2014 Group 1 Project

From CellBiology

UNSW ANAT3231 Course Coordinator Dr Mark Hill
2014 Projects: Group 1 | Group 2 | Group 3 | Group 4

Transport into the cell from the plasma membrane (Endocytosis)

Introduction

Phagocytosis is a specialised type of endocytosis where large (≥0.5 μm) solid particles are internalised through the receptor-mediated engulfment of membrane-derived vesicles called phagosomes. After the vesicles detach from the plasma membrane (scission), the phagosome matures by fusing with endosomes and lysosomes (which contain hydrolytic enzymes) to form a phagolysosome. The hydrolytic enzymes in the phagolysosome break down the internalised solid particles. The mechanism behind Phagocytosis is clathrin independent and usually requires actin polymerisation[1]. Phagocytosis is triggered by the interaction between molecules on the particle surface and receptors on the phagocytic cell. Particles may be directly recognised by the receptors or may be tagged by opsonins which can then be recognised by specific receptors.

http://www.sciencedirect.com/science/article/pii/S0960982211006117 http://www.ncbi.nlm.nih.gov/pubmed/18085665 http://www.annualreviews.org.wwwproxy0.library.unsw.edu.au/doi/full/10.1146/annurev.immunol.17.1.593 http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040190

History

Structure of Plasma Membrane

Plasma membranes surround the cell, that is composed of lipid bilayer. The lipid bilayer consists of double-layered amphipathic phospholipids that reflect each other; individually these phospholipids have a hydrophilic polar head and 2 uncharged, hydrocarbons, hydrophobic tails. The hydrophilic head faces outwards, i.e. articulates with the extracellular environment and internal cellular department. and the hydrophobic tails face towards each other, internally. This orientation is energetically favourable. This amphipothic property allows self-sealing, however this causes the plasma membrane to have no intrinsic strength.

Membrane proteins are specialized proteins that carryout specific functions for the membrane and are either embedded in the inner, outer or both phospholipid layers. Glycoproteins are proteins with polymers of oligosaccharides (sugar short chains) that coat the surface (non-cytosolic side) of all eukaryotic cells. This is called the carbohydrate layer of the cell, which functions to protect the cell surface from damage, provide lubrication for assisting with motility and cell-cell recognition and adhesion.

Cholesterol molecules are also abundant in the membrane and closely associate with the phospholipids and enhance permeability-barrier properties of the bilayer by decreasing the deformability regions of the phospholipids and prevent crystallization of hydrocarbon chains.

Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Lipid Bilayer. [1]

Mechanism of different endocytosis (how material is transported through the membrane and into the cell)

Phagocytosis

The mechanism by which relatively large particles (>~0.S µm) are taken into a cell is called Phagocytosis. Phagocytosis is a type of endocytosis that is clathrin independent and usually requires actin polymerisation.

Morphological Mechanisms

The article ‘Contemporaneous cell spreading and phagocytosis: Magneto-resistive real-time monitoring of membrane competing processes’ by Shoshi et al investigate that during phagocytosis the cell membrane expands, to engulf particles (in this case beads on surfaces), by cell spreading. The engulfment rate was additionally measured using real-time magneto-resistive monitoring, with an average of 3 beads per minute. Correspondingly, the rate of engulfment was not a linear function but is high at an early stage, then decreases steadily until saturation.


The article ‘Plasma membrane tension orchestrates membrane trafficking, cytoskeletal remodeling, and biochemical signaling during phagocytosis’ focus on the 2 phases of pseudopod extension that included actin polymerization pushing the membrane forward and increased membrane tension using high-resolution microscopy of macrophages attempting to internalize an IgG-opsonized glass surface. A 50% increase in tether force was observed in phagocytic cells, compared to the resting cells membrane tension. Additionally, inward bead movement (engulfment) and ingestion is most probably due to contraction and exocytosis activation. This confirms that membrane tension is an exocytosis activator and that exocytosis is required for phagocytosis to complete.


(Tollis, Dart et al. 2010) Tollis, S., et al., The zipper mechanism in phagocytosis: energetic requirements and variability in phagocytic cup shape. BMC Syst Biol, 2010. 4: p. 149.

The engulfment of particles by phagocytosis is demonstrated by the zipper mechanism, where plasma membrane receptors bind with ligands from particle surface, like a zipper, and then the pseudopods makes it way around the particle by attaching to each ligand (fig ) forming a phagocytic cup. The zipper model can be typed as active or passive. The passive zipper model are not supported by actin polymerization instead by ligand-receptor bonds and is therefore slower and effective on small particles, but more variable cup shape. Active zipper model involves actin polymerization is faster then passive and can take large particles as well + irreversible.

Actin fills in gaps / stops pseudopods from forming back.

{The role of actin polymerization in phagocytosis is hence to stabilize ligand receptor bonds and to rectify membrane movements in a ratchet-like fashion, leading to unidirectional movement of the leading edge of the engulfing cell.}

Engulfment can be affected by the particle shape and temperture


Champion, J. A. & Mitragotri, S. ‘Role of target geometry in phagocytosis.’ Proc. Natl Acad. Sci. USA .103, 4930–4934 (2006).

‘Shows that uniformly opsonized particles of various shapes are only ingested if the surface that contacts the macrophage membrane is less than a minimum tangent angle. This indicates a level of signal integration in forming phagocytic cups’


Joel A. Swanson, Melissa T. Johnson , Karen Beningo , Penny Post , Mark Mooseker and Nobukazu Araki ‘Contractile activity in macrophage phagosomes’ Journal of Cell Science 112, 307-316 (1999)

The phagocyte extends its pseudopodia around a particle, then a constriction occurs around at the pseudopod margin to close the phagosome. The pseudopod extension orientates around the phagosome/particle to envelope The pseudopod extension is caused by actin polymerization at the distal margin of the closing phagosome, beneath the plasma membrane. In fig 1, the fluorescent actin inside macrophages, actin concentrations were highest at the distal margins of closing phagosomes (at 1.5 minutes, 3.5 minutes and 4 minutes). Bar, 10 mm.


Professional and non-professional phagocytosis in trerms in apatocic cell clearance

Professional

Macrophages
Neutrophils

Non-Professional

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Diseases related to endocytosis

Current/future research

Complications

References

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