Sialin expression in the CNS implicates extralysosomal function in neurons.

SLC17A5 encodes a lysosomal membrane protein, sialin, which transports sialic acid from lysosomes. Mutations in sialin result in neurodegenerative sialic acid storage disorders, Salla disease (SD) and infantile sialic acid storage disease (ISSD). Here we analyzed sialin in mouse central nervous system (CNS) and primary cortical and hippocampal neurons and glia. In the CNS, sialin was predominantly expressed in neurons, especially in the proliferative zone of the prospective neocortex and the hippocampus in developing brain. In nonneuronal cells and primary glial cell cultures, mouse sialin was localized into lysosomes but interestingly, in primary neuronal cultures sialin was not targeted into lysosomes but rather revealed a punctate staining along the neuronal processes and was also seen in the plasma membrane. These data demonstrate a nonlysosomal localization of sialin in neurons and would imply a role for sialin in the secretory processes of neuronal cells.