Visualizing astrocytes in the deep mouse brain in vivo

A NIR-Activated and Mild-Temperature-Sensitive Nanoplatform with an HSP90 Inhibitor for Combinatory Chemotherapy and Mild Photothermal Therapy in Cancel Cells

Mild photothermal therapy (PTT) shows great potential to treat cancers while avoiding unwanted damage to surrounding normal cells. However, the efficacy of mild PTT is normally moderate because of the low hyperthermia temperature and limited light penetration depth. Chemotherapy has unlimited penetration but often suffers from unsatisfactory efficacy in view of the occurrence of drug resistance, suboptimal drug delivery and release profile. As a result, the combinatory of chemotherapy and mild PTT would integrate their advantages and overcome the shortcomings. Herein, we synthesized an NIR-activatable and mild-temperature-sensitive nanoplatform (BDPII-gel@TSL) composed of temperature-sensitive liposomes (TSL), heat shock protein 90 (HSP90) inhibitor (geldanamycin) and photothermal agent (BDPII), for dual chemotherapy and mild PTT in cancer cells. BDPII, constructed with donor-acceptor moieties, acts as an excellent near-infrared (NIR) photothermal agent (PTA) with a high photothermal conversion efficiency (80.75%). BDPII-containing TSLs efficiently produce a mild hyperthermia effect (42 °C) under laser irradiation (808 nm, 0.5 W cm-2). Importantly, the phase transformation of TSL leads to burst release of geldanamycin from BDPII-gel@TSL, and this contributes to down-regulation of the overexpression of HSP90, ensuring efficient inhibition of cancer cell growth. This research provides a dual-sensitive synergistic therapeutic strategy for cancer cell treatment.

Resolving arteriolar wall structures in mouse brain in vivo with three-photon microscopy

The brain arteriolar wall is a multilayered structure, whose integrity is of key significance to the brain function. However, resolving these different layers in anmial models in vivo is hampered by the lack of either labeling or imaging technology. Here, we demonstrate that three-photon microscopy (3PM) is an ideal solution. In mouse brain in vivo, excited at the 1700-nm window, label-free third-harmonic generation imaging and three-photon fluorescence (3PF) imaging with Alexa 633 labeling colocalize and resolve the internal elastic lamina. Furthermore, Alexa Fluor 594-conjugated Wheat Germ Agglutinin (WGA-594) shows time-dependent labeling behavior. As time lapses, WGA-594 first labels endothelium, and then vascular smooth muscle cells, which are readily captured and resolved with 3PF imaging. Our results show that 3PM, in combination with proper labeling, is a promising technology for investigating the structures of brain arteriolar wall in vivo.

Deep Imaging to Dissect Microvascular Contributions to White Matter Degeneration in Rodent Models of Dementia

The increasing socio-economic burden of Alzheimer disease (AD) and AD-related dementias has created a pressing need to define targets for therapeutic intervention. Deficits in cerebral blood flow and neurovascular function have emerged as early contributors to disease progression. However, the cause, progression, and consequence of small vessel disease in AD/AD-related dementias remains poorly understood, making therapeutic targets difficult to pinpoint. Animal models that recapitulate features of AD/AD-related dementias may provide mechanistic insight because microvascular pathology can be studied as it develops in vivo. Recent advances in in vivo optical and ultrasound-based imaging of the rodent brain facilitate this goal by providing access to deeper brain structures, including white matter and hippocampus, which are more vulnerable to injury during cerebrovascular disease. Here, we highlight these novel imaging approaches and discuss their potential for improving our understanding of vascular contributions to AD/AD-related dementias.

In Vivo Deep-Brain 3- and 4-Photon Fluorescence Imaging of Subcortical Structures Labeled by Quantum Dots Excited at the 2200 nm Window

