A Multicolor 3D-STORM High-Resolution Visualization Protocol for Collagen Mineralization in Self-Assembled Recombinant Type I Collagen Fibrils

Summary

Here, we present a protocol to distinguish intrafibrillar versus extrafibrillar mineralization in recombinant collagen fibrils using multicolor 3D‑STORM, integrating optimized labeling, imaging, and quantitative colocalization analysis.

Abstract

This protocol describes a multicolor three-dimensional stochastic optical reconstruction microscopy (3D-STORM) method for nanoscale visualization of collagen mineralization in a recombinant type I collagen self-assembled fibril model. The method enables simultaneous imaging of collagen, non-collagenous proteins (e.g., chondroitin sulfate), and calcium phosphate mineral phases. Sample preparation involves amino‑silanization and collagen self‑assembly, followed by mineralization using a calcium phosphate medium that forms amorphous calcium phosphate (ACP) at an early stage (30 min) and matures into hydroxyapatite (HAP) by 6 h. Multiplexed immunofluorescence labeling is then performed, and samples are first assessed by confocal microscopy before 3D-STORM image acquisition using an oxygen-scavenging imaging buffer. Data processing and analysis are carried out using publicly available software. Compared to conventional electron or confocal microscopy, this protocol combines molecular specificity with nanoscale resolution (typical lateral precision 20–30 nm, axial 50–60 nm), allowing three‑dimensional visualization of intrafibrillar versus extrafibrillar mineralization patterns. Representative results show clear visualization of collagen networks, associated non-collagenous proteins, and mineral phases within three-dimensional space. Quantitative metrics including Pearson’s correlation coefficient (0.89 ± 0.04) and Manders’ overlap coefficient (0.91 ± 0.03) are provided in the Results section. This protocol offers a powerful tool for researchers in biomaterials science, biomineralization, and bone tissue engineering who require nanoscale insight into mineralization dynamics.

Introduction

Collagen mineralization is a fundamental biological process pivotal in the formation of hard tissues such as bones and teeth¹. The intricate structure of collagen fibers, coupled with finely tuned regulation of mineral deposition, endows remarkable mechanical strength and structural integrity to these tissues². Collagen serves not merely as a passive scaffold but as an active participant, orchestrating precise mineral deposition through complex molecular and physical interactions³. Elucidating these mechanisms is crucial for understanding pathological conditions such as osteoporosis and dental car....

Protocol

All experiments involving biological samples were conducted in accordance with the guidelines and regulations of the Core Facilities, Zhejiang University School of Medicine and were approved by the Institutional Biosafety Committee (Approval Certificate No. BSL20235710079). The experimental protocol described herein utilizes commercially sourced reagents and in vitro biomimetic systems. It does not involve human participants, animal subjects, or human tissue samples, and therefore does not require ethical approval from an institutional review board.

CAUTION: All procedures involving hazardous chemicals must be performed in a fume hood with ....

Results

Successful implementation of this protocol yields a high‑resolution three‑dimensional visualization of mineralized collagen fibrils using multicolor 3D‑STORM. The following results illustrate typical outcomes, quality controls, and quantitative assessments.

Figure 1 shows a multicolor 3D‑STORM reconstruction of a collagen network mineralized with amorphous calcium phosphate (ACP). Collagen (labeled with a far‑red fluorescent dye) appears as a well‑defined fibrilla.......

Discussion

This protocol provides a comprehensive workflow for nanoscale visualization of collagen mineralization using multicolor 3D-STORM. Several critical steps require particular attention to ensure successful outcomes.

