Hipk is an essential protein that promotes Notch signal transduction in the Drosophila eye by inhibition of the global co-repressor Groucho

Functions of SRPK, CLK and DYRK kinases in stem cells, development, and human developmental disorders

Human developmental disorders encompass a wide range of debilitating physical conditions and intellectual disabilities. Perturbation of protein kinase signalling underlies the development of some of these disorders. For example, disrupted SRPK signalling is associated with intellectual disabilities, and the gene dosage of DYRKs can dictate the pathology of disorders including Down's syndrome. Here, we review the emerging roles of the CMGC kinase families SRPK, CLK, DYRK, and sub-family HIPK during embryonic development and in developmental disorders. In particular, SRPK, CLK, and DYRK kinase families have key roles in developmental signalling and stem cell regulation, and can co-ordinate neuronal development and function. Genetic studies in model organisms reveal critical phenotypes including embryonic lethality, sterility, musculoskeletal errors, and most notably, altered neurological behaviours arising from defects of the neuroectoderm and altered neuronal signalling. Further unpicking the mechanisms of specific kinases using human stem cell models of neuronal differentiation and function will improve our understanding of human developmental disorders and may provide avenues for therapeutic strategies.

HIPK2 in the physiology of nervous system and its implications in neurological disorders

HIPK2 is an evolutionary conserved serine/threonine kinase with multifunctional roles in stress response, embryonic development and pathological conditions, such as cancer and fibrosis. The heterogeneity of its interactors and targets makes HIPK2 activity strongly dependent on the cellular context, and allows it to modulate multiple signaling pathways, ultimately regulating cell fate and proliferation. HIPK2 is highly expressed in the central and peripheral nervous systems, and its genetic ablation causes neurological defects in mice. Moreover, HIPK2 is involved in processes, such as endoplasmic reticulum stress response and protein aggregate accumulation, and pathways, including TGF-β and BMP signaling, that are crucial in the pathogenesis of neurological disorders. Here, we review the data about the role of HIPK2 in neuronal development, survival, and homeostasis, highlighting the implications in the pathogenesis of neurological disorders, and pointing out HIPK2 potentiality as therapeutic target and diagnostic or prognostic marker.

HIPK2 as a Novel Regulator of Fibrosis

Fibrosis is an unmet medical problem due to a lack of evident biomarkers to help develop efficient targeted therapies. Fibrosis can affect almost every organ and eventually induce organ failure. Homeodomain-interacting protein kinase 2 (HIPK2) is a protein kinase that controls several molecular pathways involved in cell death and development and it has been extensively studied, mainly in the cancer biology field. Recently, a role for HIPK2 has been highlighted in tissue fibrosis. Thus, HIPK2 regulates several pro-fibrotic pathways such as Wnt/β-catenin, TGF-β and Notch involved in renal, pulmonary, liver and cardiac fibrosis. These findings suggest a wider role for HIPK2 in tissue physiopathology and highlight HIPK2 as a promising target for therapeutic purposes in fibrosis. Here, we will summarize the recent studies showing the involvement of HIPK2 as a novel regulator of fibrosis.

Transcriptome analysis of FOXO-dependent hypoxia gene expression identifies Hipk as a regulator of low oxygen tolerance in Drosophila

When exposed to low oxygen or hypoxia, animals must alter their metabolism and physiology to ensure proper cell-, tissue-, and whole-body level adaptations to their hypoxic environment. These alterations often involve changes in gene expression. While extensive work has emphasized the importance of the HIF-1 alpha transcription factor on controlling hypoxia gene expression, less is known about other transcriptional mechanisms. We previously identified the transcription factor FOXO as a regulator of hypoxia tolerance in Drosophila larvae and adults. Here, we use an RNA-sequencing approach to identify FOXO-dependent changes in gene expression that are associated with these tolerance effects. We found that hypoxia altered the expression of over 2,000 genes and that ∼40% of these gene expression changes required FOXO. We discovered that hypoxia exposure led to a FOXO-dependent increase in genes involved in cell signaling, such as kinases, GTPase regulators, and regulators of the Hippo/Yorkie pathway. Among these, we identified homeodomain-interacting protein kinase as being required for hypoxia survival. We also found that hypoxia suppresses the expression of genes involved in ribosome synthesis and egg production, and we showed that hypoxia suppresses tRNA synthesis and mRNA translation and reduces female fecundity. Among the downregulated genes, we discovered that FOXO was required for the suppression of many ribosomal protein genes and genes involved in oxidative phosphorylation, pointing to a role for FOXO in limiting energetically costly processes such as protein synthesis and mitochondrial activity upon hypoxic stress. This work uncovers a widespread role for FOXO in mediating hypoxia changes in gene expression.

Revealing phosphorylation regulatory networks during embryogenesis of honey bee worker and drone (Apis mellifera)

Protein phosphorylation is known to regulate a comprehensive scenario of critical cellular processes. However, phosphorylation-mediated regulatory networks in honey bee embryogenesis are mainly unknown. We identified 6342 phosphosites from 2438 phosphoproteins and predicted 168 kinases in the honey bee embryo. Generally, the worker and drone develop similar phosphoproteome architectures and major phosphorylation events during embryogenesis. In 24 h embryos, protein kinases A play vital roles in regulating cell proliferation and blastoderm formation. At 48–72 h, kinase subfamily dual-specificity tyrosine-regulated kinase, cyclin-dependent kinase (CDK), and induced pathways related to protein synthesis and morphogenesis suggest the centrality to enhance the germ layer development, organogenesis, and dorsal closure. Notably, workers and drones formulated distinct phosphoproteome signatures. For 24 h embryos, the highly phosphorylated serine/threonine-protein kinase minibrain, microtubule-associated serine/threonine-protein kinase 2 (MAST2), and phosphorylation of mitogen-activated protein kinase 3 (MAPK3) at Thr564 in workers, are likely to regulate the late onset of cell proliferation; in contrast, drone embryos enhanced the expression of CDK12, MAPK3, and MAST2 to promote the massive synthesis of proteins and cytoskeleton. In 48 h, the induced serine/threonine-protein kinase and CDK12 in worker embryos signify their roles in the construction of embryonic tissues and organs; however, the highly activated kinases CDK1, raf homolog serine/threonine-protein kinase, and MAST2 in drone embryos may drive the large-scale establishment of tissues and organs. In 72 h, the activated pathways and kinases associated with cell growth and tissue differentiation in worker embryos may promote the configuration of rudimentary organs. However, kinases implicated in cytoskeleton organization in drone embryos may drive the blastokinesis and dorsal closure. Our hitherto most comprehensive phosphoproteome offers a valuable resource for signaling research on phosphorylation dynamics in honey bee embryos.

Expression of human HIPKs in Drosophila demonstrates their shared and unique functions in a developmental model

Homeodomain-interacting protein kinases (HIPKs) are a family of four conserved proteins essential for vertebrate development, as demonstrated by defects in the eye, brain, and skeleton that culminate in embryonic lethality when multiple HIPKs are lost in mice. While HIPKs are essential for development, functional redundancy between the four vertebrate HIPK paralogues has made it difficult to compare their respective functions. Because understanding the unique and shared functions of these essential proteins could directly benefit the fields of biology and medicine, we addressed the gap in knowledge of the four vertebrate HIPK paralogues by studying them in the fruit fly Drosophila melanogaster, where reduced genetic redundancy simplifies our functional assessment. The single hipk present in the fly allowed us to perform rescue experiments with human HIPK genes that provide new insight into their individual functions not easily assessed in vertebrate models. Furthermore, the abundance of genetic tools and established methods for monitoring specific developmental pathways and gross morphological changes in the fly allowed for functional comparisons in endogenous contexts. We first performed rescue experiments to demonstrate the extent to which each of the human HIPKs can functionally replace Drosophila Hipk for survival and morphological development. We then showed the ability of each human HIPK to modulate Armadillo/β-catenin levels, JAK/STAT activity, proliferation, growth, and death, each of which have previously been described for Hipks, but never all together in comparable tissue contexts. Finally, we characterized novel developmental phenotypes induced by human HIPKs to gain insight to their unique functions. Together, these experiments provide the first direct comparison of all four vertebrate HIPKs to determine their roles in a developmental context.

