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Metabolic imaging via fluorescence lifetime imaging microscopy for egg and embryo assessment

      Current strategies for embryo assessment in the assisted reproductive technology laboratories rely primarily on morphologic parameters that have limited accuracy for determining embryo viability. Even with the addition of invasive diagnostic interventions such as preimplantation genetic testing for aneuploidy alone or in combination with mitochondrial DNA copy number assessment, at least one third of embryos fail to implant. Therefore, at a time when the clinical benefits of single ET are widely accepted, improving viability assessment of embryos is ever more important. Building on the previous work demonstrating the importance of metabolic state in oocytes and embryos, metabolic imaging via fluorescence lifetime imaging microscopy offers new and potentially useful diagnostic method by detecting natural fluorescence of FAD and NADH, the two electron transporters that play a central role in oxidative phosphorylation. Recent studies demonstrate that fluorescence lifetime imaging microscopy can detect oocyte and embryo metabolic function and dysfunction in a multitude of experimental models and provide encouraging evidence for use in scientific investigation and possibly for clinical application.

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      Soon after the application of IVF into clinical practice (
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      Birth after the reimplantation of a human embryo.
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      ). As such, it quickly became a central objective in assisted reproductive technologies to identify methods of accurately assessing embryo quality (
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      ). An association between embryo morphology and cleavage rate and IVF outcome has been observed (
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      ). However, the diagnostic accuracy of these approaches remained limited. More recently, time-lapse imaging has attempted to capture dynamic information regarding cleavage rate and morphology, in the hopes that this would be a strong predictor of viability; however, clinical trials have not demonstrated strong improvements in success rates (
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      In the absence of reliable assessment methods, clinicians resort to transferring multiple embryos to achieve higher success rates. However, as the risks associated with multiple pregnancies become increasingly more evident (
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      Human gametes and preimplantation embryos.
      ,
      • Gardner D.K.
      • Wale P.L.
      Analysis of metabolism to select viable human embryos for transfer.
      ), which can be best accomplished by single ET (
      Practice Committees of SART and ASRM
      Elective single-embryo transfer.
      ). A single ET strategy further increases the importance of precise preimplantation embryo assessment and selection tools.
      Embryo quality is determined by several factors. Chromosome copy number in embryonic cells is extremely important, as aneuploidy is highly associated with embryo failure (
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      ). Preimplantation genetic testing for aneuploidy has demonstrated some improvement in IVF success (
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      ). However, concerns exist regarding the consistency of these diagnostic methods (
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      Adjuncts in the IVF laboratory: where is the evidence for “add-on” interventions?.
      ) and around the impact of mosaicism on diagnostic accuracy (
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      ,
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      ). In addition, preimplantation genetic testing for aneuploidy does not provide information about metabolic or other nongenetic viability parameters. The mitochondrial DNA (mtDNA) copy number (
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      • Michel C.E.
      • et al.
      Altered levels of mitochondrial DNA are associated with female age, aneuploidy, and provide an independent measure of embryonic implantation potential.
      ) has been assessed as a proxy for the state of mitochondria, but results have not consistently shown a strong signal for predicting viability (
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      • Zouves C.G.
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      • et al.
      Accurate quantitation of mitochondrial DNA reveals uniform levels in human blastocysts irrespective of ploidy, age, or implantation potential.
      ). Today, at least 35%–40% of euploid embryos still fail to implant (
      • Scott R.T.
      • Upham K.M.
      • Forman E.J.
      • Hong K.H.
      • Scott K.L.
      • Taylor D.
      • et al.
      Blastocyst biopsy with comprehensive chromosome screening and fresh embryo transfer significantly increases in vitro fertilization implantation and delivery rates: a randomized controlled trial.
      ). Therefore, the need for additional approaches that may help improve implantation rates remains. As there are a number of potential risks associated with invasive methods, the demand for noninvasive assessment strategies is especially high (
      • Sanchez T.
      • Seidler E.A.
      • Gardner D.K.
      • Needleman D.
      • Sakkas D.
      Will noninvasive methods surpass invasive for assessing gametes and embryos?.
      ).
