Application of the accurate assessment of intracellular magnesium and pH from the 31P shifts of ATP to cerebral hypoxia-ischemia in neonatal rat

Magnesium (Mg2+): Essential Mineral for Neuronal Health: From Cellular Biochemistry to Cognitive Health and Behavior Regulation

Magnesium (Mg2+) is a crucial mineral involved in numerous cellular processes critical for neuronal health and function. This review explores the multifaceted roles of Mg2+, from its biochemical interactions at the cellular level to its impact on cognitive health and behavioral regulation. Mg2+ acts as a cofactor for over 300 enzymatic reactions, including those involved in ATP synthesis, nucleic acid stability, and neurotransmitter release. It regulates ion channels, modulates synaptic plasticity, and maintains the structural integrity of cell membranes, which are essential for proper neuronal signaling and synaptic transmission. Recent studies have highlighted the significance of Mg2+ in neuroprotection, showing its ability to attenuate oxidative stress, reduce inflammation, and mitigate excitotoxicity, thereby safeguarding neuronal health. Furthermore, Mg2+ deficiency has been linked to a range of neuropsychiatric disorders, including depression, anxiety, and cognitive decline. Supplementation with Mg2+, particularly in the form of bioavailable compounds such as Magnesium-L-Threonate (MgLT), Magnesium-Acetyl-Taurate (MgAT), and other Magnesium salts, has shown some promising results in enhancing synaptic density, improving memory function, and alleviating symptoms of mental health disorders. This review highlights significant current findings on the cellular mechanisms by which Mg2+ exerts its neuroprotective effects and evaluates clinical and preclinical evidence supporting its therapeutic potential. By elucidating the comprehensive role of Mg2+ in neuronal health, this review aims to underscore the importance of maintaining optimal Mg2+ levels for cognitive function and behavioral regulation, advocating for further research into Mg2+ supplementation as a viable intervention for neuropsychiatric and neurodegenerative conditions.

Molecular determinants of magnesium-dependent synaptic plasticity at electrical synapses formed by connexin36

Neuronal gap junction (GJ) channels composed of connexin36 (Cx36) play an important role in neuronal synchronization and network dynamics. Here we show that Cx36-containing electrical synapses between inhibitory neurons of the thalamic reticular nucleus are bidirectionally modulated by changes in intracellular free magnesium concentration ([Mg(2+)]i). Chimeragenesis demonstrates that the first extracellular loop of Cx36 contains a Mg(2+)-sensitive domain, and site-directed mutagenesis shows that the pore-lining residue D47 is critical in determining high Mg(2+)-sensitivity. Single-channel analysis of Mg(2+)-sensitive chimeras and mutants reveals that [Mg(2+)]i controls the strength of electrical coupling mostly via gating mechanisms. In addition, asymmetric transjunctional [Mg(2+)]i induces strong instantaneous rectification, providing a novel mechanism for electrical rectification in homotypic Cx36 GJs. We suggest that Mg(2+)-dependent synaptic plasticity of Cx36-containing electrical synapses could underlie neuronal circuit reconfiguration via changes in brain energy metabolism that affects neuronal levels of intracellular ATP and [Mg(2+)]i.

Na⁺/H⁺ exchangers and intracellular pH in perinatal brain injury

Encephalopathy consequent on perinatal hypoxia–ischemia occurs in 1–3 per 1,000 term births in the UK and frequently leads to serious and tragic consequences that devastate lives and families, with huge financial burdens for society. Although the recent introduction of cooling represents a significant advance, only 40 % survive with normal neurodevelopmental function. There is thus a significant unmet need for novel, safe, and effective therapies to optimize brain protection following brain injury around birth. The Na⁺/H⁺ exchanger (NHE) is a membrane protein present in many mammalian cell types. It is involved in regulating intracellular pH and cell volume. NHE1 is the most abundant isoform in the central nervous system and plays a role in cerebral damage after hypoxia–ischemia. Excessive NHE activation during hypoxia–ischemia leads to intracellular Na⁺ overload, which subsequently promotes Ca²⁺ entry via reversal of the Na⁺/Ca²⁺ exchanger. Increased cytosolic Ca²⁺ then triggers the neurotoxic cascade. Activation of NHE also leads to rapid normalization of pH_i and an alkaline shift in pH_i. This rapid recovery of brain intracellular pH has been termed pH paradox as, rather than causing cells to recover, this rapid return to normal and overshoot to alkaline values is deleterious to cell survival. Brain pH_i changes are closely involved in the control of cell death after injury: an alkalosis enhances excitability while a mild acidosis has the opposite effect. We have observed a brain alkalosis in 78 babies with neonatal encephalopathy serially studied using phosphorus-31 magnetic resonance spectroscopy during the first year after birth (151 studies throughout the year including 56 studies of 50 infants during the first 2 weeks after birth). An alkaline brain pH_i was associated with severely impaired outcome; the degree of brain alkalosis was related to the severity of brain injury on MRI and brain lactate concentration; and a persistence of an alkaline brain pH_i was associated with cerebral atrophy on MRI. Experimental animal models of hypoxia–ischemia show that NHE inhibitors are neuroprotective. Here, we review the published data on brain pH_i in neonatal encephalopathy and the experimental studies of NHE inhibition and neuroprotection following hypoxia–ischemia.

