Lysosomal amino acid transporter LYAAT‐1 in the rat central nervous system: An in situ hybridization and immunohistochemical study

A first mammalian lysosomal transporter (LYAAT‐1) was recently identified and functionally characterized. Preliminary immunocytochemical data revealed that LYAAT‐1 localizes to lysosomes in some neurons. In order to determine whether it is expressed in specific neuron populations and other cell types, and to confirm whether it is localized at the membrane of lysosomes, we used in situ hybridization and immunohistochemistry methods in adult rat central nervous system (CNS). We found that LYAAT‐1 is expressed in most areas of the CNS, specifically in neurons, but also in choroid plexus and ependymal epithelium cells. LYAAT‐1‐IR (immunoreactivity) levels varied among different neuroanatomical structures but were present in neurons independently of the neurotransmitter used (glutamate, GABA, acetylcholine, noradrenaline, serotonin, or glycine). Light and confocal microscopy demonstrated that LYAAT‐1 and the lysosomal marker cathepsin D colocalized throughout the brain and electron microscopy showed that LYAAT‐1‐IR was associated with lysosomal membranes. In addition, LYAAT‐1‐IR was also found associated with other membranes belonging to the Golgi apparatus and lateral saccules and less frequently with multivesicular bodies, endoplasmic reticulum, and occasionally with the plasma membrane. The localization of LYAAT‐1 at the lysosomal membrane is consistent with the view that it mediates amino acid efflux from lysosomes. Furthermore, its cell expression pattern suggests that it may contribute to specialized cellular function in the rat CNS such as neuronal metabolism, neurotransmission, and control of brain amino acid homeostasis. J. Comp. Neurol. 462:71–89, 2003. © 2003 Wiley‐Liss, Inc.