As promised, here is a journal article further justifying the nature of my summer project, which aims to expose tau protein in Alzheimer’s Disease (AD) to various static and alternating magnetic fields with the hope of changing its conformational state.
AD is a progressive neurodegenerative disorder characterized by cognitive decline, memory loss, and behavioral changes. It is the most common cause of dementia, affecting millions of people worldwide. The exact cause of AD is not fully understood, but it is known to involve the accumulation of abnormal protein aggregates in the brain, particularly amyloid-beta plaques and tau tangles.
Tau protein is a microtubule-associated protein primarily found in neurons. Under normal conditions, tau stabilizes microtubules, which are essential for maintaining the structure and function of neurons. However, in AD, tau undergoes abnormal hyperphosphorylation, leading to its detachment from microtubules. This process causes tau to aggregate into insoluble fibrils, forming neurofibrillary tangles (NFTs) within neurons. The accumulation of tau tangles is believed to disrupt neuronal function and contribute to neurodegeneration through mechanisms such as microtubule disassembly, synaptic dysfunction, neuroinflammation, and cell death.
I have compiled a list of a few critical studies, each focusing on a different aspect of my project. Importantly, none of these studies are my project; I’m not trying to reproduce results, but rather to acquire my own results. Instead, these studies serve as well-established ‘guideposts,’ all pointing to the need for further investigations in the emerging field of magnetobiology (not an academic term).
Transcranial Magnetic Stimulation (TMS) is an in vivo therapy for many neurodegenerative diseases, including Alzheimer’s (Zhu et al.). The basic notion behind TMS is Lenz’s law: a change in magnetic flux will be opposed by a conductor, such as an electromagnet. Even if there is no initial current in the electromagnet, a current will be induced to generate a magnetic field that opposes the change in flux.
Consider the brain, how does it work? Neurons transmit electrical signals to other neurons through synaptic pathways. In this way, the brain can be thought of as a highly complex electromagnet. When the brain is exposed to an external alternating magnetic field (AMF), and magnetic flux is oscillating, Lenz’s law tells us there will be induced currents in these synaptic pathways. These currents can be especially helpful for stimulating degenerative areas of the brain, like those present in AD patients. Stimulation of these regions can rescue synaptic plasticity such as that created by long-term potentiation. Previous studies have found high frequency (5Hz or above, which is actually considered very low in the magneto-mechanical field as a whole!) TMS to improve synaptic plasticity in the dorsolateral prefrontal cortex, the precuneus cortex, and the cerebellum (Zhu et al.). Since the brain is interconnected, strengthening neurons in these regions contributes to net cognition.
Additionally, while controversial, some researchers believe that magnetoreceptors respond to external magnetic fields by changing their conformations, thus regulating entire signaling pathways that may be conducive to disaggregation of tau or amyloid-beta structures (Zhu et al.). One such suspected magnetoreceptor is Cryptochrome (CRY), a nuclear protein commonly expressed in eukaryotes which is believed to undergo deformation at a particular binding domain. Additionally, ISCA1, another magnetoreceptor, has been found to contribute to neuronal activation in cultured hippocampal neurons exposed to an external magnetic field. The proposed mechanism for this effect is the crucial role ISCA1 could play in protecting mitochondrial function in AD-afflicted neurons.
The result of the application of AMFs in vivo seems promising from a physiological standpoint, but are there tangible impacts in the in vivo studies conducted thus far? Yes there are!
In all of 12 studies examining TMS in vivo, patient scores on the Mini-Mental State Examination (MMSE) were found to increase, suggesting improved cognitive function, while patient scores on the Alzheimer’s Disease Assessment Scale-Cognitive (ADAS-Cog) were found to decrease, also suggesting improved cognitive function (Zhu et al.).
These studies are incredibly inspirational when it comes to the overall usefulness of magnetic fields in mitigating AD progression, although, I won’t be doing anything in vivo, I’m just focusing on tau! So let’s dive into a guidepost a little more attuned with my project.
A static magnetic field (SMF) is, as its name denotes, time-constant. Unlike TMS, SMF effects are not attributed to Lenz’s law but to other mechanisms.
Think of a compass. It points in the direction of a magnetic field because a tiny magnet within the compass aligns with the external field. Electrons moving within their orbitals in atomic configurations constitute moving charges, thus creating a magnetic field. Therefore, every compound acts as a sort of compass, willing to align with a sufficiently strong external magnetic field. This “compass,” essentially measuring the tendency of the compound to align with an external field, is referred to as the magnetic moment. The magnitude of the magnetic moment of a compound is weakened by opposite spin ordering within electron pairs in constituent atoms and lack of alignment within the molecular lattice. Consequently, most biological compounds act as very weak magnets. Nonetheless, a sufficiently powerful SMF can subtly affect the energetic interactions of molecular bonds within compounds like tau, potentially altering its aggregation state.
For instance, one study found that a 0-2.2mT SMF applied over a span of two minutes could alter the vibrational modes within tau residues, corresponding to changes in the tau’s dynamic conformational state (Darwish et al.). Analysis by Fourier Transform Infrared (FTIR) Spectroscopy demonstrated higher overall band intensity (but lower/constant in some spectral regions!) across the entire frequency range tested in the study for the exposed tau sample than for the control sample.
