Visualization and Live Imaging of Oligodendrocyte Organelles in Organotypic Brain Slices Using Adeno-associated Virus and Confocal Microscopy

Summary

Myelinating oligodendrocytes promote rapid action potential propagation and neuronal survival. Described here is a protocol for oligodendrocyte-specific expression of fluorescent proteins in organotypic brain slices with subsequent time-lapse imaging. Further, a simple procedure for visualizing unstained myelin is presented.

Abstract

Neurons rely on the electric insulation and trophic support of myelinating oligodendrocytes. Despite the importance of oligodendrocytes, the advanced tools currently used to study neurons, have only partly been taken on by oligodendrocyte researchers. Cell type-specific staining by viral transduction is a useful approach to study live organelle dynamics. This paper describes a protocol for visualizing oligodendrocyte mitochondria in organotypic brain slices by transduction with adeno-associated virus (AAV) carrying genes for mitochondrial targeted fluorescent proteins under the transcriptional control of the myelin basic protein promoter. It includes the protocol for making organotypic coronal mouse brain slices. A procedure for time-lapse imaging of mitochondria then follows. These methods can be transferred to other organelles and may be particularly useful for studying organelles in the myelin sheath. Finally, we describe a readily available technique for visualization of unstained myelin in living slices by Confocal Reflectance microscopy (CoRe). CoRe requires no extra equipment and can be useful to identify the myelin sheath during live imaging.

Introduction

The brain's white matter is composed of nerve cell axons wrapped in myelin, a specialized extended plasma membrane formed by oligodendrocytes. Myelin is required for fast and reliable action potential propagation and long-term survival of myelinated axons, and a loss of myelin can cause neurological dysfunction. Despite their importance, the properties of oligodendrocytes are less known compared with neurons and astrocytes. Consequently, fewer tools have been developed for studying oligodendrocytes.

Live imaging of cell organelles such as mitochondria, endoplasmatic reticulum (ER) or different vesicular structures can be useful to study....

Protocol

The procedures described here have been approved by the Norwegian Animal Research Authority. Suppliers and catalog numbers for the consumables and other required equipment are available in the materials list at the end of the document.

1. Preparation of Organotypic Slices

NOTE: This recipe uses two mouse pups at postnatal day 7-9 (p7-9), which yield 24 organotypic slices divided on two six-well culture dishes. Unless stated otherwise, all procedures should be done in a sterile hood and nitril or latex gloves should be used. Only cell culture grade ingredients should be used.

1. Autoclave bottles, dissect....

Results

Organotypic brain slices that were cultured and transduced as described above showed a sparse distribution of cortical oligodendrocytes expressing mito_dsred and GFP. Immunostaining with antibodies against Olig2 and MBP confirmed that the expression was specific to oligodendrocytes (Figure 1).

For live imaging, transduced oligodendrocytes were recognized by their characteristic morphology of several.......

