A new method for in vivo visualization of vessel remodeling using a near-infrared dye

Mechanical regulation of signal transduction in angiogenesis

Biophysical and biochemical cues work in concert to regulate angiogenesis. These cues guide angiogenesis during development and wound healing. Abnormal cues contribute to pathological angiogenesis during tumor progression. In this review, we summarize the known signaling pathways involved in mechanotransduction important to angiogenesis. We discuss how variation in the mechanical microenvironment, in terms of stiffness, ligand availability, and topography, can modulate the angiogenesis process. We also present an integrated view on how mechanical perturbations, such as stretching and fluid shearing, alter angiogenesis-related signal transduction acutely, leading to downstream gene expression. Tissue engineering-based approaches to study angiogenesis are reviewed too. Future directions to aid the efforts in unveiling the comprehensive picture of angiogenesis are proposed.

In vivo imaging of hemodynamic redistribution and arteriogenesis across microvascular network

OBJECTIVE

Arteriogenesis is an important mechanism that contributes to restoration of oxygen supply in chronically ischemic tissues, but remains incompletely understood due to technical limitations. This study presents a novel approach for comprehensive assessment of the remodeling pattern in a complex microvascular network containing multiple collateral microvessels.

METHODS

We have developed a hardware-software integrated platform for quantitative, longitudinal, and label-free imaging of network-wide hemodynamic changes and arteriogenesis at the single-vessel level. By ligating feeding arteries in the mouse ear, we induced network-wide hemodynamic redistribution and localized arteriogenesis. The utility of this technology was demonstrated by studying the influence of obesity on microvascular arteriogenesis.

RESULTS

Simultaneously monitoring the remodeling of competing collateral arterioles revealed a new, inverse relationship between initial vascular resistance and extent of arteriogenesis. Obese mice exhibited similar remodeling responses to lean mice through the first week, including diameter increase and flow upregulation in collateral arterioles. However, these gains were subsequently lost in obese mice.

CONCLUSIONS

Capable of label-free, comprehensive, and dynamic quantification of structural and functional changes in the microvascular network in vivo, this platform opens up new opportunities to study the mechanisms of microvascular arteriogenesis, its implications in diseases, and approaches to pharmacologically rectify microvascular dysfunction.

Introducing a mammalian nerve-muscle preparation ideal for physiology and microscopy, the transverse auricular muscle in the ear of the mouse

A new mammalian neuromuscular preparation is introduced for physiology and microscopy of all sorts: the intrinsic muscle of the mouse ear. The great utility of this preparation is demonstrated by illustrating how it has permitted us to develop a wholly new technique for staining muscle T-tubules, the critical conductive-elements in muscle. This involves sequential immersion in dilute solutions of osmium and ferrocyanide, then tannic acid, and then uranyl acetate, all of which totally blackens the T-tubules but leaves the muscle pale, thereby revealing that the T-tubules in mouse ear-muscles become severely distorted in several pathological conditions. These include certain mouse-models of muscular dystrophy (specifically, dysferlin-mutations), certain mutations of muscle cytoskeletal proteins (specifically, beta-tubulin mutations), and also in denervation-fibrillation, as observed in mouse ears maintained with in vitro tissue-culture conditions. These observations permit us to generate the hypothesis that T-tubules are the "Achilles' heel" in several adult-onset muscular dystrophies, due to their unique susceptibility to damage via muscle lattice-dislocations. These new observations strongly encourage further in-depth studies of ear-muscle architecture, in the many available mouse-models of various devastating human muscle-diseases. Finally, we demonstrate that the delicate and defined physical characteristics of this 'new' mammalian muscle are ideal for ultrastructural study, and thereby facilitate the imaging of synaptic vesicle membrane recycling in mammalian neuromuscular junctions, a topic that is critical to myasthenia gravis and related diseases, but which has, until now, completely eluded electron microscopic analysis. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Shifts in metabolic fuel use coincide with maximal rates of ventilation and body surface rewarming in an arousing hibernator