      Embryo metabolic integrity is central to viability, and methods exist for assessing metabolism noninvasively. Attempts were made to measure glucose and pyruvate uptake by analyzing spent embryo media with microfluorometry (
      • Leese H.J.
      • Hooper M.A.K.
      • Edwards R.G.
      • Ashwood-Smith M.J.
      Uptake of pyruvate by early human embryos determined by a non-invasive technique.
      ). Gardner and Leese found that viable embryos had a significantly higher rate of glucose consumption than nonviable ones (
      • Gardner D.K.
      • Leese H.J.
      Assessment of embryo viability prior to transfer by the noninvasive measurement of glucose uptake.
      ), again highlighting the importance of metabolism. Additionally, spent embryo culture media amino acid concentration has been associated with IVF outcome (
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      • Falconer D.
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      • et al.
      Identification of viable embryos in IVF by non-invasive measurement of amino acid turnover.
      ). However, efforts to translate this into a clinical tool failed due to technical complexities and the need for highly specialized equipment (
      • Gardner D.K.
      • Wale P.L.
      Analysis of metabolism to select viable human embryos for transfer.
      ). Similarly, metabolomic assessments performing spectroscopic analysis on spent media have been attempted with some initial success in proof-of-concept studies (
      • Seli E.
      • Sakkas D.
      • Scott R.
      • Kwok S.C.
      • Rosendahl S.M.
      • Burns D.H.
      Noninvasive metabolomic profiling of embryo culture media using Raman and near-infrared spectroscopy correlates with reproductive potential of embryos in women undergoing in vitro fertilization.
      ,
      • Scott R.
      • Seli E.
      • Miller K.
      • Sakkas D.
      • Scott K.
      • Burns D.H.
      Noninvasive metabolomic profiling of human embryo culture media using Raman spectroscopy predicts embryonic reproductive potential: a prospective blinded pilot study.
      ); however, subsequent randomized controlled trials failed to show a benefit (
      • Hardarson T.
      • Ahlstrm A.
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      Non-invasive metabolomic profiling of day 2 and 5 embryo culture medium: a prospective randomized trial.
      ,
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      • Schats R.
      • Hompes P.G.
      • et al.
      Day 3 embryo selection by metabolomic profiling of culture medium with near-infrared spectroscopy as an adjunct to morphology: a randomized controlled trial.
      ).
      Metabolic imaging via fluorescence lifetime imaging microscopy (FLIM) is a new, noninvasive approach to measuring the biochemical status of embryos. It is a fluorescence technique (
      • Lakowicz J.R.
      Principles of fluorescence spectroscopy.
      ) focusing on NADH and FAD. Because these molecules are naturally fluorescent and integral to cellular respiration (
      • Ghukasyan V.V.
      • Heikal A.A.
      Natural biomarkers for cellular metabolism: biology, techniques, and applications.
      ), they provide a means of directly probing cellular mitochondrial metabolic status. This technique has been previously validated for distinguishing metabolic states in other biological systems, such as cancer cells (
      • Yu Q.
      • Heikal A.A.
      Two-photon autofluorescence dynamics imaging reveals sensitivity of intracellular NADH concentration and conformation to cell physiology at the single-cell level.
      ), cell lines (
      • Niesner R.
      • Peker B.
      • Schlüsche P.
      • Gericke K.-H.
      Noniterative biexponential fluorescence lifetime imaging in the investigation of cellular metabolism by means of NAD(P)H autofluorescence.
      ), animal tissues (
      • Vishwasrao H.D.
      • Heikal A.
      • Kasischke K.
      • Webb W.W.
      Conformational dependence of intracellular NADH on metabolic state revealed by associated fluorescence anisotropy.
      ), and during germ cell differentiation in Caenorhabditis elegans and stem cells differentiation (
      • Stringari C.
      • Cinquin A.
      • Cinquin O.
      • Digman M.A.
      • Donovan P.J.
      • Gratton E.
      Phasor approach to fluorescence lifetime microscopy distinguishes different metabolic states of germ cells in a live tissue.
      ). Preliminary animal studies on oocytes and embryos indicate sensitivity to metabolic differences that are relevant in fertility. Recently, Sanchez et al. (
      • Sanchez T.
