In situ Raman study of redox state changes of mitochondrial cytochromes in a perfused rat heart

Modern optical approaches in redox biology: Genetically encoded sensors and Raman spectroscopy

The objective of the current review is to summarize the current state of optical methods in redox biology. It consists of two parts, the first is dedicated to genetically encoded fluorescent indicators and the second to Raman spectroscopy. In the first part, we provide a detailed classification of the currently available redox biosensors based on their target analytes. We thoroughly discuss the main architecture types of these proteins, the underlying engineering strategies for their development, the biochemical properties of existing tools and their advantages and disadvantages from a practical point of view. Particular attention is paid to fluorescence lifetime imaging microscopy as a possible readout technique, since it is less prone to certain artifacts than traditional intensiometric measurements. In the second part, the characteristic Raman peaks of the most important redox intermediates are listed, and examples of how this knowledge can be implemented in biological studies are given. This part covers such fields as estimation of the redox states and concentrations of Fe-S clusters, cytochromes, other heme-containing proteins, oxidative derivatives of thiols, lipids, and nucleotides. Finally, we touch on the issue of multiparameter imaging, in which biosensors are combined with other visualization methods for simultaneous assessment of several cellular parameters.

Charged Amino Acid Substitutions Affect Conformation of Neuroglobin and Cytochrome c Heme Groups

Neuroglobin (Ngb) is a cytosolic heme protein that plays an important role in protecting cells from apoptosis through interaction with oxidized cytochrome c (Cyt c) released from mitochondria. The interaction of reduced Ngb and oxidized Cyt c is accompanied by electron transfer between them and the reduction in Cyt c. Despite the growing number of studies on Ngb, the mechanism of interaction between Ngb and Cyt c is still unclear. Using Raman spectroscopy, we studied the effect of charged amino acid substitutions in Ngb and Cyt c on the conformation of their hemes. It has been shown that Ngb mutants E60K, K67E, K95E and E60K/E87K demonstrate changed heme conformations with the lower probability of the heme planar conformation compared to wild-type Ngb. Moreover, oxidized Cyt c mutants K25E, K72E and K25E/K72E demonstrate the decrease in the probability of methyl-radicals vibrations, indicating the higher rigidity of the protein microenvironment. It is possible that these changes can affect electron transfer between Ngb and Cyt c.

Label-free analysis of the β-hydroxybutyricacid drug on mitochondrial redox states repairment in type 2 diabetic mice by resonance raman scattering

BACKGROUND

Mitochondrial redox imbalance underlies the pathophysiology of type2 diabetes mellitus (T2DM), and is closely related to tissue damage and dysfunction. Studies have shown the beneficial effects of dietary strategies that elevate β-hydroxybutyrate (BHB) levels in alleviating T2DM. Nevertheless, the role of BHB has not been clearly elucidated.

METHODS

We performed a spectral study to visualize the preventive effects of BHB on blood and multiorgan mitochondrial redox imbalance in T2DM mice via using label-free resonance Raman spectroscopy (RRS), and further explored the impact of BHB therapy on the pathology of T2DM mice by histological and biochemical analyses.

FINDINGS

Our data revealed that RRS-based mitochondrial redox states assay enabled clear and reliable identification of the improvement of mitochondrial redox imbalance by BHB, evidenced by the reduction of Raman peak intensity at 750 cm-1, 1128 cm-1 and 1585 cm-1 in blood, tissue as well as purified mitochondria of db/db mice and the increase of tissue mitochondrial succinic dehydrogenase (SDH) staining after BHB treatment. Exogenous supplementation of BHB was also found to attenuate T2DM pathology related to mitochondrial redox states, involving organ injury, blood glucose control, insulin resistance and systemic inflammation.

INTERPRETATION

Our findings provide strong evidence for BHB as a potential therapeutic strategy targeting mitochondria for T2DM.

Redox differences between rat neonatal and adult cardiomyocytes under hypoxia

It is generally accepted that oxidative stress plays a key role in the development of ischemia-reperfusion injury in ischemic heart disease. However, the mechanisms how reactive oxygen species trigger cellular damage are not fully understood. Our study investigates redox state and highly reactive substances within neonatal and adult cardiomyocytes under hypoxia conditions. We have found that hypoxia induced an increase in H2O2 production in adult cardiomyocytes, while neonatal cardiomyocytes experienced a decrease in H2O2 levels. This finding correlates with our observation of the difference between the electron transport chain (ETC) properties and mitochondria amount in adult and neonatal cells. We demonstrated that in adult cardiomyocytes hypoxia caused the significant increase in the ETC loading with electrons compared to normoxia. On the contrary, in neonatal cardiomyocytes ETC loading with electrons was similar under both normoxic and hypoxic conditions that could be due to ETC non-functional state and the absence of the electrons transfer to O2 under normoxia. In addition to the variations in H2O2 production, we also noted consistent pH dynamics under hypoxic conditions. Notably, the pH levels exhibited a similar decrease in both cell types, thus, acidosis is a more universal cellular response to hypoxia. We also demonstrated that the amount of mitochondria and the levels of cardiac isoforms of troponin I, troponin T, myoglobin and GAPDH were significantly higher in adult cardiomyocytes compared to neonatal ones. Remarkably, we found out that under hypoxia, the levels of cardiac isoforms of troponin T, myoglobin, and GAPDH were elevated in adult cardiomyocytes, while their level in neonatal cells remained unchanged. Obtained data contribute to the understanding of the mechanisms of neonatal cardiomyocytes' resistance to hypoxia and the ability to maintain the metabolic homeostasis in contrast to adult ones.

Hyperglycemia exacerbates ischemic stroke not through increased generation of hydrogen peroxide

Diabetes is one of the significant risk factors for ischemic stroke. Hyperglycemia exacerbates the pathogenesis of stroke, leading to more extensive cerebral damage and, as a result, to more severe consequences. However, the mechanism whereby the hyperglycemic status in diabetes affects biochemical processes during the development of ischemic injury is still not fully understood. In the present work, we record for the first time the real-time dynamics of H2O2 in the matrix of neuronal mitochondria in vitro in culture and in vivo in the brain tissues of rats during development of ischemic stroke under conditions of hyperglycemia and normal glucose levels. To accomplish this, we used a highly sensitive HyPer7 biosensor and a fiber-optic interface technology. We demonstrated that a high glycemic status does not affect the generation of H2O2 in the tissues of the ischemic core, while significantly exacerbating the consequences of pathogenesis. For the first time using Raman microspectroscopy approach, we have shown how a sharp increase in the blood glucose level increases the relative amount of reduced cytochromes in the mitochondrial electron transport chain in neurons under normal conditions in awake mice.

