Annalisa Nobili

Dopamine neuronal loss contributes to memory and reward dysfunction in a model of Alzheimer’s disease

Alterations of the dopaminergic (DAergic) system are frequently reported in Alzheimers disease (AD) patients and are commonly linked to cognitive and non-cognitive symptoms. However, the cause of DAergic system dysfunction in AD remains to be elucidated. We investigated alterations of the midbrain DAergic system in the Tg2576 mouse model of AD, overexpressing a mutated human amyloid precursor protein (APPswe). Here, we found an age-dependent DAergic neuron loss in the ventral tegmental area (VTA) at pre-plaque stages, although substantia nigra pars compacta (SNpc) DAergic neurons were intact. The selective VTA DAergic neuron degeneration results in lower DA outflow in the hippocampus and nucleus accumbens (NAc) shell. The progression of DAergic cell death correlates with impairments in CA1 synaptic plasticity, memory performance and food reward processing. We conclude that in this mouse model of AD, degeneration of VTA DAergic neurons at pre-plaque stages contributes to memory deficits and dysfunction of reward processing.

Calcineurin inhibition rescues early synaptic plasticity deficits in a mouse model of Alzheimer's disease.

Functional and ultrastructural investigations support the concept that altered brain connectivity, exhausted neural plasticity, and synaptic loss are the strongest correlates of cognitive decline in age-related neurodegenerative dementia of Alzheimer’s type. We have previously demonstrated that in transgenic mice, expressing amyloid-β precursor protein-Swedish mutation active caspase-3 accumulates in hippocampal postsynaptic compartments leading to altered postsynaptic density (PSD) composition, increased long-term depression (LTD), and dendritic spine loss. Furthermore, we found strong evidence that dendritic spine alteration is mediated by calcineurin activation, a calcium-dependent phosphatase involved in synapse signaling. In the present work, we analyzed the molecular mechanism linking alteration of synaptic plasticity to the increase of calcineurin activity. We found that acute treatment of young and plaque-free transgenic mice with the calcineurin inhibitor FK506 leads to a complete rescue of LTD and PSD composition. Our findings are in agreement with other results reporting that calcineurin inhibition improves memory function and restores dendritic spine density, confirming that calcineurin inhibition may be explored as a neuroprotective treatment to stop or slowdown synaptic alterations in Alzheimer’s disease.

Presynaptic c-Jun N-terminal Kinase 2 regulates NMDA receptor-dependent glutamate release

Activation of c-Jun N-terminal kinase (JNK) signaling pathway is a critical step for neuronal death occurring in several neurological conditions. JNKs can be activated via receptor tyrosine kinases, cytokine receptors, G-protein coupled receptors and ligand-gated ion channels, including the NMDA glutamate receptors. While JNK has been generally associated with postsynaptic NMDA receptors, its presynaptic role remains largely unexplored. Here, by means of biochemical, morphological and functional approaches, we demonstrate that JNK and its scaffold protein JIP1 are also expressed at the presynaptic level and that the NMDA-evoked glutamate release is controlled by presynaptic JNK-JIP1 interaction. Moreover, using knockout mice for single JNK isoforms, we proved that JNK2 is the essential isoform in mediating this presynaptic event. Overall the present findings unveil a novel JNK2 localization and function, which is likely to play a role in different physiological and pathological conditions.

Neuroprotective effects of donepezil against cholinergic depletion

IntroductionIntraparenchymal injections of the immunotoxin 192-IgG-saporin into medial septum and nucleus basalis magnocellularis causes a selective depletion of basal forebrain cholinergic neurons. Thus, it represents a valid model to mimic a key component of the cognitive deficits associated with aging and dementia. Here we administered donepezil, a potent acetylcholinesterase inhibitor developed for treating Alzheimer’s disease, 15 days before 192-IgG-saporin injection, and thus we examined donepezil effects on neurodegeneration and cognitive deficits.MethodsCaspase-3 activity and cognitive performances of lesioned rats pre-treated with donepezil or saline were analyzed and compared to the outcomes obtained in pre-treated sham-lesioned rats.ResultsCholinergic depletion increased hippocampal and neocortical caspase-3 activity and impaired working memory, spatial discrimination, social novelty preference, and ultrasonic vocalizations, without affecting anxiety levels and fear conditioning. In lesioned animals, donepezil pre-treatment reduced hippocampal and neocortical caspase-3 activity and improved working memory and spatial discrimination performances and partially rescued ultrasonic vocalizations, without preventing social novelty alterations.ConclusionsPresent data indicate that donepezil pre-treatment exerts beneficial effects on behavioral deficits induced by cholinergic depletion, attenuating the concomitant hippocampal and neocortical neurodegeneration.

