Ottavio Arancio

Cyclophilin D deficiency attenuates mitochondrial and neuronal perturbation and ameliorates learning and memory in Alzheimer's disease

Cyclophilin D (CypD, encoded by Ppif) is an integral part of the mitochondrial permeability transition pore, whose opening leads to cell death. Here we show that interaction of CypD with mitochondrial amyloid-β protein (Aβ) potentiates mitochondrial, neuronal and synaptic stress. The CypD-deficient cortical mitochondria are resistant to Aβ- and Ca2+-induced mitochondrial swelling and permeability transition. Additionally, they have an increased calcium buffering capacity and generate fewer mitochondrial reactive oxygen species. Furthermore, the absence of CypD protects neurons from Aβ- and oxidative stress–induced cell death. Notably, CypD deficiency substantially improves learning and memory and synaptic function in an Alzheimers disease mouse model and alleviates Aβ-mediated reduction of long-term potentiation. Thus, the CypD-mediated mitochondrial permeability transition pore is directly linked to the cellular and synaptic perturbations observed in the pathogenesis of Alzheimers disease. Blockade of CypD may be a therapeutic strategy in Alzheimers disease.

Picomolar amyloid-beta positively modulates synaptic plasticity and memory in hippocampus.

Amyloid-β (Aβ) peptides are produced in high amounts during Alzheimers disease, causing synaptic and memory dysfunction. However, they are also released in lower amounts in normal brains throughout life during synaptic activity. Here we show that low picomolar concentrations of a preparation containing both Aβ42 monomers and oligomers cause a marked increase of hippocampal long-term potentiation, whereas high nanomolar concentrations lead to the well established reduction of potentiation. Picomolar levels of Aβ42 also produce a pronounced enhancement of both reference and contextual fear memory. The mechanism of action of picomolar Aβ42 on both synaptic plasticity and memory involves α7-containing nicotinic acetylcholine receptors. These findings strongly support a model for Aβ effects in which low concentrations play a novel positive, modulatory role on neurotransmission and memory, whereas high concentrations play the well known detrimental effect culminating in dementia.

Learning and Memory and Synaptic Plasticity Are Impaired in a Mouse Model of Rett Syndrome

Loss-of-function mutations or abnormal expression of the X-linked gene encoding methyl CpG binding protein 2 (MeCP2) cause a spectrum of postnatal neurodevelopmental disorders including Rett syndrome (RTT), nonsyndromic mental retardation, learning disability, and autism. Mice expressing a truncated allele of Mecp2 (Mecp2308) reproduce the motor and social behavior abnormalities of RTT; however, it is not known whether learning deficits are present in these animals. We investigated learning and memory, neuronal morphology, and synaptic function in Mecp2308 mice. Hippocampus-dependent spatial memory, contextual fear memory, and social memory were significantly impaired in Mecp2308 mutant males (Mecp2308/Y). The morphology of dendritic arborizations, the biochemical composition of synaptosomes and postsynaptic densities, and brain-derived neurotrophic factor expression were not altered in these mice. However, reduced postsynaptic density cross-sectional length was identified in asymmetric synapses of area CA1 of the hippocampus. In the hippocampus of symptomatic Mecp2308/Y mice, Schaffer-collateral synapses exhibited enhanced basal synaptic transmission and decreased paired-pulse facilitation, suggesting that neurotransmitter release was enhanced. Schaffer-collateral long-term potentiation (LTP) was impaired. LTP was also reduced in the motor and sensory regions of the neocortex. Finally, very early symptomatic Mecp2308/Y mice had increased basal synaptic transmission and deficits in the induction of long-term depression. These data demonstrate a requirement for MeCP2 in learning and memory and suggest that functional and ultrastructural synaptic dysfunction is an early event in the pathogenesis of RTT.

