Disruption of Astrocyte-Dependent Dopamine Control in the Developing Medial Prefrontal Cortex Leads to Excessive Grooming in Mice.

BACKGROUND: Astrocytes control synaptic activity by modulating perisynaptic concentrations of ions and neurotransmitters including dopamine (DA) and, as such, could be involved in the modulating aspects of mammalian behavior. METHODS: We produced a conditional deletion of the vesicular monoamine transporter 2 (VMAT2) specifically in astrocytes (aVMTA2cKO mice) and studied the effects of the lack of VMAT2 in prefrontal cortex (PFC) astrocytes on the regulation of DA levels, PFC circuit functions, and behavioral processes. RESULTS: We found a significant reduction of medial PFC (mPFC) DA levels and excessive grooming and compulsive repetitive behaviors in aVMAT2cKO mice. The mice also developed a synaptic pathology, expressed through increased relative AMPA versus NMDA receptor currents in synapses of the dorsal striatum receiving inputs from the mPFC. Importantly, behavioral and synaptic phenotypes were rescued by re-expression of mPFC VMAT2 and L-DOPA treatment, showing that the deficits were driven by mPFC astrocytes that are critically involved in developmental DA homeostasis. By analyzing human tissue samples, we found that VMAT2 is expressed in human PFC astrocytes, corroborating the potential translational relevance of our observations in mice. CONCLUSIONS: Our study shows that impairment of the astrocytic control of DA in the mPFC leads to symptoms resembling obsessive-compulsive spectrum disorders such as trichotillomania and has a profound impact on circuit function and behaviors.

Mitochondrial biogenesis in developing astrocytes regulates astrocyte maturation and synapse formation.

The mechanisms controlling the post-natal maturation of astrocytes play a crucial role in ensuring correct synaptogenesis. We show that mitochondrial biogenesis in developing astrocytes is necessary for coordinating post-natal astrocyte maturation and synaptogenesis. The astrocytic mitochondrial biogenesis depends on the transient upregulation of metabolic regulator peroxisome proliferator-activated receptor gamma (PPARγ) co-activator 1α (PGC-1α), which is controlled by metabotropic glutamate receptor 5 (mGluR5). At tissue level, the loss or downregulation of astrocytic PGC-1α sustains astrocyte proliferation, dampens astrocyte morphogenesis, and impairs the formation and function of neighboring synapses, whereas its genetic re-expression is sufficient to restore the mitochondria compartment and correct astroglial and synaptic defects. Our findings show that the developmental enhancement of mitochondrial biogenesis in astrocytes is a critical mechanism controlling astrocyte maturation and supporting synaptogenesis, thus suggesting that astrocytic mitochondria may be a therapeutic target in the case of neurodevelopmental and psychiatric disorders characterized by impaired synaptogenesis.

Dysfunction of homeostatic control of dopamine by astrocytes in the developing prefrontal cortex leads to cognitive impairments.

Astrocytes orchestrate neural development by powerfully coordinating synapse formation and function and, as such, may be critically involved in the pathogenesis of neurodevelopmental abnormalities and cognitive deficits commonly observed in psychiatric disorders. Here, we report the identification of a subset of cortical astrocytes that are competent for regulating dopamine (DA) homeostasis during postnatal development of the prefrontal cortex (PFC), allowing for optimal DA-mediated maturation of excitatory circuits. Such control of DA homeostasis occurs through the coordinated activity of astroglial vesicular monoamine transporter 2 (VMAT2) together with organic cation transporter 3 and monoamine oxidase type B, two key proteins for DA uptake and metabolism. Conditional deletion of VMAT2 in astrocytes postnatally produces loss of PFC DA homeostasis, leading to defective synaptic transmission and plasticity as well as impaired executive functions. Our findings show a novel role for PFC astrocytes in the DA modulation of cognitive performances with relevance to psychiatric disorders.

Insights into the release mechanism of astrocytic glutamate evoking in neurons NMDA receptor-mediated slow depolarizing inward currents.

