Quantification of Subcellular Glycogen Distribution in Skeletal Muscle Fibers using Transmission Electron Microscopy

Summary

A modified post-fixation procedure increases the contrast of glycogen particles in tissue. This paper provides a step-by-step protocol describing how to handle the tissue, conduct the imaging, and use stereological methods to obtain unbiased and quantitative data on fiber type-specific subcellular glycogen distribution in skeletal muscle.

Abstract

With the use of transmission electron microscopy, high-resolution images of fixed samples containing individual muscle fibers can be obtained. This enables quantifications of ultrastructural aspects such as volume fractions, surface area to volume ratios, morphometry, and physical contact sites of different subcellular structures. In the 1970s, a protocol for enhanced staining of glycogen in cells was developed and paved the way for a string of studies on the subcellular localization of glycogen and glycogen particle size using transmission electron microscopy. While most analyses interpret glycogen as if it is homogeneously distributed within the muscle fibers, providing only a single value (e.g., an average concentration), transmission electron microscopy has revealed that glycogen is stored as discrete glycogen particles located in distinct subcellular compartments. Here, the step-by-step protocol from tissue collection to the quantitative determination of the volume fraction and particle diameter of glycogen in the distinct subcellular compartments of individual skeletal muscle fibers is described. Considerations on how to 1) collect and stain tissue specimens, 2) perform image analyses and data handling, 3) evaluate the precision of estimates, 4) discriminate between muscle fiber types, and 5) methodological pitfalls and limitations are included.

Introduction

Glycogen particles are composed of branched polymers of glucose and various associated proteins¹ and constitute an important fuel during high metabolic demands². Although not widely recognized, glycogen particles also constitute a local fuel, where some subcellular processes preferentially utilize glycogen despite the availability of other and more long-lasting fuels as plasma glucose and fatty acids^3,4.

The importance of storing glycogen as a subcellular specific localized fuel has been discussed in several reviews^5<....

Protocol

The present protocol using human biopsied skeletal muscle samples was approved by The Regional Committees on Health Research Ethics for Southern Denmark (S-20170198). Muscle biopsies were obtained through an incision in the skin from the vastus lateralis muscle using a Bergström needle with suction after local anesthesia was given subcutaneously (1-3 mL of Lidocaine 2% per incision). If isolated whole rat muscles were used, the animals were sacrificed by cervical dislocation before the muscle biopsies were obtained, in accordance with the guidelines of the animal ethics committee at Odense University Hospital, Denmark.

1....

Results

Using this protocol, glycogen particles appear black and distinct (Figures 1 and Figure 2). The normal values of glycogen are depicted in Figure 3. These data are based on a total of 362 fibers from 41 healthy young men as collected in different previous studies^{19,24,29,30,31}

Discussion

The critical step of the method is the use of reduced osmium by potassium ferrocyanide during post-fixation. The selectivity of this modified fixative for glycogen detection cannot be fully explained by chemistry, but also includes experimental findings demonstrating no detection of such particles in tissues known to be free of glycogen or in the extracellular space¹¹.

Critical parameters are the precision of the estimates and the fiber-to-fiber variation. By following .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1,2-Propylene oxide	Merck	75-56-9
Embedding 812 resin medium kit	Taab	T031
Glutaraldehyde solution 25%	Merck	1.04239.0250
ITEM	Olympus		Imaging software
Leica EM AC20	Leica		Automatic contrasting system
OSIS Veleta digital camera	Olympus
Osmium tetroxide 4% solution	Polysciences	0972A
Philips CM 100 Transmission EM	Philips
Potassium hexacyanoferrate (II) trihydrate	Sigma-Aldrich	455989-245G
Sodium cacodylatbuffer 0,2 M ph 7.4	Ampliqon.com	AMPQ40989.0500
Ultra-microtome Leica UC7	Leica
Ultrostain lead citrate 3%, stabilised solution	Leica	16707235
Uranyl acetate dihydrate	Polysciences	6159-44-0