Multiparametric Aging Study Across Adulthood in the Leg Through Quantitative MR Imaging, 1H Spectroscopy, and 31P Spectroscopy at 3T.

BACKGROUND: Improved characterization of healthy muscle aging is needed to establish early biomarkers in age-related diseases. PURPOSE: To quantify age-related changes on multiple MRI and clinical variables evaluated in the same cohort and identify correlations among them. STUDY TYPE: Prospective. POPULATION: 70 healthy subjects (30 men) from 20 to 81 years old. FIELD STRENGTH/SEQUENCE: 3T/water T2 (multiecho SE, multi-TE STEAM), water T1 (GRE MR Fingerprinting), fat-fraction (multiecho GRE, multi-TE STEAM), carnosine (PRESS), multicomponent water T2 (ISIS-CPMG SE train), and 31P pulse-acquire spectroscopy. ASSESSMENT: Age- and sex-related changes on: Imaging: fat-fraction (FFMRI), water T1 (T1-H2O), and T2 (T2-H2O-MRI) and their heterogeneities ΔT1-H2O and ΔT2-H2O-MRI in the posterior compartment (PC) and anterior compartment (AC) of the leg. 1H spectroscopy: Carnosine concentration, pH, water T2 components (T2-H2O-CPMG), fat-fraction (FFMRS), and water T2 (T2-H2O-MRS) in the gastrocnemius medialis. 31P spectroscopy: Phosphodiesters (PDE), phosphomonoesters, inorganic phosphates (Pi), and phosphocreatine (PCr) normalized to adenosine triphosphate (ATP) and pH in the calf. Clinical evaluation: Body-mass index (BMI), gait speed (GS), plantar flexion strength, handgrip strength (HS), HS normalized to wrist circumference (HSnorm), physical activity assessment. STATISTICAL TESTS: Multilinear regressions with sex and age as fixed factors. Spearman correlations calculated between variables. Benjamini-Hochberg procedure for false positives reduction (5% rate). A P < 0.05 significance level was used. RESULTS: Significant age-related increases were found for BMI (ρAge = 0.04), HSnorm (ρAge = -0.01), PDE/ATP (ρAge = 2.8 × 10-3), Pi/ATP (ρAge = 2.0 × 10-3), Pi/PCr (ρAge = 0.3 × 10-3), T2-H2O-MRS (ρAge = 0.051 msec), FFMRS (ρAge = 0.036) the intermediate T2-H2O-CPMG component time (ρAge = 0.112 msec), and fraction (ρAge = -0.3 × 10-3); and in both compartments for FFMRI (ρAge = 0.06, PC; ρAge = 0.06, AC), T2-H2O-MRI (ρAge = 0.05, PC; ρAge = 0.05, AC; msec), ΔT2-H2O-MRI (ρAge = 0.02, PC; ρAge = 0.02, AC; msec), T1-H2O (ρAge = 1.08, PC; ρAge = 1.06, AC; msec), and ΔT1-H2O (ρAge = 0.22, PC; ρAge = 0.37, AC; msec). The best age predictors, accounting for sex-related differences, were HSnorm (R2 = 0.52) and PDE/ATP (R2 = 0.44). In both leg compartments, the imaging measures and HSnorm were intercorrelated. In PC, T2-H2O-MRS and FFMRS also showed numerous correlations to the imaging measures. PDE/ATP correlated to T1-H2O, T2-H2O-MRI, ΔT2-H2O-MRI, FFMRI, FFMRS, the intermediate T2-H2O-CPMG, BMI, Pi/PCr, and HSnorm. DATA CONCLUSION: Our multiparametric MRI approach provided an integrative view of age-related changes in the leg and revealed multiple correlations between these parameters and the normalized HS. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 3.

GDF15 mediates renal cell plasticity in response to potassium depletion in mice.

OBJECTIVE: To understand the mechanisms involved in the response to a low-K+ diet (LK), we investigated the role of the growth factor GDF15 and the ion pump H,K-ATPase type 2 (HKA2) in this process. METHODS: Male mice of different genotypes (WT, GDF15-KO, and HKA2-KO) were fed an LK diet for different periods of time. We analyzed GDF15 levels, metabolic and physiological parameters, and the cellular composition of collecting ducts. RESULTS: Mice fed an LK diet showed a 2-4-fold increase in plasma and urine GDF15 levels. Compared to WT mice, GDF15-KO mice rapidly developed hypokalemia due to impaired renal adaptation. This is related to their 1/ inability to increase the number of type A intercalated cells (AIC) and 2/ absence of upregulation of H,K-ATPase type 2 (HKA2), the two processes responsible for K+ retention. Interestingly, we showed that the GDF15-mediated proliferative effect on AIC was dependent on the ErbB2 receptor and required the presence of HKA2. Finally, renal leakage of K+ induced a reduction in muscle mass in GDF15-KO mice fed LK diet. CONCLUSIONS: In this study, we showed that GDF15 and HKA2 are linked and play a central role in the response to K+ restriction by orchestrating the modification of the cellular composition of the collecting duct.

Long-term microdystrophin gene therapy is effective in a canine model of Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is an incurable X-linked muscle-wasting disease caused by mutations in the dystrophin gene. Gene therapy using highly functional microdystrophin genes and recombinant adeno-associated virus (rAAV) vectors is an attractive strategy to treat DMD. Here we show that locoregional and systemic delivery of a rAAV2/8 vector expressing a canine microdystrophin (cMD1) is effective in restoring dystrophin expression and stabilizing clinical symptoms in studies performed on a total of 12 treated golden retriever muscular dystrophy (GRMD) dogs. Locoregional delivery induces high levels of microdystrophin expression in limb musculature and significant amelioration of histological and functional parameters. Systemic intravenous administration without immunosuppression results in significant and sustained levels of microdystrophin in skeletal muscles and reduces dystrophic symptoms for over 2 years. No toxicity or adverse immune consequences of vector administration are observed. These studies indicate safety and efficacy of systemic rAAV-cMD1 delivery in a large animal model of DMD, and pave the way towards clinical trials of rAAV-microdystrophin gene therapy in DMD patients.

Forelimb treatment in a large cohort of dystrophic dogs supports delivery of a recombinant AAV for exon skipping in Duchenne patients.

Duchenne muscular dystrophy (DMD) is a severe muscle-wasting disorder caused by mutations in the dystrophin gene, without curative treatment yet available. Our study provides, for the first time, the overall safety profile and therapeutic dose of a recombinant adeno-associated virus vector, serotype 8 (rAAV8) carrying a modified U7snRNA sequence promoting exon skipping to restore a functional in-frame dystrophin transcript, and injected by locoregional transvenous perfusion of the forelimb. Eighteen Golden Retriever Muscular Dystrophy (GRMD) dogs were exposed to increasing doses of GMP-manufactured vector. Treatment was well tolerated in all, and no acute nor delayed adverse effect, including systemic and immune toxicity was detected. There was a dose relationship for the amount of exon skipping with up to 80% of myofibers expressing dystrophin at the highest dose. Similarly, histological, nuclear magnetic resonance pathological indices and strength improvement responded in a dose-dependent manner. The systematic comparison of effects using different independent methods, allowed to define a minimum threshold of dystrophin expressing fibers (>33% for structural measures and >40% for strength) under which there was no clear-cut therapeutic effect. Altogether, these results support the concept of a phase 1/2 trial of locoregional delivery into upper limbs of nonambulatory DMD patients.