Featured Core Publication:
Sinha R, Grimes WN, Wallin J, Ebbinghaus BN, Luu K, Cherry T, Rieke F, Rudolph U, Wong RO, Hoon M. (2021) Transient expression of a GABA receptor subunit during early development is critical for inhibitory synapse maturation and function. Curr Biol: 31(19):4314-4326.e5. PMCID: PMC8511107
Developing neural circuits, including GABAergic circuits, switch receptor types. But the role of early GABA receptor expression for establishment of functional inhibitory circuits remains unclear. Tracking the development of GABAergic synapses across axon terminals of retinal bipolar cells (BCs), we uncovered a crucial role of early GABAA receptor expression for the formation and function of presynaptic inhibitory synapses. Specifically, early α3-subunit-containing GABAA (GABAAα3) receptors are a key developmental organizer. Before eye opening, GABAAα3 gives way to GABAAα1 at individual BC presynaptic inhibitory synapses. The developmental downregulation of GABAAα3 is independent of GABAAα1 expression. Importantly, lack of early GABAAα3 impairs clustering of GABAAα1 and formation of functional GABAA synapses across mature BC terminals. This impacts the sensitivity of visual responses transmitted through the circuit. Lack of early GABAAα3 also perturbs aggregation of LRRTM4, the organizing protein at GABAergic synapses of rod BC terminals, and their arrangement of output ribbon synapses.
Figure 5 (click on image to enlarge)
GABAAR subunits are downregulated in GABAAα3KO but GABAAα1 total protein expression and promoter accessibility are unchanged in α3KO retina.
(A) Slices from adult GABAAα3KO-littermate control retinas co-immunolabeled for PKC (red) and GABAAγ2 (left) or GABAAβ3 (right panel: yellow). Bottom panels show higher-magnification views of the selected regions (white box) showing receptor immunostaining within PKC-labeled boutons.
(B) % occupancy of GABAAγ2 and GABAAβ3 within adult rod BC boutons across genotypes (N>4 GABAAα3KO-Ctrl pairs; ≥6 sections analyzed per condition).
(C) Western blot showing total protein levels of GABAAα1 compared to the reference Actin, of retinas from two pairs of adult GABAAα3KO-littermate control animals (KO-WT respectively).
(D) Normalized ATAC-Seq tracks from adult GABAAα3KO and littermate control (Ctrl) retinas showing promoter accessibility for the Gabra1 locus. Red box highlights similar Gabra1 promoter accessibility between genotypes.