Translating genomic tools to Raman spectroscopy analysis enables high-dimensional tissue characterization on molecular resolution

Spatial transcriptomics of histological sections have revolutionized research in life sciences and enabled unprecedented insights into genetic processes involved in tissue reorganization. However, in contrast to genomic analysis, the actual biomolecular composition of the sample has fallen behind, leaving a gap of potentially highly valuable information. Raman microspectroscopy provides untargeted spatiomolecular information at high resolution, capable of filling this gap. In this study we demonstrate spatially resolved Raman "spectromics" to reveal homogeneity, heterogeneity and dynamics of cell matrix on molecular levels by repurposing state-of-the-art bioinformatic analysis tools commonly used for transcriptomic analyses. By exploring sections of murine myocardial infarction and cardiac hypertrophy, we identify myocardial subclusters when spatially approaching the pathology, and define the surrounding metabolic and cellular (immune-) landscape. Our innovative, label-free, non-invasive "spectromics" approach could therefore open perspectives for a profound characterization of histological samples, while additionally allowing the combination with consecutive downstream analyses of the very same specimen.

SERS uncovers the link between conformation of cytochrome c heme and mitochondrial membrane potential

The balance between the mitochondrial respiratory chain activity and the cell's needs in ATP ensures optimal cellular function. Cytochrome c is an essential component of the electron transport chain (ETC), which regulates ETC activity, oxygen consumption, ATP synthesis and can initiate apoptosis. The impact of conformational changes in cytochrome c on its function is not understood for the lack of access to these changes in intact mitochondria. We have developed a novel sensor that uses unique properties of label-free surface-enhanced Raman spectroscopy (SERS) to identify conformational changes in heme of cytochrome c and to elucidate their role in functioning mitochondria. We have verified that molecule bond vibrations assessed by SERS are a reliable indicator of the heme conformation during changes in the inner mitochondrial membrane potential and ETC activity. We have demonstrated that cytochrome c heme reversibly switches between planar and ruffled conformations in response to the inner mitochondrial membrane potential (ΔΨ) and H⁺ concentration in the intermembrane space. This regulates the efficiency of the mitochondrial respiratory chain, thus, adjusting the mitochondrial respiration to the cell's consumption of ATP and the overall activity. We have found that under hypertensive conditions cytochrome c heme loses its sensitivity to ΔΨ that can affect the regulation of ETC activity. The ability of the proposed SERS-based sensor to track mitochondrial function opens broad perspectives in cell bioenergetics.

Label-free in vivo assessment of brain mitochondrial redox states during the development of diabetic cognitive impairment using Raman spectroscopy

Mitochondrial redox imbalance has been recognized as a unifying cause for diabetic cognitive impairment. Currently, a robust method for the in vivo assessment of brain mitochondrial redox imbalance is still lacking. Here, we conducted a spectral study to assess brain mitochondrial redox imbalance in the process of diabetic cognitive impairment by using label-free resonance Raman spectroscopy (RRS). Our findings showed that mitochondrial redox imbalance in cultured neurons and organotypic cortical slices exposed to high glucose were quantified by the reduction of Raman peak area at 750 cm^-1 and 1128 cm^-1, which were also associated with synaptic injury and neuron apoptosis. Raman peak area at 750 cm^-1 and 1128 cm^-1 were also decreased in db/db mice at the age of 8, 16 and 24 weeks, and had a high correlation with the mitochondrial NAD⁺/NADH redox couple. Of note, this mitochondrial redox imbalance occurred before measurable cognitive decline in 8-week-old diabetic mice, and might signal impending diabetic cognitive impairment. In summary, RRS-based mitochondrial redox states assay enabled the in vivo assessment of brain mitochondrial redox imbalance, and might provide an early indicator to enhance the prediction of diabetic cognitive impairment and inform on the response to therapies targeting mitochondrial redox imbalance.

DNA damage in preimplantation embryos and gametes: specification, clinical relevance and repair strategies

BACKGROUND

DNA damage is a hazard that affects all cells of the body. DNA-damage repair (DDR) mechanisms are in place to repair damage and restore cellular function, as are other damage-induced processes such as apoptosis, autophagy and senescence. The resilience of germ cells and embryos in response to DNA damage is less well studied compared with other cell types. Given that recent studies have described links between embryonic handling techniques and an increased likelihood of disease in post-natal life, an update is needed to summarize the sources of DNA damage in embryos and their capacity to repair it. In addition, numerous recent publications have detailed novel techniques for detecting and repairing DNA damage in embryos. This information is of interest to medical or scientific personnel who wish to obtain undamaged embryos for use in offspring generation by ART.

OBJECTIVE AND RATIONALE

This review aims to thoroughly discuss sources of DNA damage in male and female gametes and preimplantation embryos. Special consideration is given to current knowledge and limits in DNA damage detection and screening strategies. Finally, obstacles and future perspectives in clinical diagnosis and treatment (repair) of DNA damaged embryos are discussed.

SEARCH METHODS

Using PubMed and Google Scholar until May 2021, a comprehensive search for peer-reviewed original English-language articles was carried out using keywords relevant to the topic with no limits placed on time. Keywords included ‘DNA damage repair’, ‘gametes’, ‘sperm’, ‘oocyte’, ‘zygote’, ‘blastocyst’ and ‘embryo’. References from retrieved articles were also used to obtain additional articles. Literature on the sources and consequences of DNA damage on germ cells and embryos was also searched. Additional papers cited by primary references were included. Results from our own studies were included where relevant.

OUTCOMES

DNA damage in gametes and embryos can differ greatly based on the source and severity. This damage affects the development of the embryo and can lead to long-term health effects on offspring. DDR mechanisms can repair damage to a certain extent, but the factors that play a role in this process are numerous and altogether not well characterized. In this review, we describe the multifactorial origin of DNA damage in male and female gametes and in the embryo, and suggest screening strategies for the selection of healthy gametes and embryos. Furthermore, possible therapeutic solutions to decrease the frequency of DNA damaged gametes and embryos and eventually to repair DNA and increase mitochondrial quality in embryos before their implantation is discussed.

WIDER IMPLICATIONS

Understanding DNA damage in gametes and embryos is essential for the improvement of techniques that could enhance embryo implantation and pregnancy success. While our knowledge about DNA damage factors and regulatory mechanisms in cells has advanced greatly, the number of feasible practical techniques to avoid or repair damaged embryos remains scarce. Our intention is therefore to focus on strategies to obtain embryos with as little DNA damage as possible, which will impact reproductive biology research with particular significance for reproductive clinicians and embryologists.