It is well established that hibernating mammals rely predominantly on lipid stores to fuel metabolism throughout the hibernation season. However, it is unclear if other endogenous fuels contribute to the rapid, ~400-fold increase in metabolic rate during the early phase of arousal from torpor. To investigate this issue, we used cavity ring-down spectroscopy, a technique that provides a real-time indication of fuel use by measuring the ratio of ¹³C to ¹²C in the exhaled CO₂ of arousing 13-lined ground squirrels (Ictidomys tridecemlineatus). We used infrared thermography to simultaneously measure ventilation and surface temperature change in various body regions, and we interpreted these data in light of changing plasma metabolite abundances at multiple stages of arousal from torpor. We found that hibernating squirrels use a combination of lipids and, likely, carbohydrates to fuel the initial ~60 min of arousal before switching to predominantly lipid oxidation. This fuel switch coincided with times of maximal rates of ventilation and rewarming of different body surface regions, including brown adipose tissue. Infrared thermography revealed zonal rewarming, whereby the brown adipose tissue region was the first to warm, followed by the thoracic and head regions and, finally, the posterior half of the body. Consistent with the results from cavity ring-down spectroscopy, plasma metabolite dynamics during early arousal suggested a large reliance on fatty acids, with a contribution from carbohydrates and glycerol. Because of their high oxidative flux rates and efficient O₂ use, carbohydrates might be an advantageous metabolic fuel during the early phase of arousal, when metabolic demands are high but ventilation rates and, thus, O₂ supply are relatively low.

Intravital microscopy in the study of the tumor microenvironment: from bench to human application

Intravital microscopy (IVM) is a dynamic imaging modality that allows for the real time observation of biologic processes in vivo, including angiogenesis and immune cell interactions. In the setting of preclinical cancer models, IVM has facilitated an understanding of the tumor associated vasculature and the role of effector immune cells in the tumor microenvironment. Novel approaches to apply IVM to human malignancies have thus far focused on cancer diagnosis and tumor vessel characterization, but have the potential to provide advances in the field of personalized medicine by identifying individual patients who may respond to systemically delivered chemotherapeutic drugs or immunotherapeutic agents. In this review, we highlight the role that IVM has had in investigating tumor vasculature and the tumor microenvironment in preclinical studies and discuss its current and future applications to directly observe human tumors.

Efficient Enhancement of Blood-Brain Barrier Permeability Using Acoustic Cluster Therapy (ACT)

The blood-brain barrier (BBB) is a major obstacle in drug delivery for diseases of the brain, and today there is no standardized route to surpass it. One technique to locally and transiently disrupt the BBB, is focused ultrasound in combination with gas-filled microbubbles. However, the microbubbles used are typically developed for ultrasound imaging, not BBB disruption. Here we describe efficient opening of the BBB using the promising novel Acoustic Cluster Therapy (ACT), that recently has been used in combination with Abraxane® to successfully treat subcutaneous tumors of human prostate adenocarcinoma in mice. ACT is based on the conjugation of microbubbles to liquid oil microdroplets through electrostatic interactions. Upon activation in an ultrasound field, the microdroplet phase transfers to form a larger bubble that transiently lodges in the microvasculature. Further insonation induces volume oscillations of the activated bubble, which in turn induce biomechanical effects that increase the permeability of the BBB. ACT was able to safely and temporarily permeabilize the BBB, using an acoustic power 5-10 times lower than applied for conventional microbubbles, and successfully deliver small and large molecules into the brain.

Laser speckle flowmetry method for measuring spatial and temporal hemodynamic alterations throughout large microvascular networks

OBJECTIVES

1) To develop and validate laser speckle flowmetry (LSF) as a quantitative tool for individual microvessel hemodynamics in large networks. 2) To use LSF to determine if structural differences in the dorsal skinfold microcirculation (DSFWC) of C57BL/6 and BALB/c mice impart differential network hemodynamic responses to occlusion.

METHODS

We compared LSF velocity measurements with known/measured velocities in vitro using capillary tube tissue phantoms and in vivo using mouse DSFWCs and cremaster muscles. Hemodynamic changes induced by feed arteriole occlusion were measured using LSF in DSFWCs implanted on C57BL/6 and BALB/c mice.

RESULTS

In vitro, we found that the normalized speckle intensity (NSI) versus velocity linear relationship (R(2) ≥ 0.97) did not vary with diameter or hematocrit and can be shifted to meet an expected operating range. In vivo, DSFWC and cremaster muscle preparations (R(2) = 0.92 and 0.95, respectively) demonstrated similar linear relationships between NSI and centerline velocity. Stratification of arterioles into predicted collateral pathways revealed significant differences between C57BL/6 and BALB/c strains in response to feed arteriole occlusion.

CONCLUSIONS

These data demonstrate the applicability of LSF to intravital microscopy microcirculation preparations for determining both relative and absolute hemodynamics on a network-wide scale while maintaining the resolution of individual microvessels.