Recent Investigations on Neurotransmitters' Role in Acute White Matter Injury of Perinatal Glia and Pharmacotherapies-Glia Dynamics in Stem Cell Therapy

Periventricular leukomalacia (PVL) and cerebral palsy are two neurological disease conditions developed from the premyelinated white matter ischemic injury (WMI). The significant pathophysiology of these diseases is accompanied by the cognitive deficits due to the loss of function of glial cells and axons. White matter makes up 50% of the brain volume consisting of myelinated and non-myelinated axons, glia, blood vessels, optic nerves, and corpus callosum. Studies over the years have delineated the susceptibility of white matter towards ischemic injury especially during pregnancy (prenatal, perinatal) or immediately after child birth (postnatal). Impairment in membrane depolarization of neurons and glial cells by ischemia-invoked excitotoxicity is mediated through the overactivation of NMDA receptors or non-NMDA receptors by excessive glutamate influx, calcium, or ROS overload and has been some of the well-studied molecular mechanisms conducive to the injury of white matter. Expression of glutamate receptors (GluR) and transporters (GLT1, EACC1, and GST) has significant influence in glial and axonal-mediated injury of premyelinated white matter during PVL and cerebral palsy. Predominantly, the central premyelinated axons express extensive levels of functional NMDA GluR receptors to confer ischemic injury to premyelinated white matter which in turn invoke defects in neural plasticity. Several underlying molecular mechanisms are yet to be unraveled to delineate the complete pathophysiology of these prenatal neurological diseases for developing the novel therapeutic modalities to mitigate pathophysiology and premature mortality of newborn babies. In this review, we have substantially discussed the above multiple pathophysiological aspects of white matter injury along with glial dynamics, and the pharmacotherapies including recent insights into the application of MSCs as therapeutic modality in treating white matter injury.

REVIEW ■ : Physical Injury of Neurons: Important Roles for Sodium and Chloride Ions

There is growing evidence that ions other than Ca2+ play important roles in the deterioration of neuronal elements in both gray and white matter after physical injury. This review features information gathered with a tissue culture model of dendrite transection regarding the contributions of Na+ and CI- to ultrastructural damage and neuronal death. This information and the results of other in vitro investigations of physical and ischemic/excitotoxic injuries indicate that elevation of internal Na+ is an early event that may contribute significantly to neuronal injury through effects on Na+-driven transport mechanisms. Proposed deleterious consequences include cytoplasmic acidification, reduced mitochondrial energy production, and elevation of intracellular Ca2+ and extracellular excitatory amino acids to toxic levels. Prevention of Na+ entry into neurons after injury has been found to limit ultrastructural damage, prevent death, and preserve electrophysiological function. Although the role of CI- in neuronal injury is less well defined, there is also evidence that elevation of intracellular CI- contributes to structural damage, particularly to the smooth endoplasmic reticulum. In terventions that limit Na+- and CI--mediated damage to injured neurons may have utility in neurosurgery and as acute phase treatments for nervous system trauma and other pathological states. NEURO SCIENTIST 3:89-101, 1997

Regulation of intracellular calcium in N1E-115 neuroblastoma cells: the role of Na(+)/Ca(2+) exchange

In fura 2-loaded N1E-115 cells, regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) following a Ca(2+) load induced by 1 microM thapsigargin and 10 microM carbonylcyanide p-trifluoromethyoxyphenylhydrazone (FCCP) was Na(+) dependent and inhibited by 5 mM Ni(2+). In cells with normal intracellular Na(+) concentration ([Na(+)](i)), removal of bath Na(+), which should result in reversal of Na(+)/Ca(2+) exchange, did not increase [Ca(2+)](i) unless cell Ca(2+) buffer capacity was reduced. When N1E-115 cells were Na(+) loaded using 100 microM veratridine and 4 microg/ml scorpion venom, the rate of the reverse mode of the Na(+)/Ca(2+) exchanger was apparently enhanced, since an approximately 4- to 6-fold increase in [Ca(2+)](i) occurred despite normal cell Ca(2+) buffering. In SBFI-loaded cells, we were able to demonstrate forward operation of the Na(+)/Ca(2+) exchanger (net efflux of Ca(2+)) by observing increases (approximately 6 mM) in [Na(+)](i). These Ni(2+) (5 mM)-inhibited increases in [Na(+)](i) could only be observed when a continuous ionomycin-induced influx of Ca(2+) occurred. The voltage-sensitive dye bis-(1,3-diethylthiobarbituric acid) trimethine oxonol was used to measure changes in membrane potential. Ionomycin (1 microM) depolarized N1E-115 cells (approximately 25 mV). This depolarization was Na(+) dependent and blocked by 5 mM Ni(2+) and 250-500 microM benzamil. These data provide evidence for the presence of an electrogenic Na(+)/Ca(2+) exchanger that is capable of regulating [Ca(2+)](i) after release of Ca(2+) from cell stores.

