Low concentrations of ethanol deplete type-2 astrocytes of intracellular free magnesium

Effect of l-serine and magnesium ions on the functional properties of human phosphoserine phosphatase and its pathogenetic variants

L-Ser supply in the central nervous system of mammals mostly relies on its endogenous biosynthesis by the phosphorylated pathway (PP). Defects in any of the three enzymes operating in the pathway result in a group of neurometabolic diseases collectively known as serine deficiency disorders (SDDs). Phosphoserine phosphatase (PSP) catalyzes the last, irreversible step of the PP. Here we investigated in detail the role of physiological modulators of human PSP activity and the properties of three natural PSP variants (A35T, D32N and M52T) associated with SDDs. Our results, partially contradicting previous reports, indicate that: i. PSP is almost fully saturated with Mg2+ under physiological conditions and fluctuations in Mg2+ and Ca2+ concentrations are unlikely to play a modulatory role on PSP activity; ii. Inhibition by L-Ser, albeit at play on the isolated PSP, does not exert any effect on the flux through the PP unless the enzyme activity is severely impaired by inactivating substitutions; iii. The so-far poorly investigated A35T substitution was the most detrimental, with a 50-fold reduction in catalytic efficiency, and a reduction in thermal stability (as well as an increase in the IC50 for L-Ser). The M52T substitution had similar, but milder effects, while the D32N variant behaved like the wild-type enzyme. iv. Predictions of the structural effects of the A35T and M52T substitutions with ColabFold suggest that they might affect the structure of the flexible helix-loop region.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Ethanol Effect on BK Channels is Modulated by Magnesium

BACKGROUND

Alcoholics have been reported to have reduced levels of magnesium in both their extracellular and intracellular compartments. Calcium-dependent potassium channels (BK) are known to be one of ethanol (EtOH)'s better known molecular targets.

METHODS

Using outside-out patches from hippocampal neuronal cultures, we examined the consequences of altered intracellular Mg(2+) on the effects that EtOH has on BK channels.

RESULTS

We find that the effect of EtOH is bimodally influenced by the Mg(2+) concentration on the cytoplasmic side. More specifically, when internal Mg(2+) concentrations are ≤200 μM, EtOH decreases BK activity, whereas it increases activity when Mg(2+) is at 1 mM. Similar results are obtained when using patches from HEK cells expressing only the α-subunit of BK. When patches are made with the actin destabilizer cytochalasin D present on the cytoplasmic side, the potentiation caused by EtOH becomes independent of the Mg(2+) concentration. Furthermore, in the presence of the actin stabilizer phalloidin, EtOH causes inhibition even at Mg(2+) concentrations of 1 mM.

CONCLUSIONS

Internal Mg(2+) can modulate the EtOH effects on BK channels only when there is an intact, internal actin interaction with the channel, as is found at synapses. We propose that the EtOH-induced decrease in cytoplasmic Mg(2+) observed in frequent/chronic drinkers would decrease EtOH's actions on synaptic (e.g., actin-bound) BK channels, producing a form of molecular tolerance.

Modulation of BK Channels by Ethanol

In alcohol-naïve systems, ethanol (<100mM) exposure of calcium-gated BK channels perturbs physiology and behavior. Brief (several minutes) ethanol exposure usually leads to increased BK current, which results from ethanol interaction with a pocket mapped to the BK channel-forming slo1 protein cytosolic tail domain. The importance of this region in ethanol-induced intoxication has been independently supported by an unbiased screen of Caenorhabditis elegans slo1 mutants. However, ethanol-induced BK activation is not universal as refractoriness and inhibition have been reported. The final effect depends on many factors, including intracellular calcium levels, slo1 isoform, BK beta subunit composition, posttranslational modification of BK proteins, channel lipid microenvironment, and type of ethanol administration. Studies in Drosophila melanogaster, C. elegans, and rodents show that protracted/repeated ethanol administration leads to tolerance to ethanol-induced modification of BK-driven physiology and behavior. Unveiling the mechanisms underlying tolerance is of major importance, as tolerance to ethanol has been proposed as predictor of risk for alcoholism.

