Hv Physiology: Brain Damage
Thanks to the groundbreaking work of Hodgking and Huxley, the most well known physiological role of voltage-gated ion channels is the propagation of the action potential in neurons (Hodgkin & Huxley, 1952). What Hodgkin and Huxley demonstrated was that the action potential is generated by the sequential opening and closing of voltage-gated Na+ and K+ channels (Hodgkin & Huxley, 1952). Given this important role of voltage-gated channels in nerve cells it is natural to ask: “What role might the voltage-gated proton channel (Hv) play in neuron physiology?” However, although the first description of voltage-gated proton current was from a snail neuron (Tomas & Meech, 1982), the presence of Hv in mammalian neurons has not been demonstrated. But, of course, there are more than just neurons in the central nervous system, so maybe Hv plays a supporting role in one of the non-neuronal cell types. Indeed, there is some evidence for the presence of voltage-gated proton currents in the microglia of mammalian brain tissue (Schilling & Eder, 2007). In order to understand what Hv might be doing in the brain we first need to know what microglia are and what they do.
Microglia are a specific subtype of neuroglia, the non-neuronal cells that perform many important roles in the central nervous system. Neuroglia are responsible for supplying nutrients and oxygen to the neurons, generating the myelin coats of the neurons and providing support and protection for the neurons (check them out on wikipedia). Microglia are the neuron-protecting subtype of neuroglia: they are the immune cells of the brain, devouring and killing pathogens. Essentially, they are macrophages and are capable of engulfing bacteria into phagosomes and killing them by bombardment with reactive oxygen species (ROS), cationic peptides and proteolytic enzymes. In order to generate the high levels of ROS needed to kill a bacterium, the microglia require a membrane-bound enzyme complex know as the NADPH oxidase complex or NOX (Bedard & Krause, 2007). When NOX is activated, electrons are passed into the phagosome and the membrane is depolarized. In addition, protons are released in the cytoplasm, reducing the internal pH of the cell. One of the most well established physiological roles of Hv channels is to compensate for the activity of the NOX complex (DeCoursey, 2010). Upon membrane depolarization and cytoplasm acidification, the Hv channels in the phagosome membrane open, allowing protons to flow down their concentration gradient into the phagosome. This proton current re-polarizes the membrane and de-acidifies the cytoplasm, allowing for persistent NOX activity, as NOX is strongly inhibited at depolarized membrane potentials (DeCoursey, 2003). (I will discuss this well-known immunological role of Hv in more detail in future blog posts.)
Therefore, Hv channels have at least one physiological role in the brain: regardless of the roles Hv channels may play in neurons themselves, Hv activity is an important part of the normal functioning of microglia. This role is not surprising: it is well established that Hv channels are important for the proper functioning of macrophages in the immune system, so it follows that they would be important in microglia, “the macrophages of the brain”. Despite this, the role of Hv in charge compensation of the microglial phagosome membrane was somewhat controversial (Schilling & Eder, 2007; Simoni et al., 2008) and it wasn’t well established until very recently (Wu et al., 2012).
More surprising, perhaps, is the role that Hv might play in brain damage after ischemic stroke. When an artery that provides nutrient-rich blood to the brain becomes clogged, a region of the brain is starved. This results in damage to the afflicted tissue, a condition known as ischemic stroke (check out this animation from the American Heart Association). Part of the damage results from the fact that, when deprived of oxygen and nutrients, the NOX complex of microglia becomes mis-regulated and releases ROS into the surrounding brain tissue, killing the nearby neuronal and non-neuroanl cells (Chan, 2001; Yenari et al., 2010). The contribution to brain damage from ROS is significant: for instance, studies from mice lacking a functional phagocytic NOX showed a near-50% reduction in brain damage caused by ischemic stroke (Walder et al., 1997). This observation, combined with the fact that Hv channels are required for high levels of NOX activity, led Wu et al. to investigate what role, if any, Hv might play in brain damage from ischemic stroke.
By using mutant mice that lacked Hv channels and by experimenting on cultured human neurons, Wu et al. were able to demonstrate that Hv channel activity has a profound impact on brain damage caused by ROS. Mice lacking Hv channels had significantly less neuronal cell death after stroke than the controls. Through a series of elegant experiments, Wu et al. demonstrated that it is indeed the Hv channels in the resident microglia that contribute to the brain damage. This led the authors to propose that specific inhibitors of Hv activity would be a valuable therapeutic for treatment of patients after ischemic stroke. Blocking Hv channels could mitigate brain damage and greatly improve the prognosis of stroke victims.
