Novel phenotype characterization utilizing electrical impedance myography signatures in murine spinal cord injury neurogenic bladder models

Neurogenic bladder (NB) affects people of all ages. Electric impedance myography (EIM) assesses localized muscle abnormalities. Here, we sought to investigate whether unique detrusor EIM signatures are present in NB due to spinal cord injury (SCI). Twenty-eight, 8-10 weeks old, C57BL/6J female mice were studied. Twenty underwent spinal cord transection; 8 served as controls. Cohorts were euthanized at 4 and 6 weeks after spinal cord transection. Each bladder was measured in-situ with EIM with applied frequencies of 1 kHz to 10 MHz, and then processed for molecular and histologic study. SCI mice had greater bladder-to-body weight ratio (p < 0.0001), greater collagen deposition (p = 0.009), and greater smooth-muscle-myosin-heavy-chain isoform A/B ratio (p < 0.0001). Compared with the control group, the SCI group was associated with lower phase, reactance, and resistance values (p < 0.01). Significant correlations (p < 0.001) between bladder-to-body weight ratios and EIM measurements were observed across the entire frequency spectrum. A severely hypertrophied phenotype was characterized by even greater bladder-to-body weight ratios and more depressed EIM values. Our study demonstrated distinct EIM alterations in the detrusor muscle of mice with NB due to SCI. With further refinement, EIM may offer a potential point-of-care tool for the assessment of NB and its response to treatment.

Combining electromyographic and electrical impedance data sets through machine learning: A study in D2-mdx and wild-type mice

INTRODUCTION/AIMS

Needle impedance-electromyography (iEMG) assesses the active and passive electrical properties of muscles concurrently by using a novel needle with six electrodes, two for EMG and four for electrical impedance myography (EIM). Here, we assessed an approach for combining multifrequency EMG and EIM data via machine learning (ML) to discriminate D2-mdx muscular dystrophy and wild-type (WT) mouse skeletal muscle.

METHODS

iEMG data were obtained from quadriceps of D2-mdx mice, a muscular dystrophy model, and WT animals. EIM data were collected with the animals under deep anesthesia and EMG data collected under light anesthesia, allowing for limited spontaneous movement. Fourier transformation was performed on the EMG data to provide power spectra that were sampled across the frequency range using three different approaches. Random forest-based, nested ML was applied to the EIM and EMG data sets separately and then together to assess healthy versus disease category classification using a nested cross-validation procedure.

RESULTS

Data from 20 D2-mdx and 20 WT limbs were analyzed. EIM data fared better than EMG data in differentiating healthy from disease mice with 93.1% versus 75.6% accuracy, respectively. Combining EIM and EMG data sets yielded similar performance as EIM data alone with 92.2% accuracy.

DISCUSSION

We have demonstrated an ML-based approach for combining EIM and EMG data obtained with an iEMG needle. While EIM-EMG in combination fared no better than EIM alone with this data set, the approach used here demonstrates a novel method of combining the two techniques to characterize the full electrical properties of skeletal muscle.

Electrical impedance myography for the early detection of muscle ischemia secondary to compartment syndrome: a study in a rat model

Acute Compartment Syndrome (ACS) is one of the most devastating orthopedic conditions, affecting any of the body's many compartments, which, if sufficiently severe, may result in disability and amputation. Currently, intra-compartmental pressure measurements serve as the gold standard for diagnosing ACS. Diagnosing limbs at risk for ACS before irreversible damage to muscle and nerve is critical. Standard approaches for diagnosing impending compartment syndrome include clinical evaluation of the limb, such as assessment for "tightness" of the overlying skin, reduced pulses distally, and degree of pain, none of which are specific or sensitive. We have proposed a novel method to detect ACS via electrical impedance myography (EIM), where a weak, high-frequency alternating current is passed between one pair of electrodes through a region of tissue, and the resulting surface voltages are measured via a second pair. We evaluated the ability of EIM to detect early muscle ischemia in an established murine model of compression-induced muscle injury, where we collected resistance, reactance, and their dimensionless product, defined as Relative Injury Index (RII) during the study. Our model generated reproducible hypoxia, confirmed by Hypoxyprobe™ staining of endothelial regions within the muscle. Under conditions of ischemia, we demonstrated a reproducible, stable, and significant escalation in resistance, reactance, and RII values, compared to uninjured control limbs. These data make a reasonable argument for additional investigations into using EIM for the early recognition of muscle hypoperfusion and ischemia. However, these findings must be considered preliminary steps, requiring further pre-clinical and clinical validation.

Surface Electrical Impedance Myography Detects Skeletal Muscle Atrophy in Aged Wildtype Zebrafish and Aged gpr27 Knockout Zebrafish

Throughout a vertebrate organism's lifespan, skeletal muscle mass and function progressively decline. This age-related condition is termed sarcopenia. In humans, sarcopenia is associated with risk of falling, cardiovascular disease, and all-cause mortality. As the world population ages, projected to reach 2 billion older adults worldwide in 2050, the economic burden on the healthcare system is also projected to increase considerably. Currently, there are no pharmacological treatments for sarcopenia, and given the long-term nature of aging studies, high-throughput chemical screens are impractical in mammalian models. Zebrafish is a promising, up-and-coming vertebrate model in the field of sarcopenia that could fill this gap. Here, we developed a surface electrical impedance myography (sEIM) platform to assess skeletal muscle health, quantitatively and noninvasively, in adult zebrafish (young, aged, and genetic mutant animals). In aged zebrafish (~85% lifespan) as compared to young zebrafish (~20% lifespan), sEIM parameters (2 kHz phase angle, 2 kHz reactance, and 2 kHz resistance) robustly detected muscle atrophy (p < 0.000001, q = 0.000002; p = 0.000004, q = 0.000006; p = 0.000867, q = 0.000683, respectively). Moreover, these same measurements exhibited strong correlations with an established morphometric parameter of muscle atrophy (myofiber cross-sectional area), as determined by histological-based morphometric analysis (r = 0.831, p = 2 × 10-12; r = 0.6959, p = 2 × 10-8; and r = 0.7220; p = 4 × 10-9, respectively). Finally, the genetic deletion of gpr27, an orphan G-protein coupled receptor (GPCR), exacerbated the atrophy of skeletal muscle in aged animals, as evidenced by both sEIM and histology. In conclusion, the data here show that surface EIM techniques can effectively discriminate between healthy young and sarcopenic aged muscle as well as the advanced atrophied muscle in the gpr27 KO animals. Moreover, these studies show how EIM values correlate with cell size across the animals, making it potentially possible to utilize sEIM as a "virtual biopsy" in zebrafish to noninvasively assess myofiber atrophy, a valuable measure for muscle and gerontology research.

Electrical impedance myography detects age-related skeletal muscle atrophy in adult zebrafish

Age-related deficits in skeletal muscle function, termed sarcopenia, are due to loss of muscle mass and changes in the intrinsic mechanisms underlying contraction. Sarcopenia is associated with falls, functional decline, and mortality. Electrical impedance myography (EIM)-a minimally invasive, rapid electrophysiological tool-can be applied to animals and humans to monitor muscle health, thereby serving as a biomarker in both preclinical and clinical studies. EIM has been successfully employed in several species; however, the application of EIM to the assessment of zebrafish-a model organism amenable to high-throughput experimentation-has not been reported. Here, we demonstrated differences in EIM measures between the skeletal muscles of young (6 months of age) and aged (33 months of age) zebrafish. For example, EIM phase angle and reactance at 2 kHz showed significantly decreased phase angle (5.3 ± 2.1 versus 10.7 ± 1.5°; p = 0.001) and reactance (89.0 ± 3.9 versus 172.2 ± 54.8 ohms; p = 0.007) in aged versus young animals. Total muscle area, in addition to other morphometric features, was also strongly correlated to EIM 2 kHz phase angle across both groups (r = 0.7133, p = 0.01). Moreover, there was a strong correlation between 2 kHz phase angle and established metrics of zebrafish swimming performance, including turn angle, angular velocity, and lateral motion (r = 0.7253, r = 0.7308, r = 0.7857, respectively, p < 0.01 for all). In addition, the technique was shown to have high reproducibility between repeated measurements with a mean percentage difference of 5.34 ± 1.17% for phase angle. These relationships were also confirmed in a separate replication cohort. Together, these findings establish EIM as a fast, sensitive method for quantifying zebrafish muscle function and quality. Moreover, identifying the abnormalities in the bioelectrical properties of sarcopenic zebrafish provides new opportunities to evaluate potential therapeutics for age-related neuromuscular disorders and to interrogate the disease mechanisms of muscle degeneration.