      • Wang T.
      • Pedro M.V.
      • Zhang M.
      • Esencan E.
      • Sakkas D.
      • et al.
      Metabolic imaging with the use of fluorescence lifetime imaging microscopy (FLIM) accurately detects mitochondrial dysfunction in mouse oocytes.
      ) showed that FLIM measurements were able to sensitively distinguish between metabolic states that are known to be different: [1] old versus young mice (
      • Sanchez T.
      • Wang T.
      • Pedro M.V.
      • Zhang M.
      • Esencan E.
      • Sakkas D.
      • et al.
      Metabolic imaging with the use of fluorescence lifetime imaging microscopy (FLIM) accurately detects mitochondrial dysfunction in mouse oocytes.
      ) and [2] oocytes from wildtype and knockout mice for the gene, Clpp, a mutation affecting metabolic function and fertility (
      • Gispert S.
      • Parganlija D.
      • Klinkenberg M.
      • Dröse S.
      • Wittig I.
      • Mittelbronn M.
      • et al.
      Loss of mitochondrial peptidase clpp leads to infertility, hearing loss plus growth retardation via accumulation of CLPX, mtDNA and inflammatory factors.
      ,
      • Wang T.
      • Babayev E.
      • Jiang Z.
      • Li G.
      • Zhang M.
      • Esencan E.
      • et al.
      Mitochondrial unfolded protein response gene Clpp is required to maintain ovarian follicular reserve during aging, for oocyte competence, and development of pre-implantation embryos.
      ).

      The role of mitochondria in cellular metabolism

      Glycolysis (Fig. 1) is the metabolic pathway that converts glucose (a 6-carbon molecule [6C]) into two molecules of pyruvate (3-carbon molecule; [3C]). Glycolysis takes place in the cytoplasm and does not require oxygen. The free energy released in this process is used to gain two net molecules of ATP and to convert two molecules of NAD+ to NADH.
      Figure thumbnail gr1
      Figure 1The role of mitochondria in cellular metabolism. Glycolysis occurs in the cytoplasm and converts glucose into two molecules of pyruvate. During glycolysis, two net molecules of ATP are gained and two molecules of NAD+ are converted to NADH. Pyruvate molecules produced during glycolysis can be transported into the mitochondrial matrix and oxidized into AcCoA. A NADH is formed for each pyruvate molecule converted to AcCoA. In addition, for each acetyl group that enters the Krebs cycle, three molecules of NADH, one FADH2, and one GTP are produced. The process of OXPHOS is mediated by the ETC located in the inner mitochondrial membrane and involves five protein complexes. ETC oxidizes NADH to NAD+, FADH2 to FAD, generating three and two ATPs per molecule, respectively. Therefore, glycolysis (anaerobic) generates two net ATP molecules per glucose, and in the presence of oxygen and a functional mitochondrial ETC, a total of 38 ATP molecules (including the two ATP generated during glycolysis) can be produced per glucose. In lactate fermentation, the pyruvate generated during glycolysis undergoes a redox reaction catalyzed by lactate dehydrogenase, forming lactic acid. In this process, two NADH molecules are converted (oxidized) to two NAD+. Complex I (NADH-coenzyme Q oxidoreductase); complex II (succinate-Q oxidoreductase); complex III (Q-cytochrome c oxidoreductase); complex IV (cytochrome c oxidase); complex V (ATP synthase).
      Pyruvate molecules produced during glycolysis can be transported into the mitochondrial matrix and oxidized into acetyl CoA (AcCoA), leading to the formation of NADH (one for each pyruvate molecule converted to AcCoA) and facilitating the start of the Krebs cycle for additional energy production. For each acetyl group that enters the Krebs cycle, three additional molecules of NADH, one FADH2, and one GTP are produced. The NADH and FADH2 molecules can then be used to create additional ATP through oxidative phosphorylation (OXPHOS) (
      • Akram M.
      Citric acid cycle and role of its intermediates in metabolism.
      ).
