Cyclin-Dependent Kinase 5 Activator p25 Is Generated During Memory Formation and Is Reduced at an Early Stage in Alzheimer’s Disease Olivia Engmann, Tibor Hortobágyi, Andrew J. Thompson, Jennifer Guadagno, Claire Troakes, Salvador Soriano, Safa Al-Sarraj, Yong Kim, and Karl Peter Giese Background: The cyclin-dependent kinase 5 activator p35 can be cleaved into p25. Formation of p25 has been suggested to contribute to neurodegeneration in Alzheimer’s disease (AD). However, overexpression of low levels of p25 in mice enhances memory formation. Therefore, it has been suggested that p25 formation might be an event early in AD to compensate for impairments in synaptic plasticity. Ongoing p25 formation has been hypothesized to contribute to neurodegeneration at the later stages of AD. Methods: Here, we tested the early compensation hypothesis by analyzing the levels of p25 and its precursor p35 in AD postmortem samples from different brain regions at different stages of tau pathology, using quantitative Western blots. Furthermore, we studied p35 and p25 during spatial memory formation. By employing quantitative mass spectrometry, we identified proteins downstream of p25, which were then studied in AD samples. Results: We found that p25 is generated during spatial memory formation. Furthermore, we demonstrate that overexpression of p25 in the physiological range increases the expression of two proteins implicated in spine formation, septin 7 and optic atrophy 1. We show that the expression of p35 and p25 is reduced as an early event in AD. Moreover, expression of the p25-regulated protein optic atrophy 1 was reduced in a time course similar to p25 expression. Conclusions: Our findings suggest that p25 generation is a mechanism underlying hippocampal memory formation that is impaired in the early stages of AD. Our findings argue against the previously raised early compensation hypothesis and they propose that p25-mediated neurotoxicity does not occur in AD. Key Words: Alzheimer’s disease, Cdk5, memory, p25, p35 C yclin-dependent kinase 5 (Cdk5), a protein involved in syn- aptic plasticity and neurodevelopment (1–5), is activated by its small subunits p35 and p39 (6,7). p35 can be cleaved to p25 by calpain, thereby releasing the activator from the membrane. Additionally, p25 has a longer half-life than p35 and may thus increase Cdk5 activity ([8], but see [9]). Currently, p25 generation is believed to be a pathological event that leads to neurotoxicity (10). Accordingly, transgenic overexpression of high p25 levels results in neurodegeneration (11,12). However, overexpression of low p25 levels or transient p25 expression enhances synaptic plasticity and improves memory formation (12–14). The underlying molecular mechanisms of this memory enhancement are not well under- stood. Dysregulation of Cdk5 has been implicated in Alzheimer’s dis- ease (AD), the most common type of dementia (15). Pathologically, AD is recognized by the presence of dense extracellular plaques containing amyloid peptide and intracellular neurofibrillary tangles containing hyperphosphorylated tau (16,17). Cyclin-dependent ki- nase 5 is a tau kinase under physiological conditions and Cdk5 activity has been suggested to be upregulated in AD (18). Two postmortem studies coming from the same laboratory have sug- gested that p25 formation occurs in AD (8,19), whereas two post- mortem studies from different laboratories suggested that p25 ex- pression is decreased in AD (20,21). These contradictory results are summarized in Table S1 in Supplement 1. They may, however, be explained by changes in p25 expression over the course of the disease and the role of p25 may be resolved by analysis of samples from different AD stages. In mouse models, prolonged overexpres- sion of high p25 levels (as measured by the p25/p35 ratio) leads to neurodegeneration (11,12), while low p25 levels improve memory formation (13,14). Therefore, an early compensation hypothesis of AD has been proposed, in which low levels of p25 may be formed early in AD to compensate for memory loss, while increasing p25 levels in later stages may, in turn, contribute to an AD phenotype of neurodegeneration (22). This hypothesis has never been tested. Here, we provide evidence that p25 generation is a physiologi- cal event occurring during hippocampus-dependent spatial mem- ory formation in wild-type (WT) mice. Furthermore, transgenic overexpression of physiological p25 levels in mice results in in- creased spine number and upregulation of two synaptic proteins implicated in spine formation, septin 7 and optic atrophy 1 (OPA1). Finally, we demonstrate that p25, its precursor p35, and OPA1 are downregulated in early stages of AD, suggesting an early misregu- lation of p25-related pathways in this illness. Hence, our data pro- vide a novel perspective on p25 and downstream pathways in the context of memory and AD. Methods and Materials p25 Mouse Line Genotyping of heterozygous mice carrying the p25 transgene under the -calcium/calmodulin-dependent protein kinase II pro- From the Departments of Neuroscience (OE, SS, KPG), and Clinical Neurosci- ence (TH, AJT, JG, CT), Medical Research Council Centre for Neurodegen- eration Research, Institute of Psychiatry, King’s College London, and the Department of Clinical Neuropathology (SA-S), King’s College Hospital, London, United Kingdom; Neurodegenerative Diseases Laboratory (SS), Division of Anatomy School of Medicine, Loma Lima University, Loma Linda, California; and The Rockefeller University (YK), New York, New York. Address correspondence to Karl Peter Giese, Ph.D., Institute of Psychiatry, Department of Neuroscience, Centre for the Cellular Basis of Behaviour, 125 Coldharbour Lane, London SE5 9NU, UK; E-mail: Karl.Giese@ kcl.ac.uk. Received Feb 1, 2011; revised Mar 23, 2011; accepted Apr 6, 2011. BIOL PSYCHIATRY 2011;70:159 –168 0006-3223/$36.00 doi:10.1016/j.biopsych.2011.04.011 © 2011 Society of Biological Psychiatry