Myocyte vascular endothelial growth factor is required for exercise-induced skeletal muscle angiogenesis

Regulation of skeletal muscle capillary growth in exercise and disease

Capillaries, which are the smallest and most abundant type of blood vessel, form the primary site of gas, nutrient, and waste transfer between the vascular and tissue compartments. Skeletal muscle exhibits the capacity to generate new capillaries (angiogenesis) as an adaptation to exercise training, thus ensuring that the heightened metabolic demand of the active muscle is matched by an improved capacity for distribution of gases, nutrients, and waste products. This review summarizes the current understanding of the regulation of skeletal muscle capillary growth. The multi-step process of angiogenesis is coordinated through the integration of a diverse array of signals associated with hypoxic, metabolic, hemodynamic, and mechanical stresses within the active muscle. The contributions of metabolic and mechanical factors to the modulation of key pro- and anti-angiogenic molecules are discussed within the context of responses to a single aerobic exercise bout and short-term and long-term training. Finally, the paradoxical lack of angiogenesis in peripheral artery disease and diabetes and the implications for disease progression and muscle health are discussed. Future studies that emphasize an integrated analysis of the mechanisms that control skeletal muscle capillary growth will enable development of targeted exercise programs that effectively promote angiogenesis in healthy individuals and in patient populations.

Muscle-derived vascular endothelial growth factor regulates microvascular remodelling in response to increased shear stress in mice

AIM

The source of vascular endothelial growth factor-A (VEGF-A) may influence vascular function. Exercise-induced vascular growth has been attributed to elevated metabolic demand and to increased blood flow, involving the production of VEGF-A by skeletal muscle and by endothelial cells respectively. We hypothesized that muscle-derived VEGF-A is not required for vascular adaptations to blood flow in skeletal muscle, as this remodelling stimulus originates within the capillary.

METHODS

Myocyte-specific VEGF-A (mVEGF(-/-) ) deleted mice were treated for 7-21 days with the vasodilator prazosin to produce a sustained increase in skeletal muscle blood flow.

RESULTS

Capillary number increased in the extensor digitorum longus (EDL) muscle in response to prazosin in wild type but not mVEGF(-/-) mice. Prazosin increased the number of smooth muscle actin-positive blood vessels in the EDL of wild-type but not mVEGF(-/-) mice. The average size of smooth muscle actin-positive blood vessels also was smaller in knockout mice after prazosin treatment. In response to prazosin treatment, VEGF-A mRNA was elevated within the EDL of wild-type but not mVEGF(-/-) mice. Ex vivo incubation of wild-type EDL with a nitric oxide donor increased VEGF-A mRNA. Likewise, we demonstrated that nitric oxide donor treatment of cultured myoblasts stimulated an increase in VEGF-A mRNA and protein.

CONCLUSION

These results suggest a link through which flow-mediated endothelial-derived signals may promote myocyte production of VEGF-A. In turn, myocyte-derived VEGF-A is required for appropriate flow-mediated microvascular remodelling. This highlights the importance of the local environment and paracrine interactions in the regulation of tissue perfusion.

Capillary growth, ultrastructure remodelling and exercise training in skeletal muscle of essential hypertensive patients

AIM

The aim was to elucidate whether essential hypertension is associated with altered capillary morphology and density and to what extent exercise training can normalize these parameters.

METHODS

To investigate angiogenesis and capillary morphology in essential hypertension, muscle biopsies were obtained from m. vastus lateralis in subjects with essential hypertension (n = 10) and normotensive controls (n = 11) before and after 8 weeks of aerobic exercise training. Morphometry was performed after transmission electron microscopy, and protein levels of several angioregulatory factors were determined.

RESULTS

At baseline, capillary density and capillary-to-fibre ratio were not different between the two groups. However, the hypertensive subjects had 9% lower capillary area (12.7 ± 0.4 vs. 13.9 ± 0.2 μm(2)) and tended to have thicker capillary basement membranes (399 ± 16 vs. 358 ± 13 nm; P = 0.094) than controls. Protein expression of vascular endothelial growth factor (VEGF), VEGF receptor-2 and thrombospondin-1 were similar in normotensive and hypertensive subjects, but tissue inhibitor of matrix metalloproteinase was 69% lower in the hypertensive group. After training, angiogenesis was evident by 15% increased capillary-to-fibre ratio in the hypertensive subjects only. Capillary area and capillary lumen area were increased by 7 and 15% in the hypertensive patients, whereas capillary basement membrane thickness was decreased by 17% (P < 0.05). VEGF expression after training was increased in both groups, whereas VEGF receptor-2 was decreased by 25% in the hypertensive patients(P < 0.05).

CONCLUSION

Essential hypertension is associated with decreased lumen area and a tendency for increased basement membrane thickening in capillaries of skeletal muscle. Exercise training may improve the diffusion conditions in essential hypertension by altering capillary structure and capillary number.

An experimental study of muscular injury repair in a mouse model of notexin-induced lesion with EPI® technique

BACKGROUND

The mechanisms of muscle injury repair after EPI® technique, a treatment based on electrical stimulation, have not been described. This study determines whether EPI® therapy could improve muscle damage.

METHODS

Twenty-four rats were divided into a control group, Notexin group (7 and 14 days) and a Notexin + EPI group. To induce muscle injury, Notexin was injected in the quadriceps of the left extremity of rats. Pro-inflammatory interleukin 1-beta (IL-1beta) and tumoral necrosis factor-alpha (TNF-alpha) were determined by ELISA. The expression of receptor peroxisome gamma proliferator activator (PPAR-gamma), vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor-1 (VEGF-R1) were determined by western-blot.

RESULTS

The plasma levels of TNF-alpha and IL-1beta in Notexin-injured rats showed a significant increase compared with the control group. EPI® produced a return of TNF-alpha and IL-1beta values to control levels. PPAR-gamma expression diminished injured quadriceps muscle in rats. EPI® increased PPAR-gamma, VEGF and VEGF-R1 expressions. EPI® decreased plasma levels of pro-inflammatory TNF-alpha and IL-1beta and increased anti-inflammatory PPAR-gamma and proangiogenic factors as well as VEGF and VEGF-R1 expressions.