Multiphoton microscopy (MPM) is an enabling technology for visualizing deep-brain structures at high spatial resolution in vivo. Within the low tissue absorption window, shifting to longer excitation wavelengths reduces tissue scattering and boosts penetration depth. Recently, the 2200 nm excitation window has emerged as the last and longest window suitable for deep-brain MPM. However, multiphoton fluorescence imaging at this window has not been demonstrated, due to the lack of characterization of multiphoton properties of fluorescent labels. Here we demonstrate technologies for measuring both the multiphoton excitation and emission properties of fluorescent labels at the 2200 nm window, using (1) 3-photon (ησ₃) and 4-photon action cross sections (ησ₄) and (2) 3-photon and 4-photon emission spectra both ex vivo and in vivo of quantum dots. Our results show that quantum dots have exceptionally large ησ₃ and ησ₄ for efficient generation of multiphoton fluorescence. Besides, the 3-photon and 4-photon emission spectra of quantum dots are essentially identical to those of one-photon emission, which change negligibly subject to the local environment of circulating blood. Based on these characterization results, we further demonstrate deep-brain vasculature imaging in vivo. Due to the superb multiphoton properties of quantum dots, 3-photon and 4-photon fluorescence imaging reaches a maximum brain imaging depth of 1060 and 940 μm below the surface of a mouse brain, respectively, which enables the imaging of subcortical structures. We thus fill the last gap in multiphoton fluorescence imaging in terms of wavelength selection.

Three-photon excited fluorescence imaging in neuroscience: From principles to applications

The development of three-photon microscopy (3PM) has greatly expanded the capability of imaging deep within biological tissues, enabling neuroscientists to visualize the structure and activity of neuronal populations with greater depth than two-photon imaging. In this review, we outline the history and physical principles of 3PM technology. We cover the current techniques for improving the performance of 3PM. Furthermore, we summarize the imaging applications of 3PM for various brain regions and species. Finally, we discuss the future of 3PM applications for neuroscience.

Bright Near-Infrared π-Conjugated Oligomer Nanoparticles for Deep-Brain Three-Photon Microscopy Excited at the 1700 nm Window in Vivo

The development of three-photon fluorophores with 1700 nm excitation is pressingly desirable for in vivo imaging of tissue resided deep inside the brain. Herein, we report a designed and synthesized fluorescent molecule (OFET) for in vivo mouse brain imaging with three-photon microscopy at a record imaging depth. The OFET molecule has a relatively high fluorescence brightness and has a near-infrared (NIR) maximum emission at 820 nm after integrating as water-dispersible nanoparticles (OEFT NPs). Under 1720 nm excitation, OFET NPs show a large three-photon action cross-section of 1.06 × 10^-82 cm⁶ s²/photon², which is more than twice that of the commonly used sulforhodamine 101 (SR101) dye. Benefiting from the high tissue penetration depths for both the long excitation in the second NIR window of 1720 nm and the emission wavelength in the first NIR window of 820 nm, a high brightness, and a large action cross-section of three-photon, OFET NPs have good deep-brain imaging performance. Brain vasculatures of a mouse located at a depth of 1696 μm can be clearly resolved in vivo. With no observable cytotoxicity even in a high concentration, the present OFET NPs suggest that fluorescent π-conjugated oligomers are of great potential in high-resolution 3PM imaging of in vivo deep-tissue.

A biocompatible two-photon absorbing fluorescent mitochondrial probe for deep in vivo bioimaging

We reported a mitochondria-targeted two-photon fluorescent dye with an excellent two-photon absorption cross-section. With this dye, we reached an imaging depth of ca. 640 μm during mitochondrial imaging of cortical cells in live animals.

Advances in nonlinear optical microscopy techniques for in vivo and in vitro neuroimaging

Understanding the mechanism of the brain via optical microscopy is one of the challenges in neuroimaging, considering the complex structures. Advanced neuroimaging techniques provide a more comprehensive insight into patho-mechanisms of brain disorders, which is useful in the early diagnosis of the pathological and physiological changes associated with various neurodegenerative diseases. Recent advances in optical microscopy techniques have evolved powerful tools to overcome scattering of light and provide improved in vivo neuroimaging with sub-cellular resolution, endogenous contrast specificity, pinhole less optical sectioning capability, high penetration depth, and so on. The following article reviews the developments in various optical imaging techniques including two-photon and three-photon fluorescence, second-harmonic generation, third-harmonic generation, coherent anti-Stokes Raman scattering, and stimulated Raman scattering in neuroimaging. We have outlined the potentials and drawbacks of these techniques and their possible applications in the investigation of neurodegenerative diseases.

Single-beam multimodal nonlinear-optical imaging of structurally complex events in cell-cycle dynamics

We demonstrate a multimodal nonlinear-optical imaging that combines second- and third-harmonic generation (SHG and THG) with three-photon-excited fluorescence (3PEF) as a means to resolve fine details of the cell structure and trace its transformations throughout structurally complex episodes of cell-cycle dynamics, including the key stages and signatures in cell division. When zoomed in on cell mitosis, this technique enables a high-contrast multimodal imaging of intra- and extracellular signatures of cell division, detecting, via a multiplex, 3PEF/SHG/THG readout, a remarkable diversity of shapes, sizes, and symmetries in a truly single-beam setting, with no need for beam refocusing or field-waveform re-adjustment.

