Excitotoxicity

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Low Ca2+ buffering and excitotoxicity under physiological stress and pathophysiological conditions in motor neuron (MNs). Low Ca2+ buffering in amyotrophic lateral sclerosis (ALS) vulnerable hypoglossal MNs exposes mitochondria to higher Ca2+ loads compared to highly buffered cells. Under normal physiological conditions, the neurotransmitter opens glutamate, NMDA and AMPA receptor channels, and voltage dependent Ca2+ channels (VDCC) with high glutamate release, which is taken up again by EAAT1 and EAAT2. This results in a small rise in intracellular calcium that can be buffered in the cell. In ALS, a disorder in the glutamate receptor channels leads to high calcium conductivity, resulting in high Ca2+ loads and increased risk for mitochondrial damage. This triggers the mitochondrial production of reactive oxygen species (ROS), which then inhibit glial EAAT2 function. This leads to further increases in the glutamate concentration at the synapse and further rises in postsynaptic calcium levels, contributing to the selective vulnerability of MNs in ALS. Jaiswal et al., 2009.[1]

Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate (glutamate receptors) such as the NMDA receptor and AMPA receptor are overactivated by Glutamatergic Storm. Excitotoxins like NMDA and kainic acid which bind to these receptors, as well as pathologically high levels of glutamate, can cause excitotoxicity by allowing high levels of calcium ions[2] (Ca2+) to enter the cell. Ca2+ influx into cells activates a number of enzymes, including phospholipases, endonucleases, and proteases such as calpain. These enzymes go on to damage cell structures such as components of the cytoskeleton, membrane, and DNA.

Excitotoxicity may be involved in spinal cord injury, stroke, traumatic brain injury, hearing loss (through noise overexposure or ototoxicity) and in neurodegenerative diseases of the central nervous system (CNS) such as multiple sclerosis, Alzheimer's disease, amyotrophic lateral sclerosis (ALS), Parkinson's disease, alcoholism or alcohol withdrawal and especially benzodiazepine withdrawal, and also Huntington's disease.[3][4] Other common conditions that cause excessive glutamate concentrations around neurons are hypoglycemia[5] and status epilepticus.[6]

History[edit]

The harmful effect of glutamate upon the CNS were first observed in 1954 by T. Hayashi, a Japanese scientist who noted that direct application of glutamate to the CNS caused seizure activity, though this report went unnoticed for several years. The toxicity of glutamate was then observed by D. R. Lucas and J. P. Newhouse in 1957, when the subcutaneous injection of monosodium glutamate to newborn mice destroyed the neurons in the inner layers of the retina.[7] Later, in 1969, John Olney discovered the phenomenon was not restricted to the retina, but occurred throughout the brain, and coined the term excitotoxicity. He also assessed that cell death was restricted to postsynaptic neurons, that glutamate agonists were as neurotoxic as their efficiency to activate glutamate receptors, and that glutamate antagonists could stop the neurotoxicity.[8] Subsequent research by Mark Mattson provided evidence for the involvement of excitotoxicity in Alzheimer's disease, and other age-related neurodegenerative conditions that involve oxidative stress and cellular energy deficits.

Pathophysiology[edit]

Excitotoxicity can occur from substances produced within the body (endogenous excitotoxins). Glutamate is a prime example of an excitotoxin in the brain, and it is also the major excitatory neurotransmitter in the mammalian CNS.[9] During normal conditions, glutamate concentration can be increased up to 1mM in the synaptic cleft, which is rapidly decreased in the lapse of milliseconds.[10] When the glutamate concentration around the synaptic cleft cannot be decreased or reaches higher levels, the neuron kills itself by a process called apoptosis.[11] [12]

This pathologic phenomenon can also occur after brain injury and spinal cord injury. Within minutes after spinal cord injury, damaged neural cells within the lesion site spill glutamate into the extracellular space where glutamate can stimulate presynaptic glutamate receptors to enhance the release of additional glutamate.[13]Brain trauma or stroke can cause ischemia, in which blood flow is reduced to inadequate levels. Ischemia is followed by accumulation of glutamate and aspartate in the extracellular fluid, causing cell death, which is aggravated by lack of oxygen and glucose. The biochemical cascade resulting from ischemia and involving excitotoxicity is called the ischemic cascade. Because of the events resulting from ischemia and glutamate receptor activation, a deep chemical coma may be induced in patients with brain injury to reduce the metabolic rate of the brain (its need for oxygen and glucose) and save energy to be used to remove glutamate actively. (The main aim in induced comas is to reduce the intracranial pressure, not brain metabolism).[citation needed]

Increased extracellular glutamate levels leads to the activation of Ca2+ permeable NMDA receptors on myelin sheaths and oligodendrocytes, leaving oligodendrocytes susceptible to Ca2+ influxes and subsequent excitotoxicity. [14] [15] One of the damaging results of excess calcium in the cytosol is initiating apoptosis through cleaved caspase processing. [16] Another damaging result of excess calcium in the cytosol is the opening of the mitochondrial permeability transition pore, a pore in the membranes of mitochondria that opens when the organelles absorb too much calcium. Opening of the pore may cause mitochondria to swell and release reactive oxygen species and other proteins that can lead to apoptosis. The pore can also cause mitochondria to release more calcium. In addition, production of adenosine triphosphate (ATP) may be stopped, and ATP synthase may in fact begin hydrolysing ATP instead of producing it.[17]