Metabolic reprogramming in cancer: mechanistic insights from Drosophila

Cancer cells constantly reprogram their metabolism as the disease progresses. However, our understanding of the metabolic complexity of cancer remains incomplete. Extensive research in the fruit fly Drosophila has established numerous tumor models ranging from hyperplasia to neoplasia. These fly tumor models exhibit a broad range of metabolic profiles and varying nutrient sensitivity. Genetic studies show that fly tumors can use various alternative strategies, such as feedback circuits and nutrient-sensing machinery, to acquire and consolidate distinct metabolic profiles. These studies not only provide fresh insights into the causes and functional relevance of metabolic reprogramming but also identify metabolic vulnerabilities as potential targets for cancer therapy. Here, we review the conceptual advances in cancer metabolism derived from comparing and contrasting the metabolic profiles of fly tumor models, with a particular focus on the Warburg effect, mitochondrial metabolism, and the links between diet and cancer.

Summary: Recent research in fruit flies has demonstrated that tumors rewire their metabolism by using diverse strategies that involve feedback regulation, nutrient sensing, intercellular or even inter-organ interactions, yielding new molecules as potential cancer markers or drug targets.

Drosophila Homeodomain-Interacting Protein Kinase (Hipk) Phosphorylates the Hippo/Warts Signalling Effector Yorkie

Developmental growth and patterning are regulated by an interconnected signalling network of several pathways. In Drosophila, the Warts (Wts) kinase, a component of the Hippo signalling pathway, plays an essential role in regulating transcription and growth by phosphorylating its substrate Yorkie (Yki). The phosphorylation of Yki critically influences its localisation and activity as a transcriptional coactivator. In this study, we identified the homeodomain-interacting protein kinase (Hipk) as another kinase that phosphorylates Yki and mapped several sites of Yki phosphorylated by Hipk, using in vitro analysis: Ser168, Ser169/Ser172 and Ser255. These sites might provide auxiliary input for Yki regulation in vivo, as transgenic flies with mutations in these show prominent phenotypes; Hipk, therefore, represents an additional upstream regulator of Yki that works in concert with Wts.

A novel isoform of Homeodomain-interacting protein kinase-2 promotes YAP/TEAD transcriptional activity in NSCLC cells

Homeodomain-interacting protein kinase-2 (HIPK2) can either promote or inhibit transcription depending on cellular context. In this study, we show that a new HIPK2 isoform increases TEAD reporter activity in NSCLC cells. We detected HIPK2 copy number gain in 5/6 (83.3%) NSCLC cell lines. In NSCLC patients with high HIPK2 mRNA expression in the Human Protein Atlas, the five-year survival rate is significantly lower than in patients with low expression (38% vs 47%; p = 0.047). We also found that 70/78 (89.7%) of NSCLC tissues have moderate to strong expression of the N-terminal HIPK2 protein. We detected and cloned a novel HIPK2 isoform 3 and found that its forced overexpression promotes TEAD reporter activity in NSCLC cells. Expressing HIPK2 isoform 3_K228A kinase-dead plasmid failed to increase TEAD reporter activity in NSCLC cells. Next, we showed that two siRNAs targeting HIPK2 decreased HIPK2 isoform 3 and YAP protein levels in NSCLC cells. Degradation of the YAP protein was accelerated after HIPK2 knockdown in NSCLC cells. Inhibition of HIPK2 isoform 3 decreased the mRNA expression of YAP downstream gene CTGF. The specific HIPK2 kinase inhibitor TBID decreased TEAD reporter activity, reduced cancer side populations, and inhibited tumorsphere formation of NSCLC cells. In summary, this study indicates that HIPK2 isoform 3, the main HIPK2 isoform expressed in NSCLC, promotes YAP/TEAD transcriptional activity in NSCLC cells. Our results suggest that HIPK2 isoform 3 may be a potential therapeutic target for NSCLC.

Chromatin Targeting of HIPK2 Leads to Acetylation-Dependent Chromatin Decondensation

The protein kinase homeodomain-interacting protein kinase 2 (HIPK2) plays an important role in development and in the response to external cues. The kinase associates with an exceptionally large number of different transcription factors and chromatin regulatory proteins to direct distinct gene expression programs. In order to investigate the function of HIPK2 for chromatin compaction, HIPK2 was fused to the DNA-binding domains of Gal4 or LacI, thus allowing its specific targeting to binding sites for these transcription factors that were integrated in specific chromosome loci. Tethering of HIPK2 resulted in strong decompaction of euchromatic and heterochromatic areas. HIPK2-mediated heterochromatin decondensation started already 4 h after its chromatin association and required the functionality of its SUMO-interacting motif. This process was paralleled by disappearance of the repressive H3K27me3 chromatin mark, recruitment of the acetyltransferases CBP and p300 and increased histone acetylation at H3K18 and H4K5. HIPK2-mediated chromatin decompaction was strongly inhibited in the presence of a CBP/p300 inhibitor and completely blocked by the BET inhibitor JQ1, consistent with a causative role of acetylations for this process. Chromatin tethering of HIPK2 had only a minor effect on basal transcription, while it strongly boosted estrogen-triggered gene expression by acting as a transcriptional cofactor.

Homeodomain-interacting protein kinase (Hipk) plays roles in nervous system and muscle structure and function

Homeodomain-interacting protein kinases (Hipks) have been previously associated with cell proliferation and cancer, however, their effects in the nervous system are less well understood. We have used Drosophila melanogaster to evaluate the effects of altered Hipk expression on the nervous system and muscle. Using genetic manipulation of Hipk expression we demonstrate that knockdown and over-expression of Hipk produces early adult lethality, possibly due to the effects on the nervous system and muscle involvement. We find that optimal levels of Hipk are critical for the function of dopaminergic neurons and glial cells in the nervous system, as well as muscle. Furthermore, manipulation of Hipk affects the structure of the larval neuromuscular junction (NMJ) by promoting its growth. Hipk regulates the phosphorylation of the synapse-associated cytoskeletal protein Hu-li tai shao (Hts; adducin in mammals) and modulates the expression of two important protein kinases, Calcium-calmodulin protein kinase II (CaMKII) and Partitioning-defective 1 (PAR-1), all of which may alter neuromuscular structure/function and influence lethality. Hipk also modifies the levels of an important nuclear protein, TBPH, the fly orthologue of TAR DNA-binding protein 43 (TDP-43), which may have relevance for understanding motor neuron diseases.

