Effects of treadmill training on microvascular remodeling in the rat after spinal cord injury

Exoskeleton-Assisted Rehabilitation and Neuroplasticity in Spinal Cord Injury

Spinal cord injury (SCI) results in neurological deficits below the level of injury, causing motor dysfunction and various severe multisystem complications. Rehabilitative training plays a crucial role in the recovery of individuals with SCI, and exoskeleton serves as an emerging and promising tool for rehabilitation, especially in promoting neuroplasticity and alleviating SCI-related complications. This article reviews the classifications and research progresses of medical exoskeletons designed for SCI patients and describes their performances in practical application separately. Meanwhile, we discuss their mechanisms for enhancing neuroplasticity and functional remodeling, as well as their palliative impacts on secondary complications. The potential trends in exoskeleton design are raised according to current progress and requirements on SCI rehabilitation.

Conserved mammalian muscle mechanics during eccentric contractions

Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80 years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.

Skeletal muscle adaptation to indirect electrical stimulation: divergence between microvascular and metabolic adaptations

NEW FINDINGS

What is the central question of this study? Can we manipulate muscle recruitment to differentially enhance skeletal muscle fatigue resistance? What is the main finding and its importance? Through manipulation of muscle activation patterns, it is possible to promote distinct microvascular growth. Enhancement of fatigue resistance is closely associated with the distribution of the capillaries within the muscle, not necessarily with quantity. Additionally, at the acute stages of remodelling in response to indirect electrical stimulation, the improvement in fatigue resistance appears to be primarily driven by vascular remodelling, with metabolic adaptation of secondary importance.

ABSTRACT

Exercise involves a complex interaction of factors influencing muscle performance, where variations in recruitment pattern (e.g., endurance vs. resistance training) may differentially modulate the local tissue environment (i.e., oxygenation, blood flow, fuel utilization). These exercise stimuli are potent drivers of vascular and metabolic change. However, their relative contribution to adaptive remodelling of skeletal muscle and subsequent performance is unclear. Using implantable devices, indirect electrical stimulation (ES) of locomotor muscles of rat at different pacing frequencies (4, 10 and 40 Hz) was used to differentially recruit hindlimb blood flow and modulate fuel utilization. After 7 days, ES promoted significant remodelling of microvascular composition, increasing capillary density in the cortex of the tibialis anterior by 73%, 110% and 55% for the 4 Hz, 10 and 40 Hz groups, respectively. Additionally, there was remodelling of the whole muscle metabolome, including significantly elevated amino acid turnover, with muscle kynurenic acid levels doubled by pacing at 10 Hz (P < 0.05). Interestingly, the fatigue index of skeletal muscle was only significantly elevated in 10 Hz (58% increase) and 40 Hz (73% increase) ES groups, apparently linked to improved capillary distribution. These data demonstrate that manipulation of muscle recruitment pattern may be used to differentially expand the capillary network prior to altering the metabolome, emphasising the importance of local capillary supply in promoting exercise tolerance.

Temporal and spatial cellular and molecular pathological alterations with single-cell resolution in the adult spinal cord after injury

Spinal cord injury (SCI) involves diverse injury responses in different cell types in a temporally and spatially specific manner. Here, using single-cell transcriptomic analyses combined with classic anatomical, behavioral, electrophysiological analyses, we report, with single-cell resolution, temporal molecular and cellular changes in crush-injured adult mouse spinal cord. Data revealed pathological changes of 12 different major cell types, three of which infiltrated into the spinal cord at distinct times post-injury. We discovered novel microglia and astrocyte subtypes in the uninjured spinal cord, and their dynamic conversions into additional stage-specific subtypes/states. Most dynamic changes occur at 3-days post-injury and by day-14 the second wave of microglial activation emerged, accompanied with changes in various cell types including neurons, indicative of the second round of attacks. By day-38, major cell types are still substantially deviated from uninjured states, demonstrating prolonged alterations. This study provides a comprehensive mapping of cellular/molecular pathological changes along the temporal axis after SCI, which may facilitate the development of novel therapeutic strategies, including those targeting microglia.

