Knockout of GARPs and the β-subunit of the rod cGMP-gated channel disrupts disk morphogenesis and rod outer segment structural integrity

Abstract

Ion flow into the rod photoreceptor outer segment (ROS) is regulated by a member of the cyclic-nucleotide-gated cation-channel family; this channel consists of two subunit types, alpha and beta. In the rod cells, the Cngb1 locus encodes the channel beta-subunit and two related glutamic-acid-rich proteins (GARPs). Despite intensive research, it is still unclear why the beta-subunit and GARPs are coexpressed and what function these proteins serve. We hypothesized a role for the proteins in the maintenance of ROS structural integrity. To test this hypothesis, we created a Cngb1 5'-knockout photoreceptor null (Cngb1-X1). Morphologically, ROSs were shorter and, in most rods that were examined, some disks were misaligned, misshapen and abnormally elongated at periods when stratification was still apparent and degeneration was limited. Additionally, a marked reduction in the level of channel alpha-subunit, guanylate cyclase I (GC1) and ATP-binding cassette transporter (ABCA4) was observed without affecting levels of other ROS proteins, consistent with a requirement for the beta-subunit in channel assembly or targeting of select proteins to ROS. Remarkably, phototransduction still occurred when only trace levels of homomeric alpha-subunit channels were present, although rod sensitivity and response amplitude were both substantially reduced. Our results demonstrate that the beta-subunit and GARPs are necessary not only to maintain ROS structural integrity but also for normal disk morphogenesis, and that the beta-subunit is required for normal light sensitivity of the rods.