The nutrient sensor OGT regulates Hipk stability and tumorigenic-like activities in Drosophila

Environmental cues such as nutrients alter cellular behaviors by acting on a wide array of molecular sensors inside cells. Of emerging interest is the link observed between effects of dietary sugars on cancer proliferation. Here, we identify the requirements of hexosamine biosynthetic pathway (HBP) and O-GlcNAc transferase (OGT) for Drosophila homeodomain-interacting protein kinase (Hipk)-induced growth abnormalities in response to a high sugar diet. On a normal diet, OGT is both necessary and sufficient for inducing Hipk-mediated tumor-like growth. We further show that OGT maintains Hipk protein stability by blocking its proteasomal degradation and that Hipk is O-GlcNAcylated by OGT. In mammalian cells, human HIPK2 proteins accumulate posttranscriptionally upon OGT overexpression. Mass spectrometry analyses reveal that HIPK2 is at least O-GlcNAc modified at S852, T1009, and S1147 residues. Mutations of these residues reduce HIPK2 O-GlcNAcylation and stability. Together, our data demonstrate a conserved role of OGT in positively regulating the protein stability of HIPKs (fly Hipk and human HIPK2), which likely permits the nutritional responsiveness of HIPKs.

Hipk is required for JAK/STAT activity during development and tumorigenesis

Drosophila has been instrumental as a model system in studying signal transduction and revealing molecular functions in development and human diseases. A point mutation in the Drosophila Janus kinase JAK (called hop) causes constitutive activation of the JAK/STAT pathway. We provide robust genetic evidence that the Homeodomain interacting protein kinase (Hipk) is required for endogenous JAK/STAT activity. Overexpression of Hipk can phenocopy the effects of overactive JAK/STAT mutations and lead to melanized tumors, and loss of Hipk can suppress the effects of hyperactive JAK/STAT. Further, the loss of the pathway effector Stat92E can suppress Hipk induced overgrowth. Interaction studies show that Hipk can physically interact with Stat92E and regulate Stat92E subcellular localization. Together our results show that Hipk is a novel factor required for effective JAK/STAT signaling.

The crystal structure of the protein kinase HIPK2 reveals a unique architecture of its CMGC-insert region

The homeodomain-interacting protein kinase (HIPK) family is comprised of four nuclear protein kinases, HIPK1-4. HIPK proteins phosphorylate a diverse range of transcription factors involved in cell proliferation, differentiation, and apoptosis. HIPK2, thus far the best-characterized member of this largely understudied family of protein kinases, plays a role in the activation of p53 in response to DNA damage. Despite this tumor-suppressor function, HIPK2 is also found overexpressed in several cancers, and its hyperactivation causes chronic fibrosis. There are currently no structures of HIPK2 or of any other HIPK kinase. Here, we report the crystal structure of HIPK2's kinase domain bound to CX-4945, a casein kinase 2α (CK2α) inhibitor currently in clinical trials against several cancers. The structure, determined at 2.2 Å resolution, revealed that CX-4945 engages the HIPK2 active site in a hybrid binding mode between that seen in structures of CK2α and Pim1 kinases. The HIPK2 kinase domain crystallized in the active conformation, which was stabilized by phosphorylation of the activation loop. We noted that the overall kinase domain fold of HIPK2 closely resembles that of evolutionarily related dual-specificity tyrosine-regulated kinases (DYRKs). Most significant structural differences between HIPK2 and DYRKs included an absence of the regulatory N-terminal domain and a unique conformation of the CMGC-insert region and of a newly defined insert segment in the αC-β4 loop. This first crystal structure of HIPK2 paves the way for characterizing the understudied members of the HIPK family and for developing HIPK2-directed therapies for managing cancer and fibrosis.

Integrative genomic analysis reveals novel regulatory mechanisms of eyeless during Drosophila eye development

Eyeless (ey) is one of the most critical transcription factors for initiating the entire eye development in Drosophila. However, the molecular mechanisms through which Ey regulates target genes and pathways have not been characterized at the genomic level. Using ChIP-Seq, we generated an endogenous Ey-binding profile in Drosophila developing eyes. We found that Ey binding occurred more frequently at promoter compared to non-promoter regions. Ey promoter binding was correlated with the active transcription of genes involved in development and transcription regulation. An integrative analysis revealed that Ey directly regulated a broad and highly connected genetic network, including many essential patterning pathways, and known and novel eye genes. Interestingly, we observed that Ey could target multiple components of the same pathway, which might enhance its control of these pathways during eye development. In addition to protein-coding genes, we discovered Ey also targeted non-coding RNAs, which represents a new regulatory mechanism employed by Ey. These findings suggest that Ey could use multiple molecular mechanisms to regulate target gene expression and pathway function, which might enable Ey to exhibit a greater flexibility in controlling different processes during eye development.

A positive feedback loop between Myc and aerobic glycolysis sustains tumor growth in a Drosophila tumor model

Cancer cells usually exhibit aberrant cell signaling and metabolic reprogramming. However, mechanisms of crosstalk between these processes remain elusive. Here, we show that in an in vivo tumor model expressing oncogenic Drosophila Homeodomain-interacting protein kinase (Hipk), tumor cells display elevated aerobic glycolysis. Mechanistically, elevated Hipk drives transcriptional upregulation of Drosophila Myc (dMyc; MYC in vertebrates) likely through convergence of multiple perturbed signaling cascades. dMyc induces robust expression of pfk2 (encoding 6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase; PFKFB in vertebrates) among other glycolytic genes. Pfk2 catalyzes the synthesis of fructose-2,6-bisphosphate, which acts as a potent allosteric activator of Phosphofructokinase (Pfk) and thus stimulates glycolysis. Pfk2 and Pfk in turn are required to sustain dMyc protein accumulation post-transcriptionally, establishing a positive feedback loop. Disruption of the loop abrogates tumorous growth. Together, our study demonstrates a reciprocal stimulation of Myc and aerobic glycolysis and identifies the Pfk2-Pfk governed committed step of glycolysis as a metabolic vulnerability during tumorigenesis.

Cancer arises when cells in the body divide and grow excessively. These cells will often also take up more glucose than normal cells and break it down into another chemical known as lactate faster. This change to the chemical reactions happening within the cell, also called a metabolic change, is required to help produce the extra DNA, proteins and fatty molecules that it needs to grow.

Elevated levels of certain proteins can trigger the changes that lead to the growth of tumors. MYC (called dMyc in fruit flies) is one of these proteins. It controls cell division and increases the production of enzymes that break down glucose. Hipk is another protein that can induce tumor growth in fruit flies, but how it does so was unknown.

Here, Wong et al. show that high levels of Hipk boost glucose metabolism and accumulation of dMyc protein in fruit fly cells. They also describe the link between increased glucose metabolism and uncontrolled cell division.

First, fruit fly cells were fed a fluorescent molecule similar to glucose that cannot be broken down by the cells. This experiment established that glucose uptake increases in cells with too much Hipk. These cells also break down glucose faster, confirming that they have increased glucose metabolism.

Cells with high levels of Hipk also activate the genes that generate the enzymes involved in glucose breakdown, and increase the activity of the gene coding for dMyc. Levels of the dMyc protein are higher in these cells, which was shown by staining the cells with fluorescent molecules that specifically bind the dMyc protein. It is this buildup of dMyc protein that activates the genes coding for the enzymes responsible for glucose breakdown. PFK2 is one of these enzymes.

Finally, Wong et al. inhibited the production of the enzymes that are elevated in cells with high Hipk. Stopping the production of PFK2 prevents both tumor growth and the accumulation of dMyc protein. This shows that high levels of dMyc increase PFK2 levels, leading to further dMyc buildup, and creating a loop that links the uncontrolled cell division caused by too much dMyc and the shift to higher glucose metabolism.

These results highlight new potential targets for cancer therapy, showing that targeting glucose metabolism may reduce, or even stop, tumor growth.