The gliotransmitter glutamate in different brain regions modulates neuronal excitability and synaptic transmission through a variety of mechanisms. Among the hallmarks of astrocytic glutamate release are the slow depolarizing inward currents (SICs) in neurons mediated by N-methyl-d-aspartate receptor activation. Different stimuli that evoke Ca2+ elevations in astrocytes induce neuronal SICs suggesting a Ca2+ -dependent exocytotic glutamate release mechanism of SIC generation. To gain new insights into this mechanism, we investigated the relationship between astrocytic Ca2+ elevations and neuronal SICs in mouse hippocampal slice preparations. Here we provide evidence that SICs, occurring either spontaneously or following a hypotonic challenge, are unchanged in the virtual absence of Ca2+ signal changes at astrocytic soma and processes, including spatially restricted Ca2+ microdomains. SICs are also unchanged in the presence of Bafilomycin A1 that after prolonged slice incubation depletes glutamate from astrocytic vesicles. We also found that hemichannels and TREK family channels-that recent studies proposed to mediate astrocytic glutamate release - are not involved in SIC generation. SICs are reduced by the volume-sensitive anion channel antagonists diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), quinine and fluoxetine, suggesting a possible contribution of these channels in SIC generation. Direct measurements of astrocytic glutamate release further confirm that hypotonicity-evoked gliotransmission is impaired following DIDS, quinine and fluoxetine while the exocytotic release of glutamate-that is proposed to mediate synaptic transmission modulation by astrocytes-remains unaffected. In conclusion, our data provide evidence that the release of glutamate generating SICs occurs independently on exocytotic Ca2+ -dependent glutamate release mechanism.

Homer1 Scaffold Proteins Govern Ca2+ Dynamics in Normal and Reactive Astrocytes.

In astrocytes, the intracellular calcium (Ca2+) signaling mediated by activation of metabotropic glutamate receptor 5 (mGlu5) is crucially involved in the modulation of many aspects of brain physiology, including gliotransmission. Here, we find that the mGlu5-mediated Ca2+ signaling leading to release of glutamate is governed by mGlu5 interaction with Homer1 scaffolding proteins. We show that the long splice variants Homer1b/c are expressed in astrocytic processes, where they cluster with mGlu5 at sites displaying intense local Ca2+ activity. We show that the structural and functional significance of the Homer1b/c-mGlu5 interaction is to relocate endoplasmic reticulum (ER) to the proximity of the plasma membrane and to optimize Ca2+ signaling and glutamate release. We also show that in reactive astrocytes the short dominant-negative splice variant Homer1a is upregulated. Homer1a, by precluding the mGlu5-ER interaction decreases the intensity of Ca2+ signaling thus limiting the intensity and the duration of glutamate release by astrocytes. Hindering upregulation of Homer1a with a local injection of short interfering RNA in vivo restores mGlu5-mediated Ca2+ signaling and glutamate release and sensitizes astrocytes to apoptosis. We propose that Homer1a may represent one of the cellular mechanisms by which inflammatory astrocytic reactions are beneficial for limiting brain injury.

Brain natriuretic peptide is able to stimulate cardiac progenitor cell proliferation and differentiation in murine hearts after birth.

Brain natriuretic peptide (BNP) contributes to heart formation during embryogenesis. After birth, despite a high number of studies aimed at understanding by which mechanism(s) BNP reduces myocardial ischemic injury in animal models, the actual role of this peptide in the heart remains elusive. In this study, we asked whether BNP treatment could modulate the proliferation of endogenous cardiac progenitor cells (CPCs) and/or their differentiation into cardiomyocytes. CPCs expressed the NPR-A and NPR-B receptors in neonatal and adult hearts, suggesting their ability to respond to BNP stimulation. BNP injection into neonatal and adult unmanipulated mice increased the number of newly formed cardiomyocytes (neonatal: +23 %, p = 0.009 and adult: +68 %, p = 0.0005) and the number of proliferating CPCs (neonatal: +142 %, p = 0.002 and adult: +134 %, p = 0.04). In vitro, BNP stimulated CPC proliferation via NPR-A and CPC differentiation into cardiomyocytes via NPR-B. Finally, as BNP might be used as a therapeutic agent, we injected BNP into mice undergoing myocardial infarction. In pathological conditions, BNP treatment was cardioprotective by increasing heart contractility and reducing cardiac remodelling. At the cellular level, BNP stimulates CPC proliferation in the non-infarcted area of the infarcted hearts. In the infarcted area, BNP modulates the fate of the endogenous CPCs but also of the infiltrating CD45(+) cells. These results support for the first time a key role for BNP in controlling the progenitor cell proliferation and differentiation after birth. The administration of BNP might, therefore, be a useful component of therapeutic approaches aimed at inducing heart regeneration.