Plk4 has been termed a `suicide kinase' because it promotes its own destruction to regulate protein levels. We identified numerous autophosphorylated residues within a region of Plk4 called the Downstream regulatory element (DRE). We find that phosphorylation of a single residue is sufficient for Slimb recruitment and phosphorylation of the surrounding residues builds a high affinity Slimb-binding site. These autophosphorylation events are dependent on Plk4 homodimerization, although the domains that mediate this dimerization are unknown. We show that Plk4 homodimerization is mediated by interactions between the PB1-PB2 cassette. We find that like all Polo kinases, Plk4 encodes a mechanism of kinase autoinhibition. Unlike other Polo kinases, which rely on external inputs for relief of inhibition, Plk4 is self-sufficient in relieving kinase inhibition. This relief of autoinhibition is regulated by PB3 of Plk4 and is dependent on homodimerization, thereby making homodimerization a necessary step in formation of the Slimb phosphodegron on Plk4. Polo Boxes are known as multifunctional domains, and the Polo Boxes of Plk4 are no different. We identified numerous Slimb-mediated ubiquitination sites on PB1 as well as PB2. Furthermore, the PB1-PB2 cassette mediates the interaction between Plk4 and the N-terminus of Asterless. In Drosophila cells, Plk4 requires Asterless for centriolar localization and Asterless overexpression drives centriole amplification in a Plk4 dependent manner. This is a fascinating result as endogenous Plk4 protein levels are undetectable in S2 cells, making it hard to envision a scenario where overexpression of Asterless could shuttle a non-existent Plk4 population to the centriole to initiate duplication. We found that in addition to shuttling Plk4 to the centriole, Asterless stabilizes Plk4, likely protecting Plk4 at the centriole to allow it to `license' the centriole for duplication. Moreover, we show that Asterless encodes two distinct Plk4 binding sites: the previously described N-terminal binding site as well as a novel C-terminal binding site. We found that the interaction between the C-terminal of Asterless and Plk4 is necessary for centriole duplication while the interaction between the N-terminal of Asterless and Plk4 is expendable. Together these findings provide significant insight into Plk4 biology and the mechanisms which limit its activity.

Centrosomes are complex organelles consisting of a pair of small microtubule based structures called centrioles embedded in an amorphous cloud of pericentriolar material (PCM). These organelles are critical for proper mitotic spindle assembly and orientation, and can also migrate to the plasma membrane where, as basal bodies, they serve to nucleate cilia¹. Centrioles are the core duplicating elements of the centrosome and, similar to DNA, duplicate once per cell cycle during S-phase². Errors in this process can lead to a cell that contains too many or too few centrosomes and are thought to promote tumorigenesis by directly promoting genomic instability and loss of polarity in stem cells³⁻⁷. Polo-like kinase 4 (Plk4) and Asterless (Asl) are both essential for centriole duplication to occur and overexpression of either of these proteins leads to cells that contain too many centrosomes, a condition known as centrosome amplification. Interestingly, both of these proteins also have the unique ability to promote de novo formation of centrioles in cells that normally lack centrioles⁸⁻¹⁴. Plk4 is a member of the Polo-like kinase family of proteins and is named after the founding member Drosophila Polo¹⁵. In humans, there are four members (Plk1-4) that all share sequence similarity and an N-terminal ser/thr kinase domain. Plk's 1-3 all contain two characteristic Polo box (PB) motifs, downstream of the kinase domain, that consist of a six stranded β-sheet lying across a C-terminal α-helix¹⁶. However, Plk4 was thought to be unique among family members since it only contained one C-terminal PB domain and a larger cryptic polo box domain that showed very little sequence similarity to known PBs. However, we performed an in-depth structure/function analysis of this cryptic polo box region and were able to determine, through x-ray crystallography, that Plk4 is unique among Plk family members not because it contains one PB domain but, in fact, because it contains three polo box domains. Thus, the cryptic polo box domain contains two previously unidentified polo boxes, termed PB1-PB2, upstream of the C-terminal PB3¹⁷. Furthermore, we found that PB1-PB2 is necessary for proper localization of Plk4 and that the entire PB1-PB2 cassette is necessary for binding to Asl. Our results also indicate that the PB1-PB2 domain plays a critical role in regulating Plk4 autophosphorylation and degradation, in order to restrict centriole duplication to once and only once per cell cycle. Previously Asl has been shown to be not only a binding partner of Plk4 but also a substrate for Plk4 kinase activity, however, the functional significance of this phosphorylation has remained elusive⁹. Our work has shown that Asl phosphorylation by Plk4 is conserved from flies to humans and is restricted to the N-terminal (Asl-A) region. In total we identified 13 phosphorylated residues via mass spectrometry. Analysis of phosphorylation mutant constructs revealed that Plk4 and Asl are involved in a novel feedback loop controlling centriole duplication. This feedback loop consists of two important features: first, Asl oligomerization stimulates Plk4 kinase activity and thus promotes centriole duplication and second, the Asl-A region can be phosphorylated by Plk4, preventing further Asl oligomerization, which in turn limits the amount of active Plk4. We propose that this feedback loop is a crucial step in limiting a mother centriole to only one daughter per cell cycle. This work represents a significant advance in our understanding of the processes that govern the centriole duplication pathway and specifically the structure and function of two critical components, Plk4 and Asl. A detailed understanding of the molecular mechanisms controlling centriole biogenesis is an essential first step in our goal of understanding their role in tumorigenesis and may serve as a guide for future studies focused on targeting this pathway for the prevention or treatment of cancer.

