Towards a more relevant hind limb model of muscle ischaemia

What Is the Best Experimental Model for Developing Novel Therapeutics in Peripheral Artery Disease?

CLINICAL PROBLEM

More than 200 million people worldwide have peripheral artery disease (PAD). PAD affects the quality of life and is associated with significant morbidity and mortality. Standard treatment for severe cases of PAD is surgical or endovascular revascularization. However, up to 30% of patients are not candidates for open or endovascular procedures, due to high operative risk or unfavorable vascular involvement. Furthermore, revascularization procedures may be insufficient to adequately improve microvascular tissue perfusion, wound healing, or limb salvage. Accordingly, regardless of advances in treatment modalities, outcomes of patients with PAD have remained unfavorable. Therefore, new medical therapeutic approaches are much needed. Small animal models are indispensable tools for the understanding of PAD physiopathology and the development of novel medical therapies.

RECOMMENDATIONS FOR INCREASING TRANSLATION FROM ANIMAL MODELS

Development of animal models that more closely mimic the pathophysiology (with occlusive atherothrombosis and chronic development of limb ischemia) can incorporate the cardiovascular risk factors associated with this disease state, and focus on more clinically relevant outcomes is critical. In practice, this means using both animals that develop atherosclerosis and methods for the application of gradual arterial occlusion to induce hind limb ischemia. Doing so will likely help identify novel targets for intervention and overcome some principal challenges confronted by previous clinical trials. While various rodent models are discussed, the optimal animal model is yet to be defined.

Translational Relevance of Advanced Age and Atherosclerosis in Preclinical Trials of Biotherapies for Peripheral Artery Disease

Approximately 6% of adults worldwide suffer from peripheral artery disease (PAD), primarily caused by atherosclerosis of lower limb arteries. Despite optimal medical care and revascularization, many PAD patients remain symptomatic and progress to critical limb ischemia (CLI) and risk major amputation. Delivery of pro-angiogenic factors as proteins or DNA, stem, or progenitor cells confers vascular regeneration and functional recovery in animal models of CLI, but the effects are not well replicated in patients and no pro-angiogenic biopharmacological procedures are approved in the US, EU, or China. The reasons are unclear, but animal models that do not represent clinical PAD/CLI are implicated. Consequently, it is unclear whether the obstacles to clinical success lie in the toxic biochemical milieu of human CLI, or in procedures that were optimized on inappropriate models. The question is significant because the former case requires abandonment of current strategies, while the latter encourages continued optimization. These issues are discussed in the context of relevant preclinical and clinical data, and it is concluded that preclinical mouse models that include age and atherosclerosis as the only comorbidities that are consistently present and active in clinical trial patients are necessary to predict clinical success. Of the reviewed materials, no biopharmacological procedure that failed in clinical trials had been tested in animal models that included advanced age and atherosclerosis relevant to PAD/CLI.

Animal experimental models of ischemic limbs - A systematic review

BACKGROUND

The objective of this systematic review is to summarize the available animal models of ischemic limbs, and to provide an overview of the advantages and disadvantages of each animal model and individual method of limb ischemia creation.

METHODS

A review of literature was conducted using the PubMed and Web of Science pages. Various types of experimental animals and surgical approaches used in creating ischemic limbs were evaluated. Other outcomes of interest were the specific characteristics of the individual experimental animals, and duration of tissue ischemia.

RESULTS

The most commonly used experimental animals were mice, followed by rabbits, rats, pigs, miniature pigs, and sheep. Single or double arterial ligation and excision of the entire femoral artery was the most often used method of ischemic limb creation. Other methods comprised single or double arterial electrocoagulation, use of ameroid constrictors, photochemically induced thrombosis, and different types of endovascular methods. The shortest duration of tissue ischemia was 7 days, the longest 90 days.

CONCLUSIONS

This review shows that mice are among the most commonly used animals in limb ischemia research. Simple ligation and excision of the femoral artery is the most common method of creating an ischemic limb; nevertheless, it can result in acute rather than chronic ischemia. A two-stage sequential approach and methods using ameroid constrictors or endovascular blinded stent grafts are more suitable for creating a gradual arterial occlusion typically seen in humans. Selecting the right mouse strain or animal with artificially produced diabetes or hyperlipidaemia is crucial in chronic ischemic limb research. Moreover, the observation period following the onset of ischemia should last at least 14 days, preferably 4 weeks.

Development and Validation of a Multiparametric Semiquantitative Scoring System for the Histopathological Assessment of Ischaemia Severity in Skeletal Muscle

Skeletal muscle is one of the most abundant and dynamic tissues of the body, with a strong regenerative capacity. Muscle injuries can occur as a result of a variety of events, including tissue ischaemia. Lower limb ischaemia occurs when there is an insufficient nutrient and oxygen supply, often caused by stenosis of the arteries due to atherosclerosis. The aim of this study was to develop and validate a multiparametric scoring tool for assessing ischaemia severity in skeletal muscle in a commonly used preclinical animal model. Tissue ischaemia was surgically induced in mice by ligation and excision of the femoral artery. Calf muscles were carefully dissected, prepared for histological analysis, and scored for inflammation, fibrosis, necrosis, adipocyte infiltration, and muscle fibre degeneration/regeneration. Kendall’s coefficient of concordance (W) showed a very good agreement between the appraisers when scoring each individual histological feature: inflammation (W = 0.92, $p\le 0.001$ ), fibrosis (W = 0.94, $p\le 0.001$ ), necrosis (W = 0.77, $p\le 0.001$ ), adipocyte infiltration (W = 0.91, $p\le 0.001$ ), and fibre degeneration/regeneration (W = 0.86, $p\le 0.001$ ). Intrarater agreement was also excellent (W = 0.94 or more, $p\le 0.001$ ). There was a statistically significant negative association between the level of muscle ischaemia damage and the calf muscle weight and skeletal muscle fibre diameter. Here, we have developed and validated a new multiparametric, semiquantitative scoring system for assessing skeletal muscle damage due to ischaemia, with excellent inter- and intrarater reproducibility. This scoring system can be used for assessing treatment efficacy in preclinical models of hind limb ischaemia.

Microbubble Delivery Platform for Ultrasound-Mediated Therapy in Brain Cancers

The blood-brain barrier (BBB) is one of the most selective endothelial barriers that protect the brain and maintains homeostasis in neural microenvironments. This barrier restricts the passage of molecules into the brain, except for gaseous or extremely small hydrophobic molecules. Thus, the BBB hinders the delivery of drugs with large molecular weights for the treatment of brain cancers. Various methods have been used to deliver drugs to the brain by circumventing the BBB; however, they have limitations such as drug diversity and low delivery efficiency. To overcome this challenge, microbubbles (MBs)-based drug delivery systems have garnered a lot of interest in recent years. MBs are widely used as contrast agents and are recently being researched as a vehicle for delivering drugs, proteins, and gene complexes. The MBs are 1-10 μm in size and consist of a gas core and an organic shell, which cause physical changes, such as bubble expansion, contraction, vibration, and collapse, in response to ultrasound. The physical changes in the MBs and the resulting energy lead to biological changes in the BBB and cause the drug to penetrate it, thus enhancing the therapeutic effect. Particularly, this review describes a state-of-the-art strategy for fabricating MB-based delivery platforms and their use with ultrasound in brain cancer therapy.

Repair of Limb Ischemia Is Dependent on Hematopoietic Stem Cell Specific-SHP-1 Regulation of TGF-β1

BACKGROUND

Hematopoietic stem cell (HSC) therapy has shown promise for tissue regeneration after ischemia. Therefore, there is a need to understand mechanisms underlying endogenous HSCs activation in response to ischemic stress and coordination of angiogenesis and repair. SHP-1 plays important roles in HSC quiescence and differentiation by regulation of TGF-β1 signaling. TGF-β1 promotes angiogenesis by stimulating stem cells to secrete growth factors to initiate the formation of blood vessels and later aid in their maturation. We propose that SHP-1 responds to ischemia stress in HSC and progenitor cells (HSPC) via regulation of TGF-β1.

METHODS

A mouse hind limb ischemia model was used. Local blood perfusion in the limbs was determined using laser doppler perfusion imaging. The number of positive blood vessels per square millimeter, as well as blood vessel diameter (μm) and area (μm²), were calculated. Hematopoietic cells were analyzed using flow cytometry. The bone marrow transplantation assay was performed to measure HSC reconstitution.

RESULTS

After femoral artery ligation, TGF-β1 was initially decreased in the bone marrow by day 3 of ischemia, followed by an increase on day 7. This pattern was opposite to that in the peripheral blood, which is concordant with the response of HSC to ischemic stress. In contrast, SHP-1 deficiency in HSC is associated with irreversible activation of HSPCs in the bone marrow and increased circulating HSPCs in peripheral blood following limb ischemia. In addition, there was augmented auto-induction of TGF-β1 and sustained inactivation of SHP-1-Smad2 signaling, which impacted TGF-β1 expression in HSPCs in circulation. Importantly, restoration of normal T GF-β1 oscillations helped in the recovery of limb repair and function.

