Up-regulation of cation-independent mannose 6-phosphate receptor and endosomal-lysosomal markers in surviving neurons after 192-IgG-saporin administrations into the adult rat brain

Significance of native PLGA nanoparticles in the treatment of Alzheimer's disease pathology

Alzheimer's disease (AD) is believed to be triggered by increased levels/aggregation of β-amyloid (Aβ) peptides. At present, there is no effective disease-modifying treatment for AD. Here, we evaluated the therapeutic potential of FDA-approved native poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles on Aβ aggregation and in cellular/animal models of AD. Our results showed that native PLGA can not only suppress the spontaneous aggregation but can also trigger disassembly of preformed Aβ aggregates. Spectroscopic studies, molecular dynamics simulations and biochemical analyses revealed that PLGA, by interacting with the hydrophobic domain of Aβ_1-42, prevents a conformational shift towards the β-sheet structure, thus precluding the formation and/or triggering disassembly of Aβ aggregates. PLGA-treated Aβ samples can enhance neuronal viability by reducing phosphorylation of tau protein and its associated signaling mechanisms. Administration of PLGA can interact with Aβ aggregates and attenuate memory deficits as well as Aβ levels/deposits in the 5xFAD mouse model of AD. PLGA can also protect iPSC-derived neurons from AD patients against Aβ toxicity by decreasing tau phosphorylation. These findings provide unambiguous evidence that native PLGA, by targeting different facets of the Aβ axis, can have beneficial effects in mouse neurons/animal models as well as on iPSC-derived AD neurons - thus signifying its unique therapeutic potential in the treatment of AD pathology.

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PLGA nanoparticles by interacting with hydrophobic domain inhibit Aβ aggregation.

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PLGA-mediated inhibition of Aβ aggregation can increase viability of mouse neurons.

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PLGA administration can attenuate cognitive deficits/pathology in 5xFAD AD mouse model.

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PLGA can protect iPSC-derived neurons from AD patients against Aβ toxicity.

Blood-brain barrier delivery for lysosomal storage disorders with IgG-lysosomal enzyme fusion proteins

The majority of lysosomal storage diseases affect the brain. Treatment of the brain with intravenous enzyme replacement therapy is not successful, because the recombinant lysosomal enzymes do not cross the blood-brain barrier (BBB). Biologic drugs, including lysosomal enzymes, can be re-engineered for BBB delivery as IgG-enzyme fusion proteins. The IgG domain of the fusion protein is a monoclonal antibody directed against an endogenous receptor-mediated transporter at the BBB, such as the insulin receptor or the transferrin receptor. This receptor transports the IgG across the BBB, in parallel with the endogenous receptor ligand, and the IgG acts as a molecular Trojan horse to ferry into brain the lysosomal enzyme genetically fused to the IgG. The IgG-enzyme fusion protein is bi-functional and retains both high affinity binding for the BBB receptor, and high lysosomal enzyme activity. IgG-lysosomal enzymes are presently in clinical trials for treatment of the brain in Mucopolysaccharidosis.

Unconjugated PLGA nanoparticles attenuate temperature-dependent β-amyloid aggregation and protect neurons against toxicity: implications for Alzheimer's disease pathology

Conversion of β-amyloid (Aβ) peptides from soluble random-coil to aggregated protein enriched with β-sheet-rich intermediates has been suggested to play a role in the degeneration of neurons and development of Alzheimer’s disease (AD) pathology. Aggregation of Aβ peptide can be prompted by a variety of environmental factors including temperature which can influence disease pathogenesis. Recently, we reported that FDA-approved unconjugated poly (d,l-lactide-co-glycolide) (PLGA) nanoparticles can have beneficial effects in cellular and animal models of AD by targeting different facets of the Aβ axis. In this study, using biochemical, structural and spectroscopic analyses, we evaluated the effects of native PLGA on temperature-dependent Aβ aggregation and its ability to protect cultured neurons from degeneration. Our results show that the rate of spontaneous Aβ_1–42 aggregation increases with a rise in temperature from 27 to 40 °C and PLGA with 50:50 resomer potently inhibits Aβ aggregation at all temperatures, but the effect is more profound at 27 °C than at 40 °C. It appears that native PLGA, by interacting with the hydrophobic domain of Aβ_1–42, prevents a conformational shift towards β-sheet structure, thus precluding the formation of Aβ aggregates. Additionally, PLGA triggers disassembly of matured Aβ_1–42 fibers at a faster rate at 40 °C than at 27 °C. PLGA-treated Aβ samples can significantly enhance viability of cortical cultured neurons compared to neurons treated with Aβ alone by attenuating phosphorylation of tau protein. Injection of native PLGA is found to influence the breakdown/clearance of Aβ peptide in the brain. Collectively, these results suggest that PLGA nanoparticles can inhibit Aβ aggregation and trigger disassembly of Aβ aggregates at temperatures outside the physiological range and can protect neurons against Aβ-mediated toxicity thus validating its unique therapeutic potential in the treatment of AD 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.

