P2RX7 and osteoporosis: Especially a possible interaction between physical activity and P2X7 SNPs in relation to osteoporosis risk would have been interesting to investigate, since previous animal studies using P2X7 knock-out models demonstrated that bone formation in response to mechanical loading as a result of enhanced production of prostaglandin-E2 via P2X7R activation was diminished in P2X7 knock-outs [15, 35].