The amyloid precursor protein (APP) is a key protein involved in the development of Alzheimer's disease. We previously identified a signal transduction secretory pathway in which the small G protein Rac sets downstream of the cAMP/Epac/ Rap1 signalling cascade regulating the a cleavage of APP [Maillet, M. et al. (2003) Crosstalk between Rap and Rac regulates secretion of sAPPa. Nat. Cell Biol. 5, 633-639]. We now report that Rap1 can physically and specifically associate with the guanine nucleotide exchange factor (GEF) STEF through its TSS region. A deleted TSS domain of STEF cells fails to activate Rac1 and dramatically decreases secretion of the nonamyloidogenic soluble form of APP (sAPPa) induced by the cAMP-binding protein Epac. Altogether, our data show that upon Epac activation, Rap1 recruits STEF through its TSS region and activates Rac1, which mediates APP processing.

The cholinergic system has been widely implicated in cognitive processes and cholinergic loss is a classical hallmark in Alzheimer disease. Increasing evidence supports a role of the serotonergic system in cognition, possibly through a modulation of cholinergic activity. We compared selective cholinergic denervation by administration of the immunotoxin 192 IgG-saporin in the nucleus basalis of Meynert (NBM) with intracerebroventricular (ICV) lesions of the basal forebrain in male rats 7 days after lesioning. NBM lesions induced significant changes in cholinergic markers in the frontal cortex, whereas ICV lesions produced significant decreases in cholinergic markers both in the frontal cortex and hippocampus. Only ICV lesions lead to memory impairments in passive avoidance and Morris water maze tasks. Both models lead to reductions of serotonin levels in the frontal cortex. Similar changes in 5-hydroxytriptophan levels were observed, suggesting a downregulation of the rate-limiting enzyme for the synthesis of serotonin along with the cholinergic deficit. Neither 5-HT1A nor 5-HT1B receptors seem to mediate this process. These data imply that the serotonergic system in the frontal cortex can compensate for diminished cholinergic function and support the investigation of the serotonergic system as a therapeutic target to treat Alzheimer disease.

Small GTPases of the Ras superfamily are capable of activating E2F-dependent transcription leading to cell proliferation, but the molecular mechanisms are poorly understood. In this study, using immortalized chicken DT40 B cell lines to investigate the role of the Vav/Rac signalling cascade on B cell proliferation, it is shown that the proliferative response triggered by B cell receptor activation is dramatically reduced in the absence of Vav3 expression. Analysis of this proliferative defect shows that in the absence of Vav3 expression, retinoblastoma protein (RB) phosphorylation and the subsequent E2F activation do not take place. By combining pharmacological and genetic approaches, phosphatidylinositol-3-kinase and phospholipase Cc2 (PLCc2) were identified as the key regulatory signalling molecules upstream of the Vav3/Rac pathway leading to RB phosphorylation and E2F transcription factor activation. Additionally, vav3À/À and plcc2 À/À DT40 B cells were not able to activate the RB-E2F complex wild-type phenotype when these genetically modified cells were transfected with constitutively active forms of RhoA or Cdc42. However, when these knockout cells were transfected with different constitutively active versions of PLCc, Vav or Rac1, not only activation of the RB-E2F complex wild-type phenotype was recovered but also the cellular proliferation. Furthermore, by evaluating the effect of two known effector mutants of Rac1 (Rac1 Q61L/F37A and Rac1), the RB-E2F complex activation dependency on p21-activated kinase (PAK) and protein kinase Ce (PKCe) activities was established, being independent of both actin cytoskeleton reorganization and Ras activity. These results suggest that PAK1 and PKCe may be potential therapeutic targets to stop uncontrolled B cell proliferation mediated by the Vav/Rac pathway.Abbreviations BAPTA/AM, 1,2-bis(2-aminophenoxy)ethane-N,N,N 0 ,N 0 -tetraacetic acid tetrakis(acetoxymethyl ester); BCR, B cell receptor; CDK, cyclindependent kinase; DAG, diacylglycerol; GEF, guanine nucleotide exchange factor; PAK, p21-activated kinase; PI3K, phosphatidylinositol-3-kinase; PKC, protein kinase C; PKD, protein kinase D; PLC, phospholipase C; PMA, phorbol 12-myristate 13-acetate; RasGRP, RAS guanyl releasing protein; RB, retinoblastoma protein.