Drosophila Homeodomain-Interacting Protein Kinase (Hipk) Phosphorylates the Homeodomain Proteins Homeobrain, Empty Spiracles, and Muscle Segment Homeobox

The Drosophila homeodomain-interacting protein kinase (Hipk) is the fly representative of the well-conserved group of HIPKs in vertebrates. It was initially found through its characteristic interactions with homeodomain proteins. Hipk is involved in a variety of important developmental processes, such as the development of the eye or the nervous system. In the present study, we set Hipk and the Drosophila homeodomain proteins Homeobrain (Hbn), Empty spiracles (Ems), and Muscle segment homeobox (Msh) in an enzyme-substrate relationship. These homeoproteins are transcription factors that function during Drosophila neurogenesis and are, at least in part, conserved in vertebrates. We reveal a physical interaction between Hipk and the three homeodomain proteins in vivo using bimolecular fluorescence complementation (BiFC). In the course of in vitro phosphorylation analysis and subsequent mutational analysis we mapped several Hipk phosphorylation sites of Hbn, Ems, and Msh. The phosphorylation of Hbn, Ems, and Msh may provide further insight into the function of Hipk during development of the Drosophila nervous system.

Homeodomain-interacting protein kinase 2 suppresses proliferation and aerobic glycolysis via ERK/cMyc axis in pancreatic cancer

OBJECTIVES

To investigate the roles of the homeodomain-interacting protein kinase (HIPK) family of proteins in pancreatic cancer prognosis and the possible molecular mechanism.

MATERIALS AND METHODS

The expression of HIPK family genes and their roles in pancreatic cancer prognosis were analysed by using The Cancer Genome Atlas (TCGA). The roles of HIPK2 in pancreatic cancer proliferation and glycolysis were tested by overexpression of HIPK2 in pancreatic cancer cells, followed by cell proliferation assay, glucose uptake analysis and Seahorse extracellular flux analysis. The mechanism of action of HIPK2 in pancreatic cancer proliferation and glycolysis was explored by examining its effect on the ERK/cMyc axis.

RESULTS

Decreased HIPK2 expression indicated worse prognosis of pancreatic cancer. Overexpression of HIPK2 in pancreatic cancer cells decreased cell proliferation and attenuated aerobic glycolysis, which sustained proliferation of cancer cells. HIPK2 decreased cMyc protein levels and expression of cMyc-targeted glycolytic genes. cMyc was a mediator that regulated HIPK2-induced decrease in aerobic glycolysis. HIPK2 regulated cMyc protein stability via ERK activation, which phosphorylated and controlled cMyc protein stability.

CONCLUSIONS

HIPK2 suppressed proliferation of pancreatic cancer in part through inhibiting the ERK/cMyc axis and related aerobic glycolysis.

Homeodomain-interacting protein kinase phosphorylates the Drosophila Paired box protein 6 (Pax6) homologues Twin of eyeless and Eyeless

Homeodomain-interacting protein kinase (Hipk), the Drosophila homologue of mammalian HIPK2, plays several important roles in regulating differentiation, proliferation, apoptosis, and stress responses and acts as a mediator for signals of diverse pathways, such as Notch or Wingless signalling. The Paired box protein 6 (Pax6) has two Drosophila homologues, Twin of eyeless (Toy) and Eyeless (Ey). Both stand atop the retinal determination gene network (RDGN), which is essential for proper eye development in Drosophila. Here, we set Hipk and the master regulators Toy and Ey in an enzyme-substrate relationship. Furthermore, we prove a physical interaction between Toy and Hipk in vivo using bimolecular fluorescence complementation. Using in vitro kinase assays with different truncated Toy constructs and mutational analyses, we mapped four Hipk phosphorylation sites of Toy, one in the paired domain (Ser¹²¹ ) and three in the C-terminal transactivation domain of Toy (Thr³⁹⁵ , Ser⁴¹⁰ and Thr⁴⁵² ). The interaction and phosphorylation of the master regulator Toy by Hipk may be important for precise tuning of signalling within the RDGN and therefore for Drosophila eye development.

HIPK2 polymorphisms rs2058265, rs6464214, and rs7456421 were associated with kidney stone disease in Chinese males not females

BACKGROUND AND AIM

Recent studies have shown that genetic factors are involved in the development of kidney stone disease (KSD). A case-control association analysis was performed to investigate the association between homeodomain-interacting protein kinase 2 (HIPK2; OMIM *606868) polymorphisms and KSD.

METHODS

A total of 890 KSD patients and 920 healthy subjects were analyzed. Polymorphisms were genotyped using SNPscanTM high-throughput SNP classification technology. The genotypic and allelic frequencies in KSD patients and healthy individuals were analyzed using a Chi-square test.

RESULTS

The genotype and allele distributions of the three polymorphisms (rs2058265, rs6464214, and rs7456421 in HIPK2) displayed strong associations with KSD in males (rs2058265: odds ratio [OR] 2.480,95%confidence interval [CI] 1.205-5.106, p = 0.014; rs6464214: OR 2.466, 95%CI 1.198-5.078, p = 0.014; rs7456421: OR 2.846, 95%CI 1.362-5.947, p = 0.005; perallele: r2058265T, OR 1.357, 95%CI 1.073-1.715, p = 0.011; rs6464214G, OR 1.340, 95%CI 1.060-1.693, p = 0.014; rs7456421C, OR 1.356, 95%CI 1.073-1.713, p = 0.011). Patients carrying the T allele of rs2058265, the G allele of rs6464214, or the C allele of rs7456421 showed higher systolic blood pressure, creatinine, and uric acid levels compared with wild-genotype individuals after adjusting for age, gender, and body mass index (p < 0.005).

CONCLUSION

The association of HIPK2 gene polymorphisms with KSD was only observed in males but not in females. HIPK2 gene polymorphisms were also involved in the changes of KSD-related metabolic traits.

Homeodomain-interacting protein kinase promotes tumorigenesis and metastatic cell behavior

Aberrations in signaling pathways that regulate tissue growth often lead to tumorigenesis. Homeodomain-interacting protein kinase (Hipk) family members are reported to have distinct and contradictory effects on cell proliferation and tissue growth. From these studies, it is clear that much remains to be learned about the roles of Hipk family protein kinases in proliferation and cell behavior. Previous work has shown that Drosophila Hipk is a potent growth regulator, thus we predicted that it could have a role in tumorigenesis. In our study of Hipk-induced phenotypes, we observed the formation of tumor-like structures in multiple cell types in larvae and adults. Furthermore, elevated Hipk in epithelial cells induces cell spreading, invasion and epithelial-to-mesenchymal transition (EMT) in the imaginal disc. Further evidence comes from cell culture studies, in which we expressed Drosophila Hipk in human breast cancer cells and showed that it enhances proliferation and migration. Past studies have shown that Hipk can promote the action of conserved pathways implicated in cancer and EMT, such as Wnt/Wingless, Hippo, Notch and JNK. We show that Hipk phenotypes are not likely to arise from activation of a single target, but rather through a cumulative effect on numerous target pathways. Most Drosophila tumor models involve mutations in multiple genes, such as the well-known Ras^V12 model, in which EMT and invasiveness occur after the additional loss of the tumor suppressor gene scribble. Our study reveals that elevated levels of Hipk on their own can promote both hyperproliferation and invasive cell behavior, suggesting that Hipk family members could be potent oncogenes and drivers of EMT.

Summary: The protein kinase Hipk can promote proliferation and invasive behaviors, and can synergize with known cancer pathways, in a new Drosophila model for tumorigenesis.

