Overexpression of the Insulin-Like Growth Factor II Receptor Increases β-Amyloid Production and Affects Cell Viability

AP-1γ2 is an adaptor protein 1 variant required for endosome-to-Golgi trafficking of the mannose-6-P receptor (CI-MPR) and ATP7B copper transporter

Selective retrograde transport from endosomes back to the trans-Golgi network (TGN) is important for maintaining protein homeostasis, recycling receptors, and returning molecules that were transported to the wrong compartments. Two important transmembrane proteins directed to this pathway are the Cation-Independent Mannose-6-phosphate receptor (CI-MPR) and the ATP7B copper transporter. Among CI-MPR functions is the delivery of acid hydrolases to lysosomes, while ATP7B facilitates the transport of cytosolic copper ions into organelles or the extracellular space. Precise subcellular localization of CI-MPR and ATP7B is essential for the proper functioning of these proteins. This study shows that both CI-MPR and ATP7B interact with a variant of the clathrin adaptor 1 (AP-1) complex that contains a specific isoform of the γ-adaptin subunit called γ2. Through synchronized anterograde trafficking and cell-surface uptake assays, we demonstrated that AP-1γ2 is dispensable for ATP7B and CI-MPR exit from the TGN while being critically required for ATP7B and CI-MPR retrieval from endosomes to the TGN. Moreover, AP-1γ2 depletion leads to the retention of endocytosed CI-MPR in endosomes enriched in retromer complex subunits. These data underscore the importance of AP-1γ2 as a key component in the sorting and trafficking machinery of CI-MPR and ATP7B, highlighting its essential role in the transport of proteins from endosomes.

Native PLGA nanoparticles regulate APP metabolism and protect neurons against β-amyloid toxicity: Potential significance in Alzheimer's disease pathology

Biodegradable poly(lactic-co-glycolic acid)(PLGA) nanoparticles have been used extensively in delivering drugs to target tissues due to their excellent biocompatibility. Evidence suggests that PLGA-conjugated drugs/agents can attenuate pathology in cellular/animal models of Alzheimer's disease (AD), which is initiated by increased level/aggregation of amyloid β (Aβ) peptide generated from amyloid precursor protein (APP). The beneficial effects were attributed to conjugated-drugs rather than to PLGA nanoparticles. Interestingly, we recently reported that PLGA without any drug/agent (native PLGA) can suppress Aβ aggregation/toxicity. However, very little is known about the internalization, subcellular localization or effects of PLGA in neurons. In this study, using primary mouse cortical neurons, we first showed that native PLGA is internalized by an energy-mediated clathrin-dependent/-independent pathway and is localized in endosomal-lysosomal-autophagic vesicles. By attenuating internalization, PLGA can protect neurons against Aβ-mediated toxicity. Additionally, PLGA treatment altered expression profiles of certain AD-associated genes and decreased the levels of APP, its cleaved products α-/β-CTFs and Aβ peptides in mouse as well as iPSC-derived neurons from control and AD patients. Collectively, these results suggest that native PLGA not only protects neurons against Aβ-induced toxicity but also influences the expression of AD-related genes/proteins - highlighting PLGA's implication in normal and AD-related pathology.

Insulin-Like Growth Factor 2 As a Possible Neuroprotective Agent and Memory Enhancer-Its Comparative Expression, Processing and Signaling in Mammalian CNS

A number of studies performed on rodents suggest that insulin-like growth factor 2 (IGF-2) or its analogs may possibly be used for treating some conditions like Alzheimer’s disease, Huntington’s disease, autistic spectrum disorders or aging-related cognitive impairment. Still, for translational research a comparative knowledge about the function of IGF-2 and related molecules in model organisms (rats and mice) and humans is necessary. There is a number of important differences in IGF-2 signaling between species. In the present review we emphasize species-specific patterns of IGF-2 expression in rodents, humans and some other mammals, using, among other sources, publicly available transcriptomic data. We provide a detailed description of Igf2 mRNA expression regulation and pre-pro-IGF-2 protein processing in different species. We also summarize the function of IGF-binding proteins. We describe three different receptors able to bind IGF-2 and discuss the role of IGF-2 signaling in learning and memory, as well as in neuroprotection. We hope that comprehensive understanding of similarities and differences in IGF-2 signaling between model organisms and humans will be useful for development of more effective medicines targeting IGF-2 receptors.