Electrical Impedance Myography Correlates with Functional Measures of Disease Progression in D2-mdx Mice and Boys with Duchenne Muscular Dystrophy

BACKGROUND

Sensitive, objective, and longitudinal outcome measures applicable to both pre-clinical and clinical interventions are needed to assess muscle health in Duchenne muscular dystrophy (DMD). Electrical impedance myography (EIM) has the potential to non-invasively measure disease progression in mice and boys with DMD.

OBJECTIVE

We sought to evaluate how electrical impedance values (i.e., phase, reactance, and resistance) correlate to established measures of disease in both D2-mdx and wild type (WT) mice and boys with and without DMD.

METHODS

Histological, functional, and EIM data collected from previous studies of WT and D2-mdx mice at 6, 13, 21 and 43 weeks of age were reanalyzed. In parallel, previously collected functional outcome measures and EIM values were reanalyzed from boys with and without DMD at four different age groups from 2 to 14 years old.

RESULTS

In mice, disease progression as detected by histological, functional, and EIM measures, was appreciable over this time period and grip strength best correlated to longitudinal phase and reactance impedance values. In boys, disease progression quantified through commonly utilized functional outcome measures was significant and longitudinal phase demonstrated the strongest correlation with functional outcome measures.

CONCLUSION

Similar changes in EIM values, specifically in longitudinal reactance and phase, were found to show significant correlations to functional measures in both mice and boys. Thus, EIM demonstrates applicability in both pre-clinical and clinical settings and can be used as a safe, non-invasive, and longitudinal proxy biomarker to assess muscle health in DMD.

Assessing the therapeutic impact of resveratrol in ALS SOD1-G93A mice with electrical impedance myography

To aid in the identification of new treatments for amyotrophic lateral sclerosis (ALS), convenient biomarkers are needed to effectively and uniformly measure drug efficacy. To this end, we assessed the effects of the nutraceutical resveratrol (RSV) on disease onset and overall survival in SOD1-G93A (ALS) mice and compared several standard biomarkers including body mass, motor score (MS), paw grip endurance (PGE), and compound motor action potential (CMAP) amplitude, with the technique of electrical impedance myography (EIM) to follow disease progression. Eighteen ALS mice (nine females, nine males) received RSV in the chow (dose: 120 mg/kg/day) starting at 8 weeks of age; 19 ALS mice (nine females, 10 males) received normal chow; 10 wild type (WT) littermates (five females, five males) fed standard chow served as controls. Biomarker assessments were performed weekly beginning at 8 weeks. No differences in either disease onset or overall survival were found between RSV-treated and untreated ALS mice of either sex; moreover, all biomarkers failed to identify any beneficial effect of RSV when administered at this dose. Therefore, for the comparative evaluation of the ability of the various biomarkers to detect the earliest symptoms of disease, data from all animals (i.e., RSV-treated and untreated ALS mice of both sexes) were combined. Of the biomarkers tested, EIM impedance values, i.e., surface EIM longitudinal phase at 50 kHz (LP 50 kHz), and CMAP amplitude showed the earliest significant changes from baseline. LP 50 kHz values showed a rate of decline equivalent to that of CMAP amplitude and correlated with both PGE and CMAP amplitude [Spearman rho = 0.806 (p = 0.004) and 0.627 (p = 0.044), respectively]. Consistent with previous work, these findings indicate that surface EIM can serve as an effective non-invasive biomarker for preclinical drug testing in rodent models of ALS.

Comparison of Quantitative Ultrasound Methods to Classify Dystrophic and Obese Models of Skeletal Muscle

In this study, we compared multiple quantitative ultrasound metrics for the purpose of differentiating muscle in 20 healthy, 10 dystrophic and 10 obese mice. High-frequency ultrasound scans were acquired on dystrophic (D2-mdx), obese (db/db) and control mouse hindlimbs. A total of 248 image features were extracted from each scan, using brightness-mode statistics, Canny edge detection metrics, Haralick features, envelope statistics and radiofrequency statistics. Naïve Bayes and other classifiers were trained on single and pairs of features. The a parameter from the Homodyned K distribution at 40 MHz achieved the best univariate classification (accuracy = 85.3%). Maximum classification accuracy of 97.7% was achieved using a logistic regression classifier on the feature pair of a² (K distribution) at 30 MHz and brightness-mode variance at 40MHz. Dystrophic and obese mice have muscle with distinct acoustic properties and can be classified to a high level of accuracy using a combination of multiple features.

Using machine learning algorithms to enhance the diagnostic performance of electrical impedance myography

INTRODUCTION/AIMS

We assessed the classification performance of machine learning (ML) using multifrequency electrical impedance myography (EIM) values to improve upon diagnostic outcomes as compared to those based on a single EIM value.

METHODS

EIM data was obtained from unilateral excised gastrocnemius in eighty diseased mice (26 D2-mdx, Duchenne muscular dystrophy model, 39 SOD1G93A ALS model, and 15 db/db, a model of obesity-induced muscle atrophy) and 33 wild-type (WT) animals. We assessed the classification performance of a ML random forest algorithm incorporating all the data (multifrequency resistance, reactance and phase values) comparing it to the 50 kHz phase value alone.

RESULTS

ML outperformed the 50 kHz analysis as based on receiver-operating characteristic curves and measurement of the area under the curve (AUC). For example, comparing all diseases together versus WT from the test set outputs, the AUC was 0.52 for 50 kHz phase, but was 0.94 for the ML model. Similarly, when comparing ALS versus WT, the AUCs were 0.79 for 50 kHz phase and 0.99 for ML.

DISCUSSION

Multifrequency EIM utilizing ML improves upon classification compared to that achieved with a single-frequency value. ML approaches should be considered in all future basic and clinical diagnostic applications of EIM.

Performing In Vivo and Ex Vivo Electrical Impedance Myography in Rodents

Electrical impedance myography (EIM) is a convenient technique that can be used in preclinical and clinical studies to assess muscle tissue health and disease. EIM is obtained by applying a low-intensity, directionally focused, electrical current to a muscle of interest across a range of frequencies (i.e., from 1 kHz to 10 MHz) and recording the resulting voltages. From these, several standard impedance components, including the reactance, resistance, and phase, are obtained. When performing ex vivo measurements on excised muscle, the inherent passive electrical properties of the tissue, namely the conductivity and relative permittivity, can also be calculated. EIM has been used extensively in animals and humans to diagnose and track muscle alterations in a variety of diseases, in relation to simple disuse atrophy, or as a measure of therapeutic intervention. Clinically, EIM offers the potential to track disease progression over time and to assess the impact of therapeutic interventions, thus offering the opportunity to shorten the clinical trial duration and reduce sample size requirements. Because it can be performed noninvasively or minimally invasively in living animal models as well as humans, EIM offers the potential to serve as a novel translational tool enabling both preclinical and clinical development. This article provides step-by-step instructions on how to perform in vivo and ex vivo EIM measurements in mice and rats, including approaches to adapt the techniques to specific conditions, such as for use in pups or obese animals.

Design and pilot testing of a 26-gauge impedance-electromyography needle in wild-type and ALS mice

INTRODUCTION/AIMS

Needle impedance-electromyography (iEMG) is a diagnostic modality currently under development that combines intramuscular electrical impedance with concentric electromyography (EMG) in a single needle. We designed, manufactured, and tested a prototype iEMG needle in a cohort of wild-type (WT) and SOD1G93A amyotrophic lateral sclerosis (ALS) mice to assess its ability to record impedance and EMG data.

METHODS

A new six-electrode, 26-gauge, iEMG needle was designed, manufactured and tested. Quantitative impedance and qualitative "gestalt" EMG were performed sequentially on bilateral quadriceps of 16-wk-old SOD1G93A ALS (N = 6) and WT (N = 6) mice by connecting the needle first to an impedance analyzer (with the animal at rest) and then to a standard EMG system (with the animal fully under anesthesia to measure spontaneous activity and briefly during awakening to measure voluntary activity). The needle remained in the muscle throughout the measurement period.

