Bax Contributes to Retinal Ganglion Cell Dendritic Degeneration During Glaucoma

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.

Botulinum toxin type A relieve neuropathic pain by suppressing the expression of CXCL13/CXCR5 and GAT-1 in chronic constriction injury rats

Botulinum toxin type A (BTX-A) was widely used to treat neuropathic pain in clinic. The underlying analgesic mechanism of BTX-A involves in axonal transport. The chemokine (C-X-C motif) ligand 13 (CXCL13) and GABA transporter 1 (GAT-1) played important roles in chronic pain. We established a chronic constriction injury (CCI) model. The pain behaviors of rats were measured by testing paw withdrawal thresholds (PWTs) and paw withdrawal latencies (PWLs). The level of proteins was measured by western blots. In our results, the CCI rats showed decrease of PWTs and PWLs, which were relieved by BTX-A. BTX-A reversed the over-expression of CXCL13 and GAT-1 in spinal cord, DRG, sciatic nerve and plantar in CCI rats and characterized in dose-dependent manner. The inhibition of BTX-A on proteins we examined didn’t show significant trend among time points. The analgesic effect of BTX-A disappeared after the axon transport of sciatic nerve blocked by the colchicine. But the PWTs of the colchicine treated CCI rats were higher than non- colchicine-treated CCI rats. Colchicine decreased the levels of CXCL13 and GAT-1 in CCI rats. What’s more, the proteins we examined peaked at the sciatic nerve in the non-colchicine group, but the phenomenon disappeared in the colchicine group. In conclusion, the BTX-A and colchicine relieve neuropathic pain and suppress the increase of CXCL13 and GAT-1. Colchicine prevents the analgesic effect of BTX-A by blocking axon transport. The axon transport may play roles in the peripheral mechanisms of neuropathic pain.

Deacetylation of Miro1 by HDAC6 blocks mitochondrial transport and mediates axon growth inhibition

Inhibition of histone deacetylase 6 (HDAC6) was shown to support axon growth on the nonpermissive substrates myelin-associated glycoprotein (MAG) and chondroitin sulfate proteoglycans (CSPGs). Though HDAC6 deacetylates α-tubulin, we find that another HDAC6 substrate contributes to this axon growth failure. HDAC6 is known to impact transport of mitochondria, and we show that mitochondria accumulate in distal axons after HDAC6 inhibition. Miro and Milton proteins link mitochondria to motor proteins for axon transport. Exposing neurons to MAG and CSPGs decreases acetylation of Miro1 on Lysine 105 (K105) and decreases axonal mitochondrial transport. HDAC6 inhibition increases acetylated Miro1 in axons, and acetyl-mimetic Miro1 K105Q prevents CSPG-dependent decreases in mitochondrial transport and axon growth. MAG- and CSPG-dependent deacetylation of Miro1 requires RhoA/ROCK activation and downstream intracellular Ca2+ increase, and Miro1 K105Q prevents the decrease in axonal mitochondria seen with activated RhoA and elevated Ca2+. These data point to HDAC6-dependent deacetylation of Miro1 as a mediator of axon growth inhibition through decreased mitochondrial transport.