Membrane-Associated RING-CH1 (MARCH1) is a highly-conserved E3 ubiquitin ligase identified as a potent regulator of the immune modulatory molecules, Major Histocompatibility Complex II (MHC-II), and co-stimulatory molecules (such as CD86). Antigen-presenting cells (APCs), such as dendritic cells, are induced to mature following interaction with stimuli, including microbial ligands and cytokines, whereupon MHC-II and CD86 surface expression is significantly upregulated. At the surface of the APC, these MARCH1 substrates provide signals critical for T cell activation and induction of the appropriate adaptive immune response. As a key regulator of the T cell-APC conversation, it is imperative to gain a thorough understanding of the underlying regulatory mechanisms governing MARCH1 expression and function, and the biological consequences of dysregulation of this E3 ligase. Indeed, it has been demonstrated that MARCH1 gene transcription is negatively-regulated upon APC maturation. We have shown that MARCH1 function is regulated at additional levels. We have observed that MARCH1 is rapidly degraded under normal circumstances. Further, its activity also appears to be negatively regulated by APC maturation, possibly through post-translational modifications including phosphorylation. The fact that MARCH1 is subject to multiple levels of regulation indicates a need for precise control of antigen-presentation. Interestingly, MARCH1-deficient mice on a normal diet show changes in organismal metabolism, which is known to be regulated, in part, by immune cells. We observed gender-associated dimorphisms in weight gain, visceral adipose tissue (VAT) deposits, and increased inflammation of the VAT, all characteristic of insulin-resistance and type II diabetes progression. However, despite exhibiting these hallmark risk factors of metabolic dysregulation, MARCH1-deficient mice show increased glucose tolerance compared to wildtype mice. Collectively, the data support the hypothesis that MARCH1 is stringently regulated to ensure proper control of the adaptive immune response, and suggest a novel role for this E3 ligase in the maintenance of metabolic homeostasis.

A cell's composition is not uniform, but is comprised of many molecular gradients to compartmentalize functions into specialized subcellular domains. This organization is called polarity–the asymmetry of morphology and composition. Though it's a feature of nearly all prokaryotic and eukaryotic cells, polarity is plastic and highly dynamic, and is continuously instructed by the crosstalk between extracellular cues and internal effector pathways. One of the master regulators of polarity is the Par complex, canonically comprised of Cdc42, Par6, Par3 and atypical protein kinase C (aPKC). The Par complex defines the apical domain of epithelia and the neuronal axon, directs cell migration and the assembly of cell junctions, and restricts other polarity complexes to their respective domains. We have identified a novel polarity protein that counteracts the activities of the Par complex in cells. PKMζ, a truncated isoform of aPKC normally found in neurons, competes with full-length aPKC for substrate interactions. This competition results in the disruption of the canonical Par complex and its instruction of cell polarity, manifesting as a block in axon specification in developing neurons, or as a loss of the apical-basal axis of epithelial polarity. By eliminating PKMζ's ability to compete with aPKC for interaction with Par3, the effect on polarity is mitigated, while RNAi-mediated reduction of Par3 levels similarly rescues PKMζ-associated defects. We further report that PKMζ is aberrantly transcribed in certain epithelial cancers, and its expression correlates with grade. Malignant epithelial phenotypes are driven by PKMζ's Par3-dependent disruption of polarity, and its Par3- independent promotion of anoikis resistance. We demonstrate that PKMζ, as the catalytic fragment of aPKC, is surprisingly competent to influence polarity independently of its kinase activity, while other aPKC isoforms require their catalytic function to permit apical development. Together, this body of work presents PKMζ as an endogenous inhibitor of Par complex function, whose presence provides bistability to the dynamics of symmetry-breaking.

