Transgenic Mice Expressing Dominant-negative Activin Receptor IB in Forebrain Neurons Reveal Novel Functions of Activin at Glutamatergic Synapses

Abstract

The transforming growth factor ␤ family member activin is an important regulator of development and tissue repair. It is strongly up-regulated after acute injury to the adult brain, and application of exogenous activin protects neurons in several lesion models. To explore the role of endogenous activin in the normal and acutely damaged brain, we generated transgenic mice expressing a dominant-negative activin receptor IB (dnActRIB) mutant in forebrain neurons. The functionality of the transgene was verified in vivo. Hippocampal neurons from dnActRIB mice were significantly more vulnerable to intracerebroventricular injection of the excitotoxin kainic acid than those from control littermates, indicating a crucial role of endogenous activin in the rescue of neurons from excitotoxic insult. Because dnActRIB is only expressed in neurons, but not in glial cells, activin affords protection at least in part through a direct action on endangered neurons. Unexpectedly, the transgenic mice also revealed a prominent novel role of activin in glutamatergic neurotransmission in the intact adult brain. Electrophysiologic examination of excitatory synapses onto CA1 pyramidal cells in hippocampal slices of dnActRIB mice showed a reduced NMDA current response, which was associated with impaired long term potentiation. This is the first demonstration that activin receptor signaling is essential to optimize the performance of neuronal circuits in the mature brain under physiological conditions. Activins are members of the transforming growth factor ␤ family of proteins, which regulate proliferation and differentiation of various cell types. The predominant activin variants are homo-or heterodimers consisting of ␤A and/or ␤B subunits (activin A: ␤A␤A, activin B: ␤B␤B and activin AB: ␤A␤B). Their biological effects are mediated through heteromeric receptor complexes, consisting of type I and type II transmembrane serine/threonine kinase receptors (1). Upon ligand binding, one of the activin type II receptors (ActRII or ActRIIB) dimerizes with a type I receptor (ActRIA, ActRIB, or activin receptor-like kinase 7 (2, 3)), resulting in phosphorylation of the type I receptor by the type II receptor. The subsequently activated serine/ threonine kinase of the type I receptor phosphorylates the recruited receptor Smads (Smad2 and Smad3). The latter translocate to the nucleus upon multimerization with Smad4 and modulate as transcription factor complexes the expression of activin target genes (4). In addition, other signaling pathways are also activated by activin receptors, including mitogen activated kinase signaling (1,4,5).Activins are important regulators of development, inflammation, and repair of different tissues and organs (1, 5). In the central nervous system, activin is involved in development, but it may also participate in adaptive and protective mechanisms of the adult brain. Thus, activin ␤A mRNA is transiently upregulated after electrical stimuli that induce synaptic long term potentiation (LTP 4 ; Refs. 6 and 7) and after brie...

Activin tunes GABAergic neurotransmission and modulates anxiety-like behavior

“…In hippocampal slices of dnActRIB mice, we found that the NMDA component of glutamatergic neurotransmission was decreased and, as a consequence, synaptic plasticity was impaired, causing a significant reduction in long-term potentiation (LTP) at the Schaffer-CA1 synapse. 7 These data were the first to demonstrate that endogenously produced activin is capable of modulating the performance of the major excitatory synapse in the brain. Here, we extended our study to the role of activin at the g-aminobutyric acid (GABA)ergic synapse.…”

“…7 The founder mice were created by microinjection of fertilized oocytes from B6D2F2 Â C57BL/6 mice. Tg mice were backcrossed into the C57BL/6 strain for 6-10 generations before used for the experiments described in this study.…”

“…Experiments were performed with mice from two different lines (line 1, L1 and line 3, L3), which displayed the highest level of overexpression of the transgene compared to the endogenous gene and which revealed the same phenotype in previous experiments. 7 Only hemizygous tg mice and their wt littermates were used for experiments. The mice were housed and fed according to federal guidelines.…”

“…[8][9][10] The expression of the transgene in forebrain neurons and the efficient inhibition of activin-receptor signaling by the transgene product were verified in vivo using two independent tg mouse lines with different integration sites of the transgene. 7 These mice were macroscopically indistinguishable from their wild-type (wt) littermates. In hippocampal slices of dnActRIB mice, we found that the NMDA component of glutamatergic neurotransmission was decreased and, as a consequence, synaptic plasticity was impaired, causing a significant reduction in long-term potentiation (LTP) at the Schaffer-CA1 synapse.…”

“…To explore the functions of activin in normal adult brain, we recently generated transgenic (tg) mice expressing a dominant-negative mutant of activin receptor IB (dnActRIB) under the control of the CaMKIIa promoter. 7 The CaMKIIa promoter was chosen, because it drives expression of a transgene specifically in forebrain neurons, especially the hippocampus, and because it becomes active at a relatively late stage of development, typically the first to second postnatal week, as demonstrated in various tg mouse lines using this promoter. [8][9][10] The expression of the transgene in forebrain neurons and the efficient inhibition of activin-receptor signaling by the transgene product were verified in vivo using two independent tg mouse lines with different integration sites of the transgene.…”