Abstract
The BCL-2 (B-cell lymphoma-2) family of proteins contributes to mitochondrial-based apoptosis in models of neurodegeneration, including glaucomatous optic neuropathy (glaucoma), which degrades the retinal ganglion cell (RGC) axonal projection to the visual brain. Glaucoma is commonly associated with increased sensitivity to intraocular pressure (IOP) and involves a proximal program that leads to RGC dendritic pruning and a distal program that underlies axonopathy in the optic projection. While genetic deletion of the Bcl2-associated X protein (Bax-/-) prolongs RGC body survival in models of glaucoma and optic nerve trauma, axonopathy persists, thus raising the question of whether dendrites and the RGC light response are protected. Here, we used an inducible model of glaucoma in Bax-/- mice to determine if Bax contributes to RGC dendritic degeneration. We performed whole-cell recordings and dye filling in RGCs signaling light onset (αON-Sustained) and offset (αOFF-Sustained). We recovered RGC dendritic morphologies by confocal microscopy and analyzed dendritic arbor complexity and size. Additionally, we assessed RGC axon function by measuring anterograde axon transport of cholera toxin subunit B to the superior colliculus and behavioral spatial frequency threshold (i.e., spatial acuity). We found 1 month of IOP elevation did not cause significant RGC death in either WT or Bax-/- retinas. However, IOP elevation reduced dendritic arbor complexity of WT αON-Sustained and αOFF-Sustained RGCs. In the absence of Bax, αON- and αOFF-Sustained RGC dendritic arbors remained intact following IOP elevation. In addition to dendrites, neuroprotection by Bax-/- generalized to αON-and αOFF-Sustained RGC light- and current-evoked responses. Both anterograde axon transport and spatial acuity declined during IOP elevation in WT and Bax-/- mice. Collectively, our results indicate Bax contributes to RGC dendritic degeneration and distinguishes the proximal and distal neurodegenerative programs involved during the progression of glaucoma.
Highlights
The BCL-2 (B-cell lymphoma-2) family of proteins contributes broadly to intrinsic, mitochondrial-based, apoptosis during development, cancer, and neurodegeneration [1]
We did not detect a significant difference in baseline intraocular pressure (IOP) between WT (14.64 ± 0.13 mmHg) and Bax-/- naïve eyes (14.35 ± 0.16 mmHg, p=0.16, Fig. 1B)
Following a single injection of microbeads into the anterior chamber of the eye, we achieved a sustained increase in IOP over the duration of the experiment (1 month) for both WT (+32.6%, 19.64 ± 1.82 vs. 14.81 ± 1.54 mmHg, p
Summary
The BCL-2 (B-cell lymphoma-2) family of proteins contributes broadly to intrinsic, mitochondrial-based, apoptosis during development, cancer, and neurodegeneration [1]. Chronic cellular stress initiates apoptosis through the activation of Bcl-2-associated X (BAX) protein [1]. BAX proteins translocate from the cytosol to the mitochondrial outer membrane [2]. Dimerization facilitates the formation of BAX oligomers [5] that permeabilize the mitochondrial outer membrane, causing the release of cytochrome c and second mitochondrial-derived activator of caspases (SMAC) into the cytosol, promoting cell death [6, 7]. Genetic deletion of Bax (Bax-/-) prevents cell death in affected tissues in models of neurodegeneration, including Parkinson’s disease [8], Alzheimer’s disease [9], amyotrophic lateral sclerosis [10], and glaucomatous optic neuropathy (glaucoma) [1, 11]. Bax-/- proves protective against RGC body death in models of glaucoma and acute focal injury to the optic nerve, yet axons comprising the optic nerve still undergo significant degeneration [14, 23]
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