A Kinome RNAi Screen in Drosophila Identifies Novel Genes Interacting with Lgl, aPKC, and Crb Cell Polarity Genes in Epithelial Tissues

In both Drosophila melanogaster and mammalian systems, epithelial structure and underlying cell polarity are essential for proper tissue morphogenesis and organ growth. Cell polarity interfaces with multiple cellular processes that are regulated by the phosphorylation status of large protein networks. To gain insight into the molecular mechanisms that coordinate cell polarity with tissue growth, we screened a boutique collection of RNAi stocks targeting the kinome for their capacity to modify Drosophila “cell polarity” eye and wing phenotypes. Initially, we identified kinase or phosphatase genes whose depletion modified adult eye phenotypes associated with the manipulation of cell polarity complexes (via overexpression of Crb or aPKC). We next conducted a secondary screen to test whether these cell polarity modifiers altered tissue overgrowth associated with depletion of Lgl in the wing. These screens identified Hippo, Jun kinase (JNK), and Notch signaling pathways, previously linked to cell polarity regulation of tissue growth. Furthermore, novel pathways not previously connected to cell polarity regulation of tissue growth were identified, including Wingless (Wg/Wnt), Ras, and lipid/Phospho-inositol-3-kinase (PI3K) signaling pathways. Additionally, we demonstrated that the “nutrient sensing” kinases Salt Inducible Kinase 2 and 3 (SIK2 and 3) are potent modifiers of cell polarity phenotypes and regulators of tissue growth. Overall, our screen has revealed novel cell polarity-interacting kinases and phosphatases that affect tissue growth, providing a platform for investigating molecular mechanisms coordinating cell polarity and tissue growth during development.

MicroRNAs-mediated epithelial-mesenchymal transition in fibrotic diseases

MicroRNAs (miRNAs), a large family of small and highly conserved non-coding RNAs, regulate gene expression through translational repression or mRNA degradation. Aberrant expression of miRNAs underlies a spectrum of diseases including organ fibrosis. Recent evidence suggests that miRNAs contribute to organ fibrosis through mediating epithelial-mesenchymal transition (EMT). Alleviation of EMT has been proposed as a promising strategy against fibrotic diseases given the key role of EMT in fibrosis. miRNAs impact the expression of specific ligands, receptors, and signaling pathways, thus modulating EMT and consequently influencing fibrosis. This review summarizes the current knowledge concerning how miRNAs regulate EMT and highlights the specific roles that miRNAs-regulated EMT plays in fibrotic diseases as diverse as pulmonary fibrosis, hepatic fibrosis, renal fibrosis and cardiac fibrosis. It is desirable that a more comprehensive understanding of the functions of miRNAs-regulated EMT will facilitate the development of novel diagnostic and therapeutic strategies for various debilitating organ fibrosis.

Homeodomain-Interacting Protein Kinases: Diverse and Complex Roles in Development and Disease

The Homeodomain-interacting protein kinase (Hipk) family of proteins plays diverse, and at times conflicting, biological roles in normal development and disease. In this review we will highlight developmental and cellular roles for Hipk proteins, with an emphasis on the pleiotropic and essential physiological roles revealed through genetic studies. We discuss the myriad ways of regulating Hipk protein function, and how these may contribute to the diverse cellular roles. Furthermore we will describe the context-specific activities of Hipk family members in diseases such as cancer and fibrosis, including seemingly contradictory tumor-suppressive and oncogenic activities. Given the diverse signaling pathways regulated by Hipk proteins, it is likely that Hipks act to fine-tune signaling and may mediate cross talk in certain contexts. Such regulation is emerging as vital for development and in disease.

Homeodomain-Interacting Protein Kinase-2: A Critical Regulator of the DNA Damage Response and the Epigenome

Homeodomain-interacting protein kinase 2 (HIPK2) is a serine/threonine kinase that phosphorylates and activates the apoptotic program through interaction with diverse downstream targets including tumor suppressor p53. HIPK2 is activated by genotoxic stimuli and modulates cell fate following DNA damage. The DNA damage response (DDR) is triggered by DNA lesions or chromatin alterations. The DDR regulates DNA repair, cell cycle checkpoint activation, and apoptosis to restore genome integrity and cellular homeostasis. Maintenance of the DDR is essential to prevent development of diseases caused by genomic instability, including cancer, defects of development, and neurodegenerative disorders. Recent studies reveal a novel HIPK2-mediated pathway for DDR through interaction with chromatin remodeling factor homeodomain protein 1γ. In this review, we will highlight the molecular mechanisms of HIPK2 and show its functions as a crucial DDR regulator.

The Myopic-Ubpy-Hrs nexus enables endosomal recycling of Frizzled

Mop regulates endosomal localization and recycling of Frizzled. Hrs is ubiquitinated and degraded in the absence of Mop. Mop aids in the maintenance of Ubpy to control the ubiquitin homeostasis of Hrs. Mop and Ubpy can rescue each other. Mop’s function is not required in the cell in the absence of the ubiquitin ligase Cbl.

Endosomal trafficking of signaling proteins plays an essential role in cellular homeostasis. The seven-pass transmembrane protein Frizzled (Fz) is a critical component of Wnt signaling. Although Wnt signaling is proposed to be regulated by endosomal trafficking of Fz, the molecular events that enable this regulation are not completely understood. Here we show that the endosomal protein Myopic (Mop) regulates Fz trafficking in the Drosophila wing disk by inhibiting the ubiquitination and degradation of Hrs. Deletion of Mop or Hrs results in endosomal accumulation of Fz and therefore reduced Wnt signaling. The in situ proximity ligation assay revealed a strong association between Mop and Hrs in the Drosophila wing disk. Overexpression of Hrs rescues the trafficking defect caused by mop knockdown. Mop aids in the maintenance of Ubpy, which deubiquitinates (and thus stabilizes) Hrs. In the absence of the ubiquitin ligase Cbl, Mop is dispensable. These findings support a previously unknown role for Mop in endosomal trafficking of Fz in Wnt-receiving cells.

Homeodomain-interacting protein kinase (Hipk) phosphorylates the small SPOC family protein Spenito

The Drosophila homeodomain-interacting protein kinase (Hipk) is a versatile regulator involved in a variety of pathways, such as Notch and Wingless signalling, thereby acting in processes including the promotion of eye development or control of cell numbers in the nervous system. In vertebrates, extensive studies have related its homologue HIPK2 to important roles in the control of p53-mediated apoptosis and tumour suppression. Spenito (Nito) belongs to the group of small SPOC family proteins and has a role, amongst others, as a regulator of Wingless signalling downstream of Armadillo. In the present study, we show that both proteins have an enzyme-substrate relationship, adding a new interesting component to the broad range of Hipk interactions, and we map several phosphorylation sites of Nito. Furthermore, we were able to define a preliminary consensus motif for Hipk target sites, which will simplify the identification of new substrates of this kinase.

miR-141 regulates TGF-β1-induced epithelial-mesenchymal transition through repression of HIPK2 expression in renal tubular epithelial cells

Epithelial-mesenchymal transition (EMT) plays a critical role in embryonic development, wound healing, tissue regeneration, cancer progression and organ fibrosis. The proximal tubular epithelial cells undergo EMT, resulting in matrix-producing fibroblasts and thereby contribute to the pathogenesis of renal fibrosis. The profibrotic cytokine, TGF‑β, is now recognized as the main pathogenic driver that has been shown to induce EMT in tubular epithelial cells. Increasing evidence indicate that HIPK2 dysfunction may play a role in fibroblasts behavior, and therefore, HIPK2 may be considered as a novel potential target for anti-fibrosis therapy. Recently, members of the miR-200 family (miR‑200a, b and c and miR‑141) have been shown to inhibit EMT. However, the steps of the multifactorial renal fibrosis progression that these miRNAs regulate, particularly miR‑141, are unclear. To study the functional importance of miR‑141 in EMT, a well‑established in vitro EMT assay was used to demonstrate renal tubulointerstitial fibrosis; transforming growth factor‑β1‑induced EMT in HK-2 cells. Overexpression of miR‑141 in HK‑2 cells, either with or without TGF‑β1 treatment, hindered EMT by enhancing E‑cadherin and decreasing vimentin and fibroblast‑specific protein 1 expression. miR‑141 expression was repressed during EMT in a dose‑ and time‑dependent manner through upregulation of HIPK2 expression. Ectopic expression of HIPK2 promoted EMT by decreasing E-cadherin. Furthermore, co-transfection of miR‑141 with the HIPK2 ORF clone partially inhibited EMT by restoring E‑cadherin expression. miR‑141 downregulated the expression of HIPK2 via direct interaction with the 3'-untranslated region of HIPK2. Taken together, these findings aid in the understanding of the role and mechanism of miR‑141 in regulating renal fibrosis via the TGF‑β1/miR-141/HIPK2/EMT axis, and miR-141 may represent novel biomarkers and therapeutic targets in the treatment of renal fibrosis.

