Understanding the Pathophysiological Actions of Tau Oligomers: A Critical Review of Current Electrophysiological Approaches

Abstract

Tau is a predominantly neuronal protein that is normally bound to microtubules, where it acts to modulate neuronal and axonal stability. In humans, pathological forms of tau are implicated in a range of diseases that are collectively known as tauopathies. Kinases and phosphatases are responsible for maintaining the correct balance of tau phosphorylation to enable axons to be both stable and labile enough to function properly. In the early stages of tauopathies, this balance is interrupted leading to dissociation of tau from microtubules. This leaves microtubules prone to damage and phosphorylated tau prone to aggregation. Initially, phosphorylated tau forms oligomers, then fibrils, and ultimately neurofibrillary tangles (NFTs). It is widely accepted that the initial soluble oligomeric forms of tau are probably the most pathologically relevant species but there is relatively little quantitative information to explain exactly what their toxic effects are at the individual neuron level. Electrophysiology provides a valuable tool to help uncover the mechanisms of action of tau oligomers on synaptic transmission within single neurons. Understanding the concentration-, time-, and neuronal compartment-dependent actions of soluble tau oligomers on neuronal and synaptic properties are essential to understanding how best to counteract its effects and to develop effective treatment strategies. Here, we briefly discuss the standard approaches used to elucidate these actions, focusing on the advantages and shortcomings of the experimental procedures. Subsequently, we will describe a new approach that addresses specific challenges with the current methods, thus allowing real-time toxicity evaluation at the single-neuron level.

Keywords: electrophysiology; neuron; oligomer; tau; tauopathy.

Figure 1

Tau in physiology and pathophysiology. Tau has many physiological roles in neurons (A). (i) The most prominent function is in the modulation of microtubule stability and assembly. (ii) Tau also plays a role in the nucleus, potentially regulating the integrity of genomic DNA by binding to different chromosomal regions. (iii) The third role for tau in physiology is in the regulation of axonal transport by interfering with the binding of motor proteins kinesin and dynein to microtubules, thus altering the movement of cellular materials between the soma and axon. (iv) Finally, tau is believed to play important roles in the regulation of synaptic plasticity, for example by its contribution to the process of spine remodeling. Design concept from Wang and Mandelkow (2016). (B) Under physiological conditions, tau is bound to microtubules. The stability of microtubules is maintained by the balanced activities of kinases and phosphatases acting to regulate the binding of tau to the microtubules. (C) If the balance is altered in favor of increased kinase activity, then tau can become hyperphosphorylated and detach from the microtubules affecting stability and tau aggregation. Initially, monomers aggregate to form oligomers which are β-sheet-rich and are thought to be the smallest toxic species. Subsequently, these species aggregate further into protofibrils, fibrils, and finally into neurofibrillary tangles (NFTs). Created with BioRender.

Figure 2

Electrophysiological approaches to measuring neuronal function. A brief overview of the common electrophysiological approaches used to evaluate tau oligomer function. in vivo experiments give the greatest level of translatability to the research as the whole brain remains intact. However, in vitro experiments permit much greater control over oligomer concentration and pharmacological manipulation. (A) A schematic illustration to demonstrate extracellular field recording in the CA1 region of the hippocampus. A stimulating electrode is placed in the Schaffer collateral fiber region and a recording electrode in the CA1 region where they synapse onto pyramidal cells, to record excitatory postsynaptic potentials (EPSPs). This technique allows for the study of local circuit activity. (B) The whole-cell patch-clamp allows for the parameterization of neuronal function on a single-cell level. By forming a high resistance seal between the pipette and the membrane of the target cell, then breaking through (with negative pressure), the contents of the patch pipette will dialyze into the cell. (i) This technique can be used to deliver proteins and peptides of known concentration and defined structure directly into target cells whilst simultaneously making current and voltage measurements in a network that is free from pathology. (ii) The whole-cell patch-clamp permits the study of tau mislocalization, with fluorescently labeled tau diffusing to synaptic structures above the dendritic bifurcation. Tau oligomers can be targeted to pre or postsynaptic neurons during multiple whole-cell recordings. (iii) An example where 3 layer V pyramidal cells were simultaneously recorded. The neuron on the left has tau oligomers introduced from the patch pipette whereas the other two cells receive a vehicle. (iv) Example traces from presynaptic neurons (tau oligomer) showing action potential firing pattern to induce EPSPs in postsynaptic neurons (vehicle). Created in part with BioRender. Reproduced in part using content previously published in Hill et al. (2019); eNeuro on a CC-BY 4.0 license.