      The process of OXPHOS is mediated by the electron transport chain (ETC) located in the inner mitochondrial membrane and involves five protein complexes (Fig. 1). NADH and FADH2, are oxidized by complex I (NADH-coenzyme Q oxidoreductase) and complex II (succinate-Q oxidoreductase) of the ETC, respectively. The added electrons at complexes I and II are then relayed along the ETC and help generate a proton gradient between the mitochondrial intermembranous space (higher) and the mitochondrial matrix (lower). Finally, the movement of protons through the ATP synthase (complex V), along the proton gradient (from the mitochondrial intermembranous space to the mitochondrial matrix), results in the generation of ATP. Overall, the ETC oxidizes NADH to NAD+, and FADH2 to FAD, generating three and two ATPs per molecule, respectively (
      • Alberts B.
      • Johnson A.
      • Lewis J.
      • Morgan D.
      • Raff M.
      • Roberts K.
      • et al.
      Molecular biology of the cell.
      ). While glycolysis generates only a net total of two ATP molecules per glucose molecule, the Krebs cycle and ETC result in the synthesis of an additional 36 ATPs for each glucose metabolized (Fig. 1).
      NAD+ and FAD play a vital role in energy metabolism in eukaryotic cells by accepting hydride equivalents to form reduced NADH and FADH2. These furnish reducing equivalents to the mitochondrial ETC to fuel OXPHOS. NADH is a product of both the glycolysis (in the cytoplasm) and the Krebs cycle (in the mitochondrial matrix), while FADH2 is only produced in the Krebs cycle (Fig. 1). Since the mitochondrial membrane is not permeable to NAD+ (
      • Barile M.
      • Passarella S.
      • Danese G.
      • Quagliariello E.
      Rat liver mitochondria can synthesize nicotinamide adenine dinucleotide from nicotinamide mononucleotide and ATP via a putative matrix nicotinamide mononucleotide adenylyltransferase.
      ), the reduced form of NADH generated in the cytoplasm can be transported into the mitochondrial matrix via either the malate-aspartate shuttle or the glycerol-3-phosphate shuttle of the inner mitochondrial membrane (
      • Pittelli M.
      • Formentini L.
      • Faraco G.
      • Lapucci A.
      • Rapizzi E.
      • Cialdai F.
      • et al.
      Inhibition of nicotinamide phosphoribosyltransferase: cellular bioenergetics reveals a mitochondrial insensitive NAD pool.
      ). Importantly, even in the presence of an excess of glucose, inadequate NAD+ could block glycolysis and NADH production, leading to cell death (
      • Ying W.
      • Alano C.C.
      • Garnier P.
      • Swanson R.A.
      NAD+ as a metabolic link between DNA damage and cell death.
      ,
      • Alano C.C.
      • Garnier P.
      • Ying W.
      • Higashi Y.
      • Kauppinen T.M.
      • Swanson R.A.
      NAD+ depletion is necessary and sufficient for poly(ADP-ribose) polymerase-1-mediated neuronal death.
      ).
      When insufficient oxygen is available to support OXPHOS, pyruvate generated from glycolysis can be converted into lactate by lactate dehydrogenase through a process called lactate fermentation (Fig. 1). Fermentation allows the recycling of NADH back into NAD+ so that glycolysis can continue. This process does not require oxygen and occurs in muscle when the need for energy surpasses what OXPHOS can produce.

      FLIM microscopy

      NADH and FAD are fluorescent molecules, which means that shining light on them of one wavelength can cause them to transition to an excited state and emit light of another wavelength as they relax back to their ground state (
      • Lakowicz J.R.
      Principles of fluorescence spectroscopy.
      ). Fluorescence microscopy takes advantage of this property to specifically visualize fluorescent molecules by selectively controlling the wavelength of the exciting light and using optical filters to only view light emitted by the molecule of interest (Fig. 2A). NADH and FAD have absorption and emission spectra that are highly distinct from each other (Fig. 2B), and from other cellular components (
      • Zipfel W.R.
      • Williams R.M.
      • Christie R.
      • Nikitin A.Y.
      • Hyman B.T.
      • Webb W.W.
      Live tissue intrinsic emission microscopy using multiphoton-excited native fluorescence and second harmonic generation.
      ), making it possible to study the behavior of these two molecules in vivo. Since the pioneering work of Chance and collaborators nearly 60 years ago (
      • Chance B.