In conclusion, although Hv may not be important for the transmission of information in the brain, it is a very important component for neuronal protection against pathogens. In fact, it is because Hv’s roles are non-neuronal that its inhibition is such a tantalizing strategy for prophylactic treatment of brain damage after ischemic stroke: blocking Hv current would reduce brain damage without interfering with the normal functioning of neurons.
Works Cited in this Post and Further Reading
Bedard, K., & Krause, K.-H. (2007). The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology. Physiological Reviews, 87(1), 245–313. doi:10.1152/physrev.00044.2005
Block, M. L., Zecca, L., & Hong, J.-S. (2007). Microglia-mediated neurotoxicity: uncovering the molecular mechanisms. Nature reviews. Neuroscience, 8(1), 57–69. doi:10.1038/nrn2038
Chan, P. H. (2001). Reactive oxygen radicals in signaling and damage in the ischemic brain. Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism, 21(1), 2–14. doi:10.1097/00004647-200101000-00002
De Simoni, A., Allen, N. J., & Attwell, D. (2008). Charge compensation for NADPH oxidase activity in microglia in rat brain slices does not involve a proton current. European Journal of Neuroscience, 28(6), 1146–1156. doi:10.1111/j.1460-9568.2008.06417.x
DeCoursey, T. E. (2003). Interactions between NADPH oxidase and voltage-gated proton channels: why electron transport depends on proton transport. FEBS letters, 555(1), 57–61. doi:10.1016/S0014-5793(03)01103-7
DeCoursey, T. E. (2010). Voltage-gated proton channels find their dream job managing the respiratory burst in phagocytes. Physiology (Bethesda, Md.), 25(1), 27–40. doi:10.1152/physiol.00039.2009
DeCoursey, T. E., Morgan, D., & Cherny, V. V. (2003). The voltage dependence of NADPH oxidase reveals why phagocytes need proton channels. Nature, 422(6931), 531–534. doi:10.1038/nature01523
Hodgkin, A. L., & HUXLEY, A. F. (1952a). A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of Physiology, 117(4), 500–544.
Hodgkin, A. L., & HUXLEY, A. F. (1952b). The components of membrane conductance in the giant axon of Loligo. The Journal of Physiology, 116(4), 473–496.
Lai, A. Y., & Todd, K. G. (2006). Microglia in cerebral ischemia: molecular actions and interactions. Canadian journal of physiology and pharmacology, 84(1), 49–59. doi:10.1139/Y05-143
Moskowitz, M. A., Lo, E. H., & Iadecola, C. (2010). The science of stroke: mechanisms in search of treatments. Neuron, 67(2), 181–198. doi:10.1016/j.neuron.2010.07.002
Nathan, C., & Ding, A. (2010). SnapShot: Reactive Oxygen Intermediates (ROI). Cell, 140(6), 951–951.e2. doi:10.1016/j.cell.2010.03.008
Ramsey, I. S., Ruchti, E., Kaczmarek, J. S., & Clapham, D. E. (2009). Hv1 proton channels are required for high-level NADPH oxidase-dependent superoxide production during the phagocyte respiratory burst. Proceedings of the National Academy of Sciences of the United States of America, 106(18), 7642–7647. doi:10.1073/pnas.0902761106
Schilling, T., & Eder, C. (2007). Ion channel expression in resting and activated microglia of hippocampal slices from juvenile mice. Brain research, 1186, 21–28. doi:10.1016/j.brainres.2007.10.027
Thomas, R. C., & Meech, R. W. (1982). Hydrogen ion currents and intracellular pH in depolarized voltage-clamped snail neurones. Nature, 299(5886), 826–828.
Walder, C. E., Green, S. P., Darbonne, W. C., Mathias, J., Rae, J., Dinauer, M. C., Curnutte, J. T., et al. (1997). Ischemic stroke injury is reduced in mice lacking a functional NADPH oxidase. Stroke; a journal of cerebral circulation, 28(11), 2252–2258.
Wu, L.-J., Wu, G., Sharif, M. R. A., Baker, A., Jia, Y., Fahey, F. H., Luo, H. R., et al. (2012). The voltage-gated proton channel Hv1 enhances brain damage from ischemic stroke. Nature neuroscience. doi:10.1038/nn.3059
Yenari, M. A., Kauppinen, T. M., & Swanson, R. A. (2010). Microglial activation in stroke: therapeutic targets. Neurotherapeutics : the journal of the American Society for Experimental NeuroTherapeutics, 7(4), 378–391. doi:10.1016/j.nurt.2010.07.005