Amyloid β-peptide inhibition of the PKA/CREB pathway and long-term potentiation: Reversibility by drugs that enhance cAMP signaling

Changes in hippocampal function seem critical for cognitive impairment in Alzheimers disease (AD). Although there is eventual loss of synapses in both AD and animal models of AD, deficits in spatial memory and inhibition of long-term potentiation (LTP) precede morphological alterations in the models, suggesting earlier biochemical changes in the disease. In the studies reported here we demonstrate that amyloid β-peptide (Aβ) treatment of cultured hippocampal neurons leads to the inactivation of protein kinase A (PKA) and persistence of its regulatory subunit PKAIIα. Consistent with this, CREB phosphorylation in response to glutamate is decreased, and the decrease is reversed by rolipram, a phosphodiesterase inhibitor that raises cAMP and leads to the dissociation of the PKA catalytic and regulatory subunits. It is likely that a similar mechanism underlies Αβ inhibition of LTP, because rolipram and forskolin, agents that enhance the cAMP-signaling pathway, can reverse this inhibition. This reversal is blocked by H89, an inhibitor of PKA. These observations suggest that Αβ acts directly on the pathways involved in the formation of late LTP and agents that enhance the cAMP/PKA/CREB-signaling pathway have potential for the treatment of AD.

Persistent improvement in synaptic and cognitive functions in an Alzheimer mouse model after rolipram treatment

Evidence suggests that Alzheimer disease (AD) begins as a disorder of synaptic function, caused in part by increased levels of amyloid beta-peptide 1-42 (Abeta42). Both synaptic and cognitive deficits are reproduced in mice double transgenic for amyloid precursor protein (AA substitution K670N,M671L) and presenilin-1 (AA substitution M146V). Here we demonstrate that brief treatment with the phosphodiesterase 4 inhibitor rolipram ameliorates deficits in both long-term potentiation (LTP) and contextual learning in the double-transgenic mice. Most importantly, this beneficial effect can be extended beyond the duration of the administration. One course of long-term systemic treatment with rolipram improves LTP and basal synaptic transmission as well as working, reference, and associative memory deficits for at least 2 months after the end of the treatment. This protective effect is possibly due to stabilization of synaptic circuitry via alterations in gene expression by activation of the cAMP-dependent protein kinase (PKA)/cAMP regulatory element-binding protein (CREB) signaling pathway that make the synapses more resistant to the insult inflicted by Abeta. Thus, agents that enhance the cAMP/PKA/CREB pathway have potential for the treatment of AD and other diseases associated with elevated Abeta42 levels.

Ubiquitin Hydrolase Uch-L1 Rescues β-Amyloid-Induced Decreases in Synaptic Function and Contextual Memory

The neuronal ubiquitin/proteasomal pathway has been implicated in the pathogenesis of Alzheimers disease (AD). We now show that a component of the pathway, ubiquitin C-terminal hydrolase L1 (Uch-L1), is required for normal synaptic and cognitive function. Transduction of Uch-L1 protein fused to the transduction domain of HIV-transactivator protein (TAT) restores normal enzymatic activity and synaptic function both in hippocampal slices treated with oligomeric Abeta and in the APP/PS1 mouse model of AD. Moreover, intraperitoneal injections with the fusion protein improve the retention of contextual learning in APP/PS1 mice over time. The beneficial effect of the Uch-L1 fusion protein is associated with restoration of normal levels of the PKA-regulatory subunit IIalpha, PKA activity, and CREB phosphorylation.

Nitric Oxide Acts Directly in the Presynaptic Neuron to Produce Long-Term Potentiationin Cultured Hippocampal Neurons

Nitric oxide (NO) has been proposed to act as a retrograde messenger during long-term potentiation (LTP) in the CA1 region of hippocampus, but the inaccessibility of the presynaptic terminal has prevented a definitive test of this hypothesis. Because both sides of the synapse are accessible in cultured hippocampal neurons, we have used this preparation to investigate the role of NO. We examined LTP following intra- or extracellular application of an NO scavenger, an inhibitor of NO synthase, and a membrane-impermeant NO donor that releases NO only upon photolysis with UV light. Our results indicate that NO is produced in the postsynaptic neuron, travels through the extracellular space, and acts directly in the presynaptic neuron to produce long-term potentiation, supporting the hypothesis that NO acts as a retrograde messenger during LTP.