      • Schoener B.
      • Oshino R.
      Oxidation-reduction ratio studies of mitochondria in freeze-trapped samples. NADH and flavoprotein fluorescence signals.
      ), fluorescence microscopy of NADH and FAD has been widely used to characterize the metabolic state of mitochondria, cells, and tissues (
      • Vishwasrao H.D.
      • Heikal A.
      • Kasischke K.
      • Webb W.W.
      Conformational dependence of intracellular NADH on metabolic state revealed by associated fluorescence anisotropy.
      ,
      • Heikal A.
      Intracellular coenzymes as natural biomarkers for metabolic activities and mitochondrial anomalies.
      ,
      • Walsh A.J.
      • Cook R.S.
      • Manning H.C.
      • Hicks D.J.
      • Lafontant A.
      • Arteaga C.L.
      • et al.
      Optical metabolic imaging identifies glycolytic levels, subtypes, and early-treatment response in breast cancer.
      ,
      • Quinn K.P.
      • Sridharan G.V.
      • Hayden R.S.
      • Kaplan D.L.
      • Lee K.
      • Georgakoudi I.
      Quantitative metabolic imaging using endogenous fluorescence to detect stem cell differentiation.
      ).
      Figure thumbnail gr2
      Figure 2Schematic illustrations of FLIM-based metabolic imaging. (A) Basic components of fluorescence microscopy. Fluorophores are illuminated with excitation light of one wavelength, and fluorescence of a different wavelength is isolated using a combination of a dichroic mirror and emission filter. (B) Two-photon excitation spectra and emission spectra of NADH and FAD. Heikal A. Intracellular coenzymes as natural biomarkers for metabolic activities and mitochondrial anomalies. Biomark Med [Internet] 2010;4:241-263. Fluorescence of each molecule can be isolated using appropriate combinations of excitation wavelengths and emission filters. (C) TCSPC FLIM requires pulsed illumination, where each pulse may excite a single fluorophore. Soon after (picoseconds to nanoseconds), the molecule's emitted fluorescence photon is detected by a single photon counting detector, and fast electronics register its precise arrival time. (D) FLIM delivers pulses at 80 MHz, quickly exciting many fluorophores and recording arrival times. These arrival times are collected into arrival time histograms (at each point in space). (E) Histograms reflect the microenvironment of the fluorophores, and information can be extracted by fitting the fluorescence decays with models, such as the displayed biexponential decay function, which represents the decays of engaged and unengaged molecules. A is a normalization factor, B represents experimental background, τ1 is the short lifetime, τ2 is the long lifetime, and F is the fraction of molecule engaged with enzymes.
      Fluorescence microscopy of NADH and FAD provides morphological information, including allowing the visualization of mitochondria, in which both molecules are highly enriched. The measured fluorescence intensity also reflects the activity of the pathways that these molecules are engaged in because the brightness of the fluorescence signal from these molecules is proportional to their concentration. While such intensity measurements are highly informative, they suffer from two major limitations: [1] the concentration of NADH and FAD reflects the relative balance of biochemical pathways, so very different physiological states can give rise to similar measured values; [2] the observed intensity depends on the details of the experimental setup in ways that are difficult to calibrate, making quantitative measurements highly challenging.
      Additional metabolic information can be extracted by using FLIM to measure the distribution of times NADH and FAD spend in their excited states (
      • Becker W.
      Fluorescence lifetime imaging—techniques and applications.
      ,
      • Heikal A.A.
      A multiparametric imaging of cellular coenzymes for monitoring metabolic and mitochondrial activities.
      ), which strongly depends on the microenvironment of the fluorophores: most importantly, engagement with enzymes leads to a drastic shift in the time NADH and FAD spend in their excited state (
      • Ghukasyan V.V.
      • Heikal A.A.
      Natural biomarkers for cellular metabolism: biology, techniques, and applications.
      ,
      • Blinova K.
      • Levine R.L.
      • Boja E.S.
      • Griffiths G.L.
      • Shi Z.D.
      • Ruddy B.
      • et al.
      Mitochondrial NADH fluorescence is enhanced by complex I binding.