pH as a biomarker of neurodegeneration in Huntington's disease: a translational rodent-human MRS study

Early diagnosis and follow-up of neurodegenerative diseases are often hampered by the lack of reliable biomarkers. Neuroimaging techniques like magnetic resonance spectroscopy (MRS) offer promising tools to detect biochemical alterations at early stages of degeneration. Intracellular pH, which can be measured noninvasively by (31)P-MRS, has shown variations in several brain diseases. Our purpose has been to evaluate the potential of MRS-measured pH as a relevant biomarker of early degeneration in Huntington's disease (HD). We used a translational approach starting with a preclinical validation of our hypothesis before adapting the method to HD patients. (31)P-MRS-derived cerebral pH was first measured in rodents during chronic intoxication with 3-nitropropionic acid (3NP). A significant pH increase was observed early into the intoxication protocol (pH=7.17±0.02 after 3 days) as compared with preintoxication (pH=7.08±0.03). Furthermore, pH changes correlated with the 3NP-induced inhibition of succinate dehydrogenase and preceded striatum lesions. Using a similar MRS approach implemented on a clinical MRI, we then showed that cerebral pH was significantly higher in HD patients (n=7) than in healthy controls (n=6) (7.05±0.03 versus 7.02±0.01, respectively, P=0.026). Altogether, both preclinical and human data strongly argue in favor of MRS-measured pH being a promising biomarker for diagnosis and follow-up of HD.

Cooperative regulation of Ca(v)1.2 channels by intracellular Mg(2+), the proximal C-terminal EF-hand, and the distal C-terminal domain

L-type Ca(2+) currents conducted by Ca(v)1.2 channels initiate excitation-contraction coupling in cardiac myocytes. Intracellular Mg(2+) (Mg(i)) inhibits the ionic current of Ca(v)1.2 channels. Because Mg(i) is altered in ischemia and heart failure, its regulation of Ca(v)1.2 channels is important in understanding cardiac pathophysiology. Here, we studied the effects of Mg(i) on voltage-dependent inactivation (VDI) of Ca(v)1.2 channels using Na(+) as permeant ion to eliminate the effects of permeant divalent cations that engage the Ca(2+)-dependent inactivation process. We confirmed that increased Mg(i) reduces peak ionic currents and increases VDI of Ca(v)1.2 channels in ventricular myocytes and in transfected cells when measured with Na(+) as permeant ion. The increased rate and extent of VDI caused by increased Mg(i) were substantially reduced by mutations of a cation-binding residue in the proximal C-terminal EF-hand, consistent with the conclusion that both reduction of peak currents and enhancement of VDI result from the binding of Mg(i) to the EF-hand (K(D) approximately 0.9 mM) near the resting level of Mg(i) in ventricular myocytes. VDI was more rapid for L-type Ca(2+) currents in ventricular myocytes than for Ca(v)1.2 channels in transfected cells. Coexpression of Ca(v)beta(2b) subunits and formation of an autoinhibitory complex of truncated Ca(v)1.2 channels with noncovalently bound distal C-terminal domain (DCT) both increased VDI in transfected cells, indicating that the subunit structure of the Ca(v)1.2 channel greatly influences its VDI. The effects of noncovalently bound DCT on peak current amplitude and VDI required Mg(i) binding to the proximal C-terminal EF-hand and were prevented by mutations of a key divalent cation-binding amino acid residue. Our results demonstrate cooperative regulation of peak current amplitude and VDI of Ca(v)1.2 channels by Mg(i), the proximal C-terminal EF-hand, and the DCT, and suggest that conformational changes that regulate VDI are propagated from the DCT through the proximal C-terminal EF-hand to the channel-gating mechanism.