CONCLUSION

The EPI® technique may affect inflammatory mediators in damaged muscle tissue and influences the new vascularization of the injured area. These results suggest that EPI® might represent a useful new therapy for the treatment of muscle injuries. Although our study in rats may represent a valid approach to evaluate EPI® treatment, studies designed to determine how the EPI® treatment may affect recovery of injury in humans are needed.

Exercise-induced capillary growth in human skeletal muscle and the dynamics of VEGF

In skeletal muscle, growth of capillaries is an important adaptation to exercise training that secures adequate diffusion capacity for oxygen and nutrients even at high-intensity exercise when increases in muscle blood flow are profound. Mechanical forces present during muscle activity, such as shear stress and passive stretch, lead to cellular signaling, enhanced expression of angiogenic factors, and initiation of capillary growth. The most central angiogenic factor in skeletal muscle capillary growth is VEGF. During muscle contraction, VEGF increases in the muscle interstitium, acts on VEGF receptors on the capillary endothelium, and thereby stimulates angiogenic processes. A primary source of muscle interstitial VEGF during exercise is the skeletal muscle fibers which contain large stores of VEGF within vesicles. We propose that, during muscle activity, these VEGF-containing vesicles are redistributed toward the sarcolemma where the contents are secreted into the extracellular fluid. VEGF mRNA expression is increased primarily after exercise, which allows for a more rapid replenishment of VEGF stores lost through secretion during exercise. Future studies should focus on elucidating mechanisms and regulation of VEGF secretion.

Muscle microvascular adaptation and angiogenic gene induction in response to exercise training are attenuated in middle-aged rats

This study was designed to investigate exercise-induced changes in muscle capillarisation, the mRNA expression of angiogenic genes, and microRNA levels in young and middle-aged rats. Rats in the training groups were subjected to treadmill running 5 days a week for 3 weeks. The exercise protocol for the young (12-week old) group was 20-25 m/min, 40-60 min/day with a gradient of 15%, and for the middle-aged (12-month old) group was 18-20 m/min, 40-60 min/day with a gradient of 5%. The enzyme histochemical identification of capillary profiles was performed on cross-sections of gastrocnemius muscle. Total RNA was isolated, reverse transcription was performed, and mRNA and microRNA levels were determined by real-time PCR. The capillary-to-fibre ratio was significantly increased by exercise training in the young group (by 10%), but only slightly in the middle-aged (by 5%) group. Vascular endothecial growth factor (VEGF) mRNA levels were at significantly higher values after acute exercise (1.6-fold) and the 3-week training protocol (1.9-fold) in the young group, but not in the middle-aged group. VEGF protein expression levels were significantly increased after training in the young group only. Endothelial nitric oxide synthase, VEGF-R2 and thrombospondin-1 mRNA levels were significantly lower in the middle-aged group than in the young group. Anti-angiogenic miR-195 levels were significantly enhanced by exercise training in the middle-aged group only. These results indicated that the exercise-induced adaptation of muscle capillarity was attenuated in middle-aged rats, possibly by the lower induction of VEGF and up-regulation of anti-angiogenic miRNA expression.

The role of microRNAs in skeletal muscle health and disease

Over the last decade non-coding RNAs have emerged as importance regulators of gene expression. In particular, microRNAs are a class of small RNAs of ∼ 22 nucleotides that repress gene expression through a post-transcriptional mechanism. MicroRNAs have been shown to be involved in a broader range of biological processes, both physiological and pathological, including myogenesis, adaptation to exercise and various myopathies. The purpose of this review is to provide a comprehensive summary of what is currently known about the role of microRNAs in skeletal muscle health and disease.

Mechanisms modulating skeletal muscle phenotype

Mammalian skeletal muscles are composed of a variety of highly specialized fibers whose selective recruitment allows muscles to fulfill their diverse functional tasks. In addition, skeletal muscle fibers can change their structural and functional properties to perform new tasks or respond to new conditions. The adaptive changes of muscle fibers can occur in response to variations in the pattern of neural stimulation, loading conditions, availability of substrates, and hormonal signals. The new conditions can be detected by multiple sensors, from membrane receptors for hormones and cytokines, to metabolic sensors, which detect high-energy phosphate concentration, oxygen and oxygen free radicals, to calcium binding proteins, which sense variations in intracellular calcium induced by nerve activity, to load sensors located in the sarcomeric and sarcolemmal cytoskeleton. These sensors trigger cascades of signaling pathways which may ultimately lead to changes in fiber size and fiber type. Changes in fiber size reflect an imbalance in protein turnover with either protein accumulation, leading to muscle hypertrophy, or protein loss, with consequent muscle atrophy. Changes in fiber type reflect a reprogramming of gene transcription leading to a remodeling of fiber contractile properties (slow-fast transitions) or metabolic profile (glycolytic-oxidative transitions). While myonuclei are in postmitotic state, satellite cells represent a reserve of new nuclei and can be involved in the adaptive response.

Aging and the Skeletal Muscle Angiogenic Response to Exercise in Women

Whether aging lowers skeletal muscle basal capillarization and angiogenesis remains controversial. To investigate the effects of aging on skeletal muscle capillarization, eight young (YW) and eight aged (AW) women completed 8 weeks of exercise training. The response and relationships of muscle capillarization, interstitial vascular endothelial growth factor (VEGF), and microvascular blood flow to aerobic exercise training were investigated. Vastus lateralis biopsies were obtained before and after exercise training for the measurement of capillarization. Muscle interstitial VEGF protein and microvascular blood flow were measured at rest and during submaximal exercise at PRE, 1-WK, and 8-WKS by microdialysis. Exercise training increased (20%-25%) capillary contacts of type I, IIA, and IIB fibers in YW and AW. Interstitial VEGF protein was higher in AW than YW at rest and was higher in YW than AW during exercise independent of training status. Differences in muscle capillarization were not explained by secreted VEGF nor were differences in VEGF explained by microvascular blood flow. These results confirm that aging (57-76 years age range) does not impair the muscle angiogenic response to exercise training, although sex differences may exist in similarly trained women and men.

