Glutamic acid decarboxylase 67 expression by a distinct population of mouse vestibular supporting cells

Abstract

The function of the enzyme glutamate decarboxylase (GAD) is to convert glutamate in γ-aminobutyric acid (GABA). Glutamate decarboxylase exists as two major isoforms, termed GAD65 and GAD67, that are usually expressed in GABA-containing neurons in the central nervous system. GAD65 has been proposed to be associated with GABA exocytosis whereas GAD67 with GABA metabolism. In the present immunofluorescence study, we have investigated the presence of the two GAD isoforms in the semicircular canal cristae of wild type and GAD67-GFP knock-in mice. While no evidence for GAD65 expression was found, GAD67 was detected in a distinct population of peripherally-located supporting cells, but not in hair cells or in centrally-located supporting cells. GABA, on the other hand, was found in all supporting cells. The present result indicate that only a discrete population of supporting cells use GAD67 to synthesize GABA. This is the first report of a marker that allows to distinguish two populations of supporting cells in the vestibular epithelium. On the other hand, the lack of GABA and GAD enzymes in hair cells excludes its involvement in afferent transmission.

Spontaneous and Acetylcholine Evoked Calcium Transients in the Developing Mouse Utricle

“…The fact that the Gad2-IRES-Cre used in the present study drove expression preferentially in a distinct subset of type I hair cells could have important implications, but was not addressed in the present study. Although there is evidence of glutamate decarboxylase isoform GAD67 expression in vestibular SCs (Tavazzani et al, 2014), the pattern of tdT-G5 expression observed in the present study suggests embryonic transcriptional regulation of Gad2 in a subset of type I hair cells. Previous studies indicate that GAD65 has multiple transcription start sites (Skak and Michelsen, 1999), and that GABA might act as a morphogen during development (Owens and Kriegstein, 2002).…”

Developmental GAD2 Expression Reveals Progenitor-like Cells with Calcium Waves in Mammalian Crista Ampullaris

“…There is evidence that glutamic acid decarboxylase (GAD) is expressed in the SCs of mammalian cristae, although the specific physiological role remains uncertain. In a GAD67 transgenic mouse, SCs expressing GAD67-GFP were located primarily in the peripheral zone (PZ) of cristae ( Tavazzani et al., 2014 ). In the PZ, afferent neurons predominantly make direct synaptic contacts with type II HCs via boutons instead of calyceal synapses with type I HCs in the central zone (CZ; Lindeman, 1969 ; Lim, 1976 ; Baird et al., 1988 ; Lysakowski and Goldberg, 1997 ; Fernández et al., 1998 , 1995 ; Desai et al., 2005a ), but it is not known what role SCs may play in synaptic transmission between HCs and afferent terminals or how GAD might be involved.…”

Excitatory GABAergic modulation of calyx terminals in the vestibular sensory end organ

“…However, the efferent cell group in the brainstem cannot be labeled with anti-GABA antibodies (Perachio and Kevetter 1989). A recent study used GAD67-GFP or GAD65-GFP mice that express GFP along with the two isoforms of the enzyme glutamate decarboxylase (GAD), which transform glutamate to GABA (Tavazzani et al 2014). This study concluded that only GAD67 is expressed in vestibular end organs and only in supporting cells in the peripheral regions of the semicircular canal cristae.…”

“…The results of studies on gamma amino butyric acid (GABA) in the vestibular periphery are less conclusive. In mammals, GABA has been shown in efferent fibers (Kong et al 1998a;b;Matsubara et al 1995) and a recent study has found glutamic acid decarboxylase 67 (GAD67)which is required for generation of GABAand GABA itself, in supporting cells in mice vestibular end organs (Tavazzani et al 2014). Another study in mice has found GAD2 in supporting cells and mainly hair cells in mice (Holman et al 2019).…”