Inadequate ATP production resulting from brain trauma can eliminate electrochemical gradients of certain ions. Glutamate transporters require the maintenance of these ion gradients to remove glutamate from the extracellular space. The loss of ion gradients results in not only the halting of glutamate uptake, but also the reversal of the transporters. The Na+-glutamate transporters on neurons and astrocytes can reverse their glutamate transport and start secreting glutamate at a concentration capable of inducing excitotoxicity. [18] This results in a buildup of glutamate and further damaging activation of glutamate receptors.[19]

On the molecular level, calcium influx is not the only factor responsible for apoptosis induced by excitoxicity. Recently,[20] it has been noted that extrasynaptic NMDA receptor activation, triggered by both glutamate exposure or hypoxic/ischemic conditions, activate a CREB (cAMP response element binding) protein shut-off, which in turn caused loss of mitochondrial membrane potential and apoptosis. On the other hand, activation of synaptic NMDA receptors activated only the CREB pathway, which activates BDNF (brain-derived neurotrophic factor), not activating apoptosis.[citation needed]

See also[edit]

Sources[edit]

References[edit]

  1. ^ Jaiswal MK, Zech WD, Goos M, Leutbecher C, Ferri A, Zippelius A, Carrì MT, Nau R, Keller BU (2009). "Impairment of mitochondrial calcium handling in a mtSOD1 cell culture model of motoneuron disease". BMC Neurosci 10: 64. doi:10.1186/1471-2202-10-64. PMC 2716351. PMID 19545440. 
  2. ^ Manev H, Favaron M, Guidotti A, and Costa E. Delayed increase of Ca2+ influx elicited by glutamate: role in neuronal death. Molecular Pharmacoloy. 1989 Jul;36(1):106-112. PMID 2568579. Retrieved on January 31, 2007.
  3. ^ Kim AH, Kerchner GA, and Choi DW. Blocking Excitotoxicity or Glutamatergic Storm. Chapter 1 in CNS Neuroproteciton. Marcoux FW and Choi DW, editors. Springer, New York. 2002. Pages 3-36
  4. ^ Hughes JR (February 2009). "Alcohol withdrawal seizures". Epilepsy Behav 15 (2): 92–7. doi:10.1016/j.yebeh.2009.02.037. PMID 19249388. 
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  7. ^ Lucas, DR; Newhouse, JP (1957). "The toxic effect of sodium L-glutamate on the inner layers of the retina.". A.M.A. Archives of ophthalmology 58 (2): 193–201. PMID 13443577. 
  8. ^ Olney, JW (1969). "Brain lesions, obesity, and other disturbances in mice treated with monosodium glutamate.". Science 164 (3880): 719–21. doi:10.1126/science.164.3880.719. PMID 5778021. 
  9. ^ Temple MD, O'Leary DM, and Faden AI. The role of glutamate receptors in the pathophysiology of traumatic CNS injury. Chapter 4 in Head Trauma: Basic, Preclinical, and Clinical Directions. Miller LP and Hayes RL, editors. Co-edited by Newcomb JK. John Wiley and Sons, Inc. New York. 2001. Pages 87-113.
  10. ^ Clements, JD; Lester, RA; Tong, G; Jahr, CE; Westbrook, GL (1992). "The time course of glutamate in the synaptic cleft". Science 258 (5087): 1498–501. doi:10.1126/science.1359647. PMID 1359647. 
  11. ^ Derek D. Yang et al, "Absence of excitotoxicity-induced apoptosis in the hippocampus of mice lacking the Jnk3 gene" Nature 389, 865-870 (23 October 1997) doi:10.1038/39899.
  12. ^ Maria Ankarcrona, et al, "Glutamate-induced neuronal death: A succession of necrosis or apoptosis depending on mitochondrial function", Neuron, Volume 15, Issue 4, 961-973, 1 October 1995, doi:10.1016/0896-6273(95)90186-8
  13. ^ Hulsebosch et al. (Apr 2009). "Mechanisms of chronic central neuropathic pain after spinal cord injury". Brain Res Rev 60 (1): 202–13. 
  14. ^ Nakamura et al, "S-nitrosylation of Drp1 links excessive mitochondrial fission to neuronal injury in neurodegeneration" Mitochondrion", 2010 Aug;10(5):573-8
  15. ^ Dutta et al. (Jan 2011). "Mechanisms of neuronal dysfunction and degeneration in multiple sclerosis". Prog Neurobiol 93 (1): 1–12. 
  16. ^ Dutta et al. (Jan 2011). "Mechanisms of neuronal dysfunction and degeneration in multiple sclerosis". Prog Neurobiol 93 (1): 1–12. 
  17. ^ Stavrovskaya, IG; Kristal, BS (2005). "The powerhouse takes control of the cell: is the mitochondrial permeability transition a viable therapeutic target against neuronal dysfunction and death?". Free radical biology & medicine 38 (6): 687–97. doi:10.1016/j.freeradbiomed.2004.11.032. PMID 15721979. 
  18. ^ Li et al, "Na+-K+-ATPase inhibition and depolarization induce glutamate release via reverse Na+-dependent transport in spinal cord white matter" Neuroscience 107, 675–683
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  20. ^ Hardingham, GE; Fukunaga, Y; Bading, H (2002). "Extrasynaptic NMDARs oppose synaptic NMDARs by triggering CREB shut-off and cell death pathways". Nature Neuroscience 5 (5): 405–14. doi:10.1038/nn835. PMID 11953750.