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Developmental expression of chick cortical GABA(A) receptor alpha1 subunits in vivo and in vitro
J D Miranda1, S C Liu, M E Diaz
1Division of Neuroscience, Baylor College of Medicine, Houston, TX 77030, USA.
Insights
The GABA(A) receptor alpha1 subunit is a key component in developing chick brain regions. Its expression increases significantly in vivo and in vitro, suggesting a crucial role in cortical maturation.
Area of Science:
- Neuroscience
- Developmental Biology
- Molecular Biology
Background:
- The GABA(A) receptor is a crucial inhibitory neurotransmitter receptor in the central nervous system.
- The alpha1 subunit is a significant component of GABA(A) receptors, but its developmental expression pattern is not fully understood.
Purpose of the Study:
- To investigate the expression and developmental profile of the GABA(A) receptor alpha1 subunit in the chick cortex.
- To determine the role of the alpha1 subunit in GABA(A) receptor function during cortical development.
Main Methods:
- Utilized a highly specific polyclonal antibody (RP4) for the GABA(A) receptor alpha1 subunit.
- Employed quantitative Western blotting and immunocytochemistry to analyze protein expression in vivo and in vitro.
- Analyzed alpha1-subunit mRNA levels using quantitative reverse-transcriptase polymerase chain reaction (RT-PCR).
- Performed quantitative immunoprecipitation to assess the contribution of alpha1 subunits to total GABA(A) receptors.
Main Results:
- The RP4 antibody demonstrated monospecificity for the GABA(A) receptor alpha1 subunit.
- Alpha1 subunit expression increased significantly in cultured neurons and in the developing chick cortex from embryonic day 8 (E8) to post-hatching.
- Alpha1-subunit mRNA levels showed a corresponding increase, correlating with protein expression and [3H]flunitrazepam binding sites.
- GABA(A) receptors containing alpha1 subunits constituted a relatively constant fraction (50-65%) of total receptors throughout cortical development.
Conclusions:
- The GABA(A) receptor alpha1 subunit is expressed early and consistently during chick cortical development.
- The alpha1 subunit is a major and stable component of GABA(A) receptor oligomers throughout cortical maturation.
- These findings highlight the importance of the alpha1 subunit in the early functional development of the chick cortex.
Abstract:
In order to examine the expression of the GABA(A) receptor alpha1 subunit during chick cortical development in vivo and in vitro, we have utilized a polyclonal antibody (RP4) directed against an alpha1(331-381) fusion protein. This antibody exhibits a high titer for precipitation of [3H]flunitrazepam binding sites in chick cortical extracts, no significant cross-reactivity with GABA(A) receptor beta2- or beta4-subunit fusion proteins, and a robust reaction with a single 51-kDa polypeptide on immunoblots of cortical membranes. This indicates monospecificity of the RP4 antiserum for the GABA(A) receptor alpha1 subunit. The alpha1-subunit antibody also showed strong immunocytochemical reactions with neurons in the embryonic mediodorsal cortex and Purkinje cells of the chick cerebellum. The ontogeny of the alpha1 subunit in chick cortex and in derived neuronal cultures was examined by quantitative Western blotting. The level of the alpha1 polypeptide increased from day 2 to day 6 in culture, acquiring 50% of the maximum expression at day 4. Expression of the cortical GABA(A) receptor alpha1 subunit increased in vivo from embryonic day 8 (E8) to day 7 post-hatching, reaching 50% of adult levels at E16. Levels of the corresponding alpha1-subunit mRNA, analyzed from E8 to E20 by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR), showed a corresponding incline. These findings correlated well with previous developmental studies of GABA(A) receptor ligand binding sites both in vivo and in vitro. The parallel increase of the alpha1 subunit transcript and polypeptide with [3H]flunitrazepam binding sites suggests that this subunit may be an important component of GABA(A) receptors early in cortical ontogeny. This was investigated further by quantitative immunoprecipitation. At saturation, the RP4 antiserum consistently precipitated 50-65% of the central [3H]flunitrazepam binding sites in the developing cortex from E12 through P7, despite a 5-fold increase in the binding level. The data suggest that during cortical development the fraction of GABA(A) receptors containing alpha1 subunits remains relatively constant. Furthermore, the alpha1 polypeptide appears to be a major component of GABA(A) receptor oligomers at all stages of cortical maturation.