KazrinE is a desmosome-associated liprin that colocalises with acetylated microtubules

The Structural Dynamics, Complexity of Interactions, and Functions in Cancer of Multi-SAM Containing Proteins

SAM domains are crucial mediators of diverse interactions, including those important for tumorigenesis or metastasis of cancers, and thus SAM domains can be attractive targets for developing cancer therapies. This review aims to explore the literature, especially on the recent findings of the structural dynamics, regulation, and functions of SAM domains in proteins containing more than one SAM (multi-SAM containing proteins, MSCPs). The topics here include how intrinsic disorder of some SAMs and an additional SAM domain in MSCPs increase the complexity of their interactions and oligomerization arrangements. Many similarities exist among these MSCPs, including their effects on cancer cell adhesion, migration, and metastasis. In addition, they are all involved in some types of receptor-mediated signaling and neurology-related functions or diseases, although the specific receptors and functions vary. This review also provides a simple outline of methods for studying protein domains, which may help non-structural biologists to reach out and build new collaborations to study their favorite protein domains/regions. Overall, this review aims to provide representative examples of various scenarios that may provide clues to better understand the roles of SAM domains and MSCPs in cancer in general.

Kazrin promotes dynein/dynactin-dependent traffic from early to recycling endosomes

Kazrin is a protein widely expressed in vertebrates whose depletion causes a myriad of developmental defects, in part derived from altered cell adhesion and migration, as well as failure to undergo epidermal to mesenchymal transition. However, the primary molecular role of kazrin, which might contribute to all these functions, has not been elucidated yet. We previously identified one of its isoforms, kazrin C, as a protein that potently inhibits clathrin-mediated endocytosis when overexpressed. We now generated kazrin knock-out mouse embryonic fibroblasts to investigate its endocytic function. We found that kazrin depletion delays juxtanuclear enrichment of internalized material, indicating a role in endocytic traffic from early to recycling endosomes. Consistently, we found that the C-terminal domain of kazrin C, predicted to be an intrinsically disordered region, directly interacts with several early endosome (EE) components, and that kazrin depletion impairs retrograde motility of these organelles. Further, we noticed that the N-terminus of kazrin C shares homology with dynein/dynactin adaptors and that it directly interacts with the dynactin complex and the dynein light intermediate chain 1. Altogether, the data indicate that one of the primary kazrin functions is to facilitate endocytic recycling by promoting dynein/dynactin-dependent transport of EEs or EE-derived transport intermediates to the recycling endosomes.

Identification of a Novel Serological Marker in Seronegative Rheumatoid Arthritis Using the Peptide Library Approach

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic inflammation mainly affecting the joints leading to cartilage and bone destruction. The definition of seropositive or seronegative RA is based on the presence or absence of rheumatoid factor (RF) and anti-citrullinated peptide antibodies (ACPAs). Other autoantibodies have been identified in the last decade such as antibodies directed against carbamylated antigens, peptidyl-arginine deiminase type 4 and v-Raf murine sarcoma viral oncogene homologue B. In order to identify relevant autoantigens, we screened a random peptide library (RPL) with pooled IgGs obtained from 50 patients with seronegative RA. Patients’ sera were then used in an ELISA test to identify the most frequently recognized peptide among those obtained by screening the RPL. Sera from age- and sex-matched healthy subjects were used as controls. We identified a specific peptide (RA-peptide) recognized by RA patients’ sera, but not by healthy subjects or by patients with other immune-mediated diseases. The majority of sera from seronegative and seropositive RA patients (73.8% and 63.6% respectively) contained IgG antibodies directed against the RA-peptide. Interestingly, this peptide shares homology with some self-antigens, such as Protein-tyrosine kinase 2 beta, B cell scaffold protein, Liprin-alfa1 and Cytotoxic T lymphocyte protein 4. Affinity purified anti-RA-peptide antibodies were able to cross react with these autoantigens. In conclusion, we identified a peptide that is recognized by seropositive and, most importantly, by seronegative RA patients’ sera, but not by healthy subjects, conferring to this epitope a high degree of specificity. This peptide shares also homology with other autoantigens which can be recognized by autoantibodies present in seronegative RA sera. These newly identified autoantibodies, although present also in a percentage of seropositive RA patients, may be considered as novel serum biomarkers for seronegative RA, which lacks the presence of RF and/or ACPAs.

