Arrestins as signaling molecules involved in apoptotic pathways: a real eye opener

Global Identification of ANTH Genes Involved in Rice Pollen Germination and Functional Characterization of a Key Member, OsANTH3

Pollen in angiosperms plays a critical role in double fertilization by germinating and elongating pollen tubes rapidly in one direction to deliver sperm. In this process, the secretory vesicles deliver cell wall and plasma membrane materials, and excessive materials are sequestered via endocytosis. However, endocytosis in plants is poorly understood. AP180 N-terminal homology (ANTH) domain-containing proteins function as adaptive regulators for clathrin-mediated endocytosis in eukaryotic systems. Here, we identified 17 ANTH domain-containing proteins from rice based on a genome-wide investigation. Motif and phylogenomic analyses revealed seven asparagine-proline-phenylalanine (NPF)-rich and 10 NPF-less subgroups of these proteins, as well as various clathrin-mediated endocytosis-related motifs in their C-terminals. To investigate their roles in pollen germination, we performed meta-expression analysis of all genes encoding ANTH domain-containing proteins in Oryza sativa (OsANTH genes) in anatomical samples, including pollen, and identified five mature pollen-preferred OsANTH genes. The subcellular localization of four OsANTH proteins that were preferentially expressed in mature pollen can be consistent with their role in endocytosis in the plasma membrane. Of them, OsANTH3 represented the highest expression in mature pollen. Functional characterization of OsANTH3 using T-DNA insertional knockout and gene-edited mutants revealed that a mutation in OsANTH3 decreased seed fertility by reducing the pollen germination percentage in rice. Thus, our study suggests OsANTH3-mediated endocytosis is important for rice pollen germination.

betaARKct: a therapeutic approach for improved adrenergic signaling and function in heart disease

One of the most powerful regulators of cardiovascular function is catecholamine-stimulated adrenergic receptor (AR) signaling. The failing heart is characterized by desensitization and impaired beta-AR responsiveness as a result of upregulated G protein-coupled receptor kinase-2 (GRK2) present in injured myocardium. Deterioration of cardiac function is progressively enhanced by chronic adrenergic over-stimulation due to increased levels of circulating catecholamines. Increased GRK2 activity contributes to this pathological cycle of over-stimulation but lowered responsiveness. Over the past two decades the GRK2 inhibitory peptide betaARKct has been identified as a potential therapy that is able to break this vicious cycle of self-perpetuating deregulation of the beta-AR system and subsequent myocardial malfunction, thus halting development of cardiac failure. The betaARKct has been shown to interfere with GRK2 binding to the betagamma subunits of the heterotrimeric G protein, therefore inhibiting its recruitment to the plasma membrane that normally leads to phosphorylation and internalization of the receptor. In this article we summarize the current data on the therapeutic effects of betaARKct in cardiovascular disease and report on recent and ongoing studies that may pave the way for this peptide towards therapeutic application in heart failure and other states of cardiovascular disease.

Gene expression profiling of light-induced retinal degeneration in phototransduction gene knockout mice

Exposure to light can induce photoreceptor cell death and exacerbate retinal degeneration. In this study, mice with genetic knockout of several genes, including rhodopsin kinase (Rhok-/-), arrestin (Sag-/-), transducin (Gnat1-/-), c-Fos (c-Fos-/-) and arrestin/transducin (Sag-/-/Gnat1-/-), were examined. We measured the expression levels of thousands of genes in order to investigate their roles in phototransduction signaling in light-induced retinal degeneration using DNA microarray technology and then further explored the gene network using pathway analysis tools. Several cascades of gene components were induced or inhibited as a result of corresponding gene knockout under specific light conditions. Transducin deletion blocked the apoptotic signaling induced by exposure to low light conditions, and it did not require c-Fos/AP-1. Deletion of c-Fos blocked the apoptotic signaling induced by exposure to high intensity light. In the present study, we identified many gene transcripts that are essential for the initiation of light-induced rod degeneration and proposed several important networks that are involved in pro- and anti-apoptotic signaling. We also demonstrated the different cascades of gene components that participate in apoptotic signaling under specific light conditions.

Characterization of phototransduction gene knockouts revealed important signaling networks in the light-induced retinal degeneration

Understanding the molecular pathways mediating neuronal function in retinas can be greatly facilitated by the identification of genes regulated in the retinas of different mutants under various light conditions. We attempted to conduct a gene chip analysis study on the genes regulated during rhodopsin kinase (Rhok^−/−) and arrestin (Sag^−/−) knockout and double knockouts in mice retina. Hence, mice were exposed to constant illumination of 450 lux or 6,000 lux on dilated pupils for indicated periods. The retinas were removed after the exposure and processed for microarray analysis. Double knockout was associated with immense changes in gene expression regulating a number of apoptosis inducing transcription factors. Subsequently, network analysis revealed that during early exposure the transcription factors, p53, c-MYC, c-FOS, JUN, and, in late phase, NFκB, appeared to be essential for the initiation of light-induced retinal rod loss, and some other classical pro- and antipoptotic genes appeared to be significantly important as well.

Rhodopsin kinase activity modulates the amplitude of the visual response in Drosophila

A feature shared between Drosophila rhodopsin and nearly all other G protein-coupled receptors is agonist-dependent protein phosphorylation. Despite extensive analyses of Drosophila phototransduction, the identity and function of the rhodopsin kinase (RK) have been elusive. Here, we provide evidence that G protein-coupled receptor kinase 1 (GPRK1), which is most similar to the beta-adrenergic receptor kinases, G protein-coupled receptor kinase 2 (GRK2) and GRK3, is the fly RK. We show that GPRK1 is enriched in photoreceptor cells, associates with the major Drosophila rhodopsin, Rh1, and phosphorylates the receptor. As is the case with mammalian GRK2 and GRK3, Drosophila GPRK1 includes a C-terminal pleckstrin homology domain, which binds to phosphoinositides and the Gbetagamma subunit. To address the role of GPRK1, we generated transgenic flies that expressed higher and lower levels of RK activity. Those flies with depressed levels of RK activity displayed a light response with a much larger amplitude than WT. Conversely, the amplitude of the light response was greatly suppressed in transgenic flies expressing abnormally high levels of RK activity. These data point to an evolutionarily conserved role for GPRK1 in modulating the amplitude of the visual response.

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin-coated vesicles that plays a role in clathrin-mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1(-/-)). HIP1(-/-) mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1-containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose-dependent defect in clathrin-mediated internalization of GluR1-containing AMPA receptors was observed in neurons from HIP1(-/-) mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin-mediated endocytosis.