A review of surface-enhanced Raman spectroscopy in pathological processes

With the continuous growth of the human population and new challenges in the quality of life, it is more important than ever to diagnose diseases and pathologies with high accuracy, sensitivity and in different scenarios from medical implants to the operation room. Although conventional methods of diagnosis revolutionized healthcare, alternative analytical methods are making their way out of academic labs into clinics. In this regard, surface-enhanced Raman spectroscopy (SERS) developed immensely with its capability to achieve single-molecule sensitivity and high-specificity in the last two decades, and now it is well on its way to join the arsenal of physicians. This review discusses how SERS is becoming an essential tool for the clinical investigation of pathologies including inflammation, infections, necrosis/apoptosis, hypoxia, and tumors. We critically discuss the strategies reported so far in nanoparticle assembly, functionalization, non-metallic substrates, colloidal solutions and how these techniques improve SERS characteristics during pathology diagnoses like sensitivity, selectivity, and detection limit. Moreover, it is crucial to introduce the most recent developments and future perspectives of SERS as a biomedical analytical method. We finally discuss the challenges that remain as bottlenecks for a routine SERS implementation in the medical room from in vitro to in vivo applications. The review showcases the adaptability and versatility of SERS to resolve pathological processes by covering various experimental and analytical methods and the specific spectral features and analysis results achieved by these methods.

Non-invasive quantification of the mitochondrial redox state in livers during machine perfusion

Ischemia reperfusion injury (IRI) is a critical problem in liver transplantation that can lead to life-threatening complications and substantially limit the utilization of livers for transplantation. However, because there are no early diagnostics available, fulminant injury may only become evident post-transplant. Mitochondria play a central role in IRI and are an ideal diagnostic target. During ischemia, changes in the mitochondrial redox state form the first link in the chain of events that lead to IRI. In this study we used resonance Raman spectroscopy to provide a rapid, non-invasive, and label-free diagnostic for quantification of the hepatic mitochondrial redox status. We show this diagnostic can be used to significantly distinguish transplantable versus non-transplantable ischemically injured rat livers during oxygenated machine perfusion and demonstrate spatial differences in the response of mitochondrial redox to ischemia reperfusion. This novel diagnostic may be used in the future to predict the viability of human livers for transplantation and as a tool to better understand the mechanisms of hepatic IRI.

A Microfluidic System for Simultaneous Raman Spectroscopy, Patch-Clamp Electrophysiology, and Live-Cell Imaging to Study Key Cellular Events of Single Living Cells in Response to Acute Hypoxia

The ability to sense changes in oxygen availability is fundamentally important for the survival of all aerobic organisms. However, cellular oxygen sensing mechanisms and pathologies remain incompletely understood and studies of acute oxygen sensing, in particular, have produced inconsistent results. Current methods cannot simultaneously measure the key cellular events in acute hypoxia (i.e., changes in redox state, electrophysiological properties, and mechanical responses) at controlled partial pressures of oxygen (pO₂ ). The lack of such a comprehensive method essentially contributes to the discrepancies in the field. A sealed microfluidic system that combines i) Raman spectroscopy, ii) patch-clamp electrophysiology, and iii) live-cell imaging under precisely controlled pO₂ have therefore been developed. Merging these modalities allows label-free and simultaneous observation of oxygen-dependent alterations in multiple cellular redox couples, membrane potential, and cellular contraction. This technique is adaptable to any cell type and allows in-depth insight into acute oxygen sensing processes underlying various physiologic and pathologic conditions.

Real-time surface-enhanced Raman scattering-based live cell monitoring of the changes in mitochondrial membrane potential

Obtaining molecular information on cells in real time has been a critical challenge in studying the interaction between molecules of interest and intracellular components. Fluorescence-based methods have long served as excellent tools to study such important interactions. In this paper, we introduce a Raman scattering-based method as a promising platform to achieve the real-time monitoring of subtle molecular changes occurring within cells. We found that the Raman scattering-based method enabled monitoring changes in the mitochondrial membrane potential at the single-cell level in rheumatoid arthritis synovial fibroblasts induced by tumor necrosis factor-alpha (TNF-α) protein, various chemicals (MgCl₂, FCCP, and sodium pyruvate), and a non-chemical stimulus (i.e., light). The triphenylphosphine-modified gold nanoparticles were selectively localized in the mitochondria and showed the characteristic Raman spectrum of cytochrome C and other Raman spectra of molecular components inside the cell. The surface-enhanced Raman spectrum originating from mitochondria was sensitively changed over time when mitochondrial depolarization was induced by the addition of TNF-α, or chemicals known to induce mitochondrial depolarization. The Raman-based signal changes were well matched with results of the conventional fluorescence-based analysis. However, in contrast to the conventional approach, the Raman-based method enables monitoring such changes in real time and provides detailed molecular information in terms of the interaction of molecules. Therefore, these results highlight the possibility of surface-enhanced Raman scattering-based live cell analysis for future proteomics or drug-screening applications.

Continuous assessment of metabolic activity of mitochondria using resonance Raman microspectroscopy

Dysfunctional mitochondrial activity can lead to a variety of different diseases. As such, there exists a need to quantify changes in mitochondria function as it relates to these specific diseased states. Here, we present the use of resonance Raman (RR) spectroscopy as a tool to determine changes in isolated mitochondrial activity. RR spectroscopy, using 532 nm as the excitation source, specifically provides information on the reduction and oxidation (RedOx) state of cytochrome c, which is determined by the activity of protein complexes in the electron transport chain (ETC). In this model, injection of the substrate succinate into the mitochondrial sample is used to drive the ETC, which causes a subsequent change in cytochrome c RedOx state. This change in RedOx state is tracked by RR spectroscopy. This tool gives real-time information on the rise and fall of the amount of reduced cytochrome c within the mitochondrial sample, providing a method for rapid assessment of mitochondrial metabolism that has broad applications in both basic science and medical research.

Redox Imbalance and Biochemical Changes in Cancer by Probing Redox-Sensitive Mitochondrial Cytochromes in Label-Free Visible Resonance Raman Imaging

Simple Summary

Gliomas comprise around 30% of human brain tumors, while invasive ductal carcinoma (IDC) comprises around 80% of human breast cancers. The aim of our study was to show that cancerogenesis affects the redox status of mitochondrial cytochromes, which can be tracked by using Raman spectroscopy and imaging. Our results confirmed that human breast cancer and brain tumor demonstrate a redox imbalance compared to normal tissues. We have shown the correlation between the intensity of cytochromes Raman bands at 750, 1126, 1337 and 1584 cm⁻¹ and malignancy grade for brain and breast cancers.