Significance of cytosolic cathepsin D in Alzheimer's disease pathology: Protective cellular effects of PLGA nanoparticles against β-amyloid-toxicity

BACKGROUND

Evidence suggests that amyloid β (Aβ) peptides play an important role in the degeneration of neurons during the development of Alzheimer's disease (AD), the prevalent cause of dementia affecting the elderly. The endosomal-lysosomal system, which acts as a major site for Aβ metabolism, has been shown to exhibit abnormalities in vulnerable neurons of the AD brain, reflected by enhanced levels/expression of lysosomal enzymes including cathepsin D (CatD). At present, the implication of CatD in selective neuronal vulnerability in AD pathology remains unclear.

METHODS

We evaluated the role of CatD in the degeneration of neurons in Aβ-treated cultures, transgenic AD mouse model (that is 5xFAD) and post mortem AD brain samples.

RESULTS

Our results showed that Aβ_1-42 -induced toxicity in cortical cultured neurons is associated with impaired lysosomal integrity, enhanced levels of carbonylated proteins and tau phosphorylation. The cellular and cytosolic levels/activity of CatD are also elevated in cultured neurons following exposure to Aβ peptide. Additionally, we observed that CatD cellular and subcellular levels/activity are increased in the affected cortex, but not in the unaffected cerebellum, of 5xFAD mice and post mortem AD brains. Interestingly, treatment of cultured neurons with nanoparticles PLGA, which targets lysosomal system, attenuated Aβ toxicity by reducing the levels of carbonylated proteins, tau phosphorylation and the level/distribution/activity of CatD.

CONCLUSION

Our study reveals that increased cytosolic level/activity of CatD play an important role in determining neuronal vulnerability in AD. Additionally, native PLGA can protect neurons against Aβ toxicity by restoring lysosomal membrane integrity, thus signifying its implication in attenuating AD.

APP-Mediated Signaling Prevents Memory Decline in Alzheimer's Disease Mouse Model

Amyloid precursor protein (APP) and its metabolites play key roles in Alzheimer's disease (AD) pathophysiology. Whereas short amyloid-β (Aβ) peptides derived from APP are pathogenic, the APP holoprotein serves multiple purposes in the nervous system through its cell adhesion and receptor-like properties. Our studies focused on the signaling mediated by the APP cytoplasmic tail. We investigated whether sustained APP signaling during brain development might favor neuronal plasticity and memory process through a direct interaction with the heterotrimeric G-protein subunit Gα_S (stimulatory G-protein alpha subunit). Our results reveal that APP possesses autonomous regulatory capacity within its intracellular domain that promotes APP cell surface residence, precludes Aβ production, facilitates axodendritic development, and preserves cellular substrates of memory. Altogether, these events contribute to strengthening cognitive functions and are sufficient to modify the course of AD pathology.

Subchronic toxicity of bisphenol A on the architecture of spleen and hepatic trace metals and protein profile of adult male Wistar rats

Bisphenol A (BPA) is one of the widely used chemical as a plasticizer and regarded as endocrine disruptor because of its ability to derail body metabolic functions and adverse effect on the vital organs. The present work outlined the subchronic effect of low-dose BPA (10 mg/kg) on histology of spleen, level of hepatic trace metals, and hepatic protein profile of Wistar rats. To conduct the research work, animals were grouped into two categories (n = 5). Group 1 was labelled as the control group and group 2 was taken as an experimental group. Experimental group was exposed to low-dose BPA for 12 weeks. Histopathology of spleen highlighted dilation in splenic sinuses, follicle activation, followed by depopulation in the area of white pulp and red pulp in the experimental group. Iron staining revealed significant hemosiderosis in the experimental group when compared with the control group. Statistically significant decrease was noted in zinc and copper concentrations, while nonsignificant change was observed for magnesium concentration through atomic absorption spectroscopy. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was run for hepatic protein profiling, and as compared to control, elevated levels of different proteins were observed in the experimental group. It can be concluded from the above results that even low dose of BPA causes changes in the major organs of the body. Hence, it is suggested that BPA alternative should be used, so that public health status can be secured.

