Jae Mo Park

Background: Acute phase response is an early immunometabolic response to brain injuries, primarily coordinated by the liver via the activation of acute phase proteins. These immune responses can be both beneficial, promoting tissue repair, and detrimental, exacerbating neurological deficits, if not properly controlled. Despite the central role of the liver in immunometabolism, how hepatic metabolism dynamically adapts to traumatic brain injury remains under explored, primarily due to limited liver-specific modalities that can assess metabolic pathways in vivo . ¹³ C MRI utilizing hyperpolarized ¹³ C-pyruvate can assess key regulatory enzyme activities in hepatic metabolism. Methods: Rats with controlled cortical impact were studied in vivo using hyperpolarized [1- ¹³ C]pyruvate and [2- ¹³ C]pyruvate under fed and fasted conditions 3-4 days after injury. Hyperpolarized ¹³ C products, including [ ¹³ C]bicarbonate from [1- ¹³ C]pyruvate and [5- ¹³ C]glutamate, [1- ¹³ C]acetyl-L-carnitine, and [2- ¹³ C]phosphoenolpyruvate from [2- ¹³ C]pyruvate, were evaluated to assess mitochondrial and gluconeogenic metabolism. In parallel, liver tissues were collected following [U- ¹³ C ₃ ]pyruvate injection for NMR isotopomer analysis of phosphoenolpyruvate, glucose, and glutamate. Results: While no metabolic differences were detected under fed condition, [ ¹³ C]bicarbonate and [2- ¹³ C]phosphoenolpyruvate increased after brain injury under fasted condition, indicating an upregulation of the hepatic gluconeogenic pathway after injury. ¹³ C NMR of liver tissue extracts from injured rats showed an elevated [2,3- ¹³ C ₂ ]glutamate-to-[4,5- ¹³ C ₂ ]glutamate ratio and increased ¹³ C-labeling in phosphoenolpyruvate than controls, confirming enhanced hepatic gluconeogenic pathway. Conclusion: This study demonstrates that hepatic acute phase response to brain injuries can be monitored in vivo by hyperpolarized pyruvate, which may be further utilized for longitudinal immunometabolic evaluation of the liver during pathogenesis and therapeutic interventions.

Traumatic brain injury induces profound metabolic reprogramming across neurons, astrocytes, and microglia, yet the spatiotemporal organization of these metabolic responses remains poorly understood. Because pyruvate is uniquely positioned in cerebral metabolism by connecting glycolysis, lactate metabolism, and the tricarboxylic acid cycle, we combined matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging with in vivo administration of isotopically labeled pyruvate and immunohistochemistry to characterize cell-type-wise metabolic remodeling in a controlled cortical impact rat model during the acute and subacute phases of injury. TBI induced distinct spatiotemporal immunometabolic remodeling across neuroglial compartments. Microglia- and macrophage-enriched regions exhibited selective accumulation of citrate, succinate, and itaconate, consistent with inflammatory metabolic rewiring. In contrast, astrocyte enriched regions showed increased glutamine and malate abundance, indicative of altered neuron-astrocyte metabolic coupling such as remodeling of the glutamate-glutamine cycle and enhanced anaplerotic metabolism. These metabolic signatures evolved with distinct regional and temporal distributions, identifying compartmentalized metabolic responses. Notably, labeled isotopologues of selected metabolites, including glutamate and citrate derived from administered pyruvate, changed before the corresponding metabolite pools. Together, these findings describe the spatiotemporal landscape of immunometabolic remodeling following acute TBI, uncover metabolically distinct microglial/macrophage and astrocytic responses during secondary brain injury, and identify candidate metabolic pathways for therapeutic intervention and metabolic imaging.

The TCA cycle intermediate fumarate is converted to malate by fumarase. Because the fumarate dianion enters intact cells slowly but readily penetrates into necrotic cells with compromised membranes, hyperpolarized (HP) [1,4-13C2]fumarate has been established as a molecular imaging probe for detecting cellular necrosis based on its conversion to [1,4-13C2]malate. To broaden its utility to viable cells, we developed HP-13C-labeled fumarate esters, assuming that esterification would improve membrane permeability and cellular uptake. Since fumarate is metabolized through malate to oxaloacetate, a key precursor in gluconeogenesis, we investigated dimethyl [1,4-13C2]fumarate as a potential HP probe for imaging gluconeogenic flux. Liquid-state polarization of HP-dimethyl [1,4-13C2]fumarate exceeded 50%. The HP-13C NMR spectrum displayed a single resonance at 165.39 ppm, with longitudinal relaxation times (T1) of 44 s at 3 T and 47 s at 1 T. In vivo spectra demonstrated rapid ester hydrolysis, yielding a predominant fumarate signal (175.4 ppm) along with smaller peaks corresponding to malate and bicarbonate. Isotopomer analysis of tissue extracts showed robust labeling of the phosphoenolpyruvate (PEP) pool and incorporation into newly synthesized glucose. Glucose production was higher in fasted than in fed rats (kidney: 13% vs 6%; liver: 3.7% vs 1%) and was greater in kidney than in liver. Ex vivo experiments confirmed rapid hydrolysis in rat blood but minimal hydrolysis in human blood, consistent with species-specific differences in carboxylesterase activity.