CONCLUSIONS

HSPC-SHP-1-mediated regulation of TGF-β1 in both bone marrow and peripheral blood is required for a normal response to ischemic stress.

Pro-Calcific Environment Impairs Ischaemia-Driven Angiogenesis

Peripheral arterial disease (PAD) is characterised by accelerated arterial calcification and impairment in angiogenesis. Studies implicate vascular calcification as a contributor to PAD, but the mechanisms remain unclear. We aimed to determine the effect of calcification on ischaemia-driven angiogenesis. Human coronary artery endothelial cells (ECs) were treated with calcification medium (CM: CaCl₂ 2.7 mM, Na₂PO₄ 2.0 mM) for 24 h and exposed to normoxia (5% CO₂) or hypoxia (1.2% O₂; 5% CO₂ balanced with N₂). In normoxia, CM significantly inhibited tubule formation and migration and upregulated calcification markers of ALP, BMP2, and Runx2. CM elevated levels of calcification-protective gene OPG, demonstrating a compensatory mechanism by ECs. CM failed to induce pro-angiogenic regulators VEGFA and HIF-1α in hypoxia and further suppressed the phosphorylation of endothelial nitric oxide synthase (eNOS) that is essential for vascular function. In vivo, osteoprotegerin-deficient mice (OPG^−/−), a calcification model, were subjected to hind-limb ischaemia (HLI) surgery. OPG^−/− mice displayed elevated serum alkaline phosphatase (ALP) activity compared to wild-type controls. OPG^−/− mice experienced striking reductions in blood-flow reperfusion in both 8-week-old and 6-month-old mice post-HLI. This coincided with significant impairment in tissue ischaemia and reduced limb function as assessed by clinical scoring (Tarlov). This study demonstrated for the first time that a pro-calcific environment is detrimental to ischaemia-driven angiogenesis. The degree of calcification in patients with PAD can often be a limiting factor with the use of standard therapies. These highly novel findings require further studies for full elucidation of the mechanisms involved and have implications for the development of therapies to suppress calcification in PAD.

Collateral Arteriogenesis Involves a Sympathetic Denervation That Is Associated With Abnormal α-Adrenergic Signaling and a Transient Loss of Vascular Tone

Stimulating collateral arteriogenesis is an attractive therapeutic target for peripheral artery disease (PAD). However, the potency of arteriogenesis-stimulation in animal models has not been matched with efficacy in clinical trials. This may be because the presence of enlarged collaterals is not sufficient to relieve symptoms of PAD, suggesting that collateral function is also important. Specifically, collaterals are the primary site of vascular resistance following arterial occlusion, and impaired collateral vasodilation could impact downstream tissue perfusion and limb function. Therefore, we evaluated the effects of arteriogenesis on collateral vascular reactivity. Following femoral artery ligation in the mouse hindlimb, collateral functional vasodilation was impaired at day 7 (17 ± 3 vs. 60 ± 8%) but restored by day 28. This impairment was due to a high resting diameter (73 ± 4 μm at rest vs. 84 ± 3 μm dilated), which does not appear to be a beneficial effect of arteriogenesis because increasing tissue metabolic demand through voluntary exercise decreased resting diameter and restored vascular reactivity at day 7. The high diameter in sedentary animals was not due to sustained NO-dependent vasodilation or defective myogenic constriction, as there were no differences between the enlarged and native collaterals in response to eNOS inhibition with L-NAME or L-type calcium channel inhibition with nifedipine, respectively. Surprisingly, in the context of reduced vascular tone, vasoconstriction in response to the α-adrenergic agonist norepinephrine was enhanced in the enlarged collateral (−62 ± 2 vs. −37 ± 2%) while vasodilation in response to the α-adrenergic antagonist prazosin was reduced (6 ± 4% vs. 22 ± 16%), indicating a lack of α-adrenergic receptor activation by endogenous norepinephrine and suggesting a denervation of the neuroeffector junction. Staining for tyrosine hydroxylase demonstrated sympathetic denervation, with neurons occupying less area and located further from the enlarged collateral at day 7. Inversely, MMP2 presence surrounding the enlarged collateral was greater at day 7, suggesting that denervation may be related to extracellular matrix degradation during arteriogenesis. Further investigation on vascular wall maturation and the functionality of enlarged collaterals holds promise for identifying novel therapeutic targets to enhance arteriogenesis in patients with PAD.

Paeonol Suppresses Vasculogenesis Through Regulating Vascular Smooth Muscle Phenotypic Switching

OBJECTIVE

Smooth muscle cell (SMC) phenotypic switching is associated with development of a variety of occlusive vascular diseases. Paeonol has been reported to be involved in suppressing SMC proliferation. However, it is still unknown whether paeonol can regulate SMC phenotypic switching, and which eventually result in suppressing vasculogenesis.

METHODS

Murine left common carotid artery was injured by completely ligation, and paeonol was administrated by intraperitoneal injection. Hematoxylin and eosin (H&E) staining was performed to visualize vascular neointima formation. Rat aortic SMCs were used to determine whether paeonol suppresses cell proliferation and migration. And murine hind limb ischemia model was performed to confirm the function role of paeonol in suppressing vasculogenesis.

RESULTS

Complete ligation of murine common carotid artery successfully induced neointima formation. Paeonol treatment dramatically reduced the size of injury-induced neointima. Using rat aortic primary SMC, we identified that paeonol strongly suppressed cell proliferation, migration, and decreased extracellular matrix deposition. And paeonol treatment dramatically suppressed vasculogenesis after hind limb ischemia injury.

CONCLUSION

Paeonol could regulate SMC phenotypic switching through inhibiting proliferation and migration of SMC, which results in inhibiting ischemia-induced vasculogenesis.

Bench-to-Bedside in Vascular Medicine: Optimizing the Translational Pipeline for Patients With Peripheral Artery Disease

Peripheral arterial disease is a growing worldwide problem with a wide spectrum of clinical severity and is projected to consume >$21 billion per year in the United States alone. While vascular researchers have brought several therapies to the clinic in recent years, few of these approaches have leveraged advances in high-throughput discovery screens, novel translational models, or innovative trial designs. In the following review, we discuss recent advances in unbiased genomics and broader omics technology platforms, along with preclinical vascular models designed to enhance our understanding of disease pathobiology and prioritize targets for additional investigation. Furthermore, we summarize novel approaches to clinical studies in subjects with claudication and ischemic ulceration, with an emphasis on streamlining and accelerating bench-to-bedside translation. By providing a framework designed to enhance each aspect of future clinical development programs, we hope to enrich the pipeline of therapies that may prevent loss of life and limb for those with peripheral arterial disease.

Firearms-related skeletal muscle trauma: pathophysiology and novel approaches for regeneration

One major cause of traumatic injury is firearm-related wounds (i.e., ballistic trauma), common in both civilian and military populations, which is increasing in prevalence and has serious long-term health and socioeconomic consequences worldwide. Common primary injuries of ballistic trauma include soft-tissue damage and loss, haemorrhage, bone fracture, and pain. The majority of injuries are of musculoskeletal origin and located in the extremities, such that skeletal muscle offers a major therapeutic target to aid recovery and return to normal daily activities. However, the underlying pathophysiology of skeletal muscle ballistic trauma remains poorly understood, with limited evidence-based treatment options. As such, this review will address the topic of firearm-related skeletal muscle injury and regeneration. We first introduce trauma ballistics and the immediate injury of skeletal muscle, followed by detailed coverage of the underlying biological mechanisms involved in regulating skeletal muscle dysfunction following injury, with a specific focus on the processes of muscle regeneration, muscle wasting and vascular impairments. Finally, we evaluate novel approaches for minimising muscle damage and enhancing muscle regeneration after ballistic trauma, which may have important relevance for primary care in victims of violence.

Systematic Interrogation of Angiogenesis in the Ischemic Mouse Hind Limb: Vulnerabilities and Quality Assurance

Supplemental Digital Content is available in the text.

Objective:

There has been little success in translating preclinical studies of mouse hind limb ischemia into benefit for patients with peripheral artery disease. Using systematic strategies, we sought to define the injury and angiogenesis landscapes in mice subjected to hind limb ischemia and ascertain whether published studies to date have used an analysis strategy concordant with these data.

Approach and Results:

Maps of ischemic injury were generated from 22 different hind limb muscles and 33 muscle territories in 12-week-old C57BL/6 mice, based on loss or centralization of myofiber nuclei. Angiogenesis was similarly mapped based on CD (cluster of differentiation) 31–positive capillary content. Only 10 of 33 muscle territories displayed consistent muscle injury, with the distal anterior hind limb muscles most reliably injured. Angiogenesis was patchy and exclusively associated with zones of regenerated muscle (central nuclei). Angiogenesis was not observed in normal appearing muscle, necrotic muscle, or injury border zones. Systematic review of mouse hind limb angiogenesis studies identified 5147 unique publications, of which 509 met eligibility criteria for analysis. Only 7% of these analyzed manuscripts evaluated angiogenesis in distal anterior hind limb muscles and only 15% consistently examined for angiogenesis in zones of muscle regeneration.

