Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
22.214.171.124 Nucleonavigational Issues
Cytoplasmic medical nanorobots can perform many useful tasks without entering the nucleus. Such tasks might include:
1. physical mapping and compositional analysis of the nuclear envelope;
2. monitoring of nuclear pore traffic;
3. near-complete regulation of nuclear pore traffic using multiple manipulators or other devices;
4. monitoring, initiating, or modifying cytoskeletally-mediated mechanical signal transduction into the nuclear interior;
5. injection of enzymes, RNA or DNA fragments, or other bioactive materials through nuclear pores using hollow nanoinjectors; or
6. partial or complete nucleoplasmic replacement using artificial chromatin detachment enzymes dispensed by multiple injection and extraction nanorobots positioned at the nuclear pores, operated simultaneously and located antipodally around the nucleus to establish flowthrough.
Entering the nucleus is somewhat more difficult than entering the cytoplasmic space through the plasma membrane (Section 126.96.36.199). One reason is that the nucleus is almost completely surrounded by the membranes of the endoplasmic reticulum (Fig. 8.45), and the nearest of these membranes may lie only a few hundred nanometers from the outermost surface of the nuclear envelope (Fig. 8.46). The nuclear envelope is also a double-walled membrane. Another reason why nuclear entry is difficult is that physical forces applied to cytoskeletal elements in the immediate vicinity of the nucleus may trigger unwanted transcriptional, structural, or metabolic responses from within the nucleus. Additionally, the 10-100 nm grid size of the nuclear matrix filaments and the DNA rosette loops may allow little free maneuvering room for nanorobots, which may find it difficult to avoid tearing the nucleoskeleton during passage.
Given that the nucleus of a 20 micron human cell is only ~8 microns in diameter and thus encloses a volume of at most ~268 micron3 (Table 8.17), there is precious little maneuvering room for a medical nanorobot that may be 1-30 micron3 in size. Once inside the nucleus, a principle safety concern must be avoiding damage to the relaxed euchromatin strands that permeate the nucleoplasmic space. Only ~0.1 pN of force is required to move chromosomes around the nucleus at ~0.1 microns/sec during mitosis.1463 DNA base pair hydrogen bonds may be pulled apart with 70-75 pN/bond of force,1066 although the DNA backbone itself can withstand up to ~10 nN in tension; most conservatively, nanorobots should never apply tensile or shear forces greater than ~50 pN to chromatin strands during nucleoplasmic locomotion. Care should also be taken to design nanorobot exterior surfaces that are free of sharp edges and which possess electrochemical characteristics that tend to be nonattractive to DNA and to nucleosomal components. Since DNA is negatively charged and histones are positively charged (to bind the DNA), and since the lipid membranes are internally hydrophobic but polar on the surface, the ideal nanorobot exterior might require a surface of alternating charges which is nonhydrophobic and neutral with regard to DNA, histones, and lipid surfaces.
How can a nanorobot determine its intranuclear position? Chemonavigation is one crude approach. For example, a large number of stable RNA species -- mostly the small nuclear ribonucleoproteins (snRNPs) essential for pre-messenger RNA splicing -- are found in the nucleoplasm, the cytoplasm, or both. They range in size from 90-300 nucleotides and are present in 105-106 copies per cell (Table 8.19). snRNPs from HeLa (cancer line) cells contain ~40 proteins, 8 of which are shared by all snRNPs while the remaining ~32 proteins are snRNP-type specific.1074 Nucleolus-specific proteins include UBF (94-97 kD), Ki-67 (345-390 kD), fibrillarin (34 kD), numatrin or B23 (38 kD), and nucleolin (100-110 kD);1141 different nucleolar markers appear during mitosis. Detection of these species via chemosensors allows a nanorobot to distinguish cytoplasmic, nucleoplasmic, and nucleolar spaces, and possibly may permit even finer localizations. Various nucleic and protein products are continuously being transcribed from various chromosomes, whose distinct territories within the nucleus are relatively fixed during interphase (Sections 188.8.131.52 and 184.108.40.206). Detection of specific proteins and intermediate RNA transcription products in the nucleosol thus may allow chromosomal segment localization without having to actually sequence the segment (e.g., in early-generation medical nanorobotic systems).
Nanorobots requiring a 100-nm three-dimensional Cartesian navigation grid inside the nucleus may exploit the relatively uniform acoustic characteristics of the extranucleolar nucleoplasm by tethering a minimum of four small acoustic beacons tetrahedrally placed near the inside surface of the nuclear cortex at the antipodes of three transnuclear orthogonal coordinate axes. Each beacon emits brief ~1 GHz chirps in a distinguishable format unique to each beacon, at a very low duty cycle to avoid excessive energy consumption. From Eqn. 4.52, each chirp attenuates ~10-3 in amplitude per 100 nm of travel through the nucleoplasmic fluid. Hence an acoustic detector sensitive to 10-6 atm pressure changes can establish nanorobot position relative to each beacon to ~100 nm accuracy if the beacon output amplitude during the brief chirp is ~10-3 atm. This should be low enough to avoid any possibility of cavitation damage to the chromatin or nuclear matrix. As a practical matter, additional beacons may be needed to reduce measurement uncertainties caused by nucleoplasmic nonuniformities and post-placement beacon positional shifts.
Last updated on 20 February 2003