© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.4.3.6.5 Inhibit Enclosure and Scission

Assuming that a medical nanorobot has become partially or wholly engulfed by a phagocyte, it is likely that the vacuole can still be prevented from pinching off and separating into a free intracellular phagosome containing the nanorobot (i.e., enclosure and scission).

Cells normally internalize soluble ligands or small particles via endocytosis, and large particles via actin-based phagocytosis. The dynamin family of GTPases [3416, 3417] mediates the membrane destabilization, constriction, fission (scission) and trafficking of endocytic vesicles from the plasma membrane, but dynamin-2 also has a role in phagocytosis by macrophages [3418]. Experiments reveal that early phagosomes (vacuoles) are enriched in dynamin-2, and inactive mutant versions of this molecule, if expressed, inhibit particle internalization at the stage of membrane extension around the particle [3418]. This arrest of phagocytosis resembles that seen with PI3K inhibitors, preventing the recruitment of dynamin to the site of particle binding. Dynamin is a microtubule-binding enzyme with a microtubule-activated GTPase activity; phosphorylation engages its activity [3419]. Dynamin can interact with the actin cytoskeleton to regulate actin reorganization and subsequently cell shape [3420].

Observations suggest that dynamin mediates scission from the plasma membrane of both clathrin-coated pits and caveolae during distinct endocytic processes [3421]. For example, dynamin-1 is a 100 kD GTPase involved in scission of endocytic vesicles from the plasma membrane. It is present in solution as tetramer. Following its recruitment to coated pits, dynamin-1 undergoes self-assembly into higher-order oligomers that resemble collars around the necks of nascent coated buds. GTPase hydrolysis by dynamin in these collars is thought to accompany the pinching off of endocytic vesicles [3422]. Dynamin may use GTPase hydrolysis physically to drive vesiculation, or may act as a classical G protein switch, or both [3423]. Laboratory work shows that purified dynamin readily self-assembles into rings or spirals, suggesting that it probably wraps around the necks of budding vesicles and squeezes, pinching them off [3424-3427] – in other words, the large GTPase dynamin is a mechanoenzyme [3428]. Different dynamin isoforms may be localized to distinct cellular compartments but may provide similar scission functions during the biogenesis of nascent cytoplasmic vesicles [3421]. Once again, inhibitory tools that might be employed by medical nanorobots are potentially available. For example, anti-dynamin antibodies have been used to specifically inhibit dynamin function in cultured mammalian epithelial cells, inhibiting cellular uptake of external particles in these cells [3421]. These antibodies also have been used to inhibit clathrin-mediated endocytosis in hepatocytes [3429]. Ca⁺⁺ inhibits both dynamin I GTPase [3430] and dynamin II GTPase [3431] and may also serve as a vesiculation inhibitor for engulfed medical nanorobots. Alternatively, butanedione monoxime, a class II myosin inhibitor, has been shown to prevent the purse-string-like contraction that closes phagosomes without inhibiting the initial pseudopod extension [3432].

Another approach for trapped but not yet enclosed medical nanorobots relies upon the observation [3433] that internalization of encapsulated particles via endocytosis produces a net increase in the total cell surface area of the ingesting leukocytes. This suggests that exocytosis is occurring simultaneously [3433]. If the phagocyte’s ability to recycle plasma membrane to the cell surface is interrupted, endocytosis eventually halts. Accordingly, in one experiment [3433], selective cleavage (disablement) of components of the secretory machinery using bacterial neurotoxins induced a pronounced inhibition of phagocytosis. Unlike many other cell types, macrophages lack a morphologically distinct pericentriolar recycling compartment but instead have an extensive network of transferrin receptor-positive tubules and vesicles that participate in recycling [3434]. Transferrin is recycled rapidly: the GTPase Rab11 participates in the recruitment of a rapidly mobilizable endocytic compartment to the macrophage cell surface by mediating the transferrin efflux [3434]. Chemical inhibition of Rab11 [3435] or of phospholipid synthesis [3436] could therefore slow this efflux, ultimately restricting the turnover of phagocyte plasma membrane, which could greatly slow the rate of particle internalization and give the trapped nanorobot more time to escape.

Last updated on 30 April 2004