Conclusions:

In 12-week C57BL/6 mice, angiogenesis postfemoral artery excision proceeds exclusively in zones of muscle regeneration. Only a minority of studies to date have analyzed angiogenesis in regions of demonstrably regenerating muscle or in high-likelihood territories. Quality assurance standards, informed by the atlas and mapping data herein, could augment data reliability and potentially help translate mouse hind limb ischemia studies to patient care.

Dysregulation of ghrelin in diabetes impairs the vascular reparative response to hindlimb ischemia in a mouse model; clinical relevance to peripheral artery disease

Type 2 diabetes is a prominent risk factor for peripheral artery disease (PAD). Yet, the mechanistic link between diabetes and PAD remains unclear. This study proposes that dysregulation of the endogenous hormone ghrelin, a potent modulator of vascular function, underpins the causal link between diabetes and PAD. Moreover, this study aimed to demonstrate the therapeutic potential of exogenous ghrelin in a diabetic mouse model of PAD. Standard ELISA analysis was used to quantify and compare circulating levels of ghrelin between (i) human diabetic patients with or without PAD (clinic) and (ii) db/db diabetic and non-diabetic mice (lab). Db/db mice underwent unilateral hindlimb ischaemia (HLI) for 14 days and treated with or without exogenous ghrelin (150 µg/kg/day.) Subsequently vascular reparation, angiogenesis, hindlimb perfusion, structure and function were assessed using laser Doppler imaging, micro-CT, microangiography, and protein and micro-RNA (miRNA) analysis. We further examined hindlimb perfusion recovery of ghrelin KO mice to determine whether an impaired vascular response to HLI is linked to ghrelin dysregulation in diabetes. Patients with PAD, with or without diabetes, had significantly lower circulating levels of endogenous ghrelin, compared to healthy individuals. Diabetic db/db mice had ghrelin levels that were only 7% of non-diabetic mice. The vascular reparative capacity of diabetic db/db mice in response to HLI was impaired compared to non-diabetic mice and, importantly, comparable to ghrelin KO mice. Daily therapeutic treatment of db/db mice with ghrelin for 14 days post HLI, stimulated angiogenesis, and improved skeletal muscle architecture and cell survival, which was associated with an increase in pro-angiogenic miRNAs-126 and -132. These findings unmask an important role for endogenous ghrelin in vascular repair following limb ischemia, which appears to be downregulated in diabetic patients. Moreover, these results implicate exogenous ghrelin as a potential novel therapy to enhance perfusion in patients with lower limb PAD, especially in diabetics.

Surface Tethering of Inflammation-Modulatory Nanostimulators to Stem Cells for Ischemic Muscle Repair

Stem cell transplantation has been a promising treatment for peripheral arterial diseases in the past decade. Stem cells act as living bioreactors of paracrine factors that orchestrate tissue regeneration. Prestimulated adipose-derived stem cells (ADSCs) have been proposed as potential candidates but have been met with challenges in activating their secretory activities for clinical use. Here, we propose that tethering the ADSC surface with nanoparticles releasing tumor necrosis factor α (TNFα), named nanostimulator, would stimulate cellular secretory activity in situ. We examined this hypothesis by complexing octadecylamine-grafted hyaluronic acid onto a liposomal carrier of TNFα. Hyaluronic acid increased the liposomal stability and association to CD44 on ADSC surface. ADSCs tethered with these TNFα carriers exhibited up-regulated secretion of proangiogenic vascular endothelial growth factor and immunomodulatory prosteoglandin E2 (PGE2) while decreasing secretion of antiangiogenic pigment epithelium-derived factors. Accordingly, ADSCs tethered with nanostimulators promoted vascularization in a 3D microvascular chip and enhanced recovery of perfusion, walking, and muscle mass in a murine ischemic hindlimb compared to untreated ADSCs. We propose that this surface tethering strategy for in situ stimulation of stem cells would replace the costly and cumbersome preconditioning process and expedite clinical use of stem cells for improved treatments of various injuries and diseases.

Epigenetic regulators of the revascularization response to chronic arterial occlusion

Peripheral arterial disease (PAD) is the leading cause of lower limb amputation and estimated to affect over 202 million people worldwide. PAD is caused by atherosclerotic lesions that occlude large arteries in the lower limbs, leading to insufficient blood perfusion of distal tissues. Given the severity of this clinical problem, there has been long-standing interest in both understanding how chronic arterial occlusions affect muscle tissue and vasculature and identifying therapeutic approaches capable of restoring tissue composition and vascular function to a healthy state. To date, the most widely utilized animal model for performing such studies has been the ischaemic mouse hindlimb. Despite not being a model of PAD per se, the ischaemic hindlimb model does recapitulate several key aspects of PAD. Further, it has served as a valuable platform upon which we have built much of our understanding of how chronic arterial occlusions affect muscle tissue composition, muscle regeneration and angiogenesis, and collateral arteriogenesis. Recently, there has been a global surge in research aimed at understanding how gene expression is regulated by epigenetic factors (i.e. non-coding RNAs, histone post-translational modifications, and DNA methylation). Thus, perhaps not unexpectedly, many recent studies have identified essential roles for epigenetic factors in regulating key responses to chronic arterial occlusion(s). In this review, we summarize the mechanisms of action of these epigenetic regulators and highlight several recent studies investigating the role of said regulators in the context of hindlimb ischaemia. In addition, we focus on how these recent advances in our understanding of the role of epigenetics in regulating responses to chronic arterial occlusion(s) can inform future therapeutic applications to promote revascularization and perfusion recovery in the setting of PAD.

Impaired skeletal muscle performance as a consequence of random functional capillary rarefaction can be restored with overload-dependent angiogenesis

KEY POINTS

Loss of skeletal muscle capillaries is thought to contribute to a reduction in exercise tolerance, but the relative contribution of a compromised microcirculation with disease, in isolation of co-morbidities, to impaired muscle function is unknown. We therefore developed a novel method to randomly occlude capillaries in the rat hindlimb to mimic the capillary rarefaction observed in many conditions. We demonstrate that muscle fatigue resistance is closely coupled with functional microvascular density, independent of arterial blood flow, while disturbance of the microcirculation leads to long-term impairment of muscle function if left untreated. Mechanical stretch due to muscle overload causes a restoration of fatigue resistance via angiogenic remodelling. These observations highlight the importance of a healthy microcirculation and suggest that restoring impaired microvascular supply, regardless of disease co-morbidities, will assist recovery of exercise tolerance in a variety of conditions that limit quality of life.

ABSTRACT

To what extent microvascular rarefaction contributes to impaired skeletal muscle function remains unknown. Our understanding of whether pathological changes in the microcirculation can be reversed remains limited by a lack of basic physiological data in otherwise healthy tissue. The principal objectives here were to: (1) quantify the effect of random microvascular rarefaction on limb perfusion and muscle performance, and (2) determine if these changes could be reversed. We developed a novel protocol in rats whereby microspheres injected into the femoral artery allowed a unilateral reduction in functional capillary density in the extensor digitorum longus (EDL), and assessed acute and chronic effects on muscle function. Simultaneous bilateral EDL force and hindlimb blood flow measurements were made during electrical stimulation. Following functional capillary rarefaction there was an acute microsphere dose-dependent reduction in muscle fatigue resistance (P < 0.001), despite preserved femoral artery perfusion. Histological analysis of EDL samples taken from injected animals confirmed a positive correlation between the proportion of functional capillaries and fatigue resistance (P = 0.002). Such impaired performance persisted for at least 2 weeks (P = 0.016). Concomitant mechanical overload improved both perfused capillary density and fatigue resistance (P<0.05), confirming that the capacity for muscle remodelling was retained following chronic distributed ischaemia, and that the impact of capillary rarefaction could be alleviated. These results demonstrate that loss of functional capillaries is detrimental to muscle function, even in otherwise healthy tissue, independent of arterial perfusion. Restoration of muscle performance following a mechanical overload stimulus indicates that angiogenic treatments to alleviate microvascular rarefaction may be key to restoring exercise tolerance.

