Compartmentalization and Ca2+ buffering are essential for prevention of light-induced retinal degeneration.
Fly photoreceptors are polarized cells, each of which has an extended interface between its cell body and the light-signaling compartment, the rhabdomere. Upon intense illumination, rhabdomeric calcium concentration reaches millimolar levels that would be toxic if Ca(2+) diffusion between the rhabdomere and cell body was not robustly attenuated. Yet, it is not clear how such effective attenuation is obtained. Here we show that Ca(2+) homeostasis in the photoreceptor cell relies on the protein calphotin. This unique protein functions as an immobile Ca(2+) buffer localized along the base of the rhabdomere, separating the signaling compartment from the cell body. Generation and analyses of transgenic Drosophila strains, in which calphotin-expression levels were reduced in a graded manner, showed that moderately reduced calphotin expression impaired Ca(2+) homeostasis while calphotin elimination resulted in severe light-dependent photoreceptor degeneration. Electron microscopy, electrophysiology, and optical methods revealed that the degeneration was rescued by prevention of Ca(2+) overload via overexpression of CalX, the Na(+)-Ca(2+) exchanger. In addition, Ca(2+)-imaging experiments showed that reduced calphotin levels resulted in abnormally fast kinetics of Ca(2+) elevation in photoreceptor cells. Together, the data suggest that calphotin functions as a Ca(2+) buffer; a possibility that we directly demonstrate by expressing calphotin in a heterologous expression system. We propose that calphotin-mediated compartmentalization and Ca(2+) buffering constitute an effective strategy to protect cells from Ca(2+) overload and light-induced degeneration.
Department of Medical Neurobiology, Institute for Medical Research Israel-Canada (IMRIC) and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Faculty of Medicine, the Hebrew University, Jerusalem 91120, Israel.
Shirley
S
Weiss
Elkana
E
Kohn
Daniela
D
Dadon
Ben
B
Katz
Maximilian
M
Peters
Mario
M
Lebendiker
Mickey
M
Kosloff
Nansi Jo
NJ
Colley
Baruch
B
Minke
D000818:Animals; D030801:Animals, Genetically Modified; D002021:Buffers; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000502:physiology; D002451:Cell Compartmentation / Q000502:physiology*; D003623:Dark Adaptation / Q000502:physiology*; D029721:Drosophila Proteins / Q000502:physiology; D004331:Drosophila melanogaster; D057809:HEK293 Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008027:Light / Q000009:adverse effects*; D017956:Photoreceptor Cells, Invertebrate / Q000378:metabolism / Q000473:pathology; D012162:Retinal Degeneration / Q000209:etiology* / Q000473:pathology / Q000517:prevention & control*
D002021:Buffers; D002135:Calcium-Binding Proteins; C079658:Cpn protein, Drosophila; D029721:Drosophila Proteins; D002118:Calcium