Transmembrane Signaling Regulates the Remodeling of the Actin Cytoskeleton:Roles of PKC and Adducin

Members of the conventional protein kinase C (cPKC) family are activated by both DAG and Ca2+ and have been implicated in the regulation of the actin cytoskeleton. Drosophila contains two cPKCs, Pkc53E (Pkc1) and eye-PKC (Pkc2); mutants missing each PKC lead to retinal degeneration. While eye-PKC is critical for the visual signaling, the role of Pkc53E is not known. We identified a photoreceptor-specific isoform of Pkc53E and show Pkc53E-RNAi negatively impacts the actin cytoskeleton of rhabdomeres. Interestingly, Pkc53E-RNAi enhances the degeneration of norpAP24 photoreceptors, suggesting Pkc53E could be activated independently of NorpA/PLCβ4. We further demonstrate that in norpAP24 photoreceptors Plc21C can be activated by Gq, which is responsible for the activation of Pkc53E. We explored whether Pkc53E regulates adducin in Drosophila photoreceptors. Adducin cross-links the actin cytoskeleton to the spectrin network, which is blunted by PKC phosphorylation. Importantly, we observed that phosphorylation of adducin was greatly reduced in a null allele of pkc53E. Downregulation of hts that encodes Drosophila adducin, exerts a more severe effect than Pkc53E-RNAi to impact the actin cytoskeleton. In contrast, overexpression of a mCherry tagged Add2, one of the three Drosophila adducin isoforms, led to the apical expansion of rhabdomeres with overgrowth of the actin cytoskeleton. This phenotype is likely caused by the dominant-negative activity of the tagged Add2 as it also was observed in α-spectrin-RNAi or β-spectrin-RNAi. Interestingly, downregulation of Pkc53E does not suppress the expansion of rhabdomeres during development, but negatively affects the appearance of rhabdomeres in adult photoreceptors. We conclude that Drosophila adducin has two distinct functions: in pupal photoreceptors, it regulates rhabdomere morphogenesis, which is independent of Pkc53E. In adult photoreceptors, it promotes the maintenance of the actin cytoskeleton, which is regulated by Pkc53E in response to the light-induced activation of the PLCβ activity.

A Truncated NRIP1 Mutant Amplifies Microsatellite Instability of Colorectal Cancer by Regulating MSH2/MSH6 Expression, and Is a Prognostic Marker of Stage III Tumors

Microsatellite instability (MSI) is related to the alteration of mismatch repair (MMR) genes and plays a key role in colorectal cancer (CRC) pathogenesis. We previously reported that the transcription factor Nuclear Receptor Interacting Protein 1 (NRIP1) is involved in sporadic intestinal tumorigenesis. The aim of this study was to decipher its role in MSI CRC. By using different mouse models and engineered cell lines, we demonstrated that NRIP1 increased MSH2 and MSH6 MMR gene transcription and mRNA/protein levels. In human CRC cells, NRIP1 expression was associated with decreased MSI and the hypermutator phenotype, and with resistance to chemotherapy drugs. Using a cohort of 194 CRC patients, we detected in 22% of the cases a MSI-induced frameshift mutation in the NRIP1 coding sequence. This genetic alteration generates a truncated protein with a dominant negative activity that increased human CRC cell proliferation and impaired the regulation of MSH2 and MSH6 gene expression. Moreover, the NRIP1 mutant correlated with a decreased overall survival of patients with advanced CRC, especially when MLH1-deficient. By decreasing the expression of MSH2 and MSH6 gene expression, the NRIP1 variant may amplify MLH1-dependent CRC progression and behave as a new prognostic marker of advanced MSI CRC.

