Bcl-xL overexpression blocks bax-mediated mitochondrial contact site formation and apoptosis in rod photoreceptors of lead-exposed mice
- Lihua He*,†,
- Guy A. Perkins†,‡,
- Ann T. Poblenz*,
- Jeffrey B. Harris§,
- Michael Hung§,
- Mark H. Ellisman‡, and
- Donald A. Fox*,§,¶,‖
- §College of Optometry, Departments of *Biology and Biochemistry and ¶Pharmacology and Pharmaceutical Sciences, University of Houston, Houston, TX 77204; and ‡Department of Neurosciences, University of California, San Diego, CA 92093
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Edited by Stanley J. Korsmeyer, Dana–Farber Cancer Center, Boston, MA, and approved November 22, 2002 (received for review June 17, 2002)
Abstract
Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited and disease-, injury-, and chemical-induced retinal degeneration. A clinically relevant mouse model of progressive rod photoreceptor-selective apoptosis was produced by low-level developmental lead exposure and studied in combination with transgenic mice overexpressing Bcl-xL only in the photoreceptors. A multiparametric analysis of rod apoptosis and mitochondrial structure-function was performed. Mitochondrial cristae topography and connectivity, matrix volume, and contact sites were examined by using 3D electron tomography. Lead-induced rod-selective apoptosis was accompanied by rod Ca2+ overload, rhodopsin loss, translocation of Bax from the cytosol to the mitochondria, decreased rod mitochondrial respiration and membrane potential, mitochondrial cytochrome c release, caspase-3 activation, and an increase in the number of mitochondrial contact sites. These effects occurred without mitochondrial matrix swelling, outer membrane rupture, caspase-8 activation, or Bid cleavage. Bcl-xL overexpression completely blocked all apoptotic events, except Ca2+ overload, and maintained normal rod mitochondrial function throughout adulthood. This study presents images of mitochondrial contact sites in an in vivo apoptosis model and shows that Bcl-xL overexpression blocks increased contact sites and apoptosis. These findings extend our in vitro retinal studies with Pb2+ and Ca2+ and suggest that developmental lead exposure produced rod-selective apoptosis without mitochondrial swelling by translocating cytosolic Bax to the mitochondria, which likely sensitized the Pb2+ and Ca2+ overloaded rod mitochondria to release cytochrome c. These results have relevance for therapies in a wide variety of progressive retinal and neuronal degenerations where Ca2+ overload, lead exposure, and/or mitochondrial dysfunction occur.
Footnotes
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↵ † L.H. and G.A.P. contributed equally to this work.
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↵ ‖ To whom correspondence should be addressed. E-mail: dafox{at}uh.edu.
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This paper was submitted directly (Track II) to the PNAS office.
- Abbreviations:
- CS,
- contact site;
- Cyt c,
- cytochrome c;
- EM,
- electron microscopy;
- FIGE,
- field inversion gel electrophoresis;
- IMM,
- inner mitochondrial membrane;
- IS,
- inner segment;
- IRBP,
- interphotoreceptor retinoid binding protein;
- JC-1,
- 5,5′,6,6′-tetrachloro-1,1′3,3′-tetraethylbenzimidazolycarbo-cyanine;
- MDH,
- malate dehydrogenase;
- ΔΨm,
- mitochondrial membrane potential;
- OMM,
- outer mitochondrial membrane;
- PNn,
- postnatal day n;
- OS,
- outer segment;
- QOPR,
- rod photoreceptor oxygen consumption
- Copyright © 2003, The National Academy of Sciences
