Measurement of cerebral oxygen pressure in living mice by two-photon phosphorescence lifetime microscopy

Inhibition of mitochondrial bioenergetics and hypoxia to radiosensitize diffuse intrinsic pontine glioma

BACKGROUND

Diffuse intrinsic pontine gliomas (DIPGs) and other H3K27M-mutated diffuse midline gliomas (DMGs) are brain tumors that primarily affect children. Radiotherapy is the standard of care but only provides only temporary symptomatic relief due to radioresistance. Although hypoxia is a major driver of radioresistance in other tumors, there is no definitive evidence that DIPGs are hypoxic. Diffuse intrinsic pontine gliomas often contain histone mutations, which alter tumor metabolism and are also associated with radioresistance. Our objective was to identify the metabolic profiles of DIPG cells, detect hypoxia signatures, and uncover metabolism-linked mechanisms of radioresistance to improve tumor radiosensitivity.

METHODS

Using DIPG models combined with clinical datasets, we examined mitochondrial metabolism and signatures of hypoxia. We explored DIPG reliance on mitochondrial metabolism using extracellular flux assays and targeted metabolomics. In vitro and in vivo models were used to explore the mechanisms of targeting mitochondrial bioenergetics and hypoxia for radiosensitization. Treatment-induced transcriptomics and metabolomics were also investigated.

RESULTS

Comprehensive analyses of DIPG cells show signatures of enhanced oxidative phosphorylation (OXPHOS). We also identified increased expression of specific OXPHOS-related genes and signatures of hypoxia gene expression in datasets obtained from DIPG patients. We found the presence of hypoxia in orthotopic mouse models bearing DIPG tumors. These findings enabled us to develop a proof-of-concept treatment strategy to enhance radiosensitivity of DIPGs in vitro and in animal models.

CONCLUSIONS

Diffuse intrinsic pontine glioma cells rely on mitochondrial metabolism for growth, and targeting mitochondria disrupts bioenergetics, alleviates hypoxia, and enhances radiosensitivity. These findings warrant further exploration of OXPHOS inhibition as a radiosensitizing strategy for DIPG treatment.

A low-cost FPGA-based approach for pile-up corrected high-speed in vivo FLIM imaging

Significance

Intensity-based two-photon microscopy is a cornerstone of neuroscience research but lacks the ability to measure concentrations, a pivotal task for longitudinal studies and quantitative comparisons. Fluorescence lifetime imaging (FLIM) based on time-correlated single photon counting (TCSPC) can overcome those limits but suffers from "pile-up" distortions at high photon count rates, severely limiting acquisition speed.

Aim

We introduce the "laser period blind time" (LPBT) method to correct pile-up distortions in photon counting electronics, enabling reliable low-cost TCSPC-FLIM at high count rates.

Approach

Using a realistic simulation of the TCSPC data collection, we evaluated the LPBT method's performance in silico. The correction was then implemented on low-cost hardware based on a field programable gate array and validated using in vitro, ex vivo, and in vivo measurements.

Results

The LBPT approach achieves < 3 % error in lifetime measurements at count rates more than 10 times higher than traditional limits, allowing robust FLIM imaging of subsecond metabolite dynamics with subcellular resolution.

Conclusions

We enable high-precision, cost-effective FLIM imaging at acquisition speeds comparable with state-of-the-art commercial systems, facilitating the adoption of FLIM in neuroscience and other fields of research needing robust quantitative live imaging solutions.

Out-of-focus signal rejection for in vivo pO2 measurements using two-photon phosphorescence lifetime microscopy

Two-photon phosphorescence lifetime microscopy has been a key tool for studying cerebral oxygenation in mice. However, the accuracy of the partial pressure of oxygen (pO2) measurements is affected by out-of-focus signal. In this work, we applied reconfigurable differential aberration imaging to characterize and correct for out-of-focus signal contamination in intravascular pO2 imaging. Our results show that signal contamination is higher in more oxygenated vessels and that it could be effectively removed using the proposed method.

