31P-NMR spectroscopy of skeletal muscle in Becker dystrophy and DMD/BMD carriers. Altered rate of phosphate transport

Longitudinal metabolomic analysis of plasma enables modeling disease progression in Duchenne muscular dystrophy mouse models

Duchenne muscular dystrophy is a severe pediatric neuromuscular disorder caused by the lack of dystrophin. Identification of biomarkers is needed to support and accelerate drug development. Alterations of metabolites levels in muscle and plasma have been reported in pre-clinical and clinical cross-sectional comparisons. We present here a 7-month longitudinal study comparing plasma metabolomic data in wild-type and mdx mice. A mass spectrometry approach was used to study metabolites in up to five time points per mouse at 6, 12, 18, 24 and 30 weeks of age, providing an unprecedented in depth view of disease trajectories. A total of 106 metabolites were studied. We report a signature of 31 metabolites able to discriminate between healthy and disease at various stages of the disease, covering the acute phase of muscle degeneration and regeneration up to the deteriorating phase. We show how metabolites related to energy production and chachexia (e.g. glutamine) are affected in mdx mice plasma over time. We further show how the signature is connected to molecular targets of nutraceuticals and pharmaceutical compounds currently in development as well as to the nitric oxide synthase pathway (e.g. arginine and citrulline). Finally, we evaluate the signature in a second longitudinal study in three independent mouse models carrying 0, 1 or 2 functional copies of the dystrophin paralog utrophin. In conclusion, we report an in-depth metabolomic signature covering previously identified associations and new associations, which enables drug developers to peripherally assess the effect of drugs on the metabolic status of dystrophic mice.

Multi-parametric MR in Becker muscular dystrophy patients

Quantitative MRI and MRS of muscle are increasingly being used to measure individual pathophysiological processes in Becker muscular dystrophy (BMD). In particular, muscle fat fraction was shown to be highly associated with functional tests in BMD. However, the muscle strength per unit of contractile cross-sectional area is lower in patients with BMD compared with healthy controls. This suggests that the quality of the non-fat-replaced (NFR) muscle tissue is lower than in healthy controls. Consequently, a measure that reflects changes in muscle tissue itself is needed. Here, we explore the potential of water T₂ relaxation times, diffusion parameters and phosphorus metabolic indices as early disease markers in patients with BMD. For this purpose, we examined these measures in fat-replaced (FR) and NFR lower leg muscles in patients with BMD and compared these values with those in healthy controls. Quantitative proton MRI (three-point Dixon, multi-spin-echo and diffusion-weighted spin-echo echo planar imaging) and 2D chemical shift imaging ³¹ P MRS data were acquired in 24 patients with BMD (age 18.8-66.2 years) and 13 healthy controls (age 21.3-63.6 years). Muscle fat fractions, phosphorus metabolic indices, and averages and standard deviations (SDs) of the water T₂ relaxation times and diffusion tensor imaging (DTI) parameters were assessed in six individual leg muscles. Phosphodiester levels were increased in the NFR and FR tibialis anterior, FR peroneus and FR gastrocnemius lateralis muscles. No clear pattern was visible for the other metabolic indices. Increased T₂ SD was found in the majority of FR muscles compared with NFR and healthy control muscles. No differences in average water T₂ relaxation times or DTI indices were found between groups. Overall, our results indicate that primarily muscles that are further along in the disease process showed increases in T₂ heterogeneity and changes in some metabolic indices. No clear differences were found for the DTI indices between groups.

Free intramuscular Mg2+ concentration calculated using both 31 P and 1 H NMRS-based pH in the skeletal muscle of Duchenne muscular dystrophy patients

Early studies have demonstrated that (total) magnesium was decreased in skeletal muscle of Duchenne muscular dystrophy (DMD) patients. Free intramuscular Mg²⁺ can be derived from ³¹ P NMRS measurements. The value of free intramuscular magnesium concentration ([Mg²⁺ ]) is highly dependent on precise knowledge of intracellular pH, which is abnormally alkaline in dystrophic muscle, possibly due to an expanded interstitial space, potentially causing an underestimation of [Mg²⁺ ]. We have recently shown that intracellular pH can be derived using ¹ H NMRS of carnosine. Our aim was to determine whether ³¹ P NMRS-based [Mg²⁺ ] is, in fact, abnormally low in DMD patients, taking advantage of the ¹ H NMRS-based pH. A comparative analysis was, therefore, made between [Mg²⁺ ] values calculated with both ¹ H and ³¹ P NMRS-based approaches to determine pH in 25 DMD patients, on a 3-T clinical NMR scanner. [Mg²⁺ ] was also assessed with ³¹ P NMRS only in (forearm or leg) skeletal muscle of 60 DMD patients and 63 age-matched controls. Additionally, phosphodiester levels as well as quantitative NMRI indices including water T₂ , fat fraction, contractile cross-sectional area and one-year changes were evaluated. The main finding was that the significant difference in [Mg²⁺ ] between DMD patients and controls was preserved even when the intracellular pH determined with ¹ H NMRS was similar in both groups. Consequently, we observed that [Mg²⁺ ] is significantly lower in DMD patients compared with controls in the larger database where only ³¹ P NMRS data were obtained. Significant yet weak correlations existed between [Mg²⁺ ] and PDE, water T₂ and fat fraction. We concluded that low [Mg²⁺ ] is an actual finding in DMD, whether intracellular pH is normal or alkaline, and that it is a likely consequence of membrane leakiness. The response of Mg²⁺ to therapeutic treatment remains to be investigated in neuromuscular disorders. Free [Mg²⁺ ] determination with ³¹ P NMRS is highly dependent on a precise knowledge of intracellular pH. The pH of Duchenne muscular dystrophy (DMD) patients, as determined by ³¹ P NMRS, is abnormally alkaline. We have recently shown that intracellular pH could be determined using ¹ H NMRS of carnosine, and that intracellular pH was alkaline in a proportion of, but not all, DMD patients with a ³¹ P NMRS-based alkaline pH. Taking advantage of this ¹ H NMRS-based intracellular pH, we found that free intramuscular [Mg²⁺ ] is in fact abnormally low in DMD patients.