Limb functional recovery is impaired in fibroblast growth factor-2 (FGF2) deficient mice despite chronic ischaemia-induced vascular growth

FGF2 is a potent stimulator of vascular growth; however, even with a deficiency of FGF2 (Fgf2-/-), developmental vessel growth or ischaemia-induced revascularization still transpires. It remains to be elucidated as to what function, if any, FGF2 has during ischaemic injury. Wildtype (WT) or Fgf2-/- mice were subjected to hindlimb ischaemia for up to 42 days. Limb function, vascular growth, inflammatory- and angiogenesis-related proteins, and inflammatory cell infiltration were assessed in sham and ischaemic limbs at various timepoints. Recovery of ischaemic limb function was delayed in Fgf2-/- mice. Yet, vascular growth response to ischaemia was similar between WT and Fgf2-/- hindlimbs. Several angiogenesis- and inflammatory-related proteins (MCP-1, CXCL16, MMPs and PAI-1) were increased in Fgf2-/- ischaemic muscle. Neutrophil or monocyte recruitment/infiltration was elevated in Fgf2-/- ischaemic muscle. In summary, our study indicates that loss of FGF2 induces a pro-inflammatory microenvironment in skeletal muscle which exacerbates ischaemic injury and delays functional limb use.

Stimulation of Collateral Vessel Growth by Inhibition of Galectin 2 in Mice Using a Single-Domain Llama-Derived Antibody

Background

In the presence of arterial stenosis, collateral artery growth (arteriogenesis) can alleviate ischemia and preserve tissue function. In patients with poorly developed collateral arteries, Gal‐2 (galectin 2) expression is increased. In vivo administration of Gal‐2 inhibits arteriogenesis. Blocking of Gal‐2 potentially stimulates arteriogenesis. This study aims to investigate the effect of Gal‐2 inhibition on arteriogenesis and macrophage polarization using specific single‐domain antibodies.

Methods and Results

Llamas were immunized with Gal‐2 to develop anti–Gal‐2 antibodies. Binding of Gal‐2 to monocytes and binding inhibition of antibodies were quantified. To test arteriogenesis in vivo, Western diet‐fed LDLR.(low‐density lipoprotein receptor)–null Leiden mice underwent femoral artery ligation and received treatment with llama antibodies 2H8 or 2C10 or with vehicle. Perfusion restoration was measured with laser Doppler imaging. In the hind limb, arterioles and macrophage subtypes were characterized by histology, together with aortic atherosclerosis. Llama‐derived antibodies 2H8 and 2C10 strongly inhibited the binding of Gal‐2 to monocytes (93% and 99%, respectively). Treatment with these antibodies significantly increased perfusion restoration at 14 days (relative to sham, vehicle: 41.3±2.7%; 2H8: 53.1±3.4%, P=0.016; 2C10: 52.0±3.8%, P=0.049). In mice treated with 2H8 or 2C10, the mean arteriolar diameter was larger compared with control (vehicle: 17.25±4.97 μm; 2H8: 17.71±5.01 μm; 2C10: 17.84±4.98 μm; P<0.001). Perivascular macrophages showed a higher fraction of the M2 phenotype in both antibody‐treated animals (vehicle: 0.49±0.24; 2H8: 0.73±0.15, P=0.007; 2C10: 0.75±0.18, P=0.006). In vitro antibody treatment decreased the expression of M1‐associated cytokines compared with control (P<0.05 for each). Atherosclerotic lesion size was comparable between groups (overall P=0.59).

Conclusions

Inhibition of Gal‐2 induces a proarteriogenic M2 phenotype in macrophages, improves collateral artery growth, and increases perfusion restoration in a murine hind limb model.

Regenerative Potential of the Product "CardioCell" Derived from the Wharton's Jelly Mesenchymal Stem Cells for Treating Hindlimb Ischemia

In recent years, mesenchymal stem cells (MSCs) have emerged as a promising therapeutic modality in regenerative medicine. They hold great promise for treating civilization-wide diseases, including cardiovascular diseases, such as acute myocardial infarction and critical limb ischemia. MSCs isolated from Wharton's jelly (WJ-MSCs) may be utilized in both cell-based therapy and vascular graft engineering to restore vascular function, thereby providing therapeutic benefits for patients. The efficacy of WJ-MSCs lies in their multipotent differentiation ability toward vascular smooth muscle cells, endothelial cells and other cell types, as well as their capacity to secrete various trophic factors, which are potent in promoting angiogenesis, inhibiting apoptosis and modulating immunoreaction. Ischemic limb disease is caused by insufficient nutrient and oxygen supplies resulting from damaged peripheral arteries. The lack of nutrients and oxygen causes severe tissue damage in the limb, thereby resulting in severe morbidities and mortality. The therapeutic effects of the conventional treatments are still not sufficient. Cell transplantations in small animal model (mice) are vital for deciphering the mechanisms of MSCs' action in muscle regeneration. The stimulation of angiogenesis is a promising strategy for the treatment of ischemic limbs, restoring blood supply for the ischemic region. In the present study, we focus on the therapeutic properties of the human WJ-MSCs derived product, Cardio. We investigated the role of CardioCell in promoting angiogenesis and relieving hindlimb ischemia. Our results confirm the healing effect of CardioCell and strongly support the use of the WJ-MSCs in regenerative medicine.

Critical Limb Ischaemia Exacerbates Mitochondrial Dysfunction in ApoE-/- Mice Compared with ApoE+/+ Mice, but N-acetyl Cysteine still Confers Protection

OBJECTIVES

The current study was performed in order to determine the influence of hypercholesterolaemia on critical limb ischaemia (CLI) and whether targeting oxidative stress by antioxidant therapies such as N-acetyl cysteine (NAC), considered to be a direct scavenger of reactive oxygen species, could confer muscle protection.

METHODS

Apolipoprotein E (ApoE)-/- mice (n = 9, 29 weeks old) and their genetic controls ApoE+/+ mice (n = 9, 29 weeks old) were submitted to sequential right femoral and iliac ligations; the left limb served as control. ApoE+/+ mice were divided into two groups: Group 1 (n = 4) and Group 2 (n = 5); as well as ApoE-/- mice: Group 3 (n = 3), and Group 4 (n = 6). NAC treatment was administered to Groups 2 and 4 in drinking water. Mice were sacrificed on Day 40 and gastrocnemius muscles were harvested to study mitochondrial respiration by oxygraphy, calcium retention capacity by spectrofluorometry, and production of reactive oxygen species by electron paramagnetic resonance.

RESULTS

CLI associated with ApoE deficiency resulted in more severe mitochondrial dysfunction: maximum oxidative capacity and calcium retention capacity were decreased (-42.9% vs. -25.1%, p = .010; and -73.1% vs. -40.3%, p = .003 respectively) and production of reactive oxygen species was enhanced (+63.6% vs. +41.4%, p = .03) in ApoE-/- mice compared with ApoE+/+ mice respectively. Antioxidant treatment restored oxidative capacity, calcium retention capacity and decreased production of reactive oxygen species in both mice strands.

CONCLUSIONS

In this small murine study, hypercholesterolaemia exacerbated mitochondrial dysfunction, as clinically expected; but antioxidant therapy appeared protective, which is counter to clinical experience. Further work is clearly needed.

Subcutaneous and transcutaneous monitoring of murine hindlimb ischemia by in vivo Raman spectroscopy

We have investigated the development of murine hindlimb ischemia from day 1 to day 55 after femoral artery ligation (FAL) using blood flow analysis, functional tests, histopathological staining, and in vivo Raman spectroscopy. FAL resulted in hindlimb blood deprivation and the loss of functionality as attested by the blood flow analysis and functional tests, respectively. The limbs recovered a normal circulation progressively without recovering complete functionality. Histological analysis showed changes in the morphology of muscle fibers with intense inflammation. From day 22 to day 55 post-ischemia, regeneration of the myofibers was observed. Raman spectroscopic results related to subcutaneous analysis made the identification of modification in the biochemical constituents of hindlimb muscles possible during disease progression. Ischemia was characterized by a quantitative increase in the lipid content and a decrease in the protein content. The lipid to protein ratio can be used as a spectroscopic marker to score the severity of ischemia. Multivariate statistical analysis PC-LDA (Principal Component-Linear Discriminant Analysis) was used to classify all the data measured for the normal and ischemic tissues. This classification illustrated an excellent separation between the control and ischemic tissues at any time during the course of ischemic development. In vivo Raman spectroscopy was then applied to assess the potential of this technique as a screening tool to explore an ischemic disease non-invasively (transcutaneously). For this purpose, the influence of skin on the diagnostic accuracy was evaluated; transcutaneous analysis revealed the accuracy of this technique, indicating its potential in the in situ monitoring of muscle structural changes during ischemia.

Evaluation of skeletal muscle perfusion in canine hind limb ischemia model using color-coded digital subtraction angiography

OBJECTIVE

To evaluate perfusion alterations in skeletal muscle in a canine hind limb ischemia model using color-coded digital subtraction angiography (CC-DSA).

METHODS

Twelve beagles underwent embolization at the branch of their left deep femoral artery. Right hind limbs were used as the control group. Angiography was performed before and immediately after embolization. Upon CC-DSA analysis, time to peak (TTP) was measured before embolization in both sides of the beagles' hind limbs at the middle iliac artery, and the distant, middle and proximal femoral artery. Regions of interest (ROI) peak and ROI peak time were symmetrically computed in proximal and distal thigh muscles before and immediately after embolization. The data were analyzed and compared using the Wilcoxon signed rank test.

