Neuronal Ca2+ homeostasis and Ca2+ signaling regulate multiple neuronal functions, including synaptic transmission, plasticity, and cell survival. Therefore disturbances in Ca2+ homeostasis can affect the well‐being of the neuron in different ways and to various degrees. Ca2+ homeostasis undergoes subtle dysregulation in the physiological ageing. Products of energy metabolism accumulating with age together with oxidative stress gradually impair Ca2+ homeostasis, making neurons more vulnerable to additional stress which, in turn, can lead to neuronal degeneration. Neurodegenerative diseases related to aging, such as Alzheimer's disease, Parkinson's disease, or Huntington's disease, develop slowly and are characterized by the positive feedback between Ca2+ dyshomeostasis and the aggregation of disease‐related proteins such as amyloid beta, alfa‐synuclein, or huntingtin. Ca2+ dyshomeostasis escalates with time eventually leading to neuronal loss. Ca2+ dyshomeostasis in these chronic pathologies comprises mitochondrial and endoplasmic reticulum dysfunction, Ca2+ buffering impairment, glutamate excitotoxicity and alterations in Ca2+ entry routes into neurons. Similar changes have been described in a group of multifactorial diseases not related to ageing, such as epilepsy, schizophrenia, amyotrophic lateral sclerosis, or glaucoma. Dysregulation of Ca2+ homeostasis caused by HIV infection or by sudden accidents, such as brain stroke or traumatic brain injury, leads to rapid neuronal death. The differences between the distinct types of Ca2+ dyshomeostasis underlying neuronal degeneration in various types of pathologies are not clear. Questions that should be addressed concern the sequence of pathogenic events in an affected neuron and the pattern of progressive degeneration in the brain itself. Moreover, elucidation of the selective vulnerability of various types of neurons affected in the diseases described here will require identification of differences in the types of Ca2+ homeostasis and signaling among these neurons. This information will be required for improved targeting of Ca2+ homeostasis and signaling components in future therapeutic strategies, since no effective treatment is currently available to prevent neuronal degeneration in any of the pathologies described here. © 2008 IUBMB IUBMB Life, 60(9): 575–590, 2008

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 ␤-sandwich fold as p23 but lacks the last highly conserved ␤-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 ATP␥S. 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. Heat shock protein 90 (HSP90)1 is a molecular chaperone important for protein folding. HSP90 is different from other chaperones because most of its substrates are related to signal transduction (1). Recent studies also suggest that HSP90 plays a role in protein quality control where it facilitates the polyubiquitination and degradation of substrates through interaction with the co-chaperone C terminus of HSC70-interacting protein (CHIP) (2). Thus, HSP90 can be involved in protein regulation in quite different ways depending on the cellular context.Very recently, it has been reported that Sgt1 interacts with HSP90 (3-5). Sgt1 was originally identified as a suppressor of the G 2 allele of Skp1 and was found to be important for both the G 1 /S and G 2 /M transitions in the cell cycle (6). The G 2 /M transition involves activation of the kinetochore. Sgt1 was shown to be required for the activation of the kinetochore core complex CBF3 and to physically interact with Skp1, one component of CBF3 (6). Sgt1 also physically interacts with Skp1-Cul1-F-box (SCF) E3 ubiquitin ligase complexes through interaction with Skp1. Moreover, a yeast Sgt1 mutant was defective in Sic1 degradation through ubiquitination (6).Sequence analysis of Sgt1 proteins from yeast, human, barley, rice, and Arabidopsis shows three conserved domains (tetratricopeptide repeat (TPR), CHORD-containing proteins and Sgt1 (CS), and Sgt1-specific (SGS)) and two variable regions (VR1 and VR2) (7). TPR domains are known as heat shock protein binding domains. The SGS domain was shown to interact with S100 calcium-binding proteins (8). The CS domain has high sequence homology with p23 and is also known as a p23-like domain (9). p23 has been shown to physically and functionally interact with HSP90, apparently serving in a role as co-chaperone. Recently, HSP90 was shown to be an essential factor required for...

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.

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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 ␤-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 Ca 2؉ -dependent manner. Using a nitrocellulose overlay assay with 125 I-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.

Alzheimers disease (AD) is the most common age-related dementia. Among its major challenges is identifying molecular signatures characteristic for the early AD stage in patients with Mild Cognitive Impairment (MCI-AD), which could serve for deciphering the AD pathomechanism and also as non-invasive, easy-to-access biomarkers. Using qRT-PCR we compared the microRNA (miRNA) profiles in blood plasma of 15 MCI-AD patients, whose diagnoses were confirmed by cerebrospinal fluid (CSF) biomarkers, with 20 AD patients and 15 non-demented, age-matched individuals (CTR).To minimize methodological variability, we adhered to standardization of blood and CSF assays recommended by the international Joint Programming for Neurodegenerative Diseases (JPND) BIOMARKAPD consortium, and we employed commercially available Exiqon qRT-PCR-assays. In the first screening, we assessed 179 miRNAs of plasma. We confirmed 23 miRNAs reported earlier as AD biomarker candidates in blood and found 26 novel differential miRNAs between AD and control subjects. For representative 15 differential miRNAs, the TargetScan, MirTarBase and KEGG database analysis indicated putative protein targets among such AD hallmarks as MAPT (Tau), proteins involved in amyloidogenic proteolysis, and in apoptosis. These 15 miRNAs were verified in separate, subsequent subject groups. Finally, 6 miRNAs (3 not yet reported in AD context and 3 reported in AD blood) were selected as the most promising biomarker candidates differentiating early AD from controls with the highest fold changes (from 1.32 to 14.72), consistent significance, specificities from 0.78 to 1 and sensitivities from 0.75 to 1. (patent pending, PCT/IB2016/052440).

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In this review we describe the present knowledge about store operated Ca²⁺ entry (SOCE) in neurons and the proteins involved in this process: STIM, as well as Orai and TRP channels. We address the issue of whether SOCE is used only to refill Ca²⁺ in the ER or whether Ca²⁺ that enters the neuronal cell during SOCE also performs signaling functions. We collected the data indicating that SOCE and its components participate in the important processes in neurons. This has implications for identifying new drug targets for the treatment of brain diseases. Evidence indicates that in neurodegenerative diseases Ca²⁺ homeostasis and SOCE components become dysregulated. Thus, different targets and strategies might be identified for the potential treatment of these diseases. This article is part of a Special Issue entitled: 13th European symposium on calcium.