Imaging Study of Brain Glucose Metabolism in Patients with Pyruvate Dehydrogenase Deficiency

Background

In modern mitochondrial medicine, evaluating the spectrum of brain diseases has been a limiting challenge. This limitation hinders our ability to understand the mechanisms behind the imaging phenotypes of the brains of mitochondrial disease patients, as well as to identify new biomarkers and therapeutic targets. Particularly for pyruvate dehydrogenase deficiency (PDHD), a prototypical mitochondrial disease, patients usually exhibit severe neurological deficits. Studies on PDHD mouse models have shown that as the disease progresses, there is a significant increase in brain glucose uptake and glycolysis. Propionate has been identified as a crucial anaplerotic substrate under these conditions, and when combined with a ketogenic diet, it significantly extends the lifespan of PDHD mice and improves their neuropathology and motor deficits.

Source of Research

This paper was authored by Isaac Marin-Valencia and others, from the Abimael Laboratory of Neurometabolism at the Icahn School of Medicine at Mount Sinai in New York, Rockefeller University, and Memorial Sloan-Kettering Cancer Center in New York. The paper was published in the 2024 issue of the journal Cell Metabolism.

Purpose of the Study

The primary objective of the study was to analyze the metabolic networks driving the structural and metabolic characteristics of the brains of PDHD mice through multimodal imaging and isotope tracing. The study aimed to reveal the role of glucose metabolism in these animals and to explore how alternative substrates bypass PDH to maintain Krebs cycle activity.

Research Process

Methodology

Generation and Phenotypic Validation of the Mouse Model

The experiment utilized PDHA1-deficient mouse models created through mating, which exhibited brain pathological features similar to those of PDHD patients. The expression levels of PDHA1 in the mouse brain and cell death were confirmed using methods such as immunofluorescence staining, TUNEL staining, and Western blotting.

In Vivo and In Vitro Analysis of Glucose and Propionate Metabolism

The study conducted a detailed analysis of glucose and propionate metabolism in the brains of PDHD mice using various imaging techniques (including MRI, mass spectrometry, and NMR). The specific experimental steps included: 1. MRI and ^1H-MRS: To detect brain structure and baseline metabolic profiles. 2. FDG-PET: To evaluate brain glucose uptake using [18F]fluorodeoxyglucose positron emission tomography (PET). 3. 14C-2DG Autoradiography: To detect the distribution of glucose in the brain using 14C-2-deoxyglucose. 4. Non-steady-state ^13C Isotope Analysis: To assess glycolytic switching and Krebs cycle operation through hyperpolarized (HP) [1-^13C]pyruvate imaging. 5. Metabolite Pool Size and ^13C Enrichment Analysis: To measure the total concentration of metabolites and isotopic labeling using mass spectrometry.

Experimental Results

In Vivo and In Vitro Analysis of Glucose Metabolism

^1H-MRS and mass spectrometry analysis revealed that the abundance of glucose, 3-phosphoglycerate (3-PG), lactate, and other metabolites in the brains of PDHD mice was significantly higher than that of the control group, reflecting an accelerated glycolytic process. However, the significant reduction of downstream Krebs cycle metabolites in the brains of PDHD mice indicated a lack of acetyl-CoA input.

Propionate as an Anaplerotic Substrate

The study found a significant increase in propionate metabolism in the brains of PDHD mice, mainly occurring in glial cells. This phenomenon was confirmed through NMR isotope distribution analysis and quantitative PCR (qPCR), showing that the genes for propionate metabolism enzymes were highly expressed in the glial cells of PDHD mice.

Conclusions and Significance

Metabolic reprogramming was observed in the brains of PDHD mice, where this reprogramming successfully bypassed the PDH enzyme deficiency through the anaplerotic role of propionate, maintaining Krebs cycle activity. The combination of propionate with a ketogenic diet significantly improved the neuropathology and motor performance of PDHD mice and enhanced their survival rate. These findings not only deepen our understanding of the brain metabolic mechanisms in mitochondrial diseases but also provide new avenues for potential therapies for PDHD.

Highlights and Innovations

1. First Confirmation of Propionate’s Metabolic Role in PDHD Brain: The study for the first time addressed the metabolic role of propionate in the brain, discovering its significant biological importance as a major anaplerotic substrate in the brains of PDHD mice.
2. Combination of Multimodal Imaging Techniques with Isotope Tracing: The use of advanced imaging techniques and isotope labeling comprehensively showcased the glucose and propionate metabolic pathways in the brains of PDHD mice, providing a crucial methodological reference for future mitochondrial disease research.
3. Therapeutic Potential of Combining Propionate with a Ketogenic Diet: The study found that the combination of propionate with a ketogenic diet significantly improved the survival rate and neurological behavior of PDHD mice, offering new possibilities for the clinical treatment of this disease.

Research Limitations and Future Directions

Although the study revealed the important role of propionate in the brain metabolism of PDHD mice, further research is needed to determine the optimal timing and maximum tolerated dose of this substrate during pregnancy and lactation. Additionally, potential systemic and neurological side effects of long-term exposure to propionate (with or without a ketogenic diet) need to be explored. The ultimate goal is to translate these preclinical findings into clinical applications for PDHD patients.

This study not only elucidated the metabolic reprogramming mechanisms in the brains of PDHD mice but also provided new therapeutic insights, bearing significant scientific and clinical importance.