Abstract

To monitor redox state changes and biological mechanisms occurring in mitochondrial cytochromes in cancers improving methods are required. We used Raman spectroscopy and Raman imaging to monitor changes in the redox state of the mitochondrial cytochromes in ex vivo human brain and breast tissues at 532 nm, 633 nm, 785 nm. We identified the oncogenic processes that characterize human infiltrating ductal carcinoma (IDC) and human brain tumors: gliomas; astrocytoma and medulloblastoma based on the quantification of cytochrome redox status by exploiting the resonance-enhancement effect of Raman scattering. We visualized localization of cytochromes by Raman imaging in the breast and brain tissues and analyzed cytochrome c vibrations at 750, 1126, 1337 and 1584 cm⁻¹ as a function of malignancy grade. We found that the concentration of reduced cytochrome c becomes abnormally high in human brain tumors and breast cancers and correlates with the grade of cancer. We showed that Raman imaging provides additional insight into the biology of astrocytomas and breast ductal invasive cancer, which can be used for noninvasive grading, differential diagnosis.

Fixed versus live endothelial cells: The effect of glutaraldehyde fixation manifested by characteristic bands on the Raman spectra of cells

This work shows an impact of glutaraldehyde (GA) fixation on endothelial cells. Raman spectroscopy imaging was used as a method to monitor biochemical content of the cells due to GA fixation since this is an approach frequently used for studying cells by means of Raman imaging. To get a deeper insight into the changes and to understand them better the measurements of live and fixed cells were performed using two lasers, i.e. 488 and 532 nm. It has been demonstrated that GA fixation affects lipids, proteins, nucleic acid and carbohydrates to small extent. The application of 488 nm laser line seems to be more efficient for live cells due to the small impact of cytochrome resonance on Raman spectra, however 532 nm line is more beneficial for fixed cells due to higher quantum efficiency of the detector, thus leading to higher intensity of Raman bands. Generally, the changes due to fixation are not pronounced but cannot be ignored and the knowledge about them can help in a proper interpretation of data collected for fixed versus live cells.

Early label-free analysis of mitochondrial redox states by Raman spectroscopy predicts septic outcomes

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Resonance Raman spectroscopy was applied to in vivo detection of the mitochondrial redox state in septic mice for the first time.

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Monitoring mitochondrial redox states using resonance Raman spectroscopy had higher prognostic accuracy for mortality than the lactate level during sepsis and could be a novel diagnostic marker for predicting septic outcomes at an early time point.

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Resonance Raman spectroscopy could detect mitochondrial dysfunction in sepsis and provide a biomarker that can be a specific target of adjunctive treatment.

Background

Sepsis remains an unacceptably high mortality due to the lack of biomarkers for predicting septic outcomes in the early period. Mitochondrial redox states play a pivotal role in this condition and are disturbed early in the development of sepsis. Here, we hypothesized that visualizing mitochondrial redox states via resonance Raman spectroscopy (RRS) could identify septic outcomes at an early time point. Sepsis was induced by cecal ligation and puncture (CLP). We applied RRS analysis at baseline and 30 min, 1 h, 2 h, 4 h, and 6 h after CLP, and the mitochondrial redox states were identified. The levels of blood lactate as a predictor in sepsis were assessed. Our study is the first to reveal the possibility of in vivo detection of the mitochondrial redox state by using RRS in septic mice. The peak area for the Raman reduced mitochondrial fraction, the indicator of mitochondrial redox states, fluctuated significantly at 2 h after CLP. This fluctuation occurred earlier than the change in lactate level. Moreover, this fluctuation had higher prognostic accuracy for mortality than the lactate level during sepsis and could be a novel diagnostic marker for predicting septic outcomes according to the cutoff value of 1.059, which had a sensitivity of 80% and a specificity of 90%.

Objectives

To explore an effective indicator concerning mitochondrial redox states in the early stage of sepsis and to predict septic outcomes accurately in vivo using non-invasive and label-free Resonance Raman spectroscopy (RRS) analysis.

Methods

Mitochondria, primary skeletal muscle cells andex-vivo muscles harvested from gastrocnemius were detected mitochondrial redox states respectively by using RRS. Sepsis was induced by cecal ligation and puncture (CLP). We applied RRS analysis at baseline and 30 min, 1 h, 2 h, 4 h, and 6 h after CLP, and the mitochondrial redox states were identified. The levels of blood lactate as a predictor in sepsis were assessed. The predictive correlation of mitochondrial redox states on mortality, inflammation and organ dysfunction was further assessed.

Results

Mitochondrial redox states were clearly recognized in ex-vivo gastrocnemius muscles as well as purified mitochondria and primary skeletal muscle cells by using RRS. The peak area for the Raman reduced mitochondrial fraction, the indicator of mitochondrial redox states, fluctuated significantly at 2 h after CLP. This fluctuation occurred earlier than the change in lactate level. Moreover, this fluctuation had higher prognostic accuracy for mortality than the lactate level during sepsis and could be a novel diagnostic marker for predicting septic outcomes according to the cutoff value of 1.059, which had a sensitivity of 80% and a specificity of 90%.

Conclusions

This study demonstrated that monitoring mitochondrial redox states using RRS as early as 2 h could indicate outcomes in septic mice. These data may contribute to developing a non-invasive clinical device concerning mitochondrial redox states by using bedside-RRS.

The role of the myeloperoxidase-derived oxidant hypothiocyanous acid (HOSCN) in the induction of mitochondrial dysfunction in macrophages

A host of chronic inflammatory diseases are accelerated by the formation of the powerful oxidant hypochlorous acid (HOCl) by myeloperoxidase (MPO). In the presence of thiocyanate (SCN^-), the production of HOCl by MPO is decreased in favour of the formation of a milder oxidant, hypothiocyanous acid (HOSCN). The role of HOSCN in disease has not been fully elucidated, though there is increasing interest in using SCN^- therapeutically in different disease settings. Unlike HOCl, HOSCN can be detoxified by thioredoxin reductase, and reacts selectively with thiols to result in reversible modifications, which could potentially reduce the extent of MPO-induced damage during chronic inflammation. In this study, we show that exposure of macrophages, a key inflammatory cell type, to HOSCN results in the reversible modification of multiple mitochondrial proteins, leading to increased mitochondrial membrane permeability, decreased oxidative phosphorylation and reduced formation of ATP. The increased permeability and reduction in ATP could be reversed by pre-treatment of the macrophages with cyclosporine A, implicating a role for the mitochondrial permeability transition pore. HOSCN also drives cells to utilise fatty acids as an energetic substrate after the inhibition of oxidative phosphorylation. Raman imaging studies highlighted the ability of HOSCN to perturb the electron transport chain of mitochondria and redistribute these organelles within the cell. Taken together, these data provide new insight into the pathways by which HOSCN can induce cytotoxicity and cellular damage, which may have relevance for the development of inflammatory disease, and therapeutic strategies to reduce HOCl-induced damage by supplementation with SCN^-.