Hyperpolarized [1-¹³C]pyruvate MRI provides real-time, radiation-free insight into myocardial pyruvate metabolism and mitochondrial oxidative capacity, yet its utility in patients with severe multivessel coronary artery disease before and after surgical revascularization remains unknown. Here we show, in the first prospective study applying this technique to three-vessel disease, that serial imaging is feasible and safe with no serious adverse events in 22 injections. Healthy myocardium displayed a bicarbonate-to-total hyperpolarized carbon product ratio of 0.66 ± 0.13, indicating that the majority of pyruvate undergoes oxidation. Patients exhibited significantly lower ratios in the inferolateral segment (P = 0.03) with reciprocal increases in lactate, and global ratios correlated with left ventricular ejection fraction (r = 0.695, P = 0.006). Post-revascularization changes were heterogeneous across individuals. These findings establish hyperpolarized 13C MRI as a powerful tool for noninvasive assessment of myocardial metabolism in multivessel coronary artery disease and support its potential for longitudinal therapeutic monitoring.

Purpose: To deconvolve cardiac phase and hemodynamic effects in hyperpolarized (HP) [1‐ ¹³ C]pyruvate and to develop a cardiac phase‐adjustable reconstruction framework. Methods: Cardiac phase drift during dynamic acquisition of HP ¹³ C MRS and its effect on dynamic signal fidelity were simulated using a digital cardiac phantom. A hemodynamic model describing ventricular passages of HP pyruvate was developed and applied to dynamic cardiac ¹³ C MRS datasets acquired from healthy volunteers. Timecourses of HP pyruvate from multiphase ¹³ C MRI and ¹³ C MRS were fit to the hemodynamic model and were compared with multiphase ¹ H and ECG timing. Phase‐adjustable reconstruction was demonstrated for ECG‐gated and nongated HP ¹³ C MRS. Results: Phantom simulations demonstrated that heart rate variability induces cardiac phase misalignment, causing substantial distortion of dynamic HP pyruvate timecourses. Human studies showed transient heart rate changes during pyruvate bolus arrival, resulting in phase shifts. Multiphase ¹³ C MRI confirmed that pyruvate signal is correlated with ventricular volume ( R ² = 0.88 ± 0.08), with diastolic signals exceeding systolic signals in both ventricles and LV peak less than half of RV. The hemodynamic model accurately depicted in vivo cardiac ¹³ C MRS ( R ² > 0.978), separating LV, RV, and recirculation components and estimating pulmonary transit times consistent with image‐based measurements. Retrospective phase correction enabled reconstruction of dynamic HP pyruvate timecourses aligned to specific cardiac phases. Conclusion: We present a volume‐guided hemodynamic modeling and reconstruction framework that corrects cardiac phase misalignment. This approach improves quantitative robustness across varying heart rates and is applicable to both ECG‐gated and nongated acquisitions.

Purpose: ¹³ C MRI with hyperpolarized [1‐ ¹³ C]pyruvate enables noninvasive imaging of metabolic pathways. Considering the high dose of pyruvate used in hyperpolarized ¹³ C‐pyruvate studies, circulating ¹³ C‐pyruvate and the resulting ¹³ C‐lactate in plasma may influence subsequent pyruvate metabolism, yet the guidance on consecutive injections does not exist. This study is to characterize blood pyruvate and lactate dynamics following [1‐ ¹³ C]pyruvate injection and to evaluate the repeatability of ¹³ C neuroimaging with consecutive pyruvate injections. Methods: Sixteen healthy adults (mean age, 39.0 ± 15.9 years; 8 men) underwent blood sampling or MRI. Eight participants received bolus injections of non‐hyperpolarized [1‐ ¹³ C]pyruvate following both overnight fasting and postprandial states. Blood samples were collected at baseline and up to 60 min post‐injection to quantify pyruvate and lactate concentrations and ¹³ C fractional enrichments using mass spectrometry. In parallel, eight participants underwent ¹³ C/ ¹ H MRI that included two injections of hyperpolarized [1‐ ¹³ C]pyruvate with intervals of 6–55 min for time‐resolved measurement of ¹³ C‐pyruvate and ¹³ C‐lactate in the brain. Results: Plasma ¹³ C‐pyruvate peaked within 30 s and returned to baseline within 5–10 min, while ¹³ C‐lactate peaked at 2–3 min. Lactate ¹³ C enrichment was higher in fasting versus fed states, and total lactate concentration in blood was unchanged by pyruvate injection. Brain ¹³ C‐lactate/pyruvate showed excellent repeatability (ICC = 0.9842), with all points within the 95% Bland–Altman limits. Voxel‐wise Pearson's correlation between injections was 0.72 ± 0.22 and within‐subject coefficient of variation was 17.6% ± 4.6%. Conclusion: Consecutive injections of hyperpolarized [1‐ ¹³ C]pyruvate yield repeatable blood and brain metabolic measurements in humans, supporting multi‐injection protocols in ¹³ C MRI studies.

Aims: A ketogenic diet (KD) can suppress cardiac carbohydrate utilization, which may adversely impact heart function. However, the reversibility of KD-induced metabolic changes is poorly understood. This study aims to characterize myocardial pyruvate dehydrogenase (PDH) flux during the transition from a prolonged KD to a normal chow diet (ND). Methods and results: Cardiac metabolism was longitudinally assessed in rats using hyperpolarized [1-13C]pyruvate at baseline, during a KD (2 and 5 weeks), and a subsequent ND (1, 2, 5, and 8 days) after the 5-week KD. Hyperpolarized 13C products were compared between the KD group and age-matched ND controls. In parallel, nuclear magnetic resonance isotopomer analysis of cardiac tissue with an injection of [3-13C]pyruvate and [1,2-13C2]acetate was performed along with ex vivo enzymatic analysis of PDH activity. Myocardial [13C]bicarbonate production relative to total 13C products decreased from 8.56 ± 2.29% at baseline to 0.46 ± 0.27% after 5 weeks of KD. Reverting to ND gradually restored PDH flux (8.40 ± 1.47% by Day 8) to control levels (8.69 ± 2.10%). Ex vivo NMR analysis of glutamate C4 showed reduced pyruvate contribution to acetyl-CoA during KD (4.1 ± 2.5%), which recovered upon reverting to ND (22.7 ± 1.82% vs. control: 27.6 ± 9.5%). Although PDK4 expression normalized, PDH activity remained partially impaired in the reverted group (36.80 ± 6.07 mmol NADH/min/mg) compared to controls (90.97 ± 5.40; P = 0.00007). Conclusion: KD-induced suppression of myocardial PDH flux is reversible, but its recovery requires significant time, with prolonged metabolic inflexibility persisting after transitioning to an ND. These findings highlight the value of in vivo assessment of cardiac PDH activity, complemented by conventional enzymatic analyses, to identify persistent metabolic inflexibility following ketogenic interventions.

The enzyme γ-glutamyl transferase (GGT) plays an important role in redox homeostasis and is overexpressed in many different cancers. The ability to assess GGT activity provides critical insights into cancer prognosis and pathology. Here, we present a novel imaging agent, (¹⁵N)₂-DT-GG-PA, for in vivo assessment of GGT activity by hyperpolarized nitrogen-15 magnetic resonance imaging (HP ¹⁵N MRI). This ¹⁵N-labeled HP MRI probe is developed by introducing a long-lasting HP (¹⁵N)₂-diazirine tag to deliver essential properties for in vivo detection of GGT activity. Our studies show a large chemical shift difference (Δδ = 6.3 ppm) between (¹⁵N)₂-DT-GG-PA and its product, (¹⁵N)₂-DT-PA, and long HP lifetimes of both compounds (T₁ = 196 and 81 s, respectively, at 1 T). We also demonstrate the in vivo feasibility of detecting GGT activity using HP (¹⁵N)₂-DT-GG-PA in rat kidneys on a clinical 3 T MRI scanner. Finally, the success of (¹⁵N)₂-DT-GG-PA highlights the transformative potential of the ¹⁵N-tagging approach in designing novel HP ¹⁵N MRI probes beyond structural restrictions for ¹⁵N-isotope labeling.