Astrocytes connect and regulate synaptic and vascular brain compartments in the so-called neurovascular unit (NVU). They regulate development and functions of both neuronal networks (ie synapses) and the blood-brain barrier. Therefore, if their morphology, which correlates with their functionality, is altered by either genetic predisposition or exposition to adverse environmental conditions (ie highly stressful), astrocytes might inefficiently exert their roles with consequent onset of neuropsychiatric disorders. The lab focuses on understanding the role of astrocytes in the etiopathogenesis of major depressive disorder (MDD) and stress-induced brain disorders. Moreover, an additional major aim is to investigate how astrocytes respond to pharmacological treatments and may consequently influence their neurovascular environment to rescue disease phenotypes. Applying histologic, molecular, biochemical methods and behavioural paradigms, we aim at understanding astrocyte-specific genetic and epigenetic variations which might represent predisposing factors to develop neuropsychiatric disorders and identify pharmacological treatments that modulate such variations to restore brain homeostasis.
Among glia cells, astrocytes actively regulate the shaping and functions of the “tripartite synapse” (Parpura and Haydon, 2000). Specifically, they can regulate neurotransmission through modulation of perisynaptic astrocyte processes (PAP) via changes in cytoskeleton proteins such as ezrin (Lavialle et al., 2011). Additionally, they might induce synaptic changes via the regulation of the expression/release of neurotrophic factors, such as glial-derived neurotrophic factor (GDNF) (Ledda et al., 2007) or through activation of membrane-bound phagocytic proteins, such as MEGF10 (Chung et al., 2013). Changes in synaptic contacts might further influence learning and memory processes, whose deficits are among the hallmarks of MDD. Long-term potentiation (LTP) is considered a cellular correlate of learning and memory and relies on stabilization of such synaptic changes dependent on activation of several signalling pathways (Adams and Dudek, 2005). A recent work has shown that also the glia/neuron ephrinA/EphA signalling is an essential bidirectional mechanism that modulates glutamate signalling and proper LTP induction (Filosa et al., 2009). Specifically, the activation of the glia/neuron ephrinA3/EphA4 receptor signalling has been shown to be instrumental for shaping neuronal networks in both the developing and adult brains (Murai et al., 2003, Egea and Klein, 2007, Nishida and Okabe, 2007). Moreover, ephrinAs have been shown to influence the clustering and internalization/degradation of GluR1-enriched AMPA receptors via activation of EphA4, a mechanism that influences glutamate neurotransmission and might thereby also affect LTP and synaptic stability (Fu et al., 2011). As far as chronic AD treatments have been shown to enhance LTP in adult brain of mice, it is of particular interest how release of GDNF, regulation of the MEGF10 signalling pathway and the ephrinA/EphA signalling might be involved in the etiopathogenesis of MDD and in response to pharmacological treatments.
Project 1: Investigate how the ephrinA/EphA system is regulated at the epigenetic level in healthy and diseased brains and how it is modulated by pharmacological treatments to restore physiological brain properties (Project of Victoria Malik, in cooperation with Mira Jacovcevski at the MPI of Psychiatry, Munich).
Project 2: Examine expression and functional role of astrocyte-specific proteins for synaptic formation/pruning during brain development and in adulthood in healthy and diseased brains and how they might be influenced by antidepressant treatment to properly reshape dysfunctional or miswired neuronal networks (Project of Celia Roman)
In addition to their relation to synapses, astrocytes might regulate transport of therapeutic drugs in/out of the brain through their polarized end-feet which contact blood vessels (Pardridge, 1999). Recent studies evidenced how ablation of the gene coding for the end-feet protein aquaporin-4 (AQP4) could disrupt responses to fluoxetine on behavioral measures of depressive-like phenotype in a chronic stress model for depression (Kong et al., 2009). Furthermore, the knockout of AQP4 results in cognitive deficits similar to those implicated in mood disorders (Skucas et al., 2011) and a recent study on post-mortem brains from MDD patients revealed a reduced expression of AQP4 and diminished coverage of blood vessels by astrocytic end-feet (Rajkowska et al., 2012).
Project 1: Characterize whether differences in the morphology of astrocytes (numbers and length of processes, soma sizes) and in the expression/distribution of AQP4 occur between cells derived from healthy and diseased brains and at the glia-vasculature interface in vivo in an animal model of MDD and how such changes might be affected by AD treatments (Master thesis available)
Project 2: Characterize expression and function of modulators of astrocyte processes at the BBB and their putative properties as diagnostic or predictive biomarkers of a treatment response (Project of Victoria Malik, in collaboration with Caroline Nothdurfter, MD – Master thesis available)
In order to screen for alternative early astrocytic molecular targets of ADs, a microarray analysis was performed to determine the gene profiling of C6 cells treated for 2 hrs with ADs and non-AD drugs. With such a hypothesis-free approach, we identified few factors as common specific downstream effectors activated by both ADs DMI and FLX. The validation of our microarray results using acute brain slices treated with FLX for 2 hrs and then processed with in situ hybridization (ISH) showed that specifically the expression of one of them, belonging to the TGF-beta family of growth factors, is up-regulated in filamentous processes around blood vessels, thus suggesting that this factor might be released into the bloodstream in response to pharmacological treatments. Recently, this factor has also been proposed as a blood predictive biomarker to identify treatment responders after the administration of the antimetastatic drug Danusertib (Carpinelli et al., 2011), thus suggesting that the same might hold true for ADs. Therefore, it might be possible to use it also as a blood biomarker to screen AD treatment responder- from non-responder-patients at early phases after the beginning of the therapy. Therefore, understanding its expression and release processes might have direct clinical applications.
Project 1: Investigate the regulation of gene expression and release of trophic factors induced by ADs and their functional role in the formation and polarization of astrocyte processes at the glia-vasculature interface (Bachelor thesis available).-->