Mechanisms of Ischemic Brain Damage

1. Mechanisms of Ischemic Brain Damage Jenn Mejilla

2. 2 Hypothesis of Brain Ischemia • Calcium Hypothesis • Excitotoxic Hypothesis

3. Calcium Hypothesis • Massive Ca+2 entry into cells leads to cell death • Ca+2 catalyzed the breakdown of structural components of cells (membrane lipids and cytoskeletal proteins). • Agonist-receptor interactions at the motor end plate caused necrosis of the target, innervated by cholinergic fibers.

4. When this general hypothesis was applied to the nervous system, it was assumed that calcium entering dendritic cells, caused necrosis of selectively vulnerable neurons by ischemia or hypoxia, hypoglycemic coma, and status epilepticus. Calcium was assumed to enter cells by way of voltage-sensitive calcium channels, which are abundant at the basal dendrites of cells with a tendency to epileptogenic firing.

5. Calcium Metabolism • Presynaptic depolarization causes Ca+2 to enter the cytoplasm of the presynaptic endings • Followed by release of glutamate. This activates two types of ionotropic glutamate receptors- AMPA and NMDA. (AMPA =amino-3-hydroxy-5-methol-4-isoazole propionic acid) (NMDA = N-methyl –D- aspartate)

7. When glutamate activates the AMPA receptor, a channel is opened that • allows the passage of Na+, K+ and H+. When Na+ enters down its electrochemical gradient, it depolarizes the membrane. This allows the influx of Ca+2 by way of any voltage-sensitive calcium channels that may be localized to the postsynaptic membranes of the dendrites and cell body (eg. L and T types) • In addition, it relieves the Mg+2 block of the NMDA gated channel, allowing Ca+2 to enter this high-conductance, unselective cation channel. • The excitatory event is terminated by reuptake of glutamate into presynaptic vesicles and into glial cells.

8. Ca+2 entry via NMDA receptors has special pathophysiologic significance: • NMDA receptor-gated channel has a high calcium conductance • The channels or calcium ions they conduct are in contact with cell • structures that are vulnerable to the increase in intracellular Ca+2. • When Ca+2 ions enter cells by way of NMDA receptor-gated • channels, they are more prone to trigger the production of ROS • , reactive oxygen species, such as H2O2, O2-, OH. • Postynaptic calcium influx stimulates neuronal NO synthase, allowing • for the simultaneous appearance of O2- and NO in postsynaptic • structures.

9. Excitotoxic Hypothesis • Described in 1981 • Excitatory amino acid-related toxicity led to neuronal cell death in tissue slices or primary neuronal cell cultures. • It was initially argued that glutamate activation of AMPA receptors leads to an influx of Na+, Cl- and water- which causes osmolytic cell damage. • Later, results showed that the osmolytic damage was reversible, but the influx of calcium caused a delayed type of damage.

10. It is now clear that a single Ca+2 exposure can lead to secondary compromise of Ca+2, suggesting a delayed failure of calcium regulation.

11. Glutamate and Calcium Triggered Events • Enhanced Lipolysis • Altered Phosphorylation of Proteins • Enhanced production of reactive oxygen and reactive nitrogen species.

13. Enhanced Lipolysis • Ischemia leads to lipolysis because ATP and cytidine triphosphate are no longer present to catalyze the resynthesis of phospholipids, once they are broken down, and because calcium activates enzymes, degrading phospholipids to biologically active compounds such as FFA’s and lysophospholipids. • FFA’s and lysophospholipids are mediators of membrane dysfunction b/c they make act as ionophores and uncoupling agents.

14. Once reperfusion is initiated, the oxidative meta- bolism of arachidonic acid accumulated during the ischemia leads to the formation of cyclooxygenase and lipooxygenase products- active in triggering inflammatory responses.

15. Altered Phosphorylation of Proteins • Ca+2 is an important modulator of the phosphorylation state of many proteins. When proteins are phosphorylated and dephosphorylated, their functions are altered. So, when calcium concentration is transient,particularly when Ca+2 is excessive and sustained, membrane function and metabolic activities alteration can cause harmful effects.

16. Production of ROS and NOS • Ischemia with reperfusion leads to the production of ROS . These free radicals give rise to lipid peroxidation, protein oxidation and DNA damage. • Oxygen radicals and NO, together, exert toxicity • NO has important role in brain ischemia. • 3 types • n-NOS and e-NOS (calcium dependent and constitutively expressed) • i-NOS (expressed by activated macrophages and neutrophils)

17. Neuronal NOS is involved in synaptic signalling; however, under ischemic conditions, it mediates cell death. The same is true for i-NOS. Therefore, the production of NO by the calcium- Dependent n-NOS may be detrimental because it Allows additional and toxic ROS to be formed.

18. Dissolution of the Cytoskeleton • Increase in intracellular Ca+2 activates proteases that break down neurofilaments and contribute to the disassembly of microtubules. • This breakdown cause serious problems in intracellular communication, which depends on the integrity of the cytoskeleton as well as cause damage to the mitochondria of cells.