Aggregated beta amyloid peptide 1-40 decreases Ca2+- and cholinergic receptor-mediated phosphoinositide degradation by alteration of membrane and cytosolic phospholipase C in brain cortex

TRPM7 and its role in neurodegenerative diseases

Calcium (Ca²⁺) and magnesium (Mg²⁺) ions have been shown to play an important role in regulating various neuronal functions. In the present review we focus on the emerging role of transient potential melastatin-7 (TRPM7) channel in not only regulating Ca²⁺ and Mg²⁺ homeostasis necessary for biological functions, but also how alterations in TRPM7 function/expression could induce neurodegeneration. Although eight TRPM channels have been identified, the channel properties, mode of activation, and physiological responses of various TRPM channels are quite distinct. Among the known 8 TRPM channels only TRPM6 and TRPM7 channels are highly permeable to both Ca²⁺ and Mg²⁺; however here we will only focus on TRPM7 as unlike TRPM6, TRPM7 channels are abundantly expressed in neuronal cells. Importantly, the discrepancy in TRPM7 channel function and expression leads to various neuronal diseases such as Alzheimer disease (AD) and Parkinson disease (PD). Further, it is emerging as a key factor in anoxic neuronal death and in other neurodegenerative disorders. Thus, by understanding the precise involvement of the TRPM7 channels in different neurodegenerative diseases and by understanding the factors that regulate TRPM7 channels, we could uncover new strategies in the future that could evolve as new drug therapeutic targets for effective treatment of these neurodegenerative diseases.

Alpha-synuclein and its neurotoxic fragment inhibit dopamine uptake into rat striatal synaptosomes. Relationship to nitric oxide

Alpha-synuclein (ASN), a 140-amino acid protein, is richly expressed in presynaptic terminals in the central nervous system, where it plays a role in synaptic vesicle function. However, if it is altered and accumulated it is involved in neurodegeneration as Parkinson's disease (PD). ASN contained 35-amino acid domain known as non-amyloid beta component of Alzheimer's disease amyloid (NAC) that is probably responsible for its aggregation and toxicity. Up till now the role of ASN in dopaminergic system function and in pathogenesis of PD is unknown. The aim of this study was to determine the effect of brain aging and the role of ASN and NAC peptide on striatal dopamine transporter (DAT) function. The study was carried out using radiochemical and spectrofluorimetrical determination. It was found that DAT activity assessed by measuring [3H]-dopamine (DA) uptake into striatal synaptosomes significantly decreased in 24-month-old rats comparing to 4-month-old. ASN and NAC peptide at 10 microM concentration inhibited DAT activity by 30%. Both molecules evoked intrasynaptosomal generation of reactive oxygen species measured by fluorogenic probe, 2'7'-dichlorofluorescin diacetate. In addition, ASN activated striatal cytosolic nitric oxide synthase (NOS) by 20%. Nitric oxide (NO) donor, sodium nitroprusside (SNP) (10 microM) and oxidative stress evoked by FeCl2 (25 microM) reduced [3H]DA uptake by 28 and 41%, respectively. Potent antioxidants: Trolox and 4-hydroxy-Tempo had no effect on DAT function but NOS inhibitor Nomega-nitro-L-arginine (100 microM), prevented ASN-evoked DAT down-regulation. These data indicated an important role of ASN in alteration of DA synaptic homeostasis, probably by NO mediated DAT alteration.

Changes of enzyme activity in lipid signaling pathways related to substrate reordering

The static fluid mosaic model of biological membranes has been progressively complemented by a dynamic membrane model that includes phospholipid reordering in domains that are proposed to extend from nanometers to microns. Kinetic models for lipolytic enzymes have only been developed for homogeneous lipid phases. In this work, we develop a generalization of the well-known surface dilution kinetic theory to cases where, in a same lipid phase, both domain and nondomain phases coexist. Our model also allows understanding the changes in enzymatic activity due to a decrease of free substrate concentration when domains are induced by peptides. This lipid reordering and domain dynamics can affect the activity of lipolytic enzymes, and can provide a simple explanation for how basic peptides, with a strong direct interaction with acidic phospholipids (such as beta-amyloid peptide), may cause a complex modulation of the activities of many important enzymes in lipid signaling pathways.

