Photobleaching Enables Super-resolution Imaging of the FtsZ Ring in the Cyanobacterium Prochlorococcus

Summary

Here we describe a photobleaching method to reduce the autofluorescence of cyanobacteria. After photobleaching, stochastic optical reconstruction microscopy is used to obtain three-dimensional super-resolution images of the cyanobacterial FtsZ ring.

Abstract

Super-resolution microscopy has been widely used to study protein interactions and subcellular structures in many organisms. In photosynthetic organisms, however, the lateral resolution of super-resolution imaging is only ~100 nm. The low resolution is mainly due to the high autofluorescence background of photosynthetic cells caused by high-intensity lasers that are required for super-resolution imaging, such as stochastic optical reconstruction microscopy (STORM). Here, we describe a photobleaching-assisted STORM method which was developed recently for imaging the marine picocyanobacterium Prochlorococcus. After photobleaching, the autofluorescence of Prochlorococcus is effectively reduced so that STORM can be performed with a lateral resolution of ~10 nm. Using this method, we acquire the in vivo three-dimensional (3-D) organization of the FtsZ protein and characterize four different FtsZ ring morphologies during the cell cycle of Prochlorococcus. The method we describe here might be adopted for the super-resolution imaging of other photosynthetic organisms.

Introduction

Super-resolution microscopies can break the diffraction limit of light and provide images within sub-diffraction resolutions (< 200 nm). They have been widely used in many organisms to study protein localization and subcellular structures. Major super-resolution microscopy methods include structured illumination microscopy (SIM), stimulated emission depletion microscopy (STED), STORM, and photoactivated localization microscopy (PALM). The mechanisms and applications of these super-resolution microscopes have been reviewed elsewhere^1,2.

STORM can achieve a resolution as high as 1....

Protocol

1. Sample Preparation and Fixation

1. Inoculate 1 mL of axenic Prochlorococcus MED4 to 5 mL of the seawater-based Pro99 medium¹². Grow Prochlorococcus cultures at 23 °C under the light with an intensity of 35 µmol photons/m²s. Five days later, collect 1 mL of culture into a 1.5-mL tube.
   NOTE: Five days after the inoculation, Prochlorococcus MED4 will reach the late log phase, with approximately 10⁸ cell/mL, which is appropriate for STORM imaging.
2. Add 100 µL of formaldehyde freshly prepared from 25% paraformaldehyde (PFA) (w/v) and 2 µL of 50% glutaral....

Results

STORM achieves super-resolution imaging by activating individual photoswitchable fluorophores stochastically. The location of every fluorophore is recorded and a super-resolution image is then constructed based on these locations⁴. Therefore, the precision of the fluorophore location is important for the super-resolution image reconstruction. The absorption spectra of Prochlorococcus peak at 447 nm and 680 nm,and Prochlorococcus has a minimum abso.......

Discussion

In this protocol, we described a procedure to significantly reduce the autofluorescence of the cyanobacterium Prochlorococcus (Figure 3C) and, then, immunostain the proteins in the cells, which enabled us to utilize STORM to study the 3-D FtsZ ring morphologies in Prochlorococcus (Figure 4). This protocol might be adopted for super-resolution imaging in other photosynthetic organisms.

Previous studies on photosynthet.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Polystyrene particles	Spherotech	PP-20-10	2.0-2.4 µm
Coverslip	Marienfeld	0111580	18 mm ∅, Thickness No. 1
Ethanol	Scharlau	ET00021000
Poly-L-lysine hydrobromide	Sigma-Aldrich	P9155	mol wt 70,000-150,000
Paraformaldehyde	Sigma-Aldrich	158127
Glutaraldehyde solution, 50%	Sigma-Aldrich	340855
PBS	Sigma	P3813
Triton X-100	Sigma	T8787
EDTA Disodium Salt, 2-hydrate	Gold biotechnology	E-210-500
Trizma base	Sigma	T1503
Lysozyme	Sigma	L6876
Goat serum	Sigma	G9023
anti-Anabaena FtsZ antibody	Agrisera	AS07217
Goat anti-Rabbit IgG (H+L) Cross-Adsorbed Secondary Antibody	Life Technologies	A-21039	conjugated with Alexa Fluor 750
D-Glucose Anhydrous	Fisher Scientific	D16-1
L-Ascorbic Acid	Sigma-Aldrich	A5960
Methyl Viologen	Sigma-Aldrich	856177
Cyclooctatetraene	Sigma-Aldrich	138924
tris(2-carboxyethyl)phosphine (TCEP)	Sigma-Aldrich	646547
Glucose Oxidase	Sigma-Aldrich	G2133
Catalase	Sigma-Aldrich	C9322
XD-300 Xenon light source			250 W
STORM microscope	NBI	SRiS microscope
Rohdea	NBI	SRiS 3.0	software for imaging acquisition
Luna	NBI	SRiS 3.0	software for drifting correction
QuickPALM			https://code.google.com/archive/p/quickpalm/wikis
3D Viewer			http://132.187.25.13/ij3d/?page=Home&category=Home