Brain-derived Neurotrophic Factor (BDNF)-induced Synthesis of Early Growth Response Factor 3 (Egr3) Controls the Levels of Type A GABA Receptorα4 Subunits in Hippocampal Neurons

Abstract

Altered function of ␥-aminobutyric acid type A receptors (GABA A Rs) in dentate granule cells of the hippocampus has been associated with temporal lobe epilepsy (TLE) in humans and in animal models of TLE. Such altered receptor function (including increased inhibition by zinc and lack of modulation by benzodiazepines) is related, in part, to changes in the mRNA levels of certain GABA A R subunits, including ␣4, and may play a role in epileptogenesis. The majority of GABA A Rs that contain ␣4 subunits are extra-synaptic due to lack of the ␥2 subunit and presence of ␦. However, it has been hypothesized that seizure activity may result in expression of synaptic receptors with altered properties driven by an increased pool of ␣4 subunits. Results of our previous work suggests that signaling via protein kinase C (PKC) and early growth response factor 3 (Egr3) is the plasticity trigger for aberrant ␣4 subunit gene (GABRA4) expression after status epilepticus. We now report that brain derived neurotrophic factor (BDNF) is the endogenous signal that induces Egr3 expression via a PKC/MAPK-dependent pathway. Taken together with the fact that blockade of tyrosine kinase (Trk) neurotrophin receptors reduces basal GABRA4 promoter activity by 50%, our findings support a role for BDNF as the mediator of Egr3-induced GABRA4 regulation in developing neurons and epilepsy and, moreover, suggest that BDNF may alter inhibitory processing in the brain by regulating the balance between phasic and tonic inhibition.The type A ␥-aminobutyric acid (GABA) 5 receptor (GABA A R) is an integral ligand gated ion channel that mediates the majority of inhibition in the central nervous system. Being a hetero-oligomeric complex, it is composed of five membrane spanning subunits that are chosen from the products of 19 different genes (␣ 1-6 , ␤ 1-3 , ␥ 1-3 , ␦, ⑀, , 1-3 , and ). These genes are differentially transcribed during development and in various regions of the adult brain and spinal cord (1-5). Alteration in the function of GABA A Rs has been associated with a variety of diseases whose etiology leads to an imbalance between inhibition and excitation in specific populations of neurons (6 -8).For instance, changes in certain GABA A R subunit levels occur in dentate granule cells (DGCs) of both humans with temporal lobe epilepsy (TLE) and in animal models of TLE (6, 9). These molecular responses have been hypothesized to underlie persistent changes in GABA A R function associated with epileptogenesis. Most notably, individual DGCs display an elevation of ␣4 subunit mRNAs and a decrease in ␣1 (6). Receptors that contain ␣4 subunits have unique pharmacological properties that include heightened blockade of receptor function by zinc (11-13) and decreased benzodiazepine modulation (14). In addition, the majority of GABA A Rs that contain ␣4 subunits (co-assembled with a ␤ and ␦) are located extrasynaptically and mediate tonic GABA currents, while those containing ␣(1, 2, 3, or 5) without ␦ and with ␥2 are targeted to the synapse (1, 15). Although...

Enhancing GABAAReceptor α1 Subunit Levels in Hippocampal Dentate Gyrus Inhibits Epilepsy Development in an Animal Model of Temporal Lobe Epilepsy

“…Although gene silencing has been reported in the CNS for integrated retrovirus vectors (Prasad-Alur et al, 2002;Rosenquist et al, 2005), it is less likely to alter AAV-mediated expression because AAV vector genomes are both episomal and associated with high-molecular-weight DNA in the brain, and robust gene expression has been documented from AAV using various promoters for much longer than 4 weeks (Skorupa et al, 1999;Xu et al, 2001;Passini et al, 2002). Our previous work strongly suggests that BDNF and Egr3 (early growth response 3) are major regulators of enhanced ␣4 expression after SE (Roberts et al, 2005(Roberts et al, , 2006, so downregulation of these transcription factors could explain our findings. However, the endogenous ␣4 subunit expression in AAV-injected animals did not vary at 1, 2, or 4weeks after SE, as would be expected if Egr3 or BDNF were depleted or downregulated, making this hypothesis unlikely.…”

Neurosteroid regulation of GABAA receptors: Focus on the α4 and δ subunits

“…Recent studies have identified the α4 promoter (Ma et al, 2004;Roberts et al, 2005), which is activated by PKC and early growth factor 3 to increase α4 expression. More recent studies have shown that brain derived neurotrophic factor is the endogenous signal which initiates these events via the tyrosine kinase receptor (Roberts et al, 2006). In addition to GABAR ligands, seizure activity also increases α4 expression, suggesting that altered neuronal excitability states may underlie the expression of the receptor (Roberts et al, 2005).…”

Early Growth Response 4 Mediates BDNF Induction of Potassium Chloride Cotransporter 2 Transcription

“…Previous studies have shown that BDNF induces a relatively rapid increase in expression of Egr genes Egr1, Egr2, and Egr3 (Harada et al, 2001;Roberts et al, 2006;Calella et al, 2007). To determine whether Egr4 expression is regulated in a similar way, cultured hippocampal neurons (DIV 5-6) were treated with BDNF (50 ng/ml).…”

“…Egr1, Egr2, and Egr3 are activated by BDNF in a MAPKdependent manner (Harada et al, 2001;Roberts et al, 2006;Calella et al, 2007). We have recently demonstrated that Egr4 can induce KCC2 transcription in developing neurons (Uvarov et al, 2006).…”

“…Neurotrophins robustly induce expression of several immediate early genes including early growth response (Egr) transcription factors (Crosby et al, 1991;Harada et al, 2001;Roberts et al, 2006;Calella et al, 2007). Egr4 (or NGFI-C), which displays a neural-specific pattern of expression, is rapidly upregulated by nerve growth factor and neuronal activity (Crosby et al, 1991(Crosby et al, , 1992Honkaniemi and Sharp, 1999; for review, see Beckmann and Wilce, 1997).…”