Inhibition of Glutamate Release, but Not of Glutamine Recycling to Glutamate, Is Involved in Delaying the Onset of Initial Lithium-Pilocarpine-Induced Seizures in Young Rats by a Non-Convulsive MSO Dose.

Initial seizures observed in young rats during the 60 min after administration of pilocarpine (Pilo) were delayed and attenuated by pretreatment with a non-convulsive dose of methionine sulfoximine (MSO). We hypothesized that the effect of MSO results from a) glutamine synthetase block-mediated inhibition of conversion of Glu/Gln precursors to neurotransmitter Glu, and/or from b) altered synaptic Glu release. Pilo was administered 60 min prior to sacrifice, MSO at 75 mg/kg, i.p., 2.5 h earlier. [1,2-13C]acetate and [U-13C]glucose were i.p.-injected either together with Pilo (short period) or 15 min before sacrifice (long period). Their conversion to Glu and Gln in the hippocampus and entorhinal cortex was followed using [13C] gas chromatography-mass spectrometry. Release of in vitro loaded Glu surrogate, [3H]d-Asp from ex vivo brain slices was monitored in continuously collected superfusates. [3H]d-Asp uptake was tested in freshly isolated brain slices. At no time point nor brain region did MSO modify incorporation of [13C] to Glu or Gln in Pilo-treated rats. MSO pretreatment decreased by ~37% high potassium-induced [3H]d-Asp release, but did not affect [3H]d-Asp uptake. The results indicate that MSO at a non-convulsive dose delays the initial Pilo-induced seizures by interfering with synaptic Glu-release but not with neurotransmitter Glu recycling.

Endoplasmic Reticulum Stress in Tauopathies: Contrasting Human Brain Pathology with Cellular and Animal Models.

The accumulation and spreading of protein tau in the human brain are major features of neurodegenerative disorders known as tauopathies. In addition to several subcellular abnormalities, tau aggregation within neurons seems capable of triggering endoplasmic reticulum (ER) stress and the consequent unfolded protein response (UPR). In metazoans, full activation of a complex ER-UPR network may restore proteostasis and ER function or, if stress cannot be solved, commit cells to apoptosis. Due to these alternative outcomes (survival or death), the pharmacological manipulation of ER-UPR has become the focus of potential therapies in many human diseases, including tauopathies. Here we update and analyze the experimental data from human brain, cellular, and animal models linking tau accumulation and ER-UPR. We further discuss mechanistic aspects and put the ER-UPR into perspective as a possible therapeutic target in this group of diseases.

Amyloid-ß42 clearance and neuroprotection mediated by X-box binding protein 1 signaling decline with aging in the Drosophila brain.

The unfolded protein response (UPR) may be pathogenically related to Alzheimer's disease. Yet, the effects of chronic amyloid-ß42 (Aß42) accumulation and UPR activation upon neurotoxicity remain unclear. Here, we show that neuronal Aß42 expression in Drosophila activated the inositol-requiring protein-1/X-box binding protein 1 (XBP1) UPR branch before the onset of behavioral impairment and persisted with aging. Early upregulation of hsc3/BiP, a target of XBP1 and activating transcription factor 6 pathways, was also sustained in old animals. Downregulation of XBP1 enhanced neurotoxicity and the accumulation of Aß42 and polyubiquitinated proteins. Consistently, overexpression of spliced XBP1 reduced Aß42 and improved geotaxis in old flies. The activation of protein kinase RNA-like endoplasmic reticulum (ER) kinase contributed to the age-dependent geotaxis deficit. Spliced XBP1 gene targets ER degradation-enhancing mannosidase-like protein 1, ER degradation-enhancing mannosidase-like protein 2, and HRD1 were elevated in 5-day-old Aß42-expressing flies as compared to controls but not in 18-day-old flies. Our results indicate that inositol-requiring protein-1/XBP1 activation is neuroprotective and enhances Aß42 clearance. They also suggest that such response becomes inefficient with aging.

A Novel Genetic Screen Identifies Modifiers of Age-Dependent Amyloid ß Toxicity in the Drosophila Brain.

The accumulation of amyloid ß peptide (Aß) in the brain of Alzheimer's disease (AD) patients begins many years before clinical onset. Such process has been proposed to be pathogenic through the toxicity of Aß soluble oligomers leading to synaptic dysfunction, phospho-tau aggregation and neuronal loss. Yet, a massive accumulation of Aß can be found in approximately 30% of aged individuals with preserved cognitive function. Therefore, within the frame of the "amyloid hypothesis", compensatory mechanisms and/or additional neurotoxic or protective factors need to be considered and investigated. Here we describe a modifier genetic screen in Drosophila designed to identify genes that modulate toxicity of Aß42 in the CNS. The expression of Aß42 led to its accumulation in the brain and a moderate impairment of negative geotaxis at 18 days post-eclosion (d.p.e) as compared with genetic or parental controls. These flies were mated with a collection of lines carrying chromosomal deletions and negative geotaxis was assessed at 5 and 18 d.p.e. Our screen is the first to take into account all of the following features, relevant to sporadic AD: (1) pan-neuronal expression of wild-type Aß42; (2) a quantifiable complex behavior; (3) Aß neurotoxicity associated with progressive accumulation of the peptide; and (4) improvement or worsening of climbing ability only evident in aged animals. One hundred and ninety-nine deficiency (Df) lines accounting for ~6300 genes were analyzed. Six lines, including the deletion of 52 Drosophila genes with human orthologs, significantly modified Aß42 neurotoxicity in 18-day-old flies. So far, we have validated CG11796 and identified CG17249 as a strong candidate (whose human orthologs are HPD and PRCC, respectively) by using RNAi or mutant hemizygous lines. PRCC encodes proline-rich protein PRCC (ppPRCC) of unknown function associated with papillary renal cell carcinoma. HPD encodes 4-hydroxyphenylpyruvate dioxygenase (HPPD), a key enzyme in tyrosine degradation whose Df causes autosomal recessive Tyrosinemia type 3, characterized by mental retardation. Interestingly, lines with a partial Df of HPD ortholog showed increased intraneuronal accumulation of Aß42 that coincided with geotaxis impairment. These previously undetected modifiers of Aß42 neurotoxicity in Drosophila warrant further study to validate their possible role and significance in the pathogenesis of sporadic AD.