Could there be an experimental way to link consciousness and quantum computations of brain microtubules?

By doing mathematical analyses, Penrose proposed a relation between consciousness moments and quantum computations of the brain of microtubules. It was suggested that there is a link between brain microtubules and consciousness via the entangled stage of delocalized π electrons present in the brain microtubules (Hameroff and Penrose, 2014). Then, we will try to comment on this hypothesis step by step looking for a possible future experimental approach that probes the hypothesis.We should indicate that microtubule assembly-disassembly dynamics requires the binding of tubulin (the main component of microtubules) to GTP and the hydrolysis of this GTP to GDP (for a review, see, for example (Avila, 1990;Beckett and Voth, 2023).Brain tubulin contains a specific β subunit isotype, mainly (exclusively?) present in neurons of chordates (Sullivan and Cleveland, 1984). In addition, there is a specific post-translational phosphorylation of that neuronal β subunit, whereas no such modification was found in other β tubulin isotypes (Diaz-Nido et al., 1990).Microtubules, composed by tubulin, are very abundant in the brain. By measuring tubulin levels in the cytosol of different porcine organs, including the brain, by a sensitive radioimmunoassay, it was found that tubulin accounts for 20 ± 5% of the total soluble proteins from the porcine brain (Hiller and Weber, 1978;Diez et al., 1984). Remarkably, it is about 10 to 20 times higher the amount found for tubulin in the brain than in peripheral tissues. Also, brain microtubules contain several microtubule-associated proteins (MAPs) that stabilize those polymers, being one of the tau protein (Avila, 1990). There are three specific features for brain microtubules compared to microtubules from other sources: a) they are present in a higher relative amount, b) they can nucleate in non-centrosome/basal body-directed way, and c) they could favor the formation of subcellular neuronal structures, like for example, dendrites (Figure 1).The huge amount of tubulin is assembled into microtubules from centrosome or non-centrosome direct growth (Piehl et al., 2004). Indeed, possible types for microtubules assembly independent of the wellknown models (Margolis and Wilson, 1978;Mitchison and Kirschner, 1984;Piehl et al., 2004) may take place. In mature neurons, microtubules can nucleate randomly throughout the whole cell (Stiess et al., 2010), which could be the source of the sophisticated morphologies found in neurons that may be related to some brain-specific functions, like consciousness.Indeed, the number and arrangement of microtubules and how closely they extend to a specific zone may play critical roles in synaptic vesicle delivery and, thus, in signal transmission. The trillions of synapses could differ not only in shape and synapse area but in a multitude of differences in microtubule arrangement that could be responsible for the altered vesicular arrangement in Alzheimer's disease (Wang et al., 2023). Further, the reduced microtubule density in aging and Alzheimer's disease may fundamentally change consciousness as we age and explain its loss in disease (Cash et al., 2003;Zhang et al., 2015).Consciousness could be defined as the state of being aware of something (environment) within oneself. However, other theories indicate that consciousness and awareness are different concepts since consciousness involves several stages, like perceiving, feeling, and thinking, and those stages may require memory activity (Searle, 2000). Also, consciousness may develop a memory system to create plans for the future (Budson et al., 2022), being related to decision-making and planning (Budson et al., 2022). These definitions of consciousness may facilitate the search for its mechanism based on biological and physical bases, including the most prominent theories of consciousness: higher-order landscape, global workspace, re-entry and predictive processing, integrated information, and other emerging theories (Seth and Bayne, 2022;Lenharo, 2024). Regarding consciousness brain localization, regions like the thalamus or claustrum (Crick and Koch, 2003) connecting several cortical and subcortical areas could be involved. Faster MT vibrations (Hameroff, 2012) could be a possible source of the observed EEG consciousness activity that is found as a sequence of the discrete events in synchrony with γ EEG, although this point has been discussed. It is suggested the existence of 40 consciousness moments per second, related to fractal-like patterns