RESUMEN
In this study, we describe a novel form of anti-homeostatic plasticity produced after culturing spinal neurons with strychnine, but not bicuculline or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Strychnine caused a large increase in network excitability, detected as spontaneous synaptic currents and calcium transients. The calcium transients were associated with action potential firing and activation of gamma-aminobutyric acid (GABA(A)) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as they were blocked by tetrodotoxin (TTX), bicuculline, and CNQX. After chronic blockade of glycine receptors (GlyRs), the frequency of synaptic transmission showed a significant enhancement demonstrating the phenomenon of anti-homeostatic plasticity. Spontaneous inhibitory glycinergic currents in treated cells showed a fourfold increase in frequency (from 0.55 to 2.4 Hz) and a 184% increase in average peak amplitude compared with control. Furthermore, the augmentation in excitability accelerated the decay time constant of miniature inhibitory post-synaptic currents. Strychnine caused an increase in GlyR current density, without changes in the apparent affinity. These findings support the idea of a post-synaptic action that partly explains the increase in synaptic transmission. This phenomenon of synaptic plasticity was blocked by TTX, an antibody against brain-derived neurotrophic factor (BDNF) and K252a suggesting the involvement of the neuronal activity-dependent BDNF-TrkB signaling pathway. These results show that the properties of GlyRs are regulated by the degree of neuronal activity in the developing network.
Asunto(s)
Plasticidad Neuronal , Neuronas/efectos de los fármacos , Receptores de Glicina/fisiología , Estricnina/farmacología , Sinapsis/efectos de los fármacos , 6-Ciano 7-nitroquinoxalina 2,3-diona/farmacología , Potenciales de Acción , Animales , Bicuculina/farmacología , Factor Neurotrófico Derivado del Encéfalo/fisiología , Calcio/fisiología , Células Cultivadas , Homeostasis , Ratones , Ratones Endogámicos C57BL , Neuronas/fisiología , Receptor trkB/fisiología , Receptores de Glicina/antagonistas & inhibidores , Médula Espinal/citología , Sinapsis/fisiología , Transmisión SinápticaRESUMEN
Five-day-old cultures of mouse glycinergic spinal interneurons were chronically treated with 100 mM ethanol and the glycine and gamma-aminobutyric acid (GABA)(A) receptors were assayed using whole-cell recordings and fluorescence-imaging techniques. Control neurons displayed a glycine(50) of 19 +/- 0.6 microM and a Hill coefficient of 3.1 +/- 0.3. Chronic ethanol treatment did not significantly change these parameters. The maximal responses were 310 +/- 80 pA/pF in control and 440 +/- 19 pA/pF in treated cells, and the fluorescence intensity associated to a monoclonal glycine receptor antibody was unchanged. Strychnine inhibited the glycine current with smaller potency (29%) in treated neurons, thus the IC(50) increased from 14 +/- 2 nM in control to 18 +/- 6 nM in treated neurons. Zn(2+) (10 microM) potentiated the glycine current by 43 +/- 33% in control, but only by 18 +/- 13% in treated neurons. Interestingly, no change on the inhibition produced by a high concentration of Zn(2+) was found in treated neurons. The inhibitory effect of picrotoxin on the glycine receptor, associated to a homomeric receptor, was eliminated with chronic ethanol, suggesting a faster switch to beta-subunit-containing receptors. Unlike glycine receptors, the sensitivity of GABA(A) receptors to GABA, pentobarbital, diazepam, and Zn(2+), as well as the fluorescence intensity associated to a high-affinity benzodiazepine analog was unchanged by chronic ethanol. In conclusion, we found that glycine receptors in spinal interneurons were altered by chronic ethanol treatment and this may reflect the expression of different subunits in control and treated neurons. GABA(A) receptors were resistant to the treatment.
Asunto(s)
Etanol/toxicidad , Interneuronas/efectos de los fármacos , Receptores de GABA-A/efectos de los fármacos , Receptores de Glicina/efectos de los fármacos , Médula Espinal/efectos de los fármacos , Animales , Células Cultivadas , Ratones , Ratones Endogámicos C57BL , Picrotoxina/farmacología , Estricnina/farmacología , Zinc/farmacologíaRESUMEN
IRF2 is a transcription factor, possessing oncogenic potential, responsible for both the repression of growth-inhibiting genes (interferon) and the activation of cell cycle-regulated genes (histone H4). Surprisingly little is known about the post-translational modification of this factor. In this study, we analyze the phosphorylation of IRF2 both in vivo and in vitro. Immunoprecipitation of HA-tagged IRF2 expressed in 32P-phosphate labelled COS-7 cells demonstrates that IRF2 is phosphorylated in vivo. Amino acid sequence analysis reveals that several potential phosphorylation sites exist for a variety of serine/threonine protein kinases, including those of the mitogen activated protein (MAP) kinase family. Using a battery of these protein kinases we show that recombinant IRF2 is a substrate for protein kinase A (PKA), protein kinase C (PKC), and casein kinase II (CK2) in vitro. However, other serine/threonine protein kinases, including the MAP kinases JNK1, p38, and ERK2, do not phosphorylate IRF2. Two-dimensional phosphopeptide mapping of the sites phosphorylated by PKA, PKC, and CKII in vitro demonstrates that these enzymes are capable of phosphorylating IRF2 at multiple distinct sites. Phosphoaminoacid analysis of HA-tagged IRF2 immunoprecipitated from an asynchronous population of proliferating, metabolically phosphate-labelled cells indicates that this protein is phosphorylated exclusively upon serine residues in vivo. These results suggest that the oncogenic protein IRF2 may be regulated via multiple pathways during cellular growth.