Endogenous APP derivatives oppositely modulate apoptosis through an autocrine loop

Neuroprotective effects of aromatic turmerone on activity deprivation-induced apoptosis in cerebellar granule neurons

Ar-turmerone, which is a major bioactive component found in the essential oil derived from Curcuma longa, has been reported to inhibit proliferation and induce apoptosis in cancer cell lines. Recently, ar-turmerone has been reported to increase the proliferation of neuronal stem cells, in contrast to its actions in cancer cells. These observations raise the possibility that ar-turmerone serves specific functions in neuronal cell lineages. However, the effects of ar-turmerone on postmitotic neurons remain elusive. In the present study, we investigated the neuroprotective functions of ar-turmerone in primary cerebellar granule neuronal cultures. We found that ar-turmerone increased the survival of neurons following activity deprivation. Consistently, the induction of cleaved caspase-3, a hallmark of apoptosis, was prevented by ar-turmerone, although neither the level of reactive oxygen species nor the mitochondrial membrane potential was affected. This study reports a neuroprotective function for ar-turmerone, providing new insights into the potential therapeutic applications of ar-turmerone for neurological disorders.

Substance P activates ADAM9 mRNA expression and induces α-secretase-mediated amyloid precursor protein cleavage

Altered levels of Substance P (SP), a neuropeptide endowed with neuroprotective and anti-apoptotic properties, were found in brain areas and spinal fluid of Alzheimer's disease (AD) patients. One of the hallmarks of AD is the abnormal extracellular deposition of neurotoxic beta amyloid (Aβ) peptides, derived from the proteolytic processing of amyloid precursor protein (APP). In the present study, we confirmed, the neurotrophic action of SP in cultured rat cerebellar granule cells (CGCs) and investigated its effects on APP metabolism. Incubation with low (5 mM) potassium induced apoptotic cell death of CGCs and amyloidogenic processing of APP, whereas treatment with SP (200 nM) reverted these effects via NK1 receptors. The non-amyloidogenic effect of SP consisted of reduction of Aβ(1-42), increase of sAPPα and enhanced α-secretase activity, without a significant change in steady-state levels of cellular APP. The intracellular mechanisms whereby SP alters APP metabolism were further investigated by measuring mRNA and/or steady-state protein levels of key enzymes involved with α-, β- and γ-secretase activity. Among them, Adam9, both at the mRNA and protein level, was the only enzyme to be significantly down-regulated following the induction of apoptosis (K5) and up-regulated after SP treatment. In addition to its neuroprotective properties, this study shows that SP is able to stimulate non-amyloidogenic APP processing, thereby reducing the possibility of generation of toxic Aβ peptides in brain.

Apoptosis and in vitro Alzheimer disease neuronal models

Alzheimer disease (AD) is a human neurodegenerative disease characterized by co-existence of extracellular senile plaques (SP) and neurofibrillary tangles (NFT) associated with an extensive neuronal loss, primarily in the cerebral cortex and hippocampus. Several studies suggest that caspase(s)-mediated neuronal death occurs in cellular and animal AD models as well as in human brains of affected patients, although an etiologic role of apoptosis in such neurodegenerative disorder is still debated. This review summarizes the experimental evidences corroborating the possible involvement of apoptosis in AD pathogenesis and discusses the usefulness of ad hoc devised in vitro approaches to study how caspase(s), amyloidogenic processing and tau metabolism might reciprocally interact leading to neuronal death.

Endogenous Aβ causes cell death via early tau hyperphosphorylation

Alzheimer's disease (AD) is characterized by Aβ overproduction and tau hyperphosphorylation. We report that an early, transient and site-specific AD-like tau hyperphosphorylation at Ser262 and Thr231 epitopes is temporally and causally related with an activation of the endogenous amyloidogenic pathway that we previously reported in hippocampal neurons undergoing cell death upon NGF withdrawal [Matrone, C., Ciotti, M.T., Mercanti, D., Marolda, R., Calissano, P., 2008b. NGF and BDNF signaling control amyloidogenic route and Ab production in hippocampal neurons. Proc. Natl. Acad. Sci. 105, 13138-13143]. Such tau hyperphosphorylation, as well as apoptotic death, is (i) blocked by 4G8 and 6E10 Aβ antibodies or by specific β and/or γ-secretases inhibitors; (ii) temporally precedes tau cleavage mediated by a delayed (6-12h after NGF withdrawal) activation of caspase-3 and calpain-I; (iii) under control of Akt-GSK3β-mediated signaling. Finally, we show that such site-specific tau hyperphosphorylation causes tau detachment from microtubules and an impairment of mitochondrial trafficking. These results depict, for the first time, a rapid interplay between endogenous Aβ and tau post-translational modifications which act co-ordinately to compromise neuronal functions in the same neuronal system, under physiological conditions as seen in AD brain.