HIV Infection Induces Extracellular Cathepsin B Uptake and Damage to Neurons

HIV-associated neurocognitive disorders prevail in 20-50 percent of infected individuals. Macrophages transmigrate through the blood brain barrier during HIV-1 infection, triggering neuronal dysfunction. HIV-infected macrophages secrete cathepsin B (CATB), and serum amyloid p component (SAPC), inducing neuronal apoptosis by an unknown mechanism. We hypothesized that HIV infection facilitates CATB/SAPC secretion from macrophages followed by neuronal internalization, promoting dysfunction. SK-N-SH neuronal cells were exposed to active recombinant histidine-tagged cathepsin B (His-CATB). His-CATB entry was tracked by intracellular flow cytometry, and neuronal dysfunction was verified by western blot. Macrophage-derived extracellular vesicles (EVs) were tested for the presence of CATB and SAPC. Neurons internalized His-CATB, an effect that was partially decreased by pre-treatment with anti-CATB antibody. Pre-treatment with CATB and SAPC antibodies decreased cleavage of caspase-3 and restored synaptophysin in neurons. Neurons exposed to macrophage-conditioned media differentially internalized His-CATB, dependent on the HIV replication levels. Finally, CATB and SAPC were secreted in EVs. We report for the first time that CATB is secreted from macrophages both free and in EVs, and is internalized by neurons. Moreover, HIV-replication levels modulate the amount of CATB neuronal uptake, and neuronal dysfunction can be decreased with CATB antibodies. In conclusion, the CATB/SAPC complex represents a novel target against HIV-associated neurocognitive disorders.

The mannose 6-phosphate/insulin-like growth factor 2 receptor mediates plasminogen-induced efferocytosis

The plasminogen system is harnessed in a wide variety of physiological processes, such as fibrinolysis, cell migration, or efferocytosis; and accordingly, it is essential upon inflammation, tissue remodeling, wound healing, and for homeostatic maintenance in general. Previously, we identified a plasminogen receptor in the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222). Here, we demonstrate by means of genetic knockdown, knockout, and rescue approaches combined with functional studies that M6P/IGF2R is up-regulated on the surface of macrophages, recognizes plasminogen exposed on the surface of apoptotic cells, and mediates plasminogen-induced efferocytosis. The level of uptake of plasminogen-coated apoptotic cells inversely correlates with the TNF-α production by phagocytes indicating tissue clearance without inflammation by this mechanism. Our results reveal an up-to-now undetermined function of M6P/IGF2R in clearance of apoptotic cells, which is crucial for tissue homeostasis.

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.

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

Amyloid β (Aβ) peptides originating from amyloid precursor protein (APP) in the endosomal-lysosomal compartments play a critical role in the development of Alzheimer's disease (AD), the most common type of senile dementia affecting the elderly. Since insulin-like growth factor II (IGF-II) receptors facilitate the delivery of nascent lysosomal enzymes from the trans-Golgi network to endosomes, we evaluated their role in APP metabolism and cell viability using mouse fibroblast MS cells deficient in the murine IGF-II receptor and corresponding MS9II cells overexpressing the human IGF-II receptors. Our results show that IGF-II receptor overexpression increases the protein levels of APP. This is accompanied by an increase of β-site APP-cleaving enzyme 1 levels and an increase of β- and γ-secretase enzyme activities, leading to enhanced Aβ production. At the cellular level, IGF-II receptor overexpression causes localization of APP in perinuclear tubular structures, an increase of lipid raft components, and increased lipid raft partitioning of APP. Finally, MS9II cells are more susceptible to staurosporine-induced cytotoxicity, which can be attenuated by β-secretase inhibitor. Together, these results highlight the potential contribution of IGF-II receptor to AD pathology not only by regulating expression/processing of APP but also by its role in cellular vulnerability.

