ABSTRACT
Cholinergic basal forebrain (CBF) nucleus basalis (NB) neurons display neurofibrillary tangles (NFTs) during Alzheimer's disease (AD) progression, yet the mechanisms underlying this selective vulnerability are currently unclear. Rac1, a member of the Rho family of GTPases, may interact with the proapoptotic pan-neurotrophin receptor p75(NTR) to induce neuronal cytoskeletal abnormalities in AD NB neurons. Herein, we examined the expression of Rac1b, a constitutively active splice variant of Rac1, in NB cholinergic neurons during AD progression. CBF tissues harvested from people who died with a clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment, or AD were immunolabeled for both p75(NTR) and Rac1b. Rac1b appeared as cytoplasmic diffuse granules, loosely aggregated filaments, or compact spheres in p75(NTR)-positive NB neurons. Although Rac1b colocalized with tau cytoskeletal markers, the percentage of p75(NTR)-immunoreactive neurons expressing Rac1b was significantly increased only in AD compared with both mild cognitive impairment and NCI. Furthermore, single-cell gene expression profiling with custom-designed microarrays showed down-regulation of caveolin 2, GNB4, and lipase A in AD Rac1b-positive/p75(NTR)-labeled NB neurons compared with Rac1b-negative/p75(NTR)-positive perikarya in NCI. These proteins are involved in Rac1 pathway/cell cycle progression and lipid metabolism. These data suggest that Rac1b expression acts as a modulator or transducer of various signaling pathways that lead to NFT formation and membrane dysfunction in a subgroup of CBF NB neurons in AD.
Subject(s)
Basal Nucleus of Meynert/metabolism , Cholinergic Neurons/metabolism , Tauopathies/metabolism , rac1 GTP-Binding Protein/metabolism , Aged , Aged, 80 and over , Alzheimer Disease/genetics , Alzheimer Disease/metabolism , Cadaver , Caveolin 2/genetics , Cells, Cultured , Cerebellar Cortex/metabolism , Disease Progression , Down-Regulation , Female , GTP-Binding Protein beta Subunits/genetics , Humans , Male , RNA Splicing/physiology , RNA, Messenger/metabolism , Receptor, Nerve Growth Factor/metabolism , Signal Transduction/physiology , Sterol Esterase/genetics , Tauopathies/genetics , tau ProteinsABSTRACT
The synthesis of a series of berberine, phenantridine and isoquinoline derivatives was realized to explore their Rho GTPase nucleotide inhibitory activity. The compounds were evaluated in a nucleotide binding competition assay against Rac1, Rac1b, Cdc42 and in a cellular Rac GTPase activation assay. The insertion of 19 AA in the splice variant Rac1b is shown to be sufficient to introduce a conformational difference that allows compounds 4, 21, 22, and 26 to exhibit selective inhibition of Rac 1b over Rac1.
Subject(s)
Enzyme Inhibitors/chemistry , Nucleotides/chemistry , rac1 GTP-Binding Protein/antagonists & inhibitors , Amino Acid Sequence , Berberine/chemical synthesis , Berberine/chemistry , Berberine/pharmacology , Binding Sites , Computer Simulation , Enzyme Inhibitors/chemical synthesis , Enzyme Inhibitors/pharmacology , Isoquinolines/chemical synthesis , Isoquinolines/chemistry , Isoquinolines/pharmacology , Protein Binding , Protein Isoforms/antagonists & inhibitors , Protein Isoforms/metabolism , Structure-Activity Relationship , rac1 GTP-Binding Protein/metabolismABSTRACT
Pharmacological modulation of the GABA(A) receptor has gained increasing attention as a potential treatment for central processes affected in Alzheimer disease (AD), including neuronal survival and cognition. The proteolytic cleavage of the amyloid precursor protein (APP) through the alpha-secretase pathway decreases in AD, concurrent with cognitive impairment. This APP cleavage occurs within the beta-amyloid peptide (Abeta) sequence, precluding formation of amyloidogenic peptides and leading to the release of the soluble N-terminal APP fragment (sAPPalpha) which is neurotrophic and procognitive. In this study, we show that at nanomolar-low micromolar concentrations, etazolate, a selective GABA(A) receptor modulator, stimulates sAPPalpha production in rat cortical neurons and in guinea pig brains. Etazolate (20 nM-2 microM) dose-dependently protected rat cortical neurons against Abeta-induced toxicity. The neuroprotective effects of etazolate were fully blocked by GABA(A) receptor antagonists indicating that this neuroprotection was due to GABA(A) receptor signalling. Baclofen, a GABA(B) receptor agonist failed to inhibit the Abeta-induced neuronal death. Furthermore, both pharmacological alpha-secretase pathway inhibition and sAPPalpha immunoneutralization approaches prevented etazolate neuroprotection against Abeta, indicating that etazolate exerts its neuroprotective effect via sAPPalpha induction. Our findings therefore indicate a relationship between GABA(A) receptor signalling, the alpha-secretase pathway and neuroprotection, documenting a new therapeutic approach for AD treatment.
Subject(s)
Amyloid beta-Protein Precursor/metabolism , Cerebral Cortex/metabolism , Etazolate/pharmacology , Neurons/metabolism , Neuroprotective Agents/pharmacology , Receptors, GABA-A/metabolism , Alzheimer Disease/drug therapy , Alzheimer Disease/metabolism , Alzheimer Disease/physiopathology , Amyloid Precursor Protein Secretases/drug effects , Amyloid Precursor Protein Secretases/metabolism , Amyloid beta-Peptides/antagonists & inhibitors , Amyloid beta-Peptides/biosynthesis , Amyloid beta-Peptides/toxicity , Animals , Cells, Cultured , Cerebral Cortex/drug effects , Dose-Response Relationship, Drug , GABA Agents/pharmacology , GABA Agonists/pharmacology , GABA Antagonists/pharmacology , Guinea Pigs , Male , Neurons/drug effects , Peptide Fragments/antagonists & inhibitors , Peptide Fragments/biosynthesis , Peptide Fragments/toxicity , Phosphodiesterase Inhibitors/pharmacology , Protein Structure, Tertiary/physiology , Rats , Rats, Wistar , Receptors, GABA-A/drug effects , Signal Transduction/drug effects , Signal Transduction/physiologyABSTRACT
beta-Amyloid peptides (Abeta) that form the senile plaques of Alzheimer disease consist mainly of 40- and 42-amino acid (Abeta 40 and Abeta 42) peptides generated from the cleavage of the amyloid precursor protein (APP). Generation of Abeta involves beta-secretase and gamma-secretase activities and is regulated by membrane trafficking of the proteins involved in Abeta production. Here we describe a new small molecule, EHT 1864, which blocks the Rac1 signaling pathways. In vitro, EHT 1864 blocks Abeta 40 and Abeta 42 production but does not impact sAPPalpha levels and does not inhibit beta-secretase. Rather, EHT 1864 modulates APP processing at the level of gamma-secretase to prevent Abeta 40 and Abeta 42 generation. This effect does not result from a direct inhibition of the gamma-secretase activity and is specific for APP cleavage, since EHT 1864 does not affect Notch cleavage. In vivo, EHT 1864 significantly reduces Abeta 40 and Abeta 42 levels in guinea pig brains at a threshold that is compatible with delaying plaque accumulation and/or clearing the existing plaque in brain. EHT 1864 is the first derivative of a new chemical series that consists of candidates for inhibiting Abeta formation in the brain of AD patients. Our findings represent the first pharmacological validation of Rac1 signaling as a target for developing novel therapies for Alzheimer disease.