Calsenilin interacts with transcriptional co-repressor C-terminal binding protein(s) Nikhat F. Zaidi,* Kristy G. Kuplast,* Kevin J. Washicosky,* Yuji Kajiwara, Joseph D. Buxbaum  and Wilma Wasco* *Genetics and Aging Research Unit, Department of Neurology, MassGeneral Institute for Neurodegenerative Disease and Harvard Medical School, Charlestown, Massachusetts, USA  Laboratory of Molecular Neuropsychiatry, Departments of Psychiatry, Neuroscience and Geriatrics and Adult Development, Mount Sinai School of Medicine, New York, New York, USA Abstract Calsenilin/potassium channel-interacting protein (KChIP)3/ downstream regulatory element sequence antagonist modu- lator (DREAM) is a neuronal calcium-binding protein that has been shown to have multiple functions in the cell, including the regulation of presenilin processing, repression of transcription and modulation of A-type potassium channels. To gain a better understanding of the precise role of calsenilin in specific cellular compartments, an interactor hunt for proteins that bind to the N-terminal domain of calsenilin was carried out. Using a yeast two-hybrid system and co-immunoprecipitation studies, we have identified the transcriptional co-repressor C-terminal binding protein (CtBP)2 as an interactor for calsenilin and have shown that the two proteins can interact in vivo. In co- immunoprecipitation studies, calsenilin also interacted with CtBP1, a CtBP2 homolog. Our data also showed a calsenilin- dependent increase in c-fos protein levels in CtBP knockout fibroblasts, suggesting that CtBP may modulate the tran- scriptional repression of c-fos by calsenilin. Furthermore, the finding that histone deacetylase protein and activity were associated with the calsenilin–CtBP immunocomplex sug- gests a mechanism by which calsenilin–CtBP may act to re- press transcription. Finally, we demonstrated that calsenilin and CtBP are present in synaptic vesicles and can interact in vivo. Keywords: calsenilin, c-fos, C-terminal binding protein, syn- aptic vesicles, transcriptional repression. J. Neurochem. (2006) 98, 1290–1301. Calsenilin is a neuronal calcium-binding protein that was initially identified as a binding partner of two highly related familial Alzheimer’s disease-associated proteins, presenilin (PS)1 and PS2 (Buxbaum et al. 1998). It has been shown to play a role in modulation of the levels of b-amyloid peptide (Ab), and to enhance apoptosis in cultured cells (Jo et al. 2001, 2003, 2004; Lilliehook et al. 2002;). Raised levels of calsenilin have been reported in the cortex of patients with Alzheimer’s disease, and in the neocortex and hippocampus of transgenic mice that overexpress the Swedish mutant form of the Ab precursor protein (Jin et al. 2005). Altered Ab formation and long-term potentiation have been reported in a calsenilin knockout model (Lilliehook et al. 2003) and overexpression of calsenilin has been shown to enhance c- secretase activity associated with the presenilins (Jo et al. 2005). Calsenilin appears to be a protein with multiple functions. It has also been shown to act as a calcium-dependent transcriptional repressor by binding to the downstream regulatory element sequence (DRE) of the genes that encode prodynorphin, c-fos and Hrk. Consequently, it was termed DRE antagonist modulator (DREAM) (Carrion et al. 1999; Received January 18, 2006; revised manuscript received April 13, 2006; accepted April 18, 2006. Address correspondence and reprint request to Wilma Wasco, Gen- etics and Aging Research Unit, Department of Neurology, MassGeneral Institute for Neurodegenerative Disease and Harvard Medical School, Building 114, 16th Street, Charlestown, MA 02129, USA. E-mail: wasco@helix.mgh.harvard.edu Abbreviations used:Ab, b-amyloid; Ab, antibody; BD, binding do- main; CtBP, C-terminal binding protein; DRE, downstream regulatory element sequence; DREAM, DRE antagonist modulator; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; HDAC, histone deacety- lase; het., heterozygous; IP, immunoprecipitate; KChIP, potassium channel-interacting protein; k/o, knockout; NCS, neuronal calcium sensor; PAGE, polyacrylamide gel electrophoresis; PS, presenilin; SDS, sodium dodecyl sulfate; Sup., supernatant; Syn., ves., synaptic vesicles; TBST, Tris-buffered saline containing Tween 20; VILIP, visinin-like protein; Wt, wild type. Journal of Neurochemistry , 2006, 98, 1290–1301 doi:10.1111/j.1471-4159.2006.03972.x 1290 Journal Compilation Ó 2006 International Society for Neurochemistry, J. Neurochem. (2006) 98, 1290–1301 Ó 2006 The Authors