Alzheimer's disease is the most prominent of the tauopathies. This is a class of neurodegenerative conditions in which large enough amounts of tau protein become excessively altered by phosphorylation and aggregate into solid deposits, causing inflammation, loss of function, and cell death in the brain. The various isoforms of tau play an important role in maintaining the structure of axons that connect neurons, but aggregation would be problematic regardless of the normal function of tau.
Just as much of Alzheimer's research and development has long focused on trying to prevent, clear, or disarm misfolded amyloid-β and its toxic aggregates, a similar range of efforts is focused on finding ways to prevent, clear, or disarm hyperphosphorylated tau and its aggregates. Progress to date has been frustrating slow, just as it was for amyloid-β clearance via immunotherapy. Many of the possible paths forward appear challenging to implement well.
Today's research materials present an example of the type, an approach that potentially allows dramatic reduction in overall tau levels. Yet tau is important to axonal function, one can't just get rid of it, which presents developers with the much harder goal of achieving a balancing act with dose and outcome. Even then it tends to be the case that therapies that treat a condition in which a protein becomes altered into a toxic form by reducing overall expression of that protein tend to have unpleasant side-effects.
The research team initially hypothesized that inhibiting the enzyme activity of the OTULIN protein would enhance tau clearance through cellular garbage disposal systems. However, when they completely knocked out the OTULIN gene in neurons, tau disappeared entirely - not because it was being degraded faster, but because it wasn't being made at all. "This was a paradigm shift in our thinking. We found that OTULIN deficiency causes tau messenger RNA to vanish, along with massive changes in how the cell processes RNA and controls gene expression."
The study used neurons derived from a patient with late-onset sporadic Alzheimer's disease, which showed elevated levels of both OTULIN protein and phosphorylated tau compared to healthy control neurons. This correlation suggested OTULIN might be contributing to disease progression. "OTULIN could serve as a novel drug target, but our findings suggest we need to modulate its activity carefully rather than eliminate it completely. Complete loss causes widespread changes in cellular RNA metabolism that could have unintended consequences."
The deubiquitinase OTULIN regulates tau expression and RNA metabolism in neurons
The degradation of aggregation-prone tau is regulated by the ubiquitin-proteasome system and autophagy, which are impaired in Alzheimer's disease (AD) and related dementias (ADRD), causing tau aggregation. Protein ubiquitination, with its linkage specificity determines the fate of proteins, which can be either protein degradative or stabilizing signals. While the linear M1-linked ubiquitination on protein aggregates serves as a signaling hub that recruits various ubiquitin-binding proteins for the coordinated actions of protein aggregate turnover and inflammatory nuclear factor-kappa B (NF-κB) activation, the deubiquitinase OTULIN counteracts the M1-linked ubiquitin signaling. However, the exact role of OTULIN in neurons and tau aggregates clearance in AD are unknown.
Based on our quantitative bulk RNA sequencing analysis of human induced pluripotent stem cell-derived neurons (iPSNs) from an individual with late-onset sporadic AD (sAD2.1), a downregulation of the ubiquitin ligase activating factors (MAGE-A2/MAGE-A2B/MAGE-H1) and OTULIN long noncoding RNA (OTULIN lncRNA) was observed compared to healthy control iPSNs. The downregulated OTULIN lncRNA is concurrently associated with increased levels of OTULIN protein and phosphorylated tau.
Inhibiting the deubiquitinase activity of OTULIN with a small molecule UC495 reduced the phosphorylated tau in iPSNs and SH-SY5Y cells, whereas the CRISPR-Cas9-mediated OTULIN gene knockout (KO) in sAD2.1 iPSNs decreased both the total and phosphorylated tau levels. CRISPR-Cas9-mediated OTULIN KO in SH-SY5Y resulted in a complete loss of tau at both mRNA and protein levels, and increased levels of polyubiquitinated proteins, which are being degraded by the proteasome. In addition, SH-SY5Y OTULIN KO cells showed downregulation of various genes associated with inflammation, autophagy, ubiquitin-proteasome system, and the linear ubiquitin assembly complex that consequently may prevent development of an autoinflammation in the absence of OTULIN gene in neurons.
Together, our results suggest, for the first time, a noncanonical role for OTULIN in regulating gene expression and RNA metabolism, which may have a significant pathogenic role in exacerbating tau aggregation in neurons. Thus, OTULIN could be a novel potential therapeutic target for AD and ADRD.
View the full article at FightAging
