Profile of 6 microRNA in blood plasma distinguish early stage Alzheimer’s disease patients from non-demented subjects

Recently, a human ortholog of mouse calcyclin (S100A6)-binding protein (CacyBP) called SIP (Siah-1-interacting protein) was shown to be a component of a novel ubiquitinylation pathway regulating β-catenin degradation (Matsuzawa, S., and Reed, J. C. (2001) Mol. Cell 7, 915–926). In murine brain, CacyBP/SIP is expressed at a high level, but S100A6 is expressed at a very low level. Consequently we carried out experiments to determine if CacyBP/SIP binds to other S100 proteins in this tissue. Using CacyBP/SIP affinity chromatography, we found that S100B from the brain extract binds to CacyBP/SIP in a Ca²⁺-dependent manner. Using a nitrocellulose overlay assay with ¹²⁵I-CacyBP/SIP and CacyBP/SIP affinity chromatography, we found that this protein binds purified S100A1, S100A6, S100A12, S100B, and S100P but not S100A4, calbindin D_9k, parvalbumin, and calmodulin. The interaction of S100 proteins with CacyBP/SIP occurs via its C-terminal fragment (residues 155–229). Co-immunoprecipitation of CacyBP/SIP with S100B from brain and with S100A6 from Ehrlich ascites tumor cells suggests that these interactions are physiologically relevant and that the ubiquitinylation complex involving CacyBP/SIP might be regulated by S100 proteins.

Sgt1 has been identified as a subunit of both core kinetochore and SCF (Skp1-Cul1-F-box) ubiquitin ligase complexes and is also implicated in plant disease resistance. Sgt1 has two putative HSP90 binding domains, a tetratricopeptide repeat and a p23-like CHORD and Sgt1 (CS) domain. Using NMR spectroscopy, we show that only the CS domain of human Sgt1 physically interacts with HSP90. The tetratricopeptide repeat domain does not bind to either HSP90 or HSP70. Determination of the three-dimensional structure showed that the Sgt1-CS domain shares the same β-sandwich fold as p23 but lacks the last highly conserved β-strand in p23. Analysis of the structures of Sgt1-CS and p23 revealed a similar charge distribution on one of two opposing surfaces that suggests that it is the binding region for HSP90 in Sgt1. Although ATP is absolutely required for p23 binding to HSP90, Sgt1 binds to HSP90 also in the absence of the non-hydrolyzable analog ATPγS. Our findings suggest the CS domain is a binding module for HSP90 distinct from p23-like domains, which implies that Sgt1 and related proteins function in recruiting heat shock protein activities to multiprotein assemblies.

The interaction between Ca²⁺ sensors STIM1 and STIM2 and Ca²⁺ channel-forming protein ORAI1 is a crucial element of store-operated calcium entry (SOCE) in non-excitable cells. However, the molecular mechanism of SOCE in neurons remains unclear. We addressed this issue by establishing the presence and function of STIM proteins. Real-time polymerase chain reaction from cortical neurons showed that these cells contain significant amounts of Stim1 and Stim2 mRNA. Thapsigargin (TG) treatment increased the amount of both endogenous STIM proteins in neuronal membrane fractions. The number of YFP-STIM1/ORAI1 and YFP-STIM2/ORAI1 complexes was also enhanced by such treatment. The differences observed in the number of STIM1 and STIM2 complexes under SOCE conditions and the differential sensitivity to SOCE inhibitors suggest their distinct roles. Endoplasmic reticulum (ER) store depletion by TG enhanced intracellular Ca²⁺ levels in loaded with Fura-2 neurons transfected with YFP-STIM1 and ORAI1, but not with YFP-STIM2 and ORAI1, which correlated well with the number of complexes formed. Moreover, the SOCE inhibitors ML-9 and 2-APB reduced Ca²⁺ influx in neurons expressing YFP-STIM1/ORAI1 but produced no effect in cells transfected with YFP-STIM2/ORAI1. Moreover, in neurons transfected with YFP-STIM2/ORAI1, the increase in constitutive calcium entry was greater than with YFP-STIM1/ORAI1. Our data indicate that both STIM proteins are involved in calcium homeostasis in neurons. STIM1 mainly activates SOCE, whereas STIM2 regulates resting Ca²⁺ levels in the ER and Ca²⁺ leakage with the additional involvement of STIM1.