Phosphorylation of the Drosophila transient receptor potential ion channel is regulated by the phototransduction cascade and involves several protein kinases and phosphatases

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

Unstructural Biology of TRP Ion Channels: The Role of Intrinsically Disordered Regions in Channel Function and Regulation

The first genuine high-resolution single particle cryo-electron microscopy structure of a membrane protein determined was a transient receptor potential (TRP) ion channel, TRPV1, in 2013. This methodical breakthrough opened up a whole new world for structural biology and ion channel aficionados alike. TRP channels capture the imagination due to the sheer endless number of tasks they carry out in all aspects of animal physiology. To date, structures of at least one representative member of each of the six mammalian TRP channel subfamilies as well as of a few non-mammalian families have been determined. These structures were instrumental for a better understanding of TRP channel function and regulation. However, all of the TRP channel structures solved so far are incomplete since they miss important information about highly flexible regions found mostly in the channel N- and C-termini. These intrinsically disordered regions (IDRs) can represent between a quarter to almost half of the entire protein sequence and act as important recruitment hubs for lipids and regulatory proteins. Here, we analyze the currently available TRP channel structures with regard to the extent of these "missing" regions and compare these findings to disorder predictions. We discuss select examples of intra- and intermolecular crosstalk of TRP channel IDRs with proteins and lipids as well as the effect of splicing and post-translational modifications, to illuminate their importance for channel function and to complement the prevalently discussed structural biology of these versatile and fascinating proteins with their equally relevant 'unstructural' biology.

Functional characterization of the three Drosophila retinal degeneration C (RDGC) protein phosphatase isoforms

Drosophila retinal degeneration C (RDGC) is the founding member of the PPEF family of protein phosphatases. RDGC mediates dephosphorylation of the visual pigment rhodopsin and the TRP ion channel. From the rdgC locus, three protein isoforms, termed RDGC-S, -M, and -L, with different N-termini are generated. Due to fatty acylation, RDGC-M and -L are attached to the plasma membrane while RDGC-S is soluble. To assign physiological roles to these RDGC isoforms, we constructed flies that express various combinations of RDGC protein isoforms. Expression of the RDGC-L isoform alone did not fully prevent rhodopsin hyperphosphorylation and resulted in impaired photoreceptor physiology and in decelerated TRP dephosphorylation at Ser936. However, expression of RDGC-L alone as well as RDGC-S/M was sufficient to prevent degeneration of photoreceptor cells which is a hallmark of the rdgC null mutant. Membrane-attached RDGC-M displayed higher activity of TRP dephosphorylation than the soluble isoform RDGC-S. Taken together, in vivo activities of RDGC splice variants are controlled by their N-termini.

Solubility and subcellular localization of the three Drosophila RDGC phosphatase variants are determined by acylation

Protein phosphorylation is an abundant molecular switch that regulates a multitude of cellular processes. In contrast to other subfamilies of phosphoprotein phosphatases, the PPEF subfamily is only poorly investigated. Drosophila retinal degeneration C (RDGC) constitutes the founding member of the PPEF subfamily. RDGC dephosphorylates the visual pigment rhodopsin and the ion channel TRP.However, rdgC null mutant flies exhibit rhodopsin and TRP hyperphosphorylation, altered photoreceptor physiology, and retinal degeneration. Here, we report the identification of a third RDGC protein variant and show that the three RDGC isoforms harbor different N-termini that determine solubility and subcellular targeting due to fatty acylation. Taken together, solubility and subcellular targeting of RDGC splice variants are determined by their N-termini.

The Phosphorylation State of the Drosophila TRP Channel Modulates the Frequency Response to Oscillating Light In Vivo