Pharmacological inhibition of the Na(+)/Ca(2+) exchanger enhances depolarizations induced by oxygen/glucose deprivation but not responses to excitatory amino acids in rat striatal neurons

BACKGROUND AND PURPOSE

Neuronal Na(+)/Ca(2+) exchanger plays a relevant role in maintaining intracellular Ca(2+) and Na(+) levels under physiological and pathological conditions. However, the role of this exchanger in excitotoxicity and ischemia-induced neuronal injury is still controversial and has never been studied in the same neuronal subtypes.

METHODS

We investigated the effects of bepridil and 3',4'-dichlorobenzamil (DCB), 2 blockers of the Na(+)/Ca(2+) exchanger, in rat striatal spiny neurons by utilizing intracellular recordings in brain slice preparations to compare the action of these drugs on the membrane potential changes induced either by oxygen and glucose deprivation (OGD) or by excitatory amino acids (EAAs).

RESULTS

Bepridil (3 to 100 micromol/L) and DCB (3 to 100 micromol/L) caused a dose-dependent enhancement of the OGD-induced depolarization measured in striatal neurons. The EC(50) values for these effects were 31 micromol/L and 29 micromol/L, respectively. At these concentrations neither bepridil nor DCB altered the resting membrane properties of the recorded cells (membrane potential, input resistance, and current-voltage relationship). The effects of bepridil and DCB on the OGD-induced membrane depolarization persisted in the presence of D-2-amino-5-phosphonovalerate (50 micromol/L) plus 6-cyano-7-nitroquinoxaline-2,3-dione (20 micromol/L), which suggests that they were not mediated by an enhanced release of EAAs. Neither tetrodotoxin (1 micromol/L) nor nifedipine (10 micromol/L) affect the actions of these 2 blockers of the Na(+)/Ca(2+) exchanger, which indicates that voltage-dependent Na(+) channels and L-type Ca(2+) channels were not involved in the enhancement of the OGD-induced depolarization. Conversely, the OGD-induced membrane depolarization was not altered by 5-(N, N-hexamethylene) amiloride (1 to 3 micromol/L), an inhibitor of the Na(+)/H(+) exchanger, which suggests that this antiporter did not play a prominent role in the OGD-induced membrane depolarization recorded from striatal neurons. Bepridil (3 to 100 micromol/L) and DCB (3 to 100 micromol/L) did not modify the amplitude of the excitatory postsynaptic potentials evoked by cortical stimulation. Moreover, these blockers did not affect membrane depolarizations caused by brief applications of glutamate (0.3 to 1 mmol/L), AMPA (0. 3 to 1 micromol/L), and NMDA (10 to 30 micromol/L).

CONCLUSIONS

These results provide pharmacological evidence that the activation of the Na(+)/Ca(2+) exchanger exerts a protective role during the early phase of OGD in striatal neurons, although it does not shape the amplitude and the duration of the electrophysiological responses of these cells to EAA.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Role of Na+/Ca2+ exchange in regulation of neuronal Ca2+ homeostasis requires re-evaluation

In cultured rat cerebellar granule cells an inhibition of plasma membrane Na+/Ca2+ exchange by removal of external Na+ (replacement with NMDG) caused an increase in [Ca2+]i at rest and a considerable delay in [Ca2+]i recovery from Glu-imposed [Ca2+]i load. These effects did not result from Ca2+ influx through reversed Na+/Ca2+ exchange since they were readily abolished or prevented by using the NMDA receptor inhibitor AP-5 (100 microM) or the NMDA channel blocker memantine (25-50 microM). The effect of Na+/NMDG replacement could be enhanced by: (1) an increase in cytoplasmic Na+ concentration by monensin pretreatment of neurons; (2) external alkalinity, pH 8.5; (3) blockade of the mitochondrial Ca2+ uptake with antimycin plus oligomycin. Analysis of the data obtained led us to conclude that all the changes in [Ca2+]i caused by Na+/NMDG replacement are mainly due to a release of endogenous Glu (reversed Glu uptake) and a subsequent Ca2+ influx through NMDA receptor-mediated channels.

