Activity-dependent changes in synaptic pH are the result of proton (H+) or H+ equivalent movement across membranes temporarily overwhelming phosphate and bicarbonate buffering systems. Many pumps and exchangers are active in synaptic membranes; however, at many synapses, including the Drosophila neuromuscular junction (NMJ), the plasma membrane calcium ATPase (PMCA) is responsible for the greatest pH fluctuations by exchanging H+ for calcium ions (Ca2+).
The cover image for the December 21 issue of Biophysical Journal is an immunohistochemical staining. Confocal microscopy was used to excite and reveal fluorophore-conjugated antibodies bound to an active zone protein, Bruchpilot (green), and the PMCA (periwinkle) at the Drosophila larval NMJ. The image illustrates the distribution of PMCA around the motor nerve terminal boutons relative to the active zones. Active zones are where synaptic vesicles dock and release neurotransmitters along with H+, causing cleft acidification. The PMCA counters cleft acidification by exchanging 2H+ from the cleft for every Ca2+ from the cytosol.
We addressed the paradox inherent in the literature that reports alkalinization at conventional glutamatergic synapses in fruit flies and mice, even though acid-sensing ion channels can be activated at the very same synapses. This paradox is resolved in the predictions of our reaction-diffusion scheme implemented in MATLAB. Our model predicts substantial cleft acidification at the site of synaptic vesicle exocytosis, but only for fractions of a millisecond, followed by alkalinization driven by PMCA activity persisting for >100 ms. Therefore, the model reveals the cleft as a highly dynamic pH landscape where exceedingly rapid acidic microdomains are ephemeral elements in an otherwise alkaline environment. A better understanding of the pH dynamics within the cleft would seem to be essential for understanding the full ramifications for those pH-sensitive synaptic proteins exposed to the extracellular milieu at the cleft, especially because altered brain pH is associated with neurodegenerative disease states such as Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis.
We hope you find these findings as interesting as we did and will make use of our reaction-diffusion scheme to further investigate implications for your own studies.
-Roberto X. Hernandez, Touhid Feghhi, and Gregory T. Macleod