Role of HIPK2 in kidney fibrosis

Homeodomain interacting protein kinase 2 (HIPK2) functions as either a co-repressor or a co-activator of transcriptional regulators. Dysregulation of HIPK2 is associated with cancer and neurological disease. Recently, we found that HIPK2 is also an important driver of kidney fibrosis in the HIV-1 transgenic murine model, Tg26. HIPK2 protein levels are upregulated in the tubular epithelial cells of Tg26 mice as well as in kidney biopsies of patients with HIV-associated nephropathy, focal segmental glomerulosclerosis, diabetic nephropathy, and IgA nephropathy. We found that HIPK2 regulates pro-apoptotic, pro-fibrotic, and pro-inflammatory pathways including p53, transforming growth factor β (TGF-β)-SMAD family member 3 (Smad3), Notch, Wingless and INT-1 (Wnt)/β-catenin, and nuclear factor kappa-light-chain-enhancer of activated B cells in renal tubular epithelial cells. Our data suggest that HIPK2 may be a potential target for antifibrotic therapy. As mice with germline deletion of HIPK2 do not exhibit any phenotypic change under basal conditions, we do not expect significant side effects with specific HIPK2 inhibitors. However, potential effects of HIPK2 on tumor growth should be considered because of its tumor suppressor effects. Therefore, further understanding of structure-function relationships and post-translational modifications of HIPK2 are necessary to develop more specific drugs targeting the pro-fibrotic effects of HIPK2.

HIPK2 modification code for cell death and survival

Homeodomain-interacting protein kinase 2 (HIPK2) is a serine/threonine protein kinase that participates in the regulation of diverse cellular activities as a transcriptional cofactor and signal transducer. HIPK2 senses various signaling cues that in turn phosphorylate downstream substrates to coordinate developmental processes, cell cycle regulation, cell proliferation, differentiation, and the DNA damage response. HIPK2 functions are affected by its catalytic activity, stability, and subcellular localization, which in turn are dynamically regulated by diverse post-translational modifications such as polyubiquitination, SUMOylation, phosphorylation, and acetylation. HIPK2 is not modified with small molecules and/or peptides individually or independently, but in a combinatorial manner that is referred to as the “HIPK2 modification code.” HIPK2 integrates various signaling cues and senses different doses of DNA damage and ROS stimuli, which are reflected by unique patterns of HIPK2 modification. Hence, the HIPK2 modification code differentially contributes to cellular homeostasis and determination of cell fate depending on cellular context.

Hipk promotes photoreceptor differentiation through the repression of Twin of eyeless and Eyeless expression

Organogenesis is a complex developmental process, which requires tight regulation of selector gene expression to specify individual organ types. The Pax6 homolog Eyeless (Ey) is an example of such a factor and its expression pattern reveals it is dynamically controlled during development. Ey׳s paralog Twin of eyeless (Toy) induces its expression during embryogenesis, and the two genes are expressed in nearly identical patterns during the larval stages of development. While Ey must be expressed to initiate retinal specification, it must subsequently be repressed behind the morphogenetic furrow to allow for neuronal differentiation. Thus far, a few factors have been implicated in this repression including the signaling pathways Hedgehog (Hh) and Decapentaplegic (Dpp), and more recently downstream components of the retinal determination gene network (RDGN) Sine oculis (So), Eyes absent (Eya), and Dachshund (Dac). Homeodomain-interacting protein kinase (Hipk), a conserved serine-threonine kinase, regulates numerous factors during tissue patterning and development, including the Hh pathway. Using genetic analyses we identify Hipk as a repressor of both Toy and Ey and show that it may do so, in part, through Hh signaling. We also provide evidence that Ey repression is a critical step in ectopic eye development and that Hipk plays an important role in this process. Because Ey repression within the retinal field is a critical step in eye development, we propose that Hipk is a key link between eye specification and patterning.

Prolyl isomerase Pin1 and protein kinase HIPK2 cooperate to promote cortical neurogenesis by suppressing Groucho/TLE:Hes1-mediated inhibition of neuronal differentiation

The Groucho/transducin-like Enhancer of split 1 (Gro/TLE1):Hes1 transcriptional repression complex acts in cerebral cortical neural progenitor cells to inhibit neuronal differentiation. The molecular mechanisms that regulate the anti-neurogenic function of the Gro/TLE1:Hes1 complex during cortical neurogenesis remain to be defined. Here we show that prolyl isomerase Pin1 (peptidyl-prolyl cis-trans isomerase NIMA-interacting 1) and homeodomain-interacting protein kinase 2 (HIPK2) are expressed in cortical neural progenitor cells and form a complex that interacts with the Gro/TLE1:Hes1 complex. This association depends on the enzymatic activities of both HIPK2 and Pin1, as well as on the association of Gro/TLE1 with Hes1, but is independent of the previously described Hes1-activated phosphorylation of Gro/TLE1. Interaction with the Pin1:HIPK2 complex results in Gro/TLE1 hyperphosphorylation and weakens both the transcriptional repression activity and the anti-neurogenic function of the Gro/TLE1:Hes1 complex. These results provide evidence that HIPK2 and Pin1 work together to promote cortical neurogenesis, at least in part, by suppressing Gro/TLE1:Hes1-mediated inhibition of neuronal differentiation.

Homeodomain interacting protein kinase (HPK-1) is required in the soma for robust germline proliferation in C. elegans

BACKGROUND

Tightly regulated pathways maintain the balance between proliferation and differentiation within stem cell populations. In Caenorhabditis elegans, the germline is the only tissue that is maintained by stem-like cells into adulthood. In the current study, we investigated the role played by a member of the Homeodomain interacting protein kinase (HIPK) family of serine/threonine kinases, HPK-1, in the development and maintenance of the C. elegans germline.

RESULTS

We report that HPK-1 is required for promotion of germline proliferation during development and into adulthood. Additionally, we show that HPK-1 is required in the soma for regulation of germline proliferation. We also show that HPK-1 is a predominantly nuclear protein expressed in several somatic tissues including germline-interacting somatic cells.

CONCLUSIONS

Our observations are consistent with a conserved role for HIPKs in the control of cellular proliferation and identify a new context for such control in germ cell proliferation.

