GlyT1 determines the glycinergic phenotype of amacrine cells in the mouse retina

Abstract

The amino acid glycine acts as a neurotransmitter at both inhibitory glycinergic and excitatory glutamatergic synapses predominantly in caudal regions of the central nervous system but also in frontal brain regions and the retina. After its presynaptic release and binding to postsynaptic receptors at caudal glycinergic synapses, two high-affinity glycine transporters GlyT1 and GlyT2 remove glycine from the extracellular space. Glycinergic neurons express GlyT2, which is essential for the presynaptic replenishment of the transmitter, while glial-expressed GlyT1 was shown to control the extracellular glycine concentration. Here we show that GlyT1 expressed by glycinergic amacrine cells of the retina does not only contribute to the control of the extracellular glycine concentration in the retina but is also essential for the maintenance of the glycinergic transmitter phenotype of this cell population. Specifically, loss of GlyT1 from the glycinergic AII amacrine cells impairs AII-mediated glycinergic neurotransmission and alters regulation of the extracellular glycine concentration, without changes in the overall distribution and/or size of glycinergic synapses. Taken together, our results suggest that GlyT1 expressed by amacrine cells in the retina combines functions covered by neuronal GlyT2 and glial GlyT1 at caudal glycinergic synapses.

A three-sodium-to-glycine stoichiometry shapes the structural relationships of ATB0,+ with GlyT2 and GlyT1 in the SLC6 family

“…In contrast, GlyT1 is as a bidirectional, 2 Na + /1 Cl − -coupled transporter that is expressed primarily on astrocytes and behaves as a buffer, sink, or source of extracellular glycine ( Zafra et al, 1995 ; Supplisson and Bergman, 1997 ; Huang and Bordey, 2004 ; Aubrey et al, 2005 , 2007 ; Sipilä et al, 2014 ; Shibasaki et al, 2017 ). However, GlyT1 concentrative uptake is sufficient to specify the glycinergic phenotype of retinal amacrine cell ( Eulenburg et al, 2018 ), and critical for the cell volume regulation of early mouse embryos ( Steeves et al, 2003 ; Steeves and Baltz, 2005 ). On the extracellular side, GlyT1 tunes the basal concentration and spillover of glycine ( Sipilä et al, 2014 ; Ahmadi et al, 2003 ) that gate NMDARs activation depending on their subunits composition, synaptic location, brain structure, and developmental stage ( Supplisson and Bergman, 1997 ; Supplisson and Roux, 2002 ; Tsai et al, 2004 ; Martina et al, 2005 ; Papouin et al, 2012 ; Bail et al, 2015 ; Ferreira et al, 2017 ; Otsu et al, 2019 ).…”

Cell Types and Synapses Expressing the SNARE Complex Regulating Proteins Complexin 1 and Complexin 2 in Mammalian Retina

“…Glycinergic amacrine cells are located with their somata solely in the INL [ 21 , 22 ]. To label glycinergic amacrine cells, we employed an antibody against the glycine transporter 1 (GlyT1), which is expressed by the entire population of glycinergic amacrine cells in the mouse retina [ 23 ] ( Figure 6 B). By using GlyT1 as a marker for glycinergic amacrine cells, instead of glycine, we avoided possible staining of the ON-cone bipolar cells, which have been shown to receive glycine from AII amacrine cells by diffusion through gap junctions [ 24 ].…”

Heterogeneous Presynaptic Distribution of Munc13 Isoforms at Retinal Synapses and Identification of an Unconventional Bipolar Cell Type with Dual Expression of Munc13 Isoforms: A Study Using Munc13-EXFP Knock-in Mice

“…The presence of conventional and ribbon synaptic sites and of Munc13-3 and ubMunc13-2 in terminals of type 6 ON bipolar cells raises the intriguing possibility of a combinatory release of different types of neurotransmitters. To pursue this possibility, we labeled type 6 ON bipolar cells in combination with representative markers for glutamatergic synapses (vesicular glutamate transporters 1, 2, and 3 (VGLUT; [ 38 , 39 , 40 ]), cholinergic synapses (choline acetyltransferase (ChAT; [ 41 , 42 ]), and GABA-/glycinergic synapses (VGAT [ 43 , 44 ] and GLYT1 [ 45 , 46 ]). Syt II-positive terminals of type 6 ON bipolar cells strongly co-localized with VGLUT1 staining ( Figure 7 A).…”