Neuregulin 1 signalling modulates mGluR1 function in mesencephalic dopaminergic neurons.

Neuregulin 1 (NRG1) is a trophic factor that has an essential role in the nervous system by modulating neurodevelopment, neurotransmission and synaptic plasticity. Despite the evidence that NRG1 and its receptors, ErbB tyrosine kinases, are expressed in mesencephalic dopaminergic nuclei and their functional alterations are reported in schizophrenia and Parkinson’s disease, the role of NRG1/ErbB signalling in dopaminergic neurons remains unclear. Here we found that NRG1 selectively increases the metabotropic glutamate receptor 1 (mGluR1)-activated currents by inducing synthesis and trafficking to membrane of functional receptors and stimulates phosphatidylinositol 3-kinase-Akt-mammalian target of rapamycin (PI3K-Akt-mTOR) pathway, which is required for mGluR1 function. Notably, an endogenous NRG1/ErbB tone is necessary to maintain mGluR1 function, by preserving its surface membrane expression in dopaminergic neurons. Consequently, it enables striatal mGluR1-induced dopamine outflow in in vivo conditions. Our results identify a novel role of NRG1 in the dopaminergic neurons, whose functional alteration might contribute to devastating diseases, such as schizophrenia and Parkinson’s disease.

SAM68 is a physiological regulator of SMN2 splicing in spinal muscular atrophy

Knockout of the splicing factor SAM68 promotes SMN2 splicing, improving neuromuscular defects and viability in SMA mice.

On the properties of identified dopaminergic neurons in the mouse substantia nigra and ventral tegmental area.

We studied the properties of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and ventral tegmental area (VTA) in mice expressing the enhanced green fluorescent protein (eGFP) under the control of the tyrosine hydroxylase promoter (TH‐GFP). By using a practical map of cell positioning in distinct SNpc and VTA subregions in horizontal midbrain slices we saw that the spontaneous firing, membrane properties, cell body size and magnitude of the hyperpolarization‐activated current (Ih) in TH‐GFP‐positive neurons (TH‐GFP+) vary significantly among subregions, following a mediolateral gradient. Block of Ih with Zd7288 inhibited firing in the most lateral subregions, but had little effect in the intermediate/medial VTA. In addition, TH‐GFP+ cells were excited by Met5‐Enkephalin. Extracellular recordings from a large neuron number showed that all TH‐GFP+ cells were inhibited by dopamine, suggesting that this is a reliable approach for identifying dopaminergic neurons in vitro. Simultaneous recordings from dopamine‐sensitive and dopamine‐insensitive neurons showed that dopamine‐insensitive cells (putative non‐dopaminergic neurons) are unaffected by Zd7288 but inhibited by Met5‐Enkephalin. Under patch‐clamp, dopamine generated a quantitatively similar outward current in most TH‐GFP+ neurons, although medial VTA cells showed reduced dopamine sensitivity. Pargyline prolonged the dopamine current, whereas cocaine enhanced dopamine‐mediated responses in both the SNpc and the VTA. Our work provides new insights into the variability in mouse midbrain dopaminergic neurons along the medial–lateral axis and points to the necessity of a combination of different electrophysiological and pharmacological approaches for reliably identifying these cells to distinguish them from non‐dopaminergic neurons in the midbrain.

Persistent elevation of D-Aspartate enhances NMDA receptor-mediated responses in mouse substantia nigra pars compacta dopamine neurons

Dopamine neurons in the substantia nigra pars compacta regulate not only motor but also cognitive functions. NMDA receptors play a crucial role in modulating the activity of these cells. Considering that the amino-acid D-Aspartate has been recently shown to be an endogenous NMDA receptor agonist, the aim of the present study was to examine the effects of D-Aspartate on the functional properties of nigral dopamine neurons. We compared the electrophysiological actions of D-Aspartate in control and D-aspartate oxidase gene (Ddo(-/-)) knock-out mice that show a concomitant increase in brain D-Aspartate levels, improved synaptic plasticity and cognition. Finally, we analyzed the effects of L-Aspartate, a known dopamine neuron endogenous agonist in control and Ddo(-/-) mice. We show that D- and L-Aspartate excite dopamine neurons by activating NMDA, AMPA and metabotropic glutamate receptors. Ddo deletion did not alter the intrinsic properties or dopamine sensitivity of dopamine neurons. However, NMDA-induced currents were enhanced and membrane levels of the NMDA receptor GluN1 and GluN2A subunits were increased. Inhibition of excitatory amino-acid transporters caused a marked potentiation of D-Aspartate, but not L-Aspartate currents, in Ddo(-/-) neurons. This is the first study to show the actions of D-Aspartate on midbrain dopamine neurons, activating not only NMDA but also non-NMDA receptors. Our data suggest that dopamine neurons, under conditions of high D-Aspartate levels, build a protective uptake mechanism to compensate for increased NMDA receptor numbers and cell hyper-excitation, which could prevent the consequent hyper-dopaminergia in target zones that can lead to neuronal degeneration, motor and cognitive alterations.

