Differential expression and regulation of myristoylated alanine-rich C kinase substrate (MARCKS) in the hippocampus of C57/BL6J and DBA/2J mice Robert K. McNamara,* ,1 Patricia A. Vasquez, Aleksander A. Mathe and Robert H. Lenox* ,2 *Departments of Psychiatry, University of Pennsylvania School of Medicine, Clinical Research Building, Philadelphia, Pennsylvania, USA  Karolinska Institute, Institute of Physiology and Pharmacology, Division of Pharmacology, Stockholm, Sweden Abstract The myristoylated alanine-rich C kinase substrate (MARCKS) is a major protein kinase C (PKC) substrate in brain that binds the inner surface of the plasma membrane, calmodulin, and cross-links filamentous actin, all in a PKC phosphorylation- reversible manner. MARCKS has been implicated in hippo- campal-dependent learning and long-term potentiation (LTP). Previous studies have shown DBA/2 mice to exhibit poor spatial/contextual learning, impaired hippocampal LTP, and hippocampal mossy fiber hypoplasia, as well as reduced hippocampal PKC activity and expression relative to C57BL/6 mice. In the present study, we assessed the expression (mRNA and protein) and subcellular distribution (membrane and cytolsol) of MARCKS in the hippocampus and frontal cortex of C57BL/6 and DBA/2 mice using quantitative western blotting. In the hippocampus, total MARCKS mRNA and pro- tein levels in C57BL/6J mice were significantly lower (45%) compared with DBA/2J mice, and MARCKS protein was observed predominantly in the cytosolic fraction. MARCKS expression in frontal cortex did not differ significantly between strains. To examine the dynamic regulation of MARCKS subcellular distribution, mice from each strain were subjected to 60 min restraint stress and MARCKS subcellular distribution was determined 24 h later. Restraint stress resulted in a significant reduction in membrane MARCKS expression in C57BL/6J hippocampus but not in the DBA/2J hippocampus despite similar stress-induced increases in serum corticos- terone. Restraint stress did not affect cytosolic or total MARCKS levels in either strain. Similarly, restraint stress (30 min) in rats also induced a significant reduction in mem- brane MARCKS, but not total or cytosolic MARCKS, in the hippocampus but not in frontal cortex. In rats, chronic lithium treatment prior to stress exposure reduced hippocampal MARCKS expression but did not affect the stress-induced reduction in membrane MARCKS. Collectively these data demonstrate higher resting levels of MARCKS in the hippo- campus of DBA/2J mice compared to C57BL/6J mice, and that acute stress leads to a long-term reduction in membrane MARCKS expression in C57BL/6J mice and rats but not in DBA/2J mice. These strain differences in hippocampal MARCKS expression and subcellular translocation following stress may contribute to the differences in behaviors requiring hippocampal plasticity observed between these strains. Keywords: C57BL/6J, DBA/2J, hippocampus, myristoylated alanine-rich C kinase, protein kinase C, stress. J. Neurochem. (2003) 85, 462–468. Protein kinase C (PKC) is a gene superfamily consisting of at least 12 serine/threonine protein kinase isozymes with differing regional and subcellular distributions, second messenger activators, and substrate affinities (Tanaka and Nishizuka 1994; Mellor and Parker 1998). PKC-mediated substrate phosphorylation has been implicated in a number of long-term neuroplastic events in brain including long-term potentiation (LTP) and learning and memory (Ramakers et al. 1999; Van der Zee et al. 1997). The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary PKC Received November 7, 2002; revised manuscript received January 9, 2003; accepted January 10, 2003. Address correspondence and reprint requests to Dr Robert K. McNamara, PhD, Eli Lilly Corporate Center, 639 S Delaware St, Indi- anapolis, IN 46285–6322, USA. E-mail: mcnamara_robert_k@lilly.com 1 The present address of Robert K. McNamara is Neuroscience Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285, USA. 2 The present address of Robert H. Lenox is Aventis Pharmaceuticals, Bridgewater, NJ 08807, USA. Abbreviations used: LTP, long-term potentiation; MARCKS, myris- toylated alanine-rich C kinase substrate; PKC, protein kinase C. Journal of Neurochemistry , 2003, 85, 462–468 doi:10.1046/j.1471-4159.2003.01700.x 462 Ó 2003 International Society for Neurochemistry, J. Neurochem. (2003) 85, 462–468