Neurotoxic and neuroprotective effects of glutamate are enhanced by introduction of amyloid precursor protein cDNA

How Can Insulin Resistance Cause Alzheimer's Disease?

Alzheimer's disease (AD) is a neurodegenerative disorder associated with cognitive decline. Despite worldwide efforts to find a cure, no proper treatment has been developed yet, and the only effective countermeasure is to prevent the disease progression by early diagnosis. The reason why new drug candidates fail to show therapeutic effects in clinical studies may be due to misunderstanding the cause of AD. Regarding the cause of AD, the most widely known is the amyloid cascade hypothesis, in which the deposition of amyloid beta and hyperphosphorylated tau is the cause. However, many new hypotheses were suggested. Among them, based on preclinical and clinical evidence supporting a connection between AD and diabetes, insulin resistance has been pointed out as an important factor in the development of AD. Therefore, by reviewing the pathophysiological background of brain metabolic insufficiency and insulin insufficiency leading to AD pathology, we will discuss how can insulin resistance cause AD.

Decoding Alzheimer's disease from perturbed cerebral glucose metabolism: implications for diagnostic and therapeutic strategies

Alzheimer's disease (AD) is an age-related devastating neurodegenerative disorder, which severely impacts on the global economic development and healthcare system. Though AD has been studied for more than 100 years since 1906, the exact cause(s) and pathogenic mechanism(s) remain to be clarified. Also, the efficient disease-modifying treatment and ideal diagnostic method for AD are unavailable. Perturbed cerebral glucose metabolism, an invariant pathophysiological feature of AD, may be a critical contributor to the pathogenesis of this disease. In this review, we firstly discussed the features of cerebral glucose metabolism in physiological and pathological conditions. Then, we further reviewed the contribution of glucose transportation abnormality and intracellular glucose catabolism dysfunction in AD pathophysiology, and proposed a hypothesis that multiple pathogenic cascades induced by impaired cerebral glucose metabolism could result in neuronal degeneration and consequently cognitive deficits in AD patients. Among these pathogenic processes, altered functional status of thiamine metabolism and brain insulin resistance are highly emphasized and characterized as major pathogenic mechanisms. Finally, considering the fact that AD patients exhibit cerebral glucose hypometabolism possibly due to impairments of insulin signaling and altered thiamine metabolism, we also discuss some potential possibilities to uncover diagnostic biomarkers for AD from abnormal glucose metabolism and to develop drugs targeting at repairing insulin signaling impairment and correcting thiamine metabolism abnormality. We conclude that glucose metabolism abnormality plays a critical role in AD pathophysiological alterations through the induction of multiple pathogenic factors such as oxidative stress, mitochondrial dysfunction, and so forth. To clarify the causes, pathogeneses and consequences of cerebral hypometabolism in AD will help break the bottleneck of current AD study in finding ideal diagnostic biomarker and disease-modifying therapy.

In vitro modelling of Alzheimer's disease: degeneration and cell death induced by viral delivery of amyloid and tau

With increasing life expectancy, Alzheimer's disease (AD) and other dementias pose an increasing and as yet unresolved health problem. A variety of cellular models of AD has helped to decipher some key aspects of amyloid and tau related degeneration. The initial approach of extracellular applications of synthetic peptides has now been replaced by the introduction of amyloid precursor protein (APP) and tau genes. In the present study adenoviral transductions were exploited for gene delivery into primary rat hippocampal and dorsal root ganglion (DRG) cultures to enable comparative and mechanistic studies at the cellular level and subsequent drug testing. Time lapse experiments revealed a different pattern of cell death: apoptotic-like for APP whereas tau positive cells joined and formed clusters. Mutated human APP or tau expression caused accelerated neuronal damage and cell death (cf. EGFP: -50% for APP at 5 days; -40% for tau at 3 days). This reduction in viability was preceded by decreased excitability, monitored via responses to depolarising KCl-challenges in Ca(2+) imaging experiments. Additionally, both transgenes reduced neurite outgrowth in DRG neurones. Treatment studies confirmed that APP induced-damage can be ameliorated by β- and γ-secretase inhibitors (providing protection to 60-100% of control levels), clioquinol (80%) and lithium (100%); while anti-aggregation treatments were beneficial for tau-induced damage (60-90% recovery towards controls). Interestingly, caffeine was the most promising drug candidate for therapeutic intervention with high efficacy in both APP (77%) and tau-induced models (72% recovery). Overall, these cellular models offer advantages for mechanistic studies and target identification in AD and related disorders.

Repetitive induction of late-phase LTP produces long-lasting synaptic enhancement accompanied by synaptogenesis in cultured hippocampal slices

Long-term plasticity of synaptic transmission is assumed to underlie the formation of long-term memory. Although the cellular mechanisms underlying short-term plasticity have been analyzed in detail, the mechanisms underlying the transformation from short-term to long-term plasticity remain largely unrevealed. We propose the novel long-lasting phenomenon as a model system for the analysis of long-term plasticity. We previously reported that the repetitive activation of cAMP-dependent protein kinase (PKA) by forskolin application led to an enhancement in synaptic strength coupled with synaptogenesis that lasted more than 3 weeks in cultured rat hippocampal slices. To elucidate whether this long-lasting synaptic enhancement depended on the induction of long-term potentiation (LTP) or on the pharmacological effect of forskolin, we applied glutamate (Glu) and correlated its dose with the production of the long-lasting synaptic enhancement. When the dose of Glu was low (10, 30 muM), only transient excitation or early-phase LTP (E-LTP) was induced by a single application and no long-lasting synaptic enhancement was produced by three applications. When the dose was raised to 100 or 300 muM, late-phase LTP (L-LTP) was induced by a single application and long-lasting synaptic enhancement was produced by three applications. The Glu-produced enhancement was accompanied by an increase in the frequency (but not the amplitude) of miniature EPSC and the number of synaptic structures. The enhancement depended on the interval of repetition and protein synthesis immediately after the Glu applications. These results indicate that the repetitive induction of L-LTP, but not E-LTP or transient excitation, triggers cellular processes leading to the long-lasting synaptic enhancement and the formation of new synapses.

