Tauopathy-associated tau modifications selectively impact neurodegeneration and mitophagy in a novel C. elegans single-copy transgenic model

Abstract

Background A defining pathological hallmark of the progressive neurodegenerative disorder Alzheimer’s disease (AD) is the accumulation of misfolded tau with abnormal post-translational modifications (PTMs). These include phosphorylation at Threonine 231 (T231) and acetylation at Lysine 274 (K274) and at Lysine 281 (K281). Although tau is recognized to play a central role in pathogenesis of AD, the precise mechanisms by which these abnormal PTMs contribute to the neural toxicity of tau is unclear. Methods Human 0N4R tau (wild type) was expressed in touch receptor neurons of the genetic model organism C. elegans through single-copy gene insertion. Defined mutations were then introduced into the single-copy tau transgene through CRISPR-Cas9 genome editing. These mutations included T231E, to mimic phosphorylation of a commonly observed pathological epitope, and K274/281Q, to mimic disease-associated lysine acetylation – collectively referred as “PTM-mimetics” – as well as a T231A phosphoablation mutant. Stereotypical touch response assays were used to assess behavioral defects in the transgenic strains as a function of age. Genetically-encoded fluorescent biosensors were expressed in touch neurons and used to measure neuronal morphology, mitochondrial morphology, mitophagy, and macro autophagy. Results Unlike existing tau overexpression models, C. elegans single-copy expression of tau did not elicit overt pathological phenotypes at baseline. However, strains expressing disease associated PTM-mimetics (T231E and K274/281Q) exhibited reduced touch sensation and neuronal morphological abnormalities that increased with age. In addition, the PTM-mimetic mutants lacked the ability to engage neuronal mitophagy in response to mitochondrial stress. Conclusions Limiting the expression of tau results in a genetic model where modifications that mimic pathologic tauopathy-associated PTMs contribute to cryptic, stress-inducible phenotypes that evolve with age. These findings and their relationship to mitochondrial stress provides a new perspective into the pathogenic mechanisms underlying AD.

Tau Oligomers Neurotoxicity

“…When tau composed mostly of monomers is injected into the hippocampus, it reduces the number of synapses, causes a loss of synaptic vesicles, and exerts detrimental effects on the morphology and connectivity of newborn granule neurons of dental gyrus, and these effects correlate with impaired behavior [24]. In addition, tau modifications suppress compensatory responses to mitochondrial stress and thus lead to numerous metabolic disturbances and reduced energy production [25]. It seems that the mechanism of toxicity of modified monomers is different from that of oligomers.…”

Drug discovery: Insights from the invertebrate Caenorhabditis elegans

“…The first C. elegans model of a human ND was built 25 years ago, 178 and, since then, an extensive list of animal models for most of the NDs have been generated ( Table 3) (Table 3). [180][181][182] Since C. elegans has no orthologous gene for β-amyloid (Aβ), αsynuclein, or huntingtin, models of NDs have been essentially produced by transgenic overexpression of the disease causing-protein in either body-wall muscle cells, in all neurons or in a specific subset of neurons. 82,83,150,158,178,195,197,201 In most cases, the protein is tagged with GFP, which permits monitoring protein aggregation in whole live animals.…”

“…CRISP/Cas9 technology has been fortunately established in C. elegans 179 and recent single‐copy knock‐in models of some NDs have been developed (Table 3). 180–182 …”

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

“…In these tauopathy models, most of the pathological features of human tauopathy, such as Tau phosphorylation, axonal degeneration, neuronal loss, synapse dysfunction, and mitochondria defect, were in an age-dependent manner. Typical C. elegans behavioral phenotypes, including uncoordinated moving, touch response, or thrash rates, were affected to different degrees of severity depending on the expressed forms of human Tau species [59,[219][220][221][222][223][224]. The representative C. elegans tauopathy models and their phenotypes are listed and summarized in Table 3 (More information about C. elegans can be found at https://wormbase.org (accessed on 4 August 2021)).…”