Family 19: Purinergic and Danger Signalling
Family overview. Cells release ATP and purines in response to stress, damage, or infection. Extracellular ATP acts as a danger-associated molecular pattern (DAMP), activating P2X and P2Y purinergic receptors, triggering NLRP3 inflammasome assembly, and initiating sterile inflammation. Resolution requires enzymatic degradation of ATP to adenosine by ectonucleotidases.
Concrete mechanisms and ME/CFS evidence:
Extracellular ATP as DAMP. Under mitochondrial stress and cell damage — both present in ME/CFS — intracellular ATP leaks into the extracellular space, where it acts as a danger signal rather than an energy carrier. Metabolomics confirms altered purine metabolite profiles in ME/CFS (Robert K. Naviaux et al. 2016).
P2X7 receptor activation. High extracellular ATP activates P2X7R on microglia and macrophages, triggering ion flux, ROS production, and NLRP3 inflammasome assembly — directly linking mitochondrial dysfunction to neuroinflammation.
NLRP3 inflammasome activation. NLRP3 assembly leads to caspase-1 activation, IL-1β and IL-18 maturation and secretion, and potential pyroptosis; elevated IL-1β and IL-18 in ME/CFS (Family 4) imply upstream NLRP3 activity even without direct confirmation.
Cell danger response (CDR). Naviaux proposed that a conserved metabolic response to threat — shifting cells from OXPHOS to glycolysis and releasing purines as danger signals — when chronically engaged rather than resolved, produces many features of ME/CFS (Robert K. Naviaux et al. 2016). The CDR integrates Families 1, 2, 4, and 19 into a unified metabolic-immune framework.
Resolution failure via ectonucleotidase impairment. Normal resolution requires ATP → AMP → adenosine conversion by CD73/CD39, producing anti-inflammatory adenosine signals. Impaired CD73/CD39 activity would sustain P2X7 activation and prevent resolution. Ectonucleotidase activity in ME/CFS is poorly characterised.
Evidence status: Theoretical (CDR hypothesis mechanistically coherent and supported by metabolomics; direct P2X7, NLRP3, and ectonucleotidase studies in ME/CFS are sparse).
In health, CD73 (5′-ectonucleotidase) terminates danger signalling by converting AMP to adenosine. In parallel, CD73 converts NMN to NR, a key step in the NAD⁺ salvage pathway. Under chronic mitochondrial stress, massive ATP release through pannexin-1 and connexin hemichannels (opened by the oxidative stress documented in Family 2) floods the extracellular space with purines. If CD73 activity is overwhelmed or downregulated, a dual bottleneck emerges: neither purinergic resolution (excess extracellular ATP persists, sustaining P2X7 and immune activation) nor NAD⁺ repletion (NMN→NR conversion impaired) can occur simultaneously. This predicts a threshold phenomenon: below a critical CD73 activity level, supplemental NMN will fail as a NAD⁺ source, while NR — which bypasses the CD73 step — will succeed. It also predicts that NMN supplementation trials in ME/CFS will show heterogeneous outcomes stratified by patient CD73 activity (Robert K. Naviaux et al. 2016) (Robert K. Naviaux 2019).
Certainty: 0.30. No direct measurement of CD73 activity in ME/CFS exists. Not yet tested.
Is the cell danger response chronically engaged in ME/CFS, and if so, what prevents resolution? Measuring CD73/CD39 ectonucleotidase activity, extracellular purine ratios (ATP/AMP/adenosine), and P2X7R expression in ME/CFS PBMCs would directly test whether purinergic resolution failure sustains the disease state.