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, 54 (6), 889-903

GAD67-mediated GABA Synthesis and Signaling Regulate Inhibitory Synaptic Innervation in the Visual Cortex

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GAD67-mediated GABA Synthesis and Signaling Regulate Inhibitory Synaptic Innervation in the Visual Cortex

Bidisha Chattopadhyaya et al. Neuron.

Abstract

The development of GABAergic inhibitory circuits is shaped by neural activity, but the underlying mechanisms are unclear. Here, we demonstrate a novel function of GABA in regulating GABAergic innervation in the adolescent brain, when GABA is mainly known as an inhibitory transmitter. Conditional knockdown of the rate-limiting synthetic enzyme GAD67 in basket interneurons in adolescent visual cortex resulted in cell autonomous deficits in axon branching, perisomatic synapse formation around pyramidal neurons, and complexity of the innervation fields; the same manipulation had little influence on the subsequent maintenance of perisomatic synapses. These effects of GABA deficiency were rescued by suppressing GABA reuptake and by GABA receptor agonists. Germline knockdown of GAD67 but not GAD65 showed similar deficits, suggesting a specific role of GAD67 in the maturation of perisomatic innervation. Since intracellular GABA levels are modulated by neuronal activity, our results implicate GAD67-mediated GABA synthesis in activity-dependent regulation of inhibitory innervation patterns.

