The human prion octarepeat fragment prevents and reverses the inhibitory action of copper in the P2X4 receptor without modifying the zinc action

Action of natural products on p2 receptors: a reinvented era for drug discovery

Natural products contribute significantly to available drug therapies and have been a rich source for scientific investigation. In general, due to their low cost and traditional use in some cultures, they are an object of growing interest as alternatives to synthetic drugs. With several diseases such as cancer, and inflammatory and neuropathic diseases having been linked to the participation of purinergic (P2) receptors, there has been a flurry of investigations on ligands within natural products. Thirty-four different sources of these compounds have been found so far, that have shown either agonistic or antagonistic effects on P2 receptors. Of those, nine different plant sources demonstrated effects on P2X2, P2X3, P2X7, and possibly P2Y12 receptor subtypes. Microorganisms, which represent the largest group, with 26 different sources, showed effects on both receptor subtypes, ranging from P2X1 to P2X4 and P2X7, and P2Y1, P2Y2, P2Y4, and P2Y6. In addition, there were seventeen animal sources that affected P2X7 and P2Y1 and P2Y12 receptors. Natural products have provided some fascinating new mechanisms and sources to better understand the P2 receptor antagonism. Moreover, current investigations should clarify further pharmacological mechanisms in order to consider these products as potential new medicines.

The Cellular Prion Protein Prevents Copper-Induced Inhibition of P2X4Receptors

Although the physiological function of the cellular prion protein (PrP^C) remains unknown, several evidences support the notion of its role in copper homeostasis. PrP^C binds Cu²⁺ through a domain composed by four to five repeats of eight amino acids. Previously, we have shown that the perfusion of this domain prevents and reverses the inhibition by Cu²⁺ of the adenosine triphosphate (ATP)-evoked currents in the P2X₄ receptor subtype, highlighting a modulatory role for PrP^C in synaptic transmission through regulation of Cu²⁺ levels. Here, we study the effect of full-length PrP^C in Cu²⁺ inhibition of P2X₄ receptor when both are coexpressed. PrP^C expression does not significantly change the ATP concentration-response curve in oocytes expressing P2X₄ receptors. However, the presence of PrP^C reduces the inhibition by Cu²⁺ of the ATP-elicited currents in these oocytes, confirming our previous observations with the Cu²⁺ binding domain. Thus, our observations suggest a role for PrP^C in modulating synaptic activity through binding of extracellular Cu²⁺.

Stoichiometry and conditional stability constants of Cu(II) or Zn(II) clioquinol complexes; implications for Alzheimer's and Huntington's disease therapy

Successful trials with 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol, CQ) for Alzheimer's disease treatment prompted renewed interest in assessing whether its therapeutic action is related to the coordination of neurotoxic trace metals, such as Cu(II) and Zn(II). We now report conditional stability constants (K(C')) for CQ Cu(II) and Zn(II) complexes measured in a biological buffer containing Ca(II) and Mg(II) ions. UV-vis spectroscopy and polarography evidenced a 1:2 stoichiometry of Cu(II) and Zn(II) CQ complexes; the K(C')s calculated were: Cu(CQ)(2) 1.2x10(10), and Zn(CQ)(2) 7.0x10(8)M(-2); the CQ affinity for Cu(II) is at least an order of magnitude higher than for Zn(II). To test the possible functional relevance of the Cu(II) CQ complexes in the brain, we bioassayed free Cu(II) concentration by the metal-induced inhibition of ATP-gated currents of the P2X(4) receptor, a predominant brain P2X receptor. CQ reduced concentration-dependently the Cu(II) inhibition of the ATP-gated currents. In view that the stability constant of CQ for Zn(II) is similar to that of Abeta-amyloid for Zn(II), and the fact that CQ may form complexes with Cu(II), even in the presence of competing ions, the present results highlight that the formation of Cu(II) CQ complexes in the brain may act by diminishing free Cu(II) concentrations modifying thereby brain excitability, or favoring the degradation of beta-amyloid plaques or huntingtin, rather than through a specific effect of CQ itself.

A human prion protein peptide (PrP(59-91)) protects against copper neurotoxicity

Human cellular prion protein (PrP(C)) is involved in several neurodegenerative disorders; however, its normal function is unknown. We report here that a synthetic peptide corresponding to the four-octarepeat sequence of the PrP(C) (PrP(59-91)) protects hippocampal neurons against copper neurotoxic effects in vivo. Using a rat bilateral intrahippocampal injection model, we found that PrP(59-91) protects against copper-induced neurotoxicity, including a recovery in spatial learning performance and a reduced neuronal cell loss and astrogliosis. Previous studies from our laboratory indicated that a tryptophan (Trp) residue plays a key role in the reduction of copper(II) to copper(I); therefore several PrP(59-91) fragments lacking histidine (His) and Trp residues were tested for their capacity to protect from copper toxicity. A PrP(59-91) peptide lacking His residue shows as much neuroprotection as the native peptide; however, PrP(59-91) without Trp residues only partially protected against copper toxicity. The neuroprotective effect not only occurs with PrP(59-91), in fact a full neuroprotection was also observed using just one octamer of the N-terminal region of prion protein. We conclude that the N-terminal tandem octarepeat of the human PrP(C) protects neurons against copper toxicity by a differential contribution of the binding (His) and reducing (Trp) copper activities of PrP(59-91). Our results are consistent with the idea that PrP(C) function is related to copper homeostasis.