Kyunghee Byun

Amyloid β-protein (Aβ)1–40 protects neurons from damage induced by Aβ1–42 in culture and in rat brain

Previously, we found that amyloid β‐protein (Aβ)1–42 exhibits neurotoxicity, while Aβ1–40 serves as an antioxidant molecule by quenching metal ions and inhibiting metal‐mediated oxygen radical generation. Here, we show another neuroprotective action of nonamyloidogenic Aβ1–40 against Aβ1–42‐induced neurotoxicity in culture and in vivo. Neuronal death was induced by Aβ1–42 at concentrations higher than 2 μm, which was prevented by concurrent treatment with Aβ1–40 in a dose‐dependent manner. However, metal chelators did not prevent Aβ1–42‐induced neuronal death. Circular dichroism spectroscopy showed that Aβ1–40 inhibited the β‐sheet transformation of Aβ1–42. Thioflavin‐T assay and electron microscopy analysis revealed that Aβ1–40 inhibited the fibril formation of Aβ1–42. In contrast, Aβ1–16, Aβ25–35, and Aβ40–1 did not inhibit the fibril formation of Aβ1–42 nor prevent Aβ1–42‐induced neuronal death. Aβ1–42 injection into the rat entorhinal cortex (EC) caused the hyperphosphorylation of tau on both sides of EC and hippocampus and increased the number of glial fibrillary acidic protein (GFAP)‐positive astrocytes in the ipsilateral EC, which were prevented by the concurrent injection of Aβ1–40. These results indicate that Aβ1–40 protects neurons from Aβ1–42‐induced neuronal damage in vitro and in vivo, not by sequestrating metals, but by inhibiting the β‐sheet transformation and fibril formation of Aβ1–42. Our data suggest a mechanism by which elevated Aβ1–42/Aβ1–40 ratio accelerates the development of Alzheimers disease (AD) in familial AD.

A Systems Approach for Decoding Mitochondrial Retrograde Signaling Pathways

Transcriptional profiling and transcription factor matching reveal potential targets for therapies against diseases associated with mitochondrial dysfunction. Signaling Mitochondrial Dysfunction The mitochondrial and nuclear genomes contribute to mitochondrial function, and when mitochondrial function is compromised, mitochondrial retrograde signaling alters nuclear gene expression. Chae et al. performed gene expression profiling of engineered cells that had mitochondria containing a disease-associated mutation that causes mitochondrial dysfunction. By generating networks of transcription factors that targeted these genes, the authors revealed putative mitochondrial retrograde signaling pathways. One such pathway involved retinoic X receptor α (RXRA), the mRNA for which was reduced in the mutant cells. Network analysis and experiments in cells suggested that mitochondrial dysfunction caused by the mutation initiated a positive feedback loop that aggravated mitochondrial dysfunction: Reduced RXRA abundance further compromised expression of genes encoding products involved in mitochondrial function and translation. This gene transcription factor mapping network approach may reveal targets for therapeutic intervention of diseases associated with mitochondrial dysfunction. Mitochondrial dysfunctions activate retrograde signaling from mitochondria to the nucleus. To identify transcription factors and their associated pathways that underlie mitochondrial retrograde signaling, we performed gene expression profiling of the cells engineered to have varying amounts of mitochondrial DNA with an A3243G mutation (mt3243) in the leucine transfer RNA (tRNALeu), which reduces the abundance of proteins involved in oxidative phosphorylation that are encoded by the mitochondrial genome. The cells with the mutation exhibited reduced mitochondrial function, including compromised oxidative phosphorylation, which would activate diverse mitochondrial retrograde signaling pathways. By analyzing the gene expression profiles in cells with the mutant tRNALeu and the transcription factors that recognize the differentially regulated genes, we identified 72 transcription factors that were potentially involved in mitochondrial retrograde signaling. We experimentally validated that the mt3243 mutation induced a retrograde signaling pathway involving RXRA (retinoid X receptor α), reactive oxygen species, kinase JNK (c-JUN N-terminal kinase), and transcriptional coactivator PGC1α (peroxisome proliferator–activated receptor γ, coactivator 1 α). This RXR pathway contributed to the decrease in mRNA abundances of oxidative phosphorylation enzymes encoded in the nuclear genome, thereby aggravating the dysfunction in oxidative phosphorylation caused by the reduced abundance of mitochondria-encoded enzymes of oxidative phosphorylation. Thus, matching transcription factors to differentially regulated gene expression profiles was an effective approach to understand mitochondrial retrograde signaling pathways and their roles in mitochondrial dysfunction.