RESULTS

EMG data were qualitatively similar to that observed with a commercially available concentric EMG needle; fibrillation potentials were observed in 84% of the ALS mice and none of the WT mice; motor unit potentials were also readily identified. Impedance data revealed significant differences in resistance, reactance, and phase values between the two groups, with ALS animals having reduced reactance and resistance values.

DISCUSSION

This work demonstrates the feasibility of a single iEMG needle conforming to standard dimensions of size and function. Further progress of iEMG technology for enhanced neuromuscular diagnosis and quantification of disease status is currently in development.

Relationships between in vivo surface and ex vivo electrical impedance myography measurements in three different neuromuscular disorder mouse models

Electrical impedance myography (EIM) using surface techniques has shown promise as a means of diagnosing and tracking disorders affecting muscle and assessing treatment efficacy. However, the relationship between such surface-obtained impedance values and pure muscle impedance values has not been established. Here we studied three groups of diseased and wild-type (WT) animals, including a Duchenne muscular dystrophy model (the D2-mdx mouse), an amyotrophic lateral sclerosis (ALS) model (the SOD1 G93A mouse), and a model of fat-related atrophy (the db/db diabetic obese mouse), performing hind limb measurements using a standard surface array and ex vivo measurements on freshly excised gastrocnemius muscle. A total of 101 animals (23 D2-mdx, 43 ALS mice, 12 db/db mice, and corresponding 30 WT mice) were studied with EIM across a frequency range of 8 kHz to 1 MHz. For both D2-mdx and ALS models, moderate strength correlations (Spearman rho values generally ranging from 0.3-0.7, depending on the impedance parameter (i.e., resistance, reactance and phase) were obtained. In these groups of animals, there was an offset in frequency with impedance values obtained at higher surface frequencies correlating more strongly to impedance values obtained at lower ex vivo frequencies. For the db/db model, correlations were comparatively weaker and strongest at very high and very low frequencies. When combining impedance data from all three disease models together, moderate correlations persisted (with maximal Spearman rho values of 0.45). These data support that surface EIM data reflect ex vivo muscle tissue EIM values to a moderate degree across several different diseases, with the highest correlations occurring in the 10-200 kHz frequency range. Understanding these relationships will prove useful for future applications of the technique of EIM in the assessment of neuromuscular disorders.

Estimating myofiber cross-sectional area and connective tissue deposition with electrical impedance myography: A study in D2-mdx mice

INTRODUCTION

Surface electrical impedance myography (sEIM) has the potential for providing information on muscle composition and structure noninvasively. We sought to evaluate its use to predict myofiber size and connective tissue deposition in the D2-mdx model of Duchenne muscular dystrophy (DMD).

METHODS

We applied a prediction algorithm, the least absolute shrinkage and selection operator, to select specific EIM measurements obtained with surface and ex vivo EIM data from D2-mdx and wild-type (WT) mice (analyzed together or separately). We assessed myofiber cross-sectional area histologically and hydroxyproline (HP), a surrogate measure for connective tissue content, biochemically.

RESULTS

Using WT and D2-mdx impedance values together in the algorithm, sEIM gave average root-mean-square errors (RMSEs) of 26.6% for CSA and 45.8% for HP, which translate into mean errors of ±363 μm2 for a mean CSA of 1365 μm2 and of ±1.44 μg HP/mg muscle for a mean HP content of 3.15 μg HP/mg muscle. Stronger predictions were obtained by analyzing sEIM data from D2-mdx animals alone (RMSEs of 15.3% for CSA and 34.1% for HP content). Predictions made using ex vivo EIM data from D2-mdx animals alone were nearly equivalent to those obtained with sEIM data (RMSE of 16.59% for CSA), and slightly more accurate for HP (RMSE of 26.7%).

DISCUSSION

Surface EIM combined with a predictive algorithm can provide estimates of muscle pathology comparable to values obtained using ex vivo EIM, and can be used as a surrogate measure of disease severity and progression and response to therapy.

Predicting myofiber cross-sectional area and triglyceride content with electrical impedance myography: A study in db/db mice

BACKGROUND

Electrical impedance myography (EIM) provides insight into muscle composition and structure. We sought to evaluate its use in a mouse obesity model characterized by myofiber atrophy.

METHODS

We applied a prediction algorithm, ie, the least absolute shrinkage and selection operator (LASSO), to surface, needle array, and ex vivo EIM data from db/db and wild-type mice and assessed myofiber cross-sectional area (CSA) histologically and triglyceride (TG) content biochemically.

RESULTS

EIM data from all three modalities provided acceptable predictions of myofiber CSA with average root mean square error (RMSE) of 15% in CSA (ie, ±209 μm2 for a mean CSA of 1439 μm2 ) and TG content with RMSE of 30% in TG content (ie, ±7.3 nmol TG/mg muscle for a mean TG content of 25.4 nmol TG/mg muscle).

CONCLUSIONS

EIM combined with a predictive algorithm provides reasonable estimates of myofiber CSA and TG content without the need for biopsy.

Using Electrical Impedance Myography as a Biomarker of Muscle Deconditioning in Rats Exposed to Micro- and Partial-Gravity Analogs

As astronauts prepare to undertake new extra-terrestrial missions, innovative diagnostic tools are needed to better assess muscle deconditioning during periods of weightlessness and partial gravity. Electrical impedance myography (EIM) has been used to detect muscle deconditioning in rodents exposed to microgravity during spaceflight or using the standard ground-based model of hindlimb unloading via tail suspension (HU). Here, we used EIM to assess muscle changes in animals exposed to two new models: hindlimb suspension using a pelvic harness (HLS) and a partial weight-bearing (PWB) model that mimics partial gravity (including Lunar and Martian gravities). We also used a simple needle array electrode in lieu of surface or ex vivo EIM approaches previously employed. Our HLS results confirmed earlier findings obtained after spaceflight and tail suspension. Indeed, one EIM measure (i.e., phase-slope) that was previously reported as highly sensitive, was significantly decreased after HLS (day 0: 14.60 ± 0.97, day 7: 11.03 ± 0.81, and day 14: 10.13 ± 0.55 | Deg/MHz|, p < 0.0001), and was associated with a significant decrease in muscle grip force. Although EIM parameters such as 50 kHz phase, reactance, and resistance remained variable over 14 days in PWB animals, we identified major PWB-dependent effects at 7 days. Moreover, the data at both 7 and 14 days correlated to previously observed changes in rear paw grip force using the same PWB model. In conclusion, our data suggest that EIM has the potential to serve as biomarker of muscle deconditioning during exposure to both micro- and partial- gravity during future human space exploration.

Partial Weight-Bearing in Female Rats: Proof of Concept in a Martian-Gravity Analog

Many studies have investigated the physiological response to microgravity in both astronauts and animals. However, while space agencies have sought to deploy more women on their missions; animal models rarely include females studies or comparisons between males and females. Therefore, we exposed adult female rats to 2 weeks of partial weight-bearing at either 100% of their normal loading (PWB100) or 40% of their normal loading (PWB40), corresponding to Martian gravity-analog, and assess muscle function, force and histomorphometry. Females exposed to PWB showed an 11.62% decline in hindlimb grip force associated with an 11.84% decrease in soleus myofiber size after 14 days of exposure, while maintaining normal blood oxygenation and stress levels. This pilot study represents the first experiment designed to understand the muscular disuse associated with a partial reduction in mechanical loading in female rats, and the first step needed to develop successful mitigating strategies.

NEW AND NOTEWORTHY

This research article describes the first use of quadrupedal partial weight-bearing in female rats. This study demonstrates the feasibility of partial gravity analogs in females and allows for future investigations about the impact of sex on muscle deconditioning due to reduced mechanical loading.

Altered muscle electrical tissue properties in a mouse model of premature aging

INTRODUCTION

Improved methods are needed to detect and quantify age-related muscle change. In this study we assessed the electrical properties of muscle impacted by acquired mitochondrial DNA mutations via the PolG mouse, which exhibits typical age-associated features, and the impact of a potential therapy, nicotinamide mononucleotide (NMN).

METHODS

The gastrocnemii of 24 PolG and 30 wild-type (WT) mice (8 PolG and 17 WT treated with NMN) were studied in an electrical impedance-measuring cell. Conductivity and relative permittivity were determined from the impedance data. Myofiber cross-sectional area (CSA) was quantified histologically.

