Correlative Microscopy for 3D Structural Analysis of Dynamic Interactions

Summary

We describe a correlative microscopy method that combines high-speed 3D live-cell fluorescent light microscopy and high-resolution cryo-electron tomography. We demonstrate the capability of the correlative method by imaging dynamic, small HIV-1 particles interacting with host HeLa cells.

Abstract

Cryo-electron tomography (cryoET) allows 3D visualization of cellular structures at molecular resolution in a close-to-physiological state¹. However, direct visualization of individual viral complexes in their host cellular environment with cryoET is challenging², due to the infrequent and dynamic nature of viral entry, particularly in the case of HIV-1. While time-lapse live-cell imaging has yielded a great deal of information about many aspects of the life cycle of HIV-1^3-7, the resolution afforded by live-cell microscopy is limited (~ 200 nm). Our work was aimed at developing a correlation method that permits direct visualization of early events of HIV-1 infection by combining live-cell fluorescent light microscopy, cryo-fluorescent microscopy, and cryoET. In this manner, live-cell and cryo-fluorescent signals can be used to accurately guide the sampling in cryoET. Furthermore, structural information obtained from cryoET can be complemented with the dynamic functional data gained through live-cell imaging of fluorescent labeled target.

In this video article, we provide detailed methods and protocols for structural investigation of HIV-1 and host-cell interactions using 3D correlative high-speed live-cell imaging and high-resolution cryoET structural analysis. HeLa cells infected with HIV-1 particles were characterized first by confocal live-cell microscopy, and the region containing the same viral particle was then analyzed by cryo-electron tomography for 3D structural details. The correlation between two sets of imaging data, optical imaging and electron imaging, was achieved using a home-built cryo-fluorescence light microscopy stage. The approach detailed here will be valuable, not only for study of virus-host cell interactions, but also for broader applications in cell biology, such as cell signaling, membrane receptor trafficking, and many other dynamic cellular processes.

Introduction

Cryo-electron tomography (cryoET) is a powerful imaging technique that allows three-dimensional (3D) visualization of cells and tissues and provides insights into the organization of native organelles and cellular structures at molecular resolution in a close-to physiological state¹. However, the inherently low contrast of unstained frozen-hydrated specimen, combined with their radiation sensitivity, makes it difficult to locate areas of interest inside a cell and subsequently performing the tilt series successfully without damaging the target area. In order to overcome these problems, a correlative approach that combines light and electron microscopy is ne....

Protocol

1. HeLa Cell Culture on Carbon-coated, Gold EM Finder Grids

1. Glow discharge the carbon-side of a 200-mesh R2/2 Quantifoil gold EM finder grid under 25 mA for 25 sec.
2. Coat the EM grid with fibronectin, by floating it, carbon-side down, onto a 40 μl droplet of 50 μg/ml fibronectin solution, then disinfect it under UV light for 2 hr in a tissue culture hood.
3. Plate HeLa cells onto the grid at a density of 2 x 10⁴ cells/ml (total 2 ml culture) in a glass-bottom culture dish and grow the cells at 37 °C, with 5% CO₂, in DMEM supplemented with 4.5 g/L L-glutamine and glucose, 10% heat-inactivated fetal calf serum,....

Results

To characterize the dynamic behavior of the virus particles, HeLa cells infected with HIV-1 were imaged by high-speed confocal live-cell microscopy and the particle movements were analyzed by automated 3D particle tracking (Figure 1). To avoid the time lapse of several minutes that can occur between collection of the last confocal live-cell image and plunge-freezing (which may be long enough to lose correlated HIV-1 particles), a cryo-fluorescence light microscopy stage (Figure 3) was de.......

Discussion

We have presented a straightforward set of protocols to provide an advanced correlative approach to analyze dynamic cellular events using time-lapse confocal, live-cell fluorescence imaging followed by cryoET. Our methodological development to correlate 3D live-cell imaging with high-resolution cryoET is critical to investigate many challenging biological problems, such as visualizing rare, dynamic (not static, as previously reported), and diffraction limited viral particles and their interactions with host cells. The sp.......

Materials

Name	Company	Catalog Number	Comments
			Reagents
DMEM 4.5 g/L Glucose w/ L-Glutamine	Atlanta Biologicals, Inc. Lawrenceville, GA	12-604F
Fibronectin	Sigma, St. Louis, MO	F1141-1MG	HeLa cell culture on EM grids
Cell tracker	Invitrogen Corporation, Carlsbad, CA	C34552	Red CMTPX
Protein A gold conjugates	Ted Pella, Inc., Redding, CA	15822-1	15 nm diameter
0.2 μm fluorescent microspheres	Invitrogen Corporation, Carlsbad, CA	F8811	Yellow-green fluorescent
Heat-inactivated fetal calf bovine serum	Invitrogen Corporation, Carlsbad, CA	10082-139
Penicillin-Streptomycin	Invitrogen Corporation, Carlsbad, CA	15140-122
Glass-bottom culture dish	MatTek Corporation	P35G-1.5-14-C
Gold quantifoil finder EM grids	Quantifoil Micro Tools, Jena, Germany	R2/2 Au NH2 200 mesh
PBS	Invitrogen Corporation, Carlsbad, CA	70011-044
			Equipment
Glow-discharge device 100X	EMS, Hatfield, PA
Tecnai Polara G2 electron microscope with a Field Emission Gun	FEI, Hillsboro, OR		300 keV
Vitrobot Mark III	FEI, Hillsboro, OR
Olympus IX 71 microscope	Olympus America Inc., Center Valley, PA		LUCPlanFLN 40X/0.6 NA (2.7-4 mm working distance) objective lens
Nikon TiE microscope	Nikon Instruments, Melville, NY		using a 60X/1.35 NA oil immersion objective lens
Sweptfield confocal microscope	Prairie Technologies, Middleton, WI
Tokai Hit live cell chamber	Tokyo, Japan
Cryo-fluorescence sample stage	Home-made		Homebuilt by machine shop, see reference 15 for the design.