The lysosomal sialic acid transporter sialin is required for normal CNS myelination

Postnatal expression of sialin in the mouse submandibular gland

OBJECTIVE

Sialin has been identified as a sialic acid and aspartate/glutamate transporter. Both cytoplasmic localization and the plasma membrane labelling pattern suggested that sialin may possess multiple transport functions in different cell types. In mouse embryos, sialin expression was primarily detected in the central nervous system. However, sialin shows widespread and high-level expression in adult tissues. Despite its ubiquitous expression and important functions, the postnatal expression profile and subcellular localization of sialin in the salivary gland remains elusive. The aim of the present study was to investigate the expression and subcellular distribution of sialin during postnatal development in the mouse submandibular gland (SMG).

DESIGN

Six SMGs from both female and male C57BL/6 mice were collected at P10, P30 and P90, and the material from each littermate of either sex was pooled to extract total RNA and tissue protein. The remaining tissues were immediately fixed in 10% neutral buffered formalin for histological analysis. The mRNA and protein expression levels of sialin were examined by quantitative real-time RT-PCR and Western blot analysis. The subcellular distribution of sialin was analysed by immunohistochemistry and immunofluorescence.

RESULTS

The postnatal expression level of sialin in the mouse SMG was comparable with that in brain at each time point tested. The temporal expression of sialin in the SMG gradually increased during postnatal maturation. Immunohistochemical and immunofluorescence analysis demonstrated that sialin was predominantly expressed on the basal cytoplasmic membrane of acini and ducts, as well as in some myoepithelial cells in the SMG.

CONCLUSIONS

The high-level expression and subcellular distribution pattern of sialin in the SMG suggest that sialin may play an important role in the transport and secretion of saliva.

Aspartate release and signalling in the hippocampus

The Ca(2+)-dependent release of aspartate from hippocampal preparations was first reported 35 years ago, but the functional significance of this process remains uncertain. Aspartate satisfies all the criteria normally required for identification of a CNS transmitter. It is synthesized in nerve terminals, is accumulated and stored in synaptic vesicles, is released by exocytosis upon nerve terminal depolarization, and activates postsynaptic NMDA receptors. Aspartate may be employed as a neuropeptide-like co-transmitter by pathways that release either glutamate or GABA as their principal transmitter. Aspartate mechanisms include vesicular transport by sialin, vesicular content sensitive to glucose concentration, release mainly outside the presynaptic active zones, and selective activation of extrasynaptic NR1-NR2B NMDA receptors. Possible neurobiological functions of aspartate in immature neurons include activation of cAMP-dependent gene transcription and in mature neurons inhibition of CREB function, reduced BDNF expression, and induction of excitotoxic neuronal death. Recent findings suggest new experimental approaches toward resolving the functional significance of aspartate release.