Increased levels and activity of cathepsins B and D in kainate-induced toxicity

Administration of kainic acid induces acute seizures that result in the loss of neurons, gliosis and reorganization of mossy fiber pathways in the hippocampus resembling those observed in human temporal lobe epilepsy. Although these structural changes have been well characterized, the mechanisms underlying the degeneration of neurons following administration of kainic acid remain unclear. Since the lysosomal enzymes, cathepsins B and D, are known to be involved in the loss of neurons and clearance of degenerative materials in a variety of experimental conditions, we evaluated their potential roles in kainic acid-treated rats. In parallel, we also measured the levels and expression of insulin-like growth factor-II/mannose 6-phosphate (IGF-II/M6P) receptors, which mediate the intracellular trafficking of these enzymes, in kainic acid-treated rats. Our results showed that systemic administration of kainic acid evoked severe loss of neurons along with hypertrophy of astrocytes and microglia in the hippocampus of the adult rat brain. The levels and activity of cathepsins B and D increased with time in the hippocampus of kainic acid-treated rats compared to the saline-injected control animals. The expression of both cathepsins B and D, as evident by immunolabeling studies, was also markedly increased in activated astrocytes and microglia of the kainic acid-treated rats. Additionally, cytosolic levels of the cathepsins were enhanced along with cytochrome c and to some extent Bax in the hippocampus in kainic acid-treated rats. These changes were accompanied by appearance of cleaved caspase-3-positive neurons in the hippocampus of kainic acid-treated animals. The levels of IGF-II/M6P receptors, on the other hand, were not significantly altered, but these receptors were found to be present in a subset of reactive astrocytes following administration of kainic acid. These results, taken together, suggest that enhanced levels/expression and activity of lysosomal enzymes may have a role in the loss of neurons and/or clearance of degenerative materials observed in kainic acid-treated rats.

Role of cathepsin D in U18666A-induced neuronal cell death: potential implication in Niemann-Pick type C disease pathogenesis

Cathepsin D is an aspartyl protease that plays a crucial role in normal cellular functions and in a variety of neurodegenerative disorders, including Niemann-Pick type C (NPC) disease, which is characterized by intracellular accumulation of cholesterol and glycosphingolipids in many tissues, including the brain. There is evidence that the level and activity of cathepsin D increased markedly in vulnerable neurons in NPC pathology, but its involvement in neurodegeneration remains unclear. In the present study, using mouse hippocampal cultured neurons, we evaluated the significance of cathepsin D in toxicity induced by U18666A, a class II amphiphile, which triggers cell death by impairing the trafficking of cholesterol, as observed in NPC pathology. Our results showed that U18666A-mediated toxicity is accompanied by an increase in cathepsin D mRNA and enzyme activity but a decrease in the total peptide content. The cytosolic level of cathepsin D, on the other hand, was increased along with cytochrome c and activated caspase-3 in U18666A-treated neurons. The cathepsin D inhibitor, pepstatin A, partially protected neurons against toxicity by attenuating these signaling mechanisms. Additionally, down-regulation of cathepsin D level prevented, whereas overexpression of the protease increased, vulnerability of cultured N2a cells to U18666A-induced toxicity. We also showed that extracellular cathepsin D from U18666A-treated neurons or application of exogenous enzyme can induce neurotoxicity by activating the autophagic pathway. These results suggest that increased release/activation of cathepsin D can trigger neurodegeneration and possibly development of NPC pathology. Thus, targeting cathepsin D level/activity may provide a new therapeutic opportunity for the treatment of NPC pathology.

