Degenerative phenotypes caused by the combined deficiency of murine HIP1 and HIP1r are rescued by human HIP1

Apoptosis, Autophagy, Necrosis and Their Multi Galore Crosstalk in Neurodegeneration

The progression of neurodegenerative disorders is mainly characterized by immense neuron loss and death of glial cells. The mechanisms which are active and regulate neuronal cell death are namely necrosis, necroptosis, autophagy and apoptosis. These death paradigms are governed by a set of molecular determinants that are pivotal in their performance and also exhibit remarkable overlapping functional pathways. A large number of such molecules have been demonstrated to be involved in the switching of death paradigms in various neurodegenerative diseases. In this review, we discuss various molecules and the concurrent crosstalk mediated by them. According to our present knowledge and research in neurodegeneration, molecules like Atg1, Beclin1, LC3, p53, TRB3, RIPK1 play switching roles toggling from one death mechanism to another. In addition, the review also focuses on the exorbitant number of newer molecules with the potential to cross communicate between death pathways and create a complex cell death scenario. This review highlights recent studies on the inter-dependent regulation of cell death paradigms in neurodegeneration, mediated by cross-communication between pathways. This will help in identifying potential targets for therapeutic intervention in neurodegenerative diseases.

Huntingtin-interacting protein family members have a conserved pro-viral function from Caenorhabditis elegans to humans

Understanding host–virus interaction mechanisms is critical to identify novel drug targets for development of antiviral therapeutics. The C. elegans–Orsay virus infection system offers an innovative approach for discovering host factors required for virus infection. The hipr-1 gene was determined to be essential in C. elegans for Orsay virus infection at an early stage of the virus life cycle. Studies of its orthologs in human cell culture determined that both HIP1 and HIP1R are important for Coxsackie B3 virus infection, highlighting the evolutionarily conserved role of HIP-family proteins in virus infection from nematodes to humans.

Huntingtin-interacting protein family members are evolutionarily conserved from yeast to humans, and they are known to be key factors in clathrin-mediated endocytosis. Here we identified the Caenorhabditis elegans protein huntingtin-interacting protein-related 1 (HIPR-1) as a host factor essential for Orsay virus infection of C. elegans. Ablation of HIPR-1 resulted in a greater than 10,000-fold reduction in viral RNA, which could be rescued by ectopic expression of HIPR-1. Viral RNA replication from an endogenous transgene replicon system was not affected by lack of HIPR-1, suggesting that HIPR-1 plays a role during an early, prereplication virus life-cycle stage. Ectopic expression of HIPR-1 mutants demonstrated that neither the clathrin light chain-binding domain nor the clathrin heavy chain-binding motif were needed for virus infection, whereas the inositol phospholipid-binding and F-actin–binding domains were essential. In human cell culture, deletion of the human HIP orthologs HIP1 and HIP1R led to decreased infection by Coxsackie B3 virus. Finally, ectopic expression of a chimeric HIPR-1 harboring the human HIP1 ANTH (AP180 N-terminal homology) domain rescued Orsay infection in C. elegans, demonstrating conservation of its function through evolution. Collectively, these findings further our knowledge of cellular factors impacting viral infection in C. elegans and humans.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Behavioral and synaptic alterations relevant to obsessive-compulsive disorder in mice with increased EAAT3 expression

Obsessive-compulsive disorder (OCD) is a severe, chronic neuropsychiatric disorder with a strong genetic component. The SLC1A1 gene encoding the neuronal glutamate transporter EAAT3 has been proposed as a candidate gene for this disorder. Gene variants affecting SLC1A1 expression in human brain tissue have been associated with OCD. Several mouse models fully or partially lacking EAAT3 have shown no alterations in baseline anxiety-like or repetitive behaviors. We generated a transgenic mouse model (EAAT3^glo) to achieve conditional, Cre-dependent EAAT3 overexpression and evaluated the overall impact of increased EAAT3 expression at behavioral and synaptic levels. Mice with EAAT3 overexpression driven by CaMKIIα-promoter (EAAT3^glo/CMKII) displayed increased anxiety-like and repetitive behaviors that were both restored by chronic, but not acute, treatment with fluoxetine or clomipramine. EAAT3^glo/CMKII mice also displayed greater spontaneous recovery of conditioned fear. Electrophysiological and biochemical analyses at corticostriatal synapses of EAAT3^glo/CMKII mice revealed changes in NMDA receptor subunit composition and altered NMDA-dependent synaptic plasticity. By recapitulating relevant behavioral, neurophysiological, and psychopharmacological aspects, our results provide support for the glutamatergic hypothesis of OCD, particularly for the increased EAAT3 function, and provide a valuable animal model that may open novel therapeutic approaches to treat this devastating disorder.