Human Microglial Cells Synthesize Albumin in Brain

Albumin, an abundant plasma protein with multifunctional properties, is mainly synthesized in the liver. Albumin has been implicated in Alzheimers disease (AD) since it can bind to and transport amyloid beta (Aβ), the causative agent of AD; albumin is also a potent inhibitor of Aβ polymerization. Despite evidence of non-hepatic transcription of albumin in many tissues including kidney and pancreas, non-hepatic synthesis of albumin at the protein level has been rarely confirmed. In a pilot phase study of Human Brain Proteome Project, we found evidence that microglial cells in brain may synthesize albumin. Here we report, for the first time, the de novo synthesis of albumin in human microglial cells in brain. Furtherore, we demonstrate that the synthesis and secretion of albumin from microglial cells is enhanced upon microgial activation by Aβ1–42- or lipopolysaccharide (LPS)-treatment. These data indicate that microglial cells may play a beneficial role in AD by secreting albumin that not only inhibits Aβ polymerization but also increases its clearance.

Mitochondrial dysfunction and tissue injury by alcohol, high fat, nonalcoholic substances and pathological conditions through post-translational protein modifications

Mitochondria are critically important in providing cellular energy ATP as well as their involvement in anti-oxidant defense, fat oxidation, intermediary metabolism and cell death processes. It is well-established that mitochondrial functions are suppressed when living cells or organisms are exposed to potentially toxic agents including alcohol, high fat diets, smoking and certain drugs or in many pathophysiological states through increased levels of oxidative/nitrative stress. Under elevated nitroxidative stress, cellular macromolecules proteins, DNA, and lipids can undergo different oxidative modifications, leading to disruption of their normal, sometimes critical, physiological functions. Recent reports also indicated that many mitochondrial proteins are modified via various post-translation modifications (PTMs) and primarily inactivated. Because of the recently-emerging information, in this review, we specifically focus on the mechanisms and roles of five major PTMs (namely oxidation, nitration, phosphorylation, acetylation, and adduct formation with lipid-peroxides, reactive metabolites, or advanced glycation end products) in experimental models of alcoholic and nonalcoholic fatty liver disease as well as acute hepatic injury caused by toxic compounds. We also highlight the role of the ethanol-inducible cytochrome P450-2E1 (CYP2E1) in some of these PTM changes. Finally, we discuss translational research opportunities with natural and/or synthetic anti-oxidants, which can prevent or delay the onset of mitochondrial dysfunction, fat accumulation and tissue injury.

Induction of Neuronal Death by Microglial AGE-Albumin: Implications for Alzheimer’s Disease

Advanced glycation end products (AGEs) have long been considered as potent molecules promoting neuronal cell death and contributing to neurodegenerative disorders such as Alzheimer’s disease (AD). In this study, we demonstrate that AGE-albumin, the most abundant AGE product in human AD brains, is synthesized in activated microglial cells and secreted into the extracellular space. The rate of AGE-albumin synthesis in human microglial cells is markedly increased by amyloid-β exposure and oxidative stress. Exogenous AGE-albumin upregulates the receptor protein for AGE (RAGE) and augments calcium influx, leading to apoptosis of human primary neurons. In animal experiments, soluble RAGE (sRAGE), pyridoxamine or ALT-711 prevented Aβ-induced neuronal death in rat brains. Collectively, these results provide evidence for a new mechanism by which microglial cells promote death of neuronal cells through synthesis and secretion of AGE-albumin, thereby likely contributing to neurodegenerative diseases such as AD.

Induced pluripotent stem cells and personalized medicine: current progress and future perspectives.

Generation of induced pluripotent stem cells (iPSCs) has revolutionized the field of regenerative medicine by providing researchers with a unique tool to derive disease-specific stem cells for study. iPSCs can self-renew and can differentiate into many cell types, offering a potentially unlimited source of cells for targeted differentiation into somatic effector cells. Hence, iPSCs are likely to be invaluable for therapeutic applications and disease-related research. In this review, we summarize the recent progress of iPSC generation that has been made with an emphasis on both basic and clinical applications including disease modeling, drug toxicity screening/drug discovery and cell replacement therapy.

