Transient absorption imaging of hemes with 2-color, independently tunable visible-wavelength ultrafast source

Transient absorption spectroscopy and imaging of redox in muscle mitochondria

Mitochondrial redox is an important indicator of cell metabolism and health, with implications in cancer, diabetes, aging, neurodegenerative diseases, and mitochondrial disease. The most common method to observe redox of individual cells and mitochondria is through fluorescence of NADH and FAD+, endogenous cofactors serve as electron transport inputs to the mitochondrial respiratory chain. Yet this leaves out redox within the respiratory chain itself. To a degree, the missing information can be filled in by exogenous fluorophores, but at the risk of disturbed mitochondrial permeability and respiration. Here we show that variations in respiratory chain redox can be detected up by visible-wavelength transient absorption microscopy (TAM). In TAM, the selection of pump and probe wavelengths can provide multiphoton imaging contrast between non-fluorescent molecules. Here, we applied TAM with a pump at 520nm and probe at 450nm, 490nm, and 620nm to elicit redox contrast from mitochondrial respiratory chain hemeproteins. Experiments were performed with reduced and oxidized preparations of isolated mitochondria and whole muscle fibers, using mitochondrial fuels (malate, pyruvate, and succinate) to set up physiologically relevant oxidation levels. TAM images of muscle fibers were analyzed with multivariate curve resolution (MCR), revealing that the response at 620nm probe provides the best redox contrast and the most consistent response between whole cells and isolated mitochondria.

High-Repetition-Rate Transient Absorption Spectroscopy of Respiratory Supercomplexes

Hemeproteins are frequent subjects for ultrafast transient absorption spectroscopy (TAS) because of biological importance, strong UV-vis absorption, high photostability, and interesting transient dynamics that depend on redox, conformation, and ligand binding. TAS on hemeproteins is usually performed on isolated, purified proteins, though their response is likely to be different in their native molecular environment, which involves the formation of protein complexes and supercomplexes. Recently, we reported a transient absorption microscopy (TAM) experiment which elicited a transient response from hemeproteins in intact biological tissue using a visible-wavelength pump (530 nm) and probe (490 nm). Here, we find that adaptive noise canceling plus resonant galvanometer scanning enables a high-repetition-rate fiber laser source to make redox-sensitive measurements of cytochrome c (Cyt-c). We investigate the origins of the visible-wavelength response of biological tissue through TAS of intact mitochondrial respiratory supercomplexes, separated via gel electrophoresis. We find that each of these high-molecular-weight gel bands yields a TAS response characteristic of cytochrome hemes, implying that the TAS response of intact cells and tissue originates from not just Cyt-c but a mixture of respiratory cytochromes. We also find differences in excited-state lifetime between wild-type (WT) and a tafazzin-deficient (TAZ) mouse model of mitochondrial disease.

DR-RINS: Digital real-time relative intensity noise suppressor for pump-probe spectroscopy and microscopy

Relative intensity noise (RIN) inherent in fiber lasers poses a serious obstacle to their use in pump-probe spectroscopy and imaging. RIN can be removed through an analog balanced detector, or, as we have previously shown, software adaptive noise cancellation (ANC) on digitized signals. One major drawback to software ANC is the added time required for digitizing and post-processing. In this article, we describe a design for ANC on a field-programmable gate array (FPGA), making use of high-level synthesis tools and fixed-point arithmetic to achieve real-time laser RIN suppression at 25 MHz sample rates. Unlike the software-ANC approach, the FPGA-ANC device can serve as a dedicated drop-in denoiser, placed between the detectors and a commercial lock-in amplifier. We demonstrate its application to transient absorption spectroscopy and microscopy, lowering the noise floor to ∼17 dB above the shot noise limit. Furthermore, we demonstrate a dramatic improvement in data acquisition time from ∼6 h to ∼5 min in a real-time imaging scenario.

Ultrashort visible energetic pulses generated by nonlinear propagation of necklace beams in capillaries

The generation of ultrashort visible energetic pulses is investigated numerically by the nonlinear propagation of infrared necklace beams in capillaries. We have developed a (3+1)D model that solves the nonlinear propagation equation, including the complete spatio-temporal dynamics and the azimuthal dependence of these structured beams. Due to their singular nonlinear propagation, the spectrum broadening inside the capillary extends to the visible region in a controlled way, despite the high nonlinearity, avoiding self-focusing. The results indicate that the features of these necklace beams enable the formation of visible pulses with pulse duration below 10 fs and energies of 50 μJ by soliton self-compression dynamics for different gas pressures inside the capillary.

Adaptive noise canceling for transient absorption microscopy

SIGNIFICANCE

Ultrafast fiber lasers are an attractive alternative to bulk lasers for nonlinear optical microscopy for their compactness and low cost. The high relative intensity noise (RIN) of these lasers poses a challenge for pump-probe measurements such as transient absorption and stimulated Raman scattering, along with modalities that provide label-free contrast from the vibrational and electronic structure of molecules.

AIM

Digital adaptive filtering was applied to determine the applicability for canceling laser RIN in a transient absorption microscope with an ultrafast fiber laser source.

APPROACH

Digitized signals from the transmitted probe and reference photodetectors were fed to an adaptive filter in MATLAB, running in a noise canceling configuration. This result was then fed to a software lock-in algorithm to demodulate the pump-probe signal. Images were built up one line scan at a time with a 3.5-kHz resonant scanner, with 100  ×   averaging. The imaging target was Bi4Ge3O12, which exhibits nondegenerate two-photon absorption at the pump/probe wavelengths used (530-nm pump and 490-nm probe).