RESULTS

Untreated PolG mice demonstrated alterations in several impedance features, including 50-kHz relative permittivity and center frequency. A potential effect of NMN was also observed in these parameters in PolG but not WT animals. Impedance values correlated with myofiber CSA.

DISCUSSION

Electrical impedance is sensitive to myofiber features considered characteristic of aging and to the impact of a potential therapy.

Electrical impedance myography for the detection of muscle inflammation induced by λ-carrageenan

Electrical impedance myography (EIM) is a technique for the assessment of muscle health and composition and has been shown to be sensitive to a variety of muscle pathologies including neurogenic atrophy and connective tissue deposition. However, it has been minimally studied in pure inflammation. In this study, we sought to assess EIM sensitivity to experimental inflammation induced by the localized intramuscular injection of λ-carrageenan. A total of 91 mice underwent 1–1000 kHz EIM measurements of gastrocnemius using a needle array, followed by injection of either 0.3% λ-carrageenan in phosphate-buffered saline (PBS) or PBS alone. Animals were then remeasured with EIM at 4, 24, 48, or 72 hours and euthanized and quantitative assessment of muscle histology was performed. Parallel alterations in both 5 and 50 kHz EIM values were identified at 4 and 24 hours, including reductions in phase, reactance, and resistance. In PBS-treated animals these values normalized by 48 hours, whereas substantial reductions in phase and reactance in 5 kHz EIM values persisted at 48 and 72 hours (i.e., values of phase 72 hours post-injection were 6.51 ± 0.40 degrees for λ-carrageenan versus 8.44 ± 0.35 degrees for PBS p<0.001, n = 11 per group). The degree of basophilic area observed in muscle sections by histology correlated to the degree of phase change at these two time points (R_spearman = -0.51, p = 0.0029). Changes in low frequency EIM parameters are sensitive to the presence of inflammatory infiltrates, and have the potential of serving as a simple means of quantifying the presence and extent of muscle inflammation without the need for biopsy.

Approximate complex electrical potential distribution in the monodomain model with unequal conductivity and relative permittivity anisotropy ratios

OBJECTIVE

Electrical conductivity and relative permittivity are properties that indicate muscle health and they have different values parallel and perpendicular to the direction of the myofiber, a concept known as anisotropy. When the intrinsic electrical properties of muscle have ratios of anisotropy that are different then there is no analytical solution that can describe the electrical potential distribution in the tissue.

APPROACH

Here, we present approximate analytical solutions to monodomain equations with unequal anisotropy ratios. For this, we base our analysis on perturbation theory where the electrical potential is approximated by the sum of the zeroth- and first-order terms of an infinite series.

MAIN RESULTS

The validity of the approach is confirmed using experimental data for healthy and diseased muscle available online.

SIGNIFICANCE

A better understanding of electrical potential distribution in anisotropic skeletal muscle tissue will allow the development of improved diagnostic tools for neuromuscular diseases.

New electrical impedance methods for the in situ measurement of the complex permittivity of anisotropic skeletal muscle using multipolar needles

This paper provides a rigorous analysis on the measurement of the permittivity of two-dimensional anisotropic biological tissues such as skeletal muscle using the four-electrode impedance technique. The state-of-the-art technique requires individual electrodes placed at the same depth in contact with the anisotropic material, e.g. using monopolar needles. In this case, the minimum of measurements in different directions needed to estimate the complex permittivity and its anisotropy direction is 3, which translates into 12 monopolar needle insertions (i.e. 3 directions × 4 electrodes in each direction). Here, we extend our previous work and equip the reader with 8 new methods for multipolar needles, where 2 or more electrodes are spaced along the needle's shaft in contact with the tissue at different depths. Using multipolar needles, the new methods presented reduce the number of needle insertions by a factor of 2 with respect to the available methods. We illustrate the methods with numerical simulations and new experiments on ex vivo ovine skeletal muscle (n = 3). Multi-frequency longitudinal and transverse permittivity data from 30 kHz to 1 MHz is made publicly available in the supplementary material. The methods presented here for multipolar needles bring closer the application of needle electrical impedance to patients with neuromuscular diseases.

Estimating Myofiber Size With Electrical Impedance Myography: a Study In Amyotrophic Lateral Sclerosis MICE

INTRODUCTION

A method for quantifying myofiber size noninvasively would find wide use, including primary diagnosis and evaluating response to therapy.

METHODS

Using prediction algorithms, including the least absolute shrinkage and selection operator, we applied multifrequency electrical impedance myography (EIM) to amyotrophic lateral sclerosis superoxide dismutase 1 G93A mice of different ages and assessed myofiber size histologically.

RESULTS

Multifrequency EIM data provided highly accurate predictions of myofiber size, with a root mean squared error (RMSE) of only 14% in mean myofiber area (corresponding to ± 207 µm² for a mean area of 1,488 µm² ) and an RMSE of only 8.8% in predicting the coefficient of variation in fiber size distribution.

DISCUSSION

This impedance-based approach provides predictive variables to assess myofiber size and distribution with good accuracy, particularly in diseases in which myofiber atrophy is the predominant histological feature, without the requirement for biopsy or burdensome quantification. Muscle Nerve 58: 713-717, 2018.

A novel partial gravity ground-based analog for rats via quadrupedal unloading

Musculoskeletal deconditioning is a well-known consequence of microgravity. However, the effects of partial gravity, such as that experienced on the moon (0.16 g) or Mars (0.38 g), on musculoskeletal health remain relatively unexplored. Because Mars is being increasingly viewed as the likely next extraterrestrial site for human exploration, there is an increasing need for Earth-based models that can replicate the long-term physiological effects of microgravity. These models would also offer the opportunity to explore the potential impact of partial artificial gravity (as would be achieved by centrifugation). In this study, we describe a novel partial gravity model that can be employed in rats over extended periods of time. We demonstrate that 2 wk of partial weight bearing at 20, 40, or 70% of normal loading affects the musculoskeletal health of the animals, as evidenced by decreased trabecular bone density (ranging from -7.5 ± 2.7% at 70% of normal loading to -27.9 ± 2.9% at 20%), hindlimb muscle mass, and impaired muscle function as characterized by grip force. This new model will facilitate studies of the physiological changes occurring in partial gravity and allow for the design of potential countermeasures to mitigate these changes. NEW & NOTEWORTHY This research article describes the first quadrupedal unloading model in rats that is sustainable for investigating the physiological alterations occurring in partial gravity environments, providing a new and adaptable model for ground-based research for future space exploration.

Predicting myofiber size with electrical impedance myography: A study in immature mice

INTRODUCTION

Electrical impedance can be used to estimate cellular characteristics. We sought to determine whether it could be used to approximate myofiber size using standard prediction modeling approaches.

METHODS

Forty-four C57BL/6J wild-type immature mice of varying ages underwent electrical impedance myography (EIM) with a needle electrode array placed in the gastrocnemius. Animals were then humanely killed and muscle fixed, stained, and myofiber size quantified. Two different statistical prediction models were then applied.

RESULTS

Impedance parameters showed major variation with increasing myofiber size. The prediction models based on EIM data alone were able to predict fiber size, with errors in the range of ±69.05-78.44 µm2 (16.19%-18.40% with respect to the average myofiber size).

DISCUSSION

By using well-established statistical models, EIM data alone can provide a satisfactory estimate of myofiber size. Additional study of this approach for approximating myofiber size without the requirement of removing tissue for histological analysis is warranted. Muscle Nerve, 2018.

