Recent experimental and clinical retrospective studies support the view that reduction of brain cholesterol protects against Alzheimer's disease (AD). However, genetic and pharmacological evidence indicates that low brain cholesterol leads to neurodegeneration. This apparent contradiction prompted us to analyze the role of neuronal cholesterol in amyloid peptide generation in experimental systems that closely resemble physiological and pathological situations. We show that, in the hippocampus of control human and transgenic mice, only a small pool of endogenous APP and its β-secretase, BACE 1, are found in the same membrane environment. Much higher levels of BACE 1–APP colocalization is found in hippocampal membranes from AD patients or in rodent hippocampal neurons with a moderate reduction of membrane cholesterol. Their increased colocalization is associated with elevated production of amyloid peptide. These results suggest that loss of neuronal membrane cholesterol contributes to excessive amyloidogenesis in AD and pave the way for the identification of the cause of cholesterol loss and for the development of specific therapeutic strategies.

The cholesterol-synthesizing enzyme seladin-1, encoded by the Dhcr24 gene, is a flavin adenine dinucleotidedependent oxidoreductase and regulates responses to oncogenic and oxidative stimuli. It has a role in neuroprotection and is downregulated in affected neurons in Alzheimer's disease (AD). Here we show that seladin-1-deficient mouse brains had reduced levels of cholesterol and disorganized cholesterol-rich detergent-resistant membrane domains (DRMs). This was associated with inefficient plasminogen binding and plasmin activation, the displacement of b-secretase (BACE) from DRMs to APP-containing membrane fractions, increased b-cleavage of APP and high levels of Ab peptides. In contrast, overexpression of seladin-1 increased both cholesterol and the recruitment of DRM components into DRM fractions, induced plasmin activation and reduced both BACE processing of APP and Ab formation. These results establish a role of seladin-1 in the formation of DRMs and suggest that seladin-1-dependent cholesterol synthesis is involved in lowering Ab levels. Pharmacological enhancement of seladin-1 activity may be a novel Ab-lowering approach for the treatment of AD.

Binding of the neurotrophin brain-derived neurotrophic factor (BDNF) to the TrkB receptor is a major survival mechanism during embryonic development. In the aged brain, however, BDNF levels are low, suggesting that if TrkB is to play a role in survival at this stage additional mechanisms must have developed. We here show that TrkB activity is most robust in the hippocampus of 21-d-old BDNF-knockout mice as well as in old, wild-type, and BDNF heterozygous animals. Moreover, robust TrkB activity is evident in old but not young hippocampal neurons differentiating in vitro in the absence of any exogenous neurotrophin and also in neurons from BDNF ؊/؊ embryos. Age-associated increase in TrkB activity correlated with a mild yet progressive loss of cholesterol. This, in turn, correlated with increased expression of the cholesterol catabolic enzyme cholesterol 24-hydroxylase. Direct cause-effect, cholesterol loss-high TrkB activity was demonstrated by pharmacological means and by manipulating the levels of cholesterol 24-hydroxylase. Because reduced levels of cholesterol and increased expression of choleseterol-24-hydroxylase were also observed in the hippocampus of aged mice, changes in cellular cholesterol content may be used to modulate receptor activity strength in vivo, autonomously or as a way to complement the natural decay of neurotrophin production. INTRODUCTIONDuring development, neurotrophins are mandatory for the survival, differentiation, and growth of different neuronal populations (Reichardt, 2006). In the mature nervous system, neurotrophins are important for the modulation of neuronal connectivity and activity-dependent plasticity (Conover and Yancopoulos, 1997;Blum and Konnerth, 2005). Neurotrophins bind and activate receptor tyrosine kinases (RTKs), in turn leading to multiple intracellular signaling pathways, most notoriously those involving mitogen-activated protein kinases and phosphatidylinositol 3-kinase (PI3K) (Kaplan and Miller, 2000;Reichardt, 2006). In the hippocampus, a region of the brain critically involved in certain types of learning and memory, the most prominently expressed neurotrophin receptor is TrkB (Tokuyama et al., 1998), whose cognate ligand is BDNF (brain-derived neurotrophic factor). In agreement with a role in memory-associated processes, loss-of-function studies of both TrkB and BDNF result in changes in affective and cognitive states in mammals and humans (Minichiello et al., 1999;Pozzo-Miller et al., 1999;Xu et al., 2000;Egan et al., 2003;Yeo et al., 2004). Although there is no doubt that BDNF is the main modulator of TrkB activity, a number of evidences indicate that certain roles mediated by the activation of TrkB may occur independently from BDNF. For instance, TrkB conditional knockout mice present clear defects in pre-and postsynaptic morphogenesis in the hippocampus (Luikart et al., 2005), yet this is not the case in BDNF conditional knockout mice (Gorski et al., 2003;Hill et al., 2005). The last observations are in turn consistent with the lack of an overt effect on...

The transcription factor NURR1 regulates the dopamine (DA) signaling pathway and exerts a critical role in the development of midbrain dopaminergic neurons (mDA). NURR1 alterations have been linked to DA-associated brain disorders, such as Parkinson’s disease and schizophrenia. However, the association between NURR1 defects and the attention-deficit hyperactivity disorder (ADHD), a DA-associated brain disease characterized by hyperactivity, impulsivity and inattention, has never been demonstrated. To date, a comprehensive murine model of ADHD truly reflecting the whole complex human psychiatric disorder still does not exist. NURR1-knockout (NURR1-KO) mice have been reported to exhibit increased spontaneous locomotor activity, but their complete characterization is still lacking. In the present study a wide-ranging test battery was used to perform a comprehensive analysis of the behavioral phenotype of the male NURR1-KO mice. As a result, their hyperactive phenotype was confirmed, while their impulsive behavior was reported for the first time. On the other hand, no anxiety and alterations in motor coordination, sociability and memory were observed. Also, the number of mDA expressing tyrosine hydroxylase, a rate-limiting enzyme of catecholamines biosynthesis, and DA level in brain were not impaired in NURR1-KO mice. Finally, hyperactivity has been shown to be recovered by treatment with methylphenidate, the first line psychostimulant drug used for ADHD. Overall, our study suggests that the NURR1 deficient male mouse may be a satisfactory model to study some ADHD behavioral phenotypes and to test the clinical efficacy of potential therapeutic agents.