RESULTS

Before embolization, ROI peak in the proximal thigh was lower than in the ipsilateral distal thigh, whereas ROI peak time in the proximal thigh was longer than in the distal thigh. In the iliac femoral artery, there was no significant difference in ROI peak, ROI peak time, or TTP between right and left sides. After embolization, ROI peaks in proximal and distal skeletal muscles of the left hind limb were significantly lower than on the contralateral side. ROI peak time was significantly longer in the left proximal and left distal thigh compared to the contralateral side. There were no significant changes in ROI peak or ROI peak time in the right proximal and right distal thigh compared to pre-embolization values. Changes in ROI peak and ROI peak time were larger in the left proximal than in the left distal thigh.

CONCLUSION

CC-DSA provided real-time measurement of changes in vascular hemodynamics and skeletal muscle perfusion without increasing X-ray usage or contrast agent dose.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Mitochondrial complex I defect resulting from exercise-induced lower limb ischemia in patients with peripheral arterial disease

This study aims to compare the structural and mitochondrial alterations between muscle segments affected by exercise-induced ischemia and segments of the same muscle without ischemia, in the same subject. In a prospective analysis, 34 patients presenting either peripheral arterial disease or chronic coronary syndrome without any evidence of peripheral arterial disease were eligible for inclusion based on findings indicating a need for either a femoro-popliteal bypass or a saphenous harvesting for coronary bypass. Before surgery, we assessed the level of exercise-induced ischemia in proximal and distal sections of the thigh by the measurement of transcutaneous oxygen pressure during an exercise treadmill test. Distal and proximal biopsies of the sartorius muscle were procured during vascular surgical procedures to assess mitochondrial function and morphometric parameters of the sartorius myofibers. Comparisons were made between the distal and proximal biopsies, with respect to these parameters. Thirteen of the study patients that initially presented with peripheral arterial disease had evidence of an isolated distal thigh exercise-induced ischemia, associated with a 35% decrease in the mitochondrial complex I enzymatic activity in the distal muscle biopsy. This defect was also associated with a decreased expression of the manganese superoxide dismutase enzyme and with alterations of the shapes of the myofibers. No functional or structural alterations were observed in the patients with coronary syndrome. We validated a specific model ischemia in peripheral arterial disease characterized by muscular alterations. This "Distal-Proximal-Sartorius Model" would be promising to explore the physiopathological consequences specific to chronic ischemia. NEW & NOTEWORTHY We compared proximal versus distal biopsies of the sartorius muscle in patients with superficial femoral artery stenosis or occlusion and proof of, distal only, regional blood flow impairment with exercise oximetry. We identified a decrease in the mitochondrial complex I enzymatic activity and antioxidant system impairment at the distal level only. We validate a model to explore the physiopathological consequences of chronic muscle ischemia.

Dexamethasone Protects Against Tourniquet-Induced Acute Ischemia-Reperfusion Injury in Mouse Hindlimb

Extremity injuries with hemorrhage have been a significant cause of death in civilian medicine and on the battlefield. The use of a tourniquet as an intervention is necessary for treatment to an injured limb; however, the tourniquet and subsequent release results in serious acute ischemia-reperfusion (IR) injury in the skeletal muscle and neuromuscular junction (NMJ). Much evidence demonstrates that inflammation is an important factor to cause acute IR injury. To find effective therapeutic interventions for tourniquet-induced acute IR injuries, our current study investigated effect of dexamethasone, an anti-inflammatory drug, on tourniquet-induced acute IR injury in mouse hindlimb. In C57/BL6 mice, a tourniquet was placed on unilateral hindlimb (left hindlimb) at the hip joint for 3 h, and then released for 24 h to induce IR. Three hours of tourniquet and 24 h of release (24-h IR) caused gastrocnemius muscle injuries including rupture of the muscle sarcolemma and necrosis (42.8 ± 2.3% for infarct size of the gastrocnemius muscle). In the NMJ, motor nerve terminals disappeared, and endplate potentials were undetectable in 24-h IR mice. There was no gastrocnemius muscle contraction in 24-h IR mice. Western blot data showed that inflammatory cytokines (TNFα and IL-1β) were increased in the gastrocnemius muscle after 24-h IR. Treatment with dexamethasone at the beginning of reperfusion (1 mg/kg, i.p.) significantly inhibited expression of TNFα and IL-1β, reduced rupture of the muscle sarcolemma and infarct size (24.8 ± 2.0%), and improved direct muscle stimulation-induced gastrocnemius muscle contraction in 24-h IR mice. However, this anti-inflammatory drug did not improve NMJ morphology and function, and sciatic nerve-stimulated skeletal muscle contraction in 24-h IR mice. The data suggest that one-time treatment with dexamethasone at the beginning of reperfusion only reduced structural and functional impairments of the skeletal muscle but not the NMJ through inhibiting inflammatory cytokines.

Type 2 diabetes impairs the ability of skeletal muscle pericytes to augment postischemic neovascularization in db/db mice

Peripheral artery disease is an atherosclerotic occlusive disease that causes limb ischemia and has few effective noninterventional treatments. Stem cell therapy is promising, but concomitant diabetes may limit its effectiveness. We evaluated the therapeutic potential of skeletal muscle pericytes to augment postischemic neovascularization in wild-type and type 2 diabetic (T2DM) mice. Wild-type C57BL/6J and leptin receptor spontaneous mutation db/db T2DM mice underwent unilateral femoral artery excision to induce limb ischemia. Twenty-four hours after ischemia induction, CD45^-CD34^-CD146⁺ skeletal muscle pericytes or vehicle controls were transplanted into ischemic hindlimb muscles. At postoperative day 28, pericyte transplantation augmented blood flow recovery in wild-type mice (79.3 ± 5% vs. 61.9 ± 5%; P = 0.04), but not in T2DM mice (48.6% vs. 46.3 ± 5%; P = 0.51). Pericyte transplantation augmented collateral artery enlargement in wild-type (26.7 ± 2 μm vs. 22.3 ± 1 μm, P = 0.03), but not T2DM mice (20.4 ± 1.4 μm vs. 18.5 ± 1.2 μm, P = 0.14). Pericyte incorporation into collateral arteries was higher in wild-type than in T2DM mice ( P = 0.002). Unexpectedly, pericytes differentiated into Schwann cells in vivo. In vitro, Insulin increased Nox2 expression and decreased tubular formation capacity in human pericytes. These insulin-induced effects were reversed by N-acetylcysteine antioxidant treatment. In conclusion, T2DM impairs the ability of pericytes to augment neovascularization via decreased collateral artery enlargement and impaired engraftment into collateral arteries, potentially via hyperinsulinemia-induced oxidant stress. While pericytes show promise as a unique form of stem cell therapy to increase postischemic neovascularization, characterizing the molecular mechanisms by which T2DM impairs their function is essential to achieve their therapeutic potential.

Consideration of Sex Differences in Design and Reporting of Experimental Arterial Pathology Studies-Statement From ATVB Council

There are many differences in arterial diseases between men and women, including prevalence, clinical manifestations, treatments, and prognosis. The new policy of the National Institutes of Health, which requires the inclusion of sex as a biological variable for preclinical studies, aims to foster new mechanistic insights and to enhance our understanding of sex differences in human diseases. The purpose of this statement is to suggest guidelines for designing and reporting sex as a biological variable in animal models of atherosclerosis, thoracic and abdominal aortic aneurysms, and peripheral arterial disease. We briefly review sex differences of these human diseases and their animal models, followed by suggestions on experimental design and reporting of animal studies for these vascular pathologies.

Evaluation of a new model of hind limb ischemia in rabbits

OBJECTIVE

Various animal models of critical limb ischemia have been developed in the past. However, there is no animal model that can undergo endovascular treatment, while providing reproducible true critical limb ischemia with arterial ulcers and rest pain. We evaluated the efficacy of a new model of rabbit hindlimb ischemia created through a percutaneous approach using embolization with calibrated particles.

METHODS

Through a percutaneous transauricular artery approach and selective catheterization of the superficial femoral artery, embolization of distal limb vessels was performed using a mixture of 300- to 500-μm calibrated microparticles (Embosphere, Merit Medical, Salt Lake City, Utah), saline solution, and iodine contrast. Clinical and ultrasound imaging-based blood flow evaluation was performed before embolization and during follow-up. Histologic evaluation was performed at humane killing 14 days after the procedure.

RESULTS

The model was successfully created in 10 rabbits (10 limbs). One rabbit died of sudden death at 8 days after the procedure. The nine surviving rabbits developed hind ulcers. All rabbits had a higher pain score in the follow-up compared to baseline value (P < .0001). Blood flow in the saphenous artery decreased significantly after the procedure and later at 14 days follow-up (baseline value 63.4 ± 31.3 μL per cardiac cycle vs 32.0 ± 28.4 μL per cardiac cycle postprocedure [P = .0013] and 32.0 ± 28.4 μL per cardiac cycle at 14 days [P = .0015]). Pathology showed signs of severe limb ischemia in all rabbits with subacute and chronic injury patterns.

