Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Mol Cell Neurosci. 2005 Mar;28(3):440-51. doi: 10.1016/j.mcn.2004.10.004.


Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Transport System X-AG / genetics
  • Amino Acid Transport System X-AG / metabolism*
  • Animals
  • Animals, Newborn
  • Base Sequence
  • Cattle
  • Cell Compartmentation / physiology
  • Cell Line, Tumor
  • Excitatory Amino Acid Transporter 1
  • Excitatory Amino Acid Transporter 4
  • Feedback, Physiological / physiology
  • Glutamate Plasma Membrane Transport Proteins
  • Glutamic Acid / metabolism*
  • Humans
  • Immunohistochemistry
  • Mice
  • Mice, Inbred C57BL
  • Molecular Sequence Data
  • Phagocytosis / physiology
  • Pigment Epithelium of Eye / metabolism
  • RNA, Messenger / metabolism
  • Receptors, Glutamate / genetics
  • Receptors, Glutamate / metabolism
  • Retinal Cone Photoreceptor Cells / cytology
  • Retinal Cone Photoreceptor Cells / metabolism*
  • Retinal Rod Photoreceptor Cells / cytology
  • Retinal Rod Photoreceptor Cells / metabolism*
  • Sciuridae
  • Sequence Homology, Nucleic Acid
  • Spectrin / genetics
  • Spectrin / metabolism
  • Symporters / genetics
  • Symporters / metabolism*
  • Synapses


  • Amino Acid Transport System X-AG
  • Excitatory Amino Acid Transporter 1
  • Excitatory Amino Acid Transporter 4
  • Glutamate Plasma Membrane Transport Proteins
  • RNA, Messenger
  • Receptors, Glutamate
  • SLC1A3 protein, human
  • SLC1A6 protein, human
  • Slc1a3 protein, mouse
  • Slc1a6 protein, mouse
  • Sptbn2 protein, rat
  • Symporters
  • Spectrin
  • Glutamic Acid