Deficiency of the Endocytic Protein Hip1 Leads to Decreased Gdpd3 Expression, Low Phosphocholine, and Kypholordosis

Deficiency of huntingtin-interacting protein 1 (Hip1) results in degenerative phenotypes. Here we generated a Hip1 deficiency allele where a floxed transcriptional stop cassette and a human HIP1 cDNA were knocked into intron 1 of the mouse Hip1 locus. CMV-Cre-mediated germ line excision of the stop cassette resulted in expression of HIP1 and rescue of the Hip1 knockout phenotype. Mx1-Cre-mediated excision led to HIP1 expression in spleen, kidney and liver, and also rescued the phenotype. In contrast, hGFAP-Cre-mediated, brain-specific HIP1 expression did not rescue the phenotype. Metabolomics and microarrays of several Hip1 knockout tissues identified low phosphocholine (PC) levels and low glycerophosphodiester phosphodiesterase domain containing 3 (Gdpd3) gene expression. Since Gdpd3 has lysophospholipase D activity that results in the formation of choline, a precursor of PC, Gdpd3 downregulation could lead to the low PC levels. To test whether Gdpd3 contributes to the Hip1 deficiency phenotype, we generated Gdpd3 knockout mice. Double knockout of Gdpd3 and Hip1 worsened the Hip1 phenotype. This suggests that Gdpd3 compensates for Hip1 loss. More-detailed knowledge of how Hip1 deficiency leads to low PC will improve our understanding of HIP1 in choline metabolism in normal and disease states.

Depletion of Nsd2-mediated histone H3K36 methylation impairs adipose tissue development and function

The epigenetic mechanisms regulating adipose tissue development and function are poorly understood. In this study, we show that depletion of histone H3K36 methylation by H3.3K36M in preadipocytes inhibits adipogenesis by increasing H3K27me3 to prevent the induction of C/EBPα and other targets of the master adipogenic transcription factor peroxisome proliferator-activated receptor-γ (PPARγ). Depleting H3K36 methyltransferase Nsd2, but not Nsd1 or Setd2, phenocopies the effects of H3.3K36M on adipogenesis and PPARγ target expression. Consistently, expression of H3.3K36M in progenitor cells impairs brown adipose tissue (BAT) and muscle development in mice. In contrast, depletion of histone H3K36 methylation by H3.3K36M in adipocytes in vivo does not affect adipose tissue weight, but leads to profound whitening of BAT and insulin resistance in white adipose tissue (WAT). These mice are resistant to high fat diet-induced WAT expansion and show severe lipodystrophy. Together, these results suggest a critical role of Nsd2-mediated H3K36 methylation in adipose tissue development and function.

Novel common variants associated with body mass index and coronary artery disease detected using a pleiotropic cFDR method

Genome-wide association studies (GWAS) have been successfully applied in identifying single nucleotide polymorphisms (SNPs) associated with body mass index (BMI) and coronary heart disease (CAD). However, the SNPs to date can only explain a small percentage of the genetic variances of traits. Here, we applied a genetic pleiotropic conditional false discovery rate (cFDR) method that combines summary statistic p values from different multi-center GWAS datasets, to detect common genetic variants associated with these two traits. The enrichment of SNPs associated with BMI and CAD was assessed by conditional Q-Q plots and the common variants were identified by the cFDR method. By applying the cFDR level of 0.05, 7 variants were identified to be associated with CAD (2 variants being novel), 34 variants associated with BMI (11 variants being novel), and 3 variants associated with both BMI and CAD (2 variants being novel). The SNP rs653178 (ATXN2) is noteworthy as this variant was replicated in an independent analysis. SNP rs12411886 (CNNM2) and rs794356 (HIP1) were of note as the annotated genes may be associated with processes that are functionally important in lipid metabolism. In conclusion, the cFDR method identified novel variants associated with BMI and/or CAD by effectively incorporating different GWAS datasets.

