A distinct step in inter cellular communication involves termination of synaptic transmission via the removal of neurotransmitters by specialized transporters. There are 2 classes of transporters: plasma membrane and vesicular. The plasma membrane transporters (GABA, norepinephrine, dopamine, glycine, etc) use an electrochemical gradient of Na+ generated by an Na-K+-ATPase and Cl- may also be co-transported. Vesicular transporters (monoamines, serotonin, dopamine, norepinephrine, epinephrine, and histamine) utilized the H+ electrochemical gradient generated by a vacuolar ATP-dependent H+ pump (V-ATPase) located on vesicular plasma membrane. Most recently, vesicular type transporters for glutamate, termed VGLUTs (VGLUT1/BNPI, VGLUT2/DNPI, EAT-4), have been cloned and characterized that are related to phosphate transporters. Although neurons exhibit Na-dependent Pi transport, the biological role of Pi uptake is not clear. Inorganic phosphate is essential for various cellular metabolic functions and signal transport. Proximal tubules in the kidney reabsorb Pi in the glomeruli by the action of a group of phosphate transporters (Type 1-NaPi related, type 2-NaPi-2 related, and type 3-viral receptor-related. These receptor show weak (~20% identity) between various subtypes.

A distinct type of brain specific Na+-dependent phosphate (Pi) transporter (BNPI), originally characterized as a plasma membrane transporter has been localized in a subset of glutamatergic neurons (amygdala,cereberal cortex, and hippocmaplus) and identified as VGLUT1 (rat and human 560 aa; ~60 kDa, ~30% homology with type-1 Pi-transporters). Interestingly, a sequence induced by subtoxic levels of NMDA in cerebelar granules also belongs to this family of transporters. BNPI/VGLUT1 is expression is restricted to the brain, where it is predominantly located in synaptic vesicles. VGLUT1 shows strong sequence homology to EAT-4, a C. elegans protein that appears to have specific presynaptioc role in glutamtergic

transmission. Loss of function mutations in EAT-4 affects multiple glutamregic neurotransmission pathways. EAT4 encodes a protein of 563 aa (~48% identity with BNPI and ~30% identity with known NaPi-transporters found in mammalian kidney). Like BNPI, EAT-4 is also expressed in glutamatergic neurons.

A novel transporter VGLUT2/DNPI (Differentiation-assocaited Na-Pi cotransporter, ~75% homology with VGLUT1; human VGLUT2; human/rat/mouse 582 aa) has also been implicated in vesicular glutamate transport. It is expressed in only a subset of neurons. It is also localized to synaptic vesicles, at synapses exhibiting classical excitatory features. VGLUT2 mRNA is found in brain regions that lack VGLUT1.

Sialic acid storage disease (SASD) are autosomal recessive neurdegenerative disorders that may present as a severa infantile form (ISSD) or a slowly progressive adult form (Salla disease) prevalent in Finland. The patients exccrete large amounts of free sialic acid in urine. A H+/anionic sugar symporter mechanism for sialinc acid and glucoronic acid is impaired in lysosomal membranes from Sall and ISSD. A new gene, termed Sialin (SLC17A5; human 495 aa, chromosome 6q14-q15) belonging to the family of anion/cation symporters (ACS) has been found to be mutated in sialic acid storage disease. Sialin is predicted to contain up to 12 TM domains with N and C-termini located in the cytoplasm. Sialin has 37%, 34%, and 16% sequence identity with BNPI/VGLUT1, NPT1 (Napi-I), and E. coli hexuronate (EcHex) transporter, respectively. Sialin is found to be expressed in many human tissues.

ADI has produced highly specific rabbit-antibodies to VGLLUT1, VGLUT2, EAT4, and Sialin using peptide sequences specific to each protein. These antibodies should be useful in studying specific transporters.