Divalent cation chelators citrate and EDTA unmask an intrinsic uncoupling pathway in isolated mitochondria

We demonstrate a suppression of ROS production and uncoupling of mitochondria by exogenous citrate in Mg²⁺ free medium. Exogenous citrate suppressed H₂O₂ emission and depolarized mitochondria. The depolarization was paralleled by the stimulation of respiration of mitochondria. The uncoupling action of citrate was independent of the presence of sodium, potassium, or chlorine ions, and it was not mediated by the changes in permeability of the inner mitochondrial membrane to solutes. The citrate transporter was not involved in the citrate effect. Inhibitory analysis data indicated that several well described mitochondria carriers and channels (ATPase, IMAC, ADP/ATP translocase, mPTP, mKATP) were not involved in citrate's effect. Exogenous MgCl₂ strongly inhibited citrate-induced depolarization. The uncoupling effect of citrate was demonstrated in rat brain, mouse brain, mouse liver, and human melanoma cells mitochondria. We interpreted the data as an evidence to the existence of a hitherto undescribed putative inner mitochondrial membrane channel that is regulated by extramitochondrial Mg²⁺ or other divalent cations.

Silencing TRPM7 in mouse cortical astrocytes impairs cell proliferation and migration via ERK and JNK signaling pathways

Transient receptor potential melastatin 7 (TRPM7), a non-selective cation channel, is highly expressed expressed in the brain and plays a critical role in ischemic neuronal death. Astrocyte, the most abundant cell type in central nervous system (CNS), exerts many essential functions in the physiological and pathological conditions. Here we investigated the expression and functions of the TRPM7 channel in mouse cortical astrocytes. Using reverse transcription (RT)-PCR, immunostaining, western blot and patch clamp recording, we showed that functional TRPM7 channel is expressed in cultured mouse cortical astrocytes. Knocking down TRPM7 with specific siRNA impairs the proliferation and migration of astrocytes by 40.2% ± 3.9% and 40.1% ± 11.5%, respectively. Consistently, inhibition of TRPM7 with 2-aminoethoxydiphenyl borate (2-APB) also decreases astrocyte proliferation and migration by 46.1% ± 2.5% and 64.2% ± 2.4%. MAPKs and Akt signaling pathways have been shown to be implicated in TRPM7-mediated responses including cell proliferation and migration. Our data show that suppression of TRPM7 in astrocytes reduces the phosphorylation of extracellular signal-regulated kinases (ERK) and c-Jun N-terminal kinases (JNK), but not p38 mitogen-activated protein kinase and Akt. In addition, TRPM7, as a cation channel, has been involved in the Ca²⁺ and Mg²⁺ homeostasis in several types of cells. In our study, we found that silencing TRPM7 decreases the intracellular basal Mg²⁺ concentration without affecting Ca²⁺ concentration in astrocytes. However, an addition of Mg²⁺ to the growth medium could not rescue the impaired proliferation of astrocytes. Together, our data suggest that TRPM7 channel may play a critical role in the proliferation and migration of astrocytes via the ERK and JNK pathways.

Magnesium in health and disease

Mammalian cells tightly regulate cellular Mg(2+) content through a variety of transport and buffering mechanisms under the control of various hormones and cellular second messengers. The effect of these hormones and agents results in dynamic changes in the total content of Mg(2+) being transported across the cell membrane and redistributed within cellular compartments. The importance of maintaining proper cellular Mg(2+) content optimal for the activity of various cellular enzymes and metabolic cycles is underscored by the evidence that several diseases are characterized by a loss of Mg(2+) within specific tissues as a result of defective transport, hormonal stimulation, or metabolic impairment. This chapter will review the key mechanisms regulating cellular Mg(2+) homeostasis and their impairments under the most common diseases associated with Mg(2+) loss or deficiency.