      ). Thus, FLIM enables measurements reflecting the concentration of NADH and FAD (from intensity) and the extent to which those molecules are engaged with enzymes (from the time they spend in the excited state). There are a variety of different methods for performing FLIM measurements (
      • Becker W.
      Fluorescence lifetime imaging—techniques and applications.
      ). Of these, time-correlated single photon counting (TCSPC) has a number of advantages in terms of photon economy, signal-to-noise, and error analysis, making it well suited for robust, quantitative measurements (
      • Becker W.
      The bh TCSPC handbook.
      ). TCSPC-FLIM uses a laser that generates a high frequency of very short pulses for excitation (Fig. 2C). The power of the laser is kept low enough such that only about one in 100 laser pulses results in the fluorescence molecule producing a photon that can be detected. A sensitive detector enables these individual photons to be counted, and, for each photon, fast electronics allow the precise arrival time of the photon to be determined (Fig. 2D). The arrival times are combined to form a histogram, which represents how long the fluorophores remain in the excited state (Fig. 2E).
      Simple fluorophores, such as fluorescein, exhibit an exponential distribution of times in the excited state. In contrast, the histogram of times in the excited state for NADH and FAD are double exponentials, one corresponding to the population of molecules engaged with enzymes and the other corresponding to the population of molecules not engaged with enzymes. By fitting the histogram of photon arrival times to a double exponential, it is possible to measure the fraction of NADH and FAD molecules engaged with enzymes (Fig. 2E). The value of the characteristic lifetime associated with these two states depends on the detailed local environment of NADH and FAD, and FLIM provides information on that as well. FLIM is a microscopy-based technique, producing histograms of times in the excited state for each pixel in an image. Thus, FLIM of NADH and FAD can provide metabolic information with subcellular resolution, limited only by the signal-to-noise of the measurement. In addition, FLIM measurements are relatively robust and are not prone to the experimental artifacts that plague intensity measurements.

      Application of FLIM to the assessment of oocyte and embryo

      Since the metabolism of embryos and oocytes is central to their viability, and since FLIM provides a means of noninvasively and quantitatively measuring metabolism, FLIM is a promising technique for assessing oocyte and embryo viability. We have recently carried out a number of studies on mouse oocytes and embryos to evaluate the safety and potential utility of FLIM within this context (
      • Sanchez T.
      • Wang T.
      • Pedro M.V.
      • Zhang M.
      • Esencan E.
      • Sakkas D.
      • et al.
      Metabolic imaging with the use of fluorescence lifetime imaging microscopy (FLIM) accurately detects mitochondrial dysfunction in mouse oocytes.
      ).
      FLIM of NADH and FAD of embryos and oocytes allows their structure to be visualized (Fig. 3A). As NADH is highly concentrated in the mitochondria (
      • Stein L.R.
      • Imai S.I.
      The dynamic regulation of NAD metabolism in mitochondria.
      ), and FAD is almost entirely localized within the mitochondria (
      • Dumollard R.
      • Marangos P.
      • Fitzharris G.
      • Swann K.
      • Duchen M.
      • Carroll J.
      Sperm-triggered [Ca2+] oscillations and Ca2+ homeostasis in the mouse egg have an absolute requirement for mitochondrial ATP production.
      ), both intensity images reflect the distribution of mitochondria. Since aberrations in mitochondrial localization have been associated with mitochondrial dysfunction (
      • Nagai S.
      • Mabuchi T.
      • Hirata S.
      • Shoda T.
      • Kasai T.
      • Yokota S.
      • et al.
      Correlation of abnormal mitochondrial distribution in mouse oocytes with reduced developmental competence.
      ), these images alone may be useful for screening metabolically challenged oocytes and embryos. High-resolution images from FLIM of NADH and FAD can be automatically segmented using image processing (
      • Gonzalez R.C.
      Digital image processing.
      ) and feature recognition algorithms (
      • Breiman L.
      Random forests.
      ), allowing mitochondrial and cytoplasmic regions to be separately integrated (Fig. 3B).