The formation of the embryonic vasculature is essential for life. The components driving this process are well conserved across vertebrate species. At the core of vascular development is the specification of endothelial precursor cells from nascent mesoderm. Transcription factors of the ETS family are important regulators of endothelial specification. In this document we characterize the role of the ETS transcription factors, ETV2, during embryonic vascular development.Expression analysis shows that Etv2 is highly expressed in hematopoietic and endothelial precursor cells in the Xenopus embryo. In gain-of-function experiments, ETV2 is sufficient to activate ectopic expression of vascular endothelial markers. In addition, ETV2 activated expression of hematopoietic genes representing the myeloid but not the erythroid lineage. Loss-of-function studies indicate that ETV2 is required for expression of all endothelial markers examined. However, knockdown of ETV2 has no detectable effects on expression of either myeloid or erythroid markers. This contrasts with studies in mouse and zebrafish where ETV2 is required for development of the myeloid lineage. Our studies confirm an essential role for ETV2 in endothelial development, but also reveal important differences in hematopoietic development between organisms.Although ETV2 is a pivotal molecule in development it remains unidentified in the chicken genome. We hypothesize that chicken Etv2 is expressed in the early Gallus embryo, and is necessary for endothelial specification consistent with its role in other species. To test this hypothesis we attempted to amplify Etv2 transcripts from Gallus embryos using degenerate PCR. Disappointingly this strategy did not reveal a putative Etv2 candidate. However, some important findings were uncovered, including the cloning of a previously uncharacterized Gallus ETS protein, SPDEF. Additionally the identification of an annotation error mis-identifying Ets gene "Erf" as "Etv3" (also an Ets gene) provided details on gene arrangement previously unknown. The workflow described could be used in future studies for the identification of other members of gene families that exhibit gaps, keeping in mind the goal of the study and the limitations of each technology.

In order for cells to migrate, communicate, and facilitate attachment to the surrounding extraceullar matrix, they must form intricate protein complexes called focal adhesions. The number of identified focal adhesion components continues to grow and the field is an area of active study.Lasp-2 is a member of the nebulin family of actin-binding proteins that has been identified as a member of focal adhesion complexes. To gain further insights into the functional role of lasp-2, we identified two additional binding partners of lasp-2, the integral focal adhesion proteins, vinculin and paxillin. Interestingly, the interaction of lasp-2 with its binding partners vinculin and paxillin was significantly reduced in presence of lasp-1, another nebulin family member. The presence of lasp-2 appears to enhance the interaction of vinculin and paxillin with each other, however, as with the interaction of lasp-2 with vinculin or paxillin, this effect is greatly diminished in the presence of excess lasp-1 suggesting the interplay between lasp-2 and lasp-2 could be an adhesion regulatory mechanism. Lasp-2's potential role in metastasis was revealed as overexpression of lasp-2 in SW620 cells, a highly metastatic cancer cell line, increased cell migration, but impeded cell invasion.Lasp-2 transcript and protein is readily detected in neural tissues. Preliminary experiments involving the knockdown of lasp-2- in frog embryos revealed gross morphological abnormalities in the head region as well as the inability to move normally. Neural crest derived melanocytes also failed to migrate normally.Taken together, these data suggest that lasp-2 has an important role in coordinating and regulating the composition and dynamics of focal adhesions.

