Modulation of autophagy by RTN-1C: role in autophagosome biogenesis.

The endoplasmic reticulum (ER) is a key organelle fundamental for the maintenance of cellular homeostasis and to determine the cell's fate under stress conditions. Among the known proteins that regulate ER structure and function there is Reticulon-1C (RTN-1C), a member of the reticulon family localized primarily on the ER membrane. We previously demonstrated that RTN-1C expression affects ER function and stress condition. ER is an essential site for the regulation of apoptotic pathways and it has also been recently recognized as an important component of autophagic signaling. Based on these evidences, we have investigated the impact of RTN-1C modulation on autophagy induction. Interestingly we found that reticulon overexpression is able to activate autophagic machinery and its silencing results in a significative inhibition of both basal and induced autophagic response. Using different experimental approaches we demonstrated that RTN-1C colocalizes with ATG16L and LC3II on the autophagosomes. Considering the key role of reticulon proteins in the control of ER membrane shaping and homeostasis, our data suggest the participation of RTN-1C in the autophagic vesicle biogenesis at the level of the ER compartment. Our data indicate a new mechanism by which this structural ER protein modulates cellular stress, that is at the basis of different autophagy-related pathologies.

Reticulon protein-1C is a key component of MAMs.

The endoplasmic reticulum (ER) is a key organelle fundamental for the maintenance of cellular homeostasis and the determination of cell fate under stress conditions. Reticulon-1C (RTN-1C) is a member of the reticulon family proteins localized primarily on the ER membrane and known to regulate ER structure and function. Several cellular processes depend on the structural and functional crosstalk between different organelles, particularly on the endoplasmic reticulum and mitochondria. These dynamic contacts, called mitochondria-associated ER membranes (MAMs), are essential for the maintenance of mitochondrial structure and participate in lipid and calcium exchanges between the two organelles. In this study we investigated the impact of RTN-1C modulation on mitochondrial dynamics. We demonstrate that RTN-1C controls mitochondrial structure and function affecting intracellular Ca2+ homeostasis and lipid exchange between ER and mitochondria. We propose that these events depend on RTN-1C involvement in the regulation of ER-mitochondria cross-talk and define a role for RTN-1C in maintaining the function of contacts between the two organelles.

Insulin receptor ß-subunit haploinsufficiency impairs hippocampal late-phase LTP and recognition memory.

The insulin receptor (IR) is a protein tyrosine kinase playing a pivotal role in the regulation of peripheral glucose metabolism and energy homoeostasis. IRs are also abundantly distributed in the cerebral cortex and hippocampus, where they regulate synaptic activity required for learning and memory. As the major anabolic hormone in mammals, insulin stimulates protein synthesis partially through the activation of the PI3K/Akt/mTOR pathway, playing fundamental roles in neuronal development, synaptic plasticity and memory. Here, by means of a multidisciplinary approach, we report that long-term synaptic plasticity and recognition memory are impaired in IR ß-subunit heterozygous mice. Since IR expression is diminished in type-2 diabetes as well as in Alzheimer's disease (AD) patients, these data may provide a mechanistic link between insulin resistance, impaired synaptic transmission and cognitive decline in humans with metabolic disorders.

Reversal of metabolic and neurological symptoms of phenylketonuric mice treated with a PAH containing helper-dependent adenoviral vector.

Phenylketonuria (PKU) is one of the most common inborn errors of metabolism and is due to a deficit of phenylalanine hydroxylase, the enzyme that converts phenylalanine (Phe) into tyrosine (Tyr). The resultant hyperphenylalaninemia (HPA) leads to severe neurological impairment, whose pathogenesis has not been entirely elucidated. Treatment of PKU consists essentially in lifelong protein restriction and, in mild cases, in tetrahydrobiopterin supplementation. However, compliance to both strategies, particularly to the long-term diet, is low and therefore other therapies are desirable. We explored a gene therapy approach aimed at long-term correction of the pathologic phenotype of BTBR-PahEnu2 mice, a mouse model of PKU. To this aim, we developed a helper-dependent adenoviral (HD-Ad) vector expressing phenylalanine hydroxylase and administered it to 3-week-old PKU mice. This resulted in complete normalization of Phe and Tyr levels and reversal of coat hypopigmentation that lasted throughout the observation period of six months. The spatial learning deficits observed in PKU mice were also reversed and hippocampus levels of the N-methyl-D-Aspartate and 2-amino-3-(5-methyl-3-oxo-1,2- oxazol-4-yl) propanoic acid receptor subunits returned to normal. Long-term potentiation, which is impaired in PKU mice, was also restored by treatment. Therefore, HD-Ad vector-mediated gene therapy is a promising approach to PKU treatment.

