Muscle oxidative phosphorylation quantitation using creatine chemical exchange saturation transfer (CrCEST) MRI in mitochondrial disorders

Abstract

Systemic mitochondrial energy deficiency is implicated in the pathophysiology of many age-related human diseases. Currently available tools to estimate mitochondrial oxidative phosphorylation (OXPHOS) capacity in skeletal muscle in vivo lack high anatomic resolution. Muscle groups vary with respect to their contractile and metabolic properties. Therefore, muscle group-specific estimates of OXPHOS would be advantageous. To address this need, a noninvasive creatine chemical exchange saturation transfer (CrCEST) MRI technique has recently been developed, which provides a measure of free creatine. After exercise, skeletal muscle can be imaged with CrCEST in order to make muscle group-specific measurements of OXPHOS capacity, reflected in the recovery rate (τCr) of free Cr. In this study, we found that individuals with genetic mitochondrial diseases had significantly (P = 0.026) prolonged postexercise τCr in the medial gastrocnemius muscle, suggestive of less OXPHOS capacity. Additionally, we observed that lower resting CrCEST was associated with prolonged τPCr, with a Pearson's correlation coefficient of -0.42 (P = 0.046), consistent with previous hypotheses predicting that resting creatine levels may correlate with 31P magnetic resonance spectroscopy-based estimates of OXPHOS capacity. We conclude that CrCEST can noninvasively detect changes in muscle creatine content and OXPHOS capacity, with high anatomic resolution, in individuals with mitochondrial disorders.

Figures

Figure 1. Example resting and postexercise CrCEST recovery images and summary curves.

(A–D) (left) Images from a heathy 21-year-old female. (E–H) (right) Images from a 21-year-old female with a mitochondrial DNA mutation, m.1630G>A (tRNA-Val) with hearing impairment, short stature, and stroke (53). Panels A and E show the sequential resting creatine chemical exchange saturation transfer (CrCEST) images (1 image every 30 seconds at rest), overlaid on the manually segmented, anatomical axial calf muscle image, for the healthy and affected individuals, respectively. The images encompass the muscle region of the right calf. The intensity of the color in each image, as shown on the color bar, is in proportion to the CrCEST percentage asymmetry signal, reflecting the amount of free creatine. Resting CrCEST appears lower in the affected individual as well (e.g., in soleus, CrCEST asymmetry is 11.1% in the affected individual versus 13.2% in healthy individual). Panels B and F show the sequential postexercise images (1 image every 30 seconds after cessation of exercise) in the healthy and affected individuals, respectively. Both subjects exercised, as indicated by the increase in free creatine, although the specific muscle groups used differ, and the percentage change in CrCEST for the same exercise was higher in the affected individual (e.g., in the soleus, 28% in the affected individual versus 10% in the healthy control). By ~2 minutes after exercise, the healthy volunteer’s CrCEST image resembles the baseline image, but in the affected individual, the exponential time constant for the decline in Cr after exercise (τCr) is prolonged (e.g., in the soleus, 5.9 minutes in the affected individual versus 1.1 minutes in the healthy control). Panels C and G show the postexercise CrCEST signal recovery summarized over the anatomic region corresponding to the approximate area of the surface 31P-MRS coil. Panels D and H show the postexercise phosphocreatine (PCr) signal recovery in this same anatomic region. In both modalities, prolonged postexercise recovery is observed in the affected individual relative to the healthy individual.

Figure 2. Box plots for postexercise creatine chemical exchange saturation transfer (CrCEST) exponential time constant (τCr), an index of skeletal muscle oxidative phosphorylation capacity, are shown (red = control, blue = mitochondrial disease).

The horizontal line corresponds to the median, the lower and upper margins of the boxes correspond to the 25th and 75th percentiles, respectively, and whiskers show 1.5 × the interquartile interval (IQI). The distribution of τCr is not normal so nonparametric statistics are shown here. Log-transformed τCr values were used in statistical modeling. A prolonged τCr value corresponds to less oxidative phosphorylation capacity. Mitochondrial disease increases τCr by 0.32 SDs (P = 0.017). The range of values shown results in part from the intersubject variability of which muscle groups were engaged in exercise and to what extent.

Figure 3. Example of muscle group–specific metabolic variation captured by CrCEST.

Images are shown from the right leg of a 60-year-old man with chronic progressive external ophthalmoplegia due to a mutation in C10ORF12 (c.1110C>G; p.F370L). (A) A 1.5T clinical image, which shows nearly complete fatty replacement of the medial gastrocnemius. (B) The corresponding area is indicated with a white arrow on a resting creatine chemical exchange saturation transfer (CrCEST) image obtained at 7.0T. The intensity of the color in each image, as shown on the color bar, is in proportion to the CrCEST percentage asymmetry signal reflecting the amount of free creatine. In B, the resting CrCEST signal is much lower in the medial gastrocnemius than in other muscle groups. After exercise, there is no increase in CrCEST signal in this region (C, white arrow).

Figure 4. Correlogram of the association between intentional exercise, expressed either as hours per week (reflecting time spent exercising), or MET per week (reflecting both time and intensity spent exercising) and measured imaging parameters.

The results of nonparametric correlation analyses are shown for all participants (first group of 2 columns), for participants with mitochondrial disease (second group of 2 columns), and for control participants (third group of 2 columns). Groups are separated by horizontal black lines. The bottom 4 rows indicate baseline (i.e., preexercise) metabolite concentration: baseline creatine chemical exchange saturation transfer (CrCEST) in the soleus, medial gastrocnemius, and lateral gastrocnemius, and phosphocreatine to inorganic phosphate ratio (PCr/Pi) in the region of interest captured by 31P magnetic resonance spectroscopy (31P-MRS). A higher resting CrCEST or PCr/Pi value suggests more bioenergetic capacity at rest. The top 4 rows indicate postexercise exponential time constants for return to baseline of CrCEST in soleus, medial gastrocnemius, and lateral gastrocnemius, and PCr in the region of interest captured by 31P-MRS. A longer postexercise time constant to return to baseline suggests decreased oxidative phosphorylation capacity. The 2 types of imaging assessment (baseline, postexercise) are separated by the dashed horizontal black line. As indicated by the color bar, positive associations are shown in red, and negative associations in yellow. *Nominal P value < 0.05; **Bonferroni P value < 0.05 (adjusted for the 16 comparisons shown for each group). BL, baseline; Sol, soleus; MG, medial gastrocnemius; Mito, mitochondrial; LG, lateral gastrocnemius.