Immunolocalization of the Na(+)-Ca2+ exchanger in mammalian myelinated axons

Previous studies on the pathophysiology of white matter anoxic injury have revealed that the Na(+)-Ca2+ exchanger is an important mediator of Ca2+ overload. To date, however, the localization of this key Ca2+ transporter in myelinated axons has not been demonstrated. The present study uses immunofluorescence labeling with a monoclonal antibody (R3F1) to the canine cardiac type I Na(+)-Ca2+ exchanger to localize exchanger protein to rat peripheral and central myelinated axons. The indirect immunofluorescence labeling technique was used to study paraformaldehyde fixed frozen cryostat sections of sciatic nerve, optic nerve and spinal cord. Examination of sciatic nerve sections with both conventional and confocal microscopy revealed a staining pattern which suggested both a glial and axonal localization of the exchanger. In the rat optic nerve, positive label was associated with cell bodies and their processes, likely glia, and with numerous finer processes arranged in parallel, running longitudinally. These finer processes likely represent axonal profiles. A similar staining pattern was observed in lateral and dorsal columns from spinal cord. Immunoelectron microscopy of dorsal root axons revealed gold particles associated with the paranodal and internodal myelin, in the axoplasm, and close to the nodal/paranodal axon membrane. The high density of Na(+)-Ca2+ exchanger demonstrated in central and peripheral myelinated mammalian axons supports the importance of this transporter in Ca2+ regulation in these tissues.

Ion transport and membrane potential in CNS myelinated axons. II. Effects of metabolic inhibition

Compound resting membrane potential was recorded by the grease gap technique (37 degrees C) during glycolytic inhibition and chemical anoxia in myelinated axons of rat optic nerve. The average potential recorded under control conditions (no inhibitors) was -47 +/- 3 (SD) mV and was stable for 2-3 h. Zero glucose (replacement with sucrose) depolarized the nerve in a monotonic fashion to 55 +/- 10% of control after 60 min. In contrast, glycolytic inhibition with deoxyglucose (10 mM, glucose omitted) or iodoacetate (1 mM) evoked a characteristic voltage trajectory consisting of four distinct phases. A distinct early hyperpolarizing response (phase 1) was followed by a rapid depolarization (phase 2). Phase 2 was interrupted by a second late hyperpolarizing response (phase 3), which led to an abrupt reduction in the rate of potential change, causing nerves to then depolarize gradually (phase 4) to 75 +/- 9% and 55 +/- 6% of control after 60 min, in deoxyglucose and iodoacetate, respectively. Pyruvate (10 mM) completely prevented iodoacetate-induced depolarization. Effects of glycolytic inhibitors were delayed by 20-30 min, possibly due to continued, temporary oxidative phosphorylation using alternate substrates through the tricarboxylic acid cycle. Chemical anoxia (CN- 2 mM) immediately depolarized nerves, and phase 1 was never observed. However a small inflection in the voltage trajectory was typical after approximately 10 min. This was followed by a slow depolarization to 34 +/- 4% of control resting potential after 60 min of CN-. Addition of ouabain (1 mM) to CN--treated nerves caused an additional depolarization, indicating a minor glycolytic contribution to the Na+-K+-ATPase, which is fueled preferentially by ATP derived from oxidative phosphorylation. Phases 1 and 3 during iodoacetate exposure were diminished under nominally zero Ca2+ conditions and abolished with the addition of the Ca2+ chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM). Tetraethylammonium chloride (20 mM) also reduced phase 1 and eliminated phase 3. The inflection observed with CN- was eliminated during exposure to zero-Ca2+/EGTA. A Ca2+-activated K+ conductance may be responsible for the observed hyperpolarizing inflections. Block of Na+ channels with tetrodotoxin (TTX; 1 microM) or replacement of Na+ with the impermeant cation choline significantly reduced depolarization during glycolytic inhibition with iodoacetate or chemical anoxia. The potential-sparing effects of TTX were less than those of choline-substituted perfusate, suggesting additional, TTX-insensitive Na+ influx pathways in metabolically compromised axons. The local anesthetics, procaine (1 mM) and QX-314 (300 microM), had similar effects to TTX. Taken together, the rate and extent of depolarization of metabolically compromised axons is dependent on external Na+. The Ca2+-dependent hyperpolarizing phases and reduction in rate of depolarization at later times may reflect intrinsic mechanisms designed to limit axonal injury during anoxia/ischemia.