Spreading depolarizations (SDs) are waves of sustained near-complete neuronal and glial depolarization that actively propagate a collapse of ion gradients through the brain with associated dramatic neuronal and glial swelling that entails cytotoxic edema. Recovery of ion gradients depends on the sufficient sodium pump activity which is energy-dependent. In stroke and trauma patients SDs are thought to exacerbate tissue damage in the at-risk cortical territory supporting the view that SD may be an important mechanistic endpoint in clinical studies. SD-induced neuronal damage involves a plasma membrane conformational change including soma swelling and terminal dendritic beading with spine loss which signifies acute damage to synaptic circuitry. In the ischemic core, where energy deprivation is maintained, dendrites remain terminally injured by SD. In contrast, we have shown that in the penumbra SDs result in rapid dendritic beading which is reversible, but signals leading to neuronal death could be initiated during this time. Hence, dendritic beading is the hallmark of neuronal injury. Once the energy demands for recovery of penumbral dendrites are no longer met by the diminishing blood flow, SD terminally injures dendrites and spines are lost indicating acute damage to synaptic circuitry and core expansion. SD-induced cytotoxic edema contributes to stroke injury. Mammalian pyramidal neurons lack functional aquaporins, thus the molecular pathways by which they accumulate osmotically obligated water and rapidly swell during SD is unknown. Bulk water influx could occur through large-pore pannexin channels opened by SD. Neuronal cation-chloride cotransporters may also be responsible for water accumulation as well as recovery and this will be studied in aim 1. We have shown that persistent astroglial swelling is initiated and exacerbated during SD in brain tissue with moderate to severe energy deficits, likely disrupting astroglial maintenance of homeostatic function and thus their ability to support neurons. Astroglial failure should advance the damage of neighboring neurons contributing to SD-induced expansion of the injury and it will be examined in aim 2. Astrocytes are part of the synaptic circuitry tightly coupled with neurons combining with axons and dendrites to form the tripartite synapse. SD-induced injury at the level of single synapses will be investigated in aim 3 with quantitative serial section electron microscopy.
The specific aims are: 1) To examine possible routes of rapid water entry during SD-induced dendritic beading. 2) To test the hypothesis that SD-inflicted dendritic injury is aggravated in tissue with selectively impaired glial metabolism. 3) To test the hypothesis that SD is the mechanism implicated in rapid synapses disruption and loss. We will combine in vivo 2-photon laser scanning microscopy of fluorescent neurons, astrocytes and blood flow in adult mouse sensorimotor cortex with other sophisticated in vivo approaches such as laser speckle and functional intrinsic optical signal imaging while simultaneously monitoring occurrence of SD with electrophysiology. The results will bring new insight to the development and recovery from acute cellular injury in stroke.
The American Heart Association 2012 Heart Disease and Stroke Statistics Update reports that on average someone in the US has a stroke every 40 seconds and dies as a result every 4 minutes. Two of the major pathogenic events that exacerbate injury in stroke patients are the spreading depolarizing waves and the associated brain swelling that course across the cortex injuring brain cells. Detailed understanding of how this injury evolves early after insult will help to design treatments for brain recovery after stroe.
