Visualizing Membrane Ruffle Formation using Scanning Electron Microscopy

Summary

Macropinocytosis is a highly conserved endocytic process initiated by the formation of F-actin-rich sheet-like membrane projections, also known as membrane ruffles. Increased rate of macropinocytotic solute internalization has been implicated in various pathological conditions. This protocol presents a method to quantify membrane ruffle formation in vitro using scanning electron microscopy.

Abstract

Membrane ruffling is the formation of motile plasma membrane protrusions containing a meshwork of newly polymerized actin filaments. Membrane ruffles may form spontaneously or in response to growth factors, inflammatory cytokines, and phorbol esters. Some of the membrane protrusions may reorganize into circular membrane ruffles that fuse at their distal margins and form cups that close and separate into the cytoplasm as large, heterogeneous vacuoles called macropinosomes. During the process, ruffles trap extracellular fluid and solutes that internalize within macropinosomes. High-resolution scanning electron microscopy (SEM) is a commonly used imaging technique to visualize and quantify membrane ruffle formation, circular protrusions, and closed macropinocytic cups on the cell surface. The following protocol describes the cell culture conditions, stimulation of the membrane ruffle formation in vitro, and how to fix, dehydrate, and prepare cells for imaging using SEM. Quantification of membrane ruffling, data normalization, and stimulators and inhibitors of membrane ruffle formation are also described. This method can help answer key questions about the role of macropinocytosis in physiological and pathological processes, investigate new targets that regulate membrane ruffle formation, and identify yet uncharacterized physiological stimulators as well as novel pharmacological inhibitors of macropinocytosis.

Introduction

Macropinocytosis is an endocytic process responsible for internalizing a large amount of extracellular fluid and its content via the formation of dynamic and actin-driven plasma membrane protrusions called membrane ruffles¹. Many of these membrane ruffles form cups that close and fuse back onto the cell and separate from the plasma membrane as large, heterogeneous intracellular endosomes also known as macropinosomes¹. Although macropinocytosis is induced by growth factors such as macrophage colony-stimulating factor (M-CSF) and epidermal growth factor (EGF) in a wide range of cell types, an additional unique, calcium-dep....

Protocol

NOTE: The following is a general protocol used to quantify membrane ruffle formation in RAW 264.7 macrophages using SEM microscopy. Optimization may be required for different cell types.

1. Cell line and cell culture

1. Grow RAW 264.7 macrophages in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% (vol/vol) heat-inactivated Fetal Bovine Serum (FBS), 100 IU/mL of penicillin and 100 µg/mL streptomycin in a humidified incubator at 37 °C and 5% CO₂. R.......

Discussion

The present SEM imaging protocol provides a tool to visualize and quantify membrane ruffle formation, circular protrusions, and macropinocytic cups on the cell surface in vitro. Although the current protocol focuses on macrophages, studies have shown that membrane ruffle formation also occurs on various other cell types including dendritic cells, fibroblasts, neurons, and cancer cells^11,12,14,

Materials

Name	Company	Catalog Number	Comments
0.5% Trypsin-EDTA	Gibco	15400-054
2% Glutaraldehyde	Electron Microscopy Sciences	16320
4% Paraformaldehyde	Santa Cruz Biotechnology	281692
5-(N-ethyl-N-isopropyl)-Amiloride	Sigma Life Science	A3085
Accuri C6 Flow Cytometer
Carbon Adhesive Tabs	Electron Microscopy Sciences	77825-09
Dimethyl Sulfoxide	Corning	25-950-CQC
Dulbecco's Modified Eagle Medium	Cytiva Life Sciences	SH30022.01
Falcon 24-well Clear Flat Bottom TC-treated Multiwell Cell Culture Plate	Falcon	353047
Fetal Bovine Serum	Gemini Bio	900-108
FitC-dextran	Thermo Fisher Scientific	D1823
FM 4-64	Thermo Fisher Scientific	T13320
HERAcell 150i CO2 incubator	Thermo Fisher Scientific	51026282
Hummer Model 6.2 Sputter Coater	Anatech USA	58565
JSM-IT500HR scanning electron microscope
Microscope Cover Glass	Thermo Fisher Scientific	12-545-82
Pen Strep	Gibco	15140-122
phorbol 12-myristate 13-acetate	Millipore Sigma	524400
RAW 264.7 macrophage	ATCC	ATCC TIB-71
Recombinant Human M-CSF	Peprotech	300-25
Samdri-790 Critical Point Dryer	Tousimis Research Corporation	8778B
SEM Aluminum Specimen Mounts	Electron Microscopy Sciences	75220
Sodium Cacodylate	Electron Microscopy Sciences	12300
Texas red-dextra	Thermo Fisher Scientific	D1864
Trypan Blue Solution	Thermo Fisher Scientific	15250061
Zeiss LSM 780 confocal microscope