Drosophila photoreceptors respond to oscillating light of high frequency (∼100 Hz), while the detected maximal frequency is modulated by the light rearing conditions, thus enabling high sensitivity to light and high temporal resolution. However, the molecular basis for this adaptive process is unclear. Here, we report that dephosphorylation of the light-activated transient receptor potential (TRP) ion channel at S936 is a fast, graded, light-dependent, and Ca²⁺-dependent process that is partially modulated by the rhodopsin phosphatase retinal degeneration C (RDGC). Electroretinogram measurements of the frequency response to oscillating lights in vivo revealed that dark-reared flies expressing wild-type TRP exhibited a detection limit of oscillating light at relatively low frequencies, which was shifted to higher frequencies upon light adaptation. Strikingly, preventing phosphorylation of the S936-TRP site by alanine substitution in transgenic Drosophila (trp^S936A ) abolished the difference in frequency response between dark-adapted and light-adapted flies, resulting in high-frequency response also in dark-adapted flies. In contrast, inserting a phosphomimetic mutation by substituting the S936-TRP site to aspartic acid (trp^S936D ) set the frequency response of light-adapted flies to low frequencies typical of dark-adapted flies. Light-adapted rdgC mutant flies showed relatively high S936-TRP phosphorylation levels and light-dark phosphorylation dynamics. These findings suggest that RDGC is one but not the only phosphatase involved in pS936-TRP dephosphorylation. Together, this study indicates that TRP channel dephosphorylation is a regulatory process that affects the detection limit of oscillating light according to the light rearing condition, thus adjusting dynamic processing of visual information under varying light conditions.SIGNIFICANCE STATEMENTDrosophila photoreceptors exhibit high temporal resolution as manifested in frequency response to oscillating light of high frequency (≤∼100 Hz). Light rearing conditions modulate the maximal frequency detected by photoreceptors, thus enabling them to maintain high sensitivity to light and high temporal resolution. However, the precise mechanisms for this process are not fully understood. Here, we show by combination of biochemistry and in vivo electrophysiology that transient receptor potential (TRP) channel dephosphorylation at a specific site is a fast, light-activated and Ca²⁺-dependent regulatory process. TRP dephosphorylation affects the detection limit of oscillating light according to the adaptation state of the photoreceptor cells by shifting the detection limit to higher frequencies upon light adaptation. This novel mechanism thus adjusts dynamic processing of visual information under varying light conditions.

The LH/CG receptor activates canonical signaling pathway when expressed in Drosophila

G-protein coupled receptors (GPCRs) and their ligands provide precise tissue regulation and are therefore often restricted to specific animal phyla. For example, the gonadotropins and their receptors are crucial for vertebrate reproduction but absent from invertebrates. In mammals, LHR mainly couples to the PKA signaling pathway, and CREB is the major transcription factor of this pathway. Here we present the results of expressing elements of the human gonadotropin system in Drosophila. Specifically, we generated transgenic Drosophila expressing the human LH/CG receptor (denoted as LHR), a constitutively active form of LHR, and an hCG analog. We demonstrate activation-dependent signaling by LHR to direct Drosophila phenotypes including lethality and specific midline defects; these phenotypes were due to LHR activation of PKA/CREB pathway activity. That the LHR can act in an invertebrate demonstrates the conservation of factors required for GPCR function among phylogenetically distant organisms. This novel gonadotropin model may assist the identification of new modulators of mammalian fertility by exploiting the powerful genetic and pharmacological tools available in Drosophila.

Light-dependent phosphorylation of the Drosophila inactivation no afterpotential D (INAD) scaffolding protein at Thr170 and Ser174 by eye-specific protein kinase C

Drosophila inactivation no afterpotential D (INAD) is a PDZ domain-containing scaffolding protein that tethers components of the phototransduction cascade to form a supramolecular signaling complex. Here, we report the identification of eight INAD phosphorylation sites using a mass spectrometry approach. PDZ1, PDZ2, and PDZ4 each harbor one phosphorylation site, three phosphorylation sites are located in the linker region between PDZ1 and 2, one site is located between PDZ2 and PDZ3, and one site is located in the N-terminal region. Using a phosphospecific antibody, we found that INAD phosphorylated at Thr170/Ser174 was located within the rhabdomeres of the photoreceptor cells, suggesting that INAD becomes phosphorylated in this cellular compartment. INAD phosphorylation at Thr170/Ser174 depends on light, the phototransduction cascade, and on eye-Protein kinase C that is attached to INAD via one of its PDZ domains.

Post-Translational Modifications of TRP Channels

Transient receptor potential (TRP) channels constitute an ancient family of cation channels that have been found in many eukaryotic organisms from yeast to human. TRP channels exert a multitude of physiological functions ranging from Ca²⁺ homeostasis in the kidney to pain reception and vision. These channels are activated by a wide range of stimuli and undergo covalent post-translational modifications that affect and modulate their subcellular targeting, their biophysical properties, or channel gating. These modifications include N-linked glycosylation, protein phosphorylation, and covalent attachment of chemicals that reversibly bind to specific cysteine residues. The latter modification represents an unusual activation mechanism of ligand-gated ion channels that is in contrast to the lock-and-key paradigm of receptor activation by its agonists. In this review, we summarize the post-translational modifications identified on TRP channels and, when available, explain their physiological role.