A role for CIM6P/IGF2 receptor in memory consolidation and enhancement

Cation-independent mannose-6-phosphate receptor, also called insulin-like growth factor two receptor (CIM6P/IGF2R), plays important roles in growth and development, but is also extensively expressed in the mature nervous system, particularly in the hippocampus, where its functions are largely unknown. One of its major ligands, IGF2, is critical for long-term memory formation and strengthening. Using CIM6P/IGF2R inhibition in rats and neuron-specific knockdown in mice, here we show that hippocampal CIM6P/IGF2R is necessary for hippocampus-dependent memory consolidation, but dispensable for learning, memory retrieval, and reconsolidation. CIM6P/IGF2R controls the training-induced upregulation of de novo protein synthesis, including increase of Arc, Egr1, and c-Fos proteins, without affecting their mRNA induction. Hippocampal or systemic administration of mannose-6-phosphate, like IGF2, significantly enhances memory retention and persistence in a CIM6P/IGF2R-dependent manner. Thus, hippocampal CIM6P/IGF2R plays a critical role in memory consolidation by controlling the rate of training-regulated protein metabolism and is also a target mechanism for memory enhancement.

Kainate Receptor Activation Enhances Amyloidogenic Processing of APP in Astrocytes

Kainic acid (KA) is an analogue of the excitatory neurotransmitter glutamate that, when injected systemically into adult rats, can trigger seizures and progressive neuronal loss in a manner that mirrors the neuropathology of human mesial temporal lobe epilepsy. However, biomolecular mechanisms responsible for the neuronal loss that occurs as a consequence of this treatment remains elusive. We have recently reported that toxicity induced by KA can partly be mediated by astrocyte-derived amyloid β (Aβ) peptides, which are critical in the development of Alzheimer's disease (AD). Nonetheless, little is known how KA can influence amyloid precursor protein (APP) levels and processing in astrocytes. Thus, in the present study using human U-373 astrocytoma and rat primary astrocytes, we evaluated the role of KA on APP metabolism. Our results revealed that KA treatment increased the levels of APP and its cleaved products (α-/β-CTFs) in cultured U-373 astrocytoma and primary astrocytes, without altering the cell viability. The cellular and secretory levels of Aβ_1-40/Aβ_1-42 were markedly increased in KA-treated astrocytes. We also demonstrated that the steady-state levels of APP-secretases were not altered but the activity of γ-secretase is enhanced in KA-treated U-373 astrocytoma. Furthermore, using selective receptor antagonists, we showed that the effects of KA is mediated by activation of kainate receptors and not NMDA or AMPA receptors. These results suggest that KA can enhance amyloidogenic processing of APP by activating its own receptor leading to increased production/secretion of Aβ-related peptides from activated astrocytes which may contribute to the pathogenesis of temporal lobe epilepsy.

A role for astrocyte-derived amyloid β peptides in the degeneration of neurons in an animal model of temporal lobe epilepsy

Kainic acid, an analogue of the excitatory neurotransmitter glutamate, can trigger seizures and neurotoxicity in the hippocampus and other limbic structures in a manner that mirrors the neuropathology of human temporal lobe epilepsy (TLE). However, the underlying mechanisms associated with the neurotoxicity remain unclear. Since amyloid-β (Aβ) peptides, which are critical in the development of Alzheimer's disease, can mediate toxicity by activating glutamatergic NMDA receptors, it is likely that the enhanced glutamatergic transmission that renders hippocampal neurons vulnerable to kainic acid treatment may involve Aβ peptides. Thus, we seek to establish what role Aβ plays in kainic acid-induced toxicity using in vivo and in vitro paradigms. Our results show that systemic injection of kainic acid to adult rats triggers seizures, gliosis and loss of hippocampal neurons, along with increased levels/processing of amyloid precursor protein (APP), resulting in the enhanced production of Aβ-related peptides. The changes in APP levels/processing were evident primarily in activated astrocytes, implying a role for astrocytic Aβ in kainic acid-induced toxicity. Accordingly, we showed that treating rat primary cultured astrocytes with kainic acid can lead to increased Aβ production/secretion without any compromise in cell viability. Additionally, we revealed that kainic acid reduces neuronal viability more in neuronal/astrocyte co-cultures than in pure neuronal culture, and this is attenuated by precluding Aβ production. Collectively, these results indicate that increased production/secretion of Aβ-related peptides from activated astrocytes can contribute to neurotoxicity in kainic acid-treated rats. Since kainic acid administration can lead to neuropathological changes resembling TLE, it is likely that APP/Aβ peptides derived from astrocytes may have a role in TLE pathogenesis.