Any alteration in PPAR genomic sequence, splicing pattern, or PTM is likely to cause major alterations in its function. In personalized medicine, such data becomes more significant in gene therapy design

Proliferator-activated gamma receptor (PPAR) is a paradigm of how numerous signaling networks converge on a single component whose activity is controlled by a sophisticated system of alternative splicing, post-translation alterations and coactivator and repressor interactions. The prevalence of several regulatory layers in PPAR signaling explains its time-and tissue-specific responses, suggesting that any change in the genomic sequence, pattern of splicing, or PTM is likely to produce large changes in its activity. Because the variety of PPARG genetic variants is so wide, from common SNPs to rare nucleotide variants or mutations, understanding if and how each variation impacts PPAR signaling is crucial to evaluating disease risk. Moreover, in the era of personalized medicine, such data has become more relevant in predicting the efficacy of TZD-based therapies, enabling the building of more robust decision trees to treat metabolic illnesses.Alternative splicing and post-translation changes were previously ignored PPAR biology components, according to several independent evidence lines. In this regard, the discovery of naturally occurring PPARG isoforms expressed in adipose tissue and having dominant-negative activity towards canonical receptors underlines the importance of paying special attention when designing a PPAR study, particularly for cells or tissues where it is highly expressed. Indeed, the absence of rigorous experimental design to discriminate between functional and non-functional isoforms is highly likely to result in misinterpretation of findings. It is harder to foresee or understand the effect of PPAR-modulating drugs in the absence of data on the expression of dominant-negative isoforms or their ratio vs. canonical isoforms, making any choice based on these results particularly harmful. Similarly, multiple independent experiments have demonstrated that endogenous (or exogenous) factors have a considerable influence on variations in PPAR protein and therefore activity. As a result, it is crucial to consider all these aspects in investigating PPAR, especially in a disease setting. Considering the various interwoven regulatory levels of PPAR activity, new insights into PPAR signaling in human pathophysiology might lead to novel ways to improve existing therapies for PPAR-related metabolic diseases.

Novel Cell-Penetrating Peptides Derived From Scaffold-Attachment- Factor A Inhibits Cancer Cell Proliferation and Survival

Scaffold-attachment-factor A (SAFA) has important roles in many normal and pathologic cellular processes but the scope of its function in cancer cells is unknown. Here, we report dominant-negative activity of novel peptides derived from the SAP and RGG-domains of SAFA and their effects on proliferation, survival and the epigenetic landscape in a range of cancer cell types. The RGG-derived peptide dysregulates SAFA binding and regulation of alternatively spliced targets and decreases levels of key spliceosome proteins in a cell-type specific manner. In contrast, the SAP-derived peptide reduces active histone marks, promotes chromatin compaction, and activates the DNA damage response and cell death in a subset of cancer cell types. Our findings reveal an unprecedented function of SAFA-derived peptides in regulating diverse SAFA molecular functions as a tumor suppressive mechanism and demonstrate the potential therapeutic utility of SAFA-peptides in a wide range of cancer cells.

The most common RNF43 mutant G659Vfs*41 is fully functional in inhibiting Wnt signaling and unlikely to play a role in tumorigenesis

RNF43 is an E3 ligase that inhibits Wnt signaling by ubiquitinating Wnt receptors for degradation. It is mutated in various cancer types with the most recurrent mutation being the frameshift G659Vfs*41 with frequencies of ~5–8% in colon, stomach and endometrial cancers. This mutation, a deletion of G in a 7-G repeat, has been assumed to encode an inactive enzyme that would lead to increased Wnt signaling and drive tumorigenesis, yet no functional characterization has been reported. We analyzed the distribution of G659Vfs*41 and its association with other cancer gene mutations, and found that the mutation occurred nearly exclusively in tumors with low expression of the DNA mismatch repair gene MLH1. Mutant RNF43-G659Vfs*41 was no different from wild type RNF43 in expression, stability, localization, R-spondin binding, and inhibition of Wnt signaling. No dominant negative activity of the mutant was observed. Colon tumors with RNF43-G659Vfs*41 had low Wnt/β-catenin signaling and were frequently mutated in BRAF. A colon cancer cell line with RNF43-G659Vfs*41 and BRAF-V600E mutations was sensitive to activation of Wnt/β-catenin signaling. These findings suggest that the frequent occurrence of RNF43-G659Vfs*41 may result from error-prone replication of the 7-G repeat in MLH1-deficient tumors and that the mutation itself does not inactivate enzyme.