Electrical impedance myography detects age-related muscle change in mice

Loss of muscle mass and strength represents one of the most significant contributors to impaired function in older adults. Convenient and non-invasive biomarkers are needed that can readily identify and track age-related muscle change. Previous data has suggested electrical impedance myography (EIM) has the potential to serve in this capacity. In this study we investigated how changes in EIM compared with other standard measures of muscle structure and function in aged compared with young mice. A total of 19 male mice aged approximately 25 months and 19 male mice aged 3 months underwent surface multifrequency EIM of the right gastrocnemius muscle using standard methods. Fore and hind limb grip strength, sciatic compound muscle action potential amplitude, and in-situ force of the gastrocnemius were also measured; after sacrifice, gastrocnemius myofiber size was assessed using standard histology. Spearman correlation coefficients were calculated to investigate the association between EIM and muscle characteristics. EIM in aged animals demonstrated significantly lower 50 kHz impedance phase (p<0.001) and reactance (p<0.01) values as well as reduced multifrequency parameters. In contrast, absolute gastrocnemius muscle mass was no different between young and aged mice (p = 0.58) but was reduced in aged mice after normalization to body mass (p<0.001). Median myofiber size in the aged mice was not different from that of young mice (p = 0.72). Aged mice showed reduced muscle function on the basis of normalized fore limb (p<0.001) and normalized hind limb (p<0.001) grip strength, as well as normalized gastrocnemius twitch (p<0.001) and normalized maximal isometric force (p<0.001). Sciatic compound muscle action potential amplitude was reduced in aged mice (p<0.05). EIM parameters showed good correlation with reduced standard physiological and electrophysiological measures of muscle health. Our study suggests that EIM is sensitive to aged-related muscle change and may represent a convenient and valuable method of quantifying loss of muscle health.

Stromal-Based Signatures for the Classification of Gastric Cancer

Treatment of metastatic gastric cancer typically involves chemotherapy and monoclonal antibodies targeting HER2 (ERBB2) and VEGFR2 (KDR). However, reliable methods to identify patients who would benefit most from a combination of treatment modalities targeting the tumor stroma, including new immunotherapy approaches, are still lacking. Therefore, we integrated a mouse model of stromal activation and gastric cancer genomic information to identify gene expression signatures that may inform treatment strategies. We generated a mouse model in which VEGF-A is expressed via adenovirus, enabling a stromal response marked by immune infiltration and angiogenesis at the injection site, and identified distinct stromal gene expression signatures. With these data, we designed multiplexed IHC assays that were applied to human primary gastric tumors and classified each tumor to a dominant stromal phenotype representative of the vascular and immune diversity found in gastric cancer. We also refined the stromal gene signatures and explored their relation to the dominant patient phenotypes identified by recent large-scale studies of gastric cancer genomics (The Cancer Genome Atlas and Asian Cancer Research Group), revealing four distinct stromal phenotypes. Collectively, these findings suggest that a genomics-based systems approach focused on the tumor stroma can be used to discover putative predictive biomarkers of treatment response, especially to antiangiogenesis agents and immunotherapy, thus offering an opportunity to improve patient stratification. Cancer Res; 76(9); 2573-86. ©2016 AACR.

Abstract 4810: Identification of molecular markers of pathological vascular subtypes with differential sensitivity to therapies targeting the VEGF pathway

Therapeutic targeting of the VEGF pathway has not matched the efficacy suggested by preclinical tumor models. To date, the limitations of this form of therapy stem either from inherent or acquired resistance of the tumor vasculature to the reduction of VEGF signaling. Previous studies have described morphological phenotypes of different blood vessel types in human and mouse tumors. Recapitulation of these vascular phenotypes with a surrogate Ad-VEGFA164 model has shown differential sensitivity of these vascular subclasses to inhibitors of the VEGF pathway. The goal of the current study was to identify molecular markers for the various subtypes of pathological vessels and to assess whether similar molecular signatures are found in blood vessels in mouse tumor models and human disease. Using the Ad-VEGF-A164 flank model, microarray analysis was performed to identify molecular markers at each time-point, representing distinct stages of vessel development and maturation. We identified unique gene signatures at the various time-points and confirmed differential expression of the genes by qRT-PCR and/or immunohistochemistry. We found that vascular markers such as CD31, Ang2, and Tie1 were pan-endothelial markers at all time-points. However, other markers such as CD34, MECA-32, vWF, SMA, Tie2, CD105, and AQP1 were expressed on subclasses of endothelial cells. Expression of the molecular markers in tumors models revealed different subclasses of tumor vessels in patient-derived xenografts which were sensitive or resistant to anti-VEGF-A or anti-VEGFR-2 treatment. These vascular subtypes were also found in tissues from human breast and gastric cancers. Together these results indicate that multiple molecular markers can be used to identify unique pathological subclasses of tumor vessels. Some of these vascular subtypes may be insensitive to inhibitors targeting the VEGF pathways and their signatures may be useful biomarkers to predict vascular sensitivity to anti-angiogenic therapy.

Citation Format: Beverly L. Falcon, Marguerita O'Mahony, Julie Stewart, Jiangang Liu, Janice A. Nagy, Qi Xue, Yong Pan, Cynthia Jeffries, Emma Hatten, Tim R. Holzer, Harold F. Dvorak, Aejaz Nasir, Bronek Pytowski, Laura Benjamin, Mark T. Uhlik. Identification of molecular markers of pathological vascular subtypes with differential sensitivity to therapies targeting the VEGF pathway. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4810. doi:10.1158/1538-7445.AM2014-4810

Abstract 2359: Bioinformatics analysis of an Ad-VEGF flank angiogenesis model identifies vessel subtype gene signatures: Implications for anti-VEGF therapy

Vascular Endothelial Growth Factor A (VEGF-A) is the predominant growth factor expressed by tumor cells to drive angiogenesis and solid tumor growth. Antiangiogenesis therapies have been clinically demonstrated to be effective, however, there are still not effective means to preselect those patients most likely to derive optimal benefit. To identify blood vessel markers that may differentiate patients responsive to VEGF pathway blockade, we used a well-established in vivo model to generate pathological angiogenesis following the delivery of adenovirus engineered to express VEGF-A164 (Ad-VEGF) to a localized subcutaneous site within the flank of a nude mouse. This model provides a temporally controlled induction of angiogenesis and subsequent remodeling and maturation of the vasculature. This enabled us to study discrete subclasses of vessel which have been previously shown to have differential sensitivities to anti-angiogenic therapy. Here we report gene expression profiling on a time course of the Ad-VEGF flank model which was used to characterize molecular and genetic changes associated with these distinct vessel subtypes. Using a nearest centroid-based classification algorithm we identified unique endothelial cell specific gene signatures representing ‘early’, ‘middle' and ‘late' stages of vessel development and maturation. To evaluate the utility of these gene signatures in predicting sensitivity to anti-VEGF therapy, we profiled a publically available gene array dataset generated from a mouse xenograft model following anti-VEGF treatment with our vessel subtype gene signatures. A majority of the early and middle vessel genes were significantly reduced in bevacizumab-treated animals, demonstrating they respond to VEGF pathway inhibition. To further evaluate the potential value of these gene signatures as predictive biomarkers we profiled a dataset of 248 gastric tumors using hierarchical clustering. Our analysis shows the middle vessel signature was highly expressed in a mesenchymal molecular subtype of gastric cancer. Interestingly, the majority of the mesenchymal gastric tumors were ‘diffuse' subtype in Lauren's classification. Together, our data demonstrate the potential usefulness of our vessel subtype gene signatures in identifying tumor subtypes that may benefit from this anti-angiogenic therapy.

Citation Format: Jiangang Liu, Beverly L. Falcon, Janice A. Nagy, Shou-Ching S. Jaminet, Dan Li, Thompson N. Doman, Sudhakar Chintharlapalli, Bronek Pytowski, Mark T. Uhlik, Harold Dvorak, Laura Benjamin, Anthony S. Fischl. Bioinformatics analysis of an Ad-VEGF flank angiogenesis model identifies vessel subtype gene signatures: Implications for anti-VEGF therapy. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2359. doi:10.1158/1538-7445.AM2014-2359

RhoB controls coordination of adult angiogenesis and lymphangiogenesis following injury by regulating VEZF1-mediated transcription

Mechanisms governing the distinct temporal dynamics that characterize post-natal angiogenesis and lymphangiogenesis elicited by cutaneous wounds and inflammation remain unclear. RhoB, a stress-induced small GTPase, modulates cellular responses to growth factors, genotoxic stress and neoplastic transformation. Here we show, using RhoB null mice, that loss of RhoB decreases pathological angiogenesis in the ischaemic retina and reduces angiogenesis in response to cutaneous wounding, but enhances lymphangiogenesis following both dermal wounding and inflammatory challenge. We link these unique and opposing roles of RhoB in blood versus lymphatic vasculatures to the RhoB-mediated differential regulation of sprouting and proliferation in primary human blood versus lymphatic endothelial cells. We demonstrate that nuclear RhoB-GTP controls expression of distinct gene sets in each endothelial lineage by regulating VEZF1-mediated transcription. Finally, we identify a small-molecule inhibitor of VEZF1-DNA interaction that recapitulates RhoB loss in ischaemic retinopathy. Our findings establish the first intra-endothelial molecular pathway governing the phased response of angiogenesis and lymphangiogenesis following injury.