Targeted overexpression of tumor necrosis factor-α increases cyclin-dependent kinase 5 activity and TRPV1-dependent Ca2+ influx in trigeminal neurons

We reported earlier that TNF-α, a proinflammatory cytokine implicated in many inflammatory disorders causing orofacial pain, increases the activity of Cdk5, a key kinase involved in brain development and function and recently found to be involved in pain signaling. To investigate a potential mechanism underlying inflammatory pain in trigeminal ganglia (TGs), we engineered a transgenic mouse model (TNF) that can conditionally overexpresses TNF-α upon genomic recombination by Cre recombinase. TNF mice were bred with Nav1.8-Cre mouse line that expresses the Cre recombinase in sensory neurons to obtain TNF-α:Nav1.8-Cre (TNF-α cTg) mice. Although TNF-α cTg mice appeared normal without any gross phenotype, they displayed a significant increase in TNF-α levels after activation of NFκB signaling in the TG. IL-6 and MCP-1 levels were also increased along with intense immunostaining for Iba1 and GFAP in TG, indicating the presence of infiltrating macrophages and the activation of satellite glial cells. TNF-α cTg mice displayed increased trigeminal Cdk5 activity, and this increase was associated with elevated levels of phospho-T407-TRPV1 and capsaicin-evocated Ca influx in cultured trigeminal neurons. Remarkably, this effect was prevented by roscovitine, an inhibitor of Cdk5, which suggests that TNF-α overexpression induced sensitization of the TRPV1 channel. Furthermore, TNF-α cTg mice displayed more aversive behavior to noxious thermal stimulation (45°C) of the face in an operant pain assessment device as compared with control mice. In summary, TNF-α overexpression in the sensory neurons of TNF-α cTg mice results in inflammatory sensitization and increased Cdk5 activity; therefore, this mouse model would be valuable for investigating the mechanism of TNF-α involved in orofacial pain.

Genetic aberrations in small B-cell lymphomas and leukemias: molecular pathology, clinical relevance and therapeutic targets

Small B-cell lymphomas and leukemias (SBCLs) are a clinically, morphologically, immunophenotypically and genetically heterogeneous group of clonal lymphoid neoplasms, including entities such as chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), mantle cell lymphoma (MCL), follicular lymphoma (FL), lymphoplasmacytic lymphoma (LPL), marginal zone lymphoma (MZL) and hairy cell leukemia (HCL). The pathogenesis of some of these lymphoid malignancies is characterized by distinct translocations, for example t(11;14) in the majority of cases of MCL and t(14;18) in most cases of FL, whereas other entities are associated with a variety of recurrent but nonspecific numeric chromosomal abnormalities, as exemplified by del(13q14), del(11q22), and +12 in CLL, and yet others such as LPL and HCL that lack recurrent or specific cytogenetic aberrations. The recent surge in next generation sequencing (NGS) technology has shed more light on the genetic landscape of SBCLs through characterization of numerous driver mutations including SF3B1 and NOTCH1 in CLL, ATM and CCND1 in MCL, KMT2D and EPHA7 in FL, MYD88 (L265P) in LPL, KLF2 and NOTCH2 in splenic MZL (SMZL) and BRAF (V600E) in HCL. The identification of distinct genetic lesions not only provides greater insight into the molecular pathogenesis of these disorders but also identifies potential valuable biomarkers for prognostic stratification, as well as specific targets for directed therapy. This review discusses the well-established and recently identified molecular lesions underlying the pathogenesis of SBCLs, highlights their clinical relevance and summarizes novel targeted therapies.

Conditional TNF-α Overexpression in the Tooth and Alveolar Bone Results in Painful Pulpitis and Osteitis

