Condensed mitotic chromosome structure at nanometer resolution using PALM and EGFP- histones.
Photoactivated localization microscopy (PALM) and related fluorescent biological imaging methods are capable of providing very high spatial resolutions (up to 20 nm). Two major demands limit its widespread use on biological samples: requirements for photoactivatable/photoconvertible fluorescent molecules, which are sometimes difficult to incorporate, and high background signals from autofluorescence or fluorophores in adjacent focal planes in three-dimensional imaging which reduces PALM resolution significantly. We present here a high-resolution PALM method utilizing conventional EGFP as the photoconvertible fluorophore, improved algorithms to deal with high levels of biological background noise, and apply this to imaging higher order chromatin structure. We found that the emission wavelength of EGFP is efficiently converted from green to red when exposed to blue light in the presence of reduced riboflavin. The photon yield of red-converted EGFP using riboflavin is comparable to other bright photoconvertible fluorescent proteins that allow <20 nm resolution. We further found that image pre-processing using a combination of denoising and deconvolution of the raw PALM images substantially improved the spatial resolution of the reconstruction from noisy images. Performing PALM on Drosophila mitotic chromosomes labeled with H2AvD-EGFP, a histone H2A variant, revealed filamentous components of ∼70 nm. This is the first observation of fine chromatin filaments specific for one histone variant at a resolution approximating that of conventional electron microscope images (10-30 nm). As demonstrated by modeling and experiments on a challenging specimen, the techniques described here facilitate super-resolution fluorescent imaging with common biological samples.
Matsuda
Atsushi
A
Department of Biochemistry and Biophysics, The Keck Center for Advanced Microscopy, University of California San Francisco, San Francisco, California, United States of America.
Shao
Lin
L
Boulanger
Jerome
J
Kervrann
Charles
C
Carlton
Peter M
PM
Kner
Peter
P
Agard
David
D
Sedat
John W
JW
D000818:Animals; D022004:Chromosome Structures; D059006:Chromosomes, Insect / Q000737:chemistry* / Q000235:genetics*; D004330:Drosophila / Q000737:chemistry / Q000235:genetics*; D049452:Green Fluorescent Proteins / Q000737:chemistry; D006657:Histones / Q000737:chemistry*; D008856:Microscopy, Fluorescence / Q000295:instrumentation / Q000379:methods*; D008938:Mitosis
D006657:Histones; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins