NSJ-Spine January 2004
Total Page：16
File Type：pdf, Size：1020Kb
Neurosurg Focus 22 (5):E2, 2007 Cytotoxic edema: mechanisms of pathological cell swelling DANNY LIANG, M.D.,1 SERGEI BHATTA, M.S.1,3 VOLODYMYR GERZANICH, M.D., PH.D.,1 AND J. MARC SIMARD, M.D., PH.D.1,2,3 Departments of 1Neurosurgery, 2Pathology, and 3Physiology, University of Maryland School of Medicine, Baltimore, Maryland PCerebral edema is caused by a variety of pathological conditions that affect the brain. It is associated with two separate pathophysiological processes with distinct molecular and physiological antecedents: those related to cytotoxic (cellular) edema of neurons and astrocytes, and those related to transcapillary flux of Na+ and other ions, water, and serum macromolecules. In this review, the authors focus exclusively on the first of these two processes. Cytotoxic edema results from unchecked or uncompensated influx of cations, mainly Na+, through cation channels. The authors review the different cation channels that have been implicated in the formation of cytotoxic edema of astrocytes and neurons in different pathological states. A better understanding of these molecular mechanisms holds the promise of improved treatments of cerebral edema and of the secondary injury produced by this pathological process. KEY WORDS • cation channel • cytotoxic edema • hypoxia • stroke • sulfonylurea receptor 1 • traumatic brain injury YTOTOXIC EDEMA IN THE CNS is typically accompa- that is, cytotoxic edema. Cytotoxic edema by itself does nied by brain swelling. Edema can result from not result in brain swelling, but formation of cytotoxic C almost any insult to the brain, including trauma, in- edema depletes the extracellular space of Na+, Cl–, and farction, neoplasm, abscess, or conditions such as hypox- water, thereby creating a new gradient for these molecules ia or toxic or metabolic perturbation.10,93,94 Stroke and trau- across the capillary of the blood–brain barrier. With ap- matic brain injury are especially prevalent causes of propriate changes in capillary permeability,138 the new gra- morbidity and mortality. In the US, stroke is the third most dient created by cytotoxic edema results in driving tran- common cause of death, with more than 730,000 first- scapillary formation of ionic edema. Thus, cytotoxic edema time incidents each year.47,126 Traumatic brain injury af- is important in its own right, because it signals a premorbid flicts 1.4 million people yearly, resulting in 50,000 deaths cellular process that almost inevitably leads to oncotic or and 235,000 hospitalizations.82,83,131,151 necrotic cell death. But equally important, cytotoxic ede- Cytotoxic edema is defined as the premorbid cellular ma supplies the driving force for the formation of ionic process, otherwise known as cellular edema, oncotic cell edema, which is the process that introduces new mass + + – swelling, or oncosis, whereby extracellular Na and other (Na , Cl , H2O) that is ultimately responsible for brain cations enter into neurons and astrocytes and accumulate swelling.138 intracellularly, in part due to failure of energy-dependent Excellent reviews that summarize current knowledge on mechanisms of extrusion. Unchecked influx of cations oc- this topic have been published.10,21,37,46,71–73,112,125,166 In our curs largely through cation channels. Cation influx, in recent review,138 we examined molecular mechanisms in- turn, drives influx of anions, which maintains electrical volved in transcapillary flux of Na+, water, plasma ultrafil- neutrality, and in combination these phenomena drive in- trate, and blood that lead to brain swelling. In this paper, we flux of water, resulting in osmotic expansion of the cell, review molecular mechanisms involved in cytotoxic edema. Abbreviations used in this paper: ADP = adenosine diphosphate; AQP = aquaporin; ASIC = acid-sensing ion channel; ATP = adeno- Pathophysiology of Cytotoxic Edema sine triphosphate; CNS = central nervous system; CSF = cere- When an insult to the brain results in ischemia or hy- brospinal fluid; MCA = middle cerebral artery; NCCa-ATP = calcium- activated, ATP-sensitive nonselective cation; NKCC = Na+/K+/2Cl– poxia, very little new ATP can be produced due to abro- 56 cotransporter; NMDA = N-methyl-D-aspartate; SUR1 = sulfony- gation of oxidative phosphorylation. Cells quickly use lurea receptor 1; TRP = transient receptor potential; TRPC = TRP up their reserves of ATP and, unless normoxia is restored, canonical; TRPM = TRP melastatin; TRPV = TRP vanilloid. the deranged cellular machinery loses its ability to sustain Neurosurg. Focus / Volume 22 / May, 2007 1 Unauthenticated | Downloaded 09/28/21 10:33 AM UTC D. Liang et al. homeostasis. Primary active transport, mainly ATP-de- quence of metabolic responses of brain tissue to a