Posttranslational modifications regulate HIPK2, a driver of proliferative diseases

The serine/threonine kinase homeodomain-interacting protein kinase (HIPK2) is a tumor suppressor and functions as an evolutionary conserved regulator of signaling and gene expression. This kinase regulates a surprisingly vast array of biological processes that range from the DNA damage response and apoptosis to hypoxia signaling and cell proliferation. Recent studies show the tight control of HIPK2 by hierarchically occurring posttranslational modifications such as phosphorylation, small ubiquitin-like modifier modification, acetylation, and ubiquitination. The physiological function of HIPK2 as a regulator of cell proliferation and survival has a downside: proliferative diseases. Dysregulation of HIPK2 can result in increased proliferation of cell populations as it occurs in cancer or fibrosis. We discuss various models that could explain how inappropriate expression, modification, or localization of HIPK2 can be a driver for these proliferative diseases.

Homeodomain-interacting protein kinase 2-dependent repression of myogenic differentiation is relieved by its caspase-mediated cleavage

Differentiation of skeletal muscle cells is accompanied by drastic changes in gene expression programs that depend on activation and repression of genes at defined time points. Here we identify the serine/threonine kinase homeodomain-interacting protein kinase 2 (HIPK2) as a corepressor that inhibits myocyte enhancer factor 2 (MEF2)-dependent gene expression in undifferentiated myoblasts. Downregulation of HIPK2 expression by shRNAs results in elevated expression of muscle-specific genes, whereas overexpression of the kinase dampens transcription of these genes. HIPK2 is constitutively associated with a multi-protein complex containing histone deacetylase (HDAC)3 and HDAC4 that serves to silence MEF2C-dependent transcription in undifferentiated myoblasts. HIPK2 interferes with gene expression on phosphorylation and HDAC3-dependent deacetylation of MEF2C. Ongoing muscle differentiation is accompanied by elevated caspase activity, which results in caspase-mediated cleavage of HIPK2 following aspartic acids 916 and 977 and the generation of a C-terminally truncated HIPK2 protein. The short form of the kinase loses its affinity to the repressive multi-protein complex and its ability to bind HDAC3 and HDAC4, thus alleviating its repressive function for expression of muscle genes. This study identifies HIPK2 as a further protein that determines the threshold and kinetics of gene expression in proliferating myoblasts and during the initial steps of myogenesis.

Targeting the epithelial cells in fibrosis: a new concept for an old disease

Fibrosis, which affects millions of individuals worldwide, is a leading cause of organ failure. For 40 years myofibroblasts have been recognized to be the key cellular players in fibrosis. Currently, several pharmaceutical targets are under investigation that may contribute to the activation of myofibroblasts. Recent preclinical and clinical evidence suggests that other components in the fibrotic microenvironment can trigger myofibroblast activation, providing new targets for pharmaceutical intervention. Epithelial cells may represent the most promising cellular phenotype that could be exploited in the design of new anti-fibrotic medicines through their paracrine action on myofibroblasts. The present review briefly highlights this hypothesis and discusses some interesting related pharmacological targets.

GAL4 enhancer traps that can be used to drive gene expression in developing Drosophila spermatocytes

The Drosophila testis has proven to be a valuable model organ for investigation of germline stem cell (GSC) maintenance and differentiation as well as elucidation of the genetic programs that regulate differentiation of daughter spermatogonia. Development of germ cell specific GAL4 driver transgenes has facilitated investigation of gene function in GSCs and spermatogonia but specific GAL4 tools are not available for analysis of postmitotic spermatogonial differentiation into spermatocytes. We have screened publically available pGT1 strains, a GAL4-encoding gene trap collection, to identify lines that can drive gene expression in late spermatogonia and early spermatocytes. While we were unable to identify any germline-specific drivers, we did identify an insertion in the chiffon locus, which drove expression specifically in early spermatocytes within the germline along with the somatic cyst cells of the testis.

Homeodomain-interacting protein kinase regulates Yorkie activity to promote tissue growth

The Hippo (Hpo) tumor suppressor pathway regulates tissue size by inhibiting cell proliferation and promoting apoptosis. The core components of the pathway, Hpo, Salvador, Warts (Wts), and Mats, form a kinase cascade to inhibit the activity of Yorkie (Yki), the transcriptional effector of the pathway. Homeodomain-interacting protein kinases (Hipks) are a family of conserved serine/threonine kinases that function as regulators of various transcription factors to regulate developmental processes including proliferation, differentiation, and apoptosis. Hipk can induce tissue overgrowth in Drosophila. We demonstrate that Hipk is required to promote Yki activity. Hipk affects neither Yki stability nor its subcellular localization. Moreover, hipk knockdown suppresses the overgrowth and target gene expression caused by hyperactive Yki. Hipk phosphorylates Yki and in vivo analyses show that Hipk's regulation of Yki is kinase-dependent. To our knowledge, this is the first kinase identified to positively regulate Yki.

Homeodomain-interacting protein kinase regulates Hippo pathway-dependent tissue growth

The Salvador-Warts-Hippo (SWH) pathway is an evolutionarily conserved regulator of tissue growth that is deregulated in human cancer. Upstream SWH pathway components convey signals from neighboring cells via a core kinase cassette to the transcription coactivator Yorkie (Yki). Yki controls tissue growth by modulating activity of transcription factors including Scalloped (Sd). To date, five SWH pathway kinases have been identified, but large-scale phosphoproteome studies suggest that unidentified SWH pathway kinases exist. To identify such kinases, we performed an RNA interference screen and isolated homeodomain-interacting protein kinase (Hipk). Unlike previously identified SWH pathway kinases, Hipk is unique in its ability to promote, rather than repress, Yki activity and does so in parallel to the Yki-repressive kinase, Warts (Wts). Hipk is required for basal Yki activity and is likely to regulate Yki function by promoting its accumulation in the nucleus. Like many SWH pathway proteins, Hipk's function is evolutionarily conserved as its closest human homolog, HIPK2, promotes activity of the Yki ortholog YAP in a kinase-dependent fashion. Further, HIPK2 promotes YAP abundance, suggesting that the mechanism by which HIPK2 regulates YAP has diverged in mammals.

Homeodomain-interacting protein kinase (HIPK)-1 is required for splenic B cell homeostasis and optimal T-independent type 2 humoral response

The homeodomain-interacting protein kinase (HIPK) family is comprised of four highly related serine/threonine kinases originally identified as co-repressors for various homeodomain-containing transcription factors. The HIPKs have been shown to be involved in growth regulation and apoptosis, with numerous studies highlighting HIPK regulation of the tumor suppressor p53. In this study, we have discovered a B cell homeostatic defect in HIPK1-deficient (HIPK1^−/−) mice. Lymphopoietic populations within the thymus and bone marrow of HIPK1^−/− mice appeared normal based upon FACS analysis; however, the spleen exhibited a reduced number of total B cells with a significant loss of transitional-1 and follicular B cell populations. Interestingly, the marginal zone B cell population was expanded in HIPK1^−/− mice, yielding an increased frequency of these cells. HIPK1^−/− B cells exhibited impaired cell division in response to B cell receptor cross-linking in vitro based upon thymidine incorporation or CFSE dilution; however, the addition of CD40L rescued HIPK1^−/− proliferation to wild-type levels. Despite the expanded MZ B cell population in the HIPK1^−/− mice, the T-independent type 2 humoral response was impaired. These data identify HIPK1 as a novel kinase required for optimal B cell function in mice.