Amyloid precursor protein regulates differentiation of human neural stem cells

Although amyloid beta (Abeta) deposition has been a hallmark of Alzheimer's disease (AD), the absence of a phenotype in the beta amyloid precursor protein (APP) knockout mouse, tends to detract our attention away from the physiological functions of APP. Although much attention has been focused on the neurotoxicity of Abeta, many studies suggest the involvement of APP in neuroplasticity. We found that secreted amyloid precursor protein (sAPP) increased the differentiation of human neural stem cells (hNSCs) in vitro, while an antibody-recognizing APP dose-dependently inhibited these activities. With a high dose of sAPP treatment or wild-type APP gene transfection, hNSCs were differentiated into astrocytes rather than neurons. In vivo, hNSCs transplanted into APP-transgenic mouse brain exhibited glial differentiation rather than neural differentiation. Our results suggest that APP regulates neural stem cell biology in the adult brain, and that altered APP metabolism in Down syndrome or AD may have implications for the pathophysiology of these diseases.

In vitro cultured neurons for molecular studies correlating apoptosis with events related to Alzheimer disease

This short review analyses the possible molecular events linking a general program of death such as apoptosis to highly specific intracellular pathways involving the function and degradation of two proteins--tau and amyloid precursor protein--which in their aggregated state constitute the hallmark of Alzheimer disease. By surveying the recent studies carried out in 'in vitro' neuronal cultures--with special emphasis to cerebellar granule neurons--the apparent correlation between onset of apoptosis, tau cleavage with formation of potential toxic fragments, and activation of an amyloidogenic route are discussed. Within this framework, proteasomes seem to play a crucial role upstream of the proteolytic cascade involving calpain(s) and caspase(s) by contributing to tau and amyloid precursor protein-altered breakdown and consequent tendency to aggregation of their degradation fragments. Thus, apoptotic death due to altered supply of anti apoptotic agents, neurotrophic factors, deafferentiation or other causes, may constitute a major trigger of the onset of Alzheimer disease.

Simultaneous changes in secretory amyloid precursor protein and β-amyloid peptide release from rat hippocampus by activation of muscarinic receptors

Amyloid deposits in Alzheimer's disease (AD) are composed of beta-amyloid peptides (Abeta) that are derived from the larger amyloid precursor protein (APP). A number of studies with various transfected cell lines demonstrated that either APP secretion or Abeta production could be modulated by specific muscarinic receptor activation. In the present study, we investigated the simultaneous changes of neurotrophic secretory APP (APPs) and neurotoxic Abeta release from rat hippocampus by activation of muscarinic receptors. The treatment with carbachol (10 microM-1 mM) resulted in the increased APPs release and simultaneously reduced Abeta production from the hippocampus slices in a concentration-dependent manner. The carbachol-stimulated APPs release was blocked by the nonselective M antagonist atropine and the M(1) antagonist pirenzepine, but was not significantly affected by the M(2) antagonist methoctramine, demonstrating that APP processing was regulated mainly via M(1) receptor in the rat hippocampus. The effect of carbachol-stimulated APPs secretion was further verified in CHOm(1) cells stably expressing human muscarinic M(1) receptors. These data suggest that selective M(1) receptor agonists might execute a dual action of increasing APPs release and decreasing Abeta formation to modify the AD process.

Potential use of stem cells in neuroreplacement therapies for neurodegenerative diseases

The use of stem cells for neuroreplacement therapy is no longer science fiction--it is science fact. We have succeeded in the development of neural and mesenchymal stem cell transplantation to produce neural cells in the brain. We have also seen improvement in cognitive function following stem cell transplantation in a memory-impaired aged animal model. These results promise a bright future for stem cell therapies in neurodegenerative diseases. Before we begin to think about clinical applications beyond the present preclinical studies, we have to consider the pathophysiological environment of individual diseases and weigh the factors that affect stem cell biology. Here, I not only review potential therapeutic applications of stem cell strategies in neurodegenerative diseases, but also discuss stem cell biology regarding factors that are altered under disease conditions.

Oxidative Stress Increases Expression and Activity of BACE in NT2 Neurons

Recently an aspartyl protease with beta-secretase activity called BACE was identified. In the present paper we showed that BACE is modulated by the oxidative stress product 4-hydroxynonenal (HNE). Exposure of NT(2) neurons to the two classical pro-oxidant stimuli ascorbate/FeSO(4) and H(2)O(2)/FeSO(4) resulted in a significant generation of HNE, which is temporally followed by an increased production of BACE protein levels. HNE mediated BACE induction is accompanied by a proportional elevation of carboxy-terminal fragments of amyloid precursor protein. Moreover, the direct relationship between BACE induction and lipid peroxidation products was strongly confirmed by the protection exerted by a short pretreatment with alpha-tocopherol, the most important antioxidant known to prevent the formation of aldehydic end-products of lipid peroxidation, including HNE. Our results support the hypothesis that oxidative stress and A beta production are strictly interrelated events and suggest that inhibition of BACE may have a therapeutic effect synergic with antioxidant compounds.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

Transfer of the apoptotic message in sister cultures of cerebellar neurons

In vitro cultured cerebellar granule cells (cgc) undergo apoptotic death when the depolarising concentration of KCl 25 mM is adjusted to 5 mM. We investigated whether the apoptotic message can be transferred from a group of neurons to neighbouring but separated sister cultures by resorting to two different culture systems, one based on the use of concentric dishes, the other employing the transwells. The extent of transferred death was 38.6% in the two dishes system and 31.3% in the transwell system. The transfer of such death-inducing signal(s), accompanied by chromatin condensation, was inhibited by actinomicin D. Preliminary experiments aimed at identifying such apoptosis-inducing signal(s) suggest the involvement of beta-amyloid fragment(s).