Liprins in oncogenic signaling and cancer cell adhesion

Liprins are a multifunctional family of scaffold proteins, identified by their involvement in several important neuronal functions related to signaling and organization of synaptic structures. More recently, the knowledge on the liprin family has expanded from neuronal functions to processes relevant to cancer progression, including cell adhesion, cell motility, cancer cell invasion, and signaling. These proteins consist of regions, which by prediction are intrinsically disordered, and may be involved in the assembly of supramolecular structures relevant for their functions. This review summarizes the current understanding of the functions of liprins in different cellular processes, with special emphasis on liprins in tumor progression. The available data indicate that liprins may be potential biomarkers for cancer progression and may have therapeutic importance.

Structural insights into selective interaction between type IIa receptor protein tyrosine phosphatases and Liprin-α

Synapse formation is induced by transsynaptic interaction of neuronal cell-adhesion molecules termed synaptic organizers. Type IIa receptor protein tyrosine phosphatases (IIa RPTPs) function as presynaptic organizers. The cytoplasmic domain of IIa RPTPs consists of two phosphatase domains, and the membrane-distal one (D2) is essential for synapse formation. Liprin-α, which is an active zone protein critical for synapse formation, interacts with D2 via its C-terminal domain composed of three tandem sterile alpha motifs (tSAM). Structural mechanisms of this critical interaction for synapse formation remain elusive. Here, we report the crystal structure of the complex between mouse PTPδ D2 and Liprin-α3 tSAM at 1.91 Å resolution. PTPδ D2 interacts with the N-terminal helix and the first and second SAMs (SAM1 and SAM2, respectively) of Liprin-α3. Structure-based mutational analyses in vitro and in cellulo demonstrate that the interactions with Liprin-α SAM1 and SAM2 are essential for the binding and synaptogenic activity.

Genetic background influences susceptibility to chemotherapy-induced hematotoxicity

Hematotoxicity is a life-threatening side effect of many chemotherapy regimens. While clinical factors influence patient responses, genetic factors may also play an important role. We sought to identify genomic loci that influence chemotherapy-induced hematotoxicity by dosing Diversity Outbred mice with one of three chemotherapy drugs; doxorubicin, cyclophosphamide or docetaxel. We observed that each drug had a distinct effect on both the changes in blood cell sub-populations and the underlying genetic architecture of hematotoxicity. For doxorubicin, we mapped the change in cell counts before and after dosing and found that alleles of ATP-binding cassette B1B (Abcb1b) on chromosome 5 influence all cell populations. For cyclophosphamide and docetaxel, we found that each cell population was influenced by distinct loci, none of which overlapped between drugs. These results suggest that susceptibility to chemotherapy-induced hematotoxicity is influenced by different genes for different chemotherapy drugs.

Upregulation of kazrin F by miR-186 suppresses apoptosis but promotes epithelial-mesenchymal transition to contribute to malignancy in human cervical cancer cells

Objective

Previous studies have identified that kazrin is a constituent of desmosome and influences intercellular adhesion, growing development and morphology. We previously cloned another new isoform, kazrin F and found that it has anti-apoptotic effects on human glioma cell line. To further explore whether kazrin F is involved in tumorigenesis, we investigated its expression and role in cervical cancer (CC) cells.

Methods

The role of kazrin F and miR-186 in CC was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, colony formation, transwell, and apoptosis assays. Using enhanced green fluorescent protein (EGFP) reporter assays, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blot analysis, we identified kazrin F post-transcriptional regulation by miR-186.

Results

We demonstrate that kazrin F is highly expressed in CC tissues compared with the adjacent noncancerous tissues and promotes cell proliferation, colony formation, migration and invasion in HeLa and C33A cells by suppressing apoptosis and facilitating epithelial-to-mesenchymal transition (EMT). Furthermore, miR-186 was confirmed as a regulator of kazrin F dysregulation. An EGFP reporter assay proved that miR-186 directly targets the 3’-untranslated region (3’UTR) of kazrin F and downregulates its expression, and miR-186 expression showed an inverse correlation with kazrin F levels in CC tissues. In addition, overexpression of miR-186 suppressed the malignant behaviors of CC cells. The ectopic expression of kazrin F rescued the inhibitory effects of miR-186.

