Deletion of the Presynaptic Scaffold CAST Reduces Active Zone Size in Rod Photoreceptors and Impairs Visual Processing

Abstract

How size and shape of presynaptic active zones are regulated at the molecular level has remained elusive. Here we provide insight from studying rod photoreceptor ribbon-type active zones after disruption of CAST/ERC2, one of the cytomatrix of the active zone (CAZ) proteins. Rod photoreceptors were present in normal numbers, and the a-wave of the electroretinogram (ERG)-reflecting their physiological population response-was unchanged in CAST knock-out (CAST Ϫ/Ϫ ) mice. Using immunofluorescence and electron microscopy, we found that the size of the rod presynaptic active zones, their Ca 2ϩ channel complement, and the extension of the outer plexiform layer were diminished. Moreover, we observed sprouting of horizontal and bipolar cells toward the outer nuclear layer indicating impaired rod transmitter release. However, rod synapses of CAST Ϫ/Ϫ mice, unlike in mouse mutants for the CAZ protein Bassoon, displayed anchored ribbons, normal vesicle densities, clustered Ca 2ϩ channels, and essentially normal molecular organization. The reduction of the rod active zone size went along with diminished amplitudes of the b-wave in scotopic ERGs. Assuming, based on the otherwise intact synaptic structure, an unaltered function of the remaining release apparatus, we take our finding to suggest a scaling of release rate with the size of the active zone. Multielectrode-array recordings of retinal ganglion cells showed decreased contrast sensitivity. This was also observed by optometry, which, moreover, revealed reduced visual acuity. We conclude that CAST supports large active zone size and high rates of transmission at rod ribbon synapses, which are required for normal vision.

Mechanism for Selective Synaptic Wiring of Rod Photoreceptors into the Retinal Circuitry and Its Role in Vision

“…However, synaptic ribbons became smaller and lost their “horseshoe”-like shape. We also noticed that a few axonal terminals were located in the photoreceptor nuclear layer, as observed previously in several models of synaptic dysfunction (Liu et al, 2013; Samuel et al, 2014; tom Dieck et al, 2012). Remarkably, we observed no ELFN1 staining in the photoreceptor axonal terminals of TeNT-Cre retinas (Figure 6B).…”

LKB1 and AMPK regulate synaptic remodeling in old age

“…A third possibility is that presynaptic alterations may lead to decreases in neurotransmitter release that are sensed by postsynaptic partners and lead to their sprouting. Consistent with this model, mutation of several genes implicated in photoreceptor synaptic transmission, including Bassoon and Cacna1f , also lead to sprouting of bipolar and horizontal cells 24–27,43,44 . Thus, although loss or delocalization of Bassoon and other structural components of synapses are not detectable in old retina, we speculate that decreased AMPK activity may act in part by decreasing synaptic transmission from rods to their targets.…”

The Auxiliary Calcium Channel Subunit α2δ4 Is Required for Axonal Elaboration, Synaptic Transmission, and Wiring of Rod Photoreceptors

“…We examined the effect of ablating these key players on the expression and synaptic targeting of α2δ4. Presynaptically, we found that elimination of the active zone component CAST (tom Dieck et al, 2012), or the synaptogenic molecule ELFN1 (Cao et al, 2015), had no effect on either the abundance or localization of α2δ4 to synaptic ribbons (Fig. 1F).…”