Quantifying the Heterogeneous Distribution of a Synaptic Protein in the Mouse Brain Using Immunofluorescence

Summary

Here, we describe a quantitative approach to determining the distribution of a synaptic protein relative to a marker protein using immunofluorescence staining, confocal microscopy, and computer-based analysis.

Abstract

The presence, absence, or levels of specific synaptic proteins can severely influence synaptic transmission. In addition to elucidating the function of a protein, it is vital to also determine its distribution. Here, we describe a protocol employing immunofluorescence, confocal microscopy, and computer-based analysis to determine the distribution of the synaptic protein Mover (also called TPRGL or SVAP30). We compare the distribution of Mover to that of the synaptic vesicle protein synaptophysin, thereby determining the distribution of Mover in a quantitative manner relative to the abundance of synaptic vesicles. Notably, this method could potentially be implemented to allow for comparison of the distribution of proteins using different antibodies or microscopes or across different studies. Our method circumvents the inherent variability of immunofluorescent stainings by yielding a ratio rather than absolute fluorescence levels. Additionally, the method we describe enables the researcher to analyze the distribution of a protein on different levels: from whole brain slices to brain regions to different subregions in one brain area, such as the different layers of the hippocampus or sensory cortices. Mover is a vertebrate-specific protein that is associated with synaptic vesicles. With this method, we show that Mover is heterogeneously distributed across brain areas, with high levels in the ventral pallidum, the septal nuclei, and the amygdala, and also within single brain areas, such as the different layers of the hippocampus.

Introduction

Communication between neurons happens at specialized contact sites called synapses. Synapses contain a myriad of different proteins that orchestrate synaptic transmission. Some of those proteins show a heterogeneous distribution throughout the nervous system and are not present in every synapse¹. One example for such a protein is Munc13, which is involved in the priming process of synaptic vesicles. There are different isoforms of Munc13, which are heterogeneously distributed throughout the brain², and the presence or absence of specific isoforms can influence short-term synaptic plasticity and synaptic vesicle dynamics<....

Protocol

This protocol does not involve experiments on live animals. Experiments involving euthanizing of animals to obtain brain samples were approved by the local animal protection authorities (Tierschutzkommission der Universitätsmedizin Göttingen) under the approval number T 10/30.

NOTE: For this protocol, 3 adult male C57BL/6 mice were used.

1. Sample Preparation

1. Prepare fixative and 0.1 M phosphate buffer (PB; see Table 1).
2. Fix the animal by transcardial perfusion as described in Gage et al.²⁸. First wash out the blood with 0.9% NaCl-s....

Results

Representative staining patterns of different markers can be seen in Figure 1. The pattern varies depending on the distribution of the protein. Examples of five rostro-caudal levels are shown in columns (A)-(E). A representative DAPI staining is shown in the first row: DAPI adheres to the DNA of a cell and thus nuclei are stained. This results in a punctate pattern. Regions with a high cell density are brighter than regions w.......

Discussion

The method presented here aims at quantifying the distribution of a protein of interest relative to the abundance of a marker protein with a known distribution. Immunofluorescence staining can show a high variability of staining intensities between different slices. The quantification approach described here circumvents this problem by determining the ratio of the protein of interest to the average across the hemisphere. Therefore, different staining intensities across slices are cancelled out and allow for a quantitativ.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1.5 mL reaction tubes	Eppendorf	30120094
50 mL reaction tubes	Greiner Bio-One	227261
multiwell 24 well	Fisher Scientific	087721H
plastic pipette (disposable)	Sarstedt	861,176
1000 mL pipette	Rainin	17014382
2 ml pipette	Eppendorf	3123000012
Vortex Genius 3	IKA	3340001
Menzel microscope slides	Fisher Scientific	10144633CF
Stereoscope	Leica
LSM800	Zeiss		Confocal microscope
freezing microtome	Leica
PBS (10X)	Roche	11666789001
PFA	Sigma	P6148-1kg
NaCl	BioFroxx	1394KG001
sucrose	neoFroxx	1104KG001
Tissue Tek	Sakura	4583	OCT
Na2HPO4	BioFroxx	5155KG001
NaH2PO4	Merck	1,063,460,500
normal goat serum	Merck Millipore	S26-100ML
normal donkey serum	Merck	S30-100ML
Triton X-100	Merck	1,086,031,000
rabbit anti-Mover	Synaptic Systems	RRID: AB_10804285
guinea pig anti-Synaptophysin	Synaptic Systems	RRID: AB_1210382
donkey anti-rabbit AF647	Jackson ImmunoResearch	RRID: AB_2492288
goat anti-mouse AF488	Jackson ImmunoResearch	RRID: AB_2337438
Mowiol4-88	Calbiochem	475904
ZEN2 blue software	Zeiss		Microscopy software
FIJI	ImageJ		Analysis software
Microsoft Excel	Microsoft