In this report, we demonstrated that CSF ATP levels were higher in patients with ALS who had longer disease durations and more severe symptoms and that the levels were inversely correlated with the MRC sum score. This is the first report to evaluate CSF ATP levels in patients with ALS. ALSFRS-R and serum creatinine levels were significantly lower in the severe group than in the mild group. These results were consistent with those of a previous study suggesting that plasma creatinine level correlates with ALS functional rating scale-revised (ALSFRS-R) and percent forced vital capacity (FVC) and significantly predicted survival [23].
In the central nervous system, extracellular ATP regulates several physiological processes such as cell proliferation, differentiation, and apoptosis [24]. ATP is released by damaged neurons and is also present in the neuronal synaptic or secretory vesicles and is co-released to the extracellular space with neurotransmitters such as γ-aminobutyric acid (GABA) and glutamate [25]. ATP is released from not only neurons but also microglia and astrocytes through lysosome exocytosis [26]. The released ATP is degraded by ectonucleotidase, and an ATP metabolite binds to P2 purinergic receptors localized on the neuronal pre/post synaptic membranes, astrocytes, and microglia, which then participates in neuron-glial and inter-glial cell signaling [27] [28]. Activation of the astrocytes, derived from an ALS rat model, by stimulating the ATP receptor resulted in co-cultured motor neuron death, and this was prevented by the administration of an ATP receptor antagonist [29]. Thus, it has been assumed that ALS pathogenesis is partially attributed to the disability of neuron-glial cell interaction, that is, increased extracellular ATP released by damaged neurons is recognized as a damage-associated molecular pattern (DAMP) signal that activates the microglia and astrocytes, accompanied by the production of proinflammatory cytokines and excessive neuroinflammation and neuronal loss [30] [31].
We showed that CSF ATP levels were higher in patients with ALS with advanced muscle weakness and were inversely correlated with the MRC sum score. In this report, we assayed CSF samples from patients with iNPH as a control, because in patients with iNPH, the levels of CSF neurofilament light chain (NfL), known as a marker of neuronal damage, was not significantly different from those in healthy control [32].
It seems likely that a high concentration of extracellular ATP may induce motor neuron death and muscle weakness in patients with ALS. CSF ATP levels are elevated in patients with radiation-induced brain injury, and increased ATP exacerbates neuronal injury via the activation of microglia [33]. The mean CSF ATP level in those patients was approximately 10000 pmol/L, which is not very different from that reported in patients with severe ALS in our study (3635 ± 5465 pmol/L). Thus, it is possible that ATP is released from damaged neurons, and this increase in ATP accelerates neuronal damage, although the exact mechanism behind this increase in the ATP levels remains unclear.
This study has some limitations. This was a hospital-based cross-sectional study, and the sample size was relatively small. Further study is needed to clarify the relationship between CSF ATP levels and several inflammatory cytokines such as TNF-α, IL-6, and COX-2 as well as between CSF ATP levels and blood or CSF NfL, which reflect motor neuron degeneration in patients with ALS.