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HOSCN induces the oxidation of mitochondrial thiol proteins and cytochromes.

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HOSCN alters mitochondrial permeability and ATP production via MPTP formation.

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HOSCN increases the capacity of cells to use fatty acids as an energetic substrate.

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Raman imaging shows redistribution of mitochondria after cell exposure to HOSCN.

Bypassing mitochondrial complex III using alternative oxidase inhibits acute pulmonary oxygen sensing

Mitochondria play an important role in sensing both acute and chronic hypoxia in the pulmonary vasculature, but their primary oxygen-sensing mechanism and contribution to stabilization of the hypoxia-inducible factor (HIF) remains elusive. Alteration of the mitochondrial electron flux and increased superoxide release from complex III has been proposed as an essential trigger for hypoxic pulmonary vasoconstriction (HPV). We used mice expressing a tunicate alternative oxidase, AOX, which maintains electron flux when respiratory complexes III and/or IV are inhibited. Respiratory restoration by AOX prevented acute HPV and hypoxic responses of pulmonary arterial smooth muscle cells (PASMC), acute hypoxia-induced redox changes of NADH and cytochrome c, and superoxide production. In contrast, AOX did not affect the development of chronic hypoxia-induced pulmonary hypertension and HIF-1α stabilization. These results indicate that distal inhibition of the mitochondrial electron transport chain in PASMC is an essential initial step for acute but not chronic oxygen sensing.

Raman Scattering: From Structural Biology to Medical Applications

This is a review of relevant Raman spectroscopy (RS) techniques and their use in structural biology, biophysics, cells, and tissues imaging towards development of various medical diagnostic tools, drug design, and other medical applications. Classical and contemporary structural studies of different water-soluble and membrane proteins, DNA, RNA, and their interactions and behavior in different systems were analyzed in terms of applicability of RS techniques and their complementarity to other corresponding methods. We show that RS is a powerful method that links the fundamental structural biology and its medical applications in cancer, cardiovascular, neurodegenerative, atherosclerotic, and other diseases. In particular, the key roles of RS in modern technologies of structure-based drug design are the detection and imaging of membrane protein microcrystals with the help of coherent anti-Stokes Raman scattering (CARS), which would help to further the development of protein structural crystallography and would result in a number of novel high-resolution structures of membrane proteins—drug targets; and, structural studies of photoactive membrane proteins (rhodopsins, photoreceptors, etc.) for the development of new optogenetic tools. Physical background and biomedical applications of spontaneous, stimulated, resonant, and surface- and tip-enhanced RS are also discussed. All of these techniques have been extensively developed during recent several decades. A number of interesting applications of CARS, resonant, and surface-enhanced Raman spectroscopy methods are also discussed.

Metabolism of hyperpolarized 13 C-acetoacetate to β-hydroxybutyrate detects real-time mitochondrial redox state and dysfunction in heart tissue

Mitochondrial dysfunction is considered to be an important component of many metabolic diseases yet there is no reliable imaging biomarker for monitoring mitochondrial damage in vivo. A large prior literature on inter-conversion of β-hydroxybutyrate and acetoacetate indicates that the process is mitochondrial and that the ratio reflects a specifically mitochondrial redox state. Therefore, the conversion of [1,3-¹³ C]acetoacetate to [1,3-¹³ C]β-hydroxybutyrate is expected to be sensitive to the abnormal redox state present in dysfunctional mitochondria. In this study, we examined the conversion of hyperpolarized (HP) ¹³ C-acetoacetate (AcAc) to ¹³ C-β-hydroxybutyrate (β-HB) as a potential imaging biomarker for mitochondrial redox and dysfunction in perfused rat hearts. Conversion of HP-AcAc to β-HB was investigated using ¹³ C magnetic resonance spectroscopy in Langendorff-perfused rat hearts in four groups: control, global ischemic reperfusion, low-flow ischemic, and rotenone (mitochondrial complex-I inhibitor)-treated hearts. We observed that more β-HB was produced from AcAc in ischemic hearts and the hearts exposed to complex I inhibitor rotenone compared with controls, consistent with the accumulation of excess mitochondrial NADH. The increase in β-HB, as detected by ¹³ C MRS, was validated by a direct measure of tissue β-HB by ¹ H nuclear magnetic resonance in tissue extracts. The redox ratio, NAD⁺ /NADH, measured by enzyme assays of homogenized tissue, also paralleled production of β-HB from AcAc. Transmission electron microscopy of tissues provided direct evidence for abnormal mitochondrial structure in each ischemic tissue model. The results suggest that conversion of HP-AcAc to HP-β-HB detected by ¹³ C-MRS may serve as a useful diagnostic marker of mitochondrial redox and dysfunction in heart tissue in vivo.

Intracellular investigation on the differential effects of 4 polyphenols on MCF-7 breast cancer cells by Raman imaging

The past decades have seen significant interest in the study of polyphenolic compounds as potential therapeutic agents in medicine because they display a vast array of cellular effects beneficial to treat or manage a plethora of chronic diseases including inflammatory diseases, cardiovascular abnormalities and several types of cancer. These compounds act at different stages of carcinogenesis but deciphering their mode of action is a complex task. Live MCF-7 breast cancer cells were investigated using Raman imaging to evaluate the perturbations induced after incubating cells with four different polyphenols: EGCG, gallic acid, resveratrol and tannic acid. First, clear spectral changes could be observed between the spectra of the cytoplasm and the nucleus of live MCF-7 cancer cells demonstrating a difference in their respective global chemical composition. The treatments induced significant modifications in the cells but no clear common pattern of modifications from the 4 drugs could be observed in the cell spectra in the 1800-600 cm^-1 region. The high spatial resolution of Raman confocal microscopy enabled both the nucleus and cytoplasm to be independently targeted to study the impact of the polyphenols on the cell line. Positive spectral variations at 2851 cm^-1 and 2920 cm^-1 as well as in the 1460-1420 cm^-1 and 1660-1650 cm^-1 spectral regions inside cell cytoplasm reflected an increase of the lipid content after exposure to polyphenols. Lipid accumulation appears to be an early biomarker of drug-induced cell stress and subsequent apoptosis. Interestingly an increase of cytochrome c into the cytosol was also induced by EGCG. These multiple events are possibly associated with cell apoptosis. In conclusion, Raman micro-spectroscopy provides a complementary spectroscopic method to realize biological investigations on live cancer cells and to evaluate the effects of polyphenols at the subcellular level.