Conclusions

Our findings indicate that the upregulation of kazrin F due to downregulated miR-186 levels contributes to malignancy, and highlight the significance of kazrin F in CC tumorigenesis.

Functional Analysis of Periplakin and Envoplakin, Cytoskeletal Linkers, and Cornified Envelope Precursor Proteins

Envoplakin and periplakin are the two smallest plakin family cytoskeletal linker proteins that connect intermediate filaments to cellular junctions and other membrane locations. These two plakins have a structural role in the assembly of the cornified envelope (CE), the terminal stage of epidermal differentiation. Analysis of gene-targeted mice lacking both these plakins and the third initial CE scaffold protein, involucrin, demonstrate the importance of the structural integrity of CE for a proper epidermal barrier function. It has emerged that periplakin, which also has a wider tissue distribution than envoplakin, has additional, independent roles. Periplakin participates in the cytoskeletal organization also in other tissues and interacts with a wide range of membrane-associated proteins such as kazrin and butyrophilin BTN3A1. This review covers methods used to understand periplakin and envoplakin functions in cell culture models, including siRNA ablation of periplakin expression and the use of tagged protein domain constructs to study localization and interactions. In addition, assays that can be used to analyze CEs and epidermal barrier function in gene-targeted mice are described and discussed.

Structural and Biochemical Basis for the Inhibitory Effect of Liprin-α3 on Mouse Diaphanous 1 (mDia1) Function

Diaphanous-related formins are eukaryotic actin nucleation factors regulated by an autoinhibitory interaction between the N-terminal RhoGTPase-binding domain (mDiaN) and the C-terminal Diaphanous-autoregulatory domain (DAD). Although the activation of formins by Rho proteins is well characterized, its inactivation is only marginally understood. Recently, liprin-α3 was shown to interact with mDia1. Overexpression of liprin-α3 resulted in a reduction of the cellular actin filament content. The molecular mechanisms of how liprin-α3 exerts this effect and counteracts mDia1 activation by RhoA are unknown. Here, we functionally and structurally define a minimal liprin-α3 core region, sufficient to recapitulate the liprin-α3 determined mDia1-respective cellular functions. We show that liprin-α3 alters the interaction kinetics and thermodynamics of mDiaN with RhoA·GTP and DAD. RhoA displaces liprin-α3 allosterically, whereas DAD competes with liprin-α3 for a highly overlapping binding site on mDiaN. Liprin-α3 regulates actin polymerization by lowering the regulatory potency of RhoA and DAD on mDiaN. We present a model of a mechanistically unexplored and new aspect of mDiaN regulation by liprin-α3.

Intratumor heterogeneity and clonal evolution in an aggressive papillary thyroid cancer and matched metastases

The contribution of intratumor heterogeneity to thyroid metastatic cancers is still unknown. The clonal relationships between the primary thyroid tumors and lymph nodes (LN) or distant metastases are also poorly understood. The objective of this study was to determine the phylogenetic relationships between matched primary thyroid tumors and metastases. We searched for non-synonymous single-nucleotide variants (nsSNVs), gene fusions, alternative transcripts, and loss of heterozygosity (LOH) by paired-end massively parallel sequencing of cDNA (RNA-Seq) in a patient diagnosed with an aggressive papillary thyroid cancer (PTC). Seven tumor samples from a stage IVc PTC patient were analyzed by RNA-Seq: two areas from the primary tumor, four areas from two LN metastases, and one area from a pleural metastasis (PLM). A large panel of other thyroid tumors was used for Sanger sequencing screening. We identified seven new nsSNVs. Some of these were early events clonally present in both the primary PTC and the three matched metastases. Other nsSNVs were private to the primary tumor, the LN metastases and/or the PLM. Three new gene fusions were identified. A novel cancer-specific KAZN alternative transcript was detected in this aggressive PTC and in dozens of additional thyroid tumors. The PLM harbored an exclusive whole-chromosome 19 LOH. We have presented the first, to our knowledge, deep sequencing study comparing the mutational spectra in a PTC and both LN and distant metastases. This study has yielded novel findings concerning intra-tumor heterogeneity, clonal evolution and metastases dissemination in thyroid cancer.

