Developmentally-regulated expression of tissue-specific splice variant of rat vesicular glutamate transporter 1 in retina and pineal gland

Function and expression of a splicing variant of vesicular glutamate transporter 1

Vesicular glutamate transporter (VGLUT) is an active transporter responsible for vesicular storage of glutamate in synaptic vesicles and plays an essential role in glutamatergic neurotransmission. VGLUT consists of three isoforms, VGLUT1, VGLUT2, and VGLUT3. The VGLUT1 variant, VGLUT1v, with an additional 75-base pair sequence derived from a second intron between exons 2 and 3, which corresponds to 25 amino acid residues in the 1st loop of VGLUT1, is the only splicing variant among VGLUTs, although whether VGLUT1v protein is actually translated at the protein level remains unknown. In the present study, VGLUT1v was expressed in insect cells, solubilized, purified to near homogeneity, and its transport activity was examined. Proteoliposomes containing purified VGLUT1v were shown to accumulate glutamate upon imposition of an inside-positive membrane potential (Δψ). The Δψ-driven glutamate uptake activity requires Cl^- and its pharmacological profile and kinetics are comparable to those of other VGLUTs. The retinal membrane contained two VGLUT1 moieties with apparent molecular masses of 65 and 57kDa. VGLUT1v-specific antibodies against an inserted 25-amino acid residue sequence identified a 65-kDa immunoreactive polypeptide. Immunohistochemical analysis indicated that VGLUT1v immunoreactivity is present in photoreceptor cells and is associated with synaptic vesicles. VGLUT1v immunoreactivity is also present in pinealocytes, but not in other areas, including the brain. These results indicated that VGLUT1v exists in a functional state in rat photosensitive cells and is involved in glutamatergic chemical transmission.

The molecular architecture of ribbon presynaptic terminals

The primary receptor neurons of the auditory, vestibular, and visual systems encode a broad range of sensory information by modulating the tonic release of the neurotransmitter glutamate in response to graded changes in membrane potential. The output synapses of these neurons are marked by structures called synaptic ribbons, which tether a pool of releasable synaptic vesicles at the active zone where glutamate release occurs in response to calcium influx through L-type channels. Ribbons are composed primarily of the protein, RIBEYE, which is unique to ribbon synapses, but cytomatrix proteins that regulate the vesicle cycle in conventional terminals, such as Piccolo and Bassoon, also are found at ribbons. Conventional and ribbon terminals differ, however, in the size, molecular composition, and mobilization of their synaptic vesicle pools. Calcium-binding proteins and plasma membrane calcium pumps, together with endomembrane pumps and channels, play important roles in calcium handling at ribbon synapses. Taken together, emerging evidence suggests that several molecular and cellular specializations work in concert to support the sustained exocytosis of glutamate that is a hallmark of ribbon synapses. Consistent with its functional importance, abnormalities in a variety of functional aspects of the ribbon presynaptic terminal underlie several forms of auditory neuropathy and retinopathy.

Wolfram syndrome 1 (Wfs1) gene expression in the normal mouse visual system

Wolfram syndrome (OMIM 222300) is a neurodegenerative disorder defined by insulin-dependent diabetes mellitus and progressive optic atrophy. This syndrome has been attributed to mutations in the WFS1 gene, which codes for a putative multi-spanning membrane glycoprotein of the endoplasmic reticulum. The function of WFS1 (wolframin), the distribution of this protein in the mammalian visual system, and the pathogenesis of optic atrophy in Wolfram syndrome are unclear. In this study we made a detailed analysis of the distribution of Wfs1 mRNA and protein in the normal mouse visual system by using in situ hybridization and immunohistochemistry. The mRNA and protein were observed in the retina, optic nerve, and brain. In the retina, Wfs1 expression was strong in amacrine and Müller cells, and moderate in photoreceptors and horizontal cells. In addition, it was detectable in bipolar and retinal ganglion cells. Interestingly, moderate Wfs1 expression was seen in the optic nerve, particularly in astrocytes, while little Wfs1 was expressed in the optic chiasm or optic tract. In the brain, moderate Wfs1 expression was observed in the zonal, superficial gray, and intermediate gray layers of the superior colliculus, in the dorsomedial part of the suprachiasmatic nucleus, and in layer II of the primary and secondary visual cortices. Thus, Wfs1 mRNA and protein were widely distributed in the normal mouse visual system. This evidence may provide clues as to the physiological role of Wfs1 protein in the biology of vision, and help to explain the selective vulnerability of the optic nerve to WFS1 loss-of-function.

The ontogenic expressions of multiple vesicular glutamate transporters during postnatal development of rat pineal gland

The pineal gland expresses vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2), which are thought to transport glutamate into synaptic-like microvesicles in the pinealocytes. Recently, we reported that the rat pineal gland also expresses VGLUT1v which is a novel variant of VGLUT1 during the perinatal period. To explore the biological significance of these VGLUT expressions in pineal development, we studied the ontogeny of VGLUT in this gland by in situ hybridization, immunohistochemistry and quantitative reverse transcription-polymerase chain reaction (RT-PCR) using rats. Histological analysis revealed that intensities of VGLUT1 hybridization signal and immunostaining drastically increase by postnatal day (P) 7, whereas VGLUT2 expression exhibits high levels of mRNA and protein at birth and decreases gradually from P7 onward. Quantitative RT-PCR analysis supported these histological observations, showing that expressions of VGLUT1 and VGLUT2 exhibit opposite patterns to each other. Coinciding with VGLUT1-upregulation, RT-PCR data showed that expressions of dynamin 1 and endophilin 1, which are factors predictably involved in the endocytotic recovery of VGLUT1-associated vesicle, are also increased by P7. Quantitative RT-PCR analysis of VGLUT1v demonstrated that its mRNA expression is upregulated by P7, kept at the same level until P14, and apparently decreased at P21, suggesting its functional property required for a certain developmental event. Moreover, a comparison of mRNA expressions at daytime and nighttime revealed that neither VGLUT1 nor VGLUT1v shows any difference in both P7 and P21 glands, whereas VGLUT2 is significantly lower at daytime than at nighttime at P21 but not P7, the time point at which the melatonin rhythm is not yet generated. The present study shows that expressions of these VGLUT types are differentially regulated during postnatal pineal development, each presumably participating in physiologically distinct glutamatergic functions.