Raman Spectroscopy and Microscopy Applications in Cardiovascular Diseases: From Molecules to Organs

Noninvasive and label-free vibrational spectroscopy and microscopy methods have shown great potential for clinical diagnosis applications. Raman spectroscopy is based on inelastic light scattering due to rotational and vibrational modes of molecular bonds. It has been shown that Raman spectra provide chemical signatures of changes in biological tissues in different diseases, and this technique can be employed in label-free monitoring and clinical diagnosis of several diseases, including cardiovascular studies. However, there are very few literature reviews available to summarize the state of art and future applications of Raman spectroscopy in cardiovascular diseases, particularly cardiac hypertrophy. In addition to conventional clinical approaches such as electrocardiography (ECG), echocardiogram (cardiac ultrasound), positron emission tomography (PET), cardiac computed tomography (CT), and single photon emission computed tomography (SPECT), applications of vibrational spectroscopy and microscopy will provide invaluable information useful for the prevention, diagnosis, and treatment of cardiovascular diseases. Various in vivo and ex vivo investigations can potentially be performed using Raman imaging to study and distinguish pathological and physiological cardiac hypertrophies and understand the mechanisms of other cardiac diseases. Here, we have reviewed the recent literature on Raman spectroscopy to study cardiovascular diseases covering investigations on the molecular, cellular, tissue, and organ level.

Development of a Gas-Tight Microfluidic System for Raman Sensing of Single Pulmonary Arterial Smooth Muscle Cells Under Normoxic/Hypoxic Conditions

Acute hypoxia changes the redox-state of pulmonary arterial smooth muscle cells (PASMCs). This might influence the activity of redox-sensitive voltage-gated K⁺-channels (Kv-channels) whose inhibition initiates hypoxic pulmonary vasoconstriction (HPV). However, the molecular mechanism of how hypoxia—or the subsequent change in the cellular redox-state—inhibits Kv-channels remains elusive. For this purpose, a new multifunctional gas-tight microfluidic system was developed enabling simultaneous single-cell Raman spectroscopic studies (to sense the redox-state under normoxic/hypoxic conditions) and patch-clamp experiments (to study the Kv-channel activity). The performance of the system was tested by optically recording the O₂-content and taking Raman spectra on murine PASMCs under normoxic/hypoxic conditions or in the presence of H₂O₂. Oxygen sensing showed that hypoxic levels in the gas-tight microfluidic system were achieved faster, more stable and significantly lower compared to a conventional open system (1.6 ± 0.2%, respectively 6.7 ± 0.7%, n = 6, p < 0.001). Raman spectra revealed that the redistribution of biomarkers (cytochromes, FeS, myoglobin and NADH) under hypoxic/normoxic conditions were improved in the gas-tight microfluidic system (p-values from 0.00% to 16.30%) compared to the open system (p-value from 0.01% to 98.42%). In conclusion, the new redox sensor holds promise for future experiments that may elucidate the role of Kv-channels during HPV.

Responsive monitoring of mitochondrial redox states in heart muscle predicts impending cardiac arrest

Assessing the adequacy of oxygen delivery to tissues is vital, particularly in the fields of intensive care medicine and surgery. As oxygen delivery to a cell becomes deficient, changes in mitochondrial redox state precede changes in cellular function. We describe a technique for the continuous monitoring of the mitochondrial redox state on the epicardial surface using resonance Raman spectroscopy. We quantify the reduced fraction of specific electron transport chain cytochromes, a metric we name the resonance Raman reduced mitochondrial ratio (3RMR). As oxygen deficiency worsens, heme moieties within the electron transport chain become progressively more reduced, leading to an increase in 3RMR. Myocardial 3RMR increased from baseline values of 18.1 ± 5.9 to 44.0 ± 16.9% (P = 0.0039) after inferior vena cava occlusion in rodents (n = 8). To demonstrate the diagnostic power of this measurement, 3RMR was continuously measured in rodents (n = 31) ventilated with 5 to 8% inspired oxygen for 30 min. A 3RMR value exceeding 40% at 10 min predicted subsequent cardiac arrest with 95% sensitivity and 100% specificity [area under the curve (AUC), 0.98], outperforming all current measures, including contractility (AUC, 0.51) and ejection fraction (AUC, 0.39). 3RMR correlated with indices of intracellular redox state and energy production. This technique may permit the real-time identification of critical defects in organ-specific oxygen delivery.

Raman detection and identification of normal and leukemic hematopoietic cells

The analysis of leukocytes of peripheral blood is a crucial step in hematologic exams commonly used for disease diagnosis and, typically, requires molecular labelling. In addition, only a detailed, laborious phenotypic analysis allows identifying the presence and stage of specific pathologies such as leukemia. Most of the biochemical information is lost in the routine blood tests. In the present study, we tackle 2 important issues of label-free biochemical identification and classification of leukocytes using Raman spectroscopy (RS). First, we demonstrate that leukocyte subpopulations of lymphocytes (B, T and NK cells), monocytes and granulocytes can be identified by the unsupervised statistical approach of principal component analysis and classified by linear discriminant analysis with approximately 99% of accuracy. Second, we apply the same procedure to identify and discriminate normal B cells and transformed MN60 lymphocyte leukemic cell lines. In addition, we demonstrate that RS can be efficiently used for monitoring the cell response to low-dose chemotherapy treatment, experimentally eliciting the sensitivity to a dose-dependent cell response, which is of fundamental importance to determine the efficacy of any treatment. These results largely expand established Raman-based research protocols for label-free analysis of white blood cells, leukemic cells and chemotherapy treatment follow-up.