Phosphorus magnetic resonance spectroscopy 2 h after perinatal cerebral hypoxia-ischemia prognosticates outcome in the newborn piglet

Phosphorus magnetic resonance spectroscopy ((31)P MRS) often reveals apparently normal brain metabolism in the first hours after intrapartum hypoxia-ischemia (HI) at a time when conventional clinical assessment of injury severity is problematic. We aimed to elucidate very-early, injury-severity biomarkers. Twenty-seven newborn piglets underwent cerebral HI: (31)P-MRS measures approximately 2 h after HI were compared between injury groups defined by secondary-energy-failure severity as quantified by the minimum nucleotide triphosphate (NTP) observed after 6 h. For severe and moderate injury versus baseline, [Pi]/[total exchangeable high-energy phosphate pool (EPP)] was increased (p < 0.001 and < 0.02, respectively), and [NTP]/[EPP] decreased (p < 0.03 and < 0.006, respectively): severe-injury [Pi]/[EPP] was also increased versus mild injury (p < 0.04). Mild-injury [phosphocreatine]/[EPP] was increased (p < 0.004). Severe-injury intracellular pH was alkaline versus baseline (p < 0.002). For severe and moderate injury [total Mg]/[ATP] (p < 0.0002 and < 0.02, respectively) and [free Mg] (p < 0.0001 and < 0.02, respectively) were increased versus baseline. [Pi]/[EPP], [phosphocreatine]/[Pi] and [NTP]/[EPP] correlated linearly with injury severity (p < 0.005, < 0.005 and < 0.02, respectively). Increased [Pi]/[EPP], intracellular pH and intracellular Mg approximately 2 h after intrapartum HI may prognosticate severe injury, whereas increased [phosphocreatine]/[EPP] may suggest mild damage. In vivo(31)P MRS may have potential to provide very-early prognosis in neonatal encephalopathy.

Serum magnesium levels as related to symptomatic vasospasm and outcome following aneurysmal subarachnoid hemorrhage

INTRODUCTION

Recent evidence suggests that magnesium may be neuroprotective in the setting of cerebral ischemia, and therapeutic magnesium infusion has been proposed for prophylaxis and treatment of delayed ischemic neurological deficit (DIND) resulting from vasospasm in patients with aneurysmal subarachnoid hemorrhage (SAH). We studied the association between serum magnesium levels, the development of DIND, and the outcomes of patients with SAH.

METHODS

We studied 128 consecutive patients with aneurysmal SAH treated at our institution between 1990 and 1997 who had a serum magnesium level measured at least once during the acute phase of their hospitalization. Delayed ischemic neurological deficit was defined as severe (major focal deficit or coma), moderate (incomplete focal deficit or decreased sensorium without coma), or none.

RESULTS

There was no significant difference in mean, minimum, or maximum serum magnesium levels between patients with and without DIND (1.93, 1.83, 2.02 versus 1.91, 1.84, 1.97 mg/dL, respectively). Similarly, no difference was found in mean serum magnesium levels among patients with severe (1.94 mg/dL), moderate (1.92 mg/dL), or no DIND (1.91 mg/dL). Analyses of serum magnesium levels before (0-4 days following SAH), during (4-14 days following SAH), and after (greater than 14 days following SAH) the period of highest risk for vasospasm revealed no association with the development or severity of DIND. Permanent deficit or death resulting from vasospasm and Glasgow Outcome Scale score at longest follow-up were similarly unaffected by serum magnesium levels overall or during any time interval. Forty (31.5%) patients were hypomagnesemic (less than 1.7 mg/dL) during hospitalization, but no difference in outcome (p = 0.185) or development of DIND (p = 0.785) was found when compared to patients with normal (1.7-2.1 mg/dL) or high (greater than 2.1 mg/dL) magnesium serum levels.

CONCLUSION

We identified no relationship between serum magnesium levels and the development of DIND or outcome following aneurysmal SAH. Based on these data, magnesium supplementation to normal or high-normal physiological ranges seems unlikely to be beneficial for DIND resulting from vasospasm. However, no inference can be made regarding the value of therapeutic infusion of magnesium to supraphysiological levels.