Resveratrol modulates the angiogenic response to exercise training in skeletal muscles of aged men

In animal studies, the polyphenol resveratrol has been shown to influence several pathways of importance for angiogenesis in skeletal muscle. The aim of the present study was to examine the angiogenic effect of resveratrol supplementation with parallel exercise training in aged men. Forty-three healthy physically inactive aged men (65 ± 1 yr) were divided into 1) a training group that conducted 8 wk of intense exercise training where half of the subjects received a daily intake of either 250 mg trans-resveratrol (n = 14) and the other half received placebo (n = 13) and 2) a nontraining group that received either 250 mg trans-resveratrol (n = 9) or placebo (n = 7). The group that trained with placebo showed a ~20% increase in the capillary-to-fiber ratio, an increase in muscle protein expression of VEGF, VEGF receptor-2, and tissue inhibitor of matrix metalloproteinase (TIMP-1) but unaltered thrombospodin-1 levels. Muscle interstitial VEGF and thrombospodin-1 protein levels were unchanged after the training period. The group that trained with resveratrol supplementation did not show an increase in the capillary-to-fiber ratio or an increase in muscle VEGF protein. Muscle TIMP-1 protein levels were lower in the training and resveratrol group than in the training and placebo group. Both training groups showed an increase in forkhead box O1 protein. In nontraining groups, TIMP-1 protein was lower in the resveratrol-treated group than the placebo-treated group after 8 wk. In conclusion, these data show that exercise training has a strong angiogenic effect, whereas resveratrol supplementation may limit basal and training-induced angiogenesis.

Effects of detraining on the temporal expression of positive and negative angioregulatory proteins in skeletal muscle of mice

Temporal expression of positive and negative angiogenic factors in response to detraining is poorly understood. We report the protein expression of anti-angiogenic peptides (thrombospondin-1, TSP-1; and endostatin) as well as pro-angiogenic factors (vascular endothelial growth factor, VEGF; matrix metalloproteinases-2 and -9), and nucleolin (a nuclear protein involved with synthesis and maturation of ribosomes) in response to detraining in triceps surae muscles of C57BL/6 mice. Male mice were allowed to exercise voluntarily for 21 days, and then basal and acute response to exercise were evaluated at 1, 7, 14 and 28 days detraining (D1, D7, D14, D28, respectively, n = 12/group). As seen in the D1 mice, training resulted in the increased muscle capillary-to-fibre ratio (C/F), increased maximal running time and elevated basal expression of VEGF and matrix metalloproteinase-9 (P < 0.05). After 7 days of detraining (D7), C/F levels were similar to control levels, but both basal VEGF and TSP-1 were elevated (P < 0.05). At D14 and D28, TSP-1 protein was not different compared to baseline levels; however, VEGF was elevated in gastrocnemius (GA), but not the soleus (SOL) or plantaris (PLT) muscles, of D14 mice. Endostatin tended to decrease in D14 and D28 compared to controls. Timing of nucleolin protein expression differed between muscle groups, with increases at D1, D7 and D14 in the PLT, SOL and GA muscles, respectively. The response of VEGF and nucleolin to acute exercise was blunted with training, and remained blunted in the PLT and SOL even after 28 days of detraining, at a time point long after muscle capillarization was observed to be similar to pre-training levels. These data suggest that TSP-1 may be a mediator of capillary regression with detraining, even in the face of elevated VEGF, suggesting that pro-angiogenic regulators may not be able to prevent the regression of skeletal muscle capillaries under physiological conditions. The responses of matrix metalloproteinases, endostatin and nucleolin poorly correlated with detraining-induced capillary regression.

Expression of circulating vascular endothelial growth factor-antagonizing cytokines and vascular stabilizing factors prior to and following bypass surgery in patients with moyamoya disease

The aim of the present study was to investigate the levels of vascular endothelial growth factor (VEGF)-antagonizing cytokines and VEGF-influenced vascular stabilizing cytokines in patients with moyamoya disease (MMD) and the association with postoperative collateral vessel formation. The study population included 53 MMD patients that had undergone indirect bypass surgery and 50 healthy controls. Serum levels of VEGF, thrombospondin-1 (TSP-1), TSP-2, soluble VEGF receptor-1 (sVEGFR-1), sVEGFR-2, endostatin, angiopoietin-1 (Ang-1) and Ang-2 were measured at the baseline (preoperative) and at day seven following surgery. Postoperative collateralization assessment was conducted upon the six-month follow-up cerebral angiography. Cytokine levels were compared between patients with good or poor collateral formation. Compared with the healthy controls, MMD patients exhibited lower baseline levels of sVEGFR-1 (P<0.0001) and sVEGFR-2 (P<0.0001), but higher VEGF expression (P<0.0001). Ang-1 and Ang-2 levels did not exhibit any difference between the two groups. On day seven following surgery, MMD patients exhibited an almost unchanged sVEGFR-1 and sVEGFR-2 expression level, but upregulated expression of VEGF (P<0.0001), Ang-1 (P<0.0001) and TSP-2 (P<0.0001). The six-month follow-up angiographies revealed that 21 patients (45.65%) that had undergone the same surgical procedure achieved good collateralization. Patients with good collateral formation appeared to have lower sVEGFR-1 and sVEGFR-2 levels prior to (P=0.029 and P=0.045, respectively) and at day seven (P=0.044 and P=0.047, respectively) following bypass surgery when compared with the patients with worse collateralization. Therefore, sVEGFR-1 and sVEGFR-2 may play a role in the pathogenesis of MMD. Lower levels of sVEGFR-1 and sVEGFR-2 indicated better postoperative collateralization in the six months following indirect bypass surgery. However, Ang-1 and Ang-2 may not be specifically involved in the course of MMD.

