resulting in a decline in AlloP levels and an increase in CORT. Furthermore, our results indicate that AlloP is a critical link between inflammatory cytokines and the age-related decline in cognitive function.

Insulin-like growth factor receptor signaling regulates working memory, mitochondrial metabolism, and amyloid-β uptake in astrocytes

Objective

A decline in mitochondrial function and biogenesis as well as increased reactive oxygen species (ROS) are important determinants of aging. With advancing age, there is a concomitant reduction in circulating levels of insulin-like growth factor-1 (IGF-1) that is closely associated with neuronal aging and neurodegeneration. In this study, we investigated the effect of the decline in IGF-1 signaling with age on astrocyte mitochondrial metabolism and astrocyte function and its association with learning and memory.

Methods

Learning and memory was assessed using the radial arm water maze in young and old mice as well as tamoxifen-inducible astrocyte-specific knockout of IGFR (GFAP-Cre^TAM/igfr^f/f). The impact of IGF-1 signaling on mitochondrial function was evaluated using primary astrocyte cultures from igfr^f/f mice using AAV-Cre mediated knockdown using Oroboros respirometry and Seahorse assays.

Results

Our results indicate that a reduction in IGF-1 receptor (IGFR) expression with age is associated with decline in hippocampal-dependent learning and increased gliosis. Astrocyte-specific knockout of IGFR also induced impairments in working memory. Using primary astrocyte cultures, we show that reducing IGF-1 signaling via a 30–50% reduction IGFR expression, comparable to the physiological changes in IGF-1 that occur with age, significantly impaired ATP synthesis. IGFR deficient astrocytes also displayed altered mitochondrial structure and function and increased mitochondrial ROS production associated with the induction of an antioxidant response. However, IGFR deficient astrocytes were more sensitive to H₂O₂-induced cytotoxicity. Moreover, IGFR deficient astrocytes also showed significantly impaired glucose and Aβ uptake, both critical functions of astrocytes in the brain.

Conclusions

Regulation of astrocytic mitochondrial function and redox status by IGF-1 is essential to maintain astrocytic function and coordinate hippocampal-dependent spatial learning. Age-related astrocytic dysfunction caused by diminished IGF-1 signaling may contribute to the pathogenesis of Alzheimer's disease and other age-associated cognitive pathologies.

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Altered mitochondrial structure and function with IGFR deficiency in astrocytes is proposed.

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Increased reactive oxygen species production and susceptibility to peroxide induced cytotoxicity.

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Decreased Aβ uptake and impairment in spatial working memory.

Cell surface epidermal growth factor receptors increase Src and c-Cbl activity and receptor ubiquitylation

There is an established role for the endocytic pathway in regulation of epidermal growth factor receptor (EGFR) signaling to downstream effectors. However, because ligand-mediated EGFR endocytosis utilizes multiple "moving parts," dissecting the spatial versus temporal contributions has been challenging. Blocking all endocytic trafficking can have unintended effects on other receptors as well as give rise to compensatory mechanisms, both of which impact interpretation of EGFR signaling. To overcome these limitations, we used epidermal growth factor (EGF) conjugated to polystyrene beads (EGF beads). EGF beads simultaneously activate the EGFR while blocking its endocytosis and allow analysis of EGFR signaling from the plasma membrane. Human telomerase immortalized corneal epithelial (hTCEpi) cells were used to model normal epithelial cell biology. In hTCEpi cells, both cell surface and intracellular EGFRs exhibited dose-dependent increases in effector activity after 15 min of ligand stimulation, but only the serine phosphorylation of signal transducer and activator of transcription 3 (STAT3) was statistically significant when accounting for receptor phosphorylation. However, over time with physiological levels of receptor phosphorylation, cell surface receptors produced either enhanced or sustained mitogen-activated protein kinase kinase (MEK), Casitas B-lineage lymphoma (c-Cbl), and the pro-oncogene Src activity. These increases in effector communication by cell surface receptors resulted in an increase in EGFR ubiquitylation with sustained ligand incubation. Together, these data indicate that spatial regulation of EGFR signaling may be an important regulatory mechanism in receptor down-regulation.

A simple method for using silicone elastomer masks for quantitative analysis of cell migration without cellular damage or substrate disruption

Cell migration is fundamental to many biological processes, including development, normal tissue remodeling, wound healing, and many pathologies. However, cell migration is a complex process, and understanding its regulation in health and disease requires the ability to manipulate and measure this process quantitatively under controlled conditions. This report describes a simple in vitro assay for quantitative analysis of cell migration in two-dimensional cultures that is an inexpensive alternative to the classic "scratch" assay. The method described utilizes flexible silicone masks fabricated in the lab according to the research demands of the specific experiment to create a cell-free area for cells to invade, followed by quantitative analysis based on widely available microscopic imaging tools. This experimental approach has the important advantage of visualizing cell migration in the absence of the cellular damage and disruption of the substrate that occurs when the "wound" is created in the scratch assay. This approach allows the researcher to study the intrinsic migratory characteristics of cells in the absence of potentially confounding contributions from cellular responses to injury and disruption of cell-substrate interactions. This assay has been used with vascular smooth muscle cells, fibroblasts, and epithelial cell types, but should be applicable to the study of practically any type of cultured cell. Furthermore, this method can be easily adapted for use with fluorescence microscopy, molecular biological, or pharmacological manipulations to explore the molecular mechanisms of cell migration, live cell imaging, fluorescence microscopy, and correlative immunolabeling.

Human GGT2 does not autocleave into a functional enzyme: A cautionary tale for interpretation of microarray data on redox signaling

AIMS

Human γ-glutamyltranspeptidase 1 (hGGT1) is a cell-surface enzyme that is a regulator of redox adaptation and drug resistance due to its glutathionase activity. The human GGT2 gene encodes a protein that is 94% identical to the amino-acid sequence of hGGT1. Transcriptional profiling analyses in a series of recent publications have implicated the hGGT2 enzyme as a modulator of disease processes. However, hGGT2 has never been shown to encode a protein with enzymatic activity. The aim of this study was to express the protein encoded by hGGT2 and each of its known variants and to assess their stability, cellular localization, and enzymatic activity.

RESULTS

We discovered that the proteins encoded by hGGT2 and its variants are inactive propeptides. We show that hGGT2 cDNAs are transcribed with a similar efficiency to hGGT1, and the expressed propeptides are N-glycosylated. However, they do not autocleave into heterodimers, fail to localize to the plasma membrane, and do not metabolize γ-glutamyl substrates. Substituting the coding sequence of hGGT1 to conform to alterations in a CX3C motif encoded by hGGT2 mRNAs disrupted autocleavage of the hGGT1 propeptide into a heterodimer, resulting in loss of plasma membrane localization and catalytic activity.

INNOVATION AND CONCLUSIONS

This is the first study to evaluate hGGT2 protein. The data show that hGGT2 does not encode a functional enzyme. Microarray data which have reported induction of hGGT2 mRNA should not be interpreted as induction of a protein that has a role in the metabolism of extracellular glutathione and in maintaining the redox status of the cell.