Loss of Class III Phosphoinositide 3-Kinase Vps34 Results in Cone Degeneration.

Ammaji Rajala,Feng He,Robert E Anderson,Theodore G Wensel,Raju V S Rajala

doi:10.3390/biology9110384

Abstract

Simple SummaryCone photoreceptors are the class of neurons in the retina that support daylight and color vision. In humans and rodents, the cone photoreceptors constitute a small percentage of total retinal photoreceptors; in some retinal diseases, these cells malfunction over time and cease to work, and eventually die. Class III phosphoinositide 3-kinase, also known as vacuole protein sorting 34 (Vps34), generates phosphoinositide 3-phosphate (PI(3)P), a lipid molecule that transmits information inside of the cell. PI(3)P plays an essential role in removing injured cells, a process called autophagy, which maintains a healthy environment, as well as in protein trafficking inside of the cell. Furthermore, PI(3)P can act as a bridging molecule for proteins to bind to each other. We eliminated the class III phosphoinositide 3-kinase in mouse cones, which resulted in the loss of visual function and death of cone cells. Our studies suggest that PI(3)P generated by class III phosphoinositide 3-kinase is essential for cone photoreceptor function and survival.The major pathway for the production of the low-abundance membrane lipid phosphatidylinositol 3-phosphate (PI(3)P) synthesis is catalyzed by class III phosphoinositide 3-kinase (PI3K) Vps34. The absence of Vps34 was previously found to disrupt autophagy and other membrane-trafficking pathways in some sensory neurons, but the roles of phosphatidylinositol 3-phosphate and Vps34 in cone photoreceptor cells have not previously been explored. We found that the deletion of Vps34 in neighboring rods in mouse retina did not disrupt cone function up to 8 weeks after birth, despite diminished rod function. Immunoblotting and lipid analysis of cones isolated from the cone-dominant retinas of the neural retina leucine zipper gene knockout mice revealed that both PI(3)P and Vps34 protein are present in mouse cones. To determine whether Vps34 and PI(3)P are important for cone function, we conditionally deleted Vps34 in cone photoreceptor cells of the mouse retina. Overall retinal morphology and rod function appeared to be unaffected. However, the loss of Vps34 in cones resulted in the loss of structure and function. There was a substantial reduction throughout the retina in the number of cones staining for M-opsin, S-opsin, cone arrestin, and peanut agglutinin, revealing degeneration of cones. These studies indicate that class III PI3K, and presumably PI(3)P, play essential roles in cone photoreceptor cell function and survival.

Highlights

Phosphatidylinositol (PI) is a component of phospholipids in the cell membrane and contains aD-myoinositol head group, a glycerol backbone, and two fatty acids at the C1 and C2 acyl positions of glycerol [1,2,3]
Prefer-fixed retinal sections from Vps34-floxed and rod-Vps34 KO mice were stained for rhodopsin, cone opsin (M-opsin), cone arrestin, and peanut agglutinin (PNA), the latter being used to label cone outer segments
The results showed that by 8 weeks, rhodopsin expression was absent from rod-Vps34 KO mice compared with Vps34-floxed mice (Figure 1C–E), whereas cone markers, medium-wavelength opsins (M-opsin), cone arrestin, and PNA could still be observed in rod-Vps34 KO mice at 8 weeks in the rod-degenerated retina (Figure 1C–H)

Summary

Introduction

Phosphatidylinositol (PI) is a component of phospholipids in the cell membrane and contains aD-myoinositol head group, a glycerol backbone, and two fatty acids at the C1 and C2 acyl positions of glycerol [1,2,3]. The action of phosphoinositide kinases and phosphoinositide phosphatases, which can rapidly convert one specific PIP into another, results in the generation of seven distinct PIPs [7]. These seven PIPs serve as site-specific signals on membranes that recruit and regulate protein complexes at the interface of the cytosol [2]. Phosphoinositide 3-kinases (PI3K) are a group of enzymes that phosphorylate PI at position 3 to generate 30 or D-3 phosphoinositides [6]. These PI3Ks have been grouped into three distinct classes, depending on subunit interactions and substrate specificity: class I, class II, and class

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