Skeletal myofiber VEGF is essential for the exercise training response in adult mice

Vascular endothelial growth factor (VEGF) is exercise responsive, pro-angiogenic, and expressed in several muscle cell types. We hypothesized that in adult mice, VEGF generated within skeletal myofibers (and not other cells within muscle) is necessary for the angiogenic response to exercise training. This was tested in adult conditional, skeletal myofiber-specific VEGF gene-deleted mice (skmVEGF-/-), with VEGF levels reduced by >80%. After 8 wk of daily treadmill training, speed and endurance were unaltered in skmVEGF-/- mice, but increased by 18% and 99% (P < 0.01), respectively, in controls trained at identical absolute speed, incline, and duration. In vitro, isolated soleus and extensor digitorum longus contractile function was not impaired in skmVEGF-/- mice. However, training-induced angiogenesis was inhibited in plantaris (wild type, 38%, skmVEGF-/- 18%, P < 0.01), and gastrocnemius (wild type, 43%, P < 0.01; skmVEGF-/-, 7%, not significant). Capillarity was maintained (different from VEGF gene deletion targeted to multiple cell types) in untrained skmVEGF-/- mice. Arteriogenesis (smooth muscle actin+, artery number, and diameter) and remodeling [vimentin+, 5'-bromodeoxycytidine (BrdU)+, and F4/80+ cells] occurred in skmVEGF-/- mice, even in the absence of training. skmVEGF-/- mice also displayed a limited oxidative enzyme [citrate synthase and β-hydroxyacyl CoA dehydrogenase (β-HAD)] training response; β-HAD activity levels were elevated in the untrained state. These data suggest that myofiber expressed VEGF is necessary for training responses in capillarity and oxidative capacity and for improved running speed and endurance.

Angiotensin II evokes angiogenic signals within skeletal muscle through co-ordinated effects on skeletal myocytes and endothelial cells

Skeletal muscle overload induces the expression of angiogenic factors such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP)-2, leading to new capillary growth. We found that the overload-induced increase in angiogenesis, as well as increases in VEGF, MMP-2 and MT1-MMP transcripts were abrogated in muscle VEGF KO mice, highlighting the critical role of myocyte-derived VEGF in controlling this process. The upstream mediators that contribute to overload-induced expression of VEGF have yet to be ascertained. We found that muscle overload increased angiotensinogen expression, a precursor of angiotensin (Ang) II, and that Ang II signaling played an important role in basal VEGF production in C2C12 cells. Furthermore, matrix-bound VEGF released from myoblasts induced the activation of endothelial cells, as evidenced by elevated endothelial cell phospho-p38 levels. We also found that exogenous Ang II elevates VEGF expression, as well as MMP-2 transcript levels in C2C12 myotubes. Interestingly, these responses also were observed in skeletal muscle endothelial cells in response to Ang II treatment, indicating that these cells also can respond directly to the stimulus. The involvement of Ang II in muscle overload-induced angiogenesis was assessed. We found that blockade of AT1R-dependent Ang II signaling using losartan did not attenuate capillary growth. Surprisingly, increased levels of VEGF protein were detected in overloaded muscle from losartan-treated rats. Similarly, we observed elevated VEGF production in cultured endothelial cells treated with losartan alone or in combination with Ang II. These studies conclusively establish the requirement for muscle derived VEGF in overload-induced angiogenesis and highlight a role for Ang II in basal VEGF production in skeletal muscle. However, while Ang II signaling is activated following overload and plays a role in muscle VEGF production, inhibition of this pathway is not sufficient to halt overload-induced angiogenesis, indicating that AT1-independent signals maintain VEGF production in losartan-treated muscle.

Angiogenic response to passive movement and active exercise in individuals with peripheral arterial disease

Peripheral arterial disease (PAD) is caused by atherosclerosis and is associated with microcirculatory impairments in skeletal muscle. The present study evaluated the angiogenic response to exercise and passive movement in skeletal muscle of PAD patients compared with healthy control subjects. Twenty-one PAD patients and 17 aged control subjects were randomly assigned to either a passive movement or an active exercise study. Interstitial fluid microdialysate and tissue samples were obtained from the thigh skeletal muscle. Muscle dialysate vascular endothelial growth factor (VEGF) levels were modestly increased in response to either passive movement or active exercise in both subject groups. The basal muscle dialysate level of the angiostatic factor thrombospondin-1 protein was markedly higher (P < 0.05) in PAD patients compared with the control subjects, whereas soluble VEGF receptor-1 dialysate levels were similar in the two groups. The basal VEGF protein content in the muscle tissue samples was ∼27% lower (P < 0.05) in the PAD patients compared with the control subjects. Analysis of mRNA expression for a range of angiogenic and angiostatic factors revealed a modest change with active exercise and passive movement in both groups, except for an increase (P < 0.05) in the ratio of angiopoietin-2 to angiopoietin-1 mRNA in the PAD group with both interventions. PAD patients and aged individuals showed a similar limited angiogenic response to active exercise and passive movement. The limited increase in muscle extracellular VEGF combined with an elevated basal level of thrombospondin-1 in muscle extracellular fluid of PAD patients may restrict capillary growth in these patients.

Temporal response of positive and negative regulators in response to acute and chronic exercise training in mice

Angiogenesis is controlled by a balance between positive and negative angiogenic factors, but temporal protein expression of many key angiogenic regulators in response to exercise are still poorly defined. In C57BL/6 mice, we evaluated the temporal protein expression of several pro-angiogenic and anti-angiogenic factors in response to (1) a single acute bout of exercise and (2) chronic exercise training resulting from 3, 5, 7, 14 and 28 days of voluntary wheel running. Following acute exercise, protein levels of vascular endothelial growth factor-A (VEGF), endostatin and nucleolin were increased at 2-4 h (P < 0.05), whereas matrix metalloproteinase (MMP)-2 was elevated within a 12-24 h window (P < 0.05). Training increased muscle capillarity 11%, 15% and 22% starting with 7, 14 and 28 days of training, respectively (P < 0.01). Basal VEGF and MMP-2 were increased by 31% and 22%, respectively, compared to controls (P < 0.05) after 7 days (7d) training, but decreased to back to baseline after 14d training. After 28d training VEGF fell 49% below baseline control (P < 0.01). Basal muscle expression of thrombospondin 1 (TSP-1) was ∼900% greater in 14d- and 28d-trained mice compared to either 5d- and 7d-trained mice (P < 0.05), and tended to increase by ∼180-258% compared to basal control levels (P < 0.10). The acute responsiveness of VEGF to exercise in untrained mice (i.e. 161% increase, P < 0.001) was lost with capillary adaptation occurring after 7, 14 and 28d training. Taken together, these data support the notion that skeletal muscle angiogenesis is controlled by a balance between positive and negative mitogens, and reveals a complex, highly-coordinated, temporal scheme whereby these factors can differentially influence capillary growth in response to acute versus chronic exercise.

