© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.4.5.4 Vesicle Fusion and Endocytotic Entry

A nanorobot may also gain entry to a cell by releasing amphipathic lipid molecules from its surface, which will self-assemble into a thin lipid bilayer coat, enveloping the entire nanorobot. Natural viruses¹⁷²¹ and synthetic carriers¹⁷²² use similar methods to convey extracellular DNA across outer cell plasma membranes and nuclear membranes. The materials comprising a 10-nm thick lipid bilayer surrounding a spherical object ~1 micron in radius can be stored in a ~0.1 micron³ internal tank, occupying only ~3% of total internal device volume. The fusion of two distinct lipid bilayers is energetically unfavorable in the absence of specialized proteins.¹⁵³⁰ Thus after approaching the target plasma membrane, the enveloped nanorobot must emit specialized plasma membrane fusion proteins, known as fusogens,^1587,3658 similar to the sperm protein PH-30a-b that allows sperm and egg to merge,¹⁰⁸² the HIV virus envelope protein gp120-gp41 with a typical contact area of ~8 nm²,^1531,1587 the envelope glycoprotein H employed by the herpesvirsuses,³⁶⁵⁷ and the hemagglutinin protein found on the surface of influenza virus.¹⁵³² (Nonprotein fusogens also are well-known, including lipids such as N-acyl phosphatidylethanolamines (NAPEs),³⁶⁵⁹ fatty acids such as arachidonic acid,³⁶⁶⁰ carbohydrates such as polyethylene glycol,³⁶⁶¹ and simple solvents such as dimethyl sulfoxide (DMSO).³⁶⁶²) The nanorobot lipid bilayer, now fusion active, can join with the plasma membrane layer and unfold, bringing the formerly fully enclosed nanorobot into direct sealed contact with the cytosol with a negligible energy expenditure (Fig. 9.31). This process will add 0.5%-1.0% new lipid, by volume, to the existing ~14-24 micron³ plasma membrane volume (Table 8.17) of a 20-micron tissue cell. Added lipids that don't exactly match the native lipids of the cell might disrupt the normal protein/lipid membrane ratio and modify membrane fluidity, possibly altering signal timing and other cell behaviors. If this is a problem, nanorobots employing this entry technique could be designed to reabsorb and recycle the foreign lipid, once inside the cell.

Other means of cell entry, such as "induced uptake," are well known. Like bacterial invaders, a nanorobot may emit cytochemical "entry mediator" signals (e.g., the outer wall membrane protein invasin, in Yersinia bacteria) to induce the target cell to form an endocytotic vacuole around the device and suck it into the interior of the cell. A membrane fusion-type tool may subsequently be used to escape from the resulting endosome. Macrophages ingest up to ~25% of their volume per hour,⁹⁹⁶ a volume equivalent to hundreds of micron-sized nanorobots. Many bacteria^1012,1561 and viruses^1530,1533 gain entry to cells by this means, and phagocytosis is mechanically similar. Both are initiated by ligand-receptor interactions that activate host signaling, with the actin cytoskeleton providing the necessary force to internalize the particle into a membrane-bound vacuole.¹⁰¹² Invasive bacteria utilize two major types of induced uptake:

1. a "zipper" type mechanism involving direct contact between bacterial ligands and cellular receptors that sequentially encircle the organism (e.g., Yersinia, Listeria); and

2. a "trigger" mechanism in which bacterial signals to the cell induce dramatic plasma membrane ruffling and cytoskeletal rearrangements resulting in macropinocytosis with virtually passive entry of the bacterium (e.g., Salmonella, Shigella) into the cell.^{1012,1561,3191}

Last updated on 21 February 2003