Desmosomes: regulators of cellular signaling and adhesion in epidermal health and disease

Desmosomes are intercellular junctions that mediate cell-cell adhesion and anchor the intermediate filament network to the plasma membrane, providing mechanical resilience to tissues such as the epidermis and heart. In addition to their critical roles in adhesion, desmosomal proteins are emerging as mediators of cell signaling important for proper cell and tissue functions. In this review we highlight what is known about desmosomal proteins regulating adhesion and signaling in healthy skin-in morphogenesis, differentiation and homeostasis, wound healing, and protection against environmental damage. We also discuss how human diseases that target desmosome molecules directly or interfere indirectly with these mechanical and signaling functions to contribute to pathogenesis.

A new role of multi scaffold protein Liprin-α: Liprin-α suppresses Rho-mDia mediated stress fiber formation

Regulation of the actin cytoskeleton is crucial for cell morphology and migration. One of the key molecules that regulates actin remodeling is the small GTPase Rho. Rho shuttles between the inactive GDP-bound form and the active GTP-bound form, and works as a molecular switch in actin remodeling in response to both extra- and intra-cellular stimuli. Mammalian homolog of Diaphanous (mDia) is one of the Rho effectors and produces unbranched actin filaments. While Rho GTPases activate mDia, the mechanisms of how the activity of mDia is downregulated in cells remains largely unknown. In our recent paper, we identified Liprin-α as an mDia interacting protein and found that Liprin-α negatively regulates the activity of mDia in the cell by displacing it from the plasma membrane through binding to the DID-DD region of mDia. Here, we review these findings and discuss how Liprin-α regulates the Rho-mDia pathway and how the mDia-Liprin-α complex functions in vivo.

Exons 5-15 of kazrin are dispensable for murine epidermal morphogenesis and homeostasis

Kazrin binds to periplakin and ARVCF catenin, and regulates adhesion and differentiation of cultured human keratinocytes. To explore kazrin function in vivo, we generated a kazrin gene-trap mouse in which only exons 1-4 were expressed, fused to β-galactosidase. On transient transfection, the protein encoded by exons 1-4 did not enter the nucleus, but did cause keratinocyte shape changes. The mice had no obvious defects in skin development or homeostasis, and periplakin and desmoplakin localization was normal. Expression of the kazrin-β-galactosidase fusion protein faithfully reported endogenous kazrin expression. Kazrin was not expressed in embryonic epidermis and was first detected at postnatal day 1. In adult mice, epidermal kazrin expression was less widespread than in humans and Xenopus, being confined to the bulb of anagen hair follicles, the infundibulum, and parakeratotic tail epidermis. In anagen bulbs, kazrin was expressed by a band of cells with elongated morphology and low desmoplakin levels, suggesting a role in morphogenetic cell movements. We conclude that exons 5-15 of kazrin, encoding the nuclear localization signal and C-terminal domain, are not required for epidermal development and function. The previously reported role of kazrin in regulating cell shape appears to reside within the N-terminal coiled-coil domain encoded by exons 1-4.

Annexin A9 is a periplakin interacting partner in membrane-targeted cytoskeletal linker protein complexes

Periplakin regulates keratin organisation and participates in the assembly of epidermal cornified envelopes. A proteomic approach identified annexin A9 as a novel interacting partner for periplakin N-terminus. The presence of annexin A9 in complexes with periplakin was confirmed by immunoblotting of proteins immunoprecipitated by anti-HA or anti-annexin A9 antibodies. Both endogenous and GFP-tagged annexin A9 co-localise with endogenous periplakin and transfected periplakin N-terminus at MCF-7 cell borders and aggregate after Okadaic acid treatment. Annexin A9 and periplakin co-localise in the epidermis and annexin A9 is up-regulated in differentiating keratinocytes, but the epidermal annexin A9 expression does not require periplakin.