Cerebrovascular development: mechanisms and experimental approaches

The cerebral vasculature plays a central role in human health and disease and possesses several unique anatomic, functional and molecular characteristics. Despite their importance, the mechanisms that determine cerebrovascular development are less well studied than other vascular territories. This is in part due to limitations of existing models and techniques for visualisation and manipulation of the cerebral vasculature. In this review we summarise the experimental approaches used to study the cerebral vessels and the mechanisms that contribute to their development.

Self-phase-modulated femtosecond laser source at 1603 nm and its application to deep-brain 3-photon microscopy in vivo

3-photon microscopy (3PM) excited at the 1700 nm window enables deep-tissue imaging in vivo, especially in brain. PC rod soliton source has previously been exclusively used as the excitation source, which is rather costly and difficult to align. Here we demonstrate a novel nonlinear optical technique to build femtosecond laser source at the 1700 nm window, based on self-phase modulation (SPM) in a short span of large-mode-area fiber. The spectral broadening experienced by the pump pulse leads to the generation of a red-shifted sidelobe at 1603 nm. After spectral filtering, this sidelobe corresponds to 170-fs, 167-nJ pulses at 1603 nm. Using this SPM source, we further demonstrate deep-brain 3 PM to a depth of 1500 μm below the mouse brain surface in vivo. Our SPM femtosecond laser source thus provides a cost effective and easy-to-align alternative excitation source to the PC rod soliton source.

3-photon microscopy of myelin in mouse digital skin excited at the 1700-nm window

Myelin is a key component of the peripheral nervous system, whose structure anomaly in the digital skin is implicated in neuropathy. Here we demonstrate an in vivo labeling and imaging technique, capable of visualizing myelin sheaths deep in the mouse digital skin. Through material characterization, we verify that 3-photon fluorescence (3PF) can be generated from a commonly used dye- FluoroMyelin Red for labeling myelin, excited at the 1700-nm window. Topical injection of FluoroMyelin Red in the mouse digit leads to bright labeling of myelin sheaths. Harnessing the deep-penetration capability of 3-photon microscopy excited at the 1700-nm window, we demonstrate that 3PF imaging of FluoroMyelin Red-labeled myelin sheaths in the mouse digit in vivo can be achieved to a depth 340 μm below the skin surface, revealing both branching bundle of and individual myelin sheaths.

Measurement of two-photon properties of indocyanine green in water and human plasma excited at the 1700-nm window

Indocyanine green (ICG) is a human compatible dye and is ideal for deep-tissue two-photon fluorescence (2PF) microscopy excited at the 1700-nm window in vivo. However, the two-photon excitation and emission properties of this dye remain unknown. Here we demonstrate measurement of the two-photon excitation and emission properties of ICG in both water and human plasma, using home-built two-photon action cross-sectional measurement and two-photon emission spectrum measurement systems. Our results show that excited from 1600 to 1800 nm, 2PF can be generated from ICG dissolved in both water and human plasma. The measured two-photon action cross-sectional ησ₂ of ICG dissolved in human plasma is an order-of-magnitude larger than that dissolved in water. The measured two-photon emission spectrum overlaps with the one-photon emission spectrum for ICG dissolved in both human plasma and water. Our results will provide key two-photon parameters for the clinical use of ICG.

In vivo deep-brain imaging of microglia enabled by three-photon fluorescence microscopy

Microglia act as the first and main form of active immune defense in brain. However, in animal models, research on these cells is limited to the superficial layer of the brain, due to the lack of a deep-imaging technique. Here we break this depth limit using three-photon fluorescence (3PF) microscopy excited at the 1700-nm window. Three-photon action cross-section (ησ₃) measurement lays the basis for dye selection and the resultant maximization of 3PF generation. 3PF imaging suppresses the surface background, leading to a much improved signal-to-background ratio compared to the commonly used two-photon microscopy (2PM). We can image microglia 1124 µm below the brain surface in vivo, 3.7 times deeper than previous results using 2PM for microglia imaging. This technique enables us to visualize microglia in the white matter layer in vivo for the first time.