Advances in Quantitative Imaging of Genetic and Acquired Myopathies: Clinical Applications and Perspectives

In the last years, magnetic resonance imaging (MRI) has become fundamental for the diagnosis and monitoring of myopathies given its ability to show the severity and distribution of pathology, to identify specific patterns of damage distribution and to properly interpret a number of genetic variants. The advances in MR techniques and post-processing software solutions have greatly expanded the potential to assess pathological changes in muscle diseases, and more specifically of myopathies; a number of features can be studied and quantified, ranging from composition, architecture, mechanical properties, perfusion, and function, leading to what is known as quantitative MRI (qMRI). Such techniques can effectively provide a variety of information beyond what can be seen and assessed by conventional MR imaging; their development and application in clinical practice can play an important role in the diagnostic process and in assessing disease course and treatment response. In this review, we briefly discuss the current role of muscle MRI in diagnosing muscle diseases and describe in detail the potential and perspectives of the application of advanced qMRI techniques in this field.

Skeletal Muscle Quantitative Nuclear Magnetic Resonance Imaging and Spectroscopy as an Outcome Measure for Clinical Trials

Recent years have seen tremendous progress towards therapy of many previously incurable neuromuscular diseases. This new context has acted as a driving force for the development of novel non-invasive outcome measures. These can be organized in three main categories: functional tools, fluid biomarkers and imagery. In the latest category, nuclear magnetic resonance imaging (NMRI) offers a considerable range of possibilities for the characterization of skeletal muscle composition, function and metabolism. Nowadays, three NMR outcome measures are frequently integrated in clinical research protocols. They are: 1/ the muscle cross sectional area or volume, 2/ the percentage of intramuscular fat and 3/ the muscle water T2, which quantity muscle trophicity, chronic fatty degenerative changes and oedema (or more broadly, “disease activity”), respectively. A fourth biomarker, the contractile tissue volume is easily derived from the first two ones. The fat fraction maps most often acquired with Dixon sequences have proven their capability to detect small changes in muscle composition and have repeatedly shown superior sensitivity over standard functional evaluation. This outcome measure will more than likely be the first of the series to be validated as an endpoint by regulatory agencies. The versatility of contrast generated by NMR has opened many additional possibilities for characterization of the skeletal muscle and will result in the proposal of more NMR biomarkers. Ultra-short TE (UTE) sequences, late gadolinium enhancement and NMR elastography are being investigated as candidates to evaluate skeletal muscle interstitial fibrosis. Many options exist to measure muscle perfusion and oxygenation by NMR. Diffusion NMR as well as texture analysis algorithms could generate complementary information on muscle organization at microscopic and mesoscopic scales, respectively. ³¹P NMR spectroscopy is the reference technique to assess muscle energetics non-invasively during and after exercise. In dystrophic muscle, ³¹P NMR spectrum at rest is profoundly perturbed, and several resonances inform on cell membrane integrity. Considerable efforts are being directed towards acceleration of image acquisitions using a variety of approaches, from the extraction of fat content and water T2 maps from one single acquisition to partial matrices acquisition schemes. Spectacular decreases in examination time are expected in the near future. They will reinforce the attractiveness of NMR outcome measures and will further facilitate their integration in clinical research trials.

Elevated phosphodiester and T2 levels can be measured in the absence of fat infiltration in Duchenne muscular dystrophy patients

Quantitative MRI and MRS are increasingly important as non-invasive outcome measures in therapy development for Duchenne muscular dystrophy (DMD). Many studies have focussed on individual measures such as fat fraction and metabolite levels in relation to age and functionality, but much less attention has been given to how these indices relate to each other. Here, we assessed spatially resolved metabolic changes in leg muscles of DMD patients, and classified muscles according to the degree of fat replacement compared with healthy controls. Quantitative MRI (three-point Dixon and multi-spin echo without fat suppression and a tri-exponential fit) and 2D-CSI ³¹ P MRS scans were obtained from 18 DMD patients and 12 healthy controls using a 3 T and a 7 T MR scanner. Metabolite levels, T₂ values and fat fraction were individually assessed for five lower leg muscles. In muscles with extensive fat replacement, phosphodiester over adenosine triphosphate (PDE/ATP), inorganic phosphate over phosphocreatine, intracellular tissue pH and T₂ were significantly increased compared with healthy controls. In contrast, in muscles without extensive fat replacement, only PDE/ATP and T₂ values were significantly elevated. Overall, our results show that PDE levels and T₂ values increase prior to the occurrence of fat replacement and remain elevated in later stages of the disease. This suggests that these individual measures could not only function as early markers for muscle damage but also reflect potentially reversible pathology in the more advanced stages.

Mr Spectroscopy in Clinical Research

MR spectroscopy (MRS) offers unique possibilities for non-invasive evaluation of biochemistry in vivo. During recent years there has been a growing body of evidence from clinical research studies on human beings using 31P and 1H MRS. The results indicate that it is possible to evaluate phosphorous energy metabolism, loss of neurones, and lactate production in a large number of brain diseases. Furthermore, 31P and 1H MRS may be particularly clinically useful in evaluation of various disorders in skeletal muscle. In the heart 31P MRS seems at the moment the most suitable for evaluation of global affections of the myocardium. In the liver 31P MRS appears to be rather insensitive and non-specific, but absolute quantification of metabolite concentrations and using metabolic “stress models” may prove useful in the future. The clinical role of MRS in oncology is still unclear, but it may be useful for noninvasive follow-up of treatment.

Taken together, the evidence obtained so far certainly shows some trends for clinical applications of MRS. Methods are now available for the clinical research necessary for establishing routine clinical MRS examinations.

Longitudinal functional and NMR assessment of upper limbs in Duchenne muscular dystrophy

OBJECTIVE

To explore the value of nuclear magnetic resonance (NMR) and functional assessments for follow-up of ambulatory and nonambulatory patients with Duchenne muscular dystrophy (DMD).

METHODS

Twenty-five 53-skippable patients with DMD were included in this study; 15 were nonambulatory at baseline. All patients underwent clinical and functional assessments every 6 months using the Motor Function Measure (MFM), hand grip and key pinch strength, MoviPlate, and NMR spectroscopy and imaging studies.

