Insulin-like growth factor-I (IGF-I) is a pleiotropic polypeptide with a wide range of actions in both central and peripheral nervous sytems. Over the past few years, we studied the trophic as well as neuromodulatory roles of IGF-I in the brain. Accumulated evidence indicates that IGF-I, apart from regulating growth and development, protects neurons against cell death induced by amyloidogenic derivatives, glucose or serum deprivation via the activation of intracellular pathways implicating phosphatidylinositide 3/Akt kinase, winged-helix family of transcription factor FKHRL1 phosphorylation or production of free radicals. The effects of IGF-I on neuroprotection, glucose metabolism and activity-dependent plasticity suggest the potential usefulness of this growth factor or related mimetics in the treatment of Alzheimer's disease and other neurodegenerative disorders.
INSULIN-LIKE growth factor II/mannose-6-phosphate (IGF II/Man-6-P) receptors participate in the trafficking of lysosomal enzymes and also in the transduction of the effects of the growth factor via transmembrane-anchored receptor protein. During ligand-induced endocytosis, this receptor interacts with clathrin-associated protein (AP-2) which can lead to their assembly and subsequent transport in coated vesicles to the lysosomes. Only recently has it been suggested that AP-2 itself may also act as one of the receptor sites for inositol hexakisphosphate (IP). This evidence, together with autoradiographic data showing that [3H]IP6 binding sites in rat brain are similarly distributed to [125I]IGF II sites, led us to examine the possible interaction between IP6 and [125I]GF II receptor binding sites using an autoradiographic approach. Our results indicate that IP6, at μM concentrations, competes for [125I]IGF II, but not [125I]IGF I or [125I]insulin binding sites in the rat brain. These results, in keeping with other evidence, suggest that IP6 may be able to regulate the [125I]IGF II receptor binding sites either directly or indirectly, possibly through clathrin-associated AP-2 sites.
Evidence suggests that increased level/aggregation of β-amyloid (Aβ) peptide, together with enhanced phosphorylation/aggregation of tau protein, play a critical role in the development of Alzheimer's disease (AD), the leading cause of dementia in the elderly. At present, AD diagnosis is based primarily on cognitive assessment, neuroimaging, and immunological assays to detect altered levels/deposition of Aβ peptides and tau protein. While measurement of Aβ and tau in the cerebrospinal fluid/blood can indicate disease status, neuroimaging of aggregated Aβ and tau protein in the brain using positron emission tomography (PET) enable to monitor the pathological changes in AD patients. With advancements in nanomedicine, several nanoparticles, apart from drug-delivery, have been used as a diagnostic agent to identify more accurately changes in AD patients. Recently, we reported that FDA approved native PLGA nanoparticles can interact with Aβ to inhibit its aggregation/toxicity in cellular and animal models of AD. Here, we reveal that fluorescence labelled native PLGA following acute intracerebellar injection can identify majority of the immunostained Aβ as well as Congo red labelled neuritic plaques in the cortex of 5xFAD mice. Labelling of plaques by PLGA is apparent at 1 h, peak around 3 h and then start declining by 24 h after injection. No fluorescent PLGA was detected in the cerebellum of 5xFAD mice or in any brain regions of wild-type control mice following injection. These results provide the very first evidence that native PLGA nanoparticles can be used as a novel nano-theragnostic agent in the treatment as well as diagnosis of AD pathology.
In the preceding paper, we showed that GSK3β phosphorylates tau at S202, T231, S396, and S400 in vivo. Phosphorylation of S202 occurs without priming. Phosphorylation of T231, on the other hand, requires priming phosphorylation of S235. Similarly, priming phosphorylation of S404 is essential for the sequential phosphorylation of S400 and S396 by GSK3β. The priming kinase that phosphorylates tau at S235 and S404 in the brain is not known. In this study, we find that in HEK-293 cells cotransfected with tau, GSK3β, and Cdk5, Cdk5 phosphorylates tau at S202, S235, and S404. S235 phosphorylation enhances GSK3β-catalyzed T231 phosphorylation. Similarly, Cdk5 by phosphorylating S404 stimulates phosphorylation of S400 and S396 by GSK3β. These data indicate that Cdk5 primes tau for GSK3β in intact cells. To evaluate if Cdk5 primes tau for GSK3β in mammalian brain, we examined localizations of Cdk5, tau, and GSK3β in rat brain. We also analyzed the interaction of Cdk5 with tau and GSK3β in brain microtubules. We found that Cdk5, GSK3β, and tau are virtually colocalized in rat brain cortex. When bovine brain microtubules are analyzed by FPLC gel filtration, Cdk5, GSK3β, and tau coelute within an ∼450 kDa complex. From the fractions containing the ∼450 kDa complex, tau, Cdk5, and GSK3β co-immunoprecipitate with each other. In HEK-293 cells transfected with tau, Cdk5, and GSK3β in different combinations, tau binds to Cdk5 in a manner independent of GSK3β and to GSK3β in a manner independent of Cdk5. However, Cdk5 and GSK3β bind to each other only in the presence of tau, suggesting that tau connects Cdk5 and GSK3β. Our results suggest that in the brain, tau, Cdk5, and GSK3β are components of an ∼450 kDa complex. Within the complex, Cdk5 phosphorylates tau at S235 and primes it for phosphorylation of T231 by GSK3β. Similarly, Cdk5 by phosphorylating tau at S404 primes tau for a sequential phosphorylation of S400 and S396 by GSK3β.
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.
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