Figures

Figure 1
Figure 1. Gad1 deficiency in single basket interneuron reduces GABA levels
A, Exon 2 of the Gad1 gene was flanked with loxP sites to create a conditional Gad1lox allele. Cortical organotypic cultures were transfected with biolistics with PG67-GFP-ires-Cre to generate GAD67-null (Gad1−/−) or PG67-GFP for control (Ctrl) basket cells. B1, A control basket cell (green, arrowhead) with prominent GABA immunostaining (red) in soma. B2, Two GAD67-null basket cells (arrowheads) with no detectable GABA staining in their somata while neighboring untransfected interneurons (arrow) show prominent GABA staining. Scale bar: 10 μm. C, Only 4% of PG67-GFP-ires-Cre-transfected basket cells show detectable levels of GABA 2 days after transfection as compared to 98% of control cells (2d-Ctrl); none of PG67-GFP-ires-Cre-transfected basket interneurons show detectable levels of GABA 6 days after transfection. Bars represent the average from 3 experiments±SEM.
Figure 2
Figure 2. GAD67-mediated GABA synthesis is limiting for perisomatic synapse maturation but not its maintenance
A1, Control basket cell (green) at EP24 with exuberant innervation field characterized by extensive branching, dense boutons along axons (A2), and terminal branches (asterisks, A3) with prominent and clustered boutons (arrowheads) around pyramidal cell somata (NeuN immunostaining, red). B, Gad1−/− basket cell show overall similar axon arbor size and morphology (B1) but reduced axon branching (B2), terminal branching, bouton size and density (B3, arrow indicates larger inter-bouton intervals). C, Inactivation of only one Gad1 allele in basket cells yields a phenotype comparable to of Gad1−/− cells. A3, B3, C3 are from boxed regions in A2, B2, C2. A4, Immuno-EM of a typical GFP-labeled axonal bouton in a control basket cell, which forms symmetric synaptic contacts (arrowheads) with synaptic clefts, vesicle clustering, and thickening of the postsynaptic membrane. In Gad1−/− basket cells (B4), it is difficult to identify vesicles within the bouton-like structures and evidence of synaptic specialization throughout the bouton (arrow). Note that a neighboring GFP-negative symmetric synapse (arrowhead) from a Gad1+/+ cell appears entirely normal. Scale bar: A1, B1, C1: 50 μm; A2, B2, C2: 10 μm; A3, B3, C3: 5 μm; A4, B4: 0.5 μm. D–E, When transfected from EP16-24, bouton density and size are significantly reduced in Gad1−/− cells (7 basket cells and 50 pyramidal somata) and Gad1+/ cells (7 basket cells and 45 pyramidal somata) compared to aged-matched controls (8 basket cells and 50 pyramidal somata; One-way Anova, posthoc Dunn’s test p<0.05; also see Sppl Fig 3). When transfected from EP26-32, however, both parameters are unaffected in Gad1−/− cells (7 basket cells and 50 pyramidal somata) as compared to controls (7 basket cells and 45 pyramidal somata; Mann-Whitney U test, p>0.1; also see images in Sppl Fig 4). F, When transfected from EP16-24, Gad1−/− and Gad1+/ cells show reduced terminal branching compared to controls quantified as in Chattopadhyaya et al., 2004 (One-way Anova, posthoc Dunn’s test, p<0.05). When transfected from EP26-32, however, terminal branch complexity is maintained in Gad1−/− cells (Mann-Whitney U test, P>0.1).
Figure 3
Figure 3. Overexpression of GAD67 increases basket cell axon branching and perisomatic innervation
A, At EP18, perisomatic synapses in control basket cells is immature (A2), with a single terminal branch and only a few putative boutons (A3, arrowheads) around each pyramidal soma (NeuN staining, red). B, A GAD67-overexpressing basket cell from EP12-18 shows increased axon branching (B2), terminal branching and bouton density around pyramidal cell soma (B3). A3, B3, C3 are from boxed regions in A2, B2, C2. Scale bar: A1, B1, C1: 50 μm; A2, B2, C2: 10 μm; A3, B3, C3: 2 μm. When transfected from EP12-18, bouton density (C) and terminal branching (E) are significantly increased in GAD67-overexpressing basket cells (7 basket cells and 88 pyramidal somata) compared to controls (5 basket cells and 48 pyramidal somata; Mann-Whitney U test, p<0.05), while bouton size is unaffected (Mann-Whitney U test, p>0.1).
Figure 4
Figure 4. GABA signaling regulates basket cell axon branching and perisomatic synapse formation through GABAA and GABAB receptors
A, GAT1 antagonist rescues bouton formation, bouton size and terminal branching in Gad1+/cells. NO711-treated (EP16-24) Gad1+/ basket cells (7 cells and 51 pyramidal somata) show increased bouton density (A1), bouton size (A2) and terminal branching (A3) compared to untreated Gad+/ basket cells (5 cells and 54 pyramidal somata; one-way Anova, posthoc Dunn’s test, p< 0.05). Control basket cells treated with NO711 (3 cells and 36 pyramidal somata) show no changes in these parameters compared to untreated controls (6 cells and 58 pyramidal somata; one-way Anova, posthoc Dunn’s test, p> 0.05; See also Sppl Fig 6). B, Diazepam (DZ) or Baclofen (Bcl) treatment (EP16-24) rescued different aspects of perisomatic innervation in Gad1−/− basket cells. DZ-treated Gad1−/− basket cells (7 cells and 50 pyramidal somata) show bouton density (B1) and terminal branching (B3) similar to control cells (7 cells and 44 pyramidal somata; one-way Anova, posthoc Dunn’s test, p> 0.05). Bcl-treated Gad1−/− basket cells (6 cells and 45 pyramidal somata) show higher bouton density compared to untreated Gad1−/− cells (7 cells and 50 pyramidal somata, B1; One-way Anova, posthoc Dunn’s test p<0.05), but poor terminal branching similar to untreated Gad1−/− (B2; One-way Anova, posthoc Dunn’s test p>0.05). Control basket cells treated with Bcl (6 cells and 45 pyramidal somata) or DZ (7 cells and 50 pyramidal somata) show no significant changes in bouton density (B1) or terminal branching (B2, B3; Mann Whitney test, p>0.1; see also Sppl Fig 7).