Importance of magnesium ions in development of tolerance to ethanol: studies on cultured cerebral vascular smooth muscle cells, type-2 astrocytes and intact rat brain

This study was designed to examine the roles of intracellular free magnesium ion concentration ([Mg(2+)](i)) in ethanol-induced intoxication and development of tolerance in cultured canine cerebral vascular smooth muscle cells and astrocytes as well as intact rat brain. The basal, resting level of [Mg(2+)](i) in cerebrovascular cells was 732.5 +/- 82.4 microM. Exposure of cultured canine cerebral vascular smooth muscle cells to ethanol (10 and 25 mM) for 24 h reduced the concentrations of [Mg(2+)](i) to 521.1 +/- 59.6 microM, and 308.2 +/- 37.8 microM, respectively. However, exposure of these cultured vascular cells to the same concentrations of ethanol, after initial pretreatment with ethanol for 24 h, failed to interfere with the levels of [Mg(2+)](i). Measurement of [Mg(2+)](i) at 48 h and 72 h indicated that the decreased levels of [Mg(2+)](i) induced by ethanol at 24 h treatment returned toward baseline. Similar experiments were performed in cultured type-2 astrocytes isolated from neonatal rat brain. The basal level of [Mg(2+)](i) in type-2 astrocytes was about 125 microM. Incubation of these cells with 10 mM ethanol for 10 min resulted in a 27% reduction in the level of [Mg(2+)](i), whereas incubation with 25 mM ethanol resulted in almost a 50% reduction in [Mg(2+)](i). The decreased levels of [Mg(2+)](i) lasted around 30 min, until the measurement finished. Continuous incubation of these cultured astrocytes, with ethanol (either 10 mM or 25 mM), for more than 24 h, indicated that the concentrations of [Mg(2+)](i) in type-2 astrocytes were equivalent to those at basal, resting levels. In vivo 31P-NMR spectroscopy, performed on intact rat brains, indicated that an initial administration of 4 mg/kg ethanol ( approximately 20-25 mM blood alcohol level) resulted (after 20-40 min of exposure) in severe deficits in whole brain [Mg(2+)](i) (550 +/- 33 microM to 358 +/- 24 microM). Repeated injections of ethanol (4 mg/kg) over the next 24-72 h resulted in progressively diminishing effects on brain [Mg(2+)](i). These experimental data indicate that chronic ethanol treatment can induce a tolerance to depletion of [Mg(2+)](i) in cerebrovascular smooth muscle cells, type-2 astrocytes as well as intact rat brain. The results suggest that [Mg(2+)](i) might play a major role in alcohol-induced tolerance in the brain.

Antioxidants prevent depletion of [Mg2+]i induced by alcohol in cultured canine cerebral vascular smooth muscle cells: possible relationship to alcohol-induced stroke

Low serum concentrations of Mg(2+) ions have been reported, recently, in patients with coronary disease, atherosclerosis, and stroke as well as in patients with cerebral hemorrhage. The aim of the present study was to determine whether potent antioxidants [alpha-tocopherol and pyrrolidine dithiocarbamate (PDTC)] can prevent or ameliorate intracellular Mg(2+) ([Mg(2+)](i)) depletion associated with cerebral vascular injury induced by alcohol. Exposure of cultured canine cerebral vascular smooth muscle cells to alcohol (10-100 mM) for 24 h induced marked depletion in [Mg(2+)](i) (i.e., approximately 30-65%, depending upon alcohol concentration). Treatment of the cultured cells with either PDTC (0.1 microM) or alpha-tocopherol (15 microM) for 24 h, alone, failed to interfere with basal [Mg(2+)](i) levels. However, preincubation of the cells with either alpha-tocopherol or PDTC for 24 h completely inhibited the depletion of [Mg(2+)](i) induced by exposure to 10-100 mM ethanol. These results indicate that alpha-tocopherol and PDTC prevent decreases in [Mg(2+)](i) produced by ethanol. Moreover, these new results suggest that such protective effects of alpha-tocopherol and PDTC on cerebral vascular cells might be useful therapeutic tools in prevention and amelioration of cerebral vascular injury and stroke in alcoholics.

