CentrioleMT Signaling AlbrechtBuehler

Northwestern University biologist Guenter Albrecht-Buehler has considered the general question of intelligence in cytoplasm (Chapter 5) as well as two specific models of cytoskeletal information processing: centriole signal detection, and propagation of MT impulses. The walls of centrioles are composed of nine MT doublets or triplets arrayed in a cylinder. Each MT triplet can consist of two incomplete C-shaped MT and a complete O-shaped one fused longitudinally. The triplets are arranged at an angle of 30-45 degrees from the main cylinder, pitched to form a blade which advances to a final increment of one ninth of the perimeter just below the position of the next blade. A line drawn through any blade connects with the inner edge of the preceding blade (Figure 8.4).

Collective behavior of cytoplasm would seem to require some communicative format, and Albrecht-Buehler suggests that centrioles are perfectly designed to detect both intensity and direction of linear signals. One possible example of a highly specific, yet ubiquitous signal propagated in a straight line in the cellular environment is infrared radiation of the molecules inside and around cells (Albrecht-Buehler, 1981). As discussed in Chapter 6, transmission of biomolecular infrared energy would require shielding from bulk water, and perhaps nonlinear coupling to structural conformational states. Both of these requirements may be met by the ordered water and ions surrounding the cytoskeleton and the electron-dense pericentriolar material.

Cellular navigators, centrioles are involved in directional orientation of moving cells, establishment of cell architecture in cell growth and differentation, and all dynamic rearrangements of cytoplasm. In an attempt to understand how centrioles could navigate and orient, Albrecht-Buehler asks how an optimally designed, technological spatial signal detector would appear. Instruments such as radar scanners can determine direction of signals by scanning different directions sequentially. However, such scanners miss signals which arrive from one direction while another is being scanned. A properly designed nonscanning instrument can listen simultaneously to all directions with no moving parts. Albrecht-Buehler points out some geometric features of an optimally designed, nonscanning "angular" detector. With signals arriving from arbitrary directions, a detector designed as a circle with a number of regularly spaced marks around its circumference would be accessible and capable of identifying direction. Nine fold symmetry provides small size with sufficient angular resolution. To locate a signal source, a detector must prevent an emitted signal from arriving at more than one receptor. A simple circular arrangement would fail to meet this requirement because about half the receptors are accessible to a signal emitted from any source. A simple way to improve the design is to attach blinds or "blades" to one side of each receptor to absorb or deflect a signal wherever it interacts with them. A radial arrangement of straight blinds would be inadequate because at least two receptors would remain accessible to signals from the same source. However, if the blinds are bent circumferentially, an optimal angle is achieved when the blinds prevent access to all but one receptor without producing "blank spots," areas from which signal sources cannot reach any of the receptors. Blinds that restrict access to single receptors are even better if their shape is concaved. Used by manufacturers of "Venetian blinds," this curvature averts the possibility that a signal located directly in line with a straight blind could reach two adjacent receptors. Consequently, a signal is received not by the receptor closest to the signal source, but by one located at a fixed angle off the incident direction. Centriolar "blades" extend above and below the centriolar cylinder and are pitched, or twisted. Albrecht-Buehler sees this torque as further angular resolution, but the "propeller-like" arrangement could also serve to screw centrioles through the cytoplasm, assuming a centriolar rotation as suggested by Bornens (Record, 1986). Two detectors are best placed at right angles to each other so that one of them can locate the "longitude" while the other locates the "latitude" of the signal source. Centriole-like basal bodies fixed perpendicularly into a two dimensional cell surface are usually not accompanied by a second basal body at right angles. Thus the right angle paired cylinder formation permits global exposure and three dimensional reckoning unnecessary in basal bodies. Albrecht-Buehler's view of centrioles as perfectly designed signal detectors is complementary with Bornens' concept of a gyroscopic oscillator and signaling center. Combination of the two models results in a dynamic cell center capable of piloting cytoplasmic activities.

Figure 8.4: Centriole in cross section is comprised of nine triplet MT angled like "Venetian blinds" from centriole axis. Centriole pairs consist of perpendicular cylinders. Albrecht-Buehler (1985) contends these features are ideally suited for signal detection. Bornens suggests rotatory oscillations of centrioles leading to gyroscopic function. By Paul Jablonka.

Figure 8.4: Centriole in cross section is comprised of nine triplet MT angled like "Venetian blinds" from centriole axis. Centriole pairs consist of perpendicular cylinders. Albrecht-Buehler (1985) contends these features are ideally suited for signal detection. Bornens suggests rotatory oscillations of centrioles leading to gyroscopic function. By Paul Jablonka.

Albrecht-Buehler (1985) has also considered a mechanism for signal propagation along microtubules. He considers that each MT protofilament is a chain of alpha-beta tubulin dimers: AB, AB, AB, ..., AB. Within the wall of a microtubule each monomer is in contact with other monomers of the same protofilament and with those of adjacent protofilaments. Each monomer is consequently subject to attractive Van der Waals forces from surrounding tubulin monomers which hold together the protofilaments and MT cylinders. Albrecht-Buehler proposed that each of these interactions must weaken the A-B dimer bond; consequently the wall of a microtubule exists in a state of resonance as to the relative strengths of the intermonomeric bonds. For example, (A-B) (A-B) ... (A-B) (A-B) could resonate with (A) (B-A) (B-A) ... (B-A) (b). Such a resonating chain could propagate information at close to the speed of light.

Albrecht-Buehler is suggesting a coherent communicative resonance among protein conformational states within MT assemblies.

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