In vivo deep-brain blood flow speed measurement through third-harmonic generation imaging excited at the 1700-nm window

Measurement of the hemodynamic physical parameter blood flow speed in the brain in vivo is key to understanding brain physiology and pathology. 2-photon fluorescence microscopy with single blood vessel resolution is typically used, which necessitates injection of toxic fluorescent dyes. Here we demonstrate a label-free nonlinear optical technique, third-harmonic generation microscopy excited at the 1700-nm window, that is promising for such measurement. Using a simple femtosecond laser system based on soliton self-frequency shift, we can measure blood flow speed through the whole cortical grey matter, even down to the white matter layer. Together with 3-photon fluorescence microscopy, we further demonstrate that the blood vessel walls generate strong THG signals, and that plasma and circulating blood cells are mutually exclusive in space. This technique can be readily applied to brain research.

3-photon fluorescence imaging of sulforhodamine B-labeled elastic fibers in the mouse skin in vivo

Elastic fibers are key constituents of the skin. The commonly adopted optical technique for visualizing elastic fibers in the animal skin in vivo is 2-photon microscopy (2 PM) of autofluorescence, which typically suffers from low signal level. Here we demonstrate a new optical methodology to image elastic fibers in animal models in vivo: 3-photon microscopy (3 PM) excited at the 1700-nm window combining with preferential labeling of elastic fibers using sulforhodamine B (SRB). First, we demonstrate that intravenous injection of SRB can circumvent the skin barrier (encountered in topical application) and preferentially label elastic fibers, as verified by simultaneous 2 PM of both autofluorescence and SRB fluorescence from skin structures. Then through 3-photon excitation property characterization, we show that 3-photon fluorescence can be excited from SRB at the 1700-nm window, and 1600-nm excitation is most efficient according to our 3-photon action cross section measurement. Based on these results and using our developed 1600-nm femtosecond laser source, we finally demonstrate 3 PM of SRB-labeled elastic fibers through the whole dermis in the mouse skin in vivo, with only 3.7-mW optical power deposited on the skin surface. We expect our methodology will provide novel optical solution to elastic fiber research.

Insights From Dynamic Neuro-Immune Imaging on Murine Immune Responses to CNS Damage

Evolving technologies and increasing understanding of human physiology over the past century have afforded our ability to intervene on human diseases using implantable bio-materials. These bio-electronic devices present a unique challenge through the creation of an interface between the native tissue and implantable bio-materials: the generation of host immune response surrounding such devices. While recent developments in cancer immunology seek to stimulate the immune system against cancer, successful long-term application of implantable bio-material devices need to durably minimize reactive immune processes at involved anatomical sites. Peripheral immune system response has been studied extensively for implanted bio-materials at various body sites. Examples include tooth composites (Gitalis et al., 2019), inguinal hernia repair (Heymann et al., 2019), and cardiac stents and pacemaker leads (Slee et al., 2016). Studies have also been extended to less well-studied immune reactivity in response to CNS neural-electronic implant devices. Recent technological advances in 2-Photon Laser Scanning Microscopy (2P-LSM) have allowed novel insights into in vivo immune response in a variety of tissue microenvironments. While imaging of peripheral tissues has provided an abundance of data with regards to immune cell dynamics, central nervous system (CNS) imaging is comparatively complicated by tissue accessibility and manipulation. Despite these challenges, the results of dynamic intravital neuro-immune imaging thus far have provided foundational insights into basic CNS biology. Utilizing a combination of intravital and ex vivo 2P-LSM, we have observed novel pathways allowing immune cells, stromal cells, cancer cells and proteins to communicate between the CNS parenchyma and peripheral vasculature. Similar to what has been reported in the intestinal tract, we have visualized myeloid cells extend dendritic processes across the blood brain barrier (BBB) into pial blood vessels. Furthermore, transient vessel leaks seen during systemic inflammation provide opportunities for cellular protein to be exchanged between the periphery and CNS. These insights provide new, visual information regarding immune surveillance and antigen presentation within the CNS. Furthermore, when combining intravital 2P-LSM and microfluidic devices complexed with mathematical modeling, we are gaining new insights into the intravascular behavior of circulating immune cells. This new knowledge into the basic mechanisms by which cells migrate to and interact with the CNS provide important considerations for the design of neuro-electronic biomaterials that have the potential to connect the peripheral-neural microenvironments into a unique, artificial interface.