Impaired skeletal muscle fatigue resistance during cardiac hypertrophy is prevented by functional overload- or exercise-induced functional capillarity

KEY POINTS

Capillary rarefaction is hypothesized to contribute to impaired exercise tolerance in cardiovascular disease, but it remains a poorly exploited therapeutic target for improving skeletal muscle performance. Using an abdominal aortic coarctation rat model of compensatory cardiac hypertrophy, we determine the efficacy of aerobic exercise for the prevention of, and mechanical overload for, restoration of hindlimb muscle fatigue resistance and microvascular impairment in the early stages of heart disease. Impaired muscle fatigue resistance was found after development of cardiac hypertrophy, but this impairment was prevented by low-intensity aerobic exercise and recovered after mechanical stretch due to muscle overload. Changes in muscle fatigue resistance were closely related to functional (i.e. perfused) microvascular density, independent of arterial blood flow, emphasizing the critical importance of optimal capillary diffusion for skeletal muscle function. Pro-angiogenic therapies are an important tool for improving skeletal muscle function in the incipient stages of heart disease.

ABSTRACT

Microvascular rarefaction may contribute to declining skeletal muscle performance in cardiac and vascular diseases. It remains uncertain to what extent microvascular rarefaction occurs in the earliest stages of these conditions, if impaired blood flow is an aggravating factor and whether angiogenesis restores muscle performance. To investigate this, the effects of aerobic exercise (voluntary wheel running) and functional muscle overload on the performance, femoral blood flow (FBF) and microvascular perfusion of the extensor digitorum longus (EDL) were determined in a chronic rat model of compensatory cardiac hypertrophy (CCH, induced by surgically imposed abdominal aortic coarctation). CCH was associated with hypertension (P = 0.001 vs. Control) and increased relative heart mass (P < 0.001). Immediately upon placing the aortic band (i.e. before development of CCH), post-fatigue test FBF was reduced (P < 0.003), coinciding with attenuated fatigue resistance (P = 0.039) indicating an acute arterial perfusion constraint on muscle performance. While FBF was normalized during CCH in chronic groups (P > 0.05) fatigue resistance remained reduced (P = 0.039) and was associated with reduced (P = 0.009) functional capillarity after development of CCH without intervention, indicating a microvascular limitation to muscle performance. Normalization of functional capillarity after aerobic exercise (P = 0.065) and overload (P = 0.329) in CCH coincided with restoration to control levels of muscle fatigue resistance (P > 0.999), although overload-induced EDL hypertrophy (P = 0.027) and wheel-running velocity and duration (both P < 0.05) were attenuated after aortic banding. These data show that reductions in skeletal muscle performance during CCH can be countered by improving functional capillarity, providing a therapeutic target to improve skeletal muscle function in chronic diseases.

Revascularization After Traumatic Spinal Cord Injury

Traumatic spinal cord injury (SCI) is a complex pathological process. The initial mechanical damage is followed by a progressive secondary injury cascade. The injury ruptures the local microvasculature and disturbs blood-spinal cord barriers, exacerbating inflammation and tissue damage. Although endogenous angiogenesis is triggered, the new vessels are insufficient and often fail to function normally. Numerous blood vessel interventions, such as proangiogenic factor administration, gene modulation, cell transplantation, biomaterial implantation, and physical stimulation, have been applied as SCI treatments. Here, we briefly describe alterations and effects of the vascular system on local microenvironments after SCI. Therapies targeted at revascularization for SCI are also summarized.

Exercise-mitigated sex-based differences in aging: from genetic alterations to heart performance