CONCLUSIONS

A rabbit hind limb ischemia model created by percutaneous transauricular distal femoral artery embolization with calibrated particles may overcome some of the limitations of existing animal models. As such, this model could prove useful for assessing therapies designed to improve arterial perfusion and collateral growth.

Acute and chronic effects of exercise on mRNA expression in the skeletal muscle of two mouse models of peripheral artery disease

Endurance exercise improves walking performance in patients with peripheral artery disease (PAD), which is characterized by skeletal muscle dysfunction caused by lower extremity ischemia. Although transcriptional analyses of exercise-induced changes in normal animals and healthy volunteers have been reported, no detailed study has explored exercise-induced alterations in gene expression in PAD animal models. Here, we determined the acute and chronic effects of exercise on mRNA expression in the skeletal muscles of two mouse models of PAD. Three particular gene categories were investigated: known exercise-responsive genes (Pgc1a, Il6, Nr4a1, Nr4a2, and Nr4a3); myogenic and muscle regeneration-related genes (Myf5, Myogenin, Myomaker, and Myh3); and Gpr56 and its ligand Col3a1. PAD was induced by bilateral femoral artery ligation in normal C57BL/6 and diabetic KK-A^y mice. From 1 week after surgery, repetitive twice-weekly 30-min treadmill endurance exercise sessions were applied. Altered mRNA expression in the soleus muscles was measured in both the acute and chronic phases. In the acute phase, transcript levels of exercise-inducible genes showed significant increases in both C57BL/6 and diabetic KK-A^y PAD mice; levels of regeneration-related genes showed little alteration, and those of Gpr56 increased immediately and significantly after exercise in both models. In the chronic phase, transcript levels of Pgc1a, Myf5, Myogenin, Myomaker, Myh3, Gpr56, and Col3a1 were upregulated significantly in sedentary C57BL/6 PAD mice compared with that in sham-operated mice. Exercise training inhibited the upregulation of Col3a1, Myf5, and Myogenin significantly. In KK-A^y PAD mice, only Gpr56 mRNA levels increased significantly compared with those in sham-operated mice. RNA sequence analysis revealed 33 and 166 differentially upregulated, and 363 and 99 downregulated, genes after exercise training in C57BL/6 PAD and KK-A^y PAD mice, respectively. In summary, we detected significant alterations of skeletal muscle genes after exercise in PAD mouse models and characterized their expression patterns.

Moderate Exercise Allows for shorter Recovery Time in Critical Limb Ischemia

Whether and how moderate exercise might allow for accelerated limb recovery in chronic critical limb ischemia (CLI) remains to be determined. Chronic CLI was surgically induced in mice, and the effect of moderate exercise (training five times per week over a 3-week period) was investigated. Tissue damages and functional scores were assessed on the 4th, 6th, 10th, 20th, and 30th day after surgery. Mice were sacrificed 48 h after the last exercise session in order to assess muscle structure, mitochondrial respiration, calcium retention capacity, oxidative stress and transcript levels of genes encoding proteins controlling mitochondrial functions (PGC1α, PGC1β, NRF1) and anti-oxidant defenses markers (SOD1, SOD2, catalase). CLI resulted in tissue damages and impaired functional scores. Mitochondrial respiration and calcium retention capacity were decreased in the ischemic limb of the non-exercised group (V_max = 7.11 ± 1.14 vs. 9.86 ± 0.86 mmol 02/min/g dw, p < 0.001; CRC = 7.01 ± 0.97 vs. 11.96 ± 0.92 microM/mg dw, p < 0.001, respectively). Moderate exercise reduced tissue damages, improved functional scores, and restored mitochondrial respiration and calcium retention capacity in the ischemic limb (V_max = 9.75 ± 1.00 vs. 9.82 ± 0.68 mmol 02/min/g dw; CRC = 11.36 ± 1.33 vs. 12.01 ± 1.24 microM/mg dw, respectively). Exercise also enhanced the transcript levels of PGC1α, PGC1β, NRF1, as well as SOD1, SOD2, and catalase. Moderate exercise restores mitochondrial respiration and calcium retention capacity, and it has beneficial functional effects in chronic CLI, likely by stimulating reactive oxygen species-induced biogenesis and anti-oxidant defenses. These data support further development of exercise therapy even in advanced peripheral arterial disease.

Underlying chronic inflammation alters the profile and mechanisms of acute neutrophil recruitment

Chronically inflamed tissues show altered characteristics that include persistent populations of inflammatory leukocytes and remodelling of the vascular network. As the majority of studies on leukocyte recruitment have been carried out in normal healthy tissues, the impact of underlying chronic inflammation on ongoing leukocyte recruitment is largely unknown. Here, we investigate the profile and mechanisms of acute inflammatory responses in chronically inflamed and angiogenic tissues, and consider the implications for chronic inflammatory disorders. We have developed a novel model of chronic ischaemia of the mouse cremaster muscle that is characterized by a persistent population of monocyte‐derived cells (MDCs), and capillary angiogenesis. These tissues also show elevated acute neutrophil recruitment in response to locally administered inflammatory stimuli. We determined that Gr1^lowMDCs, which are widely considered to have anti‐inflammatory and reparative functions, amplified acute inflammatory reactions via the generation of additional proinflammatory signals, changing both the profile and magnitude of the tissue response. Similar vascular and inflammatory responses, including activation of MDCs by transient ischaemia–reperfusion, were observed in mouse hindlimbs subjected to chronic ischaemia. This response demonstrates the relevance of the findings to peripheral arterial disease, in which patients experience transient exercise‐induced ischaemia known as claudication.These findings demonstrate that chronically inflamed tissues show an altered profile and altered mechanisms of acute inflammatory responses, and identify tissue‐resident MDCs as potential therapeutic targets. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Improving the therapeutic efficacy of mesenchymal stromal cells to restore perfusion in critical limb ischemia through pulsed focused ultrasound

Mesenchymal stem cells (MSC) are promising therapeutics for critical limb ischemia (CLI). Mechanotransduction from pulsed focused ultrasound (pFUS) upregulates local chemoattractants to enhance homing of intravenously (IV)-infused MSC and improve outcomes. This study investigated whether pFUS exposures to skeletal muscle would improve local homing of iv-infused MSCs and their therapeutic efficacy compared to iv-infused MSCs alone. CLI was induced by external iliac arterial cauterization in 10–12-month-old mice. pFUS/MSC treatments were delayed 14 days, when surgical inflammation subsided. Mice were treated with iv-saline, pFUS alone, IV-MSC, or pFUS and IV-MSC. Proteomic analyses revealed pFUS upregulated local chemoattractants and increased MSC tropism to CLI muscle. By 7 weeks post-treatment, pFUS + MSC significantly increased perfusion and CD31 expression, while reducing fibrosis compared to saline. pFUS or MSC alone reduced fibrosis, but did not increase perfusion or CD31. Furthermore, MSCs homing to pFUS-treated CLI muscle expressed more vascular endothelial growth factor (VEGF) and interleukin-10 (IL-10) than MSCs homing to non-pFUS-treated muscle. pFUS + MSC improved perfusion and vascular density in this clinically-relevant CLI model. The molecular effects of pFUS increased both MSC homing and MSC production of VEGF and IL-10, suggesting microenvironmental changes from pFUS also increased potency of MSCs in situ to further enhance their efficacy.

Optimization of Vascular Casting for Three-Dimensional Fluorescence Cryo-Imaging of Collateral Vessels in the Ischemic Rat Hindlimb

Development of collateral vessels, arteriogenesis, may protect against tissue ischemia, however, quantitative data on this process remain scarce. We have developed a technique for replicating the entire arterial network of ischemic rat hindlimbs in three dimensions (3D) based on vascular casting and automated sequential cryo-imaging. Various dilutions of Batson's No. 17 with methyl methacrylate were evaluated in healthy rats, with further protocol optimization in ischemic rats. Penetration of the resin into the vascular network greatly depended on dilution; the total length of casted vessels below 75 µm was 13-fold higher at 50% dilution compared with the 10% dilution. Dilutions of 25-30%, with transient clamping of the healthy iliac artery, were optimal for imaging the arterial network in unilateral ischemia. This protocol completely filled the lumina of small arterioles and collateral vessels. These appeared as thin anastomoses in healthy legs and increasingly larger vessels during ligation (median diameter 1 week: 63 µm, 4 weeks: 127 µm). The presented combination of quality casts with high-resolution cryo-imaging enables automated, detailed 3D analysis of collateral adaptation, which furthermore can be combined with co-registered 3D distributions of fluorescent molecular imaging markers reflecting biological activity or perfusion.