RESULTS

Upper limb distal strength decreased in nonambulatory patients over the period of 1 year; ambulatory patients showed improvement during the same period. The same applied for several NMRS indices, such as phosphocreatine/adenosine triphosphate, which decreased in older patients but increased in younger ambulatory patients. Fat infiltration in the upper limbs increased linearly with age. Almost all NMR and functional assessment results correlated.

CONCLUSIONS

Our results underscore complementarity of functional and NMR assessments in patients with DMD. Sensitivity to change of various indices may differ according to disease stage.

Revisiting the dystrophin-ATP connection: How half a century of research still implicates mitochondrial dysfunction in Duchenne Muscular Dystrophy aetiology

Duchenne Muscular Dystrophy (DMD) is a fatal neuromuscular disease that is characterised by dystrophin-deficiency and chronic Ca(2+)-induced skeletal muscle wasting, which currently has no cure. DMD was once considered predominantly as a metabolic disease due to the myriad of metabolic insufficiencies evident in the musculature, however this aspect of the disease has been extensively ignored since the discovery of dystrophin. The collective historical and contemporary literature documenting these metabolic nuances has culminated in a series of studies that importantly demonstrate that metabolic dysfunction exists independent of dystrophin expression and a mild disease phenotype can be expressed even in the complete absence of dystrophin expression. Targeting and supporting metabolic pathways with anaplerotic and other energy-enhancing supplements has also shown therapeutic value. We explore the hypothesis that DMD is characterised by a systemic mitochondrial impairment that is central to disease aetiology rather than a secondary pathophysiological consequence of dystrophin-deficiency.

Quantitative NMRI and NMRS identify augmented disease progression after loss of ambulation in forearms of boys with Duchenne muscular dystrophy

Quantitative NMRI and (31)P NMRS indices are reported in the forearms of 24 patients with Duchenne muscular dystrophy (DMD) (6-18 years, 14 non-ambulant) amenable to exon 53 skipping therapy and in 12 age-matched male controls (CONT). Examinations carried out at 3 T comprised multi-slice 17-echo measurements of muscle water T2 and heterogeneity, three-point Dixon imaging of fat fraction in flexor and extensor muscles (FLEX, EXT), and non-localised spectroscopy of phosphate metabolites. We studied four imaging indices, eight metabolic ratios combining ATP, phosphocreatine, phosphomonoesters and phosphodiesters, the cytosolic inorganic phosphate (Pia ) and an alkaline (Pib) pool present in dystrophic muscle, and average pH. All indices differed between DMD and CONT, except for muscle water T2 . Measurements were outside the 95th percentile of age-matched CONT values in over 65% of cases for percentage fat signal (%F), and in 78-100% of cases for all spectroscopic indices. T2 was elevated in one-third of FLEX measurements, whereas %pixels > 39 ms and T2 heterogeneity were abnormal in one-half of the examinations. The FLEX muscles had higher fat infiltration and T2 than EXT muscle groups. All indices, except pH, correlated with patient age, although the correlation was negative for T2 . However, in non-ambulant patients, the correlation with years since loss of ambulation was stronger than the correlation with age, and the slope of evolution per year was steeper after loss of ambulation. All indices except Pi/gATP differed between ambulant and non-ambulant patients; however, T2 and %pixels > 39 ms were highest in ambulant patients, possibly owing to the greater extent of inflammatory processes earlier in the disease. All other indices were worse in non-ambulant subjects. Quantitative measurements obtained from patients at different disease stages covered a broad range of abnormalities that evolved with the disease, and metabolic indices were up to 10-fold above normal from the onset, thus establishing a variety of potential markers for future therapy.

Defects in mitochondrial ATP synthesis in dystrophin-deficient mdx skeletal muscles may be caused by complex I insufficiency

Duchenne Muscular Dystrophy is a chronic, progressive and ultimately fatal skeletal muscle wasting disease characterised by sarcolemmal fragility and intracellular Ca2+ dysregulation secondary to the absence of dystrophin. Mounting literature also suggests that the dysfunction of key energy systems within the muscle may contribute to pathological muscle wasting by reducing ATP availability to Ca2+ regulation and fibre regeneration. No study to date has biochemically quantified and contrasted mitochondrial ATP production capacity by dystrophic mitochondria isolated from their pathophysiological environment such to determine whether mitochondria are indeed capable of meeting this heightened cellular ATP demand, or examined the effects of an increasing extramitochondrial Ca2+ environment. Using isolated mitochondria from the diaphragm and tibialis anterior of 12 week-old dystrophin-deficient mdx and healthy control mice (C57BL10/ScSn) we have demonstrated severely depressed Complex I-mediated mitochondrial ATP production rate in mdx mitochondria that occurs irrespective of the macronutrient-derivative substrate combination fed into the Kreb's cycle, and, which is partially, but significantly, ameliorated by inhibition of Complex I with rotenone and stimulation of Complex II-mediated ATP-production with succinate. There was no difference in the MAPR response of mdx mitochondria to increasing extramitochondrial Ca2+ load in comparison to controls, and 400 nM extramitochondrial Ca2+ was generally shown to be inhibitory to MAPR in both groups. Our data suggests that DMD pathology is exacerbated by a Complex I deficiency, which may contribute in part to the severe reductions in ATP production previously observed in dystrophic skeletal muscle.