New insight into the mechanism of mitochondrial cytochrome c function

We investigate functional role of the P⁷⁶GTKMIFA⁸³ fragment of the primary structure of cytochrome c. Based on the data obtained by the analysis of informational structure (ANIS), we propose a model of functioning of cytochrome c. According to this model, conformational rearrangements of the P⁷⁶GTKMIFA⁸³ loop fragment have a significant effect on conformational mobility of the heme. It is suggested that the conformational mobility of cytochrome c heme is responsible for its optimal orientation with respect to electron donor and acceptor within ubiquinol–cytochrome c oxidoreductase (complex III) and cytochrome c oxidase (complex IV), respectively, thus, ensuring electron transfer from complex III to complex IV. To validate the model, we design several mutant variants of horse cytochrome c with multiple substitutions of amino acid residues in the P⁷⁶GTKMIFA⁸³ sequence that reduce its ability to undergo conformational rearrangements. With this, we study the succinate–cytochrome c reductase and cytochrome c oxidase activities of rat liver mitoplasts in the presence of mutant variants of cytochrome c. The electron transport activity of the mutant variants decreases to different extent. Resonance Raman spectroscopy (RRS) and surface-enhanced Raman spectroscopy (SERS) data demonstrate, that all mutant cytochromes possess heme with the higher degree of ruffling deformation, than that of the wild-type (WT) cytochrome c. The increase in the ruffled deformation of the heme of oxidized cytochromes correlated with the decrease in the electron transport rate of ubiquinol–cytochrome c reductase (complex III). Besides, all mutant cytochromes have lower mobility of the pyrrol rings and methine bridges, than WT cytochrome c. We show that a decrease in electron transport activity in the mutant variants correlates with conformational changes and reduced mobility of heme porphyrin. This points to a significant role of the P⁷⁶GTKMIFA⁸³ fragment in the electron transport function of cytochrome c.

Label-free detection of myocardial ischaemia in the perfused rat heart by spontaneous Raman spectroscopy

Raman spectroscopy, which identifies intrinsic molecular constituents, has a potential for determining myocardial viability under label-free conditions. However, its suitability for evaluating myocardial ischaemia is undetermined. Focusing on cytochromes, i.e., representative molecules reflecting mitochondrial activity, we tested whether Raman spectroscopy is applicable for evaluating myocardial ischaemia especially during early ischaemic phase. We obtained spontaneous Raman spectra of the subepicardial myocardium in the Langendorff-perfused rat heart upon 532-nm excitation before and during the “stopped-flow,” global ischaemia. Semi-quantitative values of the peak intensities at 750 and 1127 cm⁻¹, which reflect reduced cytochromes c and b, increased immediately and progressively after induction of the stopped flow, indicating progressive reduction of the mitochondrial respiration. Such spectral changes emerged before the loss of 1) mitochondrial membrane potentials measured by the fluorescence intensity of tetramethyl rhodamine ethyl ester or 2) staining of the triphenyl tetrazolium chloride dye in the myocardium. The progressive increases in the Raman peaks by stopped flow were significantly retarded by ischaemic preconditioning. Sequential measurements of the peak intensities at 750 and 1127 cm⁻¹ enabled early detection of the myocardial ischaemia based on the mitochondrial functions. These data suggest that Raman spectroscopy offers the potential to evaluate acute ischaemic heart under label-free conditions.

New nanocomposites for SERS studies of living cells and mitochondria

A great enhancement in Raman scattering (SERS) from heme-containing submembrane biomolecules inside intact erythrocytes and functional mitochondria is demonstrated for the first time using silver-silica beads prepared using a new method involving aerosol pyrolysis with aqueous diamminesilver(i) hydroxide as a unique source of plasmonic nanoparticles for SiO₂ microspheres. The recorded SERS spectra reveal a set of characteristic peaks at 750, 1127, 1170, 1371, 1565, 1585 and 1638 cm^-1, resulting from the normal group vibrations of the pyrrole rings, methine bridges and side radicals in the heme molecules. The SERS spectra of functional mitochondria are sensitive to the activity of the mitochondrial electron transport chain, thus making the method a novel label-free approach to monitor the redox state and conformation of cytochromes in their natural cell environment. The developed nanocomposites are highly suitable for the analysis of biological objects due to their robust synthesis and superior spatial and temporal signal reproducibility, which was preserved for a period of at least one year.

Spectroscopic analysis of myoglobin and cytochrome c dynamics in isolated cardiomyocytes during hypoxia and reoxygenation

Raman microspectroscopy was applied to monitor the intracellular redox state of myoglobin and cytochrome c from isolated adult rat cardiomyocytes during hypoxia and reoxygenation. The nitrite reductase activity of myoglobin leads to the production of nitric oxide in cells under hypoxic conditions, which is linked to the inhibition of mitochondrial respiration. In this work, the subsequent reoxygenation of cells after hypoxia is shown to lead to increased levels of oxygen-bound myoglobin relative to the initial levels observed under normoxic conditions. Increased levels of reduced cytochrome c in ex vivo cells are also observed during hypoxia and reoxygenation by Raman microspectroscopy. The cellular response to reoxygenation differed dramatically depending on the method used in the preceding step to create hypoxic conditions in the cell suspension, where a chemical agent, sodium dithionite, leads to reduction of cytochromes in addition to removal of dissolved oxygen, and bubbling-N2 gas leads to displacement of dissolved oxygen only. These results have an impact on the assessment of experimental simulations of hypoxia in cells. The spectroscopic technique employed in this work will be used in the future as an analytical method to monitor the effects of varying levels of oxygen and nutrients supplied to cardiomyocytes during either the preconditioning of cells or the reperfusion of ischaemic tissue.

Monitoring Changes in the Redox State of Myoglobin in Cardiomyocytes by Raman Spectroscopy Enables the Protective Effect of NO Donors to Be Evaluated

Raman microspectroscopy has been used to monitor changes in the redox and ligand-coordination states of the heme complex in myoglobin during the preconditioning of ex vivo cardiomyocytes with pharmacological drugs that release nitric oxide (NO). These chemical agents are known to confer protection on heart tissue against ischemia-reperfusion injury. Subsequent changes in the redox and ligand-coordination states during experimental simulations of ischemia and reperfusion have also been monitored. We found that these measurements, in real time, could be used to evaluate the preconditioning treatment of cardiomyocytes and to predict the likelihood of cell survival following a potentially lethal period of ischemia. Evaluation of the preconditioning treatment was done at the single-cell level. The binding of NO to myoglobin, giving a 6-coordinate ferrous-heme complex, was inferred from the measured Raman bands of a cardiomyocyte by comparison to pure solution of the protein in the presence of NO. A key change in the Raman spectrum was observed after perfusion of the NO-donor was completed, where, if the preconditioning treatment was successful, the bands corresponding to the nitrosyl complex were replaced by bands corresponding to metmyoglobin, Mb(III). An observation of Mb(III) bands in the Raman spectrum was made for all of the cardiomyocytes that recovered contractile function, whereas the absence of Mb(III) bands always indicated that the cardiomyocyte would be unable to recover contractile function following the simulated conditions of ischemia and reperfusion in these experiments.

