Albumin induces excitatory synaptogenesis through astrocytic TGF-β/ALK5 signaling in a model of acquired epilepsy following blood–brain barrier dysfunction
Introduction
Traumatic, ischemic, and infectious brain injuries often initiate a cascade of epileptogenic events, ultimately leading to post-injury epilepsy (PIE) after a latent period of months to years. Accumulating evidence suggests a key role of post-injury dysfunction of the blood–brain barrier (BBB) in the development of PIE: BBB dysfunction is a well-documented finding in patients following brain trauma (occurring within hours of the event and often persisting for days to weeks; for a review, see Abbott and Friedman, 2012, Cunningham et al., 2005, Rosenberg, 2012, Shlosberg et al., 2010) and is more frequent in post-traumatic patients with epilepsy (Raabe et al., 2012, Schmitz et al., 2013, Tomkins et al., 2008). In experimental animals, BBB dysfunction was associated with increased propensity for symptomatic seizures and the development of epilepsy (Friedman et al., 2009, Frigerio et al., 2012, Marchi et al., 2007, Seiffert et al., 2004, Van Vliet et al., 2007).
Previous studies have linked the brain's immune response with seizures and epilepsy, as activation of the pro-inflammatory IL-1 receptor/Toll-like receptor (IL1R/TLR) system was shown to promote seizure onset and recurrence in mice and rats (for a review, see Devinsky et al., 2013, Marchi et al., 2013, Vezzani et al., 2011a, Vezzani et al., 2011b). Recent findings have highlighted the involvement of a specific inflammatory pathway in PIE, identifying the epileptogenic role of transforming growth factor beta (TGF-β) signaling in animal models of BBB dysfunction. Serum albumin was shown to enter BBB-disrupted brain tissue and bind to TGF-β receptors in astrocytes (Ivens et al., 2007), inducing inflammatory signaling (Cacheaux et al., 2009) and SMAD-2/3 phosphorylation (Bar-Klein et al., 2014), thereby modifying astrocytic function (Braganza et al., 2012, David et al., 2009, Seiffert et al., 2004). TGF-β signaling was also associated with immediate changes in extracellular potassium and glutamate, and a lower threshold for neuronal activation in slices (David et al., 2009, Lapilover et al., 2012). However, the immediate and short-lived nature of these changes (Ivens et al., 2007, Lapilover et al., 2012) suggests the involvement of additional modifications, underlying permanent network changes that sustain chronic recurrent seizures. While axonal sprouting and synaptogenesis were shown to contribute to seizures and persistent network alterations (Babb et al., 1991, Bragin et al., 2000, Marco and DeFelipe, 1997), the detailed changes and mechanisms underlying network reorganization in PIE are not well understood.
Glial cells are known to play a key role in the formation and elimination of both excitatory and inhibitory synapses in developing and mature brain (Chung and Barres, 2012, Clarke and Barres, 2013, Elmariah et al., 2005, Eroglu and Barres, 2010). Specifically, astrocytes were shown to regulate synaptogenesis through secretion of thrombospondins (TSPs) (Christopherson et al., 2005, Crawford et al., 2012, Eroglu et al., 2009, Liauw et al., 2008), Hevin (Kucukdereli et al., 2011) and glypicans 4 and 6 (Allen et al., 2012). Moreover, synapse formation was also associated with activation of TGF-β signaling in Schwann cells (Feng and Ko, 2008, Packard et al., 2003), and more recently in astrocytes via induction of Smad3 (Yu et al., 2014), secretion of d-serine (Diniz et al., 2012), and CaM kinase II signaling (Diniz et al., 2014). Since the dysfunction of astroglia has been shown following brain injury, in models of acquired epilepsy (Crunelli et al., 2014, Devinsky et al., 2013, Friedman et al., 1996, Marchi et al., 2013) and in the BBB-compromised brain, here we set out to study the role of albumin-induced TGF-β pathway activation in synaptogenesis and chronic seizures. We further challenged the hypothesis that the inhibition of the TGF-β ALK5 receptor is sufficient to prevent the development of epilepsy. Through a combination of in vitro and in vivo models of inflammation post-BBB dysfunction, our study sheds light on the epileptogenic cascade following brain injury and highlights novel targets for PIE prevention in at-risk patients.
Section snippets
Materials and methods
All animal procedures were approved by the UC Berkeley Animal Care and Use Committee or the animal care and use ethical committees at the Ben-Gurion University of the Negev, Beer-Sheva.
ICV albumin induces epileptogenesis in vivo
To simulate BBB dysfunction in naïve animals and to study the role of serum albumin in epileptogenesis, mice were infused with albumin (0.4 mM in ACSF, 1 μL/h) through an ICV mini-osmotic pump for 7 days. ECoG data were acquired for up to 32 days (32 days: n = 4; 14 days: n = 7; 10 days: n = 2) and analyzed using an automated seizure detection algorithm (see Materials and Methods and Bar-Klein et al., 2014). Albumin treatment did not induce status epilepticus (SE) nor seizures during the first 72 h after
Discussion
Here we demonstrate an albumin-induced and TGF-β-mediated synaptogenic mechanism critical in the epileptogenic process. We show both in vitro and in vivo that albumin induces excitatory synapse formation, and that this effect is astrocyte and ALK5 dependent.