Muscle MRS detects elevated PDE/ATP ratios prior to fatty infiltration in Becker muscular dystrophy

Becker muscular dystrophy (BMD) is characterized by progressive muscle weakness. Muscles show structural changes (fatty infiltration, fibrosis) and metabolic changes, both of which can be assessed using MRI and MRS. It is unknown at what stage of the disease process metabolic changes arise and how this might vary for different metabolites. In this study we assessed metabolic changes in skeletal muscles of Becker patients, both with and without fatty infiltration, quantified via Dixon MRI and (31) P MRS. MRI and (31) P MRS scans were obtained from 25 Becker patients and 14 healthy controls using a 7 T MR scanner. Five lower-leg muscles were individually assessed for fat and muscle metabolite levels. In the peroneus, soleus and anterior tibialis muscles with non-increased fat levels, PDE/ATP ratios were higher (P < 0.02) compared with controls, whereas in all muscles with increased fat levels PDE/ATP ratios were higher compared with healthy controls (P ≤ 0.05). The Pi /ATP ratio in the peroneus muscles was higher in muscles with increased fat fractions (P = 0.005), and the PCr/ATP ratio was lower in the anterior tibialis muscles with increased fat fractions (P = 0.005). There were no other significant changes in metabolites, but an increase in tissue pH was found in all muscles of the total group of BMD patients in comparison with healthy controls (P < 0.05). These findings suggest that (31) P MRS can be used to detect early changes in individual muscles of BMD patients, which are present before the onset of fatty infiltration.

Distinct disease phases in muscles of facioscapulohumeral dystrophy patients identified by MR detected fat infiltration

Facioscapulohumeral muscular dystrophy (FSHD) is an untreatable disease, characterized by asymmetric progressive weakness of skeletal muscle with fatty infiltration. Although the main genetic defect has been uncovered, the downstream mechanisms causing FSHD are not understood. The objective of this study was to determine natural disease state and progression in muscles of FSHD patients and to establish diagnostic biomarkers by quantitative MRI of fat infiltration and phosphorylated metabolites. MRI was performed at 3T with dedicated coils on legs of 41 patients (28 men/13 women, age 34-76 years), of which eleven were re-examined after four months of usual care. Muscular fat fraction was determined with multi spin-echo and T1 weighted MRI, edema by TIRM and phosphorylated metabolites by 3D (31)P MR spectroscopic imaging. Fat fractions were compared to clinical severity, muscle force, age, edema and phosphocreatine (PCr)/ATP. Longitudinal intramuscular fat fraction variation was analyzed by linear regression. Increased intramuscular fat correlated with age (p<0.05), FSHD severity score (p<0.0001), inversely with muscle strength (p<0.0001), and also occurred sub-clinically. Muscles were nearly dichotomously divided in those with high and with low fat fraction, with only 13% having an intermediate fat fraction. The intramuscular fat fraction along the muscle's length, increased from proximal to distal. This fat gradient was the steepest for intermediate fat infiltrated muscles (0.07±0.01/cm, p<0.001). Leg muscles in this intermediate phase showed a decreased PCr/ATP (p<0.05) and the fastest increase in fatty infiltration over time (0.18±0.15/year, p<0.001), which correlated with initial edema (p<0.01), if present. Thus, in the MR assessment of fat infiltration as biomarker for diseased muscles, the intramuscular fat distribution needs to be taken into account. Our results indicate that healthy individual leg muscles become diseased by entering a progressive phase with distal fat infiltration and altered energy metabolite levels. Fat replacement then relatively rapidly spreads over the whole muscle.

Magnetic resonance imaging of pediatric muscular disorders: recent advances and clinical applications

This review describes various quantitative magnetic resonance imaging techniques that can be used to objectively analyze the composition (T2 relaxation time mapping, Dixon imaging, and diffusion-weighted imaging), architecture (diffusion tensor imaging), mechanical properties (magnetic resonance elastography), and function (magnetic resonance spectroscopy) of normal and pathologic skeletal muscle in the pediatric population.

Splitting of Pi and other ³¹P NMR anomalies of skeletal muscle metabolites in canine muscular dystrophy

Many anomalies exist in the resting (31) P muscle spectra of boys with Duchenne muscular dystrophy (DMD) but few have been reported in Golden Retriever muscular dystrophy (GRMD), the closest existing animal model for DMD. Because GRMD is recommended for preclinical evaluation of therapies and quantitative outcome measures are needed, we investigated anomalies of (31) P NMRS in tibial cranial and biceps femoris muscles from 14 GRMD compared to 9 control (CONT) dogs. Alterations observed in DMD children - low phosphocreatine and high phospho-monoesters and -diesters - were all found in GRMD but increased pH was not. More surprisingly, inorganic phosphate (Pi) appeared to present a prominent splitting with an enhanced Pi(b) resonance at 0.3 ppm downfield of Pi(a) . Assuming that both resonances are Pi, the pH for Pi(a) in GRMD corresponded to a physiological intracellular pH(a) (6.97 ± 0.05), while pH(b) approached the extracellular range (7.27 ± 0.10) and correlated with pH(a) in GRMD (R(2)  = 0.65). Both Pi(a) and Pi(b) were elevated compared to CONT and Pi(a) increased with age for GRMD (R(2)  = 0.48, p < 0.001). Magnetisation transfer experiments between γATP and Pi were conducted to better characterise Pi pools. Equal T1 relaxation times for Pi(b) and Pi(a) did not support a mitochondrial origin of Pi(b) . We suggest that Pi(b) could originate from degenerating hypercontracted cells that have a leaky membrane and inadequate cell homeostasis and pH regulation. Pi(b) showed minimal chemical exchange in all dogs, while the exchange rate of Pi(a) was reduced in GRMD and might extraneously reflect low glycolytic activity in DMD. Taken together, the ensemble of (31) P NMRS alterations identifies muscle dysfunction and could provide useful biomarkers of therapeutic efficacy. Furthermore, among these, two might relate more specifically to dystrophic processes and merit further investigation: one is the existence of the enhanced alkaline Pi(b) pool; the other, mechanisms by which membrane disruption might increase phosphodiesters in dystrophy.

Muscle metabolic alterations assessed by 31-phosphorus magnetic resonance spectroscopy in mild Becker muscular dystrophy

Although the molecular defect causing Becker muscular dystrophy (BMD) has been identified, the biochemical mechanisms that lead to muscle necrosis remain unclear. Exercise-related muscle metabolism in 9 mildly affected BMD patients was assessed by muscle 31-phosphorus magnetic resonance spectroscopy ((31)P MRS) during an incremental workload. Compared with normal controls, BMD patients showed deregulation of resting pH and intramuscular membrane breakdown. We also observed increased reliance upon anaerobic metabolism during sustained submaximal contraction and maintenance of oxidative function during recovery.