PGC-1α induces SPP1 to activate macrophages and orchestrate functional angiogenesis in skeletal muscle

RATIONALE

Mechanisms of angiogenesis in skeletal muscle remain poorly understood. Efforts to induce physiological angiogenesis hold promise for the treatment of diabetic microvascular disease and peripheral artery disease but are hindered by the complexity of physiological angiogenesis and by the poor angiogenic response of aged and patients with diabetes mellitus. To date, the best therapy for diabetic vascular disease remains exercise, often a challenging option for patients with leg pain. Peroxisome proliferation activator receptor-γ coactivator-1α (PGC-1α), a powerful regulator of metabolism, mediates exercise-induced angiogenesis in skeletal muscle.

OBJECTIVE

To test whether, and how, PGC-1α can induce functional angiogenesis in adult skeletal muscle.

METHODS AND RESULTS

Here, we show that muscle PGC-1α robustly induces functional angiogenesis in adult, aged, and diabetic mice. The process involves the orchestration of numerous cell types and leads to patent, nonleaky, properly organized, and functional nascent vessels. These findings contrast sharply with the disorganized vasculature elicited by induction of vascular endothelial growth factor alone. Bioinformatic analyses revealed that PGC-1α induces the secretion of secreted phosphoprotein 1 and the recruitment of macrophages. Secreted phosphoprotein 1 stimulates macrophages to secrete monocyte chemoattractant protein-1, which then activates adjacent endothelial cells, pericytes, and smooth muscle cells. In contrast, induction of PGC-1α in secreted phosphoprotein 1(-/-) mice leads to immature capillarization and blunted arteriolarization. Finally, adenoviral delivery of PGC-1α into skeletal muscle of either young or old and diabetic mice improved the recovery of blood flow in the murine hindlimb ischemia model of peripheral artery disease.

CONCLUSIONS

PGC-1α drives functional angiogenesis in skeletal muscle and likely recapitulates the complex physiological angiogenesis elicited by exercise.

Vascular hyperpermeability, angiogenesis, and stroma generation

It has been known for more than half a century that the tumor microvasculature is hyperpermeable to plasma proteins. However, the identity of the leaky vessels and the consequences of vascular hyperpermeability have received little attention. This article places tumor vascular hyperpermeability in a broader context, relating it to (1) the low-level "basal" permeability of the normal vasculature; (2) the "acute," short-term hyperpermeability induced by vascular permeability factor/vascular endothelial growth factor (VPF/VEGF-A) and other vascular permeabilizing agents; and (3) the "chronic" hyperpermeability associated with longer-term exposure to agents such as VPF/VEGF-A that accompanies many types of pathological angiogenesis. Leakage of plasma protein-rich fluids is important because it activates the clotting system, depositing an extravascular fibrin gel provisional matrix that serves as the first step in stroma generation.

Anti-VEGF/VEGFR therapy for cancer: reassessing the target

Judah Folkman recognized that new blood vessel formation is important for tumor growth and proposed antiangiogenesis as a novel approach to cancer therapy. Discovery of vascular permeability factor VEGF-A as the primary tumor angiogenesis factor prompted the development of a number of drugs that targeted it or its receptors. These agents have often been successful in halting tumor angiogenesis and in regressing rapidly growing mouse tumors. However, results in human cancer have been less impressive. A number of reasons have been offered for the lack of greater success, and, here, we call attention to the heterogeneity of the tumor vasculature as an important issue. Human and mouse tumors are supplied by at least 6 well-defined blood vessel types that arise by both angiogenesis and arterio-venogenesis. All 6 types can be generated in mouse tissues by an adenoviral vector expressing VEGF-A(164). Once formed, 4 of the 6 types lose their VEGF-A dependency, and so their responsiveness to anti-VEGF/VEGF receptor therapy. If therapies directed against the vasculature are to have a greater impact on human cancer, targets other than VEGF and its receptors will need to be identified on these resistant tumor vessels.

Tumor-surrogate blood vessel subtypes exhibit differential susceptibility to anti-VEGF therapy

Antivascular therapy directed against VEGF or its receptors (VEGFR) has been successful when administered at early stages of tumor vessel growth but is less effective when administered later. Tumor blood vessels are heterogeneous, so vessel subpopulations may differ in their requirements for tumor cell-secreted VEGF and in their susceptibility to anti-VEGF/VEGFR therapy. Human cancers contain several distinct blood vessel types, including mother vessels (MV), glomeruloid microvascular proliferations (GMP), vascular malformations (VM), feeding arteries (FA), and draining veins (DV), all of which can be generated in mice in the absence of tumor cells using expression vectors for VEGF-A(164). In this study, we investigated the sensitivity of each of these vessel types to anti-VEGF therapy with Aflibercept (VEGF Trap), a potent inhibitor of VEGF-A(164). Administering VEGF Trap treatment before or shortly after injection of a recombinant VEGF-A(164)-expressing adenovirus could prevent or regress tumor-free neovasculature, but it was progressively less effective if initiated at later times. Early-forming MVs and GMPs in which the lining endothelial cells expressed high levels of VEGFR-2 were highly susceptible to blockade by VEGF Trap. In contrast, late-forming VMs, FAs, and DVs that expressed low levels of VEGFR-2 were largely resistant. Together, our findings define the susceptibility of different blood vessel subtypes to anti-VEGF therapy, offering a possible explanation for the limited effectiveness of anti-VEGF-A/VEGFR treatment of human cancers, which are typically present for months to years before discovery and are largely populated by late-forming blood vessels.

Proteolytic cleavage of versican and involvement of ADAMTS-1 in VEGF-A/VPF-induced pathological angiogenesis

Malignant tumors and chronic inflammatory diseases induce angiogenesis by overexpressing vascular endothelial growth factor A (VEGF-A/VPF). VEGF-A-induced pathological angiogenesis can be mimicked in immunoincompetent mice with an adenoviral vector expressing VEGF-A(164) (Ad-VEGF-A(164)). The initial step is generation of greatly enlarged "mother" vessels (MV) from preexisting normal venules by a process involving degradation of their rigid basement membranes. Immunohistochemical and Western blot analyses revealed that versican, an extracellular matrix component in the basement membranes of venules, is degraded early in the course of MV formation, resulting in the appearance of a versican N-terminal DPEAAE fragment associated with MV endothelial cells. The protease ADAMTS-1, known to cleave versican near its N terminus to generate DPEAAE, is also upregulated by VEGF-A in parallel with MV formation and localizes to the endothelium of the developing MV. The authors also show that MMP-15 (MT-2 MMP), a protease that activates ADAMTS-1, is upregulated by VEGF-A in endothelial cells in vitro and in vivo. These data suggest VEGF-A initiates MV formation, in part, by inducing the expression of endothelial cell proteases such as ADAMTS-1 and MMP-15 that act in concert to degrade venular basement membrane versican. Thus, versican is actively processed during the early course of VEGF-A-induced pathological angiogenesis.

Moderation of calpain activity promotes neovascular integration and lumen formation during VEGF-induced pathological angiogenesis

Background

Successful neovascularization requires that sprouting endothelial cells (ECs) integrate to form new vascular networks. However, architecturally defective, poorly integrated vessels with blind ends are typical of pathological angiogenesis induced by vascular endothelial growth factor-A (VEGF), thereby limiting the utility of VEGF for therapeutic angiogenesis and aggravating ischemia-related pathologies. Here we investigated the possibility that over-exuberant calpain activity is responsible for aberrant VEGF neovessel architecture and integration. Calpains are a family of intracellular calcium-dependent, non-lysosomal cysteine proteases that regulate cellular functions through proteolysis of numerous substrates.