Chronic exposure to arsenic-contaminated drinking water is correlated with obstructive lung disease (i.e. chronic obstructive pulmonary disease (COPD), bronchiectasis), reduced lung function and other respiratory effects (e.g. chronic cough, chest sounds). Researchers have associated arsenic exposure with reduced airway immunity. The airway epithelial innate immune system protects underlying tissue from inhaled particulates and pathogens through a variety of mechanisms. Such defects in innate immunity are associated with chronic bacterial infections and development of obstructive airway diseases, including COPD and bronchiectasis. We hypothesize that arsenic exposure may lead to recurrent lung infection and eventual obstructive lung disease by compromising mechanisms essential in airway innate immunity. In the work presented herein we evaluated the effects of arsenic on airway epithelial barrier properties, wound repair capacity, and signaling pathways essential in innate immunity. We previously published that acute (24 hr) arsenic (0.4-3.9 μM as Naarsenite) slowed wound repair in a human bronchial epithelial cell line (16HBE14o-). In the first study we hypothesized arsenic may be affecting wound repair by altering Ca²⁺ signaling that is important in multiple aspects of wound repair, including cell migration. We found wound-induced Ca²⁺ signaling was largely mediated by paracrine ATP in 16HBE14o- cells, and acute (24 hr) arsenic (0.8, 3.9 μM) exposure reduced ATPmediated Ca²⁺ signaling. We identified functional reductions in the ATP receptors P2Y₂ and P2X₄ following arsenic exposure. Both of these receptors are essential in airway innate immunity (e.g. mucociliary clearance). In the second study we found similar reductions in wound repair capacity and ATP-mediated Ca²⁺ signaling in 16HBE14o cells using a chronic (4-5 week) low-dose (0.13, 0.33 μM) arsenic exposure representative of U.S. drinking water standards. Further, wound-induced Ca²⁺ signaling was reduced in primary cultured tracheal cells derived from mice fed arsenic-free or arsenic-supplemented (50 ppb; 1μM=75 ppb) water for four weeks prior to experimentation. In the last study we demonstrated that the structure and function of the airway epithelial barrier was altered by a five-day exposure of arsenic (0.8, 3.9 μM). We conclude that arsenic at environmentally relevant levels compromises key functions in airway epithelial innate immunity that may underlie development of lung disease.

A leading cause of morbidity and mortality in the developed world is cardiovascular disease (CVD). Like many other disease processes the etiology of CVD has origins in both genetic and environmental factors. These factors affect the development of the heart and vasculature and how they respond to physiological and pathological stress. Abnormal heart development can lead to cardiac pathologies that manifest in a shift from normal cardiac geometry and physiology to what is called pathological cardiac remodeling. Often though, pathological remodeling can result from cardiovascular stress even when heart development is normal. Growth factors are essential mediators of cardiac development and physiology and a good number of clinical and experimental studies have implicated growth factors and their signaling effectors as potential therapeutic targets for pathological cardiac remodeling. Of those is Fibroblast Growth Factor 2 (FGF2) which is a potent inducer of fibroblast and cardiomyocyte proliferation in vitro. FGF2 is made in high molecular weight and low molecular weight isoforms (Hi FGF2 and Lo FGF2, respectively). It has already been demonstrated that, in the context of the heart, FGF2 modulates cardiac hypertrophy, cardiac fibrosis and mediates protection against cardiac injury. However, the isoform specific role of FGF2 in cardiac development, physiology and pathological remodeling has not been disclosed, and in this dissertation I address the hypothesis that FGF2 has isoform-specific function in cardiac physiology and remodeling. To test this hypothesis I used mice that are either deficient in Hi FGF2 (Hi KO) or Lo FGF2 (Lo KO) and subjected them to echocardiographic analysis and isoproterenol (Iso) treatment and compared them to wildtype (WT) cohorts. At baseline echocardiographic measurements, female Lo KO hearts are smaller and present with increased peak E-wave velocity, a diminutive A wave, and shortened mitral-flow deceleration time consistent with a restricted filling pattern and myocardial stiffness. Conversely, male Lo KO hearts present with a lower E wave and a higher A-wave velocity and a prolonged isovolumic-relaxation time consistent with impaired left ventricular (LV) relaxation. Female Hi KO hearts display no significant deviation from WT, while male Hi KO hearts exhibit increased systolic function. Hence, a deficiency in Lo FGF2 results in a shift from normal diastolic parameters and geometric measurements which is gender specific. Conversely, a deficiency in Hi FGF2 produces a phenotype in male hearts only. Histological and gravimetric analysis of Lo KO and Hi KO hearts post-Iso treatment reveals that female Lo KO hearts remain smaller even though their cardiomyocytes are hypertrophied while female Hi KO hearts present with a blunted hypertrophic response indicating a hypoplastic myocardium. Male Lo KO hearts present with an exacerbated fibrotic response and increased alpha-smooth muscle actin protein expression while Hi KO hearts exhibit a resistance to the fibrotic response and an induction of atrial natriuretic factor protein expression. Thus, in female hearts Hi FGF2 mediate cardiac hypertrophy while in male hearts Lo FGF2 and Hi FGF2 display an antithetical role in cardiac fibrosis where Lo FGF2 is protective while Hi FGF2 is damaging. Hence, cardiac remodeling following catecholamine overactivation is modulated by FGF2 in isoform- and gender-specific manners. In conclusion, the results presented here provide novel evidence on the interaction of gender and endogenous FGF2 isoforms as modulators of cardiac development, physiology and remodeling. Lo FGF2 signaling is necessary in the male heart for normal myocardial relaxation and for amelioration of the fibrotic response induced by beta-adrenergic stress, while in female hearts Lo FGF2 is necessary for normal cardiac growth and normal myocardial compliance. Hi FGF2 is necessary only in female hearts for mediating the hypertrophic response. Hence, I demonstrate that Lo FGF2 and Hi FGF2 have non-redundant roles in cardiac physiology and remodeling which are gender-specific.