Only fat infiltrated muscles in resting lower leg of FSHD patients show disturbed energy metabolism

Facioscapulohumeral muscular dystrophy (FSHD) is characterized by asymmetric dysfunctioning of individual muscles. Currently, it is unknown why specific muscles are affected before others and more particularly what pathophysiology is causing this differential progression. The aim of our study was to use a combination of (31)P magnetic resonance spectroscopic imaging (MRSI) and T1-weighted MRI to uncover metabolic differences in fat infiltrated and not fat infiltrated muscles in patients with FSHD. T1-weighted images and 3D (31)P MRSI were obtained from the calf muscles of nine patients with diagnosed FSHD and nine healthy age and sex matched volunteers. Muscles of patients were classified as fat infiltrated (PFM) and non fat-infiltrated (PNM) based on visual assessment of the MR images. Ratios of phosphocreatine (PCr), phosphodiesters (PDE) and inorganic phosphate (Pi) over ATP and tissue pH were compared between PFM and PNM and the same muscles in healthy volunteers. Of all patients, seven showed moderate to severe fatty infiltration in one or more muscles. In these muscles, decreases in PCr/ATP and increases in tissue pH were observed compared to the same muscles in healthy volunteers. Interestingly, these differences were absent in the PNM group. Our data show that differences in metabolite ratios and tissue pH in skeletal muscle between healthy volunteers and patients with FSHD appear to be specific for fat infiltrated muscles. Normal appearing muscles on T1 weighted images of patients showed normal phosphoryl metabolism, which suggests that in FSHD disease progression is truly muscle specific.

[Non-invasive investigation of muscle function using 31P magnetic resonance spectroscopy and 1H MR imaging]

31P MRS and 1H MRI of skeletal muscle have become major new tools allowing a complete non invasive investigation of muscle function both in the clinical setting and in basic research. The comparative analysis of normal and diseased muscle remains a major requirement to further define metabolic events surrounding muscle contraction and the metabolic anomalies underlying pathologies. Also, standardized rest-exercise-recovery protocols for the exploration of muscle metabolism by P-31 MRS in healthy volunteers as well as in patients with intolerance to exercise have been developed. The CRMBM protocol is based on a short-term intense exercise, which is very informative and well accepted by volunteers and patients. Invariant metabolic parameters have been defined to characterize the normal metabolic response to the protocol. Deviations from normality can be directly interpreted in terms of specific pathologies in some favorable cases. This protocol has been applied to more than 4,000 patients and healthy volunteers over a period of 15 years. On the other hand, MRI investigations provide anatomical and functional information from resting and exercising muscle. From a diagnostic point of view, dedicated pulse sequences can be used in order to detect and quantify muscle inflammation, fatty replacement, muscle hyper and hypotrophy. In most cases, MR techniques provide valuable information which has to be processed in conjunction with traditional invasive biochemical, electrophysiological and histoenzymological tests. P-31 MRS has proved particularly useful in the therapeutic follow-up of palliative therapies (coenzyme Q treatment of mitochondriopathies) and in family investigations. It is now an accepted diagnostic tool in the array of tests which are used to characterize muscle disorders in clinical routine. As a research tool, it will keep bringing new information on the physiopathology of muscle diseases in animal models and in humans and should play a role in the metabolic characterization of gene and cell therapy.

Biochemical characterization of muscle tissue of limb girdle muscular dystrophy: an 1H and 13C NMR study

The metabolic differences between the muscle biopsies of patients with limb girdle muscular dystrophy (LGMD) and normal controls were characterized using high-resolution 1H and 13C NMR spectroscopy. In all, 44 metabolites were unambiguously assigned in the perchloric acid extracts of skeletal muscle tissue, using 2D double quantum filtered (DQF COSY), total correlation (TOCSY), and 1H/13C heteronuclear multiple quantum coherence (HMQC) spectroscopy. The concentrations of glycolytic substrate, glucose (p=0.03), gluconeogenic amino acids, glutamine (p=0.02) and alanine (p=0.009) together with glycolytic product, lactate (p=0.04), were found to be significantly lowered in LGMD patients as compared with controls. The reduction in the concentration of glucose may be attributed to the decrease in the concentration of gluconeogenic amino acids in the degenerated muscle. Reduction in the rate of anaerobic glycolysis and lowered substrate concentration appear to be the possible reasons for the decrease in the concentration of lactate. A significant reduction in the concentration of choline in LGMD patients was also observed compared with controls. Lower concentration of choline may be the result of decreased rate of membrane turnover in LGMD patients. The data presented here provide an insight into the potentials of in-vitro NMR spectroscopy in the study of muscle metabolism.

Skeletal muscle metabolism in Duchenne muscular dystrophy (DMD): an in-vitro proton NMR spectroscopy study

The metabolic differences in the skeletal muscle of patients with Duchenne muscular dystrophy (DMD) and normal subjects (controls) were investigated using in-vitro high-resolution proton NMR spectroscopy. In all, 56 metabolites were unambiguously identified in the perchloric acid extract of muscle tissue using one- and two-dimensional NMR. The concentrations of glycolytic substrate glucose (Glc; p < 0.05), gluconeogenic amino acids such as glutamine (Gln; p < 0.05) and alanine (Ala; p < 0.05) and the glycolytic product lactate (Lac; p < 0.05) were statistically significantly lower in DMD patients as compared to controls. A significant reduction in the concentrations of total creatine (TCr; p < 0.05), glycerophosphoryl choline + phosphoryl choline + carnitine (GPC/PC/Car; p < 0.05), choline (Cho; p < 0.05) and acetate (Ace; p < 0.05) was also observed in these patients. Decrease in the level of glucose may be attributed to the reduction in the concentrations of gluconeogenic substrates or membrane abnormalities in degenerated muscle of DMD patients. Lower levels of choline containing compounds indicate membrane abnormalities. Decrease in the concentration of lactate in the muscle of DMD patients may be due to the reduction in anaerobic glycolytic activity or lower substrate concentration. The decrease in the concentration of acetate may reflect reduced transport of fatty acids into mitochondria due to decreased concentration of carnitine in DMD patients. Kreb's cycle intermediate alpha-ketoglutarate was observed only in the diseased muscle, which is suggestive of predominant oxidative metabolism for energy generation.

