Acidification of tissue pH has long been recognized as a component of the
injury process for many diseases and injuries that affect the brain and spinal cord.
Acute injuries like stroke and trauma induce severe long-lasting acidosis in the
brain, and low tissue pH has been observed in brain lesions associated with
neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease. How
exactly low extracellular pH contributes to the loss of neurons associated with these
injuries remains an unanswered question. Recently, a family of ion channels was
discovered that respond directly to extracellular free protons (low extracellular pH).
This family, known as the Acid-Sensing Ion Channels (ASICs) mediates acid-induced
death of neurons, but little is known about how ASICs activate during
pathological acidosis, and what endogenous factors control their activity.
I began my thesis studying the ASIC1a and ASIC2a channels specifically, in an
effort to determine how ASICs are gated by protons, and how extracellular calcium
regulates their activity. This work lead to the discovery that two regions of the ASIC
protein control high-affinity proton sensing in ASIC1a. I also discovered a novel area of
the human ASIC1a protein that controls calcium-dependent modulation of channel
activation. These results allow us to better understand how ASICs respond to both
protons and calcium, and may help to develop treatments which prevent ASIC-mediated
cell death during injury conditions.
Normally, ASICs require acute rapid decreases in extracellular pH to activate.
Slow incremental decreases in pH, which are expected to occur during brain injury, cause
ASICs to desensitize to acid, and limit channel activity. Yet, ASICs clearly contribute to
neuronal death. To gain a better understanding of how ASIC desensitization occurs, and
what impact it has on acid-induced death, I researched areas of the ASIC protein that
control this form of channel desensitization. This work lead to the discovery of a protein
region that regulates desensitization of ASIC1a. I also determined that desensitizing
ASICs, by gradually reducing pH, can protect neurons from prolonged acidosis.
Interestingly, I discovered that several specific classes of endogenous neuropeptides
prevent ASIC desensitization, allowing for much greater channel activation. I found
these peptides also enhance ASIC-mediated cell death. This has important implications
for acid-induced death in vivo, suggesting that induction of ASIC desensitization could
prevent acid-dependent brain damage. It also demonstrates that certain neuropeptides
enhance cell death by stopping ASIC desensitization.
There are multiple ASIC subunits expressed in the brain. Only some of these
ASICs are known to contribute acid-evoked current in neurons. I investigated the role of
ASIC2b, a subunit with unknown function, in proton-gated current of central neurons. I
found that ASIC2b pairs with ASIC1a to form functional channels with some distinct
properties. I found that ASIC2b/1a channels are present in many hippocampal neurons.
Further, I found that ASIC2b/1a is calcium permeable, and contributes to acid-induced
neuronal death. These experiments are the first to describe novel characteristics of ASIC2b/1a channels, and offer a mechanism for ASIC2-dependent toxicity in central
neurons.
Taken together, this work has helped to establish how ASICs activate in different
extracellular conditions, and what factors impact their activity. I have discovered
previously unknown protein domains in ASICs that contribute to their function. I also
describe, for the first time, a heteromeric ASIC channel present in hippocampal neurons
that contributes to acid-induced neuronal death. I discovered a novel aspect of
neuropeptide modulation of ASICs, and describe how this modulation influences acid-dependent cell death. This work has significant implications for understanding how acid
contributes to brain injury, and how ASICs can be utilized as a target to limit acid-dependent cell death.