Isolation and Characterization of Exosomes from Skeletal Muscle Fibroblasts

Summary

This protocol illustrates the 1) the isolation and culture of primary fibroblasts from the adult mouse gastrocnemius muscle as well as 2) purification and characterization of exosomes using a differential ultracentrifugation method combined with sucrose density gradients followed by western blot analyses.

Abstract

Exosomes are small extracellular vesicles released by virtually all cells and secreted in all biological fluids. Many methods have been developed for the isolation of these vesicles, including ultracentrifugation, ultrafiltration, and size exclusion chromatography. However, not all are suitable for large scale exosome purification and characterization. Outlined here is a protocol for establishing cultures of primary fibroblasts isolated from adult mouse skeletal muscles, followed by purification and characterization of exosomes from the culture media of these cells. The method is based on the use of sequential centrifugation steps followed by sucrose density gradients. Purity of the exosomal preparations is then validated by western blot analyses using a battery of canonical markers (i.e., Alix, CD9, and CD81). The protocol describes how to isolate and concentrate bioactive exosomes for electron microscopy, mass spectrometry, and uptake experiments for functional studies. It can easily be scaled up or down and adapted for exosome isolation from different cell types, tissues, and biological fluids.

Introduction

Exosomes are heterogeneous extracellular vesicles ranging in size from 30–150 nm. They are established key players in physiological and pathological processes, given their ubiquitous distribution in tissues and organs^1,2. Exosomes carry a complex cargo of proteins, lipids, DNA types, and RNA types, which vary according to the type of cells from which they are derived^1,2,3. Exosomes are enriched in proteins that have different functions (i.e., tetraspanins, including CD9 and CD63) are responsible for fusion event....

Protocol

All procedures in mice were performed according to animal protocols approved by the St. Jude Children’s Research Hospital Institutional Animal Care and Use Committee and National Institutes of Health guidelines.

1. Preparation of solutions and media

1. Prepare digestion solution by mixing 15.4 mL of PBS with 2.5 mL of 20 mg/mL collagenase P (5 mg/mL final concentration), 2 mL of 11 U/mL dispase II (1.2 U/mL final concentration), and 100 µL of 1.0 M CaCl₂ (5 mM final concentration).
2. Prepare 500 mL of primary fibroblasts medium (DMEM complete) by mixing 440 mL of DMEM with 50 mL of FBS (10%), 5.0 mL of....

Results

This protocol is suitable for the purification of exosomes from large volumes of conditioned medium in a cost-effective manner. The procedure is highly reproducible and consistent. Figure 1 shows transmission electron microscopy (TEM) image of exosomes purified from the culture medium of mouse primary fibroblasts. Figure 2 shows the protein expression pattern of canonical exosomal markers, and the absence of cytosolic (LDH) and ER (calnexin) protein contaminants.......

Discussion

A critical step for the successful isolation of exosomes from the culture media, as outlined in this protocol, is the proper establishment and maintenance of primary mouse fibroblast cultures from adult skeletal muscle. These cultures need to be maintained at a low oxygen level to ensure physiological-like conditions (O₂ level in skeletal muscle is ~2.5%)¹⁵. Primary fibroblasts will change characteristics when passed in culture too many times. Hence, a low passage number is imperative f.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
10 cm dishes	Midwest Scientific, TPP	TP93100
15 cm dishes	Midwest Scientific	TP93150
BCA protein assay kit	Thermo Fisher Scientific, Pierce	23225
Bovine serum albumin Fraction V	Roche	10735094001
CaCl2	Sigma	C1016-100G
Centrifuge 5430R with rotors FA-35-6-30/ FA-45-48-11	Eppendorf	022620659/5427754008
Chemidoc MP imaging system	BioRad	12003154
Collagenase P	Sigma, Roche	11 213 857 001	100 mg
cOmplete protease inhibitor cocktail	Millipore/Sigma, Roche	11697498001
Criterion Blotter with plate electrodes	BioRad	1704070
Criterion TGX stain-free protein gel	BioRad	5678034	10% 18-well, midi-gel
Criterion vertical electrophoresis cell (midi)	BioRad	NC0165100
Dispase II	Sigma, Roche	04 942 078 001	neutral protease, grade II
Dithiothreitol	Sigma/Millipore, Roche	10708984001
Dulbecco’s Modification Eagles Medium	Corning	15-013-CV
Dulbecco’s Phosphate Buffered Saline	Corning	21-031-CV
Ethanol 200 proof	Pharmco by Greenfield Global	111000200
Falcon 50 mL conical centrifuge tubes	Corning	352070
Fetal Bovine Serum	Gibco	10437-028
Fluostar Omega multi-mode microplate reader	BMG Labtech
GlutaMAX supplement	Thermo Fisher Scientific, Gibco	35050-061
Hydrochloric acid	Fisher Scientific	A144S-500
Immobilon-P Transfer membranes	Millipore	IPVH00010
Laemmli sample buffer (4x)	BioRad	1610747
Magnesium acetate solution	Sigma	63052-100ml
Non-fat dry milk	LabScientific	M-0842
O2/CO2 incubator	Sanyo	MC0-18M
Penicillin-Streptomycin	Thermo Fisher Scientific, Gibco	15140-122	10,000 U/ml
Premium microcentrifuge tubes	Fisher Scientific, Midwest Scientific	AVSC1510	1.7 mL
Protected disposable scalpels	Fisher Scientific, Aspen Surgical Bard-Parker	372610
Running buffer	BioRad	1610732
Sodium Chloride	Fisher Scientific, Fisher Chemical	S271-3
Stericup Quick release-GP sterile vacuum filtration system	Millipore	S2GPU05RE	500 mL
Sterile cell strainer (70 mm)	Fisher Scientific, Fisher brand	22-363-548
Sucrose	Fisher Scientific, Fisher Chemical	S5-500
SuperSignal west Femto	Thermo Fisher Scientific	34096
Thin wall Polypropylene tubes	Beckman Coulter	326823
Transfer buffer	BioRad	16110734
Trichloroacetic Acid	Sigma	91228-100G
Tris base	BioRad	1610719
Triton-X100 solution	Sigma	93443-100mL
TrypLE Express Enzyme	Thermo Fisher Scientific, Gibco	12604-013	No phenol red
Tween-20	BioRad	#1610781
Ultra-centrifuge Optima XPM	Beckman Coulter	A99842
Ultra-clear tube (14x89 mm)	Beckman Coulter	344059
Ultra-clear tubes (25x89 mm)	Beckman Coulter	344058
Water bath Isotemp 220	Fisher Scientific	FS220