TGF-β signaling in astrocytes following exposure to albumin was previously shown to promote several functional changes that may be associated with increased neuronal excitability, including induction of pro-ictogenic inflammatory cytokines (
Competing interests
The authors declare no competing interests.
Acknowledgments
The research leading to these results has received funding from the European Union's Seventh Framework Program (FP7/2007-2013) under grant agreement no.602102 (EPITARGET, AF), the Israel Science Foundation (713/11, AF), the National Institute of Health (RO1/NINDS NS066005, DK, AF), and the German Science Foundation (DFG-SFB TR3, AF).
References (68)
- et al.
Gene expression profile analysis of epilepsy-associated gangliogliomas
Neuroscience
(2008) - et al.
Synaptic reorganization by mossy fibers in human epileptic fascia dentata
Neuroscience
(1991) - et al.
Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis
Cell
(2005) - et al.
The role of glial cells in synapse elimination
Curr. Opin. Neurobiol.
(2012) - et al.
Glia and epilepsy: excitability and inflammation
Trends Neurosci.
(2013) - et al.
Astrocyte-induced synaptogenesis is mediated by transforming growth factor β signaling through modulation of d-serine levels in cerebral cortex neurons
J. Biol. Chem.
(2012) - et al.
Gabapentin receptor alpha2delta-1 is a neuronal thrombospondin receptor responsible for excitatory CNS synaptogenesis
Cell
(2009) - et al.
Blood–brain barrier breakdown-inducing astrocytic transformation: novel targets for the prevention of epilepsy
Epilepsy Res.
(2009) - et al.
Peri-infarct blood–brain barrier dysfunction facilitates induction of spreading depolarization associated with epileptiform discharges
Neurobiol. Dis.
(2012) - et al.
Age-dependent vascular changes induced by status epilepticus in rat forebrain: implications for epileptogenesis
Neurobiol. Dis.
(2009)
Vascular changes in epilepsy: functional consequences and association with network plasticity in pilocarpine-induced experimental epilepsy
Neuroscience
Early and transient increase of rat hippocampal blood–brain barrier permeability to amino acids during kainic acid-induced seizures
Neurosci. Lett.
Blood–brain barrier dysfunctions following systemic injection of kainic acid in the rat
Life Sci.
Albumin storage in neoplastic astroglial elements of gangliogliomas
Seizure
Blood–brain barrier disruption results in delayed functional and structural alterations in the rat neocortex
Neurobiol. Dis.
Longitudinal assessment of blood–brain barrier leakage during epileptogenesis in rats. A quantitative MRI study
Neurobiol. Dis.
IL-1 receptor/Toll-like receptor signaling in infection, inflammation, stress and neurodegeneration couples hyperexcitability and seizures
Brain Behav. Immun.
Blood–brain barrier changes with kainic acid-induced limbic seizures
Exp. Neurol.
Overview and introduction: the blood–brain barrier in health and disease
Epilepsia
Astrocyte glypicans 4 and 6 promote formation of excitatory synapses via GluA1 AMPA receptors
Nature
A novel non-transcriptional pathway mediates the proconvulsive effects of interleukin-1beta
Brain
Losartan prevents acquired epilepsy via TGF-β signaling suppression
Ann. Neurol.
Albumin is taken up by hippocampal NG2 cells and astrocytes and decreases gap junction coupling
Epilepsia
Chronic epileptogenesis requires development of a network of pathologically interconnected neuron clusters: a hypothesis
Epilepsia
Transcriptome profiling reveals TGF-beta signaling involvement in epileptogenesis
J. Neurosci.
Astrocytic transforming growth factor-beta signaling reduces subacute neuroinflammation after stroke in mice
Glia
Emerging roles of astrocytes in neural circuit development
Nat. Rev. Neurosci.
Astrocyte-derived thrombospondins mediate the development of hippocampal presynaptic plasticity in vitro
J. Neurosci.
Astrocytic targets provide new avenues for the therapeutic treatment of epilepsy
Neuroscientist
Physiological thresholds for irreversible tissue damage in contusional regions following traumatic brain injury
Brain
Astrocytic dysfunction in epileptogenesis: consequence of altered potassium and glutamate homeostasis?
J. Neurosci.
Histopathology of Human Epilepsy
Jasper's Basic Mechanisms of the Epilepsies [Internet]. 4th edition. Bethesda (MD): National Center for Biotechnology Information (US)
Astrocyte transforming growth factor beta 1 promotes inhibitory synapse formation via CaM kinase II signaling
Glia
Astrocytes regulate inhibitory synapse formation via Trk-mediated modulation of postsynaptic GABAA receptors
J. Neurosci.
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