Na+ -Ca2+ exchanger (NCX3) knock-out mice display an impairment in hippocampal long-term potentiation and spatial learning and memory.

Long-term potentiation (LTP) depends on the coordinated regulation of an ensemble of proteins related to Ca(2+) homeostasis, including Ca(2+) transporters. One of the major players in the regulation of intracellular Ca(2+) ([Ca(2+)](i)) homeostasis in neurons is the sodium/calcium exchanger (NCX), which represents the principal mechanism of Ca(2+) clearance in the synaptic sites of hippocampal neurons. Because NCX3, one of the three brain isoforms of the NCX family, is highly expressed in the hippocampal subfields involved in LTP, we hypothesized that it might represent a potential candidate for LTP modulation. To test this hypothesis, we first examined the effect of ncx3 gene ablation on NCX currents (I(NCX)) and Ca(2+) homeostasis in hippocampal neurons. ncx3(-/-) neurons displayed a reduced I(NCX), a higher basal level of [Ca(2+)](i), and a significantly delayed clearance of [Ca(2+)](i) following depolarization. Furthermore, measurement of field EPSPs, recorded from the CA1 area, revealed that ncx3(-/-) mice had an impaired basal synaptic transmission. Moreover, hippocampal slices from ncx3(-/-) mice exhibited a worsening in LTP compared with congenic ncx3(+/+). Consistently, immunohistochemical and immunoblot analysis indicated that in the hippocampus of ncx3(-/-) mice both Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) expression and the phosphoCaMKIIα/CaMKIIα ratio were significantly reduced compared with ncx3(+/+). Interestingly, ncx3(-/-) mice displayed a reduced spatial learning and memory performance, as revealed by the novel object recognition, Barnes maze, and context-dependent fear conditioning assays. Collectively, our findings demonstrate that the deletion of the ncx3 gene in mice has detrimental consequences on basal synaptic transmission, LTP regulation, spatial learning, and memory performance.

The γ-secretase modulator CHF5074 restores memory and hippocampal synaptic plasticity in plaque-free Tg2576 mice.

Abnormal amyloid-ß (Aß) production and deposition is believed to represent one of the main causes of Alzheimer's disease (AD). γ-Secretase is the enzymatic complex responsible for Aß generation from its precursor protein. Inhibition or modulation of γ-secretase represents an attractive therapeutic approach. CHF5074 is a new γ-secretase modulator that has been shown to inhibit brain plaque deposition and to attenuate memory deficit in adult AD transgenic mice after chronic treatment. To date, it is not known whether the positive behavioral effects of this compound also occur in young transgenic mice without plaque deposition. Here, we evaluated the effects of acute and subchronic treatment with CHF5074 on contextual and recognition memory and on hippocampal synaptic plasticity in plaque-free Tg2576 mice. We found that at 5 months of age, contextual memory impairment was significantly attenuated after acute subcutaneous administration of 30 mg/kg CHF5074. At 6 months of age, recognition memory impairment was fully reversed after a 4-week oral treatment in the diet (≈60 mg/kg/day). These cognitive effects were associated with a reversal of long-term potentiation (LTP) impairment in the hippocampus. A significant reduction in brain intraneuronal AßPP/Aß levels and hyperphosphorylated tau, but no change in soluble or oligomeric Aß levels was detected in Tg2576 mice showing functional recovery following CHF5074 treatment. We conclude that the beneficial effects of CHF5074 treatment in young transgenic mice occurred at a stage that precedes plaque formation and were associated with a reduction in intraneuronal AßPP/Aß and hyperphosphorylated tau.

Characterization of gene expression induced by RTN-1C in human neuroblastoma cells and in mouse brain.

The endoplasmic reticulum (ER) stress-mediated pathway is involved in a wide range of human neurodegenerative disorders. Hence, molecules that regulate the ER stress response represent potential candidates as drug targets to tackle these diseases. In previous studies we demonstrated that upon acetylation the reticulon-1C (RTN-1C) variant of the reticulon family leads to inhibition of histone deacetylase (HDAC) enzymatic activity and endoplasmic reticulum stress-dependent apoptosis. Here, by microarray analysis of the whole human genome we found that RTN-1C is able to specifically regulate gene expression, modulating transcript clusters which have been implicated in the onset of neurodegenerative disorders. Interestingly, we show that some of the identified genes were also modulated in vivo in a brain-specific mouse model overexpressing RTN-1C. These data provide a basis for further investigation of RTN-1C as a potential molecular target for use in therapy and as a specific marker for neurological diseases.