Loss of mTOR-Dependent Macroautophagy Causes Autistic-like Synaptic Pruning Deficits

Developmental alterations of excitatory synapses are implicated in autism spectrum disorders (ASDs). Here, we report increased dendritic spine density with reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. These spine deficits correlate with hyperactivated mTOR and impaired autophagy. In Tsc2 ± ASD mice where mTOR is constitutively overactive, we observed postnatal spine pruning defects, blockade of autophagy, and ASD-like social behaviors. The mTOR inhibitor rapamycin corrected ASD-like behaviors and spine pruning defects in Tsc2 ± mice, but not in Atg7(CKO) neuronal autophagy-deficient mice or Tsc2 ± :Atg7(CKO) double mutants. Neuronal autophagy furthermore enabled spine elimination with no effects on spine formation. Our findings suggest that mTOR-regulated autophagy is required for developmental spine pruning, and activation of neuronal autophagy corrects synaptic pathology and social behavior deficits in ASD models with hyperactivated mTOR.Developmental alterations of excitatory synapses are implicated in autism spectrum disorders (ASDs). Here, we report increased dendritic spine density with reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. These spine deficits correlate with hyperactivated mTOR and impaired autophagy. In Tsc2 ± ASD mice where mTOR is constitutively overactive, we observed postnatal spine pruning defects, blockade of autophagy, and ASD-like social behaviors. The mTOR inhibitor rapamycin corrected ASD-like behaviors and spine pruning defects in Tsc2 ± mice, but not in Atg7(CKO) neuronal autophagy-deficient mice or Tsc2 ± :Atg7(CKO) double mutants. Neuronal autophagy furthermore enabled spine elimination with no effects on spine formation. Our findings suggest that mTOR-regulated autophagy is required for developmental spine pruning, and activation of neuronal autophagy corrects synaptic pathology and social behavior deficits in ASD models with hyperactivated mTOR.

Chapter 11 Nitric oxide as a retrograde messenger during long-term potentiation in hippocampus

Nitric oxide (NO) is widespread in the nervous system and is thought to play a role in a variety of different neuronal functions, including learning and memory (see other chapters, this volume). A number of behavioral studies have indicated that NO is involved in several types of learning such as motor learning (Yanagihara and Kondo, 1996), avoidance learning (Barati and Kopf, 1996; Myslivecek et al., 1996), olfactory learning (Okere et. al., 1996; Kendrick et al., 1997), and spatial learning (Holscher et al., 1995; Yamada et al., 1996) (for review of earlier papers see Hawkins, 1996). Moreover, NO is thought to be involved in neuronal plasticity contributing to these different types of learning in different brain areas including the cerebellum (chapter by R. Tsien, this volume) and hippocampus. In this chapter we review evidence on the role of NO in long-term potentiation (LTP), a type of synaptic plasticity in hippocampus that is believed to contribute to declarative forms of learning such as spatial learning.

RAGE potentiates Aβ‐induced perturbation of neuronal function in transgenic mice

Receptor for Advanced Glycation Endproducts (RAGE), a multiligand receptor in the immunoglobulin superfamily, functions as a signal‐transducing cell surface acceptor for amyloid‐beta peptide (Aβ). In view of increased neuronal expression of RAGE in Alzheimers disease, a murine model was developed to assess the impact of RAGE in an Aβ‐rich environment, employing transgenics (Tgs) with targeted neuronal overexpression of RAGE and mutant amyloid precursor protein (APP). Double Tgs (mutant APP (mAPP)/RAGE) displayed early abnormalities in spatial learning/memory, accompanied by altered activation of markers of synaptic plasticity and exaggerated neuropathologic findings, before such changes were found in mAPP mice. In contrast, Tg mice bearing a dominant‐negative RAGE construct targeted to neurons crossed with mAPP animals displayed preservation of spatial learning/memory and diminished neuropathologic changes. These data indicate that RAGE is a cofactor for Aβ‐induced neuronal perturbation in a model of Alzheimers‐type pathology, and suggest its potential as a therapeutic target to ameliorate cellular dysfunction.