Nanopodia - Thin, Fragile Membrane Projections with Roles in Cell Movement and Intercellular Interactions

Summary

Nanopodia are thin but fragile membrane channels that extend up to 100 μm from a cell's leading front or trailing rear and sense the cellular environment. Direct fixation at 37 °C, gentle washing, and avoidance of organic solvents like ethanol, methanol, or acetone and of higher Triton X-100 concentrations are required to observe these cellular structures.

Abstract

Adherent cells in culture maintain a polarized state to support movement and intercellular interactions. Nanopodia are thin, elongated, largely F-actin-negative membrane projections in endothelial and cancer cells that can be visualized through TM4SF1 (Transmembrane-4-L-six-family-1) immunofluorescence staining. TM4SF1 clusters in 100-300 μm diameter TMED (TM4SF1 enriched microdomains) containing 3 to as many as 14 individual TM4SF1 molecules. TMED are arranged intermittently along nanopodia at a regular spacing of 1 to 3 TMED per μm and firmly anchor nanopodia to matrix. This enables nanopodia to extend more than 100 μm from the leading front or trailing rear of polarized endothelial or tumor cells, and causes membrane residues to be left behind on matrix when the cell moves away. TMED and nanopodia have been overlooked because of their extreme fragility and sensitivity to temperature. Routine washing and fixation disrupt the structure. Nanopodia are preserved by direct fixation in paraformaldehyde (PFA) at 37 °C, followed by brief exposure to 0.01% Triton X-100 before staining. Nanopodia open new vistas in cell biology: they promise to reshape our understanding of how cells sense their environment, detect and identify other cells at a distance, initiate intercellular interactions at close contact, and of the signaling mechanisms involved in movement, proliferation, and cell-cell communications. The methods that are developed for studying TM4SF1-derived nanopodia may be useful for studies of nanopodia that form in other cell types through the agency of classic tetraspanins, notably the ubiquitously expressed CD9, CD81, and CD151.

Introduction

During polarization for movement, animal cells extend a variety of dynamic, membrane protrusions from their surfaces, including filopodia, lamellipodia, retraction fibers, and ruffles¹. Recently added to this list were nanopodia, a newly recognized type of thin (100-300 μm in diameter), elongated (up to 50-100 μm long) membrane projection that provide membrane channels for the extension of F-actin structures such as filopodia and retraction fiber, and that stain positively with TM4SF1 (Transmembrane-4-L-six-family-1) in cultured endothelial and tumor cells^2,3.

TM4SF1 is a protein with tetraspanin-like topolog....

Protocol

1. Cell Culture on Collagen Coated Glass Disk

1. Place glass disks (12 mm in diameter) in a glass jar (4 oz) and autoclave to sterilize the discs.
2. Put 25 ml 70% ethanol in a 50 ml Falcon tube and place it in a cell culture hood. This solution can be reused multiple times until the level of the solution drops to 20 ml.
3. Place a sharp forceps with an extra fine point in the 70% ethanol for 5 min before using it to handle the glass disk.
4. Carefully remove the forceps out of the tube and close the cap, then gently place the ethanol treated forceps on top of the tube to air dry for 2 min. Do not allow the tip of the forceps to touc....

Results

For Step 1:

If cells (such as HUVEC and PC3 used in this study) are able to grow normally, cells will attach to the collagen-coated disc within 30 min after they are seeded, polarize and become mobile soon afterward, and extend nanopodia ahead of their path of movement. Figures 1A and 1B respectively shows a polarized and proliferating HUVEC. Figure 2A shows polarized PC3 cells in a mobile state.