Functional alterations of the dopaminergic and glutamatergic systems in spontaneous α-synuclein overexpressing rats

&NA; The presence of &agr;‐synuclein (&agr;‐syn) in Lewy bodies and Lewy neurites is an important characteristic of the neurodegenerative processes of substantia nigra pars compacta (SNpc) dopaminergic (DAergic) neurons in Parkinsons disease (PD) and other synucleinopathies. Here we report that Berlin‐Druckrey rats carrying a spontaneous mutation in the 3′ untranslated region of &agr;‐syn mRNA (m/m rats) display a marked accumulation of &agr;‐syn in the mesencephalic area, striatum and frontal cortex, accompanied to severe dysfunctions in the dorsolateral striatum. Despite a small reduction in the number of SNpc and ventral tegmental area DAergic cells, the surviving dopaminergic neurons of the m/m rats do not show clear‐cut alterations of the spontaneous and evoked firing activity, DA responses and somatic amphetamine‐induced firing inhibition. Interestingly, mutant DAergic neurons display diminished whole‐cell Ih conductance and a reduced frequency of spontaneous excitatory synaptic currents. By contrast, m/m rats show a severe impairment of DA and glutamate release in the dorsolateral striatum, as revealed by amperometric measure of DA currents and by electrophysiological recordings of glutamatergic synaptic events in striatal medium spiny neurons. These functional impairments are paralleled by a decreased expression of the DA transporter and VGluT1 proteins in the same area. Thus, together with &agr;‐syn overload in the mesencephalic region, striatum and frontal cortex, the main functional alterations occur in the DAergic and glutamatergic terminals in the dorsal striatum of the m/m rats. Highlights&agr;‐syn accumulates in the mesencephalon, striatum and frontal cortex of m/m rats.SNpc DAergic neurons from m/m rats show normal functional activity.The number of SNpc DAergic neurons from m/m rats is 9% reduced.Dopamine and glutamate release in the striatum from m/m rats is impaired.Loss of striatal DAT‐ and VGluT1‐expressing terminals occurs in m/m rats.

Neuregulin 1/ErbB signalling modulates hippocampal mGluRI-dependent LTD and object recognition memory

&NA; The neurotrophic factors neuregulins (NRGs) and their receptors, ErbB tyrosine kinases, regulate neurotransmission, synaptic plasticity and cognitive functions and their alterations have been associated to different neuropsychiatric disorders. Group 1 metabotropic glutamate receptors (mGluRI)‐dependent mechanisms are also altered in animal models of neuropsychiatric diseases, especially mGluRI‐induced glutamatergic long‐term depression (mGluRI‐LTD), a form of synaptic plasticity critically involved in learning and memory. Despite this evidence, a potential link between NRGs/ErbB signalling and mGluRI‐LTD has never been considered. Here, we aimed to test the hypothesis that NRGs/ErbB signalling regulates mGluRI functions in the hippocampus, thus controlling CA1 pyramidal neurons excitability and synaptic plasticity as well as mGluRI‐dependent behaviors. We investigated the functional interaction between NRG1/ErbB signalling and mGluRI in hippocampal CA1 pyramidal neurons, by analyzing the effect of a pharmacological modulation of NRG1/ErbB signalling on the excitation of pyramidal neurons and on the LTD at CA3‐CA1 synapses induced by an mGluRI agonist. Furthermore, we verified the involvement of ErbB signalling in mGluRI‐dependent learning processes, by evaluating the consequence of an intrahippocampal in vivo injection of a pan‐ErbB inhibitor in the object recognition test in mice, a learning task dependent on hippocampal mGluRI. We found that NRG1 potentiates mGluRI‐dependent functions on pyramidal neurons excitability and synaptic plasticity at CA3‐CA1 synapses. Further, endogenous ErbB signalling per se regulates, through mGluRI, neuronal excitability and LTD in CA1 pyramidal neurons, since ErbB inhibition reduces mGluRI‐induced neuronal excitation and mGluRI‐LTD. In vivo intrahippocampal injection of the ErbB inhibitor, PD158780, impairs mGluRI‐LTD at CA3‐CA1 synapses and affects the exploratory behavior in the object recognition test. Thus, our results identify a key role for NRG1/ErbB signalling in the regulation of hippocampal mGluRI‐dependent synaptic and cognitive functions, whose alteration might contribute to the pathogenesis of different brain diseases. Graphical abstract Figure. No caption available.