Global/temporal gene expression in diaphragm and hindlimb muscles of dystrophin-deficient (mdx) mice

The mdx mouse is a model for human Duchenne muscular dystrophy (DMD), an X-linked degenerative disease of skeletal muscle tissue characterized by the absence of the dystrophin protein. The mdx mice display a much milder phenotype than DMD patients. After the first week of life when all mdx muscles evolve like muscles of young DMD patients, mdx hindlimb muscles substantially compensate for the lack of dystrophin, whereas mdx diaphragm muscle becomes progressively affected by the disease. We used cDNA microarrays to compare the expression profile of 1,082 genes, previously selected by a subtractive method, in control and mdx hindlimb and diaphragm muscles at 12 time points over the first year of the mouse life. We determined that 1) the dystrophin gene defect induced marked expression remodeling of 112 genes encoding proteins implicated in diverse muscle cell functions and 2) two-thirds of the observed transcriptomal anomalies differed between adult mdx hindlimb and diaphragm muscles. Our results showed that neither mdx diaphram muscle nor mdx hindlimb muscles evolve entirely like the human DMD muscles. This finding should be taken under consideration for the interpretation of future experiments using mdx mice as a model for therapeutic assays.

Expression profiling in the muscular dystrophies: identification of novel aspects of molecular pathophysiology

We used expression profiling to define the pathophysiological cascades involved in the progression of two muscular dystrophies with known primary biochemical defects, dystrophin deficiency (Duchenne muscular dystrophy) and alpha-sarcoglycan deficiency (a dystrophin-associated protein). We employed a novel protocol for expression profiling in human tissues using mixed samples of multiple patients and iterative comparisons of duplicate datasets. We found evidence for both incomplete differentiation of patient muscle, and for dedifferentiation of myofibers to alternative lineages with advancing age. One developmentally regulated gene characterized in detail, alpha-cardiac actin, showed abnormal persistent expression after birth in 60% of Duchenne dystrophy myofibers. The majority of myofibers ( approximately 80%) remained strongly positive for this protein throughout the course of the disease. Other developmentally regulated genes that showed widespread overexpression in these muscular dystrophies included embryonic myosin heavy chain, versican, acetylcholine receptor alpha-1, secreted protein, acidic and rich in cysteine/osteonectin, and thrombospondin 4. We hypothesize that the abnormal Ca(2)+ influx in dystrophin- and alpha-sarcoglycan-deficient myofibers leads to altered developmental programming of developing and regenerating myofibers. The finding of upregulation of HLA-DR and factor XIIIa led to the novel identification of activated dendritic cell infiltration in dystrophic muscle; these cells mediate immune responses and likely induce microenvironmental changes in muscle. We also document a general metabolic crisis in dystrophic muscle, with large scale downregulation of nuclear-encoded mitochondrial gene expression. Finally, our expression profiling results show that primary genetic defects can be identified by a reduction in the corresponding RNA.

Insights into muscle diseases gained by phosphorus magnetic resonance spectroscopy

Phosphorus magnetic resonance spectroscopy (P-MRS) has now been used in the investigation of muscle energy metabolism in health and disease for over 15 years. The present review describes the basics of the metabolic observations made by P-MRS including the assumptions and problems associated with the use of this technique. Extramuscular factors, which may affect the P-MRS results, are detailed. The important P-MRS observations in patients with mitochondrial myopathies, including the monitoring of experimental therapies, are emphasized. The findings in other metabolic myopathies (those associated with glycolytic defects or endocrine disturbances) and in the destructive myopathies (the dystrophies and the inflammatory myopathies) are also described. Observations made in normal and abnormal fatigue, fibromyalgia, and malignant hyperthermia are considered. Finally, a summary of the possible diagnostic use of P-MRS in exercise intolerance is provided.

Reduced cytosolic acidification during exercise suggests defective glycolytic activity in skeletal muscle of patients with Becker muscular dystrophy. An in vivo 31P magnetic resonance spectroscopy study

Becker muscular dystrophy is an X-linked disorder due to mutations in the dystrophin gene, resulting in reduced size and/or content of dystrophin. The functional role of this subsarcolemma protein and the biochemical mechanisms leading to muscle necrosis in Becker muscular dystrophy are still unknown. In particular, the role of a bioenergetic deficit is still controversial. In this study, we used 31p magnetic resonance spectroscopy (31p-MRS) to investigate skeletal muscle mitochondrial and glycolytic ATP production in vivo in 14 Becker muscular dystrophy patients. Skeletal muscle glycogenolytic ATP production, measured during the first minute of exercise, was similar in patients and controls. On the other hand, during later phases of exercise, skeletal muscle in Becker muscular dystrophy patients was less acidic than in controls, the cytosolic pH at the end of exercise being significantly higher in Becker muscular dystrophy patients. The rate of proton efflux from muscle fibres of Becker muscular dystrophy patients was similar to that of controls, pointing to a deficit in glycolytic lactate production as a cause of higher end-exercise cytosolic pH in patients. The maximum rate of mitochondrial ATP production was similar in muscle of Becker muscular dystrophy patients and controls. The results of this in vivo 31P-MRS study are consistent with reduced glucose availability in dystrophin-deficient muscles.