Tumor necrosis factor-α (TNF-α) is a proalgesic cytokine that is commonly expressed following tissue injury. TNF-α expression not only promotes inflammation but can also lead to pain hypersensitivity in nociceptors. With the established link between TNF-α and inflammatory pain, we identified its increased expression in the teeth of patients affected with caries and pulpitis. We generated a transgenic mouse model (TNF-α(glo)) that could be used to conditionally overexpress TNF-α. These mice were bred with a dentin matrix protein 1 (DMP1)-Cre line for overexpression of TNF-α in both the tooth pulp and bone to study oral pain that would result from subsequent development of pulpitis and bone loss. The resulting DMP1/TNF-α(glo) mice show inflammation in the tooth pulp that resembles pulpitis while also displaying periodontal bone loss. Inflammatory infiltrates and enlarged blood vessels were observed in the tooth pulp. Pulpitis and osteitis affected the nociceptive neurons innervating the orofacial region by causing increased expression of inflammatory cytokines within the trigeminal ganglia. With this new mouse model morphologically mimicking pulpitis and osteitis, we tested it for signs of oral pain with an oral function assay (dolognawmeter). This assay/device records the time required by a mouse to complete a discrete gnawing task. The duration of gnawing required by the DMP1/TNF-α(glo) mice to complete the task was greater than that for the controls; extended gnaw time in a dolognawmeter indicates reduced orofacial function. With the DMP1/TNF-α(glo) mice, we have shown that TNF-α expression alone can produce inflammation similar to pulpitis and osteitis and that this mouse model can be used to study dental inflammatory pain.

Epsin deficiency impairs endocytosis by stalling the actin-dependent invagination of endocytic clathrin-coated pits

Epsin is an evolutionarily conserved endocytic clathrin adaptor whose most critical function(s) in clathrin coat dynamics remain(s) elusive. To elucidate such function(s), we generated embryonic fibroblasts from conditional epsin triple KO mice. Triple KO cells displayed a dramatic cell division defect. Additionally, a robust impairment in clathrin-mediated endocytosis was observed, with an accumulation of early and U-shaped pits. This defect correlated with a perturbation of the coupling between the clathrin coat and the actin cytoskeleton, which we confirmed in a cell-free assay of endocytosis. Our results indicate that a key evolutionary conserved function of epsin, in addition to other roles that include, as we show here, a low affinity interaction with SNAREs, is to help generate the force that leads to invagination and then fission of clathrin-coated pits.

Endosomal signaling and oncogenesis

The endosomal system provides a route whereby nutrients, viruses, and receptors are internalized. During the course of endocytosis, activated receptors can accumulate within endosomal structures and certain signal-transducing molecules can be recruited to endosomal membranes. In the context of signaling and cancer, they provide platforms within the cell from which signals can be potentiated or attenuated. Regulation of the duration of receptor signaling is a pivotal means of refining growth responses in cells. In cancers, this is often considered in terms of mutations that affect receptor tyrosine kinases and maintain them in hyperactivated states of dimerization and/or phosphorylation. However, disruption to the regulatory control exerted by the assembly of protein complexes within the endosomal network can also contribute to disease among which oncogenesis is characterized in part by dysregulated growth, enhanced cell survival, and changes in the expression of markers of differentiation. In this chapter, we will discuss the role of proteins that regulate in endocytosis as tumor suppressors or oncogenes and how changing the fate of internalized receptors and concomitant endosomal signaling can contribute to cancer.

Autophagy and apoptosis dysfunction in neurodegenerative disorders

Autophagy and apoptosis are basic physiologic processes contributing to the maintenance of cellular homeostasis. Autophagy encompasses pathways that target long-lived cytosolic proteins and damaged organelles. It involves a sequential set of events including double membrane formation, elongation, vesicle maturation and finally delivery of the targeted materials to the lysosome. Apoptotic cell death is best described through its morphology. It is characterized by cell rounding, membrane blebbing, cytoskeletal collapse, cytoplasmic condensation, and fragmentation, nuclear pyknosis, chromatin condensation/fragmentation, and formation of membrane-enveloped apoptotic bodies, that are rapidly phagocytosed by macrophages or neighboring cells. Neurodegenerative disorders are becoming increasingly prevalent, especially in the Western societies, with larger percentage of members living to an older age. They have to be seen not only as a health problem, but since they are care-intensive, they also carry a significant economic burden. Deregulation of autophagy plays a pivotal role in the etiology and/or progress of many of these diseases. Herein, we briefly review the latest findings that indicate the involvement of autophagy in neurodegenerative diseases. We provide a brief introduction to autophagy and apoptosis pathways focusing on the role of mitochondria and lysosomes. We then briefly highlight pathophysiology of common neurodegenerative disorders like Alzheimer's diseases, Parkinson's disease, Huntington's disease and Amyotrophic lateral sclerosis. Then, we describe functions of autophagy and apoptosis in brain homeostasis, especially in the context of the aforementioned disorders. Finally, we discuss different ways that autophagy and apoptosis modulation may be employed for therapeutic intervention during the maintenance of neurodegenerative disorders.