For Step 2:

Discussion

Nanopodia are thin cellular membrane channels that firmly attach to matrix through TMED and can extend more than 100 μm from a polarized mobile cell to sense the environment and mediate intercellular interactions^2,3. Nanopodia adhere to matrix so firmly that residues are left behind as the cell moves away (Figures 1 and 2). Nanopodia thus allow us to study how cells sense their environment and determine the path of cell movement, and how gene expression or drug treatment .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Glass disks	Fisher Scientific	12-545-82	12 mm diameter
Glass jar	Fisher Scientific	02-912-310	Certified Clean Clear Glass Straight-Sided Jars, 4 oz
Glass slide	Fisher Scientific	12-544-1	Fisherfinest Premium Plain Glass Microscope Slides
50 ml Falcon tube	BD Falcon	352070	50 ml high-clarity polypropylene conical centrifuge tube, 16,000 rcf rating. Sterile.
15 ml Falcon tube with Styrofoam rack	Corning	430790	Corning 15 ml PP Centrifuge Tubes
Sharp forceps	Fisher Scientific	13-812-42	Dissecting Extra Fine Pointed Splinter Forceps
24-well Cell culture plate	BD Bioscience	353047	24-well Cell Culture Plate
150 mm Cell culture plate	BD Bioscience	353025	150 mm cell culture plate
19 G 1 ½ Syringe needle	BD Bioscience	309635	19 G 1 ½
Ethanol	Decon Laboratories, Inc.	2701	Decon's Pure Ethanol 200 Proof
Collagen solution	BD Bioscience	354249	Collagen I, High Concentration, rat tail, 100 mg
Trypsin/EDTA	Cellgro	25-053-CI	0.25% Trypsin-EDTA 1x
DMEM	Life Technologies	11965-092	DMEM high glucose (1x), liquid, with L-glutamine, without sodium pyruvate
10x PBS	Affymetrix	75889 5 LT	PBS, 10x Solution, pH 7.4
PFA (paraformaldehyde)	Affymetrix	19943 1 LT	4% in PBS
FBS	Sigma-Aldrich	F4135-500ML	Fetal Bovine Serum
EGM2-MV	Lonza	CC-3162	EGM-2 BulletKit, EBM-2 Basal Medium 500 ml and EGM-2 SingleQuot Kit Supplement & Growth Factors
Triton X-100	Sigma-Aldrich	T8787	Triton X-100
NaN3	Sigma-Aldrich	S2002-25G	Solubilized in water. 4% stock solution and working concentration is 0.4%.
Mouse anti-human TM4SF1 antibody	Millipore	MAB3127	Epitope is located in extracellular domain
Mouse anti-human CD9 antibody	BD Bioscience	555370	Epitope is located in extracellular domain
Alexa Fluor 488 Donkey anti-mouse 2nd antibody	Life Technologies	A-21202	Alexa Fluor 488 Donkey Anti-Mouse IgG (H+L)
Phalloidin	Chemicon	90324	Rhodamine-conjugated Phalloidin
Anti-fade mounting media	Life Technologies	P-36931	ProLong Gold Antifade Reagent with DAPI
70% Ethanol			17.5 ml of ethanol (200 proof) 7.5 ml of double-deionized water
10x PBS (make 200 ml)			20 ml of 10x PBS 180 ml of double-deionized water
20% Triton X-100 (make 20 ml)			4 ml Triton X-100 16 ml double-deionized water
4% NaN3 (make 25 ml)			1 g of NaN3 25 ml of double-deionized water
50 ng/ml Bovine collagen solution in PBS (make 5 ml)			Stock solution of 50 μg/ml (50 ml) 830 μl of Collagen I (3 mg) 50 ml PBS
ICC Blocking Buffer (50 ml)			49 ml PBS 2% FBS (add 1 ml) 0.04% NaN3 (add 100 μl of 4% NaN3)
ICC Blocking Buffer/0.01% Triton X-100 (make 50 ml)			50 ml of ICC Blocking Buffer 25 μl of 20% Triton X-100
HUVEC	Lonza	C2517A	www.lonza.com
PC3	ATCC	CRL-1435	www.atcc.org
Cell culture hood	NuAIRE	Nu-425-600	NU-425 (Series 60) Biological Safety Cabinet
37 °C, 5% CO2 Cell culture incubator	CellStar	QWJ300DABA	Cellstar CO2 Water Jacketed Incubator
Heating pad	K&H Manufacturing	1020	K&H Lectro Kennel Heated Pad with Free Fleece Cover (www.amazon.com)
Centrifuge	Sorvall	T6000B	Sorvall T6000 (B) Benchtop Centrifuge
Hemocytometer	Sigma-Aldrich	Z359629	Bright-Line Hemocytometer