Association of alcohol in brain injury, headaches, and stroke with brain-tissue and serum levels of ionized magnesium: a review of recent findings and mechanisms of action

Although there is general agreement that chronic ingestion of alcohol poses great risks for normal cardiovascular functions and peripheral-vascular homeostasis, a direct cause and effect between the real phenomena of alcohol-induced headache and risk of brain injury and stroke is not appreciated. "Binge drinking" of alcohol is associated with an ever-growing number of strokes and sudden death. It is becoming clear that alcohol ingestion can result in profoundly different actions on the cerebral circulation (e.g., vasodilation, vasoconstriction-spasm, vessel rupture), depending upon dose and physiologic state of host. Using rats, it has been demonstrated that acute, high doses of ethanol can result in stroke-like events concomitant with alterations in brain bioenergetics. We review recent in vivo findings obtained with 31P-NMR spectroscopy, optical reflectance spectroscopy, and direct in vivo microcirculatory studies on the intact brain. Alcohol-induced hemorrhagic stroke is preceded by a rapid fall in brain intracellular free magnesium ions ([Mg2+]i) followed by cerebrovasospasm and reductions in phosphocreatine (PCr)/ATP ratio, intracellular pH, and the cytosolic phosphorylation potential (CPP) with concomitant rises in deoxyhemoglobin (DH), mitochondrial reduced cytochrome oxidase aa3 (rCOaa3), blood volume, and intracellular inorganic phosphate (Pi). Using osmotic mini-pumps implanted in the third cerebral ventricle, containing 30% ethanol, it was found that brain [Mg2+]i is reduced 30% after 14 days; brain PCr fell 15%, whereas the CPP fell 40%. Such animals became susceptible to stroke from nonlethal doses of ethanol. Human subjects with mild head injury have been found to exhibit early deficits in serum ionized Mg (IMg2+); the greater the degree of early head injury (30 min-8 h), the greater and more profound the deficit in serum IMg2+ and the greater the ionized Ca (ICa2+) to IMg2+ ratio. Patients with histories of alcohol abuse or ingestion of alcohol prior to head injury exhibited greater deficits in IMg2+ (and higher ICa2+/IMg2+ ratios) and, unlike the subjects without alcohol, did not leave the hospital for at least several days. Women, for some unknown reason, exhibit a much higher incidence of morbidity and mortality from subarachnoid hemorrhage (SAH) than men. Data on 105 men and women with different types of stroke indicate that, on the average, a 20% deficit in serum IMg2+ is seen; total Mg (TMg) or blood pH is usually near normal. Women with SAH, however, exhibit much lower IMg2+ and higher ICa2+/IMg2+ ratios; the presence of ethanol in the blood is associated with even more depression in IMg2+ in SAH in women. It is possible that prior alcohol ingestion is, in large measure, responsible for a great deal of this unexplained higher incidence of SAH in women. It has recently been reported that the cyclical changes in estrogenic hormones appear to control the serum IMg2+ level in young women. A surge in estrogenic levels prior to SAH could thus precipitate, in part, the SAH. In other human studies, it has been shown that migraines and headache, dizziness, and hangover, which accompany ethanol ingestion, are associated with rapid deficits in serum IMg2+ but not in TMg. The former, and the alcohol-associated headache, can be ameliorated with IV administration of MgSO4. Premenstrual tension-headache (PTH) and its exacerbation by alcohol in women is also accompanied by deficits in IMg2+, and elevation in serum ICa2+/IMg2+; IV MgSO4 corrects the PTH and the serum deficit in IMg2+. Animal experiments show that IV Mg2+ can prevent alcohol-induced hemorrhagic stroke and the subsequent fall in brain [Mg2+]i, [PCr], pHi, and CPP. Other recent data indicate that alcohol-induced cellular loss of [Mg2+]i is associated with cellular Ca2+ overload and generation of oxygen-derived free radicals; chronic pretreatment with vitamin E prevents alcohol-induced vascular injury and pathology in the brain. (ABSTRACT TRUNCATED)