Key tissue targets responsible for anthrax-toxin-induced lethality

Bacillus anthracis, the causative agent of anthrax disease, is lethal owing to the actions of two exotoxins: anthrax lethal toxin (LT) and oedema toxin (ET). The key tissue targets responsible for the lethal effects of these toxins are unknown. Here we generated cell-type-specific anthrax toxin receptor capillary morphogenesis protein-2 (CMG2)-null mice and cell-type-specific CMG2-expressing mice and challenged them with the toxins. Our results show that lethality induced by LT and ET occurs through damage to distinct cell types; whereas targeting cardiomyocytes and vascular smooth muscle cells is required for LT-induced mortality, ET-induced lethality occurs mainly through its action in hepatocytes. Notably, and in contradiction to what has been previously postulated, targeting of endothelial cells by either toxin does not seem to contribute significantly to lethality. Our findings demonstrate that B. anthracis has evolved to use LT and ET to induce host lethality by coordinately damaging two distinct vital systems.

Region-specific epithelial cell dynamics during branching morphogenesis

BACKGROUND

Epithelial cells of developing embryonic organs, such as salivary glands, can display substantial motility during branching morphogenesis. Their dynamic movements and molecules involved in their migration are not fully characterized.

RESULTS

We generated transgenic mice expressing photo-convertible KikGR and tracked the movements of individual cells highlighted by red fluorescence in different regions of developing salivary glands. Motility was highest for outer bud epithelial cells adjacent to the basement membrane, lower in inner bud cells, and lowest in duct cells. The highly motile outer cells contacting the basement membrane were pleomorphic, whereas inner cells were rounded. Peripheral cell motility was disrupted by antibodies inhibiting α6+β1 integrins and the nonmuscle myosin II inhibitor blebbistatin. Inner bud cell migration was unaffected by these inhibitors, but their rate of migration was stimulated by inhibiting E-cadherin.

CONCLUSIONS

Cell motility in developing salivary glands was highest in cells in contact with the basement membrane. The basement membrane-associated motility of these outer bud cells depended on integrins and myosin II, but not E-cadherin. In contrast, motility of inner bud cells was restrained by E-cadherin. These findings identify the importance of integrin-dependent basement membrane association for the morphology, tissue organization, and lateral motility of morphogenetic epithelial cells.

An integrative, translational approach to understanding rare and orphan genetically based diseases

PhenomeNet is an approach for integrating phenotypes across species and identifying candidate genes for genetic diseases based on the similarity between a disease and animal model phenotypes. In contrast to ‘guilt-by-association’ approaches, PhenomeNet relies exclusively on the comparison of phenotypes to suggest candidate genes, and can, therefore, be applied to study the molecular basis of rare and orphan diseases for which the molecular basis is unknown. In addition to disease phenotypes from the Online Mendelian Inheritance in Man (OMIM) database, we have now integrated the clinical signs from Orphanet into PhenomeNet. We demonstrate that our approach can efficiently identify known candidate genes for genetic diseases in Orphanet and OMIM. Furthermore, we find evidence that mutations in the HIP1 gene might cause Bassoe syndrome, a rare disorder with unknown genetic aetiology. Our results demonstrate that integration and computational analysis of human disease and animal model phenotypes using PhenomeNet has the potential to reveal novel insights into the pathobiology underlying genetic diseases.

Huntingtin-interacting protein 1 phosphorylation by receptor tyrosine kinases

Huntingtin-interacting protein 1 (HIP1) binds inositol lipids, clathrin, actin, and receptor tyrosine kinases (RTKs). HIP1 is elevated in many tumors, and its expression is prognostic in prostate cancer. HIP1 overexpression increases levels of the RTK epidermal growth factor receptor (EGFR) and transforms fibroblasts. Here we report that HIP1 is tyrosine phosphorylated in the presence of EGFR and platelet-derived growth factor β receptor (PDGFβR) as well as the oncogenic derivatives EGFRvIII, HIP1/PDGFβR (H/P), and TEL/PDGFβR (T/P). We identified a four-tyrosine "HIP1 phosphorylation motif" (HPM) in the N-terminal region of HIP1 that is required for phosphorylation mediated by both EGFR and PDGFβR but not by the oncoproteins H/P and T/P. We also identified a tyrosine residue (Y152) within the HPM motif of HIP1 that inhibits HIP1 tyrosine phosphorylation. The HPM tyrosines are conserved in HIP1's only known mammalian relative, HIP1-related protein (HIP1r), and are also required for HIP1r phosphorylation. Tyrosine-to-phenylalanine point mutations in the HPM of HIP1 result in proapoptotic activity, indicating that an intact HPM may be necessary for HIP1's role in cellular survival. These data suggest that phosphorylation of HIP1 by RTKs in an N-terminal region contributes to the promotion of cellular survival.

