New study identifies potential treatment for tauopathies using carbonic anhydrase inhibitors

By Dr. Chinta SidharthanReviewed by Lily Ramsey, LLMNov 4 2024

Carbonic anhydrase inhibitors show promise in reducing tau protein buildup and neurodegeneration, paving the way for new treatments in neurodegenerative diseases.

Study: Carbonic anhydrase inhibition ameliorates tau toxicity via enhanced tau secretion. Image Credit: YURIMA/Shutterstock.com

In a recent study published in Nature Chemical Biology, University of Cambridge scientists used zebrafish models of tauopathies — a group of neurodegenerative diseases driven by tau protein accumulation — to screen over 1,400 chemicals to find potential treatments for various tauopathies.

The researchers identified carbonic anhydrase inhibitors as a promising option for reducing tau toxicity.

Background

Tauopathies are a group of neurodegenerative diseases caused by the accumulation of tau proteins inside brain cells. They include diseases such as corticobasal degeneration and progressive supranuclear palsy.

Many of these diseases are not the direct result of mutations in primary tau genes but are often caused by some genetic variants that drive the toxic build-up of tau proteins.

The toxic accumulation of tau protein seen in tau pathologies shares similarities with other neurodegenerative disorders, such as Huntington’s disease and familial Parkinson’s disease, where mutations in specific protein-coding genes cause tau accumulation.

Similarly, in diseases such as chronic traumatic encephalopathy and Alzheimer’s disease, tau build-up is a secondary occurrence but contributes significantly to the damage to brain cells.

However, despite the high prevalence of various neurodegenerative diseases associated with toxic tau build-up, effective pharmacological treatments have been lacking. Furthermore, many of the previous drug screens have been conducted in cell cultures, which do not fully replicate the complexity of the brain.

About the study

In the present study, the researchers used zebrafish models of tauopathies that could closely mimic tau toxicity in the brain and allow for high-throughput screening of a wide range of potential drugs.

The study employed several methods to explore the compounds that could potentially lower the toxic accumulation of tau proteins. A comprehensive compound screening was first performed using a zebrafish model expressing enhanced green fluorescent protein-labeled human tau protein in the photoreceptors in rod cells.

This model of zebrafish undergoes progressive neurodegeneration, and the researchers used it to screen 1,437 compounds administered at concentrations of 10 micromolar.

Of these compounds, 94 lethal ones were excluded, and the researchers used Western blot analysis to measure the degree of rod degeneration post-treatment by analyzing the ratio of rhodopsin to arrestin, which would indicate the survival of rod cells.

The compounds that increased the survival of rod cells, with rhodopsin to arrestin ratios above the cut-off level, were selected, and of these, the researchers focused on drugs already approved by the United States (U.S.) Food and Drug Administration (FDA) has fewer than five known biological targets.

Subsequent concentration-response assays and validation methods resulted in the selection of 16 compounds, which were then tested on a second zebrafish model expressing a mutant form of tau protein throughout its central nervous system.

Based on their observations, the researchers further examined the potential neuroprotective activity of carbonic anhydrase inhibitors using mutant tau and wild-type zebrafish models. Murine models were also used to investigate the impact of carbonic anhydrase inhibitors on tau protein accumulation.

They also used clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) to target the genes encoding carbonic anhydrase to determine whether the knockdown of specific carbonic anhydrase isoforms would also reduce tau-associated degeneration.

Results

The researchers found that carbonic anhydrase inhibitors could reduce the toxicity due to tau accumulation, indicating their potential use as pharmacological agents for the treatment of tauopathies.

Specifically, they found that the inhibition of carbonic anhydrase promoted the clearance of toxic tau proteins from the neurons by enhancing the cell-clearing pathway of lysosomal exocytosis.

Furthermore, carbonic anhydrase inhibitors such as methazolamide, which is an approved glaucoma drug, were found to effectively reduce the levels of total and phosphorylated tau proteins, decrease neurodegeneration, and improve neuronal survival.

Moreover, these effects were observed at dosages of carbonic anhydrase inhibitors that are used for human therapies.

The gene knockout studies using CRISPR-Cas9 found that the deletion of the gene encoding carbonic anhydrase mimicked the effects of carbonic anhydrase inhibitors in the tauopathy models, which validated the finding that carbonic anhydrase was a potential target for managing tau toxicity.

The inhibition of carbonic anhydrase was also found to increase the secretion of tau proteins into the extracellular space, lowering the intracellular levels of tau proteins.

Furthermore, although autophagic and proteasomal degradation pathways are more common, the tau proteins were secreted out of the cell through lysosomal exocytosis. This finding was also supported by the elevated activity levels of the lysosomal enzyme and the increase in the peripheral distribution of lysosomes after the inhibition of carbonic anhydrase.

Conclusions

Overall, the findings indicated that carbonic anhydrase inhibitors, especially the FDA-approved glaucoma drug methazolamide, were a promising therapeutic option for mitigating the toxic accumulation of tau proteins.

The evidence provided in this study for the potential use of carbonic anhydrase inhibitors for treating tauopathies could facilitate the repurposing of existing, approved carbonic anhydrase inhibitors for addressing tau protein-related neurodegenerative diseases.