Dual Role for Astroglial Copper-Assisted Polyamine Metabolism during Intense Network Activity

Abstract

Astrocytes serve essential roles in human brain function and diseases. Growing evidence indicates that astrocytes are central players of the feedback modulation of excitatory Glu signalling during epileptiform activity via Glu-GABA exchange. The underlying mechanism results in the increase of tonic inhibition by reverse operation of the astroglial GABA transporter, induced by Glu-Na+ symport. GABA, released from astrocytes, is synthesized from the polyamine (PA) putrescine and this process involves copper amino oxidase. Through this pathway, putrescine can be considered as an important source of inhibitory signaling that counterbalances epileptic discharges. Putrescine, however, is also a precursor for spermine that is known to enhance gap junction channel communication and, consequently, supports long-range Ca2+ signaling and contributes to spreading of excitatory activity through the astrocytic syncytium. Recently, we presented the possibility of neuron-glia redox coupling through copper (Cu+/Cu2+) signaling and oxidative putrescine catabolism. In the current work, we explore whether the Cu+/Cu2+ homeostasis is involved in astrocytic control on neuronal excitability by regulating PA catabolism. We provide supporting experimental data underlying this hypothesis. We show that the blockade of copper transporter (CTR1) by AgNO3 (3.6 µM) prevents GABA transporter-mediated tonic inhibitory currents, indicating causal relationship between copper (Cu+/Cu2+) uptake and the catabolism of putrescine to GABA in astrocytes. In addition, we show that MnCl2 (20 μM), an inhibitor of the divalent metal transporter DMT1, also prevents the astrocytic Glu-GABA exchange. Furthermore, we observed that facilitation of copper uptake by added CuCl2 (2 µM) boosts tonic inhibitory currents. These findings corroborate the hypothesis that modulation of neuron-glia coupling by copper uptake drives putrescine → GABA transformation, which leads to subsequent Glu-GABA exchange and tonic inhibition. Findings may in turn highlight the potential role of copper signaling in fine-tuning the activity of the tripartite synapse.

Putrescine Intensifies Glu/GABA Exchange Mechanism and Promotes Early Termination of Seizures

“…It is this inhibitory halo that prevents propagation of the excitatory core and seizure generalization in the end [ 31 ]. The ability of astrocytes to produce significant and highly localized inhibitory constrain is confirmed by (i) the observation that SNAP-5114, a specific blocker of astrocytic GAT-2/3, could reverse the antiepileptic effect of PUT ( Figure 2 ) and (ii) only excitatory synaptic potentials were reduced by PUT application ( Figure 3 ) By turning excitation into inhibition, the astroglial Glu-GABA exchange mechanism not only drives against ictal fatigue comprising impaired metabolism, mitochondrial malfunctioning and cellular energy depletion [ 5 , 15 , 32 , 33 ] but can prevent propagation and generalization of seizures. We conclude that the enhancement of GABA release through astroglial GAT-2/3 transporters entitles PUT with therapeutic potential in absence epilepsy [ 34 , 35 , 36 ].…”

Critical Role of Astrocytic Polyamine and GABA Metabolism in Epileptogenesis

“…This Glu/GABA exchange mechanism could be prevented by blocking MAOB and DAO ( Héja et al, 2012 ), suggesting that PUT is a key player in the mechanism. In addition, MAOB and DAO activity may also be decreased by limiting the astrocytic copper concentration as blockade of copper transporter was shown to decrease tonic inhibitory currents ( Szabó et al, 2021 ). Moreover, not only tonic ( Héja et al, 2012 ), but also induced inhibitory synaptic transmission in hippocampal brain slices was depressed by applying alfa-difluoromethylornithine (DFMO), known to block production of PUT ( Ferchmin et al, 1995 ).…”

Astrocytes: GABAceptive and GABAergic Cells in the Brain

“…Besides, hippocampal astrocytes utilize two glutamate decarboxylases (GAD65 and GAD67, 65-and 67-kD isoforms, respectively) to convert glutamate into GABA (Kwak et al, 2020). Putrescine in astrocytes can be catabolized to GABA under the catalysis of copper amine oxidases (CAOs; Szabó et al, 2021).…”

“…For example, intracellular copper levels affect the amount of GABA accumulated in astrocytes. After being taken up by copper transporter (CTR1), copper enhances CAOs' activities and GABA production, causing GABA release and tonic inhibition (Szabó et al, 2021). Astrocyte GABA can also be released into the extrasynaptic space by Na + influx mediated by glutamate transporter (EAAT; Szabó et al, 2021).…”

“…After being taken up by copper transporter (CTR1), copper enhances CAOs' activities and GABA production, causing GABA release and tonic inhibition (Szabó et al, 2021). Astrocyte GABA can also be released into the extrasynaptic space by Na + influx mediated by glutamate transporter (EAAT; Szabó et al, 2021). Besides, activation of α-2 adrenergic receptors also stimulates the release of GABA by astrocytes through G iβγ subunit-associated signaling pathways (Gaidin et al, 2020a).…”