Large-scale association analyses identify new loci influencing glycemic traits and provide insight into the underlying biological pathways

Through genome-wide association meta-analyses of up to 133,010 individuals of European ancestry without diabetes, including individuals newly genotyped using the Metabochip, we have increased the number of confirmed loci influencing glycemic traits to 53, of which 33 also increase type 2 diabetes risk (q < 0.05). Loci influencing fasting insulin concentration showed association with lipid levels and fat distribution, suggesting impact on insulin resistance. Gene-based analyses identified further biologically plausible loci, suggesting that additional loci beyond those reaching genome-wide significance are likely to represent real associations. This conclusion is supported by an excess of directionally consistent and nominally significant signals between discovery and follow-up studies. Functional analysis of these newly discovered loci will further improve our understanding of glycemic control.

Diversity of clathrin function: new tricks for an old protein

Clathrin is considered the prototype vesicle coat protein whose self-assembly mediates sorting of membrane cargo and recruitment of lipid modifiers. Detailed knowledge of clathrin biochemistry, structure, and interacting proteins has accumulated since the first observation, almost 50 years ago, of its role in receptor-mediated endocytosis of yolk protein. This review summarizes that knowledge, and focuses on properties of the clathrin heavy and light chain subunits and interaction of the latter with Hip proteins, to address the diversity of clathrin function beyond conventional receptor-mediated endocytosis. The distinct functions of the two human clathrin isoforms (CHC17 and CHC22) are discussed, highlighting CHC22's specialized involvement in traffic of the GLUT4 glucose transporter and consequent role in human glucose metabolism. Analysis of clathrin light chain function and interaction with the actin-binding Hip proteins during bacterial infection defines a novel actin-organizing function for CHC17 clathrin. By considering these diverse clathrin functions, along with intracellular sorting roles and influences on mitosis, further relevance of clathrin function to human health and disease is established.

Low levels of human HIP14 are sufficient to rescue neuropathological, behavioural, and enzymatic defects due to loss of murine HIP14 in Hip14-/- mice

Huntingtin Interacting Protein 14 (HIP14) is a palmitoyl acyl transferase (PAT) that was first identified due to altered interaction with mutant huntingtin, the protein responsible for Huntington Disease (HD). HIP14 palmitoylates a specific set of neuronal substrates critical at the synapse, and downregulation of HIP14 by siRNA in vitro results in increased cell death in neurons. We previously reported that mice lacking murine Hip14 (Hip14-/-) share features of HD. In the current study, we have generated human HIP14 BAC transgenic mice and crossed them to the Hip14-/- model in order to confirm that the defects seen in Hip14-/- mice are in fact due to loss of Hip14. In addition, we sought to determine whether human HIP14 can provide functional compensation for loss of murine Hip14. We demonstrate that despite a relative low level of expression, as assessed via Western blot, BAC-derived human HIP14 compensates for deficits in neuropathology, behavior, and PAT enzyme function seen in the Hip14-/- model. Our findings yield important insights into HIP14 function in vivo.

Huntingtin-interacting protein 1-related is required for accurate congression and segregation of chromosomes

Huntingtin-interacting protein 1-related (HIP1r) is known to function in clathrin-mediated endocytosis and regulation of the actin cytoskeleton, which occurs continuously in non-dividing cells. This study reports a new function for HIP1r in mitosis. Green fluorescent protein-fused HIP1r localizes to the mitotic spindles. Depletion of HIP1r by RNA interference induces misalignment of chromosomes and prolonged mitosis, which is associated with decreased proliferation of HIP1r-deficeint cells. Chromosome misalignment leads to missegregation and ultimately production of multinucleated cells. Depletion of HIP1r causes persistent activation of the spindle checkpoint in misaligned chromosomes. These findings suggest that HIP1r plays an important role in regulating the attachment of spindle microtubules to chromosomes during mitosis, an event that is required for accurate congression and segregation of chromosomes. This finding may provide new insights that improve the understanding of various human diseases involving HIP1r as well as its fusion genes.