Tandem SAM domain structure of human Caskin1: a presynaptic, self-assembling scaffold for CASK

The synaptic scaffolding proteins CASK and Caskin1 are part of the fibrous mesh of proteins that organize the active zones of neural synapses. CASK binds to a region of Caskin1 called the CASK interaction domain (CID). Adjacent to the CID, Caskin1 contains two tandem sterile α motif (SAM) domains. Many SAM domains form polymers so they are good candidates for forming the fibrous structures seen in the active zone. We show here that the SAM domains of Caskin1 form a new type of SAM helical polymer. The Caskin1 polymer interface exhibits a remarkable segregation of charged residues, resulting in a high sensitivity to ionic strength in vitro. The Caskin1 polymers can be decorated with CASK proteins, illustrating how these proteins may work together to organize the cytomatrix in active zones.

Kazrin, and its binding partners ARVCF- and delta-catenin, are required for Xenopus laevis craniofacial development

The novel adaptor protein Kazrin associates with multifunctional entities including p120-subfamily members (ARVCF-, delta-, and p0071-catenin). Critical contributions of Kazrin to development or homeostasis are indicated with respect to ectoderm formation, integrity and keratinocyte differentiation, whereas its presence in varied tissues suggests broader roles. We find that Kazrin is maternally loaded, is expressed across development and becomes enriched in the forming head. Kazrin's potential contributions to craniofacial development were probed by means of knockdown in the prospective anterior neural region. Cartilaginous head structures as well as eyes on injected sides were reduced in size, with molecular markers suggesting an impact upon neural crest cell establishment and migration. Similar effects followed the depletion of ARVCF (or delta-catenin), with Kazrin:ARVCF functional interplay supported upon ARVCF partial rescue of Kazrin knockdown phenotypes. Thus, Kazrin and its associating ARVCF- and delta-catenins, are required to form craniofacial tissues originating from cranial neural crest and precordal plate.

Crystal structure of the central coiled-coil domain from human liprin-β2

Liprins are a conserved family of scaffolding proteins important for the proper regulation and development of neuronal synapses. Humans have four liprin-αs and two liprin-βs which all contain long coiled-coil domains followed by three tandem SAM domains. Complex interactions between the coiled-coil and SAM domains are thought to create liprin scaffolds, but the structural and biochemical properties of these domains remain largely uncharacterized. In this study we find that the human liprin-β2 coiled-coil forms an extended dimer. Several protease-resistant subdomains within the liprin-β1 and liprin-β2 coiled-coils were also identified. A 2.0 Å crystal structure of the central, protease-resistant core of the liprin-β2 coiled-coil reveals a parallel helix orientation. These studies represent an initial step toward determining the overall architecture of liprin scaffolds and understanding the molecular basis for their synaptic functions.

Three Drosophila liprins interact to control synapse formation

Liprin-α proteins are adaptors that interact with the receptor protein tyrosine phosphatase leukocyte common antigen-related (LAR) and other synaptic proteins to promote synaptic partner selection and active zone assembly. Liprin-β proteins bind to and share homology with Liprin-α proteins, but their functions at the synapse are unknown. The Drosophila genome encodes single Liprin-α and Liprin-β homologs, as well as a third related protein that we named Liprin-γ. We show that both Liprin-β and Liprin-γ physically interact with Liprin-α and that Liprin-γ also binds to LAR. Liprin-α mutations have been shown to disrupt synaptic target layer selection by R7 photoreceptors and to reduce the size of larval neuromuscular synapses. We have generated null mutations in Liprin-β and Liprin-γ to investigate their role in these processes. We find that, although Liprin-α mutant R7 axons terminate before reaching the correct target layer, Liprin-β mutant R7 axons grow beyond their target layer. Larval neuromuscular junction size is reduced in both Liprin-α and Liprin-β mutants, and further reduced in double mutants, suggesting independent functions for these Liprins. Genetic interactions demonstrate that both Liprin proteins act through the exchange factor Trio to promote stable target selection by R7 photoreceptor axons and growth of neuromuscular synapses. Photoreceptor and neuromuscular synapses develop normally in Liprin-γ mutants; however, removing Liprin-γ improves R7 targeting in Liprin-α mutants, and restores normal neuromuscular junction size to Liprin-β mutants, suggesting that Liprin-γ counteracts the functions of the other two Liprins. We propose that context-dependent interactions between the three Liprins modulate their functions in synapse formation.