Centrosomes are organelles that promote microtubule growth. Normally, a single centrosome duplicates once each cell cycle to guide assembly of a bipolar mitotic spindle, ensuring that each daughter cell inherits an equal complement of the genome and a single centrosome. Centrosomes are composed of a pair of ‘mother-daughter’ centrioles and, during duplication, each mother centriole assembles one daughter at a single site. However, mother centrioles can inappropriately assemble multiple daughters, thereby generating centriole amplification (or overduplication), resulting in multipolar spindle assembly and, consequently, chromosome missegration - a driving force for chromosomal instability/aneuploidy which induces birth defects, miscarriage, and tumorigenesis. We have elucidated how the cell cycle control program regulates the centriole duplication machinery to limit centriole duplication to one event per cell cycle via the cell cycle-dependent regulation of Ana2/STIL and PLK4 degradation. In the case of the centrosome licensing factor Plk4, we found that autophosphorylation promotes its own destruction during interphase, which is then counteracted by the Protein Phosphatase 2A (PP2A) in complex with its Twins (tws) regulatory subunit during mitosis. This promotes stabilization of Plk4 and thus allows for the licensing of the mother centriole, making it competent to duplicate during the proceeding S-phase. While PP2Atws plays a positive role in regulating Plk4 to promote centriole duplication, we found that PP2A complexed with the Well-rounded (wrd) and Widerborst (wdb) regulatory subunits negatively regulates Ana2 by promoting its degradation to limit centriole duplication. PP2Awrd/wdb dephosphorylates numerous serine/threonine residues residing in Ana2, including several CDK phosphorylation consensus motifs. We found that CDK1/cycA and CDK2/cycE phosphorylate these residues to promote Ana2 stabilization from S-phase, the start of centriole duplication, to M-phase, the start of centriole duplication licensing. Interestingly, we found that the tumorigenic SV40 virus protein Small Tumor Antigen (ST) amplifies centrioles by targeting the PP2A complex to stabilize Plk4 as well as Ana2, underscoring the oncogenic importance of these newly discovered centriole duplication pathways. Finally, we shed insight into the mechanism for centriole amplification upon Ana2 stabilization by showing that Ana2 associates with Plk4 to promote Plk4 kinase activity as well as Plk4 stabilization.

Having an in vivo model of development that develops quickly and efficiently is important for investigators to elucidate the critical steps, components and signaling pathways involved in building a myofibril; hence a compliant in vivo model would provide a pivotal foundation for deciphering muscle disease mechanisms as well as the development of myopathy-related therapeutics. Here, we take advantage of a relatively quick, cost effective, and molecularly pliable developmental model system in the Xenopus laevis (frog) embryo and establish it as an in vivo model to study the roles of sarcomeric proteins during de novo myofibrillogenesis.Using the Xenopus model, we elucidated the functional roles of Leiomodin3 (Lmod3) and Tropomodulin 4 (Tmod4) during de novo skeletal myofibrillogenesis. Tmods have been demonstrated to contribute to thin filament length uniformity by regulating both elongation and depolymerization of actin-thin filaments' pointed-ends. Lmods, which are structurally related to Tmod proteins also localize to actin filament pointed-ends. In situ hybridization studies demonstrated that of their respective families, only tmod4 and lmod3 transcripts were expressed at high levels in skeletal muscle from the earliest stages of development. When reducing their protein levels via morpholino (MO) treatment, thin filament regulation and sarcomere assembly were compromised. Surprisingly, alternate rescues (i.e., lmod3 mRNA co-injected with Tmod4 MO and vice versa) partially restored myofibril structure and actin-thin filament organization. Thus, our results not only indicate that both Tmod4 and Lmod3 are critical for myofibrillogenesis during Xenopus skeletal muscle development, but also revealed that they may share redundant functions during skeletal muscle thin filament assembly.