Cathepsin S increases tau oligomer formation through limited cleavage, but only IL-6, not cathespin S serum levels correlate with disease severity in the neurodegenerative tauopathy progressive supranuclear palsy

Limited cleavage promotes the aggregation propensity of protein tau in neurodegenerative tauopathies. Cathepsin S (CatS) is overexpressed in brains of patients suffering from tauopathies such as Alzheimer's disease (AD). Furthermore, CatS serum levels correlate with survival in the elderly. The current study investigates whether limited cleavage by CatS promotes tau aggregation, and whether CatS serum levels may correlate with disease severity in tauopathies. Oligomer formation of fluorescently labeled protein tau was monitored by single particle fluorescence spectroscopy after coincubation with CatS. Tau cleavage patterns were investigated by SDS-PAGE. For serum analyses, samples were collected from 42 patients with probable progressive supranuclear palsy (PSP) according to NINDS-PSP criteria. Disease severity was assessed by PSP rating scale (PSP-RS), PSP staging system (PSP-S) and Schwab and England Activities of Daily Living (SEADL). CatS, cystatin C (CysC) and interleukin 6 (IL-6) serum levels were determined by ELISA, ECLIA and turbidimetry, respectively. SDS-PAGE demonstrated a distinct cleavage pattern of protein tau after coincubation with CatS. Furthermore, tau oligomer formation was increased 2.4-fold (p < 0.05) after limited cleavage. Serum CatS and CysC levels did not correlate with disease severity in PSP. Of note, IL-6 correlated with PSP-S (r = 0.41; 95% CI 0.11-0.65; p = 0.008), SEADL (r = -0.37; 95% CI -0.61 to -0.06; p = 0.017) and the history and gait/midline subdomains of the PSP-RS. While CatS facilitates tau aggregation in vitro, serum levels of CatS appear not to correlate with disease severity. The observed correlation of IL-6 with disease severity warrants further investigation of inflammatory markers in PSP.