Beneficial vs. detrimental actions of ethanol on heart and coronary vascular muscle: roles of Mg2+ and Ca2+

Epidemiologic studies suggest that daily ingestion of small amounts of alcohol may protect the heart, whereas higher intake may be detrimental. We studied: 1) cardiac performance, bioenergetics, and [Mg2+]i of isolated working rat hearts during perfusion with Krebs-Henseleit medium containing different concentrations of ethanol (EtOH), 2) mechanical responses. Ca2+ metabolism and Mg content of isolated coronary arteries obtained from dogs, sheep, and piglets subjected to varying concentrations of EtOH and [Mg2+]o and 3) intracellular free Ca2+ of isolated rat cardiac myocytes. In intact hearts, EtOH produced a biphasic hemodynamic change, depending upon concentration; 15 mM EtOH (0.07 g/dl) and 45 mM EtOH (0.21 g/dl) were stimulatory: 90 (0.42 g/dl), 135 (0.63 g/dl), and 170 mM (0.79 g/dl) EtOH were depressive. EtOH 15 and 45 mM increased coronary flow up to 150%, cardiac output up to 130%, stroke volume up to 135%, and oxygen consumption (VO2) up to 130%. However, 90 mM and higher EtOH depressed most hemodynamic parameters (except for heart rate) dose dependently. Lactic acid, lactic acid dehydrogenase, and creatine phosphokinase levels in the perfusate tended to be elevated progressively with increasing duration of EtOH perfusion and pH tended to be reduced (p < 0.05). [31P]NMR spectroscopy on hearts revealed that EtOH > or = 90 mM resulted in rises in Pi/ATP concentration ratio with no significant change in PCr/ATP ratio; [Mg2+]i levels fell and cytosolic pH tended to become slightly acidotic [19F]NMR spectroscopy of isolated myocytes revealed that [Ca2+]i rises at high concentrations of EtOH. With respect to coronary vascular muscle (CVM), low concentrations of EtOH resulted in a concentration-dependent reduction in contractions induced by K+, angiotensin II, and 5-HT; concentration-effect curves were shifted rightward to higher concentrations. Low [Mg2+]o potentiated contractions of CVM induced by EtOH. Low EtOH also resulted in reductions in exchangeable and membrane-bound 45Ca in CVM; medium to high concentrations of EtOH reduced Mg content in CVM and increased 45Ca. In the absence of [Ca2+]o, caffeine and EtOH induced similar, transient contractions followed by relaxation in K(+)-depolarized coronary arterial tissues. EtOH-induced contractions were completely abolished by pretreatment of tissues with caffeine. These results on isolated coronary vessels suggest that in addition to a need for [Ca2+]o, an intracellular release of Ca2+ is needed for EtOH to induce contractions. Overall, the data indicate that low concentrations of EtOH (15, 45 mM) are beneficial on cardiac performance, at least in the intact rat heart and coronary arteries: higher concentrations of EtOH (90, 135 mM) are detrimental. High concentrations of EtOH decrease coronary flow, lead to loss of cellular Mg2+, hypoxia, metabolic acidosis of the myocardium, cell membrane damage, and Ca2+ overload, which could result in cardiac failure. Cellular loss of Mg2+ appears to be causative in the detrimental actions of EtOH on the heart.

