The deubiquitinase OTULIN regulates tau expression and RNA metabolism in neurons

The degradation of aggregation-prone tau is regulated by the ubiquitin-proteasome system (UPS) 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 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 sequence analysis of human inducible pluripotent stem cell (iPSC)-derived neurons (iPSNs) from an individual with late-onset sporadic AD (sAD2.1), a downregulation of the ubiquitin ligase activating factors (MAGE-A2/A2B/H1) and OTULIN long non-coding RNA (OTULIN lncRNA) was observed compared to healthy control WTC11 iPSNs. The down-regulated OTULIN lncRNA is concurrently associated with increased levels of OTULIN protein and phosphorylated tau at p-S202/p-T205 (AT8), p-T231 (AT180), and p-S396/p-S404 (PHF-1) in sAD2.1 iPSNs. 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 in sAD2.1 iPSNs decreased both the total and phosphorylated tau levels. CRISPR-Cas9-mediated OTULIN knockout 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 as confirmed with an inhibitor, Lactacystin. 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 (LUBAC) 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 non-canonical role for OTULIN in regulating the 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.