decrease pendent Na+/K+ ATPase that requires continuous expendi- in blood flow.60–62,66,69 The brain area where blood flow is ture of ATP, is necessary to maintain homeostais.146,162 The either absent or measures less than 10 ml/100 g (brain tis- balance between survival and death is determined by the sue)/min is rapidly and irreversibly damaged in less than struggle between electrogenic pump activity and channels 6 minutes, forming an “ischemic core.” This infarcted tis- that enable Na+ to enter into cells.137,161 Cellular survival sue is surrounded by the “penumbra” of hypoxic but liv- requires that Na+ be continuously extruded from the intra- ing tissue with blood flow greater than 20 ml/100 g (brain cellular compartment, because this is critical to maintain- tissue)/min. Cells in the penumbra undergo cytotoxic ede- ing normal cell volume. ma and other changes that are potentially reversible if per- Depletion of ATP is accompanied by unchecked influx fusion is restored within the first few hours after injury. If of extracellular ions, primarily Na+, down their electro- hypoxic conditions persist, however, penumbral cells with chemical gradients. This influx is driven by energy stored cytotoxic edema eventually die, extending the course of in preexisting ionic gradients across the cell membrane.84 cell death deeper into the parenchyma than the originally Sodium ion influx in turn drives Cl– influx via chloride involved core. The penumbra, therefore, is the main ther- channels, and the resultant increase in intracellular osmo- apeutic target in the prevention of ischemic stroke and in- larity drives inflow of water via AQP channels (among jury. others).8,11,12,46,71–73,75 Extracellular water flows into the cell A number of studies have now shown that pharmaco- interior, resulting in an increase in intracellular fluid vol- logical inhibition of ion channels, including nonselective ume, at the expense of the extracellular space. Morpho- cation channels, reduces focal ischemic injury in rodent logically, this process results in alterations in membrane models of ischemic stroke.59,98,118,172 Nonselective cation surface architecture with prominent bleb formation. In the channels are distinguished from selective cation channels initial stages of cytotoxic edema, the blood–brain barrier by their permeability properties; ion selective channels are is intact and largely impermeable to ions and fluids, so typically permeable to a single cation, such as Na+, K+, or extracellular ions and water loss are not replenished. Thus, Ca2+, whereas a nonselective cation channel may allow fluid movement involved in formation of cytotoxic edema flux of any monovalent cation, or even a mixture of mono- does not lead to any change in total brain volume, despite valent and divalent cations. It is likely that these channels the observable increase in cell size. play an important role in secondary injury in the penum- Cells in both gray and white matter are affected by cyto- bra,139 and thus targeting these channels offers the possi- toxic edema.52 Cellular swelling begins within 30 minutes bility of reducing secondary injury. In the sections that fol- of MCA occlusion, particularly around capillaries, per- low, we review several of the nonselective cation channels sists for up to 24 hours after reperfusion,43,44 and results in that have been implicated in cytotoxic edema and sec- an average reduction of extracellular space from the nor- ondary injury in the penumbra. mal 20% down to 4 to 10%.86,147 Astrocytic swelling is much more prominent than neuronal swelling. Astrocytes The ASIC Channel are more prone to pathological swelling than neurons, because they are involved in clearance of K+ and gluta- Acid-sensing ion channels are members of the recently mate, which cause osmotic overload that in turn promotes discovered epithelial sodium channel/degenerin gene fam- water inflow. Astrocytic but not neuronal NKCC is upreg- ily of ion channels. Acid-sensing ion channel genes en- + code proton-gated cation channels in both the central and ulated by elevated extracellular K and cell swel- 153,154 ling.100,142,143 Expression of high levels of the water channel peripheral nervous system. Six different ASIC sub- 90,91 units have been cloned to date, which are encoded by four AQP4 is also important. 16 When compensatory mechanisms such as ionic pumps in genes, ASIC1–ASIC4. the plasma membrane are exceeded or fail altogether, the Acid-sensing ion channels are hydrogen ion–gated swollen cell dies.148 This pathway to cell death was called cation channels that are activated as pH falls but are gen- erally inactive at physiological pH (7.4). All ASICs are oncosis (derived from the Greek word “onkos,” which + 2+ means swelling) by von Recklinghausen, specifically to permeable to Na and,