Role of Ca2+/Calmodulin-dependent Protein Kinase II in Drosophila Photoreceptors

Abstract

Ca2؉ modulates the visual response in both vertebrates and invertebrates. In Drosophila photoreceptors, an increase of cytoplasmic Ca 2؉ mimics light adaptation. Little is known regarding the mechanism, however. We explored the role of the sole Drosophila Ca 2؉ /calmodulin-dependent protein kinase II (CaMKII) to mediate light adaptation. CaMKII has been implicated in the phosphorylation of arrestin 2 (Arr2). However, the functional significance of Arr2 phosphorylation remains debatable. We identified retinal CaMKII by anti-CaMKII antibodies and by its Ca 2؉ -dependent autophosphorylation. Moreover, we show that phosphorylation of CaMKII is greatly enhanced by okadaic acid, and indeed, purified PP2A catalyzes the dephosphorylation of CaMKII. Significantly, we demonstrate that antiCaMKII antibodies co-immunoprecipitate, and CaMKII fusion proteins pull down the catalytic subunit of PP2A from fly extracts, indicating that PP2A interacts with CaMKII to form a protein complex. To investigate the function of CaMKII in photoreceptors, we show that suppression of CaMKII in transgenic flies affects light adaptation and increases prolonged depolarizing afterpotential amplitude, whereas a reduced PP2A activity brings about reduced prolonged depolarizing afterpotential amplitude. Taken together, we conclude that CaMKII is involved in the negative regulation of the visual response affecting light adaptation, possibly by catalyzing phosphorylation of Arr2. Moreover, the CaMKII activity appears tightly regulated by the co-localized PP2A.Visual transduction is the process that converts the signal of light (photons) into a change of membrane potential in photoreceptors (see Ref. 1 for review). Visual signaling is initiated upon the activation of rhodopsins by light: light switches on rhodopsin to generate metarhodopsin, which activates the heterotrimeric G q in Drosophila (2). Subsequently, the GTPbound G␣ q subunit activates phospholipase C␤4 encoded by the norpA (no receptor potential A) gene (3). Phospholipase C␤4 catalyzes the breakdown of phosphoinositol 4,5-bisphosphate to generate diacylglycerol, which or its metabolite has been implicated in gating the transient receptor potential (TRP) 2 and TRP-like channels (4, 5). TRP is the major Ca 2ϩ channel that mediates the light-dependent depolarization response leading to an increase of cytosolic Ca 2ϩ in photoreceptors. The rise of intracellular Ca 2ϩ modulates several aspects of the visual response including activation, deactivation, and light adaptation (6). For example, Ca 2ϩ together with diacylglycerol activates a classical protein kinase C, eye-PKC, which is critical for the negative regulation of visual signaling by modulating deactivation and light adaptation (7-11).Light adaptation is the process by which photoreceptors adjust the visual sensitivity in response to ambient background light by down-regulating rhodopsin-mediated signaling. Light adaptation can be arbitrarily subdivided into long term and short term adaptation and may involve multiple regulations to red...

Role of Rhodopsin and Arrestin Phosphorylation in Retinal Degeneration of Drosophila

“…Arr2 is phosphorylated by CaMKII at Ser 366 (Matsumoto et al, 1994; Kahn and Matsumoto, 1997; Kahn et al, 1998), which has been implicated in regulating membrane dissociation of Arr2 following inactivation of photoactivated Rh1 (Alloway and Dolph, 1999). Our laboratory previously proposed that the Rh1 interaction with Arr2 is positively regulated by Arr2 phosphorylation, as reduced Arr2 phosphorylation diminishes its ability to uncouple Rh1 (Lu et al, 2009). …”

Calmodulin Enhances Cryptochrome Binding to INAD in Drosophila Photoreceptors

“…The rhodopsin phosphatase Retinal degeneration C (RdgC) also binds CaM, and this interaction is important for photoresponse termination (Lee and Montell, 2001 ). CaM interaction regulates the activity of Ca 2+ /CaM dependent kinase II (CaMKII), which is abundant in fly retina and involved in the negative regulation of visual responses (Lu et al, 2009 ). A direct interaction is also reported between CaM and INAD, involving the region upstream of the PDZ2 domain of the latter (Chevesich et al, 1997 ; Tsunoda et al, 1997 ; Xu et al, 1998 ).…”

The Steroid Hormone 20-Hydroxyecdysone via Nongenomic Pathway Activates Ca2+/Calmodulin-dependent Protein Kinase II to Regulate Gene Expression

“…CaMKII can be activated by Ca 2ϩ and calmodulin, and activation leads to the autophosphorylation of CaMKII at amino acid threonine 287 (or 286 in different isoforms) in mammalian cells (21). CaMKII, which may be located in the cytosol, cytoskeleton (22), and nucleus (23), responds to the elevation of intracellular calcium ion concentration (24) and mediates a variety of biological processes, including neurotransmitter synthesis (25), neurotransmitter exocytosis (26), and ion channel regulation in mammalian (27) and insect cells (28). CaMKII induces histone deacetylase 4 (HDAC4) phosphorylation and nuclear export, which keep the target protein acetylation regulated by histone acetyltransferases (29).…”