Quantitative Optical Imaging of Oxygen in Brain Vasculature

The intimate relationship between neuronal activity and cerebral oxygenation underpins fundamental brain functions like cognition, sensation, and motor control. Optical imaging offers a noninvasive approach to assess brain oxygenation and often serves as an indirect proxy for neuronal activity. However, deciphering neurovascular coupling─the intricate interplay between neuronal activity, blood flow, and oxygen delivery─necessitates independent, high spatial resolution, and high temporal resolution measurements of both microvasculature oxygenation and neuronal activation. This Perspective examines the established optical techniques employed for brain oxygen imaging, specifically functional near-infrared spectroscopy, photoacoustic imaging, optical coherence tomography, and two-photon phosphorescent lifetime microscopy, highlighting their fundamental principles, strengths, and limitations. Several other emerging optical techniques are also introduced. Finally, we discuss key technological challenges and future directions for quantitative optical oxygen imaging, paving the way for a deeper understanding of oxygen metabolism in the brain.

Assessment of blood-brain barrier leakage and brain oxygenation in Connexin-32 knockout mice with systemic neuroinflammation using pulse electron paramagnetic resonance imaging techniques

PURPOSE

The determination of blood-brain barrier (BBB) integrity and partial pressure of oxygen (pO2) in the brain is of substantial interest in several neurological applications. This study aimed to assess the feasibility of using trityl OX071-based pulse electron paramagnetic resonance imaging (pEPRI) to provide a quantitative estimate of BBB integrity and pO2 maps in mouse brains as a function of neuroinflammatory disease progression.

METHODS

Five Connexin-32 (Cx32)-knockout (KO) mice were injected with lipopolysaccharide to induce neuroinflammation for imaging. Three wild-type mice were also used to optimize the imaging procedure and as control animals. An additional seven Cx32-KO mice were used to establish the BBB leakage of trityl using the colorimetric assay. All pEPRI experiments were performed using a preclinical instrument, JIVA-25 (25 mT/720 MHz), at times t = 0, 4, and 6 h following lipopolysaccharide injection. Two pEPRI imaging techniques were used: (a) single-point imaging for obtaining spatial maps to outline the brain and calculate BBB leakage using the signal amplitude, and (b) inversion-recovery electron spin echo for obtaining pO2 maps.

RESULTS

A statistically significant change in BBB leakage was found using pEPRI with the progression of inflammation in Cx32 KO animals. However, the change in pO2 values with the progression of inflammation for these animals was not statistically significant.

CONCLUSIONS

For the first time, we show the ability of pEPRI to provide pO2 maps in mouse brains noninvasively, along with a quantitative assessment of BBB leakage. We expect this study to open new queries from the field to explore the pathology of many neurological diseases and provide a path to new treatments.

Effects of Photodynamic Therapy on Tumor Metabolism and Oxygenation Revealed by Fluorescence and Phosphorescence Lifetime Imaging

This work was aimed at the complex analysis of the metabolic and oxygen statuses of tumors in vivo after photodynamic therapy (PDT). Studies were conducted on mouse tumor model using two types of photosensitizers-chlorin e6-based drug Photoditazine predominantly targeted to the vasculature and genetically encoded photosensitizer KillerRed targeted to the chromatin. Metabolism of tumor cells was assessed by the fluorescence lifetime of the metabolic redox-cofactor NAD(P)H, using fluorescence lifetime imaging. Oxygen content was assessed using phosphorescence lifetime macro-imaging with an oxygen-sensitive probe. For visualization of the perfused microvasculature, an optical coherence tomography-based angiography was used. It was found that PDT induces different alterations in cellular metabolism, depending on the degree of oxygen depletion. Moderate decrease in oxygen in the case of KillerRed was accompanied by an increase in the fraction of free NAD(P)H, an indicator of glycolytic switch, early after the treatment. Severe hypoxia after PDT with Photoditazine resulted from a vascular shutdown yielded in a persistent increase in protein-bound (mitochondrial) fraction of NAD(P)H. These findings improve our understanding of physiological mechanisms of PDT in cellular and vascular modes and can be useful to develop new approaches to monitoring its efficacy.