Liprin-α1 regulates breast cancer cell invasion by affecting cell motility, invadopodia and extracellular matrix degradation

Migration of cells and degradation of the extracellular matrix (ECM) are required for efficient tumor cell invasion, but the underlying molecular mechanisms are only partially known. The PPFIA1 gene for liprin-α1 is frequently amplified in human breast cancers. We recently demonstrated that liprin-α1 is an important regulator of cell edge dynamics during motility. We show, herein, that the liprin-α1 protein is highly expressed in human breast tumors. Functional analysis shows that liprin-α1 is specifically required for the migration and invasion of highly invasive human breast cancer MDA-MB-231 cells. We used time-lapse analysis to demonstrate defects in the motility of liprin-α1-depleted cells that include a striking instability of the lamellipodia. Liprin-α1 levels altered by either RNA interference or overexpression affected also cell spreading and the number of invadopodia per cell, but not the density of invadopodia per unit of surface area. On the other hand, silencing of liprin-α1 inhibited the degradation of the ECM, whereas its overexpression enhanced degradation, resulting in significant negative or positive effects, respectively, on the area of degradation per invadopodium. Transfection of fluorescent-labeled cortactin revealed that depletion of liprin-α1 also affected the assembly and disassembly of invadopodia, with decrease of their lifetime. Our results strongly support a novel important role of liprin-α1 in the regulation of human tumor cell invasion.

Xenopus Kazrin interacts with ARVCF-catenin, spectrin and p190B RhoGAP, and modulates RhoA activity and epithelial integrity

In common with other p120-catenin subfamily members, Xenopus ARVCF (xARVCF) binds cadherin cytoplasmic domains to enhance cadherin metabolic stability or, when dissociated, modulates Rho-family GTPases. We report here that xARVCF binds and is stabilized by Xenopus KazrinA (xKazrinA), a widely expressed conserved protein that bears little homology to established protein families, and which is known to influence keratinocyte proliferation and differentiation and cytoskeletal activity. Although we found that xKazrinA binds directly to xARVCF, we did not resolve xKazrinA within a larger ternary complex with cadherin, nor did it co-precipitate with core desmosomal components. Instead, screening revealed that xKazrinA binds spectrin, suggesting a potential means by which xKazrinA localizes to cell-cell borders. This was supported by the resolution of a ternary biochemical complex of xARVCF-xKazrinA-xβ2-spectrin and, in vivo, by the finding that ectodermal shedding followed depletion of xKazrin in Xenopus embryos, a phenotype partially rescued with exogenous xARVCF. Cell shedding appeared to be the consequence of RhoA activation, and thereby altered actin organization and cadherin function. Indeed, we also revealed that xKazrinA binds p190B RhoGAP, which was likewise capable of rescuing Kazrin depletion. Finally, xKazrinA was found to associate with δ-catenins and p0071-catenins but not with p120-catenin, suggesting that Kazrin interacts selectively with additional members of the p120-catenin subfamily. Taken together, our study supports the essential role of Kazrin in development, and reveals the biochemical and functional association of KazrinA with ARVCF-catenin, spectrin and p190B RhoGAP.

Liprin-α4 is a new hypoxia-inducible target gene required for maintenance of cell-cell contacts

Liprin-α1 to liprin-α4 constitute a family of cytoplasmic proteins, which have been found in various multiprotein complexes. For liprin-α1 roles in synapse formation and cell spreading were described but other liprin family members are not well characterized. We show here that liprin-α4 is upregulated in human clear cell renal cell carcinomas (RCC) as compared to normal kidney tissue. Liprin-α4 expression is downregulated by the von Hippel-Lindau tumor suppressor (VHL) and upregulated by hypoxia in RCC cell lines. The liprin-α4 gene promoter is directly activated by binding of the hypoxia-inducible factor 1α (HIF-1α) to HRE consensus binding sites as shown by reporter assays and chromatin immunoprecipitations. RNAi mediated knockdown of liprin-α4 leads to reduced E-cadherin and β-catenin levels at cell junctions and to dissociation of epithelial cell contacts. Our data describe for the first time liprin-α4 as a hypoxia-induced gene potentially involved in cell-cell adhesion.