(-)-Epicatechin is associated with increased angiogenic and mitochondrial signalling in the hindlimb of rats selectively bred for innate low running capacity

Alternative approaches to reduce congenital muscle dysfunction are needed in cases where the ability to exercise is limited. (-)-Epicatechin is found in cocoa and may stimulate capillarity and mitochondrial proliferation in skeletal muscle. A total of 21 male rats bred for LCR (low running capacity) from generation 28 were randomized into three groups: vehicle for 30 days (control); (-)-epicatechin for 30 days; and (-)-epicatechin for 30 days followed by 15 days without (-)-epicatechin. Groups 2 and 3 received 1.0 mg of (-)-epicatechin/kg of body mass twice daily, whereas water was given to the control group. The plantaris muscle was harvested for protein and morphometric analyses. In addition, in vitro experiments were conducted to examine the role of (-)-epicatechin on mitochondrial respiratory kinetics at different incubation periods. Treatment for 30 days with (-)-epicatechin increased capillarity (P<0.001) and was associated with increases in protein expression of VEGF (vascular endothelial growth factor)-A with a concomitant decrease in TSP-1 (thrombospondin-1) and its receptor, which remained after 15 days of (-)-epicatechin cessation. Analyses of the p38 MAPK (mitogen-activated protein kinase) signalling pathway indicated an associated increase in phosphorylation of MKK3/6 (MAPK kinase 3/6) and p38 and increased protein expression of MEF2A (myocyte enhancer factor 2A). In addition, we observed significant increases in protein expression of PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α), PGC-1β, Tfam and cristae abundance. Interestingly, these increases associated with (-)-epicatechin treatment remained after 15 days of cessation. Lastly, in vitro experiments indicated that acute exposure of LCR muscle to (-)-epicatechin incubation was not sufficient to increase mitochondrial respiration. The results suggest that increases in skeletal muscle capillarity and mitochondrial biogenesis are associated with 30 days of (-)-epicatechin treatment and sustained for 15 days following cessation of treatment. Clinically, the use of this natural compound may have potential application in populations that experience muscle fatigue and are unable to perform endurance exercise.

Mesenchymal stem cells contribute to vascular growth in skeletal muscle in response to eccentric exercise

The α7β1-integrin is an adhesion molecule highly expressed in skeletal muscle that can enhance regeneration in response to eccentric exercise. We have demonstrated that mesenchymal stem cells (MSCs), predominantly pericytes, accumulate in muscle (mMSCs) overexpressing the α7B-integrin (MCK:α7B; α7Tg) and contribute to new fiber formation following exercise. Since vascularization is a common event that supports tissue remodeling, we hypothesized that the α7-integrin and/or mMSCs may stimulate vessel growth following eccentric exercise. Wild-type (WT) and α7Tg mice were subjected to single or multiple (3 times/wk, 4 wk) bouts of downhill running exercise. Additionally, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) -labeled mMSCs were intramuscularly injected into WT recipients. A subset of recipient mice were run downhill before injection to recapitulate the exercised microenvironment. While total number of CD31+ vessels declined in both WT and α7Tg muscle following a single bout of exercise, the number of larger CD31+ vessels with a visible lumen was preferentially increased in α7Tg mice following eccentric exercise training ( P < 0.05). mMSC transplantation similarly increased vessel diameter and the total number of neuron-glial antigen-2 (NG2+) arterioles postexercise. Secretion of arteriogenic factors from mMSCs in response to mechanical strain, including epidermal growth factor and granulocyte macrophage-colony stimulating factor, may account for vessel remodeling. In conclusion, this study demonstrates that the α7-integrin and mMSCs contribute to increased vessel diameter size and arteriolar density in muscle in response to eccentric exercise. The information in this study has implications for the therapeutic treatment of injured muscle and disorders that result in vessel occlusion, including peripheral artery disease.

On aerobic exercise and behavioral and neural plasticity

Aerobic exercise promotes rapid and profound alterations in the brain. Depending upon the pattern and duration of exercise, these changes in the brain may extend beyond traditional motor areas to regions and structures normally linked to learning, cognition, and emotion. Exercise-induced alterations may include changes in blood flow, hormone and growth factor release, receptor expression, angiogenesis, apoptosis, neurogenesis, and synaptogenesis. Together, we believe that these changes underlie elevations of mood and prompt the heightened behavioral plasticity commonly observed following adoption of a chronic exercise regimen. In the following paper, we will explore both the psychological and psychobiological literatures relating to exercise effects on brain in both human and non-human animals and will attempt to link plastic changes in these neural structures to modifications in learned behavior and emotional expression. In addition, we will explore the therapeutic potential of exercise given recent reports that aerobic exercise may serve as a neuroprotectant and can also slow cognitive decline during normal and pathological aging.

Understanding vascular development

The vasculature of an organism has the daunting task of connecting all the organ systems to nourish tissue and sustain life. This complex network of vessels and associated cells must maintain blood flow, but constantly adapt to acute and chronic changes within tissues. While the vasculature has been studied for over a century, we are just beginning to understand the processes that regulate its formation and how genetic hierarchies are influenced by mechanical and metabolic cues to refine vessel structure and optimize efficiency. As we gain insights into the developmental mechanisms, it is clear that the processes that regulate blood vessel development can also enable the adult to adapt to changes in tissues that can be elicited by exercise, aging, injury, or pathology. Thus, research in vessel development has provided tremendous insights into therapies for vascular diseases and disorders, cancer interventions, wound repair and tissue engineering, and in turn, these models have clearly impacted our understanding of development. Here we provide an overview of the development of the vascular system, highlighting several areas of active investigation and key questions that remain to be answered.