of microtubules (Hameroff and Penrose, 2014). These 40 Hz (γ waves) consciousness moments could be located at some cortex regions (Hameroff and Penrose, 2014) and being related to a very fast MT-assembly disassembly dynamic. Furthermore, it has been reported that microtubules have inside the cell endogenous oscillations in the range of 100 Hz ("high γ") EGG (Cantero and Cantiello, 2020), in the other of interneuronal γ connections (Singh et al., 2021). Also, EEG γ -waves may be not generated by axonal firing but by dendritic and soma interneuronal connections, suggesting that, in fact, consciousness may be related to those changes in microtubules present in dendrites and cell somas (Hameroff, 2010). Indeed, in neurodegenerative disorders like Alzheimer's disease, resulting in progressive of awareness (consciousness), the dynamics features for microtubule assembly-disassembly also are decreased (Peris et al., 2022), together with a decrease in γ waves (Mably and Colgin, 2018). Thus, the possible correlation of microtubule assembly dynamics, γ waves, and lack of consciousness could be compatible with the proposed Penroses's hypothesis.The entangled stage could be defined as an ensemble of particles that cannot be described through the individual particles but as a set. The ensembled is the result of entanglement of two or more components, even if they are separated in space. The entanglement could occur through qubits. A qubit is a subatomic particle, like the spins of electrons of the spins of nuclear components like protons or neutrons. The spin of all of those fermions (electrons, protons or neutrons) could contain, in individual particles, a positive or negative charge. Upon entanglement of two of those particles with different charges, the result is a null charge. Spins changes could be used to look at an entangled stage. Also, qubit is the basic unit in quantum computing, showing two relevant features: superposition and entanglement (Horodecki et al., 2009).Electrons are moving around the nucleus of an atom in different orbitals located at different distances of the nucleus, being π electrons those present in π orbital. As previously described, π electrons have spin configurations (Fang et al., 1995) that could act as qubits. However, spins of π electrons are difficult to measure since they can entangle with the surrounding wet environment, causing de-phasing of any putative quantum coherent phenomena. However, an exception was suggested for a subatomic particle: the nuclear spin in phosphate atoms (Fisher, 2015). Thus, we have a subatomic level with π electrons and atomic nucleus, these elementary particles have intrinsic quantum properties, for example they have their spins. It was described that spin is the intrinsic angular momentum associated with these particles (Uhlenbeck and Goudsmit, 1925). For example, spin up or spin down states of these subatomic particles could be present and quantum bits (qubits), which can exist in both states at once, permitting simultaneous answers to the computation they encode. Lately, there is a dawn of quantum biology in different biological processes (Ball, 2011). Although, traditionally nuclear spin was not considered to play a role in biological processes, more recently, this view has changed (Vardi et al., 2023). For brain studies, it has been proposed that only elements with a nuclear spin I=1/2 (traditionally labeled like spin up and spin down) should be used (Fisher, 2015), being phosphorus nucleus the only brain element with that particular spin (Fisher, 2015), a putative qubit1 . We will discuss below that GTP/GDP molecules are involved in microtubule assembly/disassembly. GTP/GDP are composed of guanine, ribose, and phosphates. Guanine contains 10 π electrons and phosphates have their nuclear spin (Figure 1). Nevertheless, there are some difficulties in using phosphorus (nuclear spin) as a suitable qubit transporter, when memory storage is required. Phosphate ion (as qubit transporter) spreads out about 10 μm in 10-2 sec (Nicholson and Sykova, 1998), but for qubit memory storage measurements, it may require times of seconds (or longer ones) as indicated by (Fisher, 2015).In Penrose's hypothesis, a role of the interior of microtubules was proposed. But, in the interior of microtubules, not only tubulin is present. Also, brain microtubules associated Tau protein is located (Kar et al., 2003), independently of its presence in the outer surface (Ackmann et al., 2000). In addition, Tau protein could be modified by phosphorylation (Hanger et al., 2009) and the phosphorus (nuclear spin) of modified