The prevalence of cardiovascular diseases dramatically increases with age; therefore, striving to maintain a physiological heart function is particularly important. Our aim was to study the voluntary exercise-evoked cardioprotective effects in aged male and female rats, from genetic alterations to changes in heart performance. We divided 20-month-old female and male Wistar rats to control and running groups. After the 12-wk-long experimental period, echocardiographic measurements were performed. Afterwards, hearts were either removed for biochemical measurements or mounted into a Langendorff-perfusion system to detect infarct size. The following genes and their proteins were analyzed from heart: catechol-O-methyltransferase (Comt), endothelin-1 (Esm1), Purkinje cell protein-4 (Pcp4), and osteoglycin (Ogn). Recreational exercise caused functional improvements; however, changes were more prominent in males. Cardiac expression of Comt and Ogn was reduced as a result of exercise in aged males, whereas Pcp4 and Esm1 showed a marked overexpression, along with a markedly improved diastolic function. The key result of this study is that exercise enhanced the expression of the Pcp4 gene and protein, a recently described regulator of calcium balance in cardiomyocytes, and suppressed Comt and Ogn gene expression, which has been associated with impaired cardiac function. In addition, as a result of exercise, a significant improvement was observed in the size of infarct elicited by left anterior descending coronary artery occlusion. Our results clearly show that age and sex-dependent changes were both apparent in key proteins linked to cardiovascular physiology. Exercise-moderated fundamental genetic alterations may have contributed to the functional adaptation of the heart.NEW & NOTEWORTHY Voluntary exercise has proved to be an effective therapeutic tool to improve cardiac function in aged rats with clearly visible sex differences. Long-term exercise is associated with decreased Ogn and Comt expression and enhanced presence of Pcp4 and Esm1 genes. Sex-dependent changes were also observed in the expression of the cardiovascular key proteins. Fundamental alterations in gene and protein expression may contribute to the improvement of cardiac performance.

Distinct structural and functional angiogenic responses are induced by different mechanical stimuli

Objective

Adequacy of the microcirculation is essential for maintaining repetitive skeletal muscle function while avoiding fatigue. It is unclear, however, whether capillary remodelling after different angiogenic stimuli is comparable in terms of vessel distribution and consequent functional adaptations. We determined the physiological consequences of two distinct mechanotransductive stimuli: (1) overload‐mediated abluminal stretch (OV); (2) vasodilator‐induced shear stress (prazosin, PR).

Methods

In situ EDL fatigue resistance was determined after 7 or 14 days of intervention, in addition to measurements of femoral artery flow. Microvascular composition (muscle histology) and oxidative capacity (citrate synthase activity) were quantified, and muscle PO₂ calculated using advanced mathematical modelling.

Results

Compared to controls, capillary‐to‐fiber ratio was higher after OV14 (134%, p < .001) and PR14 (121%, p < .05), although fatigue resistance only improved after overload (7 days: 135%, 14 days: 125%, p < .05). In addition, muscle overload improved local capillary supply indices and reduced CS activity, while prazosin treatment failed to alter either index of aerobic capacity.

Conclusion

Targeted capillary growth in response to abluminal stretch is a potent driver of improved muscle fatigue resistance, while shear stress‐driven angiogenesis has no beneficial effect on muscle function. In terms of capillarity, more is not necessarily better.

Water treadmill training attenuates blood-spinal cord barrier disruption in rats by promoting angiogenesis and inhibiting matrix metalloproteinase-2/9 expression following spinal cord injury

Background

The permeability of the blood-spinal cord barrier (BSCB) is mainly determined by junction complexes between adjacent endothelial cells (ECs), including tight junctions (TJs) and adherens junctions (AJs), which can be severely damaged after spinal cord injury (SCI). Exercise training is a recognized method for the treatment of SCI. The destruction of the BSCB mediated by matrix metalloproteinases (MMPs) leads to inflammation, neurotoxin production, and neuronal apoptosis. The failure of new blood vessels to effectively regenerate is also an important cause of delayed recovery after SCI. For the first time, we introduced water treadmill training (TT) to help SCI rats successfully exercise and measured the effects of TT in promoting recovery after SCI and the possible mechanisms involved.

Methods

Sprague-Dawley (200–250 g) rats were randomly divided into the following three groups: sham operated, SCI, and SCI + TT. Animals were sacrificed at 7 or 14 days post-surgery. The degree of neurological deficit, tissue morphology and BSCB permeability were assessed by the Basso-Beattie-Bresnahan (BBB) motor function scale and appropriate staining protocols, and apoptosis, protein expression and vascular EC ultrastructure were assessed by TUNEL staining, Western blotting, immunofluorescence and transmission electron microscopy (TEM).

