Truncation mutation in HRG4 (UNC119) leads to mitochondrial ANT-1-mediated photoreceptor synaptic and retinal degeneration by apoptosis

Invest Ophthalmol Vis Sci. 2006 Apr;47(4):1281-92. doi: 10.1167/iovs.05-0493.

Abstract

Purpose: To characterize the time course of apoptosis and degeneration in a transgenic mouse model of retinal degeneration based on truncated mutant HRG4; to investigate the nature of binding of the mutant HRG4 to its target, ADP-ribosylation factor-like (ARL)2; to study its effects on the downstream molecules Binder-of-ARL2 (BART) and adenine nucleotide transporter (ANT)-1 and on the induction of apoptosis.

Methods: Saturation binding, microscopic morphometric, Western blot, immunofluorescence, and TUNEL analyses were used.

Results: Increased apoptosis did not occur until 20 months in the transgenic retina, consistent with the delayed-onset degeneration in this model. The truncated HRG4 protein exhibited approximately threefold greater affinity for ARL2 than the wild-type HRG4, likely resulting in nonfunctional sequestration of ARL2. A significant decrease in ARL2 was present by 20 months, accompanied by a 50% decrease in ANT-1 in the photoreceptor synaptic mitochondria, with evidence of mitochondrial dysfunction. Preapoptotic degeneration in the photoreceptor synapse was demonstrated with cytochrome c release and caspase 3 activation within the synapse-without evidence of TUNEL-positive apoptosis in the photoreceptor cell body-indicating an initial event in the synapse leading to apoptosis. Caspase 3 was activated in the accompanying secondary neuron, consistent with transsynaptic degeneration.

Conclusions: The results support a novel mechanism of retinal degeneration in which preapoptotic degeneration starts in the photoreceptor synapse because of a deficiency in ANT-1 and spreads to the secondary neuron transsynaptically, followed by apoptosis and degeneration in the cell body of the photoreceptor.

Publication types

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

MeSH terms

  • Adaptor Proteins, Signal Transducing
  • Adenine Nucleotide Translocator 1 / deficiency*
  • Animals
  • Apoptosis*
  • Blotting, Western
  • Caspase 3
  • Caspases / metabolism
  • Codon, Nonsense*
  • Cytochromes c / metabolism
  • Enzyme Activation
  • In Situ Nick-End Labeling
  • Intracellular Signaling Peptides and Proteins / genetics*
  • Mice
  • Mice, Transgenic
  • Microscopy, Fluorescence
  • Microtubule Proteins / genetics*
  • Mitochondria / metabolism*
  • Photoreceptor Cells, Vertebrate / metabolism*
  • Retinal Degeneration / genetics*
  • Retinal Degeneration / metabolism
  • Synapses / metabolism*

Substances

  • Adaptor Proteins, Signal Transducing
  • Adenine Nucleotide Translocator 1
  • Codon, Nonsense
  • Intracellular Signaling Peptides and Proteins
  • Microtubule Proteins
  • Unc119 protein, mouse
  • Cytochromes c
  • Casp3 protein, mouse
  • Caspase 3
  • Caspases