A novel human aquaporin-4 splice variant exhibits a dominant-negative activity: a new mechanism to regulate water permeability

Two major isoforms of aquaporin-4 (AQP4) have been described in human tissue. Here we report the identification and functional analysis of an alternatively spliced transcript of human AQP4, AQP4-Δ4, that lacks exon 4. In transfected cells AQP4-Δ4 is mainly retained in the endoplasmic reticulum and shows no water transport properties. When AQP4-Δ4 is transfected into cells stably expressing functional AQP4, the surface expression of the full-length protein is reduced. Furthermore, the water transport activity of the cotransfectants is diminished in comparison to transfectants expressing only AQP4. The observed down-regulation of both the expression and water channel activity of AQP4 is likely to originate from a dominant-negative effect caused by heterodimerization between AQP4 and AQP4-Δ4, which was detected in coimmunoprecipitation studies. In skeletal muscles, AQP4-Δ4 mRNA expression inversely correlates with the level of AQP4 protein and is physiologically associated with different types of skeletal muscles. The expression of AQP4-Δ4 may represent a new regulatory mechanism through which the cell-surface expression and therefore the activity of AQP4 can be physiologically modulated.

Familial Glucocorticoid Resistance Caused by a Novel Frameshift Glucocorticoid Receptor Mutation

ABSTRACT Context Familial glucocorticoid resistance is a rare condition with a typical presentation of women with hirsutism and hypertension, with or without hypokalemia. Objective The aim was to determine the cause of apparent glucocorticoid resistance in a young woman. Patients and Methods We studied a family with a novel glucocorticoid receptor (GR) mutation and a surprisingly mild phenotype. Their discovery resulted from serendipitous measurement of serum cortisol with little biochemical or clinical evidence for either hyperandrogenism or mineralocorticoid excess. Results The causative mutation was identified as a frameshift mutation in exon 6. Transformed peripheral blood lymphocytes were generated to analyze GR expression in vitro. Carriers of the mutation had less full-length GR, but the predicted mutant GR protein was not detected. However, this does not exclude expression in vivo, and so the mutant GR (D612GR) was expressed in vitro. Simple reporter gene assays suggested that Δ612GR has dominant negative activity. Δ612GR was not subject to ligand-dependent Ser211 phosphorylation or to ligand-dependent degradation. A fluorophore-tagged construct showed that Δ612GR did not translocate to the nucleus in response to ligand and retarded translocation of the wild-type GR. These data suggest that Δ612GR is not capable of binding ligand and exerts dominant negative activity through heterodimerization with wild-type GR. Conclusion Therefore, we describe a novel, naturally occurring GR mutation that results in familial glucocorticoid resistance. The mutant GR protein, if expressed in vivo, is predicted to exert dominant negative activity by impairing wild-type GR nuclear translocation.

Familial Platelet Disorder and Predisposition to Myeloid Leukemia (FPD/AML) in the Absence of RUNX1 Mutations- A Report of Three Families.

Abstract 4689 Familial Platelet Disorder with Predisposition to Acute Myeloid Leukemia (FPD/AML) has been attributed to abnormalities of the RUNX1 gene, located at human chromosome 21q22. The protein product of this gene forms a critical component of the core binding factor (CBF) complex, which is required for normal hematopoiesis. Truncating mutations and deletions of RUNX1 (particularly in exons 3, 4, and 5, encoding the DNA binding and CBF-beta interaction domains) that result in haploinsufficiency or dominant-negative activity have been identified as the genetic mechanisms underlying FPD/AML. Clinically, these patients present with mild to moderate thrombocytopenia, platelet dysfunction, bleeding, and about a 35% risk of developing AML. We report on three unrelated families who share histories of autosomal dominant thrombocytopenia over two to five generations, bleeding, platelet aggregation defects, and development of myelodysplastic syndrome, AML or intriguingly, chronic myeloid leukemia (CML). Detailed genetic interrogation of the RUNX1 locus in the proband from each family, including complete sequencing of all 8 exons, flanking 50 base pair regions, and P1 and P2 promoters, as well as gene dosage studies, failed to demonstrate a causative lesion in the RUNX1 gene. Our findings strongly suggest that genetic loci other than RUNX1 are involved in some cases of autosomal dominant thrombocytopenia with a predisposition to both AML and CML. Identification of the disease-causing genes in these families will allow for prospective testing of family members, appropriate surveillance and early intervention in affected individuals, and potentially new molecular insights into leukemogenesis. We have broadened the genotype-phenotype correlation in FPD/AML beyond the RUNX1 gene and suggest that this syndrome may be more genetically heterogeneous than initially described. Disclosures: No relevant conflicts of interest to declare.