Conditional overexpression of TGF-beta1 disrupts mouse salivary gland development and function

Transforming growth factor-beta (TGF-beta) signaling is known to affect salivary gland physiology by influencing branching morphogenesis, regulating ECM deposition, and controlling immune homeostasis. To study the role of TGF-beta1 in the salivary gland, we created a transgenic mouse (beta1(glo)) that conditionally overexpresses active TGF-beta1 upon genomic recombination by Cre recombinase. beta1(glo) mice were bred with an MMTV (mouse mammary tumor virus)-Cre (MC) transgenic line that expresses the Cre recombinase predominantly in the secretory cells of both the mammary and salivary glands. Although most of the double positive (beta1(glo)/MC) pups die either in utero or just after birth, clear defects in salivary gland morphogenesis such as reduced branching and increased mesenchyme could be seen. Those beta1(glo)/MC mice that survived into adulthood, however, had hyposalivation due to salivary gland fibrosis and acinar atrophy. Increased TGF-beta signaling was observed in the salivary gland with elevated phosphorylation of Smad2 and concomitant increase in ECM deposition. In particular, aberrant TGF-beta1 overexpression caused salivary gland hypofunction in this mouse model because of the replacement of normal glandular parenchyma with interstitial fibrous tissue. These results further implicate TGF-beta in pathological cases of salivary gland inflammation and fibrosis that occur with chronic infections in the glands or with the autoimmune disease, Sjögren's syndrome, or with radiation therapy given to head-and-neck cancer patients.

The Sla2p/HIP1/HIP1R family: similar structure, similar function in endocytosis?

HIP1 (huntingtin interacting protein 1) has two close relatives: HIP1R (HIP1-related) and yeast Sla2p. All three members of the family have a conserved domain structure, suggesting a common function. Over the past decade, a number of studies have characterized these proteins using a combination of biochemical, imaging, structural and genetic techniques. These studies provide valuable information on binding partners, structure and dynamics of HIP1/HIP1R/Sla2p. In general, all suggest a role in CME (clathrin-mediated endocytosis) for the three proteins, though some differences have emerged. In this mini-review we summarize the current views on the roles of these proteins, while emphasizing the unique attributes of each family member.

Transcription regulation of caspase-1 by R393 of HIPPI and its molecular partner HIP-1

Earlier we have shown that exogenous expression of HIPPI, a molecular partner of Huntingtin interacting protein HIP-1, induces apoptosis and increases expression of caspases-1, -8 and -10 in HeLa and Neuro2A cells. The C-terminal pseudo death effector domain of HIPPI (pDED-HIPPI) specifically interacts with the putative promoter sequences of these genes. In the present manuscript, we predict from structural modeling of pDED-HIPPI that R393 of HIPPI is important for such interaction. R393E mutation in pDED-HIPPI decreases the interaction with the putative promoter of caspase-1 in cells. Expression of caspase-1 is decreased in cells expressing mutant pDED-HIPPI in comparison to that observed in cells expressing wild type pDED-HIPPI. Using HIP-1 knocked down cells as well as over expressing HIP-1 with mutation at its nuclear localization signal and other deletion mutations, we demonstrate that translocation of HIPPI to the nucleus is mediated by HIP-1 for the increased expression of caspase-1. HIPPI-HIP-1 heterodimer is detected in cytoplasm as well as in the nucleus and is associated with transcription complex in cells. Taking together, we are able to show the importance of R393 of HIPPI and the role of HIPPI-HIP-1 heterodimer in the transcription regulation of caspase-1.

Hip1r is expressed in gastric parietal cells and is required for tubulovesicle formation and cell survival in mice

Huntingtin interacting protein 1 related (Hip1r) is an F-actin- and clathrin-binding protein involved in vesicular trafficking. In this study, we demonstrate that Hip1r is abundantly expressed in the gastric parietal cell, predominantly localizing with F-actin to canalicular membranes. Hip1r may provide a critical function in vivo, as demonstrated by extensive changes to parietal cells and the gastric epithelium in Hip1r-deficient mice. Electron microscopy revealed abnormal apical canalicular membranes and loss of tubulovesicles in mutant parietal cells, suggesting that Hip1r is necessary for the normal trafficking of these secretory membranes. Accordingly, acid secretory dynamics were altered in mutant parietal cells, with enhanced activation and acid trapping, as measured in isolated gastric glands. At the whole-organ level, gastric acidity was reduced in Hip1r-deficient mice, and the gastric mucosa was grossly transformed, with fewer parietal cells due to enhanced apoptotic cell death and glandular hypertrophy associated with cellular transformation. Hip1r-deficient mice had increased expression of the gastric growth factor gastrin, and mice mutant for both gastrin and Hip1r exhibited normalization of both proliferation and gland height. Taken together, these studies demonstrate that Hip1r plays a significant role in gastric physiology, mucosal architecture, and secretory membrane dynamics in parietal cells.