Exercise training and peripheral arterial disease

Peripheral arterial disease (PAD) is a common vascular disease that reduces blood flow capacity to the legs of patients. PAD leads to exercise intolerance that can progress in severity to greatly limit mobility, and in advanced cases leads to frank ischemia with pain at rest. It is estimated that 12 to 15 million people in the United States are diagnosed with PAD, with a much larger population that is undiagnosed. The presence of PAD predicts a 50% to 1500% increase in morbidity and mortality, depending on severity. Treatment of patients with PAD is limited to modification of cardiovascular disease risk factors, pharmacological intervention, surgery, and exercise therapy. Extended exercise programs that involve walking approximately five times per week, at a significant intensity that requires frequent rest periods, are most significant. Preclinical studies and virtually all clinical trials demonstrate the benefits of exercise therapy, including improved walking tolerance, modified inflammatory/hemostatic markers, enhanced vasoresponsiveness, adaptations within the limb (angiogenesis, arteriogenesis, and mitochondrial synthesis) that enhance oxygen delivery and metabolic responses, potentially delayed progression of the disease, enhanced quality of life indices, and extended longevity. A synthesis is provided as to how these adaptations can develop in the context of our current state of knowledge and events known to be orchestrated by exercise. The benefits are so compelling that exercise prescription should be an essential option presented to patients with PAD in the absence of contraindications. Obviously, selecting for a lifestyle pattern that includes enhanced physical activity prior to the advance of PAD limitations is the most desirable and beneficial.

Deletion of heart-type cytochrome c oxidase subunit 7a1 impairs skeletal muscle angiogenesis and oxidative phosphorylation

Oxidative metabolism is needed for sustained skeletal muscle function. A key component of such metabolism is cytochrome c oxidase, the 13-subunit terminal complex of the mitochondrial electron transport chain. We used mice null for one of the two isoforms of Cox subunit 7a, heart/skeletal muscle-specific Cox7a1, to examine the cellular and functional responses of muscle adaptation in response to mitochondrial dysfunction. Specifically we determined if deletion of Cox7a1 would (1) limit exercise capacity, and (2) alter genes responsible for skeletal muscle capillarity and mitochondrial biogenesis. Sixteen male mice (Cox7a1 null mice, n = 8, and littermate controls, n = 8) performed incremental and run-to-exhaustion treadmill tests. The hindlimb muscles for both groups were analysed. The results indicated that capillary indices were reduced (by 30.7–44.9%) in the Cox7a1 null mice relative to controls. In addition, resting ATP levels and Cox specific activity were significantly reduced (>60%) in both glycolytic and oxidative muscle fibre types despite an increase in a major regulator of mitochondrial biogenesis, PGC-1β. These changes in the skeletal muscle resulted in exercise intolerance for the Cox7a1 null mice. Thus, our data indicate that deletion of the Cox7a1 isoform results in reduced muscle bioenergetics and hindlimb capillarity, helping to explain the observed impairment of muscle structure and function.

Exposure to cigarette smoke induces overexpression of von Hippel-Lindau tumor suppressor in mouse skeletal muscle

Cigarette smoke (CS) is a well-established risk factor in the development of chronic obstructive pulmonary disease (COPD). In contrast, the extent to which CS exposure contributes to the development of the systemic manifestations of COPD, such as skeletal muscle dysfunction and wasting, remains largely unknown. Decreased skeletal muscle capillarization has been previously reported in early stages of COPD and might play an important role in the development of COPD-associated skeletal muscle abnormalities. To investigate the effects of chronic CS exposure on skeletal muscle capillarization and exercise tolerance, a mouse model of CS exposure was used. The 129/SvJ mice were exposed to CS for 6 mo, and the expression of putative elements of the hypoxia-angiogenic signaling cascade as well as muscle capillarization were studied. Additionally, functional tests assessing exercise tolerance/endurance were performed in mice. Compared with controls, skeletal muscles from CS-exposed mice exhibited significantly enhanced expression of von Hippel-Lindau tumor suppressor (VHL), ubiquitin-conjugating enzyme E2D1 (UBE2D1), and prolyl hydroxylase-2 (PHD2). In contrast, hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) expression was reduced. Furthermore, reduced muscle fiber cross-sectional area, decreased skeletal muscle capillarization, and reduced exercise tolerance were also observed in CS-exposed animals. Taken together, the current results provide evidence linking chronic CS exposure and induction of VHL expression in skeletal muscles leading toward impaired hypoxia-angiogenesis signal transduction, reduced muscle fiber cross-sectional area, and decreased exercise tolerance.

Murine double minute-2 expression is required for capillary maintenance and exercise-induced angiogenesis in skeletal muscle

Exercise-induced angiogenesis is a key determinant of skeletal muscle function. Here, we investigated whether the E3 ubiquitin ligase murine double minute-2 (Mdm2) exerts a proangiogenic function in exercised skeletal muscle. Mdm2 hypomorphic (Mdm2(Puro/Δ7-9)) mice have a 60% reduction in Mdm2 expression compared with that in wild-type animals. Capillary staining on muscle sections from Mdm2(Puro/Δ7-9) sedentary mice with a wild-type or knockout background for p53 revealed that deficiency in Mdm2 resulted in 20% capillary regression independently of p53 status. In response to one bout of exercise, protein expression of the proangiogenic vascular endothelial growth factor-A (VEGF-A) was increased by 64% in muscle from wild-type animals, and endothelial cell outgrowth from exercised muscle biopsy samples cultured in a 3-dimensional collagen gel was enhanced by 37%. These proangiogenic responses to exercise were impaired in exercised Mdm2(Puro/Δ7-9) mice. Prolonged exercise training resulted in increased Mdm2 protein expression (+49%) and capillarization (+24%) in wild-type muscles. However, exercise training-induced angiogenesis was abolished in Mdm2(Puro/Δ7-9) mice. Finally, exercise training restored Mdm2, VEGF-A, and capillarization levels in skeletal muscles from obese Zucker diabetic fatty rats compared with those in healthy animals. Our results define Mdm2 as a crucial regulator of capillary maintenance and exercise-induced angiogenesis in skeletal muscle.