      Figure thumbnail gr3
      Figure 3FLIM provides quantitative, multiparametric measures of embryo metabolic state. (A) An NADH FLIM intensity image reflects the mitochondrial spatial distribution of a mouse blastocyst, as NADH is highly concentrated in the mitochondria. Scale bar = 25 μm. (B) Image processing and machine learning allow for automated recognition of mitochondrial and cytoplasmic regions. (C) For each of these two segments, all photon arrival times can be binned into a single histogram and fit to a biexponential decay. (D) Fits return quantitative fit parameters, reflecting embryo metabolic state.
      Combining photon arrival times from all pixels in each region into a single histogram leads to high signal-to-noise measurements, which can be fit by a double exponential model (Fig. 3C). Each of these fits provides four metabolic parameters: fraction engaged (F), short lifetime (τ1), long lifetime (τ2), and average intensity (I). With four parameters each from mitochondrial NADH, cytosolic NADH, and mitochondrial FAD (there is no appreciable cytosolic FAD), a single metabolic acquisition yields up to 12 parameters for measuring embryo or oocyte metabolic state (Fig. 3D shows eight metabolic parameters extracted from the NADH measurement). These parameters are highly sensitive to differences and deficiencies in the metabolic state of oocytes and embryos.
      In a recent proof-of-concept study, mouse oocytes with significant metabolic dysfunction due to a mutation in a mitochondrial stress response gene exhibited highly significantly different FLIM parameter values compared with wild-type (normal) oocytes (
      • Sanchez T.
      • Wang T.
      • Pedro M.V.
      • Zhang M.
      • Esencan E.
      • Sakkas D.
      • et al.
      Metabolic imaging with the use of fluorescence lifetime imaging microscopy (FLIM) accurately detects mitochondrial dysfunction in mouse oocytes.
      ). Within the same experimental system, mtDNA copy number was only marginally different between the groups. FLIM was also used to compare oocytes from old (1-year-old) versus young mice as a model for mild metabolic dysfunction and showed highly significant differences; mtDNA copy number was not significantly different between the groups (
      • Sanchez T.
      • Wang T.
      • Pedro M.V.
      • Zhang M.
      • Esencan E.
      • Sakkas D.
      • et al.
      Metabolic imaging with the use of fluorescence lifetime imaging microscopy (FLIM) accurately detects mitochondrial dysfunction in mouse oocytes.
      ). Furthermore, FLIM metabolic parameters change over the course of preimplantation embryo development, as the embryo's metabolism reconfigures. Parameters also undergo a large shift in response to mitochondria poisons and changing culture media and oxygen tension (unpublished). Taken together, these results show that FLIM of NADH and FAD can detect biologically relevant differences in the metabolism of oocytes and embryos.
      Metabolic imaging with FLIM serves as a powerful research tool for elucidating fundamental aspects of embryo and oocyte metabolism. Studies aimed at determining how oocytes and preimplantation embryos respond to environmental cues such as changes in nutrient and gas content in the culture environment would largely benefit from this sensitive assay. We can also be cautiously optimistic for a potential application in clinical IVF, despite the failure of previous attempts at exploiting metabolic and metabolomics parameters as an embryo viability test. One advantage of FLIM is that the metabolic assessment is done directly in the cell, without being affected by the dilution and variation associated with spent culture media analyses. Nevertheless, clinical application will require a number of challenging steps, including development of sophisticated algorithms for viability prediction, nonselection studies to determine the diagnostic accuracy of the technique, and randomized clinical trials to demonstrate benefit.

      Conclusions

      Metabolism is a key determinant of cell survival, and metabolic parameters could be exploited to improve our understanding of oocyte and embryo viability. Within this context, metabolic imaging via FLIM offers new and potentially useful diagnostic potential by detecting natural fluorescence of FAD and NADH, the two electron transporters that play a central role in OXPHOS. FLIM has been used for metabolic imaging of a variety of systems and most recently has been shown to effectively assess oocyte metabolic state in mouse models of severe and mild metabolic dysfunction. It is likely that FLIM technology will be very useful for experimental studies aimed at improving our understanding of oocyte and embryo metabolism. In addition, FLIM could potentially be implemented as a noninvasive embryo viability test in assisted reproduction, pending appropriate studies.

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