Coordinated development of the mitochondrial reticulum in skeletal muscle

Abstract

Skeletal muscle cells form a mitochondrial network, or reticulum, in a fiber‐type specific manner with grid‐like and fiber‐perpendicular mitochondrial network configurations found in oxidative and glycolytic fibers, respectively. However, it is not well characterized how the networks form during postnatal muscle development. Using focused ion beam scanning electron microscopy (FIB‐SEM) and machine learning based 3D image segmentation and analysis, we have sought to understand the 3D mitochondrial development in oxidative (soleus) and glycolytic (gastrocnemius) muscles of mice at different postnatal days 1 (P1), 14 (P14), and 42 (P42). Both oxidative and glycolytic fibers show tortuous, parallel mitochondrial formations at P1. At P14, oxidative fibers have more elongated, fiber‐parallel mitochondria, while glycolytic fibers have a more grid‐like mitochondrial reticulum. To better understand the intra‐ and inter‐organelle interactions of the mitochondrial reticulum during development, we additionally assessed intermitochondrial junctions (IMJ) and mitochondrial interactions with lipid droplets (LD). During early maturation (P1‐P14), many IMJ in oxidative fibers are arranged in both diagonal (~40%) and parallel (~40%) directions, yet matured oxidative fibers have almost half (~50%) of IMJ in the perpendicular direction. In contrast, after having ~50% of IMJ along the parallel axis at P1, glycolytic fibers gradually change IMJ orientations more into both perpendicular and diagonal axis and IMJ are located in all directions at P42. Although mitochondria in glycolytic fibers maintain similar sizes of the IMJ contact sites between P1 and P14 and have smaller areas at P42, oxidative fibers have ~2‐fold larger IMJ areas at P14 as compared to early (P1) and matured (P42) fibers. Meanwhile, both oxidative and glycolytic fibers at P1 have many, big LD that sharply decrease in size during the course of postnatal development. In the matured muscles, oxidative fibers have few, tiny LD and glycolytic fibers have no LD. Both fibers maintain similar LD contract areas per mitochondrion across P1‐P42. Notably, mitochondria directly in contact with LD have volumes ~2–3‐fold larger than non‐LD‐contacted mitochondria at all time points. In addition, the LD‐contacted mitochondria have ~2‐fold bigger IMJ areas as compared to non‐contacted one, with the largest differences observed at P14. In conclusion, development of the muscle mitochondrial reticulum occurs in coordination with altered interactions among mitochondria and with LD. However, future studies are warranted to study possible interactive roles of calcium handling structures such as sarcoplasmic reticulum and t‐tubules.Support or Funding InformationThis study is supported by intramural programs within the National Heart, Lung, and Blood Institute and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (grant 1ZIAHL006221‐02).