Significance of vascular endothelial growth factor in growth and peritoneal dissemination of ovarian cancer

Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis which drives endothelial cell survival, proliferation, and migration while increasing vascular permeability. Playing an important role in the physiology of normal ovaries, VEGF has also been implicated in the pathogenesis of ovarian cancer. Essentially by promoting tumor angiogenesis and enhancing vascular permeability, VEGF contributes to the development of peritoneal carcinomatosis associated with malignant ascites formation, the characteristic feature of advanced ovarian cancer at diagnosis. In both experimental and clinical studies, VEGF levels have been inversely correlated with survival. Moreover, VEGF inhibition has been shown to inhibit tumor growth and ascites production and to suppress tumor invasion and metastasis. These findings have laid the basis for the clinical evaluation of agents targeting VEGF signaling pathway in patients with ovarian cancer. In this review, we will focus on VEGF involvement in the pathophysiology of ovarian cancer and its contribution to the disease progression and dissemination.

Vascular remodelling in human skeletal muscle

Exercise-induced angiogenesis in skeletal muscle involves both non-sprouting and sprouting angiogenesis and results from the integrated responses of multiple systems and stimuli. VEGF-A (vascular endothelial growth factor A) levels are increased in exercised muscle and have been demonstrated to be critical for exercise-induced capillary growth. Only limited information is available regarding the role of other angiogenic and angiostatic factors in exercise, but changes in the angiopoietin family following repetitive bouts of exercise occur in a pattern that is favourable for angiogenesis. Results from other angiogenic model systems, indicate that miRNAs (microRNAs) are important factors in the regulation of angiogenesis and thus to explore their role as regulators of exercise induced angiogenesis will be an important avenue of study in the future. ECM (extracellular matrix) remodelling and activation of MMPs (matrix metalloproteinases) are, to some extent, overlooked players in skeletal muscle adaptation. Degradation of ECM proteins liberates angiogenic factors from immobilized matrix stores and make cell migration possible. In fact, it is known that MMPs become activated by a single bout of exercise in humans, rapid interstitial changes occur long before any changes in gene transcription could result in protein synthesis and inhibition of MMP activity completely abolishes sprouting angiogenesis. A growing body of evidence suggests that circulating and resident progenitor cells, in addition to other cell types located in skeletal muscle tissue, participate in skeletal muscle angiogenesis by various mechanisms. However, more studies are needed before these can be confirmed as mechanisms of exercise-induced capillary growth.

The critical role of VEGF in skeletal muscle angiogenesis and blood flow

VEGF (vascular endothelial growth factor) is well known as an important molecule in angiogenesis. Its inhibition is pursued as an anticancer therapy; its enhancement as therapy for tissue ischaemia. In the present paper, its role in skeletal muscle is explored, both at rest and after exercise. Muscle VEGF mRNA and protein are increased severalfold after heavy exercise. Whereas global VEGF knockout is embryonically lethal, muscle-specific knockout is not, providing models for studying its functional significance. Its deletion in adult mouse skeletal muscle: (i) reduces muscle capillarity by more than 50%, (ii) decreases exercise endurance time by approximately 80%, and (iii) abolishes the angiogenic response to exercise training. What causes VEGF to increase with exercise is not clear. Despite regulation by HIF (hypoxia-inducible factor), increased HIF on exercise, and PO2 falling to single digit values during exercise, muscle-specific HIF knockout does not impair performance or capillarity, leaving many unanswered questions.

Regular exercise cures depression-like behavior via VEGF-Flk-1 signaling in chronically stressed mice

In animals, chronic stress leads to the development of depression-like behavior and decreases neurogenesis and blood vessel density in hippocampus, whereas antidepressants increase adult neurogenesis in hippocampus. Regular exercise training also has antidepressant action and increases hippocampal neurogenesis; however, whether exercise-induced antidepressant action is related to hippocampal microvasculature is unclear. To address this issue, we compared depression-like behavior, blood vessel density, and neurogenesis in hippocampal dentate gyrus between stressed and exercised mice with or without administration of inhibitor of vascular endothelial growth factor (VEGF) receptor. Chronic stress led to the development of depression-like behavior, decreased blood vessel density, and neurogenesis in hippocampus. Regular exercise training improved depression-like behavior, the decrease of hippocampal blood vessel density, and neurogenesis in the stress state, whereas the combination of regular exercise and administration of SU1498, VEGF receptor Flk-1 inhibitor, canceled the exercise-induced antidepressant effect. These findings suggested that the improvement of hippocampal blood vessel and adult neurogenesis via VEGF signaling pathway is necessary for exercise-induced antidepressant effect.

Pro- and anti-angiogenic factors in human skeletal muscle in response to acute exercise and training

This study examined the effect of acute exercise and 4 weeks of aerobic training on skeletal muscle gene and protein expression of pro- and anti-angiogenic factors in 14 young male subjects. Training consisted of 60 min of cycling (∼60% of ), 3 times/week. Biopsies were obtained from vastus lateralis muscle before and after training. Muscle interstitial fluid was collected during cycling at weeks 0 and 4. Training increased (P < 0.05) the capillary: fibre ratio and capillary density by 23% and 12%, respectively. The concentration of interstitial vascular endothelial growth factor (VEGF) in response to acute exercise increased similarly (>6-fold; P < 0.05) before and after training. Resting protein levels of soluble VEGF receptor-1 in interstitial fluid, and of VEGF, thrombospondin-1 (TSP-1) and tissue inhibitor of matrix metalloproteinase-1 (TIMP1) in muscle were unaffected by training, whereas endothelial nitric oxide synthase protein levels in muscle increased by 50% (P < 0.05). Before and after training, acute exercise induced a similar increase (P < 0.05) in the mRNA level of angiopoietin 2, matrix metalloproteinase 9 and TSP-1. After training, TIMP1 mRNA content increased with exercise (P < 0.05). In conclusion, acute exercise induced a similar increase in the gene-expression of both pro- and anti-angiogenic factors in untrained and trained muscle. We propose that the increase in anti-angiogenic factors with exercise is important for modulation of angiogenesis. The lack of effect of training on basal muscle VEGF protein levels and VEGF secretion during exercise suggests that increased VEGF levels are not a prerequisite for exercise-induced capillary growth in healthy muscle.

