Neurodegenerative diseases affect millions of people worldwide and as our life expectancy increases, more individuals are expected to be affected in the coming decades. Tauopathies such as Alzheimer’s disease are a class of neurodegenerative disorders involving a pathological accumulation of tau proteins which eventually results in massive loss of brain cells. There is little consensus about the underlying pathogenesis and no effective treatments are available currently for these disorders. A recent study by researchers at Texas Children’s Hospital and Baylor College of Medicine has now identified new tau regulators that can serve as viable and effective therapeutic targets for Alzheimer’s disease and other tauopathies.
This exciting study, published in the journal Neuron, was led by Dr Huda Zoghbi, professor at Baylor College of Medicine and founding director of the Jan and Dan Duncan Neurological Research Institute (Duncan NRI) at Texas Children’s Hospital and involved multi-disciplinary collaborations with other Duncan NRI faculty, Drs Juan Botas and Zhandong Liu.
A cross-species screen reveals three new tau regulators
The researchers’ goal was to undertake an unbiased screen to find genes whose inhibition can reduce the levels of tau protein. First, the Liu lab performed computational modelling and prediction analysis of the known ~17,000 human genes and generated a compendium of 6600 genes that were deemed to be – ‘druggable’ – which they defined as those proteins whose functional domains can be modified by chemical compounds.
“Next, we used a cross-species approach involving mammalian cells and fruit flies to ‘comb’ through this large collection to find genes that impact tau levels,” said Dr Ji-Yoen Kim, assistant professor in the Zoghbi lab and lead author of the study.
In both screens, the genes were down-regulated using RNA interference technology, with a small subset of genes targeted by CRISPR technology in the cell-based screen.
“Our strategy of performing parallel loss-of-function genetic screens in mammalian cells and fruit flies allowed us to select targets that showed up as top hits in both species,” explained Dr Ismael Al-Ramahi, assistant professor at Baylor and co-author of the study.
This approach led them to 11 new in vivo validated tau regulators. Of these, three targets – ubiquitin-specific protease 7 (USP 7), RING-Type E3 Ubiquitin Transferase (RNF130), and RING-Type E3 Ubiquitin Transferase (RN149) – converged on the ubiquitin protein degradation pathway. “The cross-species approach led us to reliable tau regulators whose functions are critical enough to be evolutionarily conserved from fruit flies to humans,” added Al-Ramahi.
The team further investigated these targets with the premise that understanding how these proteins regulate the ubiquitin pathway will likely reveal mechanistic insights into tau degradation.
USP7, RNF130 and RNF149 regulate tau levels
The majority of intracellular proteins within all tissues are degraded by the ubiquitin-proteasomal pathway. This is a complex, tightly regulated process involving several discrete and successive steps. Ubiquitin molecules are first activated and transferred to carrier proteins. Multiple ubiquitin molecules are attached to the protein substrate via a group of enzymes called the E3 ubiquitin ligases. Finally, the ubiquitinated substrate is degraded.
Previous studies have implicated the ubiquitin ligase, CHIP (C-terminus of Hsc70-interacting protein), as an important regulator of tau turnover and a critical player in the selective elimination of abnormal tau species.
Interestingly, in this study, the Duncan NRI team discovered that USP7 stabilizes tau by protecting it from CHIP-mediated degradation. They also found that RNF130 and RNF149 decrease the levels of the tau degrader (CHIP) and that their inhibition increases CHIP which in turn decreases tau levels. To test if these target genes can regulate CHIP and tau levels in the brain, the team turned off their expression in adult mice that overexpress mutant tau.
“Turning off the expression of USP7, RNF130, or RNF149 in adult mice with tauopathy using a doxycycline-inducible system increased CHIP level, and reduced total and phosphorylated-tau proteins,” Dr Kim said. “We also saw a decrease in other tell-tale signs of tau pathology and neuroinflammation. Most excitingly, these mice performed as well as age-matched normal mice in tasks that require learning and memory – a strong indicator that increasing CHIP levels in addition to a concomitant reduction in tau levels can improve neuronal and overall brain function in these mice.”
Although these three proteins have never been linked with each other before, it is notable that their functions converged on CHIP, which highlights the central role CHIP plays in maintaining tau levels in check.
“While previous studies have used antisense oligonucleotides to target human tau mRNA, we rationalized that identifying tau regulators that can be inhibited by small-molecule drugs will be worthwhile given the likelihood that treatments to prevent dementia are best initiated in the pre-symptomatic phase and are likely to go on for decades.” Dr Zoghbi said.
“We are excited to have found three targets that reduce tau level and show marked improvements in pathology and learning and memory. This discovery opens the exciting possibility of leveraging small-molecule inhibitors to lower tau levels and hopefully, prevent memory deficits in those at risk for Alzheimer’s disease and other tauopathies,” Dr Zoghbi concluded.