Proc Natl Acad Sci U S A. 2018 Mar 6;115(10):E2393-E2402. doi: 10.1073/pnas.1714888115.

Axogenic mechanism enhances retinal ganglion cell excitability during early progression in glaucoma.

Risner ML, Pasini S, Cooper ML, Lambert WS, Calkins DJ.
Department of Ophthalmology and Visual Sciences, Vanderbilt University Medical Center, Nashville, TN 37232-0654. Email: david.j.calkins@vanderbilt.edu.

Abstract

Diseases of the brain involve early axon dysfunction that often precedes outright degeneration. Pruning of dendrites and their synapses represents a potential driver of axonopathy by reducing activity. Optic nerve degeneration in glaucoma, the world’s leading cause of irreversible blindness, involves early stress to retinal ganglion cell (RGC) axons from sensitivity to intraocular pressure (IOP). This sensitivity also influences survival of RGC dendrites and excitatory synapses in the retina. Here we tested in individual RGCs identified by type the relationship between dendritic organization and axon signaling to light following modest, short-term elevations in pressure. We found dendritic pruning occurred early, by 2 wk of elevation, and independent of whether the RGC responded to light onset (ON cells) or offset (OFF cells). Pruning was similarly independent of ON and OFF in the DBA/2J mouse, a chronic glaucoma model. Paradoxically, all RGCs, even those with significant pruning, demonstrated a transient increase in axon firing in response to the preferred light stimulus that occurred on a backdrop of generally enhanced excitability. The increased response was not through conventional presynaptic signaling, but rather depended on voltage-sensitive sodium channels that increased transiently in the axon. Pruning, axon dysfunction, and deficits in visual acuity did not progress between 2 and 4 wk of elevation. These results suggest neurodegeneration in glaucoma involves an early axogenic response that counters IOP-related stress to excitatory dendritic architecture to slow progression and maintain signaling to the brain. Thus, short-term exposure to elevated IOP may precondition the neural system to further insult. Copyright © 2018 the Author(s).

KEYWORDS: axonopathy; dendritic pruning; glaucoma; neurodegeneration; retinal ganglion cells

PMID:29463759

 

Supplement:

Glaucomatous optic neuropathy (glaucoma) is the leading cause of irreversible blindness worldwide (1). This age-related disease affects retinal ganglion cells (RGCs) through sensitivity to intraocular pressure (IOP). Evidence supports the idea of compartmentalized neurodegeneration, where degenerative programs separately affect dendrites and axons (2, 3).  Furthermore, results indicate raising IOP leads to degeneration of select RGCs (4-6). RGCs excited by light offset (OFF cells) have been identified as particularly sensitive to increased IOP compared to RGCs excited by light onset (ON cells). Here, we examined how modest IOP elevation (~30%) for two to four weeks affects dendrites and axons of distinct RGC types.

Following two weeks of raised IOP, we found RGCs show significant dendritic pruning. Also, IOP elevation significantly increases RIBEYE expression. RIBEYE is a key protein involved in ribbon synapse formation within photoreceptor and bipolar cell terminals (7). Despite, IOP-induced dendritic pruning, we found no change in postsynaptic density protein (PSD-95) expression, suggesting an increase in the amount of PSD-95 on the remaining RGC dendrites. After four weeks of elevated IOP, we did not observe further dendritic pruning. However, both RIBEYE and PSD-95 expression decreased (Fig. 1).

Next, we assessed how increasing IOP affects excitatory synaptic input to RGC light responses by blocking inhibitory mechanisms and voltage-gated channels. We also examined light-evoked spike rate under normal bath conditions.  Control ON RGCs show a strong inward current during light onset, and OFF RGCs exhibit a sustained outward current throughout light stimulation and an inward current following light offset (Fig. 2A-B). Two weeks of raised IOP had no effect on the synaptic input to the RGC light response for neither ON nor OFF cells despite significant dendritic pruning (Fig. 2A-B). Paradoxically, increasing IOP for two weeks led to a significant increase in light-evoked spike rate for ON and OFF RGCs (Fig. 2C-D). This increase in light-induced spiking was associated with an increase in voltage-gated sodium channel (NaV1.6) expression. Following four weeks of ocular hypertension, we observed no change in the synaptic input to ON RGCs and a decrease in OFF RGC light responses (Fig. 2A-B). We also found a decrease in light-evoked spike rate in these cells after four weeks of raised IOP (Fig. 2C-D). At this time point, we also saw a decrease in NaV1.6 expression along intraretinal ganglion cell axons. We found no significant change in axon transport or behavioral optomotor visual acuity between two and four weeks of ocular hypertension, suggesting that early enhanced electrical activity may slow axon pathology.

Our key finding is raising IOP leads to significant RGC dendritic pruning and concurrent enhanced axogenic light responses independent light response bias. Hyperexcitability of affected tissues has also been observed during early progression in other age-related neurodegenerative diseases, including Alzheimer’s, amyotrophic lateral sclerosis, and Huntington’s disease (8-10). Thus, enhanced excitability may be physiological signature of pre-clinical neurodegeneration.

 

 

Fig. 1: Two weeks of increased intraocular pressure (IOP) leads to retinal ganglion cell dendritic pruning. Despite dendritic pruning, postsynaptic dendritic protein (PSD-95) is unchanged. However, the ribbon synapse protein RIBEYE within bipolar cells terminals increases following two weeks of increased pressure. After four weeks of increased IOP, both RIBEYE and PSD-95 decreases, but dendritic pruning does not progress.

 

 

 

Fig. 2: (A-B) Two weeks of raised IOP does not affect light-induced pre- and post-synaptic excitation of ON and OFF retinal ganglion cells (RGCs, cyan). Prolonged IOP elevation does not change light-evoked current of ON cells but decreases light-induced excitation OFF cells (orange).  (C-D) Raising IOP for two weeks significantly increases light-evoked spike rate in both ON and OFF RGCs (cyan). Four weeks of ocular hypertension decreases light-induced spike rate for both ON and OFF RGCs (orange). Retinal circuit diagram illustrates basic ON and OFF pathway neurons: depolarizing bipolar cell (DBC), starburst amacrine cell (SAC), hyperpolarizing bipolar cells, ON ganglion cell (ON GC), and OFF ganglion cell (OFF GC). Red ovals indicate NaV1.6 expression within RGC proximal axon segments.

 

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