Effects of aging and amyloid-beta peptides on choline acetyltransferase activity in rat brain

Choline acetyltransferase (ChAT, acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6), involved in the learning and memory processes is responsible for the synthesis of acetylcholine. There are many discrepancies in literature concerning ChAT activity during brain aging and the role of amyloid beta peptides in modulation of this enzyme. The aim of the study was to investigate the mechanism of ChAT regulation and age-related alteration of ChAT activity in different parts of the brain. Moreover the effect of Abeta peptides on ChAT activity in adult and aged brain was investigated. The enzyme activity was determined in the brain cortex, hippocampus and striatum in adult (4-months-old), adult-aged (14-months-old) and aged (24-months-old) animals. The highest ChAT activity was observed in the striatum. We found that inhibitors of protein kinase C, A, G and phosphatase A2 have no effect on ChAT activity and that this enzyme is not dependent on calcium ions. About 70% of the total ChAT activity is present in the cytosol. Arachidonic acid significantly inhibited cytosolic form of this enzyme. In the brain cortex and striatum from aged brain ChAT activity is inhibited by 50% and 37%, respectively. The aggregated form of Abeta 25-35 decreased significantly ChAT activity only in the aged striatum and exerted inhibitory effect on this enzyme in adult, however, statistically insignificant. ChAT activity in the striatum was diminished after exposure to 1 mM H2O2. The results from our study indicate that aging processes play a major role in inhibition of ChAT activity and that this enzyme in striatum is selectively sensitive for amyloid beta peptides.

Nanomolar amyloid beta protein activates a specific PKC isoform mediating phosphorylation of MARCKS in Neuro2A cells

Myristoylated alanine-rich C kinase substrate (MARCKS), a protein associated with cell growth, neurosecretion and macrophage activation, is activated by protein kinase C (PKC) phosphorylation. We reported that amyloid beta protein (Abeta) activated MARCKS through a tyrosine kinase and PKC-delta in rat cultured microglia. Here we report that Abeta signaling pathway through a specific PKC isoform is involved in the phosphorylation of MARCKS in Neuro2A cells. Selective PKC inhibitors but not tyrosine kinase inhibitors significantly inhibited the phosphorylation of MARCKS induced by Abeta. Abeta selectively activated PKC-alpha among the four PKC isoforms localized in Neuro2A cells. PKC-alpha activated by Abeta directly phosphorylated a recombinant MARCKS in vitro, Translocation of PKC-alpha from the cytoplasm to the membrane and accumulation of phospho-MARCKS in the cytoplasm were induced by Abeta. These results suggest involvement of a phosphoinositide signaling system through PKC-alpha in the phosphorylation of MARCKS in neurons, an event which may be associated with mechanisms underlying neurotrophic and neurotoxic effects of Abeta.

Apoptosis induced in neuronal cells by C-terminal amyloid beta-fragments is correlated with their aggregation properties in phospholipid membranes

A number of findings suggest that lipophilic monomeric Abeta peptides can interact with the cellular lipid membranes. These interactions can affect the membrane integrity and result in the initiation of apoptotic cell death. The secondary structure of C-terminal Abeta peptides (29-40) and the longer (29-42) variant have been investigated in solution by circular dichroism measurements. The secondary structure of lipid bound Abeta (29-40) and (29-42) peptides prepared at different lipid/peptide ratio's, was investigated by ATR-FTIR spectroscopy. Finally, the changes in secondary structure (i.e. the transition of alpha-helix to beta-sheet) of the lipid bound peptides were correlated with the induction of neurotoxic and apoptotic effects in neuronal cells. The data suggest that the C-terminal fragments of the Abeta peptide induce a significant apoptotic cell death, as demonstrated by caspase-3 measurements and DNA laddering, with consistently a stronger effect of the longer Abeta (29-42) variant. Moreover, the induction of apoptotic death induced by these peptides can be correlated with the secondary structure of the lipid bound amyloid beta peptides. Based on these observations, it is proposed that membrane bound aggregated Abeta peptides (produced locally as the result of gamma-secretase cleavage) can accumulate and aggregate in the membrane. These membrane bound beta-sheet aggregated amyloid peptides induce neuronal apoptotic cell death.