Amyloid-beta precursor protein expression and modulation in human embryonic stem cells: a novel role for human chorionic gonadotropin

The amyloid-beta precursor protein (AbetaPP) is a ubiquitously expressed adhesion and neuritogenic protein whose processing has previously been shown to be regulated by reproductive hormones including the gonadotropin luteinizing hormone (LH) in human neuroblastoma cells. We report for the first time the expression of AbetaPP in human embryonic stem (hES) cells at the mRNA and protein levels. Using N- and C-terminal antibodies against AbetaPP, we detected both the mature and immature forms of AbetaPP as well as truncated variants ( approximately 53kDa, 47kDa, and 29kDa) by immunoblot analyses. Expression of AbetaPP is regulated by both the stemness of the cells and pregnancy-associated hormones. Addition of human chorionic gonadotropin, the fetal equivalent of LH that is dramatically elevated during pregnancy, markedly increased the expression of all AbetaPP forms. These results indicate a critical molecular signaling link between the hormonal environment of pregnancy and the expression of AbetaPP in hES cells that is suggestive of an important function for this protein during early human embryogenesis prior to the formation of neural precursor cells.

Structure and functions of the human amyloid precursor protein: the whole is more than the sum of its parts

The amyloid precursor protein (APP) is a transmembrane protein that plays major roles in the regulation of several important cellular functions, especially in the nervous system, where it is involved in synaptogenesis and synaptic plasticity. The secreted extracellular domain of APP, sAPPalpha, acts as a growth factor for many types of cells and promotes neuritogenesis in post-mitotic neurons. Alternative proteolytic processing of APP releases potentially neurotoxic species, including the amyloid-beta (Abeta) peptide that is centrally implicated in the pathogenesis of Alzheimer's disease (AD). Reinforcing this biochemical link to neuronal dysfunction and neurodegeneration, APP is also genetically linked to AD. In this review, we discuss the biological functions of APP in the context of tissue morphogenesis and restructuring, where APP appears to play significant roles both as a contact receptor and as a diffusible factor. Structural investigation of APP, which is necessary for a deeper understanding of its roles at a molecular level, has also been advancing rapidly. We summarize recent progress in the determination of the structure of isolated APP fragments and of the conformations of full-length sAPPalpha, in both monomeric and dimeric states. The potential role of APP dimerization for the regulation of its biological functions is also discussed.

Restraint stress induces beta-amyloid precursor protein mRNA expression in the rat basolateral amygdala

Several studies have shown the involvement of beta-amyloid precursor proteins (APP) isoforms in physiological process like development of the central nervous system (CNS), functional roles in mature brain, and in pathological process like Alzheimer's disease, neuronal experimental damage, and stress, among others. However, the APP functions are still not clear. In the brain, APP(695) isoform is predominantly found in neurons while APP(751/770) isoforms are predominantly found in astroglial cells and have been associated to neurodegenerative processes. Acute or chronic stress in rats may trigger specific response mechanisms in several brain areas such as amygdala, hippocampus and cortex with the involvement of multiple neurotransmitters. Chronic stress may also induce neuronal injury in rat hippocampus. In situ hybridization (ISH) was used to investigate the expression of APP(695) and APP(751/770) mRNA in amygdala and hippocampus of male Wistar rats (n=4-6 per group) after acute (2 or 6h) or chronic (2h daily/7 days or 6h daily/21 days) restraint stress. Only the APP(695) mRNA expression was significantly increased in the basolateral amygdaloid nuclei following acute or chronic restraint. No APP isoform changed in hippocampus after any stress condition. These results suggest that restraint stress induces changes in gene expression of APP(695) in basolateral amygdaloid nucleus, an area related to stress response.

Upregulation and antiapoptotic role of endogenous Alzheimer amyloid precursor protein in dorsal root ganglion neurons

The amyloid precursor protein (APP) is a transmembrane protein whose abnormal processing is associated with the pathogenesis of Alzheimer's disease. In this study, we examined the expression and role of cell-associated APP in primary dorsal root ganglion (DRG) neurons. When dissociated DRG cells prepared from mouse embryos were treated with nerve growth factor (NGF), neuronal APP levels were transiently elevated. DRG neurons treated with an antibody against cell surface APP failed to mature and underwent apoptosis. When NGF was withdrawn from the cultures after a 36-h NGF treatment, virtually all neurons underwent apoptosis by 48 h. During the course of apoptosis, some neurons with intact morphology contained increased levels of APP immunoreactivity, whereas the APP levels were greatly reduced in apoptotic neurons. Furthermore, affected neurons contained immunoreactivities for activated caspase-3, a caspase-cleaved APP fragment (APPDeltaC31), and Abeta. Downregulation of endogenous APP expression by treatment with an APP antisense oligodeoxynucleotide significantly increased the number of apoptotic neurons in NGF-deprived DRG cultures. Furthermore, overexpression of APP by adenovirus vector-mediated gene transfer reduced the number of apoptotic neurons deprived of NGF. These results suggest that endogenous APP is upregulated to exert an antiapoptotic effect on neurotrophin-deprived DRG neurons and subsequently undergoes caspase-dependent proteolysis.