P-Type Lectins: Cation-Dependent Mannose-6-Phosphate Receptor

In eukaryotic cells, post-translational modification of secreted proteins and intracellular protein transport between organelles are ubiquitous features. One of the most studied systems is the N-linked glycosylation pathway in the synthesis of secreted glycoproteins (Schrag et al. 2003). The N-linked glycoproteins are subjected to diverse modifications and are transported through ER and Golgi apparatus to their final destinations in- and outside the cell. Incorporation of cargo glycoproteins into transport vesicles is mediated by transmembrane cargo receptors, which have been identified as intracellular lectins. For example, mannose 6-phosphate receptors (Ghosh et al. 2003) function as a cargo receptor for lysosomal proteins in the trans-Golgi network, whereas ERGIC-53 (Zhang et al. 2003) and its yeast orthologs Emp46/47p (Sato and Nakano 2002) are transport lectins for glycoproteins that are transported out of ER.

Increased activity and altered subcellular distribution of lysosomal enzymes determine neuronal vulnerability in Niemann-Pick type C1-deficient mice

Niemann-Pick disease type C (NPC), caused by mutations in the Npc1 or Npc2 genes, is a progressive neurodegenerative disorder characterized by intracellular accumulation/redistribution of cholesterol in a number of tissues including the brain. This is accompanied by a severe loss of neurons in selected brain regions. In this study, we evaluated the role of lysosomal enzymes, cathepsins B and D, in determining neuronal vulnerability in NPC1-deficient (Npc1(-/-)) mouse brains. Our results showed that Npc1(-/-) mice exhibit an age-dependent degeneration of neurons in the cerebellum but not in the hippocampus. The cellular level/expression and activity of cathepsins B and D are increased more predominantly in the cerebellum than in the hippocampus of Npc1(-/-) mice. In addition, the cytosolic levels of cathepsins, cytochrome c, and Bax2 are higher in the cerebellum than in the hippocampus of Npc1(-/-) mice, suggesting a role for these enzymes in the degeneration of neurons. This suggestion is supported by our observation that degeneration of cultured cortical neurons treated with U18666A, which induces an NPC1-like phenotype at the cellular level, can be attenuated by inhibition of cathepsin B or D enzyme activity. These results suggest that the increased level/activity and altered subcellular distribution of cathepsins may be associated with the underlying cause of neuronal vulnerability in Npc1(-/-) brains. Therefore, their inhibitors may have therapeutic potential in attenuating NPC pathology.

Hepatic gap junctions in the hepatocarcinogen-resistant DRH rat

Although the gap junction or connexin (Cx) is considered to be a tumor-suppressor, it is also required for tumor promotion. Therefore, we examined hepatic gap junctions in hepatocarcinogen-resistant (DRH) rats. Specifically, we investigated gap junction structure and Cx32 expression during normal conditions and in response to a hepatocarcinogen, 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB). On a basal diet without 3'-MeDAB, hepatic gap junctions and Cx32 protein expression were greater in DRH rats than in control Donryu rats, as evidenced by morphometry, immunohistochemistry and immunoblotting. On a diet containing 3'-MeDAB, gap junctions and expressed Cx32 were increased significantly in Donryu rats, but not in DRH rats. In this condition, Donryu rats lost weight but DRH rats increased relative liver weight. After 3'-MeDAB treatment, cathepsin D expression in hepatocytes was significantly increased only in Donryu rats, indicating that DRH rats were less susceptible to 3'-MeDAB. The abundance of mitogen-activated protein kinase, some constituent of which might be associated with the degree of Cx protein phosphorylation, was reduced to a greater extent in Donryu than in DRH rats after 3'-MeDAB treatment. The resistance of DRH rats to carcinogenesis may be due partially to their stabilized gap junctions, which could coordinate metabolic coupling to evade 3'-MeDAB toxicity.

