Glial glycine transporter 1 function is essential for early postnatal survival but dispensable in adult mice

Abstract

The glycine transporter 1 (GlyT1) is expressed in astrocytes and selected neurons of the mammalian CNS. In newborn mice, GlyT1 is crucial for efficient termination of glycine-mediated inhibitory neurotransmission. Furthermore, GlyT1 has been implicated in the regulation of excitatory N-methyl-D-asparate (NMDA) receptors. To evaluate whether glial and neuronal GlyT1 have distinct roles at inhibitory synapses, we inactivated the GlyT1 gene cell type-specifically using mice carrying floxed GlyT1 alleles GlyT1((+)/+)). GlyT1((+)/(+)) mice expressing Cre recombinase in glial cells developed severe neuromotor deficits during the first postnatal week, which mimicked the phenotype of conventional GlyT1 knock-out mice and are consistent with glycinergic over-inhibition. In contrast, Cre-mediated inactivation of the GlyT1 gene in neuronal cells did not result in detectable motor impairment. Notably, some animals deficient for glial GlyT1 survived the first postnatal week and did not develop neuromotor deficits throughout adulthood, although GlyT1 expression was efficiently reduced. Thus, glial GlyT1 is critical for the regulation of glycine levels at inhibitory synapses only during early postnatal life.

Glycine receptors and brain development

“…In contrast, later during postnatal development glycine level is exclusively controlled by GlyT1 independently of GlyT2 in the cortex (Gomeza et al, 2003; Kunz et al, 2012). Similar findings are observed in the hippocampus where the prominent function of GlyT1 found at P2 (Eulenburg et al, 2010) is also present in the adult formation (Martina et al, 2004; Yee et al, 2006). Nevertheless, GlyT2 is expressed by hippocampal interneurons and may contribute to additional functions (Danglot et al, 2004; Song et al, 2006).…”

Adenoviral-mediated Cre expression effectively suppresses GlyT1 binding in the thalamic area of GlyT1 conditional knock-out mice

“…It is therefore not unexpected to achieve a lower level of gene deletion using a viral approach in adult mice than what can be achieved utilizing a conventional germline deletion strategy. It is noted that in the recent report of Eulenburg et al (2010), such a germline approach yielded a 60-70% reduction of GlyT1-specific uptake. A viral approach has the advantage of allowing one to be much more specific in the delivery of Cre protein and hence mediate deletion in specific, predetermined locations in the brain, critical where a gene has different functions in different brain regions.…”

“…Recently, Eulenburg and colleagues reported the use of a conventional germline deletion approach to generate mice exhibiting a disruption in the GlyT1 gene in their glial cells (GlyT1 (+)/(+) /GFAP-Cre) (Eulenburg et al, 2010). This was achieved by crossing the GlyT1-targeted mice with mice expressing Cre under the control of the glial-specific GFAP promoter.…”

Intact working memory in the absence of forebrain neuronal glycine transporter 1

“…It is possible that both contribute to the observed effects on working memory reported by Singer et al [10, 16]. This hypothesis can be readily tested in mouse models with selective disruption of brain glial-GlyT1 (c.f., [29]). Although glial-GlyT1 function is essential for early life survival, its deletion in adult mice is not associated with any gross abnormality [29] and would be congenial for cognitive assays.…”

“…This hypothesis can be readily tested in mouse models with selective disruption of brain glial-GlyT1 (c.f., [29]). Although glial-GlyT1 function is essential for early life survival, its deletion in adult mice is not associated with any gross abnormality [29] and would be congenial for cognitive assays. It would be even more instructive to target separately glial-GlyT1 associated with NMDAR versus those near glycinergic synapses when the relevant molecular technique becomes available.…”