Figure 5
Figure 5. GAD67-mediated GABA synthesis regulates basket cell axon density, branching, target coverage and innervation field
A, Reconstruction of local axon arbor of control and Gad1−/− basket cells at EP24 (axons: green, soma and dendrite: gray, dots on axon: boutons). Gad1−/− basket cells show reduced axon density, branching, and bouton density. Scale bar: 100 μm. B, Quantitative analysis of local axon arbor and innervation field from Gad1−/− (n=6) and control (n=5) basket cells. Mann Whitney U test, * p<0.01; ** p<0.001; *** p<0.0001. C, Schematic summarizing the effects of reduced GAD67 and GABA levels in a basket cell, on its axon branching, target coverage, synapse density and innervation field. Red: pyramidal cell somata; green, basket cell.
Figure 6
Figure 6. Deficiency of GAD67-mediated GABA synthesis in basket interneurons affects perisomatic synapse maturation in primary visual cortex of adolescent mice
A, Visual cortex of Gad1lox/lox mice was injected with AAV-GFP-ires-Cre virus at P13 and analyzed at P20 or P32. B, Infected basket interneurons [arrowheads; blue: parvalbumin (Pv) immunostaining], show reduction of GABA immunostaining (red) 7 days after injection compared to neighboring untransfected basket cells (arrows). Scale bar: 50 μm. C, At P32, infected basket cell axons, identified by the colocalization of GFP (green) and Pv (red), have smaller perisomatic boutons (arrowheads) around pyramidal cell somata (NeuN, blue) in Gad1lox/lox mice (C2) as compared to those in wild-type mice (C1). Terminal branches from infected Gad1lox/lox basket cell axons frequently have large intervals without boutons (arrows). Scale bar: 5 μm. D1, Perisomatic boutons of infected basket cells from Gad1lox/lox mice (n=4) are significantly smaller compared to those from wild-type mice (n=4, Mann-Whitney test, p<0.001). Bars represent average ± SEM from 120 boutons in each genotype. D2, Cumulative distribution of perisomatic bouton size from infected basket cells in Gad1lox/lox mice is shifted towards lower values compared to those from wild type mice (K-S test, p<0.001).
Figure 7
Figure 7. Perisomatic GAD65-positive bouton density is reduced in germ-line Gad1+/− mice A, B
Low magnification images in left panels show similar levels of GAD65 expression (green) in P28 visual cortex of a Gad1+/ mouse and its Gad1+/+ littermate. Right panels are from boxed regions. GAD65 puncta always form distinct ring-like structures around neuronal somata in Gad1+/+ mice (arrowheads) but often not in Gad1+/ mice, Scale bar: left panels, 20 μm; right panels, 5 μm. C, The density of perisomatic GAD65 puncta was reduced in Gad1+/mice (n=8) compared to Gad1+/+ (n=5) littermates (t-test, p<0.01; data from two different litters were not significantly different and were pooled). D, Western blot analysis of somatosensory (SSCX) and visual (VCX) cortex using GAD65, GAD67, and β-actin antibodies showed that GAD65 levels were very similar in Gad1+/ (+/) compared to their Gad1+/+ (+/+) littermates. Note that GAD67 levels were reduced by 40% in Gad1+/ samples (see text). E, Perisomatic GAD65 positive puncta (green) colocalize with Pv immunostaining (red), suggesting that these puncta represent presynaptic boutons of basket interneurons. Scale bar: 5 μm.
Figure 8
Figure 8. Perisomatic innervation is deficient in Gad1+/−but not Gad2−/− cortical organotypic cultures
A, B, At EP27, basket interneurons in organotypic cultures from Gad1+/ mice were not different from those of wild-type littermates in their axon morphology and size (A1-B1), but show reduced axon branching (A2-B2), terminal branches, synapse size and density (A3-B3, red: NeuN immunostaining). A3 and B3 are from boxed region in A2, B2. Arrowheads: boutons, asterisk: terminal branching. Scale bar: A1, B1: 50 μm; A2, B2: 10 μm; A3, B3: 5 μm. C, bouton density (C1), size (C2) and terminal branching complexity (C3) are significantly reduced in basket cells from Gad1+/ mice (6 basket cells and 50 pyramidal somata) compared to aged-matched control littermates (6 basket cells and 60 pyramidal somata; Mann-Whitney U test p<0.001). Transfection with P67-GAD67 from EP16-24 increased bouton density (C1) and terminal branching (C3) of basket cells from Gad1+/ mice (6 basket cells and 57 pyramidal somata) compared to basket cells transfected with PG67-GFP alone (Mann-Whitney U test p<0.01). D, basket cells from GAD65-null mice are not significantly different from those of wild type littermate in their bouton density (D1), size (D2) and axon terminal branching complexity (D3) (Mann Whitney U test, p>0.1; Gad2−/− mice: 7 basket cells and 50 pyramidal somata; control littermates: 7 basket cells and 50 pyramidal somata; also see Sppl Fig 9).

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