RESULTS

Without adaptive noise cancellation, the lock-in output primarily passes the laser RIN within its detection bandwidth, resulting in images that closely follow the linear transmissivity and lack sensitivity to pump-probe time delay. With adaptive noise cancellation in front of the lock-in, the RIN rejection is enough to restore the z-sectioning and sensitivity to pump-probe delay, as expected for transient absorption. Results were limited primarily by noise from the photodetector and analog-to-digital converter.

CONCLUSIONS

Digital adaptive noise cancellation, even when limited by electronics noise, can recover pump-probe signals by removal of laser RIN, under conditions where averaging alone fails.

Heme Determination and Quantification Methods and Their Suitability for Practical Applications and Everyday Use

Many research institutions, clinical diagnostic laboratories, and blood banks are desperately searching for a possibility to identify and quantify heme in different physiological and pathological settings as well as various research applications. The reasons for this are the toxicity of the heme and the fact that it acts as a hemolytic and pro-inflammatory molecule. Heme only exerts these severe and undesired effects when it is not incorporated in hemoproteins. Upon release from the hemoproteins, it enters a biologically available state (labile heme), in which it is loosely associated with proteins, lipids, nucleic acids, or other molecules. While the current methods and procedures for quantitative determination of heme have been used for many years in different settings, their value is limited by the challenging chemical properties of heme. A major cause of inadequate quantification is the separation of labile and permanently bound heme and its high aggregation potential. Thus, none of the current methods are utilized as a generally applicable, standardized approach. The aim of this Feature is to describe and summarize the most common and frequently used chemical, analytical, and biochemical methods for the quantitative determination of heme. Based on this overview, the most promising approaches for future solutions to heme quantification are highlighted.

From Synthesis to Utilization: The Ins and Outs of Mitochondrial Heme

Heme is a ubiquitous and essential iron containing metallo-organic cofactor required for virtually all aerobic life. Heme synthesis is initiated and completed in mitochondria, followed by certain covalent modifications and/or its delivery to apo-hemoproteins residing throughout the cell. While the biochemical aspects of heme biosynthetic reactions are well understood, the trafficking of newly synthesized heme—a highly reactive and inherently toxic compound—and its subsequent delivery to target proteins remain far from clear. In this review, we summarize current knowledge about heme biosynthesis and trafficking within and outside of the mitochondria.

Transient absorption microscopy: Technological innovations and applications in materials science and life science

Transient absorption (TA) spectroscopy has been extensively used in the study of excited state dynamics of various materials and molecules. The transition from TA spectroscopy to TA microscopy, which enables the space-resolved measurement of TA, is opening new investigations toward a more complete picture of excited state dynamics in functional materials, as well as the mapping of crucial biopigments for precision diagnosis. Here, we review the recent instrumental advancement that is pushing the limit of spatial resolution, detection sensitivity, and imaging speed. We further highlight the emerging application in materials science and life science.

Quantitative imaging of intraerythrocytic hemozoin by transient absorption microscopy

Hemozoin, the heme detoxification end product in malaria parasites during their growth in the red blood cells (RBCs), serves as an important marker for diagnosis and treatment target of malaria disease. However, the current method for hemozoin-targeted drug screening mainly relies on in vitro β-hematin inhibition assays, which may lead to false-positive events due to under-representation of the real hemozoin crystal. Quantitative in situ imaging of hemozoin is highly desired for high-throughput screening of antimalarial drugs and for elucidating the mechanisms of antimalarial drugs. We present transient absorption (TA) imaging as a high-speed single-cell analysis platform with chemical selectivity to hemozoin. We first demonstrated that TA microscopy is able to identify β-hematin, the artificial form of hemozoin, from the RBCs. We further utilized time-resolved TA imaging to in situ discern hemozoin from malaria-infected RBCs with optimized imaging conditions. Finally, we quantitatively analyzed the hemozoin amount in RBCs at different infection stages by single-shot TA imaging. These results highlight the potential of TA imaging for efficient antimalarial drug screening and drug mechanism investigation.

Label-free quantitation of glycated hemoglobin in single red blood cells by transient absorption microscopy and phasor analysis

As a stable and accurate biomarker, glycated hemoglobin (HbA1c) is clinically used to diagnose diabetes with a threshold of 6.5% among total hemoglobin (Hb). Current methods such as boronate affinity chromatography involve complex processing of large-volume blood samples. Moreover, these methods cannot measure HbA1c fraction at single-red blood cell (RBC) level, thus unable to separate the contribution from other factors such as RBC lifetime. Here, we demonstrate a spectroscopic transient absorption imaging approach that is able to differentiate HbA1c from Hb on the basis of their distinct excited-state dynamics. HbA1c fraction inside a single RBC is derived quantitatively through phasor analysis. HbA1c fraction distribution of diabetic blood is apparently different from that of healthy blood. A mathematical model is developed to derive the long-term blood glucose concentration. Our technology provides a unique way to study heme modification and to derive clinically important information void of bloodstream glucose fluctuation.

Label-Free Imaging of Heme Dynamics in Living Organisms by Transient Absorption Microscopy

Heme, a hydrophobic and cytotoxic macrocycle, is an essential cofactor in a large number of proteins and is important for cell signaling. This must mean that heme is mobilized from its place of synthesis or entry into the cell to other parts of the cell where hemoproteins reside. However, the cellular dynamics of heme movement is not well understood, in large part due to the inability to image heme noninvasively in live biological systems. Here, using high-resolution transient absorption microscopy, we showed that heme storage and distribution is dynamic in Caenorhabditis elegans. Intracellular heme exists in concentrated granular puncta which localizes to lysosomal-related organelles. These granules are dynamic, and their breaking down into smaller granules provides a mechanism by which heme stores can be mobilized. Collectively, these direct and noninvasive dynamic imaging techniques provide new insights into heme storage and transport and open a new avenue for label-free investigation of heme function and regulation in living systems.