Neuronic system inside neurons

Molecular biology and biophysics of neuronal microtubules

Danko Georgiev, Stelios Papaioanou, & James Glazebrook, Dept. of Anesthesiology, Varna, Medical University of Varna, Eastern Illinois University

Biomedical Reviews (2004), 15, pp. 67-75

‘We dance round in a ring and suppose
but the secrets sits in the middle and knows’ – Robert Frost

Introduction

This is possibly the clearest version of Georgiev’s ideas on quantum information processing in microtubules that I have come across to date. Amino acid tails projecting 4-5 nanometres out from the surface of microtubules play a key role in the theory. The tubulin tails interact with the electric field, with water molecules, and with ions bound to the microtubular surface, to produce solitons (solitary quantum waves that, even in collisions with other waves, retain their shape and velocity). Collisions of these solitons act as logic gates, and the conformation of the tubulin tails controls microtubule associated proteins (MAPs) and motor proteins, which in turn constitute a computational output. It is also suggested that tubulin tails could regulate the output of neurotransmitters from synapses via presynaptic scaffold proteins. This would bring microtubules centre stage, within the conventional model of the brain’s information processing. One advantage of this model is that it does not require the shielding from decoherence envisaged in the better known Hameroff theory, because the process suggested could occur within the normal time to decoherence in the brain.

The authors’ detailed investigation of the interior structure of neurons is stated to indicate that microtubule networks inside neurons are suited to carrying out computation. Microtubules in neurons are more stable than in other body cells, and this makes them more suitable for information processing and signalling.

The authors discuss the better known ideas of Stuart Hameroff and his coworkers, as to how microtubules might process information. In Hameroff’s version, the conformation of the tubulin is governed by electrons in hydrophobic pockets between the alpha and beta monomers of the tubulin dimer. Electrons in different pockets are suggested to be quantum entangled.

The authors argue against this particular model for information processing in microtubules. Hameroff’s version assumes that the process derives its energy from the hydrolysis of GTP. However, the authors claim that this form of energy generation is impossible, once a stable microtubule has been assembled. Instead, they look for an alternative form of energy. On the basis of a mathematical model developed by the quantum consciousness researchers Jibui & Yasue, they claim to show that signals from the local electric field could govern the conformation of the so-called tubulin ‘tails’, which are amino acid carboxyl or C-terminals projecting 4-5 nanometres out from the surface of the microtubules. These projections are negatively charged, and the study suggests that they would attract positive ions, and thus form a Debye layer. They suggest that the projecting tubulin tails plus the hydration shells (water molecules orientated by ions) around the tails could make the microtubules very sensitive to their environment and particularly to the local electric field. It is suggested that the interaction between the tubulin tails and the local electric field could induce conformational waves in the tubulin tails. Tubulin tail interactions with MAPs, motor protiens and presynaptic scaffold proteins could allow the output of this computation.

Mathematical modelling suggests the feasibility of solitons formed by collective tubulin tail behaviour. The authors are interested in a particular type of soliton known as a ‘breather’, where internal oscillations create a more complicated structure than in other solitons. The shape and velocity of the soliton is not changed by collisions, but it can be shifted along the microtubule, and these collisions and shifts are suggested to act as logic gates for computation. The resulting conformation of the tubulin tails can become the output of computation, by determining the position of MAPs and motor proteins.

A study by Fujii & Koisumi (1.) showed that MAPs bind to tubulin C-terminals and the initial segment of the tubulin tail, and this makes it reasonable to assume that the tubulin tails are the main regulators of MAP binding. Another study (Skiniotis, 2) has shown that tubulin tails interact with the motor protien kinesin, and are involved in tethering motor protiens between active steps. Microtubules are involved in the transport of organelles that may form synaptic vesicles and membranes, and these objects may also contain neurotransmitters. Intracellular transport involves motor proteins, and the best studies of these are kinesin and dynien that use microtubular tracks, and are essential for transport within neurons. The authors claim that their studies change the view of microtubules from being passive tracks for transport, to being controllers of intracellular transport.

Synaptic control: It is further suggested that tubulin tail conformations can control the presynaptic scaffold proteins that organise synapses, and regulate the release of neurotransmitters. Studies (3. & 4.) showed that tubulin tails interacted directly with synaptotagmin-1. It is suggested that this protein could bind to microtubules, and prevent the depolymerisation that would otherwise occur as a result of Ca2+ ions in the presynaptic space. It is suggested that microtubules could be linked to synaptic vesicles by synaptotagmin or other scaffold proteins, and could thus regulate neurotransmitter release. The probability of an axon spike leading to the actual firing of a synapse ranges from 15-70%, so the relationship is far from one-to-one. This makes external regulation by microtubules feasible.

References:-
1.) Fujii, T. & Koizumi, Y. (1999) – Identification of the binding region of basic calponin on alpha and beta tubulins – Journal of Biochemistry (Tokyo), 125, pp. 869-75
2.) Skiniotis, G. et al (2004) – Modulation of kinesin binding by the C-termini of tubulin – EMBO Journal, 23, pp. 989-99
3.) Hirokawa, N. et al (1989) – The cytoskeletal architecture of the presynaptic terminal and the molecular structure of synapsin-1 – Journal of Cell Biology, 108, pp. 111-126
4.) Schmoranzser, J. & Simon, S. (2003) – Role of microtubules in fusion of post-Gologi vesicles to the plasma membrane – Molecular Biology Cell, 14, pp. 1558-1569

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