P2X Electrophysiology and Surface Trafficking in Xenopus Oocytes

Increased surface P2X4 receptors by mutant SOD1 proteins contribute to ALS pathogenesis in SOD1-G93A mice

Amyotrophic lateral sclerosis (ALS) is a fatal motoneuron (MN) disease characterized by protein misfolding and aggregation leading to cellular degeneration. So far neither biomarker, nor effective treatment has been found. ATP signaling and P2X4 receptors (P2X4) are upregulated in various neurodegenerative diseases. Here we show that several ALS-related misfolded proteins including mutants of SOD1 or TDP-43 lead to a significant increase in surface P2X4 receptor density and function in vitro. In addition, we demonstrate in the spinal the cord of SOD1-G93A (SOD1) mice that misfolded SOD1-G93A proteins directly interact with endocytic adaptor protein-2 (AP2); thus, acting as negative competitors for the interaction between AP2 and P2X4, impairing constitutive P2X4 endocytosis. The higher P2X4 surface density was particularly observed in peripheral macrophages of SOD1 mice before the onset and during the progression of ALS symptoms positioning P2X4 as a potential early biomarker for ALS. P2X4 expression was also upregulated in spinal microglia of SOD1 mice during ALS and affect microglial inflammatory responses. Importantly, we report using double transgenic SOD1 mice expressing internalization-defective P2X4mCherryIN knock-in gene or invalidated for the P2X4 gene that P2X4 is instrumental for motor symptoms, ALS progression and survival. This study highlights the role of P2X4 in the pathophysiology of ALS and thus its potential for the development of biomarkers and treatments. We also decipher the molecular mechanism by which misfolded proteins related to ALS impact P2X4 trafficking at early pathological stage in cells expressing-P2X4.

Increased surface P2X4 receptor regulates anxiety and memory in P2X4 internalization-defective knock-in mice

ATP signaling and surface P2X4 receptors are upregulated selectively in neurons and/or glia in various CNS disorders including anxiety, chronic pain, epilepsy, ischemia, and neurodegenerative diseases. However, the cell-specific functions of P2X4 in pathological contexts remain elusive. To elucidate P2X4 functions, we created a conditional transgenic knock-in P2X4 mouse line (Floxed P2X4mCherryIN) allowing the Cre activity-dependent genetic swapping of the internalization motif of P2X4 by the fluorescent mCherry protein to prevent constitutive endocytosis of P2X4. By combining molecular, cellular, electrophysiological, and behavioral approaches, we characterized two distinct knock-in mouse lines expressing noninternalized P2X4mCherryIN either exclusively in excitatory forebrain neurons or in all cells natively expressing P2X4. The genetic substitution of wild-type P2X4 by noninternalized P2X4mCherryIN in both knock-in mouse models did not alter the sparse distribution and subcellular localization of P2X4 but increased the number of P2X4 receptors at the surface of the targeted cells mimicking the pathological increased surface P2X4 state. Increased surface P2X4 density in the hippocampus of knock-in mice altered LTP and LTD plasticity phenomena at CA1 synapses without affecting basal excitatory transmission. Moreover, these cellular events translated into anxiolytic effects and deficits in spatial memory. Our results show that increased surface density of neuronal P2X4 contributes to synaptic deficits and alterations in anxiety and memory functions consistent with the implication of P2X4 in neuropsychiatric and neurodegenerative disorders. Furthermore, these conditional P2X4mCherryIN knock-in mice will allow exploring the cell-specific roles of P2X4 in various physiological and pathological contexts.