The calcyclin binding protein and Siah-1 interacting protein (CacyBP/SIP) protein was shown to play a role in the organization of microtubules. In this work we have examined the neuronal distribution and possible function of CacyBP/SIP in cytoskeletal pathophysiology. We have used brain tissue from Alzheimer's disease (AD) patients and from transgenic mice modeling 2 different pathologies characteristic for AD: amyloid and tau. In the brain from AD patients, CacyBP/SIP was found to be almost exclusively present in neuronal somata, and in control patients it was seen in the somata and neuronal processes. In mice doubly transgenic for amyloid precursor protein and presenilin 1 there was no difference in CacyBP/SIP neuronal localization in comparison with the nontransgenic animals. By contrast in tau transgenic mice, localization of CacyBP/SIP was similar to that observed for AD patients. To find the relation between CacyBP/SIP and tau we examined dephosphorylation of tau by CacyBP/SIP. We found that indeed it exhibited phosphatase activity toward tau. Altogether, our results suggest that CacyBP/SIP might play a role in AD pathology.
Alzheimer's disease (AD) is the most common neurodegenerative disorder. It is characterized by a progressive loss of neurons and cognitive functions. Pathologic hallmarks of AD are extracellular amyloid plaques, composed of the amyloid-β (Aβ) peptide and intracellular neurofibrillary tangles (NFTs) that predominantly comprise tau protein (Braithwaite et al., 2012). The pathogenic mechanism of AD is still poorly understood and thus further studies on factors or proteins that might be important in the development of AD are necessary. One such factor might be calcyclin binding protein and Siah-1 interacting protein (CacyBP/SIP), a protein expressed at high levels in the brain (Jastrzebska et al., 2000) and having an age-dependent subcellular localization in neurons (Filipek et al., 2008).
CacyBP/SIP was discovered as a S100A6 (calcyclin) target (Filipek and Kuznicki, 1998; Schneider and Filipek, 2011) but later shown to interact with some other S100 proteins (Filipek et al., 2002a) and with Siah-1 (Matsuzawa and Reed, 2001), Skp1 (Bhattacharya et al., 2005), tubulin (Schneider et al., 2007), actin (Schneider et al., 2010), and extracellular signal-related kinase (ERK)1/2 (Kilańczyk et al., 2009). CacyBP/SIP is present in various tissues with the highest level in brain (Jastrzebska et al., 2000) and in some cell lines such as mouse neuroblastoma NB2a (Filipek et al., 2002b). The high level of CacyBP/SIP in neurons (Jastrzebska et al., 2000) and its interaction with tubulin (Schneider et al., 2007) suggested its involvement in developmental changes of the neuronal cytoskeleton. This supposition has been confirmed by the finding that subcellular compartmentalization of CacyBP/SIP in rat brain neurons depends on age. In young rats CacyBP/SIP was present in the cytoplasm and in neuronal processes, and in aged animals it was seen only in the cytoplasm and not in neuronal processes (Filipek et al., 2008). Such changes in localization of CacyBP/SIP were similar to those of tau, a major neuronal microtubule-associated protein, and suggested a possible role of CacyBP/SIP in cytoskeletal physiology and pathophysiology (Filipek et al., 2008). Interaction of CacyBP/SIP with actin, another cytoskeletal protein (Schneider et al., 2010), might also implicate a role for CacyBP/SIP in cytoskeletal organization. Furthermore, a substantial sequence similarity between CacyBP/SIP and phosphatases from the mitogen activated protein (MAP) kinase phosphatase family and its ability to dephosphorylate ERK1/2 kinase (Kilanczyk et al., 2011) suggest that the involvement of CacyBP/SIP in cytoskeletal dynamics might rely on its dephosphorylating activity toward key cytoskeletal components.
Microtubules, composed predominantly of α- and β-tubulin subunits, are the major structural components of the neuronal cytoskeleton that plays an important role in neurodevelopment, aging, and neurodegenerative disorders (Baas and Qiang, 2005; Brandt et al., 2005). Tau protein is important in the assembly and disassembly of dynamic microtubules and is associated normally with axonal rather than somatodendritic microtubules (Binder et al., 1985). AD is characterized by a substantial redistribution of tau into the somatodendritic compartment as neurofibrillary pathology, which is in the form of intra- and extracellular tangles, and as dystrophic neurites either throughout the neuropil or within neuritic plaques (Harrington, 2012). Neurofibrillary tau pathology involves the formation of paired helical filaments (PHFs) that comprised tau protein. Tau is an ideal substrate for phosphorylation by multiple kinases in the somatodendritic compartment in AD (Gong et al., 2005).
In the present work we have examined immunohistochemically the neuronal localization of CacyBP/SIP in association with phosphorylated tau (p-Tau) and β-tubulin, in the brains from AD patients and from 2 lines of mice separately transgenic for amyloid and tau. Moreover, we checked the level of CacyBP/SIP and the ratio of the major CacyBP/SIP forms in control and pathologic mouse brain material by applying sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and 2-dimensional (2D) electrophoresis, respectively. Because CacyBP/SIP has recently been shown to exhibit phosphatase activity toward ERK1/2, we also analyzed whether it could dephosphorylate tau, a protein that becomes hyperphosphorylated in AD pathology.