Probing cytochrome c in living mitochondria with surface-enhanced Raman spectroscopy

Selective study of the electron transport chain components in living mitochondria is essential for fundamental biophysical research and for the development of new medical diagnostic methods. However, many important details of inter- and intramembrane mitochondrial processes have remained in shadow due to the lack of non-invasive techniques. Here we suggest a novel label-free approach based on the surface-enhanced Raman spectroscopy (SERS) to monitor the redox state and conformation of cytochrome c in the electron transport chain in living mitochondria. We demonstrate that SERS spectra of living mitochondria placed on hierarchically structured silver-ring substrates provide exclusive information about cytochrome c behavior under modulation of inner mitochondrial membrane potential, proton gradient and the activity of ATP-synthetase. Mathematical simulation explains the observed enhancement of Raman scattering due to high concentration of electric near-field and large contact area between mitochondria and nanostructured surfaces.

Complementary analysis of tissue homogenates composition obtained by Vis and NIR laser excitations and Raman spectroscopy

Raman spectroscopy and four excitation lines in the visible (Vis: 488, 532, 633 nm) and near infrared (NIR: 785 nm) were used for biochemical analysis of rat tissue homogenates, i.e. myocardium, brain, liver, lung, intestine, and kidney. The Vis Raman spectra are very similar for some organs (brain/intestines and kidney/liver) and dominated by heme signals when tissues of lung and myocardium were investigated (especially with 532 nm excitation). On the other hand, the NIR Raman spectra are specific for each tissue and more informative than the corresponding ones collected with the Vis excitations. The spectra analyzed without any special pre-processing clearly illustrate different chemical composition of each tissue and give information about main components e.g. lipids or proteins, but also about the content of some specific compounds such as amino acid residues, nucleotides and nucleobases. However, in order to obtain the whole spectral information about tissues complex composition the spectra of Vis and NIR excitations should be collected and analyzed together. A good agreement of data gathered from Raman spectra of the homogenates and those obtained previously from Raman imaging of the tissue cross-sections indicates that the presented here approach can be a method of choice for an investigation of biochemical variation in animal tissues. Moreover, the Raman spectral profile of tissue homogenates is specific enough to be used for an investigation of potential pathological changes the organism undergoes, in particular when supported by the complementary FTIR spectroscopy.

Disruption of cytochrome c heme coordination is responsible for mitochondrial injury during ischemia

BACKGROUND

It was recently suggested that electron flow into cyt c, coupled with ROS generation, oxidizes cyt c Met(80) to Met(80) sulfoxide (Met-O) in isolated hearts after ischemia-reperfusion, and converts cyt c to a peroxidase. We hypothesize that ischemia disrupts Met(80)-Fe ligation of cyt c, forming pentacoordinated heme Fe(2+), which inhibits electron transport (ET) and promotes oxygenase activity.

METHODS

SS-20 (Phe-D-Arg-Phe-Lys-NH2) was used to demonstrate the role of Met(80)-Fe ligation in ischemia. Mitochondria were isolated from ischemic rat kidneys to determine sites of respiratory inhibition. Mitochondrial cyt c and cyt c Met-O were quantified by western blot, and cristae architecture was examined by electron microscopy.

RESULTS

Biochemical and structural studies showed that SS-20 selectively targets cardiolipin (CL) and protects Met(80)-Fe ligation in cyt c. Ischemic mitochondria showed 17-fold increase in Met-O cyt c, and dramatic cristaeolysis. Loss of cyt c was associated with proteolytic degradation of OPA1. Ischemia significantly inhibited ET initiated by direct reduction of cyt c and coupled respiration. All changes were prevented by SS-20.

CONCLUSION

Our results show that ischemia disrupts the Met(80)-Fe ligation of cyt c resulting in the formation of a globin-like pentacoordinated heme Fe(2+) that inhibits ET, and converts cyt c into an oxygenase to cause CL peroxidation and proteolytic degradation of OPA1, resulting in cyt c release.

GENERAL SIGNIFICANCE

Cyt c heme structure represents a novel target for minimizing ischemic injury. SS-20, which we show to selectively target CL and protect the Met(80)-Fe ligation, minimizes ischemic injury and promotes ATP recovery.

Photobleaching of the resonance Raman lines of cytochromes in living yeast cells

The photobleaching of the resonance cytochrome Raman lines in living Saccharomyces cerevisiae cells was studied. The photobleaching rate versus the irradiation power was described by square function plus a constant in contrast to the linear dependence of the photoinjury rate. This difference distinguishes the cytochrome photooxidation from other processes of the cell photodamage. The square dependence is associated with the reaction involving two photogenerated intermediates while the constant with the dark redox balance rates. This work demonstrates a potential of Raman spectroscopy to characterize the native cytochrome reaction rates and to study the cell photodamage precursors.

Label-free evaluation of myocardial infarction and its repair by spontaneous Raman spectroscopy

Raman spectroscopy, which provides information about molecular species and structures of biomolecules via intrinsic molecular vibrations, can analyze physiological and pathological states of tissues on the basis of molecular constituents without staining. In this study, we analyzed Raman spectra of myocardial infarction and its repair in rats using the hypothesis that the myocardium in the course of myocardial infarction and its repair could be recognized by spontaneous Raman spectroscopy on the basis of chemical changes in myocardial tissues. Raman spectra were acquired from unfixed frozen cross sections of normal and infarcted heart tissues upon excitation at 532 nm. Raman spectra of the infarcted tissues were successfully obtained at characteristic time points: days 2, 5, and 21 after coronary ligation, at which the main components of the infarcted region were coagulation necrosis, granulation tissue, and fibrotic tissue, respectively. The latent variable weights calculated by a multivariate classification method, partial least-squares-discriminant analysis (PLS-DA), revealed fundamental information about the spectral differences among the types of tissues on the basis of molecular constituents. A prediction model for the evaluation of these tissue types was established via PLS-DA. Cross-validated sensitivities of 99.3, 95.3, 96.4, and 91.3% and specificities of 99.4, 99.5, 96.5, and 98.3% were attained for the discrimination of normal, necrotic, granulation, and fibrotic tissue, respectively. A two-dimensional image of a marginal area of infarction was successfully visualized via PLS-DA. Our results demonstrated that spontaneous Raman spectroscopy combined with PLS-DA is a novel label-free method of evaluating myocardial infarction and its repair.