Dexamethasone promotes long-term functional recovery of neuromuscular junction in a murine model of tourniquet-induced ischaemia-reperfusion

AIM

Tourniquet-induced ischaemia and subsequent reperfusion cause serious ischaemia-reperfusion (IR) injury in the neuromuscular junction (NMJ) and skeletal muscle. Here, we investigated whether dexamethasone (Dex) promotes long-term functional recovery of the NMJ and skeletal muscle in tourniquet-induced hindlimb IR.

METHODS

Unilateral hindlimb of C57/BL6 mice was subjected to 3 h of ischaemia following 6 weeks of reperfusion (6-wk IR). Dex treatment began on the day of IR induction and lasted for different periods. Sciatic nerve-stimulated gastrocnemius muscle contraction was detected in situ. Function of the NMJ was measured in situ using electrophysiological recording of the miniature endplate potential (mEPP) and endplate potential (EPP). Western blot was used to detect protein expression of nicotinic acetylcholine receptors (nAChRs) in gastrocnemius muscles.

RESULTS

Gastrocnemius muscle contraction in mice with 6-wk IR was about 60% of normal skeletal muscle contraction recorded in age-matched sham mice. The amplitude of the mEPP and EPP was lower in mice with 6-wk IR, compared to sham mice. Dex treatment for 1 or 3 days did not restore the function of the NMJ and improve gastrocnemius muscle contraction in mice with 6-wk IR. Dex treatment for 1 week exerted a maximum effect on improving the function of the NMJ and skeletal muscle, with the effect of Dex gradually lessening with prolonged Dex treatment. There are no significant differences in protein expression of nAChR-α1 and nAChR-β1 subunits in the gastrocnemius muscle among all groups.

CONCLUSION

Dex promotes repair of the NMJ and subsequently restores skeletal muscle contractile function in tourniquet-induced 6-wk IR.

An endovascular model of ischemic myopathy from peripheral arterial disease

OBJECTIVE

Peripheral arterial disease (PAD) is a significant age-related medical condition with limited pharmacologic options. Severe PAD, termed critical limb ischemia, can lead to amputation. Skeletal muscle is the end organ most affected by PAD, leading to ischemic myopathy and debility of the patient. Currently, there are not any therapeutics to treat ischemic myopathy, and proposed biologic agents have not been optimized owing to a lack of preclinical models of PAD. Because a large animal model of ischemic myopathy may be useful in defining the optimal dosing and delivery regimens, the objective was to create and to characterize a swine model of ischemic myopathy that mimics patients with severe PAD.

METHODS

Yorkshire swine (N = 8) underwent acute right hindlimb ischemia by endovascular occlusion of the external iliac artery. The effect of ischemia on limb function, perfusion, and degree of ischemic myopathy was quantified by weekly gait analysis, arteriography, hindlimb blood pressures, femoral artery duplex ultrasound scans, and histologic examination. Animals were terminated at 5 (n = 5) and 6 (n = 3) weeks postoperatively. Ossabaw swine (N = 8) fed a high-fat diet were used as a model of metabolic syndrome for comparison of arteriogenic recovery and validation of ischemic myopathy.

RESULTS

There was persistent ischemia in the right hindlimb, and occlusion pressures were significantly depressed compared with the untreated left hindlimb out to 6 weeks (systolic blood pressure, 31 ± 21 vs 83 ± 15 mm Hg, respectively; P = .0007). The blood pressure reduction resulted in a significant increase of ischemic myopathy in the gastrocnemius muscle in the treated limb. Gait analysis revealed a functional deficit of the right hindlimb immediately after occlusion that improved rapidly during the first 2 weeks. Peak systolic velocity values in the right common femoral artery were severely diminished throughout the entire study (P < .001), and the hemodynamic environment after occlusion was characterized by low and oscillatory wall shear stress. Finally, the internal iliac artery on the side of the ischemic limb underwent significant arteriogenic remodeling (1.8× baseline) in the Yorkshire but not in the Ossabaw swine model.

CONCLUSIONS

This model uses endovascular technology to produce the first durable large animal model of ischemic myopathy. Acutely (first 2 weeks), this model is associated with impaired gait but no tissue loss. Chronically (2-6 weeks), this model delivers persistent ischemia, resulting in ischemic myopathy similar to that seen in PAD patients. This model may be of use for testing novel therapeutics including biologic therapies for promoting neovascularization and arteriogenesis.

Vascular Regeneration in Ischemic Hindlimb by Adeno-Associated Virus Expressing Conditionally Silenced Vascular Endothelial Growth Factor

BACKGROUND

Critical limb ischemia (CLI) is the extreme manifestation of peripheral artery disease, a major unmet clinical need for which lower limb amputation is the only option for many patients. After 2 decades in development, therapeutic angiogenesis has been tested clinically via intramuscular delivery of proangiogenic proteins, genes, and stem cells. Efficacy has been modest to absent, and the largest phase 3 trial of gene therapy for CLI reported a worsening trend of plasmid fibroblast growth factor. In all clinical trials to date, gene therapy has used unregulated vectors with limited duration of expression. Only unregulated extended expression vectors such as adeno-associated virus (AAV) and lentivirus have been tested in preclinical models.

METHODS AND RESULTS

We present preclinical results of ischemia (hypoxia)-regulated conditionally silenced (CS) AAV-human vascular endothelial growth factor (hVEGF) gene delivery that shows efficacy and safety in a setting where other strategies fail. In a BALB/c mouse model of CLI, we show that gene therapy with AAV-CS-hVEGF, but not unregulated AAV or plasmid, vectors conferred limb salvage, protection from necrosis, and vascular regeneration when delivered via intramuscular or intra-arterial routes. All vector treatments conferred increased capillary density, but organized longitudinal arteries were selectively generated by AAV-CS-hVEGF. AAV-CS-hVEGF therapy reversibly activated angiogenic and vasculogenic genes, including Notch, SDF1, Angiopoietin, and Ephrin-B2. Reoxygenation extinguished VEGF expression and inactivated the program with no apparent adverse side effects.

CONCLUSIONS

Restriction of angiogenic growth factor expression to regions of ischemia supports the safe and stable reperfusion of hindlimbs in a clinically relevant murine model of CLI.

Chronology of mitochondrial and cellular events during skeletal muscle ischemia-reperfusion

Peripheral artery disease (PAD) is a common circulatory disorder of the lower limb arteries that reduces functional capacity and quality of life of patients. Despite relatively effective available treatments, PAD is a serious public health issue associated with significant morbidity and mortality. Ischemia-reperfusion (I/R) cycles during PAD are responsible for insufficient oxygen supply, mitochondriopathy, free radical production, and inflammation and lead to events that contribute to myocyte death and remote organ failure. However, the chronology of mitochondrial and cellular events during the ischemic period and at the moment of reperfusion in skeletal muscle fibers has been poorly reviewed. Thus, after a review of the basal myocyte state and normal mitochondrial biology, we discuss the physiopathology of ischemia and reperfusion at the mitochondrial and cellular levels. First we describe the chronology of the deleterious biochemical and mitochondrial mechanisms activated by I/R. Then we discuss skeletal muscle I/R injury in the muscle environment, mitochondrial dynamics, and inflammation. A better understanding of the chronology of the events underlying I/R will allow us to identify key factors in the development of this pathology and point to suitable new therapies. Emerging data on mitochondrial dynamics should help identify new molecular and therapeutic targets and develop protective strategies against PAD.

Evaluation of the clinical relevance and limitations of current pre-clinical models of peripheral artery disease

Peripheral artery disease (PAD) has recognized treatment deficiencies requiring the discovery of novel interventions. This article describes current animal models of PAD and discusses their advantages and disadvantages. There is a need for models which more directly simulate the characteristics of human PAD, such as acute-on-chronic presentation, presence of established risk factors and impairment of physical activity.

The efficacy of extraembryonic stem cells in improving blood flow within animal models of lower limb ischaemia

BACKGROUND

Stem cell (SC) administration is a potential therapeutic strategy to improve blood supply in patients with peripheral artery disease (PAD). The aim of this systematic review and meta-analysis was to investigate the efficacy of extraembryonic tissue-derived SC (ETSC) in improving blood flow within animal models of hindlimb ischaemia (HLI).

METHODS

PubMed, ScienceDirect and Web of Science were searched to identify studies which investigated ETSCs within animal HLI models. A meta-analysis was performed focusing on the effect of ETSCs on limb blood flow assessed by laser Doppler imaging using a random effects model. Methodological quality was assessed using a newly devised quality assessment tool.

RESULTS

Five studies investigating umbilical cord-derived SCs (three studies), placental SCs (one study), amnion and chorionic SCs (one study) were included. A meta-analysis suggested that administration of ETSCs improved the restoration of blood flow within the HLI models used. The methodological quality of the included studies was assessed as poor. Problems identified included lack of randomised design and blinding of outcome assessors; that the animal models did not incorporate recognised risk factors for human PAD or atherosclerosis; the models used did not have established chronic ischaemia as is the cases in most patients presenting with PAD; and the studies lacked a clear rationale for the dosage and frequency of SCs administered.