A systems approach identifies HIPK2 as a key regulator of kidney fibrosis

Kidney fibrosis is a common process that leads to the progression of various types of kidney disease. We used an integrated computational and experimental systems biology approach to identify protein kinases that regulate gene expression changes in the kidneys of human immunodeficiency virus (HIV) transgenic mice (Tg26 mice), which have both tubulointerstitial fibrosis and glomerulosclerosis. We identified homeo-domain interacting protein kinase 2 (HIPK2) as a key regulator of kidney fibrosis. HIPK2 was upregulated in the kidneys of Tg26 mice and in those of patients with various kidney diseases. HIV infection increased the protein concentrations of HIPK2 by promoting oxidative stress, which inhibited the seven in absentia homolog 1 (SIAH1)-mediated proteasomal degradation of HIPK2. HIPK2 induced apoptosis and the expression of epithelial-to-mesenchymal transition markers in kidney epithelial cells by activating the p53, transforming growth factor β (TGF-β)-SMAD family member 3 (Smad3) and Wnt-Notch pathways. Knockout of HIPK2 improved renal function and attenuated proteinuria and kidney fibrosis in Tg26 mice, as well as in other murine models of kidney fibrosis. We therefore conclude that HIPK2 is a potential target for anti-fibrosis therapy.

The unconserved groucho central region is essential for viability and modulates target gene specificity

Groucho (Gro) is a Drosophila corepressor required by numerous DNA-binding repressors, many of which are distributed in gradients and provide positional information during development. Gro contains well-conserved domains at its N- and C-termini, and a poorly conserved central region that includes the GP, CcN, and SP domains. All lethal point mutations in gro map to the conserved regions, leading to speculation that the unconserved central domains are dispensable. However, our sequence analysis suggests that the central domains are disordered leading us to suspect that the lack of lethal mutations in this region reflects a lack of order rather than an absence of essential functions. In support of this conclusion, genomic rescue experiments with Gro deletion variants demonstrate that the GP and CcN domains are required for viability. Misexpression assays using these same deletion variants show that the SP domain prevents unrestrained and promiscuous repression by Gro, while the GP and CcN domains are indispensable for repression. Deletion of the GP domain leads to loss of nuclear import, while deletion of the CcN domain leads to complete loss of repression. Changes in Gro activity levels reset the threshold concentrations at which graded repressors silence target gene expression. We conclude that co-regulators such as Gro are not simply permissive components of the repression machinery, but cooperate with graded DNA-binding factors in setting borders of gene expression. We suspect that disorder in the Gro central domains may provide the flexibility that allows this region to mediate multiple interactions required for repression.

Groucho: a corepressor with instructive roles in development

Drosophila Groucho (Gro) is the founding member of a family of metazoan corepressors. Gro mediates repression through interactions with a myriad of DNA-binding repressor proteins to direct the silencing of genes involved in many developmental processes, including neurogenesis and patterning of the main body axis, as well as receptor tyrosine kinase/Ras/MAPK, Notch, Wingless (Wg)/Wnt, and Decapentaplegic (Dpp) signaling. Gro mediates repression by multiple molecular mechanisms, depending on the regulatory context. Because Gro is a broadly expressed nuclear factor, whereas its repressor partners display restricted temporal and spatial distribution, it was presumed that this corepressor played permissive rather than instructive roles in development. However, a wide range of studies demonstrates that this is not the case. Gro can sense and integrate many cellular inputs to modulate the expression of variety of genes, making it a versatile corepressor with crucial instructive roles in development and signaling.

Drosophila Smt3 negatively regulates JNK signaling through sequestering Hipk in the nucleus

Post-translational modification by the small ubiquitin-related modifier (SUMO) is important for a variety of cellular and developmental processes. However, the precise mechanism(s) that connects sumoylation to specific developmental signaling pathways remains relatively less clear. Here, we show that Smt3 knockdown in Drosophila wing discs causes phenotypes resembling JNK gain of function, including ectopic apoptosis and apoptosis-induced compensatory growth. Smt3 depletion leads to an increased expression of JNK target genes Mmp1 and puckered. We show that, although knockdown of the homeodomain-interacting protein kinase (Hipk) suppresses Smt3 depletion-induced activation of JNK, Hipk overexpression synergistically enhances this type of JNK activation. We further demonstrate that Hipk is sumolylated in vivo, and its nuclear localization is dependent on the sumoylation pathway. Our results thus establish a mechanistic connection between the sumoylation pathway and the JNK pathway through the action of Hipk. We propose that the sumoylation-controlled balance between cytoplasmic and nuclear Hipk plays a crucial role in regulating JNK signaling.

Degringolade, a SUMO-targeted ubiquitin ligase, inhibits Hairy/Groucho-mediated repression

Transcriptional cofactors are essential for proper embryonic development. One such cofactor in Drosophila, Degringolade (Dgrn), encodes a RING finger/E3 ubiquitin ligase. Dgrn and its mammalian ortholog RNF4 are SUMO-targeted ubiquitin ligases (STUbLs). STUbLs bind to SUMOylated proteins via their SUMO interaction motif (SIM) domains and facilitate substrate ubiquitylation. In this study, we show that Dgrn is a negative regulator of the repressor Hairy and its corepressor Groucho (Gro/transducin-like enhancer (TLE)) during embryonic segmentation and neurogenesis, as dgrn heterozygosity suppresses Hairy mutant phenotypes and embryonic lethality. Mechanistically Dgrn functions as a molecular selector: it targets Hairy for SUMO-independent ubiquitylation that inhibits the recruitment of its corepressor Gro, without affecting the recruitment of its other cofactors or the stability of Hairy. Concomitantly, Dgrn specifically targets SUMOylated Gro for sequestration and antagonizes Gro functions in vivo. Our findings suggest that by targeting SUMOylated Gro, Dgrn serves as a molecular switch that regulates cofactor recruitment and function during development. As Gro/TLE proteins are conserved universal corepressors, this may be a general paradigm used to regulate the Gro/TLE corepressors in other developmental processes.

Regulation of TCF3 by Wnt-dependent phosphorylation during vertebrate axis specification

A commonly accepted model of Wnt/β-catenin signaling involves target gene activation by a complex of β-catenin with a T-cell factor (TCF) family member. TCF3 is a transcriptional repressor that has been implicated in Wnt signaling and plays key roles in embryonic axis specification and stem cell differentiation. Here we demonstrate that Wnt proteins stimulate TCF3 phosphorylation in gastrulating Xenopus embryos and mammalian cells. This phosphorylation event involves β-catenin-mediated recruitment of homeodomain-interacting protein kinase 2 (HIPK2) to TCF3 and culminates in the dissociation of TCF3 from a target gene promoter. Mutated TCF3 proteins resistant to Wnt-dependent phosphorylation function as constitutive inhibitors of Wnt-mediated activation of Vent2 and Cdx4 during anteroposterior axis specification. These findings reveal an alternative in vivo mechanism of Wnt signaling that involves TCF3 phosphorylation and subsequent derepression of target genes and link this molecular event to a specific developmental process.

A live zebrafish-based screening system for human nuclear receptor ligand and cofactor discovery

Nuclear receptors (NRs) belong to a superfamily of transcription factors that regulate numerous homeostatic, metabolic and reproductive processes. Taken together with their modulation by small lipophilic molecules, they also represent an important and successful class of drug targets. Although many NRs have been targeted successfully, the majority have not, and one third are still orphans. Here we report the development of an in vivo GFP-based reporter system suitable for monitoring NR activities in all cells and tissues using live zebrafish (Danio rerio). The human NR fusion proteins used also contain a new affinity tag cassette allowing the purification of receptors with bound molecules from responsive tissues. We show that these constructs 1) respond as expected to endogenous zebrafish hormones and cofactors, 2) facilitate efficient receptor and cofactor purification, 3) respond robustly to NR hormones and drugs and 4) yield readily quantifiable signals. Transgenic lines representing the majority of human NRs have been established and are available for the investigation of tissue- and isoform-specific ligands and cofactors.