Aspects of human bioenergetics as studied in vivo by magnetic resonance spectroscopy

We outline the relevant capabilities of in vivo phosphorus MR spectroscopy by discussing some aspects of normal human biochemistry as studied by this technique. The transport of inorganic phosphate from cytosol into mitochondria in the human skeletal muscle was studied by exploiting a new experimental protocol. We found that Pi was transported into mitochondria in the absence of ATP biosynthesis and in the presence of a pH gradient. The control of CoQ on the efficiency of oxidative phosphorylation in the skeletal muscle and brain was studied by administering CoQ to patients with mitochondrial cytopathies due to known enzyme defects. Before CoQ we had detected a relevant reduction of mitochondrial functionality in the skeletal muscle as shown by the reduced rate of phosphocreatine recovery from exercise, and in the occipital lobes by reduced [phosphocreatine] and a high [ADP] and [Pi]. After CoQ all brain variables were remarkably improved. Treatment with CoQ also improved the rate of muscle phosphocreatine recovery from exercise. Our in vivo findings support the hypothesis that the concentration of CoQ rather than the rate of its lateral diffusion in the mitochondrial membrane controls the efficiency of oxidative phosphorylation. Other experiments were undertaken to clarify the functional relationship between cytosolic free [Mg2+] and cell bioenergetics in the intact human brain. In the same group of patients with mitochondrial cytopathies we found decreased delta G of ATP hydrolysis and low cytosolic free [Mg2+]. Treatment with CoQ resulted in improved brain bioenergetics and increased free [Mg2+]. These findings strongly indicate that decreased free magnesium was secondary to defective mitochondrial respiration, and support the hypothesis that cytosolic free [Mg2+] is regulated in the intact brain cell to equilibrate, at least in part, any changes in rapidly available free energy.

Correlative MR imaging and 31P-MR spectroscopy study in sarcoglycan deficient limb girdle muscular dystrophy

We combined magnetic resonance (MR) imaging and phosphorus magnetic resonance spectroscopy (31P-MRS) to study skeletal muscle in seven patients with limb girdle muscular dystrophy (LGMD) with a variable deficiency of the alpha-, beta-, and gamma-sarcoglycan but normal dystrophin expression on muscle biopsy. T1- and T2-weighted spin-echo axial leg images showed the highest degree of fat replacement in soleus, tibialis anterior and peroneal muscles while gastrocnemius and tibialis posterior were less affected. In LGMD patients as a group, calf muscle phosphorylated compound content did not differ from controls, but the cytosolic pH was increased (P = 0.02). The degree of calf muscle fat replacement correlated inversely with cytosolic pH (r = 0.74) and directly with PCr/ATP (r = 0.74). Muscle oxidative metabolism was normal in LGMD patients. Our findings show that primary deficits of sarcoglycan complex lead to specific morphological and metabolic patterns of skeletal muscle involvement.

Brain and skeletal muscle bioenergetic failure in familial hypobetalipoproteinaemia

OBJECTIVE

To determine whether a multisystemic bioenergetic deficit is an underlying feature of familial hypobetalipoproteinaemia.

METHODS

Brain and skeletal muscle bioenergetics were studied by in vivo phosphorus MR spectroscopy (31P-MRS) in two neurologically affected members (mother and son) and in one asymptomatic member (daughter) of a kindred with familial hypobetalipoproteinaemia. Plasma concentrations of vitamin E and coenzyme Q10 (CoQ10) were also assessed.

RESULTS

Brain 31P-MRS disclosed in all patients a reduced phosphocreatine (PCr) concentration whereas the calculated ADP concentration was increased. Brain phosphorylation potential was reduced in the members by about 40%. Skeletal muscle was studied at rest in the three members and during aerobic exercise and recovery in the son and daughter. Only the mother showed an impaired mitochondrial function at rest. Both son and daughter showed an increased end exercise ADP concentration whereas the rates of postexercise recovery of PCr and ADP were slow in the daughter. The rate of inorganic phosphate recovery was reduced in both cases. Plasma concentration of vitamin E and CoQ10 was below the normal range in all members.

CONCLUSIONS

Structural changes in mitochondrial membranes and deficit of vitamin E together with reduced availability of CoQ10 can be responsible for the multisystemic bioenergetic deficit. Present findings suggest that CoQ10 supplementation may be important in familial hypobetalipoproteinaemia.

Influence of cytosolic pH on in vivo assessment of human muscle mitochondrial respiration by phosphorus magnetic resonance spectroscopy

The authors present an in vivo phosphorus magnetic resonance spectroscopy systematic study on the effects of cytosolic pH on skeletal muscle mitochondrial respiration in human calf muscle. In 49 normal subjects, the effect of cytosolic pH on kinetics of phosphocreatine and adenosine diphosphate recovery and on maximum rate of mitochondrial adenosine triphosphate production (Q(MAX)) was evaluated. The results show a strong relationship between the rate of postexercise phosphocreatine recovery and the lowest value of cytosolic pH reached during recovery from exercise (termed minimum pH; r = 0.89); in contrast, both adenosine diphosphate recovery halftime and Q(MAX) were independent of cytosolic pH at the end of exercise.

Inflammatory myopathies: issues in diagnosis and management

The techniques of magnetic resonance imaging and spectroscopy have been shown to have utility in the diagnosis and management of inflammatory muscle diseases. But perhaps more important have been the new insights into the pathophysiology of these diseases which MR studies, along with new immunologic data on autoantibodies and cellular infiltrates, have afforded. Pathologic subsets of inflammatory muscle disorders have been identified, suggesting, for example, that PM and DM are distinct disorders, thereby challenging the idea that these are relatively homogeneous syndromes. Further insights into disease pathogenesis which are likely to emerge from these new findings may allow identification of etiologic factors and improved approaches to treatment.

Magnetic resonance spectroscopy in migraine

31-phosphorus Magnetic Resonance Spectroscopy (MRS) is a technique developed for the non-invasive study of energy metabolism in living subjects. It determines the concentrations of high and low energy phosphates in resting and activated conditions, and of intracellular pH. 31P-MRS has been applied to the study of migraine, both during and in between attacks. Intracellular brain pH remains unchanged during the migraine attack, suggesting that ischemia does not play a relevant role in the origin of the neurological signs. During and in-between attacks, migraineurs display abnormalities in energy metabolism of brain and muscle, consisting of reduced levels of phosphocreatine, reduced cellular-free energy and increased rate of ATP biosynthesis. We suggest that these abnormalities in energy metabolism predispose migraineurs to develop an attack under conditions of increased brain energy demand.