CONCLUSIONS

The identified studies suggest that ETSCs improve recovery of limb blood supply within current animal HLI models. Improved study quality is, however, needed to provide support for the likelihood of translating these findings to patients with PAD.

Magnetic Resonance Imaging Allows the Evaluation of Tissue Damage and Regeneration in a Mouse Model of Critical Limb Ischemia

Magnetic resonance imaging (MRI) provides non-invasive, repetitive measures in the same individual, allowing the study of a physio-pathological event over time. In this study, we tested the performance of 7 Tesla multi-parametric MRI to monitor the dynamic changes of mouse skeletal muscle injury and regeneration upon acute ischemia induced by femoral artery dissection. T2-mapping (T2 relaxation time), diffusion-tensor imaging (Fractional Anisotropy) and perfusion by Dynamic Contrast-Enhanced MRI (K-trans) were measured and imaging results were correlated with histological morphometric analysis in both Gastrocnemius and Tibialis anterior muscles. We found that tissue damage positively correlated with T2-relaxation time, while myofiber regeneration and capillary density positively correlated with Fractional Anisotropy. Interestingly, K-trans positively correlated with capillary density. Accordingly, repeated MRI measurements between day 1 and day 28 after surgery in ischemic muscles showed that: 1) T2-relaxation time rapidly increased upon ischemia and then gradually declined, returning almost to basal level in the last phases of the regeneration process; 2) Fractional Anisotropy dropped upon ischemic damage induction and then recovered along with muscle regeneration and neoangiogenesis; 3) K-trans reached a minimum upon ischemia, then progressively recovered. Overall, Gastrocnemius and Tibialis anterior muscles displayed similar patterns of MRI parameters dynamic, with more marked responses and less variability in Tibialis anterior. We conclude that MRI provides quantitative information about both tissue damage after ischemia and the subsequent vascular and muscle regeneration, accounting for the differences between subjects and, within the same individual, between different muscles.

A systematic approach to assess locoregional differences in angiogenesis

Skeletal muscle tissue differs with regard to the abundance of glycolytic and oxidative fiber types. In this context, capillary density has been described to be higher in muscle tissue with more oxidative metabolism as compared to that one with more glycolytic metabolism, and the highest abundance of capillaries has been found in boneward-oriented moieties of skeletal muscle tissue. Importantly, capillary formation is often analyzed as a measure for angiogenesis, a process that describes neo-vessel formation emanating from preexisting vessels, occurring, i.e., after arterial occlusion. However, a standardized way for investigation of calf muscle capillarization after surgically induced unilateral hind limb ischemia in mice, especially considering these locoregional differences, has not been provided so far. In this manuscript, a novel, methodical approach for reliable analysis of capillary density was established using anatomic-morphological reference points, and a software-assisted way of capillary density analysis is described. Thus, the systematic approach provided conscientiously considers intra-layer differences in capillary formation and therefore guarantees for a robust, standardized analysis of capillary density as a measure for angiogenesis. The significance of the methodology is further supported by the observation that capillary density in the calf muscle layers analyzed negatively correlates with distal lower limb perfusion measured in vivo.

Novel method to assess arterial insufficiency in rodent hind limb

BACKGROUND

Lack of techniques to assess maximal blood flow capacity thwarts the use of rodent models of arterial insufficiency to evaluate therapies for intermittent claudication. We evaluated femoral vein outflow (VO) in combination with stimulated muscle contraction as a potential method to assess functional hind limb arterial reserve and therapeutic efficacy in a rodent model of subcritical limb ischemia.

MATERIALS AND METHODS

VO was measured with perivascular flow probes at rest and during stimulated calf muscle contraction in young, healthy rats (Wistar Kyoto, WKY; lean Zucker rats, LZR) and rats with cardiovascular risk factors (spontaneously hypertensive [SHR]; obese Zucker rats [OZR]) with acute and/or chronic femoral arterial occlusion. Therapeutic efficacy was assessed by administration of Ramipril or Losartan to SHR after femoral artery excision.

RESULTS

VO measurement in WKY demonstrated the utility of this method to assess hind limb perfusion at rest and during calf muscle contraction. Although application to diseased models (OZR and SHR) demonstrated normal resting perfusion compared with contralateral limbs, a significant reduction in reserve capacity was uncovered with muscle stimulation. Administration of Ramipril and Losartan demonstrated significant improvement in functional arterial reserve.

CONCLUSIONS

The results demonstrate that this novel method to assess distal limb perfusion in small rodents with subcritical limb ischemia is sufficient to unmask perfusion deficits not apparent at rest, detect impaired compensation in diseased animal models with risk factors, and assess therapeutic efficacy. The approach provides a significant advance in methods to investigate potential mechanisms and novel therapies for subcritical limb ischemia in preclinical rodent models.

A new murine model of sustainable and durable chronic critical limb ischemia fairly mimicking human pathology

OBJECTIVE

To establish a chronic mouse model of critical limb ischemia (CLI) with in vivo and ex vivo validation, closely mimicking human pathology.

METHODS

Swiss mice (n = 28) were submitted to sequential unilateral femoral (day 0) and iliac (day 4) ligatures. Ischemia was confirmed by clinical scores (tissue and functional damages) and methoxyisobutylisonitrile (MIBI) scintigraphies at days 0, 4, 6, 10, 20, and 30. At days 10, 20, and 30, muscle mitochondrial respiration, calcium retention capacity (CRC), and production of reactive oxygen species (ROS) were investigated, together with transcripts of mitochondrial biogenesis and antioxidant enzymes. Histological analysis was also performed.

RESULTS

Clinical and functional damage confirmed CLI. MIBI scintigraphies showed hypoperfusion of the ischemic limb, which remained stable until day 30. Mitochondrial respiration was impaired in ischemic muscles compared with controls (Vmax = 7.93 ± 0.99 vs. 10.09 ± 2.87 mmol/L O2/minute/mg dry weight [dw]; p = .01), together with impaired CRC (7.4 ± 1.6 mmol/L minute/mg dw vs. 11.9 ± 0.9 mmol/L minute/mg dw; p < .001) and biogenesis (41% decrease in peroxisome proliferator-activated receptor gamma coactivator [PGC]-1α, 49% decrease in PGC-1β, and 41% decrease in nuclear respiratory factor-1). Ischemic muscles also demonstrated increased production of ROS under electron paramagnetic resonance (0.084 ± 0.029 vs. 0.051 ± 0.031 mmol/L minute/mg dw; p = .03) and with dihydroethidium staining (3622 ± 604 arbitrary units of fluorescence vs. 1224 ± 324; p < .01), decreased antioxidant enzymes (32% decrease in superoxide dismutase [SOD]1, 41% decrease in SOD2, and 49% decrease in catalase), and myopathic features (wider range in fiber size, rounded shape, centrally located nuclei, and smaller cross-sectional areas). All defects were stable over time.

CONCLUSION

Sequential femoral and iliac ligatures closely mimic human functional, clinical, scintigraphic, and skeletal muscle mitochondrial characteristics, and could prove useful for testing therapeutic approaches.

Glutathione adducts on sarcoplasmic/endoplasmic reticulum Ca2+ ATPase Cys-674 regulate endothelial cell calcium stores and angiogenic function as well as promote ischemic blood flow recovery

The sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) is key to Ca(2+) homeostasis and is redox-regulated by reversible glutathione (GSH) adducts on the cysteine (C) 674 thiol that stimulate Ca(2+) uptake activity and endothelial cell angiogenic responses in vitro. We found that mouse hind limb muscle ischemia induced S-glutathione adducts on SERCA in both whole muscle tissue and endothelial cells. To determine the role of S-glutathiolation, we used a SERCA 2 C674S heterozygote knock-in (SKI) mouse lacking half the key thiol. Following hind limb ischemia, SKI animals had decreased SERCA S-glutathione adducts and impaired blood flow recovery. We studied SKI microvascular endothelial cells in which total SERCA 2 expression was unchanged. Cultured SKI microvascular endothelial cells showed impaired migration and network formation compared with wild type (WT). Ca(2+) studies showed decreased nitric oxide (·NO)-induced (45)Ca(2+) uptake into the endoplasmic reticulum (ER) of SKI cells, while Fura-2 studies revealed lower Ca(2+) stores and decreased vascular endothelial growth factor (VEGF)- and ·NO-induced Ca(2+) influx. Adenoviral overexpression of calreticulin, an ER Ca(2+) binding protein, increased ionomycin-releasable stores, VEGF-induced Ca(2+) influx and endothelial cell migration. Taken together, these data indicate that the redox-sensitive Cys-674 thiol on SERCA 2 is required for normal endothelial cell Ca(2+) homeostasis and ischemia-induced angiogenic responses, revealing a novel redox control of angiogenesis via Ca(2+) stores.