Passive Exercise Using Whole-Body Periodic Acceleration Enhances Blood Supply to Ischemic Hindlimb

Objective—

Whole-body periodic acceleration (WBPA) has been developed as a passive exercise technique to improve endothelial function by increasing shear stress through repetitive movements in spinal axis direction. We investigated the effects of WBPA on blood flow recovery in a mouse model of hindlimb ischemia and in patients with peripheral arterial disease.

Methods and Results—

After unilateral femoral artery excision, mice were assigned to either the WBPA (n=15) or the control (n=13) group. WBPA was applied at 150 cpm for 45 minutes under anesthesia once a day. WBPA significantly increased blood flow recovery after ischemic surgery, as determined by laser Doppler perfusion imaging. Sections of ischemic adductor muscle stained with anti-CD31 antibody showed a significant increase in capillary density in WBPA mice compared with control mice. WBPA increased the phosphorylation of endothelial nitric oxide synthase (eNOS) in skeletal muscle. The proangiogenic effect of WBPA on ischemic limb was blunted in eNOS-deficient mice, suggesting that the stimulatory effects of WBPA on revascularization are eNOS dependent. Quantitative real-time polymerase chain reaction analysis showed significant increases in angiogenic growth factor expression in ischemic hindlimb by WBPA. Facilitated blood flow recovery was observed in a mouse model of diabetes despite there being no changes in glucose tolerance and insulin sensitivity. Furthermore, both a single session and 7-day repeated sessions of WBPA significantly improved blood flow in the lower extremity of patients with peripheral arterial disease.

Conclusion—

WBPA increased blood supply to ischemic lower extremities through activation of eNOS signaling and upregulation of proangiogenic growth factor in ischemic skeletal muscle. WBPA is a potentially suitable noninvasive intervention to facilitate therapeutic angiogenesis.

Muscle intracellular oxygenation during exercise: optimization for oxygen transport, metabolism, and adaptive change

Exercise is the example par excellence of the body functioning as a physiological system. Conventionally we think of the O(2) transport process as a major manifestation of that system linking and integrating pulmonary, cardiovascular, hematological and skeletal muscular contributions to the task of getting O(2) from the air to the mitochondria, and this process has been well described. However, exercise invokes system responses at levels additional to those of macroscopic O(2) transport. One such set of responses appears to center on muscle intracellular PO(2), which falls dramatically from rest to exercise. At rest, it approximates 4 kPa, but during heavy endurance exercise it falls to about 0.4-0.5 kPa, an amazingly low value for a tissue absolutely dependent on the continual supply of O(2) to meet very high energy demands. One wonders why intracellular PO(2) is allowed to fall to such levels. The proposed answer, to be presented in the review, is that a low intramyocyte PO(2) is pivotal in: (a) optimizing oxygen's own physiological transport, and (b) stimulating adaptive gene expression that, after translation, enables greater exercise capacity-all the while maintaining PO(2) at levels sufficient to allow oxidative phosphorylation to operate sufficiently fast enough to support intense muscle contraction. Thus, during exercise, reductions of intracellular PO(2) to less than 1% of that in the atmosphere enables an integrated response that fundamentally and simultaneously optimizes physiological, biochemical and molecular events that support not only the exercise as it happens but the adaptive changes to increase exercise capacity over the longer term.

Module-based multiscale simulation of angiogenesis in skeletal muscle

Background

Mathematical modeling of angiogenesis has been gaining momentum as a means to shed new light on the biological complexity underlying blood vessel growth. A variety of computational models have been developed, each focusing on different aspects of the angiogenesis process and occurring at different biological scales, ranging from the molecular to the tissue levels. Integration of models at different scales is a challenging and currently unsolved problem.

Results

We present an object-oriented module-based computational integration strategy to build a multiscale model of angiogenesis that links currently available models. As an example case, we use this approach to integrate modules representing microvascular blood flow, oxygen transport, vascular endothelial growth factor transport and endothelial cell behavior (sensing, migration and proliferation). Modeling methodologies in these modules include algebraic equations, partial differential equations and agent-based models with complex logical rules. We apply this integrated model to simulate exercise-induced angiogenesis in skeletal muscle. The simulation results compare capillary growth patterns between different exercise conditions for a single bout of exercise. Results demonstrate how the computational infrastructure can effectively integrate multiple modules by coordinating their connectivity and data exchange. Model parameterization offers simulation flexibility and a platform for performing sensitivity analysis.

Conclusions

This systems biology strategy can be applied to larger scale integration of computational models of angiogenesis in skeletal muscle, or other complex processes in other tissues under physiological and pathological conditions.

Regulation of exercise-induced fiber type transformation, mitochondrial biogenesis, and angiogenesis in skeletal muscle

Skeletal muscle exhibits superb plasticity in response to changes in functional demands. Chronic increases of skeletal muscle contractile activity, such as endurance exercise, lead to a variety of physiological and biochemical adaptations in skeletal muscle, including mitochondrial biogenesis, angiogenesis, and fiber type transformation. These adaptive changes are the basis for the improvement of physical performance and other health benefits. This review focuses on recent findings in genetically engineered animal models designed to elucidate the mechanisms and functions of various signal transduction pathways and gene expression programs in exercise-induced skeletal muscle adaptations.