hyperphosphorylated-Tau may also play a role. About the role of Tau in consciousness, a recent comment has been published (Kosik, 2023). Also, phosphorylated neuronal β tubulin subunit could play a role (Diaz-Nido et al., 1990) (Figure 1).On the other hand, the proposed role of the microtubule in consciousness-unconsciousness could take place in other events, such as anesthesia (see below). In unconsciousness or anesthesia, γ waves are missing, and δ waves are present (Frohlich et al., 2021). Inter-or intra-cellular wave changes may take place in processes like unconsciousness, reversible coma, or sleep, which are some similarities and differences among those processes. Among similarities, there is a presence in those processes of δ waves but not of γ waves (Frohlich et al., 2021). Also, in a model of unconsciousness, like propofolinduced anesthesia (Hameroff, 2021), δ waves are present (Frohlich et al., 2021). On the other hand, it could be possible to be awake and unconscious. As previously indicated, there is a neurological disorder, Alzheimer's disease (AD), that has been considered as a disorder of consciousness (Salmon et al., 2005;Huntley et al., 2021) found in awake persons. Indeed, a characteristic of unconsciousness, like anosognosia, can be present in some AD patients in advanced stages (Prigatano, 2009). About the possible relation between changes in consciousness and intraneuronal changes, it was suggested, as indicated, that microtubules may play a role at the cellular-molecular-quantum level in the consciousness process (Penrose, 2001). It was described that neuron microtubules could form functional assembles with specific frequencies (Frohlich et al., 2021) that can be regulated by neuronal brain microtubule-associated proteins like tau protein. Tau protein role in consciousness disorders, like AD, can be analyzed in AD mouse models or in anesthetized mouse models. A correlation between tau modifications and anesthesia has been described (Chen et al., 2023). Propofol-induced anesthesia may activate protein kinase-like GSK3β (Huang et al., 2016), also known as tau kinase I, and the kinase will modify tau protein at specific residues that are found in AD (Hanger et al., 2009), preventing the normal assembly of microtubules. Thus, tau protein may play a role in consciousness (see also (Kosik, 2023)). In addition, looking at the effect of phosphor-tau in a transgenic mouse model overexpressing GSK3β, some features related to unconsciousness were found (Debski, 1976;Engel et al., 2006;Hooper et al., 2007). These features could be reversed by decreasing the level of phosphorylated tau (Llorens-Martin et al., 2013), and those mouse models could probably be used for further analysis of consciousnessunconsciousness transitions in both directions. In conclusion, different levels to analyze the influence of brain microtubules on consciousness can be carried out: a) at the cellular (neuronal) level, where microtubule dynamics is regulated by GTP, and/or by the presence of microtubule-associated proteins, like Tau protein; b) at molecular level, exploring the role of GTP hydrolysis and the GTP/GDP binding to tubulin (the main component of microtubules; c) at molecular-atomic level, deciphering the role of kinases and phosphates from GTP/GDP bound to tubulin; d) at the subatomic level, by the proposed roles of π electrons and phosphorus spin nucleus as qubit transporters. The first three conclusions have been or could be further analyzed, but the main difficulty today is analyzing the subatomic level. However, this is the main point for testing Penrose's hypothesis. A proposal that should be experimentally improved (or forgotten?) using innovative multidisciplinary approaches and novel instrumentation available.Brain microtubule dynamics, Tubulin GTP/GDP, and consciousness. A) Tubulin accounts for around 20% of the total soluble brain protein. B) Brain tubulin-GTP can assemble into microtubules. Upon GTP hydrolysis, brain tubulin-GDP depolymerizes from microtubules. We should indicate the presence of 10 π electrons in the guanine of GTP/GDP and the presence of nuclear spin in the phosphor of GTP/GDP. C) There is a very dynamic microtubule assembly/disassembly in which microtubules can nucleate randomly through the whole neuron (Stiess et al., 2010), yielding different neuron morphologies. The role of subatomic phenomena in that process, like changes in the nuclear spin of the phosphorus present in GTP/GDP bound to tubulin, is unknown but should be analysed, if it is possible. D) Proposed association of consciousness with microtubule (MT) dynamics.