Discussion

The protocol for making organotypic cultures described here is a modified version¹⁸ of the protocol described by De Simoni and Yu (2006)⁵. The most important changes have been outlined below. Tris buffer is added to the culture medium, which improves the survival of the slices when outside of the incubator during viral transduction and changing of the cell medium. The sterilization procedure for confetti is also changed. While other protocols sterilize confetti by autoclavi.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Agarose	Sigma	A9539
BD Microlance 19G	BD	301500	Needles used for in- and outlet of bath
Bioxide gas	AGA	105701
Brand pipette bulbs	Sigma-Aldrich	Z615927	Pipette bulbs
Bunsen burner (Liquid propane burner)	VWR	89038-530
Cable assembly for heater controllers	Warner Instruments	64-0106	Temperature controller - thermometer part
CaCl2	Fluka	21100
CO2	AGA	100309	CO2 for incubator
Cover glass, square Corning	Thermo  Fischer Scientific	13206778	To attach under bath for live imaging. Seal with glue or petrolium jelly.
D-(+)-Glucose	Sigma	G7021
Delicate forceps	Finescience	11063-07	For dissection
Diamond scriber pen	Ted Pella Inc.	54463
Disposable Glass Pasteur Pipettes 230 mm	VWR	612-1702	Glass pipettes
Double edge stainless steel razor blade	Electron Microscopy Sciences	#7200	Razor blade for vibratome
Earle's Balanced Salt Solution (EBSS)	Gibco-Invitrogen	24010-043
Filter paper circles	Schleicher & Schuell	300,220	Filter paper used for filtration of PFA
Fun tack	Loctite	1270884	Use to connect/adjust position of in- and outlets in bath
Hand towel C-Fold 2	Katrin	344388
Harp, Flat for RC-41 Chamber,	Warner Instruments	64-1418	Harp to hold down confetti in bath. Cut off strings before use with organotypic slices. 1.5 mm, 13mm, SHD-41/15
HEPES, FW: 260.3	Sigma	H-7006
Holten LaminAir, Model 1.2	Heto-Holten	96004000M	Laminar flow hood
Horse serum, heat inactivated	Gibco-Invitrogen	26050-088
KCl	Sigma	P9541
LCR Membrane, PTFE,	Millipore	FHLC0130	Confetti
Leica VT1200	Leica	14048142065	Vibratome
MEM-Glutamax with HEPES	Thermo  Fischer Scientific	42360024
MgCl2	R.P. Normapur	25 108.295
Micro Spoon Heyman Type B	Electron Microscopy Sciences	62411-B	Small, rounded spatula with sharpened end for dissection
Millex-GP filter unit	Millipore	SLGPM33RA	Syringe filter unit
Millicell cell culture insert, 30 mm	Millipore	PICM03050	Cell culture inserts
Minipuls 3 Speed Control Module	GILSON	F155001	Peristaltic pump for live imaging - Control module part (connect to two-cannel head)
Na2HPO4	Sigma-Aldrich	S7907
NaCl	Sigma-Aldrich	S9888
NaH2PO4	Sigma-Aldrich	S8282
NaHCO3	Fluka	71628
Nunclon Delta Surface	Thermo  Fischer Scientific	140675	Culture plate
Nystatin Suspension	Sigma-Aldrich	N1638
Objective W "Plan-Apochromat" 40x/1.0 DIC	Zeiss	441452-9900-000	Water immersion objective used for live imaging. (WD=2.5mm), VIS-IR
Parafilm	VWR	291-1211
Paraformaldehyde, granular	Electron Microscopy Sciences	#19208
PC-R perfusion chamber	SiSkiYou	15280000E	Bath for live imaging
Penicillin-Streptomycin, liquid	Invitrogen	15070-063
Petri dish 140 mm	Heger	1075	Large Petri dish
Petri dish 92x16 mm	Sarstedt	82.1473	Medium Petri dish
Petridish 55x14,2 mm	VWR	391-0868	Small Petri dish
Phosphate buffered saline (PBS)	Sigma	P4417	PBS tablets
R2 Two Channel Head	GILSON	F117800	Peristaltic pump for live imaging - Two channel head part (requires control module)
Round/Flat Spatulas, Stainless Steel	VWR	82027-528	Large spatula for dissection
Sand paper	VWR	MMMA63119	Optional, for smoothing broken glass pipettes
Scissors,  17,5 cm	Finescience	14130-17	Large scissors for dissection
Scissors, 8,5	Finescience	14084-08	Small, sharp scissors for  dissection
Single edge, gem blade	Electron Microscopy Sciences	#71972	Single edge razor blade
Single inline solution heater SH-27B	Warner Instruments	64-0102	Temperature controller - heater part
Steritop-GP Filter unit, 500 ml , 45mm	Millipore	SCGPT05RE	Filter to sterilize solutions
Super glue precision	Loctite	1577386
Surgical scalpel blade no. 22	Swann Morton Ltd.	209	Rounded scalpel blade
Temperature controller TC324B	Warner Instruments	64-0100	Temperature controller for live imaging (requires solution heater and cable assembly)
Trizma base	Sigma	T1503
Trizma HCl	Sigma	T3253
Water jacketed incubator series II	Forma Scientific	78653-2882	Incubator