Dystrophin-dependent efficiency of metabolic pathways in mouse skeletal muscles

Muscles from the mdx mouse (X-linked genetic disorder similar to Duchenne muscular dystrophy) lack dystrophin-associated transsarcolemmal proteins and show reduced maintenance metabolic rates. Here, microcalorimetric comparisons of metabolic stimulation by exogenous substrates in isolated muscles revealed substrate-selective limitation of chemical reaction rates through both glycolytic and TCA-cycle pathways, identical in slow- and fast-twitch mdx muscles. This systemic approach, as opposed to comparisons of single-enzyme activities, sheds new light on the function of dystrophin and associated proteins. The in vivo efficiency of metabolic pathways may depend on stabilization of enzyme complexes by dystrophin-associated elements of the cytoskeleton.

Further impairment of muscle phosphate kinetics by lengthening exercise in DMD/BMD carriers. An in vivo 31P-NMR spectroscopy study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of exercise-induced muscle injury in the calf muscle of 7 DMD/BMD carriers and 6 non-carrier females. All subjects performed 50-80 lengthening contractions with the right calf muscles. 48 h after lengthening exercise non-carriers showed increased sensitivity to pressure in their gastrocnemius accompanied by increased T2 relaxation times and by elevated Pi/PCr ratios at rest. DMD/BMD carriers did not show any effect of lengthening exercise on these measurements. In-magnet exercise revealed in all carriers a reduced initial rate of Pi recovery and an increased time to fully recovery the resting value of intracellular pH. Lengthening exercise further decreased the initial rate of Pi recovery. Non-carriers did not show any variation attributable to lengthening exercise either during in-magnet work or during recovery from exercise. We found that lengthening exercise contractions causes: (1) less muscle injury in carriers compared to non-carriers, (2) even slower rate of Pi recovery, but (3) no effect on Pi recovery in non-carriers. The use of lengthening exercise and measurements of Pi recovery may be a useful method to evaluate the disease process in DMD/BMD.

In vivo assessment of mitochondrial functionality in human gastrocnemius muscle by 31P MRS. The role of pH in the evaluation of phosphocreatine and inorganic phosphate recoveries from exercise

In this study we compared the kinetics of phosphocreatine (PCr) and P(i) recovery, and their dependency on cytosolic pH in 38 normal individuals. Spectra were acquired during rest, work and recovery. A time resolution of 10 s was used to obtain detailed information. The kinetics of PCr and P(i) recovery almost overlapped when the lowest value of cytosolic pH reached during recovery (termed the minimum pH) was < 6.95, while they were completely dissociated when the minimum pH was > 6.95. This result is interpreted as indirect in vivo evidence of the kinetic control exerted by ADP on mitochondrial oxidation. Our results represent a rationale for new experimental conditions to be used in clinical routine studies of pathologies due to primary or secondary mitochondrial malfunction.

Effect of fasting, hypocaloric feeding, and refeeding on the energetics of stimulated rat muscle as assessed by nuclear magnetic resonance spectroscopy

A previous study suggested that muscles from hypocalorically fed rats were limited in their ability to rephosphorylate ADP. During muscle contraction hydrolysis of ATP results in an increase in phosphorus, free ADP, delta GATP, and a reduction in phosphocreatine levels that is reversed during rest by rephosphorylation of ADP to ATP and the resynthesis of phosphocreatine by ATP. We therefore hypothesized that these changes would be restored more slowly during postcontraction rest in hypocalorically fed rats as compared with controls. We compared controls fed ad lib to 2-d fasted and hypocalorically fed rats, losing 20% of their weight. We also compared hypocalorically fed rats that had been refed ad lib for 7 d with age-matched controls fed ad lib. The results showed that ATP, muscle pH, and total muscle creatine levels were not different in all groups. The raised phosphorus and delta GATP levels and lower phosphocreatine/phosphorus ratio at the end of contraction changed more slowly during rest in the hypocaloric rats. These abnormalities were partially corrected by refeeding. The data taken as a whole support the concept of impaired rephosphorylation of ADP in malnourished muscle that is not completely restored by refeeding in stimulated muscle.

Cellular energetics of dystrophic muscle

Cytosolic pH and phosphorus metabolite ratios in skeletal muscle were measured by 31P magnetic resonance spectroscopy in patients with Duchenne muscular dystrophy (DMD) and Becker's muscular dystrophy (BMD) and in Duchenne/Becker carriers. In resting dystrophin-deficient muscle, there was a decrease in phosphocreatine (PCr) and increase in orthophosphate (Pi) relative to ATP, and an increase in calculated free [ADP]. Phosphomonester and phosphodiester were also increased relative to ATP. These changes were largest in DMD, smaller in BMD and small or absent in carriers. Cytosolic pH was increased substantially in DMD, moderately in BMD and slightly but significantly in gastrocnemius of carriers. Raised intracellular pH thus appears to be the most characteristic abnormality in dystrophin-deficient muscle. Responses to erobic exercise were studied in the forearm muscle flexor digitorum superficialis of carriers. PCr depletion during exercise was greater than normal but the fall in pH was disproportionately small, resulting in increased [ADP]. This is likely to result either from reduced anaerobic glycogenolysis to lactic acid or from increased proton efflux (as is seen in mitochondrial myopathy). Detailed analysis suggests: (1) at the start of exercise, calculated lactic acid production was increased, as was the rate of PCr depletion, suggesting that there was no absolute defect of glycogenolysis. (2) At the start of recovery, calculated proton efflux was not increased, although as the pH at the end of exercise was higher than in controls and proton efflux is normally pH-dependent, an up-regulation of proton efflux cannot be excluded. (3) Recovery of PCr, Pi and ADP after exercise were not impaired, suggesting that mitochondrial function is normal.

Energy metabolism in muscle paresis and recovery studied by 31P-MR spectroscopy: a case report

We report a 31P Magnetic Resonance Spectroscopy (31P-MRS) study on the changes of energy metabolism in human leg anterior and lateral compartment muscles with paresis due to compression from a herniated lumbar disc at the L4-L5 level and recovery to normality. A low phosphocreatine to inorganic phosphate ratio due to both decreased phosphocreatine and increased inorganic phosphate contents and a normal intracellular pH were the features of muscles with paresis. Changes of 31P-MRS parameters were followed during 18 weeks of treatment with physical therapy until complete recovery. Results show that 31P-MRS is a useful clinical tool for detecting even small biochemical changes that may occur in muscles and for checking the effects of therapy.