Methodology/Principal Findings

In a mouse skin model of VEGF-driven angiogenesis, retroviral transduction with dominant-negative (DN) calpain-I promoted neovessel integration and lumen formation, reduced blind ends, and improved vascular perfusion. Moderate doses of calpain inhibitor-I improved VEGF-driven angiogenesis similarly to DN calpain-I. Conversely, retroviral transduction with wild-type (WT) calpain-I abolished neovessel integration and lumen formation. In vitro, moderate suppression of calpain activity with DN calpain-I or calpain inhibitor-I increased the microtubule-stabilizing protein tau in endothelial cells (ECs), increased the average length of microtubules, increased actin cable length, and increased the interconnectivity of vascular cords. Conversely, WT calpain-I diminished tau, collapsed microtubules, disrupted actin cables, and inhibited integration of cord networks. Consistent with the critical importance of microtubules for vascular network integration, the microtubule-stabilizing agent taxol supported vascular cord integration whereas microtubule dissolution with nocodazole collapsed cord networks.

Conclusions/Significance

These findings implicate VEGF-induction of calpain activity and impairment of cytoskeletal dynamics in the failure of VEGF-induced neovessels to form and integrate properly. Accordingly, calpain represents an important target for rectifying key vascular defects associated with pathological angiogenesis and for improving therapeutic angiogenesis with VEGF.

Cdc42-mediated inhibition of GSK-3β improves angio-architecture and lumen formation during VEGF-driven pathological angiogenesis

Vascular endothelial growth factor-A (VEGF) typically induces abnormal angiogenesis in the adult, thereby aggravating disease pathology and limiting utility of VEGF for therapeutic angiogenesis. To identify strategies for rectifying defects in pathological VEGF neovessels, we investigated consequences of modulating the Rho GTPase Cdc42. In a mouse skin model of VEGF-driven pathological angiogenesis, transduction with active Cdc42 (L28Cdc42) markedly improved VEGF neovessels, as measured by increased lumen formation, enlarged vessel diameter, and enhanced perfusion of macromolecular tracers. Conversely, transduction with dominant negative Cdc42 (N17Cdc42) impaired endothelial cell (EC) assembly into lumenized blood vessels and reduced neovessel diameter and tracer perfusion. In vitro, active Cdc42 improved coordination between actin filaments and microtubules and enhanced formation of vascular cords, suggesting that active Cdc42 rectifies defects in angiogenesis by improving cytoskeletal dynamics and capillary morphogenesis. Analyses of Cdc42 signaling in microvascular ECs indicated that active Cdc42 also inhibits glycogen synthase kinase-3β (GSK-3β), a multi-functional serine/threonine protein kinase. Pharmacological inhibition of GSK-3β improved vascular cord formation in vitro and promoted proper neovessel formation in vivo comparably to active Cdc42, thus linking GSK-3β inhibition to the mechanism by which active Cdc42 rectifies pathological neovascularization. These studies identify activation of Cdc42 and inhibition of GSK-3β as novel strategies for correcting abnormalities associated with VEGF-driven angiogenesis, and they suggest new approaches for achieving improved therapeutic neovascularization with VEGF.

Heterogeneity of the tumor vasculature

The blood vessels supplying tumors are strikingly heterogeneous and differ from their normal counterparts with respect to organization, structure, and function. Six distinctly different tumor vessel types have been identified, and much has been learned about the steps and mechanisms by which they form. Four of the six vessel types (mother vessels, capillaries, glomeruloid microvascular proliferations, and vascular malformations) develop from preexisting normal venules and capillaries by angiogenesis. The two remaining vessel types (feeder arteries and draining veins) develop from arterio-venogenesis, a parallel, poorly understood process that involves the remodeling of preexisting arteries and veins. All six of these tumor vessel types can be induced to form sequentially in normal mouse tissues by an adenoviral vector expressing vascular endothelial growth factor (VEGF)-A164. Current antiangiogenic cancer therapies directed at VEGF-A or its receptors have been of only limited benefit to cancer patients, perhaps because they target only the endothelial cells of the tumor blood vessel subset that requires exogenous VEGF-A for maintenance. A goal of future work is to identify therapeutic targets on tumor blood vessel endothelial cells that have lost this requirement.

Vascular permeability and pathological angiogenesis in caveolin-1-null mice

Caveolin-1, the signature protein of endothelial cell caveolae, has many important functions in vascular cells. Caveolae are thought to be the transcellular pathway by which plasma proteins cross normal capillary endothelium, but, unexpectedly, cav-1(-/-) mice, which lack caveolae, have increased permeability to plasma albumin. The acute increase in vascular permeability induced by agents such as vascular endothelial growth factor (VEGF)-A occurs through venules, not capillaries, and particularly through the vesiculo-vacuolar organelle (VVO), a unique structure composed of numerous interconnecting vesicles and vacuoles that together span the venular endothelium from lumen to ablumen. Furthermore, the hyperpermeable blood vessels found in pathological angiogenesis, mother vessels, are derived from venules. The present experiments made use of cav-1(-/-) mice to investigate the relationship between caveolae and VVOs and the roles of caveolin-1 in VVO structure in the acute vascular hyperpermeability induced by VEGF-A and in pathological angiogenesis and associated chronic vascular hyperpermeability. We found that VVOs expressed caveolin-1 variably but, in contrast to caveolae, were present in normal numbers and with apparently unaltered structure in cav-1(-/-) mice. Nonetheless, VEGF-A-induced hyperpermeability was strikingly reduced in cav-1(-/-) mice, as was pathological angiogenesis and associated chronic vascular hyperpermeability, whether induced by VEGF-A(164) or by a tumor. Thus, caveolin-1 is not necessary for VVO structure but may have important roles in regulating VVO function in acute vascular hyperpermeability and angiogenesis.

Thrombospondin-1 modulates vascular endothelial growth factor activity at the receptor level

Vascular endothelial growth factor (VEGF) is a well-established stimulator of vascular permeability and angiogenesis, whereas thrombospondin-1 (TSP-1) is a potent angiogenic inhibitor. In this study, we have found that the TSP-1 receptors CD36 and beta1 integrin associate with the VEGF receptor 2 (VEGFR2). The coclustering of receptors that regulate angiogenesis may provide the endothelial cell with a platform for integration of positive and negative signals in the plane of the membrane. Thus, this complex may represent a molecular switch that regulates angiogenesis and determines endothelial cell behavior. In this context, physiological levels of TSP-1 appear to support VEGFR2 function on both the cellular and tissue level, because phosphorylation of VEGFR2 and vascular permeability in response to VEGF are decreased in TSP-1-null mice and isolated endothelial cells. A therapeutic agent based on the antiangiogenic domain of TSP-1, designated 3TSR (for three TSP-1 type 1 repeats), has significant antiangiogenic and antitumor efficacy. Systemic treatment of wild-type mice with 3TSR significantly decreased VEGF-induced permeability. Consistent with this result, VEGF-stimulated phosphorylation of VEGFR2 was also significantly decreased in lung extracts from 3TSR-treated mice. Moreover, 3TSR significantly decreased VEGF-stimulated VEGFR2 phosphorylation in human dermal microvascular endothelial cells in culture. Taken together, the results indicate that TSP-1 and 3TSR modulate the function of VEGFR2.

Rapamycin inhibition of the Akt/mTOR pathway blocks select stages of VEGF-A164-driven angiogenesis, in part by blocking S6Kinase

OBJECTIVE

We evaluated the stages of VEGF-A(164) driven angiogenesis that are inhibited by therapeutic doses of rapamycin and the potential role of S6K1 in that response.

METHODS AND RESULTS

We assessed the effects of rapamycin on the several stages of angiogensis and lymphangiogenesis induced with an adenovirus expressing VEGF-A(164) (Ad-VEGF-A(164)) in the ears of adult nude mice. Rapamycin (0.5 mg/kg/d) effectively inhibited mTOR and downstream S6K1 signaling and partially inhibited Akt signaling, likely through effects on TORC2. The earliest stages of angiogenesis, including mother vessel formation and increased vascular permeability, were strikingly inhibited by rapamycin, as was subsequent formation of daughter glomeruloid microvasular proliferations. However, later stage formation of vascular malformations and lymphangiogenesis were unaffected. Retrovirally delivered isoforms and shRNAs demonstrated that S6K1 signaling plays an important role in early VEGF-A(164)-angiogenesis.

CONCLUSIONS

Rapamycin potently inhibited early and mid stages of VEGF-A(164)-driven angiogenesis, but not late-stage angiogenesis or lymphangiogenesis. Rapamycin decreased phosphorylation of both Akt and S6, suggesting that both the TORC1 and TORC2 pathways are impacted. Inhibition of S6K1 signaling downstream of mTOR is a major component of the antiangiogenesis action of rapamycin.

