title: Bcl-xL-mediated remodeling of rod and cone synaptic mitochondria after postnatal lead exposure: electron microscopy, tomography and oxygen consumption.
abstract: OBJECTIVE
  Postnatal lead exposure produces rod-selective and Bax-mediated apoptosis, decreased scotopic electroretinograms (ERGs), and scotopic and mesopic vision deficits in humans and/or experimental animals. Rod, but not cone, inner segment mitochondria were considered the primary site of action. However, photoreceptor synaptic mitochondria were not examined. Thus, our experiments investigated the structural and functional effects of environmentally relevant postnatal lead exposure on rod spherule and cone pedicle mitochondria and whether Bcl-xL overexpression provided neuroprotection.
  
  
  METHODS
  C57BL/6N mice pups were exposed to lead only during lactation via dams drinking water containing lead acetate. The blood [Pb] at weaning was 20.6±4.7 µg/dl, which decreased to the control value by 2 months. To assess synaptic mitochondrial structural differences and vulnerability to lead exposure, wild-type and transgenic mice overexpressing Bcl-xL in photoreceptors were used. Electron microscopy, three-dimensional electron tomography, and retinal and photoreceptor synaptic terminal oxygen consumption (QO(2)) studies were conducted in adult control, Bcl-xL, lead, and Bcl-xL/lead mice.
  
  
  RESULTS
  The spherule and pedicle mitochondria in lead-treated mice were swollen, and the cristae structure was markedly changed. In the lead-treated mice, the mitochondrial cristae surface area and volume (abundance: measure correlated with ATP (ATP) synthesis) were decreased in the spherules and increased in the pedicles. Pedicles also had an increased number of crista segments per volume. In the lead-treated mice, the number of segments/crista and fraction of cristae with multiple segments (branching) similarly increased in spherule and pedicle mitochondria. Lead-induced remodeling of spherule mitochondria produced smaller cristae with more branching, whereas pedicle mitochondria had larger cristae with more branching and increased crista junction (CJ) diameter. Lead decreased dark- and light-adapted photoreceptor and dark-adapted photoreceptor synaptic terminal QO(2). Bcl-xL partially blocked many of the lead-induced alterations relative to controls. However, spherules still had partially decreased abundance, whereas pedicles still had increased branching, increased crista segments per volume, and increased crista junction diameter. Moreover, photoreceptor and synaptic QO(2) were only partially recovered.
  
  
  CONCLUSIONS
  These findings reveal cellular and compartmental specific differences in the structure and vulnerability of rod and cone inner segment and synaptic mitochondria to postnatal lead exposure. Spherule and pedicle mitochondria in lead-exposed mice displayed complex and distinguishing patterns of cristae and matrix damage and remodeling consistent with studies showing that synaptic mitochondria are more sensitive to Ca(2+) overload, oxidative stress, and ATP loss than non-synaptic mitochondria. The lead-induced decreases in QO(2) likely resulted from the decreased spherule cristae abundance and smaller cristae, perhaps due to Bax-mediated effects as they occurred in apoptotic rod inner segments. The increase in pedicle cristae abundance and CJ diameter could have resulted from increased Drp1-mediated fission, as small mitochondrial fragments were observed. The mechanisms of Bcl-xL-mediated remodeling might occur via interaction with formation of CJ protein 1 (Fcj1), whereas the partial protection of synaptic QO(2) might result from the enhanced efficiency of energy metabolism via Bcl-xL's direct interaction with the F1F0 ATP synthase and/or regulation of cellular redox status. These lead-induced alterations in photoreceptor synaptic terminal mitochondria likely underlie the persistent scotopic and mesopic deficits in lead-exposed children, workers, and experimental animals. Our findings stress the clinical and scientific importance of examining synaptic dysfunction following injury or disease during development, and developing therapeutic treatments that prevent synaptic degeneration in retinal and neurodegenerative disorders even when apoptosis is blocked.
authors.affiliation: National Center for Microscopy and Imaging Research, University of California San Diego, La Jolla, CA, USA.
authors.forename: Guy A
authors.initials: GA
authors.lastname: Perkins
authors.forename: Ray
authors.initials: R
authors.lastname: Scott
authors.forename: Alex
authors.initials: A
authors.lastname: Perez
authors.forename: Mark H
authors.initials: MH
authors.lastname: Ellisman
authors.forename: Jerry E
authors.initials: JE
authors.lastname: Johnson
authors.forename: Donald A
authors.initials: DA
authors.lastname: Fox
mesh_terms: D000221:Adaptation, Ocular / Q000187:drug effects; D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism; D003623:Dark Adaptation / Q000187:drug effects; D004781:Environmental Exposure; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects; D007774:Lactation; D051379:Mice; D008822:Mice, Transgenic; D008854:Microscopy, Electron; D008928:Mitochondria / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D025261:Mitochondrial Proton-Translocating ATPases / Q000235:genetics / Q000378:metabolism; D009942:Organometallic Compounds / Q000633:toxicity*; D018384:Oxidative Stress; D010101:Oxygen Consumption; D017949:Retinal Cone Photoreceptor Cells / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D015398:Signal Transduction / Q000187:drug effects; D013569:Synapses / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D014057:Tomography, X-Ray Computed; D051020:bcl-X Protein / Q000235:genetics* / Q000378:metabolism
chemical_list: C494813:Bcl2l1 protein, rat; D009942:Organometallic Compounds; D051020:bcl-X Protein; C474451:F1F0-ATP synthase; D025261:Mitochondrial Proton-Translocating ATPases; C008261:lead acetate; D002118:Calcium