Implications of Tau Dysregulation in Huntington's Disease and Potential for New Therapeutics

Abstract

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. The disease, characterized by motor, cognitive, and psychiatric impairments, is caused by the expansion of a CAG repeat in the huntingtin gene. Despite the discovery of the mutation in 1993, no disease-modifying treatments are yet available. Understanding the molecular and cellular mechanisms involved in HD is therefore crucial for the development of novel treatments. Emerging research has found that HD might be classified as a secondary tauopathy, with the presence of tau insoluble aggregates in late HD. Increased total tau protein levels have been observed in both HD patients and animal models of HD. Tau hyperphosphorylation, the main feature of tau pathology, has also been investigated and our own published results suggest that the protein phosphorylation machinery is dysregulated in the early stages of HD in R6/1 transgenic mice, primarily in the cortex and striatum. Protein phosphorylation, catalysed by kinases, regulates numerous cellular mechanisms and has been shown to be dysregulated in other neurodegenerative disorders, including Alzheimer's disease. While it is still unclear how the mutation in the huntingtin gene leads to tau dysregulation in HD, several hypotheses have been explored. Evidence suggests that the mutant huntingtin does not directly interact with tau, but instead interacts with tau kinases, phosphatases, and proteins involved in tau alternative splicing, which could result in tau dysregulation as observed in HD. Altogether, there is increasing evidence that tau is undergoing pathological changes in HD and may be a good therapeutic target.

Keywords: Huntington’s disease; aggregates; phosphorylation; tau; tauopathy.

Fig. 1

Tau protein with its 4 functional domains and the localization of the most studied phosphorylation residues.

Fig. 2

Potential mechanisms leading to tau hyperphosphorylation in Huntington’s disease. Since mutant huntingtin does not directly interact with tau, several hypotheses have been explored to explain the tau hyperphosphorylation seen in HD, including an interaction between the mutant huntingtin and tau kinases/phosphatases, and an interaction between mutant huntingtin and proteins involved in tau alternative splicing (FUS, SRSF6).