Results

Our experiments showed that TT reduced permeability of the BSCB and decreased structural tissue damage. TT significantly improved functional recovery when compared with that in the SCI group; TJ and AJ proteins expression increased significantly after TT, and training reduced apoptosis induced by SCI. TT could promote angiogenesis, and MMP-2 and MMP-9 expression was significantly inhibited by TT.

Conclusions

The results of this study indicate that TT promotes functional recovery for the following reasons: TT (1) protects residual BSCB structure from further damage, (2) promotes vascular regeneration, and (3) inhibits MMP-2/9 expression to mitigate BSCB damage.

Oxygen transport kinetics underpin rapid and robust diaphragm recovery following chronic spinal cord injury

Key points

Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied.

Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype.

Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity‐induced spinal reconnection.

Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties.

There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function.

Abstract

Months after spinal cord injury (SCI), respiratory deficits remain the primary cause of morbidity and mortality for patients. It is possible to induce partial respiratory motor functional recovery in chronic SCI following 2 weeks of spinal neuroplasticity. However, the peripheral mechanisms underpinning this recovery are largely unknown, limiting development of new clinical treatments with potential for complete functional restoration. Utilizing a rat hemisection model, diaphragm function and paralysis was assessed and recovered at chronic time points following trauma through chondroitinase ABC induced neuroplasticity. We simulated the diaphragm's in vivo cyclical length change and activity patterns using the work loop technique at the same time as assessing global and local measures of the muscles histology to quantify changes in muscle phenotype, microvascular composition, and oxidative capacity following injury and recovery. These data were fed into a physiologically informed model of tissue oxygen transport. We demonstrate that hemidiaphragm paralysis causes muscle fibre hypertrophy, maintaining global oxygen supply, although it alters isolated muscle kinetics, limiting respiratory function. Treatment induced recovery of respiratory activity normalized these effects, increasing oxygen supply, restoring optimal diaphragm functional properties. However, metabolic demands of the diaphragm were significantly reduced following both injury and recovery, potentially limiting restoration of normal muscle performance. The mechanism of rapid respiratory muscle recovery following spinal trauma occurs through oxygen transport, metabolic demand and functional dynamics of striated muscle. Overall, these data support a systems‐wide approach to the treatment of SCI, and identify new targets to mediate complete respiratory recovery.

Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied.

Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype.

Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity‐induced spinal reconnection.

Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties.

There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function.

Medial gastrocnemius muscles fatigue but do not atrophy in paralyzed cat hindlimb after long-term spinal cord hemisection and unilateral deafferentation

This study of medial gastrocnemius (MG) muscle and motor units (MUs) after spinal cord hemisection and deafferentation (HSDA) in adult cats, asked 1) whether the absence of muscle atrophy and unaltered contractile speed demonstrated previously in HSDA-paralyzed peroneus longus (PerL) muscles, was apparent in the unloaded HSDA-paralyzed MG muscle, and 2) how ankle unloading impacts MG muscle and MUs after dorsal root sparing (HSDA-SP) with foot placement during standing and locomotion. Chronic isometric contractile forces and speeds were maintained for up to 12 months in all conditions, but fatigability increased exponentially. MU recordings at 8-11½ months corroborated the unchanged muscle force and speed with significantly increased fatigability; normal weights of MG muscle confirmed the lack of disuse atrophy. Fast MUs transitioned from fatigue resistant and intermediate to fatigable accompanied by corresponding fiber type conversion to fast oxidative (FOG) and fast glycolytic (FG) accompanied by increased GAPDH enzyme activity in absolute terms and relative to oxidative citrate synthase enzyme activity. Myosin heavy chain composition, however, was unaffected. MG muscle behaved like the PerL muscle after HSDA with maintained muscle and MU contractile force and speed but with a dramatic increase in fatigability, irrespective of whether all the dorsal roots were transected. We conclude that reduced neuromuscular activity accounts for increased fatigability but is not, in of itself, sufficient to promote atrophy and slow to fast conversion. Position and relative movements of hindlimb muscles are likely contributors to sustained MG muscle and MU contractile force and speed after HSDA and HSDA-SP surgeries.