Alzheimer's disease (AD) is characterized by progressive cognitive decline, increasingly attributed to neuronal dysfunction induced by amyloid-␤ oligomers (A␤Os). Although the impact of A␤Os on neurons has been extensively studied, only recently have the possible effects of A␤Os on astrocytes begun to be investigated. Given the key roles of astrocytes in synapse formation, plasticity, and function, we sought to investigate the impact of A␤Os on astrocytes, and to determine whether this impact is related to the deleterious actions of A␤Os on synapses. We found that A␤Os interact with astrocytes, cause astrocyte activation and trigger abnormal generation of reactive oxygen species, which is accompanied by impairment of astrocyte neuroprotective potential in vitro. We further show that both murine and human astrocyte conditioned media (CM) increase synapse density, reduce A␤Os binding, and prevent A␤O-induced synapse loss in cultured hippocampal neurons. Both a neutralizing anti-transforming growth factor-␤1 (TGF-␤1) antibody and siRNA-mediated knockdown of TGF-␤1, previously identified as an important synaptogenic factor secreted by astrocytes, abrogated the protective action of astrocyte CM against A␤O-induced synapse loss. Notably, TGF-␤1 prevented hippocampal dendritic spine loss and memory impairment in mice that received an intracerebroventricular infusion of A␤Os. Results suggest that astrocyte-derived TGF-␤1 is part of an endogenous mechanism that protects synapses against A␤Os. By demonstrating that A␤Os decrease astrocyte ability to protect synapses, our results unravel a new mechanism underlying the synaptotoxic action of A␤Os in AD.

The balance between excitatory and inhibitory synaptic inputs is critical for the control of brain function. Astrocytes play important role in the development and maintenance of neuronal circuitry. Whereas astrocytes-derived molecules involved in excitatory synapses are recognized, molecules and molecular mechanisms underlying astrocyte-induced inhibitory synapses remain unknown. Here, we identified transforming growth factor beta 1 (TGF-β1), derived from human and murine astrocytes, as regulator of inhibitory synapse in vitro and in vivo. Conditioned media derived from human and murine astrocytes induce inhibitory synapse formation in cerebral cortex neurons, an event inhibited by pharmacologic and genetic manipulation of the TGF-β pathway. TGF-β1-induction of inhibitory synapse depends on glutamatergic activity and activation of CaM kinase II, which thus induces localization and cluster formation of the synaptic adhesion protein, Neuroligin 2, in inhibitory postsynaptic terminals. Additionally, intraventricular injection of TGF-β1 enhanced inhibitory synapse number in the cerebral cortex. Our results identify TGF-β1/CaMKII pathway as a novel molecular mechanism underlying astrocyte control of inhibitory synapse formation. We propose here that the balance between excitatory and inhibitory inputs might be provided by astrocyte signals, at least partly achieved via TGF-β1 downstream pathways. Our work contributes to the understanding of the GABAergic synapse formation and may be of relevance to further the current knowledge on the mechanisms underlying the development of various neurological disorders, which commonly involve impairment of inhibitory synapse transmission.

Brain accumulation of the amyloid-β peptide (Aβ) and oxidative stress underlie neuronal dysfunction and memory loss in Alzheimer's disease (AD). Hexokinase (HK), a key glycolytic enzyme, plays important pro-survival roles, reducing mitochondrial reactive oxygen species (ROS) generation and preventing apoptosis in neurons and other cell types. Brain isozyme HKI is mainly associated with mitochondria and HK release from mitochondria causes a significant decrease in enzyme activity and triggers oxidative damage. We here investigated the relationship between Aβ-induced oxidative stress and HK activity. We found that Aβ triggered HKI detachment from mitochondria decreasing HKI activity in cortical neurons. Aβ oligomers further impair energy metabolism by decreasing neuronal ATP levels. Aβ-induced HKI cellular redistribution was accompanied by excessive ROS generation and neuronal death. 2-deoxyglucose blocked Aβ-induced oxidative stress and neuronal death. Results suggest that Aβ-induced cellular redistribution and inactivation of neuronal HKI play important roles in oxidative stress and neurodegeneration in AD.

AMP-activated kinase (AMPK) is a key player in energy sensing and metabolic reprogramming under cellular energy restriction. Several studies have linked impaired AMPK function to peripheral metabolic diseases such as diabetes. However, the impact of neurological disorders, such as Alzheimer disease (AD), on AMPK function and downstream effects of altered AMPK activity on neuronal metabolism have been investigated only recently. Here, we report the impact of Aβ oligomers (AβOs), synaptotoxins that accumulate in AD brains, on neuronal AMPK activity. Short-term exposure of cultured rat hippocampal neurons or human cortical slices to AβOs transiently decreased intracellular ATP levels and AMPK activity, as evaluated by its phosphorylation at threonine residue 172 (AMPK-Thr(P)). The AβO-dependent reduction in AMPK-Thr(P) levels was mediated by glutamate receptors of the -methyl-d-aspartate (NMDA) subtype and resulted in removal of glucose transporters (GLUTs) from the surfaces of dendritic processes in hippocampal neurons. Importantly, insulin prevented the AβO-induced inhibition of AMPK. Our results establish a novel toxic impact of AβOs on neuronal metabolism and suggest that AβO-induced, NMDA receptor-mediated AMPK inhibition may play a key role in early brain metabolic defects in AD.

Lithium is a drug widely used to treat bipolar disorder. It has been shown to inhibit the total activity of phosphoglucomutase (PGM) from rat brains. In this work, we show that lithium inhibits in vitro PGM activity in the cortex, hippocampus, striatum, brainstem and cerebellum. As a compensatory effect, chronic lithium treatment of Wistar rats for 6 weeks caused a 1.6-fold upregulation of cortex PGM activity. No difference was observed in the other areas tested. Another effect of chronic lithium administration was a drastic reduction of glycogen content in rat brains, as PGM activity is essential for its synthesis. In a primary culture of astrocytes, which are the main cellular components of the brain that produce glycogen, administration of 1mM lithium for 3 days markedly reduced the steady state of glycogen content. In agreement with this result, lithium did not cause insulin-like effects as previously observed in hepatocytes where lithium activated glycogen synthesis. Reduction of glycogen content was due to inhibition of glycogen synthesis, as incorporation of [(14)U(-)C]-glucose into glycogen was impaired by lithium. Consistent with these results, incubation of glucose-starved astrocytes with lithium did not stimulate dephosphorylation of glycogen synthase, which normally occurs with re-feeding of glucose. Furthermore, in a chronically treated astrocyte culture, glycogen synthase was phosphorylated constitutively. Our results indicate that chronic lithium treatment can inhibit glycogen synthesis in brain suggesting that this effect might contribute to lithium's therapeutic effect.