Leptin-promoted cilia assembly is critical for normal energy balance

The majority of mammalian cells have nonmotile primary cilia on their surface that act as antenna-like sensory organelles. Genetic defects that result in ciliary dysfunction are associated with obesity in humans and rodents, which suggests that functional cilia are important for controlling energy balance. Here we demonstrated that neuronal cilia lengths were selectively reduced in hypothalami of obese mice with leptin deficiency and leptin resistance. Treatment of N1 hypothalamic neuron cells with leptin stimulated cilia assembly via inhibition of the tumor suppressors PTEN and glycogen synthase kinase 3β (GSK3β). Induction of short cilia in the hypothalamus of adult mice increased food intake and decreased energy expenditure, leading to a positive energy balance. Moreover, mice with short hypothalamic cilia exhibited attenuated anorectic responses to leptin, insulin, and glucose, which indicates that leptin-induced cilia assembly is essential for sensing these satiety signals by hypothalamic neurons. These data suggest that leptin governs the sensitivity of hypothalamic neurons to metabolic signals by controlling the length of the cells antenna.

Olig2-Induced Neural Stem Cell Differentiation Involves Downregulation of Wnt Signaling and Induction of Dickkopf-1 Expression

Understanding stem cell-differentiation at the molecular level is important for clinical applications of stem cells and for finding new therapeutic approaches in the context of cancer stem cells. To investigate genome-wide changes involved in differentiation, we have used immortalized neural stem cell (NSC) line (HB1.F3) and Olig2-induced NSC differentiation model (F3.Olig2). Using microarray analysis, we revealed that Olig2-induced NSC differentiation involves downregulation of Wnt pathway, which was further confirmed by TOPflash/FOPflash reporter assay, RT-PCR analysis, immunoblots, and immunocytochemistry. Furthermore, we found that Olig2-induced differentiation induces the expression of Dickkopf-1(Dkk1), a potent antagonist of Wnt signaling. Dkk1 treatment blocked Wnt signaling in HB1.F3 in a dosage-dependent manner, and induced differentiation into astrocytes, oligodendrocytes, and neurons. Our results support cancer stem cell hypothesis which implies that signaling pathway for self-renewal and proliferation of stem cells is maintained till the late stage of differentiation. In our proposed model, Dkk1 may play an important role in downregulating self-renewal and proliferation pathway of stem cells at the late stage of differentiation, and its failure may lead to carcinogenesis.

Advanced glycation end-products produced systemically and by macrophages: A common contributor to inflammation and degenerative diseases.

ABSTRACT Advanced glycation end products (AGEs) and their receptor have been implicated in the progressions of many intractable diseases, such as diabetes and atherosclerosis, and are also critical for pathologic changes in chronic degenerative diseases, such as Alzheimers disease, Parkinsons disease, and alcoholic brain damage. Recently activated macrophages were found to be a source of AGEs, and the most abundant form of AGEs, AGE‐albumin excreted by macrophages has been implicated in these diseases and to act through common pathways. AGEs inhibition has been shown to prevent the pathogenesis of AGEs‐related diseases in human, and therapeutic advances have resulted in several agents that prevent their adverse effects. Recently, anti‐inflammatory molecules that inhibit AGEs have been shown to be good candidates for ameliorating diabetic complications as well as degenerative diseases. This review was undertaken to present, discuss, and clarify current understanding regarding AGEs formation in association with macrophages, different diseases, therapeutic and diagnostic strategy and links with RAGE inhibition.

Clusterin and LRP2 are critical components of the hypothalamic feeding regulatory pathway

Hypothalamic feeding circuits are essential for the maintenance of energy balance. There have been intensive efforts to discover new biological molecules involved in these pathways. Here we report that central administration of clusterin, also called apolipoprotein J, causes anorexia, weight loss and activation of hypothalamic signal transduction-activated transcript-3 in mice. In contrast, inhibition of hypothalamic clusterin action results in increased food intake and body weight, leading to adiposity. These effects are likely mediated through the mutual actions of the low-density lipoprotein receptor-related protein-2, a potential receptor for clusterin, and the long-form leptin receptor. In response to clusterin, the low-density lipoprotein receptor-related protein-2 binding to long-form leptin receptor is greatly enhanced in cultured neuronal cells. Furthermore, long-form leptin receptor deficiency or hypothalamic low-density lipoprotein receptor-related protein-2 suppression in mice leads to impaired hypothalamic clusterin signalling and actions. Our study identifies the hypothalamic clusterin-low-density lipoprotein receptor-related protein-2 axis as a novel anorexigenic signalling pathway that is tightly coupled with long-form leptin receptor-mediated signalling.