This abstract was presented today at the 2015 Association for Research in Vision and Opthalmology (ARVO) meetings in Denver, Colorado by Rebecca L. Pfeiffer, Bryan W. Jones and Robert E. Marc.
Purpose: Müller cells (MCs) play a critical role in glutamate (E) metabolism and carbon skeleton cycling in retina. MCs demonstrate changes in metabolism and morphology during retinal degeneration. The timing, extent, regulation, and impacts of these changes are not yet known. We evaluated metabolic phenotypes of MCs and evaluated their capacity to transport glutamate during degeneration.
Methods: Retinas were harvested from wild-type (WT) and rhodopsin Tg P347L rabbits, divided into chips mounted on filters, and incubated in Ames medium with 5 mM D-aspartate (D-Asp), D-glutamate (D-Glu), or D-glutamine (D-Gln) for 10 min at 35 deg to explore transport and metabolism. Chips were fixed in mixed aldehydes and resin embedded for computational molecular phenotyping (CMP) of a range of L- and D-amino acid markers and selected proteins including glutamine synthetase (GS) (J Comp Neurol. 464:1, 2003).
Results: CMP revealed wide variations in metabolite levels across individual MCs from Tg P347L retinas, generating chaotic patterns. GS decreased significantly while glutamine levels (Q) increased, although to varying degrees. Remarkably, E levels were variable and much higher in some MCs than normal, but did not correlate (inversely) with GS levels. Transport experiments using D-Glu, D-Asp, and D-Gln showed that alterations in MC metabolites are not the product of defective transporters, in contrast to previous reports. These results are also inconsistent with conventional models of GS-based E-Q metabolism and microenvironmental regulation of MC phenotypes.
Conclusions: These observations suggest three conclusions. (1) Although degeneration of the retina is certainly the trigger, MC phenotype changes are not a coherent response to the surrounding microenvironment but are, rather, uncoordinated individual MC responses. (2) Although GS is accepted as the primary enzyme responsible for the conversion of E to Q in the normal retina, alternative pathways appear unmasked in the degenerate state. (3) It has been previously hypothesized that MCs in retinal degenerations exhibit deficient E transport. Our experiments show no transport deficiency. This indicates that chaotic metabolite levels emerge from changes in individual MC metabolic processing.