Opioid and non-opioid activities of the dynorphins

Abstract

Endogenous opioid peptides a-neoendorphin, dynorphin A (Dyn A), dynorphin B (Dyn B) and big dynorphin (Big Dyn) consisting of Dyn A and Dyn B, collectively known as dynorphins are derived from the precursor protein prodynorphin (PDYN). Dynorphins regulate pain processing and memory acquisition and modulate reward induced by intake of addictive substances. Such actions are generally mediated through the k-opioid receptors. However, excitotoxic effects of Dyn A and Big Dyn relevant for neuropathic pain are non-opioid; the mechanisms of these effects are unknown but glutamate receptors are apparently involved. PDYN mRNA and dynorphin peptides have been extensively characterized in the rat and human brain, whereas little is known about PDYN, its biochemical properties, and localization and processing in the brain.

The general aim of the present study was (1) to characterize mechanisms of non-opioid interactions of dynorphins with cells, (2) to identify non-opioid biochemical targets for these peptides, (3) to characterize biochemical properties, intracellular localization and distribution in the brain of the precursor protein PDYN, and (4) to assess role of dynorphins in human neurodegenerative disorders including Alzheimer disease.

Our analysis revealed similarity of dynorphins with cell-penetrating peptides (CPPs) capable of translocating into cells. The ability of dynorphins to translocate across the plasma membrane was tested using immunofluorescence, confocal fluorescence microscopy, and fluorescence correlation spectroscopy on fixed and live cells. Big Dyn and Dyn A but not Dyn B were found to be capable to penetrate into cells. Big Dyn showed higher translocation potential compared with that of Dyn A, while Dyn A and transportan-10, a prototypical CPP translocated into cells with similar efficacy. The translocated dynorphins were predominantly located in the cytoplasm where they were associated with the endoplasmic reticulum. Fluorescence and circular dichroism spectroscopy imply that two structural features, the ability to form a-helix and high positive charge, may in conjunction determine the membrane translocation potential of Big Dyn and Dyn A. Translocation of dynorphins into cells followed by interactions with intracellular targets might represent an evolutionary ancient mechanism of intercellular communications and signal transduction (Paper I).

For identification of intracellular dynorphin targets - non-opioid binding sites in neuronal cells and brain, radioligand-binding assay was used. A novel soluble factor that binds Dyn A and Big Dyn with high specificity and affinity (IC 50 5-10 nM) was found. Binding of Dyn A to the factor was virtually irreversible and resulted in conversion of Dyn A into Leu-enkephalin, suggesting that the Dyn A-binding factor (DABF) functions as an oligopeptidase. This enzyme may potentially degrade other neuropeptides. Dynorphins may regulate this degradation acting as competitors when they bind to the enzyme (Paper II).

Dynorphins generation may be regulated at the levels of gene transcription, translation, protein trafficking and processing. As the first step in the analysis of these processes, the biochemical properties of PDYN, its intracellular localization and distribution in the rat and human brain were characterized. The focus was on structures where PDYN is synthesized and where mature dynorphins are located. PDYN distribution pattern in rat brain determined by immunohistochemistry and western blot was similar to that of dynorphin peptides with highest levels in the amygdala, hippocampus and striatum and lower levels in the cerebral cortex.

PDYN in unprocessed form was also present in the ventral tegmental area (VTA) and the hippocampal CA3 region that do not have cell bodies of neurons producing PDYN but contain axons and axon terminals originating from PDYN-ergic neurons. Thus PDYN is transported to and stored in axon terminals. PDYN in axon terminals may be processed to mature peptides prior to their release from cells. This notion is supported by the observation that K+-evoked depolarization of PDYN producing cells increases the total amounts of dynorphins in cells and medium demonstrating that PDYN processing is activated. Stimulation of PDYN processing in axon terminals and dendrites by neuronal activity and extracellular signals may represent a mechanism for the local regulation of synaptic transmission (Paper III).

Dyn A through non-opioid glutamate receptor-mediated mechanism may induce excitotoxicity resulting in neuronal death (related paper 1). Neuronal death in Alzheimer disease (AD) and other neurodegenerative diseases may be associated with upregulation of Dyn A, which may potentially contribute to neurodegeneration. To assess the relationship between neurodegeneration and dynorphins, the status of the dynorphin system was evaluated in AD. The levels of Dyn A and Dyn B and the related neuropeptide nociceptin in the inferior parietal lobule (Brodmann area 7) of control, AD, Parkinson disease (PD) and cerebro-vascular disease (CVD) groups were determined. AD subjects displayed robustly elevated levels of Dyn A, whereas the levels of Dyn B and nociceptin were unaltered. Subjects with PD and CVD showed no changes in all three peptides. The levels of PDYN and the activity of DABF did not change in AD, whereas the levels of the processing enzyme pro-hormone convertase 2 (PC2), which processes PDYN, were elevated. The levels of Dyn A correlated with the density of neuritic plaques. These results suggest a non-opioid role of Dyn A in the pathogenesis of AD (Paper IV).