Energy and Information in Microtubules

Direct support for the propagation of signals in MT has been generated by Vassilev, Kanazirska, and Tien (1985) who reconstituted bilayer membranes from brain lipids and studied their electrical excitability. They suspended membranes as parallel unconnected plane surfaces separated several millimeters apart in a buffer solution which contained depolymerized tubulin, GTP, and other physiological components. Each membrane was monitored electrically and baseline recording of the two membranes showed no electrical coupling; when one membrane was electrically stimulated it depolarized, but the other membrane remained silent. When the tubulin was caused to polymerize into MT (by lowering calcium ion concentration) MT bridges formed between the two membranes and electrical coupling between the two membranes was observed. Electrical stimulation of one membrane then resulted in depolarization in both membranes. The addition of the MT destabilizing drug colchicine prevented coupling, demonstrating that intact microtubules were necessary. The authors concluded that intermembrane signaling occurred by electrically induced polarization and conformational changes of MT components which linked the two membranes. They suggested that similar communication functions occurred routinely within the cytoskeleton.

Another series of experiments which supports the notion of MT mediated signaling is fluorescence resonance transfer among MT and membrane components. Becker, Oliver and Berlin (1975) developed a technique to study energy transfer among fluorescent groups separately attached to different MT subunits or to membranes. Resonance energy transfer occurs when a fluorescent portion of a molecule ("chromophore") which is electronically excited by the absorption of light energy transmits that energy to another "acceptor" chromophore some distance away. This transmission requires the overlap of the emission spectrum of the "donor" chromophore with the absorption spectrum of the acceptor, without involving the actual reabsorption of light by the acceptor. The process is therefore referred to as "nonradiative" resonance energy transfer and occurs if the distance between the chromophores is relatively close, not to exceed about 10 nanometers. In the study by Becker, Oliver, and Berlin, fluorescein isothiocyanate (FITC) was used to fluorescently label one population of unpolymerized MT subunits or membranes, and another fluorescent label, rhodamine isothiocyanate (RITC) was used to label a second population of tubulin. They chose these chromophores because they bind covalently to tubulin or membranes, and because the emission spectrum of FITC "donors" extensively overlaps the absorption spectrum of RITC acceptors. Recordings of fluorescence spectra reveal the "resonance transfer" when it occurs. When MT were depolymerized, a mixture of donor labeled and acceptor labeled tubulin did not show resonance transfer. With polymerization or aggregation of MT subunits, fluorescent excitation of fluorescein labeled tubulin resulted in fluorescent emission by rhodamine labeled tubulin, as the chromophores were brought sufficiently close together in a common lattice to permit resonance energy transfer. The energy transfer occurred not only among tubulin subunits in MT, but among MT subunits and membrane components.

Evidence for another mode with communicative implications in microtubules is suggested by the parallel alignment of MT in applied electric and magnetic fields (Vassilev, Dronzine, Vassileva, and Georgiev, 1982). They cite the postulated existence of low intensity electric fields (Jaffe and Nuccitelli, 1977; Adey, 1975) in the range of 20 to 500 millivolts per centimeter within cells (one millionth of the field strength across polarized membranes). Vassilev and colleagues isolated rat brain tubulin and created polymerizing conditions in the presence of pulsed electric fields of about 25 millivolts per centimeter. Electron micrographs showed that the MT polymerized in perfect parallel alignment with the applied field. Similar results were obtained when low intensity (0.02 Tesla) magnetic fields were applied. If assemblies of MT can also generate electric and/or magnetic fields of similar intensity via an electret effect, then a cooperative communication comparable to the "Indian rope trick" may be utilized in cellular growth, differentiation, and synaptic plasticity. MT could then generate their own pathways for cytoplasmic movement.

Other data (Matsumoto and Sakai, 1979; Alvarez and Ramirez, 1979) suggest that the intraneuronal cytoskeleton is necessary for nerve membrane excitability and synaptic transmission. Nerve membrane proteins including ion channels and receptors which are anchored to the cytoskeleton may be the "tips of an iceberg" of a cytoskeletal communicative medium which could utilize a number of possible modes to achieve collective cooperativity and intelligent cellular behavior. The following models suggest some possible strategies.

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