The effect of serum withdrawal on the protein profile of quiescent human dermal fibroblasts in primary cell culture

The effect of serum deprivation on proliferating cells is well known, in contrast its role on primary cell cultures, at confluence, has not been deeply investigated. Therefore, in order to explore the response of quiescent cells to serum deprivation, ubiquitous mesenchymal cells, as normal human dermal fibroblasts, were grown, for 48 h after confluence, in the presence or absence of 10% FBS. Fibroblast behaviour (i.e. cell morphology, cell viability, ROS production and elastin synthesis) was evaluated morphologically and biochemically. Moreover, the protein profile was investigated by 2-DE and differentially expressed proteins were identified by MS. Serum withdrawal caused cell shrinkage but did not significantly modify the total cell number. ROS production, as evaluated by the dihydroethidium (DH2) probe, was increased after serum deprivation, whereas elastin synthesis, measured by a colorimetric method, was markedly reduced in the absence of serum. By proteome analysis, 41 proteins appeared to significantly change their expression, the great majority of protein changes were related to the cytoskeleton, the stress response and the glycolytic pathway. Data indicate that human dermal fibroblasts in primary cell culture can adapt themselves to environmental changes, without significantly altering cell viability, at least after a few days of treatment, even though serum withdrawal represents a stress condition capable to increase ROS production, to influence cell metabolism and to interfere with cell behaviour, favouring the expression of several age-related features.

Altered levels and distribution of IGF-II/M6P receptor and lysosomal enzymes in mutant APP and APP + PS1 transgenic mouse brains

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor participates in the trafficking of lysosomal enzymes from the trans-Golgi network or the cell surface to lysosomes. In Alzheimer's disease (AD) brains, marked up-regulation of the lysosomal system in vulnerable neuronal populations has been correlated with altered metabolic functions. To establish whether IGF-II/M6P receptors and lysosomal enzymes are altered in the brain of transgenic mice harboring different familial AD mutations, we measured the levels and distribution of the receptor and lysosomal enzymes cathepsins B and D in select brain regions of transgenic mice overexpressing either mutant presenilin 1 (PS1; PS1(M146L+L286V)), amyloid precursor protein (APP; APP(KM670/671NL+V717F)) or APP+PS1 (APP(KM670/671NL+V717F)+PS1(M146L+L286V)) transgenes. Our results revealed that levels and expression of the IGF-II/M6P receptor and lysosomal enzymes are increased in the hippocampus and frontal cortex of APP and APP+PS1, but not in PS1, transgenic mouse brains compared with wild-type controls. The changes were more prominent in APP+PS1 than in APP single transgenic mice. Additionally, all beta-amyloid-containing neuritic plaques in the hippocampal and cortical regions of APP and APP+PS1 transgenic mice were immunopositive for both lysosomal enzymes, whereas only a subset of the plaques displayed IGF-II/M6P receptor immunoreactivity. These results suggest that up-regulation of the IGF-II/M6P receptor and lysosomal enzymes in neurons located in vulnerable regions reflects an altered functioning of the endosomal-lysosomal system which may be associated with the increased intracellular and/or extracellular A beta deposits observed in APP and APP+PS1 transgenic mouse brains.

Cellular distribution of gamma-secretase subunit nicastrin in the developing and adult rat brains

Nicastrin and presenilin 1 are integral components of the high molecular weight gamma-secretase complexes that regulate proteolytic processing of various type I membrane proteins including amyloid precursor protein and Notch. At present, there is little information regarding the cellular distribution of nicastrin in the developing or adult rat brain. We report here, using immunoblotting and immunohistochemical methods, that nicastrin in the adult rat brain is widely expressed and co-localized with presenilin 1 in select neuronal populations within all major areas, including the basal forebrain, striatum, cortex, hippocampus, amygdala, thalamus, hypothalamus, cerebellum and brainstem. We also observed dense neuropil labeling in many regions in the brain, suggesting that nicastrin gets transported to dendrites and/or axon terminals in the central nervous system. The levels of nicastrin are found to be relatively high at the early stages of postnatal development and then declined gradually to reach the adult profile. At the cellular level, nicastrin is localized predominantly in neuronal cell bodies at early postnatal stages, but is apparent both in cell bodies and dendrites/neuropil in all brain regions at the later stages. The regulation of nicastrin expression and localization during development and its distribution in a wide spectrum of neurons in the postnatal and adult rat brains provide an anatomical basis to suggest a multifunctional role for the gamma-secretase complex in the developing and adult rat brains.