VEGF-A induces angiogenesis by perturbing the cathepsin-cysteine protease inhibitor balance in venules, causing basement membrane degradation and mother vessel formation

Tumors initiate angiogenesis primarily by secreting vascular endothelial growth factor (VEGF-A(164)). The first new vessels to form are greatly enlarged, pericyte-poor sinusoids, called mother vessels (MV), that originate from preexisting venules. We postulated that the venular enlargement necessary to form MV would require a selective degradation of their basement membranes, rigid structures that resist vascular expansion. To identify the specific proteases responsible for MV formation, we induced angiogenesis in mouse tissues with an adenoviral vector expressing VEGF-A(164) (Ad-VEGF-A(164)) or with VEGF-A-secreting TA3/St mammary tumors. We found that MV formation resulted from greatly increased activity of cathepsins (B>S>L) in venules transitioning into MV, as well as from a reciprocal decrease in the expression of several cysteine protease inhibitors (CPI), stefin A and cystatins B and C, by these same venules. Using a fluorescence probe that selectively binds cellular sites of cathepsin protease activity in vivo, we showed that increased cathepsin activity was localized exclusively to perivenular cells, not to venule endothelial cells. CPI strikingly inhibited angiogenesis in the Matrigel assay, and Ad-VEGF-A(164)-induced angiogenesis was reduced by approximately 50% in cathepsin B-null mice. Thus, VEGF-A, whether expressed by interstitial cells infected with an adenoviral vector or by tumor cells, upsets the normal cathepsin-CPI balance in nearby venules, leading to degradation of their basement membranes, an important first step in angiogenesis.

The L6 protein TM4SF1 is critical for endothelial cell function and tumor angiogenesis

Transmembrane-4-L-six-family-1 (TM4SF1) was originally described as a cancer cell protein. Here, we show that it is highly expressed in the vascular endothelium of human cancers and in a banded pattern in the filopodia of cultured endothelial cells (EC). TM4SF1 knockdown prevented filopodia formation, inhibited cell mobility, blocked cytokinesis, and rendered EC senescent. Integrin-alpha5 and integrin-beta1 subunits gave a similar staining pattern and interacted constitutively with TM4SF1, whereas integrin subunits often associated with angiogenesis (alphaV, beta3, beta5) interacted with TM4SF1 only after vascular endothelial growth factor (VEGF)-A or thrombin stimulation. TM4SF1 knockdown substantially inhibited maturation of VEGF-A(164)-induced angiogenesis. Thus, TM4SF1 is a key regulator of EC function in vitro and of pathologic angiogenesis in vivo and is potentially an attractive target for antiangiogenesis therapy.

Why are tumour blood vessels abnormal and why is it important to know?

Tumour blood vessels differ from their normal counterparts for reasons that have received little attention. We report here that they are of at least six distinct types, we describe how each forms, and, looking forward, encourage the targeting of tumour vessel subsets that have lost their vascular endothelial growth factor-A (VEGF-A) dependency and so are likely unresponsive to anti-VEGF-A therapies.

VEGF-A and the induction of pathological angiogenesis

Tumors, wounds, and chronic inflammatory disorders generate a new vascular supply by a process known as pathological angiogenesis. Whereas formation of the normal blood vasculature requires the interaction of many different agonists and inhibitors, including vascular endothelial growth factor-A (VEGF-A) and other members of the vascular permeability factor/VEGF family, pathological angiogenesis is a cruder, simpler process that can be replicated by a single VEGF-A isoform, VEGF-A(164/5). VEGF-A(164/5) induces the formation of several distinctly different types of new blood vessels that differ from normal blood vessels with respect to organization, structure, and function. Elucidating the properties of these new vessels has led to a better understanding of angiogenesis and will hopefully lead to new approaches to antiangiogenic therapy.

Vascular permeability, vascular hyperpermeability and angiogenesis

The vascular system has the critical function of supplying tissues with nutrients and clearing waste products. To accomplish these goals, the vasculature must be sufficiently permeable to allow the free, bidirectional passage of small molecules and gases and, to a lesser extent, of plasma proteins. Physiologists and many vascular biologists differ as to the definition of vascular permeability and the proper methodology for its measurement. We review these conflicting views, finding that both provide useful but complementary information. Vascular permeability by any measure is dramatically increased in acute and chronic inflammation, cancer, and wound healing. This hyperpermeability is mediated by acute or chronic exposure to vascular permeabilizing agents, particularly vascular permeability factor/vascular endothelial growth factor (VPF/VEGF, VEGF-A). We demonstrate that three distinctly different types of vascular permeability can be distinguished, based on the different types of microvessels involved, the composition of the extravasate, and the anatomic pathways by which molecules of different size cross-vascular endothelium. These are the basal vascular permeability (BVP) of normal tissues, the acute vascular hyperpermeability (AVH) that occurs in response to a single, brief exposure to VEGF-A or other vascular permeabilizing agents, and the chronic vascular hyperpermeability (CVH) that characterizes pathological angiogenesis. Finally, we list the numerous (at least 25) gene products that different authors have found to affect vascular permeability in variously engineered mice and classify them with respect to their participation, as far as possible, in BVP, AVH and CVH. Further work will be required to elucidate the signaling pathways by which each of these molecules, and others likely to be discovered, mediate the different types of vascular permeability.

Chapter 3. The adenoviral vector angiogenesis/lymphangiogenesis assay

Adenoviral vectors expressing vascular permeability factor/vascular endothelial growth factor (VPF/VEGF, VEGF-A(164)) offer a powerful method for elucidating the mechanisms of pathological angiogenesis and lymphangiogenesis and for evaluating the effectiveness of pro- and anti-angiogenesis therapies. When injected into any of a variety of tissues in nude mice or rats, adenoviral vectors expressing VEGF-A(164) (Ad-VEGF-A(164)) induce the formation of six structurally and functionally distinct types of new blood vessels: mother vessels (MV), capillaries, glomeruloid microvascular proliferations (GMP), vascular malformations (VM), feeding arteries (FA), and draining veins (DV). Each of these abnormal vessel types may be found in tumors and in other examples of pathological angiogenesis. In addition, Ad-VEGF-A(164) induces the formation of highly abnormal and poorly functional "giant" lymphatics. The Ad-VEGF-A(164) assay has provided a means of elucidating the steps and mechanisms by which each type of new blood and lymphatic vessel forms, and for generating at defined times and in large numbers each of these different types of vessels for molecular study. Ear injection sites are advantageous in that the angiogenic and lymphangiogenic responses can be followed visually over time in intact animals, thus providing a convenient, inexpensive global screening assay for assessing the efficacy and toxicity of anti- or pro-angiogenic therapies. The assay can be readily extended to the study of the new blood vessels/lymphatics induced by adenoviral vectors expressing other growth factors and cytokines.

Historical and ecological determinants of genetic structure in arctic canids

Wolves (Canis lupus) and arctic foxes (Alopex lagopus) are the only canid species found throughout the mainland tundra and arctic islands of North America. Contrasting evolutionary histories, and the contemporary ecology of each species, have combined to produce their divergent population genetic characteristics. Arctic foxes are more variable than wolves, and both island and mainland fox populations possess similarly high microsatellite variation. These differences result from larger effective population sizes in arctic foxes, and the fact that, unlike wolves, foxes were not isolated in discrete refugia during the Pleistocene. Despite the large physical distances and distinct ecotypes represented, a single, panmictic population of arctic foxes was found which spans the Svalbard Archipelago and the North American range of the species. This pattern likely reflects both the absence of historical population bottlenecks and current, high levels of gene flow following frequent long-distance foraging movements. In contrast, genetic structure in wolves correlates strongly to transitions in habitat type, and is probably determined by natal habitat-biased dispersal. Nonrandom dispersal may be cued by relative levels of vegetation cover between tundra and forest habitats, but especially by wolf prey specialization on ungulate species of familiar type and behaviour (sedentary or migratory). Results presented here suggest that, through its influence on sea ice, vegetation, prey dynamics and distribution, continued arctic climate change may have effects as dramatic as those of the Pleistocene on the genetic structure of arctic canid species.