The amyloid precursor protein fragment His 657-Lys 676 inhibits noradrenaline- and enkephaline-induced suppression of voltage sensitive calcium currents in NG108-15 hybrid cells

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Mechanisms of amyloid beta protein-induced modification in ion transport systems: implications for neurodegenerative diseases

1. Alzheimer's disease (AD) is a neurodegenerative disorder that affects the cognitive function of the brain. Pathological changes in AD are characterized by the formation of amyloid plaques and neurofibrillary tangles as well as extensive neuronal loss. Abnormal proteolytic processing of amyloid precursor protein (APP) is the central step that leads to formation of amyloid plaque, neurofibrillary tangles, and neuronal loss. 2. The plaques, which accumulate extracellularly in the brain, are composed of aggregates and cause direct neurotoxic effects and/or increase neuronal vulnerability to excitotoxic insults. The aggregates consist of soluble pathologic amyloid beta peptides AbetaP[1-42] and AbetaP[1-43] and soluble nonpathologic AbetaP[1-40]. Both APP and AbetaP interact with ion transport systems. AbetaP induces a wide range of effects as the result of activating a cascade of mechanisms. 3. The major mechanisms proposed for AbetaP-induced cytotoxicity involve the loss of Ca2+ homeostasis and the generation of reactive oxygen species (ROS). The changes in Ca2+ homeostasis could be the result of (1) changes in endogenous ion transport systems, e.g. Ca2+ and K+ channels and Na+/K+-ATPase, and membrane receptor proteins, such as ligand-driven ion channels and G-protein-driven releases of second messengers, and (2) formation of heterogeneous ion channels. 4. The consequences of changes in Ca2+-homeostasis-induced generation of ROS are (a) direct modification of intrinsic ion transport systems and their regulatory mechanisms, and (b) indirect effects on ion transport systems via peroxidation of phospholipids in the membrane, inhibition of phosphorylation, and reduction of ATP levels and cytoplasmic pH. 5. We propose that in AD, AbetaP with its different conformations alters cell regulation by modifying several ion transport systems and also by forming heterogeneous ion channels. The changes in membrane transport systems are proposed as early steps in impairing neuronal function preceding plaque formation. We conclude that these changes damage the membrane by compromising its integrity and increasing its ion permeability. This mechanism of membrane damage is not only central for AD but also may explain other malfunctioned protein-processing-related pathologies.

Heparin-insensitive calcium release from intracellular stores triggered by the recombinant human parathyroid hormone receptor

1. In the present study we have characterized the parathyroid hormone (PTH)-induced calcium signalling in 293 cells stably transfected with the human PTH receptor cDNA. In these cells, human PTH-1(1-38) strongly stimulates adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation (EC50 = 0.39 nM) but fails to activate phosphoinositide (PI) turnover. The latter pathway is strongly activated, however, by carbachol (CCh) acting through endogenous M3-muscarinic receptors. 2. Despite the lack of detectable inositol phosphate (IP) formation, hPTH-(1-38) elicited calcium transients (EC50 = 11.2 nM) which were comparable to the signals evoked by CCh. These signals are independent of cyclic AMP generation as cyclic AMP elevating agents did not mimic or modify the PTH response. 3. The PTH-stimulated calcium signal still occurred in calcium-free medium but was absent in cells pretreated with thapsigargin, an inhibitor of the calcium pump of the endoplasmic reticulum (ER). hPTH-(1-38) did not accelerate Mn(2+)-influx through the plasma membrane. These data indicate that PTH releases calcium from intracellular stores. 4. Using heparin, an inhibitor of the IP3-activated calcium release channel of the ER, we tested whether the formation of a low amount of IP3, escaping detection by our biochemical assay, might be the origin of the PTH-induced calcium response. However, intracellular infusion of heparin through patch pipettes in voltage clamp experiments failed to block hPTH-(1-38)-induced calcium signals, whereas it abolished the CCh response. 5. The PTH response, like the CCh response, was insensitive to micromolar concentrations of ryanodine and ruthenium red, eliminating the possibility that hPTH-(1-38) stimulates calcium-induced calcium release through ryanodine receptors.6. We conclude that the recombinant human PTH receptor stimulates calcium release from intracellular stores through a novel pathway not involving IP3- or ryanodine receptors.