Hyperammonemia and chronic hepatic encephalopathy: an in vivo PMRS study of the rat brain

The brain energy metabolism of rats affected by chronic hepatic encephalopathy due to portacaval shunting was monitored by in vivo 31P-nuclear magnetic resonance spectroscopy before and after ammonium acetate administration. With respect to healthy unoperated and to sham operated controls, portacaval shunting decreased the levels of the nuclear magnetic resonance (NMR) visible brain phosphocreatine and nucleoside phosphates, and the intracellular [free Mg(2+)]. Ammonium acetate induced a further decrease of the levels of the NMR detectable phosphocreatine and nucleoside triphosphates and of the [free Mg(2+)], while the PMR spectra of the brain of non-shunted rats did not show any significant change even after treatment with ammonium acetate.

Neuroprotective effects of MgSO4 and MgCl2 in closed head injury: a comparative phosphorus NMR study

Previous studies have shown that free magnesium levels decline after traumatic brain injury and that magnesium salt administration improves posttraumatic outcome. These earlier studies, however, have been limited to models of injury that do not produce a significant degree of diffuse axonal injury and have used either MgSO4 or MgCl2 as the magnesium salt. The present study compares the neuroprotective efficacy of MgSO4 and MgCl2 in a severe model of diffuse axonal injury in rats using phosphorus nuclear magnetic resonance spectroscopy and the rotarod test to monitor effects on metabolism and neurologic outcome, respectively. Both MgSO4 and MgCl2 given as a bolus of 100 micromoles/kg at 30 min after severe, closed head injury significantly improved brain intracellular free magnesium concentration and neurologic outcome. These findings suggest that both salts penetrate the blood-brain barrier after brain trauma, enter injured tissue, and subsequently improve neurologic outcome.

Modest hypothermia preserves cerebral energy metabolism during hypoxia-ischemia and correlates with brain damage: a 31P nuclear magnetic resonance study in unanesthetized neonatal rats

Recent studies have shown that mild to moderate (modest) hypothermia decreases the damage resulting from hypoxic-ischemic insult (HI) in the immature rat. To determine whether suppression of oxidative metabolism during HI is central to the mechanism of neuroprotection, 31P nuclear magnetic resonance (NMR) spectroscopy was used to measure high energy metabolites in 7-d postnatal rats under conditions of modest hypothermia during the HI. The rats underwent unilateral common carotid artery ligation followed by exposure to hypoxia in 8% oxygen for 3 h. Environmental temperature was decreased by 3 or 6 degrees C from the control temperature, 37 degrees C, which reliably produces hemispheric damage in over 90% of pups. The metabolite parameters and tissue swelling (edema) at 42 h recovery varied very significantly with the three temperatures. Tissue swelling was 26.9, 5.3, and 0.3% at 37, 34, and 31 degrees C, respectively. Core temperature and swelling were also measured, with similar results, in parallel experiments in glass jars. Multislice magnetic resonance imaging, histology, and triphenyltetrazolium chloride staining confirmed the fairly uniform damage, confined to the hemisphere ipsilateral to the ligation. The NMR metabolite levels were integrated over the last 2.0 h out of 3.0 h of HI, and were normalized to their baseline for all surviving animals (n = 25). ATP was 47.9, 69.0, and 83.0% of normal, whereas the estimator of phosphorylation potential (phosphocreatinine/inorganic phosphorus) was 16.9, 27.8, and 42.6% of normal at 37, 34, and 31 degrees C, respectively. There was a significant correlation of both phosphocreatinine/inorganic phosphorus (p < 0.0001) and ATP levels (p < 0.0001) with brain swelling. Abnormal brain swelling and thus damage can be reliably predicted from a threshold of these metabolite levels (p < 0.0001). Thus for all three temperatures, a large change in integrated high energy metabolism during HI is a prerequisite for brain damage. With a moderate hypothermia change of 6 degrees C, where there is an insufficient change in metabolites, there is no subsequent HI brain damage. In general, treatment for HI in our 7-d-old rat model should be aimed at preserving energy metabolism.

Calculation of intracellular cerebral [Mg2+] during hypoxic ischemia by in vivo 31P NMR

Several algorithms for the calculation of ionized intracellular magnesium concentration from the chemical shifts of MgATP were compared, using in vivo 31P NMR data obtained from swine brain during and following hypoxic ischemia plus i.v. MgSO4 infusion. This analysis reveals that both the absolute ionized intracellular magnesium and relative changes in magnesium may vary widely between algorithms used. The calculated intracellular pH, used in algorithms to determine ionized magnesium concentration was found to be a critical parameter that governs the extent of these differences.