Endosomal-Lysosomal Cholesterol Sequestration by U18666A Differentially Regulates Amyloid Precursor Protein (APP) Metabolism in Normal and APP-Overexpressing Cells

Amyloid β (Aβ) peptide, derived from amyloid precursor protein (APP), plays a critical role in the development of Alzheimer's disease. Current evidence indicates that altered levels or subcellular distribution of cholesterol can regulate Aβ production and clearance, but it remains unclear how cholesterol sequestration within the endosomal-lysosomal (EL) system can influence APP metabolism. Thus, we evaluated the effects of U18666A, which triggers cholesterol redistribution within the EL system, on mouse N2a cells expressing different levels of APP in the presence or absence of extracellular cholesterol and lipids provided by fetal bovine serum (FBS). Our results reveal that U18666A and FBS differentially increase the levels of APP and its cleaved products, the α-, β-, and η-C-terminal fragments, in N2a cells expressing normal levels of mouse APP (N2awt), higher levels of human wild-type APP (APPwt), or "Swedish" mutant APP (APPsw). The cellular levels of Aβ_1-40/Aβ_1-42 were markedly increased in U18666A-treated APPwt and APPsw cells. Our studies further demonstrate that APP and its cleaved products are partly accumulated in the lysosomes, possibly due to decreased clearance. Finally, we show that autophagy inhibition plays a role in mediating U18666A effects. Collectively, these results suggest that altered levels and distribution of cholesterol and lipids can differentially regulate APP metabolism depending on the nature of APP expression.

Ectopic upregulation of membrane-bound IL6R drives vascular remodeling in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by a progressive accumulation of pulmonary artery smooth muscle cells (PA-SMCs) in pulmonary arterioles leading to the narrowing of the lumen, right heart failure, and death. Although most studies have supported the notion of a role for IL-6/glycoprotein 130 (gp130) signaling in PAH, it remains unclear how this signaling pathway determines the progression of the disease. Here, we identify ectopic upregulation of membrane-bound IL-6 receptor (IL6R) on PA-SMCs in PAH patients and in rodent models of pulmonary hypertension (PH) and demonstrate its key role for PA-SMC accumulation in vitro and in vivo. Using Sm22a-Cre Il6rfl/fl, which lack Il6r in SM22A-expressing cells, we found that these animals are protected against chronic hypoxia-induced PH with reduced PA-SMC accumulation, revealing the potent pro-survival potential of membrane-bound IL6R. Moreover, we determine that treatment with IL6R-specific antagonist reverses experimental PH in two rat models. This therapeutic strategy holds promise for future clinical studies in PAH.

Effects of cholesterol transport inhibitor U18666A on APP metabolism in rat primary astrocytes

Amyloid β (Aβ) peptides generated from the amyloid precursor protein (APP) play an important role in the degeneration of neurons and development of Alzheimer's disease (AD). Current evidence indicates that high levels of cholesterol-which increase the risk of developing AD-can influence Aβ production in neurons. However, it remains unclear how altered level/subcellular distribution of cholesterol in astrocytes can influence APP metabolism. In this study, we evaluated the effects of cholesterol transport inhibitor U18666A-a class II amphiphile that triggers redistribution of cholesterol within the endosomal-lysosomal (EL) system-on APP levels and metabolism in rat primary cultured astrocytes. Our results revealed that U18666A increased the levels of the APP holoprotein and its cleaved products (α-/β-/η-CTFs) in cultured astrocytes, without altering the total levels of cholesterol or cell viability. The cellular levels of Aβ1-40 were also found to be markedly increased, while secretory levels of Aβ1-40 were decreased in U18666A-treated astrocytes. We further report a corresponding increase in the activity of the enzymes regulating APP processing, such as α-secretase, β-secretase, and γ-secretase as a consequence of U18666A treatment. Additionally, APP-cleaved products are partly accumulated in the lysosomes following cholesterol sequestration within EL system possibly due to decreased clearance. Interestingly, serum delipidation attenuated enhanced levels of APP and its cleaved products following U18666A treatment. Collectively, these results suggest that cholesterol sequestration within the EL system in astrocytes can influence APP metabolism and the accumulation of APP-cleaved products including Aβ peptides, which can contribute to the development of AD pathology.