Actin binding by Hip1 (huntingtin-interacting protein 1) and Hip1R (Hip1-related protein) is regulated by clathrin light chain

The huntingtin-interacting protein family members (Hip1 and Hip1R in mammals and Sla2p in yeast) link clathrin-mediated membrane traffic to actin cytoskeleton dynamics. Genetic data in yeast have implicated the light chain subunit of clathrin in regulating this link. To test this hypothesis, the biophysical properties of mammalian Hip1 and Hip1R and their interaction with clathrin light chain and actin were analyzed. The coiled-coil domains (clathrin light chain-binding) of Hip1 and Hip1R were found to be stable homodimers with no propensity to heterodimerize in vitro. Homodimers were also predominant in vivo, accounting for cellular segregation of Hip1 and Hip1R functions. Coiled-coil domains of Hip1 and Hip1R differed in their stability and flexibility, correlating with slightly different affinities for clathrin light chain and more markedly with effects of clathrin light chain binding on Hip protein-actin interactions. Clathrin light chain binding induced a compact conformation of both Hip1 and Hip1R and significantly reduced actin binding by their THATCH domains. Thus, clathrin is a negative regulator of Hip-actin interactions. These observations necessarily change models proposed for Hip protein function.

Use of a cryptic splice site for the expression of huntingtin interacting protein 1 in select normal and neoplastic tissues

Huntingtin interacting protein 1 (HIP1) is a 116-kDa endocytic protein, which is necessary for the maintenance of several tissues in vivo as its deficiency leads to degenerative adult phenotypes. HIP1 deficiency also inhibits prostate tumor progression in mice. To better understand how deficiency of HIP1 leads to such phenotypes, we analyzed tumorigenic potential in mice homozygous for a Hip1 mutant allele, designated Hip1(Delta 3-5), which is predicted to result in a frame-shifted, nonsense mutation in the NH(2) terminus of HIP1. In contrast to our previous studies using the Hip1 null allele, an inhibition of tumorigenesis was not observed as a result of the homozygosity of the nonsense Delta 3-5 allele. To further examine the contrasting results from the prior Hip1 mutant mice, we cultured tumor cells from homozygous Delta 3-5 allele-bearing mice and discovered the presence of a 110-kDa form of HIP1 in tumor cells. Upon sequencing of Hip1 DNA and message from these tumors, we determined that this 110-kDa form of HIP1 is the product of splicing of a cryptic U12-type AT-AC intron. This event results in the insertion of an AG dinucleotide between exons 2 and 6 and restoration of the reading frame. Remarkably, this mutant protein retains its capacity to bind lipids, clathrin, AP2, and epidermal growth factor receptor providing a possible explanation for why tumorigenesis was not altered after this knockout mutation. Our data show how knowledge of the transcript that is produced by a knockout allele can lead to discovery of novel types of molecular compensation at the level of splicing.

Aberrant Huntingtin interacting protein 1 in lymphoid malignancies

Huntingtin interacting protein 1 (HIP1) is an inositol lipid, clathrin, and actin binding protein that is overexpressed in a variety of epithelial malignancies. Here, we report for the first time that HIP1 is elevated in non-Hodgkin's and Hodgkin's lymphomas and that patients with lymphoid malignancies frequently had anti-HIP1 antibodies in their serum. Moreover, p53-deficient mice with B-cell lymphomas were 13 times more likely to have anti-HIP1 antibodies in their serum than control mice. Furthermore, transgenic overexpression of HIP1 was associated with the development of lymphoid neoplasms. The HIP1 protein was induced by activation of the nuclear factor-kappaB pathway, which is frequently activated in lymphoid malignancies. These data identify HIP1 as a new marker of lymphoid malignancies that contributes to the transformation of lymphoid cells in vivo.