Traumatic brain axonal injury produces sustained decline in intracellular free magnesium concentration

Decline in brain intracellular free magnesium concentration following experimental traumatic brain injury has been widely reported in a number of studies. However, to date, these studies have been confined to focal models of brain injury and temporally limited to the immediate 8-h period post-trauma. Recently, a new model of impact-acceleration brain injury has been developed which produces nonfocal diffuse axonal injury more typical of severe clinical trauma. The present study has used phosphorus magnetic resonance spectroscopy and the rotarod motor test to characterise magnesium homeostasis and neurologic outcome over a period of 8 days after induction of severe impact-acceleration injury in rats. Severe impact-acceleration induced injury resulted in a highly significant and sustained decline in intracellular free magnesium concentration that was apparent for 4 days post-trauma with recovery to preinjury levels by day six. There were no significant changes in pH or ATP concentration at any time point post-injury. All animals demonstrated a significant neurologic deficit over the assessment period. The extended period of magnesium decline after severe diffuse brain trauma suggests that repeated administration may be required for pharmacotherapies targeted at restoring magnesium homeostasis.

Phosphorus nuclear magnetic resonance spectroscopy: in vivo magnesium measurements in the skeletal muscle of normal subjects

31P nuclear magnetic resonance spectroscopy was used to study the skeletal muscle of 33 normal males and 32 females. Free intracellular magnesium levels and the ratios of the phosphorus metabolites were determined. Males had significantly lower free magnesium levels (499.8 microM +/- 26.3 microM vs. 530.7 microM +/- 36.0 microM, P = 0.001, d.f. = 63, analysis of variance). The free magnesium level (rs = -0.5431, P = 0.001) and the phosphocreatin/Inorganic phosphate ratio in males (rs = -0.4102, P = 0.018), and the phosphocreatine/Inorganic phosphate ratio in females (rn = -0.4759, P = 0.009) fell with the increasing Minnesota Heart Health Program Questionnaire score.

In vivo assessment of free magnesium concentration in human brain by 31P MRS. A new calibration curve based on a mathematical algorithm

Free cytosolic [Mg2+] can be assessed in vivo by 31P MRS from the chemical shift of beta-ATP which in turn depends on the fraction of total ATP complexed to Mg2+ ions. The reliability of these in vivo measurements depends on the availability of an appropriate in vitro calibration to determine the limits of chemical shifts of unbound ATP and Mg-ATP complexes, using solutions that mimic the in vivo cytosolic conditions as far as possible. We used an algorithm and software to allow a quantitative definition of the Mg(2+)-binding molecules to build a semi-empirical equation that correlates the chemical shift of the beta-ATP signal to the [Mg2+] taking into account the amount of Mg2+ bound to all other constituents in solution. Our experiments resulted in a simple and reliable equation directly usable to assess in vivo the free cytosolic magnesium concentration of human brain by 31P MRS. Our method is also flexible enough to make it suitable for in vivo measurements of [Mg2+] in other organs and tissues.

A critical assessment of noise-induced errors in 31P MRS: application to the measurement of free intracellular magnesium in vivo

Phosphorus magnetic resonance spectroscopy (31P MRS) is a noninvasive technique that has been used to estimate free intracellular magnesium concentration (free [Mg2+]). Free [Mg2+] is computed from the chemical shift separation between the alpha- and beta-phosphate resonances of ATP. The current study was undertaken to critically assess the influence of noise effects in estimating free [Mg2+] in rat brain subjected to moderate parasagittal fluid percussion-induced injury. We show that contrary to published data, free [Mg2+] does not significantly change for up to 4 h after moderate trauma in different rat strains and using different surface coils. Before injury, free [Mg2+] = 0.56 +/- 0.11 (mean +/- SD, n = 36) and 4 h post-trauma, free [Mg2+] = 0.56 +/- 0.28. Our results suggest that explanations for this discrepancy comprise errors of chemical shift assignments accompanying low signal-to-noise ratios and the method of analysis employed. Indeed, the authors propose that spectra of beta-ATP signal-to-noise ratio less than 5:1 will produce significant noise-induced errors. We conclude that without knowledge of the inherent errors in 31P MRS spectroscopy and appropriate statistical analysis, caution should be exercised in calculating free [Mg2+] and using these changes as a basis for proposing pharmacotherapeutic interventions.