In this case, band intensity between the ground state and the (k+1)th eigenvalue be expressed as
\[ I = \frac{8{\pi}^{3}N}{3hc(4\pi\epsilon_0)}(E_k - E_0)\sum_{\alpha = x, y, z}(R_{\alpha,k})^2 \]
where most symbols represent constants with the exception of \( E_k \), the energy that a molecule achieves upon being excited by the infrared light, and \( R_{\alpha,k} \), the variation of the dipole moment between the ground state and the (k+1)th eigenvalue. The increase in overall band intensity observed in FTIR Spectroscopy of the exposed sample suggests an increase in the magnitude of the variation of the dipole moment of the exposed tau sample upon being exposed to IR light (since \( E_k \) is presumed constant post-SMF exposure and during FTIR Spectroscopy). The change in the dipole moment variation implies a shift in the conformational state within tau residues, whether that be the distance separating charges, spatial arrangement of bonds (i.e. dipole moments may be being aligned in a particular direction), or other conformational changes.
Additionally, the study found a shift in the location of band peaks in the exposed sample toward generally higher wavenumbers via FTIR Spectroscopy. These shifts primarily occurred in the fingerprint region (700 cm^-1 to 1200 cm^-1) of the absorption spectrum. The vibrational mode of a molecule is the way(s) in which its bonds vibrate with respect to time; some vibrational modes include stretching and bending. Since the frequency of a vibrational mode of a molecule excited by light is given as
\[ \nu = \frac{1}{2\pi}\sqrt{\frac{k}{\mu}} \]
where \(\mu\) is the reduced mass of constituent atoms and \(k\) is the force constant of the bond, the shift in band peaks in the exposed sample correspond to a greater force constant \(k\). This implies changes in the vibrational modes, such as bond stiffness, angles, and length. One can certainly deduce that the transition to higher wavenumbers indicates higher energy bonds.
Although the weak magnetic dipoles of biological structures like tau will not allow them to be disaggregated strictly by an SMF of realistic strength, this study demonstrates that it’s still possible to achieve conformational changes within tau substructures with a relatively weak SMF, suggesting their potential as a therapeutic complement, and serving as a crucial guidepost for my project (Darwish et al.).
It would be nice to be able to directly disaggregate tau fibrils with a magnetic field though, wouldn’t it? Luckily, there’s yet another effect to explore!
Magneto-mechanical deformation involves using magnetic fields to physically deform biological structures such as tau. Biological structures generally do not respond to external magnetic fields of realistic strength. However, introducing magnetic nanoparticles (MNPs) can facilitate this process. MNPs are fragments of paramagnetic materials that lie along an easy axis of magnetization; however, upon exposure to an external magnetic field, MNPs align with said field.
Superparamagnetic iron oxides like magnetite (Fe3O4) are commonly used in biomedical research due to their non-toxicity and strong magnetic moments (Nikitin et al.). I will be using Ferumoxytol, which is essentially magnetite wrapped in a polyglucose sorbitol carboxymethyl ether coating. Ferumoxytol’s ability to bind to tau fibrils is particularly significant, as is its high magnetization.
AMFs can stimulate oscillatory motion of these MNPs, aligning with their therapeutic goal: to exert time-variant stretching, shearing, or compressive forces that ultimately lead to structure deformation. As an AMF changes direction sinusoidally, so do the magnetic moments within MNPs, albeit with a phase lag (Nikitin et al.). One can think of MNPs in an AMF as numerous children pushing and pulling on a man’s coat until it’s ripped off. The ability of Ferumoxytol to bind to tau fibrils enhances the potential effectiveness of this approach.
Once the magnetic field direction suddenly changes in the AMF configuration, two types of relaxation (i.e. realignment with the new magnetic field direction, in this case) occur: Brownian relaxation and Néel relaxation (Nikitin et al.). In Néel relaxation, the MNP doesn’t physically move, but rather its magnetic moment simply flips along the easy axis of magnetization, which is not really conducive to mechanical effects on the structure it is bound to, and tends to waste energy on heat dissipation instead. Conversely, Brown relaxation, which can occur when an MNP is suspended in a fluid, corresponds with physical movement of the MNP in addition to the alternating magnetic moment vector, which is clearly much more conducive to mechanical effects.
One such mechanical effect is magnetic torque. MNP-aligned magnetic torque at maximal AMF amplitude in J may be expressed as
\[ L = \mu B \]
where \(B\) is the strength of the magnetic field in T, and \(\mu\) is the magnitude of the magnetic moment in J/T. I calculated that, under ideal conditions, magnetic torque on a single MNP should be 25-200 pN*nm for my AMF generator configuration, which is obviously extraordinarily small, yet sufficient nonetheless, as the threshold for dsDNA denaturation is a mere 10 pN*nm!
In fact, I’m particularly excited about this approach because magneto-mechanical therapy is very up and coming as of now, for applications such as denaturing DNA.
One study examined a magnetite-chymotrypsin (CT) conjugate exposed to a 16-410Hz AMF of amplitude 88mT, finding a decrease in CT enzyme activity of 63% (Efremova et al.).
Another study found that the magneto-mechanical approach could separate complementary ssDNA strands with a binding energy of 90 kcal/mol (Veselov et al.)!
So, I’d say magneto-mechanical deformation of tau protein is far from a wild dream!
Though slightly less relevant to my project, magneto-mechanical-assisted drug delivery is promising. This technique uses MNPs conjugated to therapeutic compounds to enhance their delivery to the brain.
The Blood-Brain Barrier (BBB) often prevents drugs from effectively targeting hyperphosphorylated AD tau. However, drugs attached to MNP carriers, accelerated by an SMF towards the brain region, can more easily cross the BBB. This approach, though not the focus of my project, offers an intriguing avenue for enhancing therapeutic delivery to AD-affected regions.
Armed with these guideposts, which serve as inspirations for the use of both SMFs and AMFs in mitigating tauopathy, I hope to generate successful and novel research this summer at my USC internship!
Thank you so much for reading!