Transient receptor potential melastatin 2 channels are overexpressed in myalgic encephalomyelitis/chronic fatigue syndrome patients

Abstract

Background: Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is hallmarked by a significant reduction in natural killer (NK) cell cytotoxicity, a mechanism tightly regulated by calcium (Ca²⁺). Interestingly, interleukin-2 (IL-2) increases NK cell cytotoxicity. Transient receptor potential melastatin 2 (TRPM2) ion channels are fundamental for Ca²⁺ signalling in NK cells. This pilot investigation aimed to characterise TRPM2 and CD38 surface expression in vitro on NK cells in ME/CFS patients. This investigation furthermore examined the pharmaceutical effect of 8-bromoadenosine phosphoribose (8-Br-ADPR) and N₆-Benzoyladenosine-3',5'-cyclic monophosphate (N₆-Bnz-cAMP) on TRPM2 and CD38 surface expression and NK cell cytotoxicity between ME/CFS and healthy control (HC) participants.

Methods: Ten ME/CFS patients (43.45 ± 12.36) and 10 HCs (43 ± 12.27) were age and sex-matched. Isolated NK cells were labelled with fluorescent antibodies to determine baseline and drug-treated TRPM2 and CD38 surface expression on NK cell subsets. Following IL-2 stimulation, NK cell cytotoxicity was measured following 8-Br-ADPR and N₆-Bnz-cAMP drug treatments by flow cytometry.

Results: Baseline TRPM2 and CD38 surface expression was significantly higher on NK cell subsets in ME/CFS patients compared with HCs. Post IL-2 stimulation, TRPM2 and CD38 surface expression solely decreased on the CD56^DimCD16⁺ subset. 8-Br-ADPR treatment significantly reduced TRPM2 surface expression on the CD56^BrightCD16^Dim/- subset within the ME/CFS group. Baseline cell cytotoxicity was significantly reduced in ME/CFS patients, however no changes were observed post drug treatment in either group.

Conclusion: Overexpression of TRPM2 on NK cells may function as a compensatory mechanism to alert a dysregulation in Ca²⁺ homeostasis to enhance NK cell function in ME/CFS, such as NK cell cytotoxicity. As no improvement in NK cell cytotoxicity was observed within the ME/CFS group, an impairment in the TRPM2 ion channel may be present in ME/CFS patients, resulting in alterations in [Ca²⁺]_i mobilisation and influx, which is fundamental in driving NK cell cytotoxicity. Differential expression of TRPM2 between NK cell subtypes may provide evidence for their role in the pathomechanism involving NK cell cytotoxicity activity in ME/CFS.

Keywords: Adenosine diphosphate ribose; Calcium; Interleukin-2; Myalgic encephalomyelitis/chronic fatigue syndrome; Natural killer cells; Transient receptor potential melastatin 2.

Fig. 1

TRPM2 and CD38 surface expression on CD56^BrightCD16^Dim/− and CD56^DimCD16⁺ NK cell subsets between groups post IL-2 stimulation. At baseline, TRPM2 surface expression was significantly higher in the ME/CFS group compared to HCs on CD56^BrightCD16^Dim/− (a) and CD56^DimCD16⁺ NK cells (b). A consistent finding was found at dual expression with CD38 on both NK cell subsets (c, d). Post IL-2 stimulation, TRPM2 with and without CD38 significantly decreased on the CD56^DimCD16⁺ subset within the ME/CFS group (b, d). No significant differences in TRPM2 and CD38 surface expression were found within the HC group pre and post IL-2 stimulation in either NK cell subset

Fig. 2

CD38 surface expression on CD56^BrightCD16^Dim/− and CD56^DimCD16⁺ NK cell subsets between groups post IL-2 stimulation. No significant difference in CD38 surface expression was found between groups or within either NK cell subset pre and post IL-2 stimulation (a, b). No significant changes in CD38 surface expression was observed post drug treatment between or within groups on either NK cell subset (c, d)

Fig. 3

Natural killer cell cytotoxicity after treatment with 8-Br-ADPR and N₆-Bnz-cAMP between groups. At baseline, a significant increase in NK cell cytotoxicity was observed in HC participants compared with ME/CFS patients. Within the HC group, NK cell lysis decreased post IL-2 stimulation. This observation was not found within the ME/CFS group. No significant difference in NK cell cytotoxicity was observed between groups after drug treatments

Fig. 4

Pharmacological effect of 8-Br-ADPR and N₆-Bnz-cAMP drug treatment on TRPM2 and CD38 surface expression on NK cell subsets. On CD56^BrightCD15^Dim/− NK cells (a), TRPM2 surface expression significantly decreased following treatment with 8-Br-ADPR within the ME/CFS group. No significant difference was observed on the CD56^DimCD16⁺ NK cell subset for TRPM2 surface expression between and within groups (b). Moreover, no significant differences between or within groups were found on theCD56^BrightCD15^Dim/− (c) and CD56^DimCD16⁺ (d) NK cell subsets for dual expression with TRPM2