Alcohol-associated acute head trauma in human subjects is associated with early deficits in serum ionized Mg and Ca

Acute head trauma (AHT) (caused by motor vehicle accidents that did not produce loss of consciousness or observed brain lesions on CT scan, or falls) was found to result in early (1-8 h after injury) serum deficits in ionized magnesium (IMg2+) and ionized calcium (ICa2+) assessed with ion-selective electrodes (ISEs). Total Mg (TMg) and other electrolytes as well as serum biochemical analytes were all within the normal reference ranges. AHT patients with acute alcohol intoxication (BAC > or = 150 mg/dl) or alcohol abuse (BAC > 200 mg/dl) demonstrated deficits (15-35% less than normal) in IMg2+, but serum TMg levels were normal as were electrolytes and serum biochemical analytes. AHT patients with alcohol intoxication or alcohol abuse required hospitalization for 1-3 days prior to release, whereas AHT patients without alcohol intoxication were released in less than 24 h. The ICa2+/IMg2+ ratio, a sign of increased vascular tone and vascular reactivity, was significantly elevated in AHT patients with alcohol intoxication but not in AHT patients without alcohol intoxication or abuse. These serum divalent cation changes early after traumatic brain injury could be of considerable practicable diagnostic value in the assessment of alcohol-associated head injury. Use of ion-selective electrodes to accurately measure IMg2+ could serve as a logical basis for monitoring the response of the body to AHT.

Alcohols induce rapid depletion of intracellular free Mg2+ in cerebral vascular muscle cells: relation to chain length and partition coefficient

Acute effects of a series of alcohols (methanol, ethanol, n-butanol) on intracellular free magnesium concentration ([Mg2+]i) in canine cerebral vascular smooth muscle cells was studied using mag-fura-2 and digital imaging microscopy. In 1.2 mM [Mg2+]o, basal [Mg2+]i was 500 +/- 30 microM. Exposure of cells to a low concentration (25 mM) of ethanol, but not methanol, for only 30 s resulted in significant loss of [Mg2+]i. Exposure to 100 mM methanol, ethanol, and butanol for 30 s resulted in a relative order of potency for [Mg2+]i depletion, where butanol >> ethanol > methanol. The heterogeneous and relative subcellular compartmented concentrations of [Mg2+]i, where perinuclear > nuclear >> peripheral (cytosolic) region, was not significantly altered by the alcohols. The degree of cellular depletion of [Mg2+]i was directly a function of each alcohol's partition coefficient and chain length. The latter is suggestive of the probability that alcohols promote intracellular depletion of Mg2+ by partitioning in membranes and disordering lipid bilayers.

Role of brain [Mg2+]i in alcohol-induced hemorrhagic stroke in a rat model: a 31P-NMR in vivo study

One hundred percent of anesthetized rats administered 6.6 gm/kg of ethanol IP died within 10-35 min of alcohol injection; upon autopsy of the brain all demonstrated profound subarachnoid and intracranial bleeding, clear signs of hemorrhagic stroke. Pretreatment of rats with 4 mumol/min MgCl2, but not saline, via IV administration (for 30-45 min), prevented hemorrhagic stroke in all animals so treated with 6.6 gm/kg ethanol. Administration of the stroke dose of alcohol resulted in rapid (within 3-5 min) and marked deficits in whole brain intracellular free Mg ([Mg2++]i) as observed by in vivo 31P-NMR spectroscopy. Intracellular pH (pHi) and the phosphocreatine [PCr]/[ATP] ratio also fell following a significant fall in brain [Mg2+]i). Brains of rats that exhibited strokelike events, upon death and autopsy, demonstrated continued and marked intracellular acidosis with progressive fall in the [PCr]/[ATP] ratio and elevation of inorganic phosphate (Pi) and [H+]i; these events were not accompanied by any rises in systemic arterial blood pressure. Rats pretreated with MgCl2 exhibited relatively stable brain [Mg2+]i, and essentially unchanged pHi, [PCr], [ATP], or [Pi] following alcohol administration, although such animals exhibited threefold alterations in plasma Mg2+, as measured by ion selective electrodes. These observations suggest that high alcohol ingestion can result in severe vasospasm, ischemia, and rupture of blood vessels probably as a consequence of depletion of brain [Mg2+]i, events that can be prevented by Mg2+ pretreatment.