P2X7 Receptor Agonist 2'(3')-O-(4-Benzoylbenzoyl)ATP Differently Modulates Cell Viability and Corticostriatal Synaptic Transmission in Experimental Models of Huntington's Disease

Abstract

Huntington's disease (HD) is a life-threatening neurodegenerative disorder. Altered levels and functions of the purinergic ionotropic P2X7 receptors (P2X7Rs) have been found in animal and cellular models of HD, suggesting their possible role in the pathogenesis of the disease; accordingly, the therapeutic potential of P2X7R antagonists in HD has been proposed. Here we further investigated the effects of P2X7R ligands in in vitro and ex vivo HD experimental models. In ST14A/Q120 rat striatal cells, we found a reduction of P2X7R expression; however, the P2X7R agonist 2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate (BzATP) induced cellular death, and this effect was fully reversed by the antagonist periodate-oxidized adenosine 5'-triphosphate (OxATP). Moreover, in corticostriatal slices from symptomatic R6/2 mice, BzATP reduced the synaptic transmission to a larger extent than in wild-type (WT) mice. Such an effect was accompanied by a concomitant increase of the paired-pulse ratio, suggesting a presynaptic inhibitory action. This was confirmed to be the case, since while the effects of BzATP were unaffected by the P2X7R antagonist OxATP, they were blocked by the adenosine A₁ receptor (A₁R) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), suggesting possible BzATP hydrolysis to 2'(3')-O-(4-benzoylbenzoyl)adenosine (Bz-adenosine) and consequent activation of A₁Rs as a mechanism. Taken together, these data point out that 1) P2X7R expression and activity are confirmed to be altered in the presence of HD mutation; 2) in some experimental settings, such an abnormal functioning can be ascribed to presynaptic A₁Rs activation.

Keywords: BzATP; DPCPX; Huntington’s disease; OxATP; P2X7 receptor; adenosine; adenosine A1 receptor; corticostriatal slices.

FIGURE 1

P2X7R expression is reduced in HD cells. (A) Western blot analysis in HD cells and relative control shows that P2X7R expression is reduced in presence of mutant htt. Lamin B1 is used as a loading control. The result is expressed as the mean ± SEM of four independent biological replicates (**p < 0.01 according to One-Sample t-test). BzATP effects in HD cell line. (B) The treatment with BzATP 300 μM was ineffective in ST14A/Q15 whereas it reduced cell viability in ST14A/Q120 (**p < 0.01 vs. Q120/CTR and *p < 0.05 vs. Q15/BzATP, according to Mann–Whitney test). (C) The BzATP effect in ST14A/Q120 (**p < 0.01 vs. Q120/CTR) was blocked by the P2X7R antagonist OxATP (1 μM; °p < 0.01 vs. BzATP-treated Q120 cells, according to Mann–Whitney test) but not by the A₁R antagonist DPCPX (D).

FIGURE 2

BzATP effects on synaptic transmission in corticostriatal slices from WT and R6/2 mice. (A) BzATP 50 μM induced a larger depression of field potential (FP) amplitude in R6/2 than in WT mice. The horizontal bars indicate the period of drug application. (B) The histograms show the mean ± SEM of the FP amplitude expressed as a mean percentage variation of baseline during the last 5 min of drug perfusion. BzATP effect was significantly larger in R6/2 mice; °p < 0.05 vs. WT and vs. basal, according to Mann–Whitney test. (C) Effect of BzATP on presynaptic neurotransmitter release. While basal R2/R1 ratio was comparable, after BzATP application it was significantly higher in R6/2 compared to WT mice (2.13 ± 0.39%, n = 6 and 1.29 ± 0.08%, respectively).

FIGURE 3

(A,B) BzATP inhibitory effect was not prevented by the P2X7R antagonist OxATP, in neither WT nor R6/2 mice (insets in panels A,B). (C,D) The BzATP-induced depression was reduced in WT and completely abolished in R6/2 slices if DPCPX was pre-applied (°p < 0.01 vs. BzATP, according to Mann–Whitney test; inset in panel D). (E) In R6/2 slices, the increase of PPR caused by BzATP was completely abolished by DPCPX.