First, sample preparation is foundational for high-quality STORM imaging. The amino-silanization of glass-bottom dishes must be thorough to ensure stable attachment of collagen fibrils throughout subsequent washing and labeling steps. Residual APTES can cause nonspecific binding and h.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Polyaspartic acid (p-Asp)	Sigma-Aldrich	P9903	Stabilizer for amorphous calcium phosphate
Calcium chloride (CaCl2)	Sigma-Aldrich	C1016	Calcium source
Sodium phosphate dibasic (Na2HPO4)	Sigma-Aldrich	S0876	Phosphate source
Sodium chloride (NaCl)	Sigma-Aldrich	S9888	Ionic strength adjuster
Polyacrylic acid (PAA)	Sigma-Aldrich	323667	Stabilizer for high-concentration calcium
Tris base	Sigma-Aldrich	T1503	Buffer component
Sodium azide (NaN3)	Sigma-Aldrich	S2002	Antimicrobial agent
(3-Aminopropyl)triethoxysilane (APTES)	Sigma-Aldrich	440140	Glass surface functionalization agent
Absolute ethanol	Sigma-Aldrich	459836	Solvent
Type I collagen solution (50 μg/mL in 0.1 M acetic acid)	Corning	354249	Self-assembly scaffold
Chondroitin sulfate (CS)	Sigma-Aldrich	C9819	Non-collagenous protein mimic
EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide)	Sigma-Aldrich	E7750	Crosslinker
NHS (N-hydroxysuccinimide)	Sigma-Aldrich	130672	Crosslinker activator
MES free acid	Sigma-Aldrich	M5287	Buffer for crosslinking
Phosphate-buffered saline (PBS)	Gibco	10010023	Washing and dilution buffer
Bovine serum albumin (BSA)	Sigma-Aldrich	A3059	Blocking agent
Rabbit anti-collagen-I antibody	Abcam	ab34710	Primary antibody for collagen
Mouse anti-chondroitin sulfate antibody	Sigma-Aldrich	C8035	Primary antibody for CS
Goat anti-rabbit IgG conjugated to far-red fluorescent dye (Alexa Fluor 647)	Thermo Fisher Scientific	A-21244	Secondary antibody for collagen
Goat anti-mouse IgM conjugated to red fluorescent dye (Alexa Fluor 568)	Thermo Fisher Scientific	A-11031	Secondary antibody for CS
Calcein (calcium indicator dye)	Sigma-Aldrich	C0875	Calcium phosphate label
Tween-20	Sigma-Aldrich	P1379	Detergent for washing buffer
Glycerol	Sigma-Aldrich	G5516	Imaging buffer component
Glucose oxidase (GOx)	Sigma-Aldrich	G7141	Oxygen scavenger
Catalase	Sigma-Aldrich	C1345	Oxygen scavenger
Cysteamine (MEA)	Sigma-Aldrich	M6500	Thiol for fluorophore blinking
D-Glucose	Sigma-Aldrich	G6152	Substrate for glucose oxidase
Sodium acetate	Sigma-Aldrich	S2889	Buffer for GOx stock
Hydrochloric acid (HCl)	Sigma-Aldrich	320331	pH adjustment
Sodium hydroxide (NaOH)	Sigma-Aldrich	71690	pH adjustment
Phosphotungstic acid	Sigma-Aldrich	P4006	Negative stain for TEM
Glass-bottom culture dishes (35 mm, #1.5H)	MatTek	P35G-1.5-14-C	Sample substrate; thickness 0.17 mm
Ultrasonic cleaner (40 kHz)	Branson	B200	Cleaning device
Humidity chamber	Thermo Fisher Scientific	11-432-10	For collagen self-assembly
Transmission electron microscope	Hitachi	HT7800	TEM imaging
Formvar/carbon-coated TEM grids (200 mesh)	Sigma-Aldrich	FCF200-Cu	TEM sample support
Horizontal shaker platform	Labnet	S2030-RC	Gentle washing
Confocal laser scanning microscope	Nikon	A1	Preliminary screening
3D-STORM microscope system (with 405/488/647 nm lasers, cylindrical lens, EMCCD)	Nikon	N-STORM	Super-resolution imaging
100× oil immersion objective (NA 1.49)	Nikon	MRD01991	High-resolution imaging
pH meter	Mettler Toledo	FiveGo F2	pH control
STORM acquisition and analysis software	Nikon	NIS-Elements (STORM module)	STORM data acquisition and processing
.nd2 file format (raw microscopy image file)	Nikon	N/A	Raw image file format generated by Nikon microscopes.
Publicly available image analysis software	Open source	N/A	e.g., ImageJ with ThunderSTORM plugin for single-molecule localization analysis (colocalization, drift correction)
Parafilm	Bemis	PM996	Sample covering during incubation
Aluminum foil	Any laboratory supplier	N/A	For light protection (e.g., wrapping samples)
Amber microcentrifuge tubes	Fisher Scientific	05-669-21	For light protection of fluorophores
Coverslips (No. 1.5)	Corning	2855-18	Sample mounting