Cooperative Repression of Insulin-Like Growth Factor Type 2 Receptor Translation by MicroRNA 195 and RNA-Binding Protein CUGBP1

Insulin-like growth factor type 2 (IGF2) receptor (IGF2R) recognizes mannose 6-phosphate-containing molecules and IGF2 and plays an important role in many pathophysiological processes, including gut mucosal adaptation. However, the mechanisms that control cellular IGF2R abundance are poorly known. MicroRNAs (miRNAs) and RNA-binding proteins (RBPs) critically regulate gene expression programs in mammalian cells by modulating the stability and translation of target mRNAs. Here we report that miRNA 195 (miR-195) and RBP CUG-binding protein 1 (CUGBP1) jointly regulate IGF2R expression at the posttranscriptional level in intestinal epithelial cells. Both miR-195 and CUGBP1 interacted with the 3' untranslated region (3'-UTR) of Igf2r mRNA, and the association of CUGBP1 with Igf2r mRNA enhanced miR-195 binding to Igf2r mRNA. Ectopically expressed CUGBP1 and miR-195 repressed IGF2R translation cooperatively without altering the stability of Igf2r mRNA. Importantly, the miR-195- and CUGBP1-repressed levels of cellular IGF2R led to a disruption in the structure of the trans-Golgi network. These findings indicate that IGF2R expression is controlled posttranscriptionally by two factors that associate with Igf2r mRNA and suggest that miR-195 and CUGBP1 dampen IGF signaling by inhibiting IGF2R translation.

Insulin-Like Growth Factor-II/Cation-Independent Mannose 6-Phosphate Receptor in Neurodegenerative Diseases

The insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein. Recent studies have advanced our understanding of the structure, ligand-binding properties, and trafficking of the IGF-II/M6P receptor. This receptor has been implicated in a variety of important cellular processes including growth and development, clearance of IGF-II, proteolytic activation of enzymes, and growth factor precursors, in addition to its well-known role in the delivery of lysosomal enzymes. The IGF-II/M6P receptor, distributed widely in the central nervous system, has additional roles in mediating neurotransmitter release and memory enhancement/consolidation, possibly through activating IGF-II-related intracellular signaling pathways. Recent studies suggest that overexpression of the IGF-II/M6P receptor may have an important role in regulating the levels of transcripts and proteins involved in the development of Alzheimer's disease (AD)-the prevalent cause of dementia affecting the elderly population in our society. It is reported that IGF-II/M6P receptor overexpression can increase the levels/processing of amyloid precursor protein leading to the generation of β-amyloid peptide, which is associated with degeneration of neurons and subsequent development of AD pathology. Given the significance of the receptor in mediating the transport and functioning of the lysosomal enzymes, it is being considered for therapeutic delivery of enzymes to the lysosomes to treat lysosomal storage disorders. Notwithstanding these results, additional studies are required to validate and fully characterize the function of the IGF-II/M6P receptor in the normal brain and its involvement in various neurodegenerative disorders including AD. It is also critical to understand the interaction between the IGF-II/M6P receptor and lysosomal enzymes in neurodegenerative processes, which may shed some light on developing approaches to detect and prevent neurodegeneration through the dysfunction of the receptor and the endosomal-lysosomal system.

miR-422a Inhibits Glioma Proliferation and Invasion by Targeting IGF1 and IGF1R

Glioma is a common type of malignant brain tumor characterized by aggressive metastasis capability. Recent evidence has suggested that noncoding RNAs, including microRNAs, have important functions in the pathophysiology of glioma development. In this study, we investigated the biological function of miR-422a in human glioma. We found that miR-422a was downregulated in glioma tissues. We also demonstrated that expression of miR-422a in glioma cells markedly suppressed cell proliferation, migration, and invasion. In addition, we identified insulin-like growth factor 1 (IGF1) and IGF1 receptor (IGF1R) as inhibitory targets of miR-422a in glioma cells. We established that the expression levels of miR-422a were negatively correlated with the expression levels of IGF1/IGF1R and the clinical parameters in glioma patients. An IGFR inhibitor, AG1024, completely blocked the activity of miR-442a on glioma cell proliferation and invasion, which further confirmed that miR-422a functions through IGF1 and IGF1R.