Recombinant Xenopus tropicalis Protein KIBRA (wwc1), partial

What is KIBRA (WWC1) and what are its primary functions?

KIBRA (WWC1) encodes a multi-domain scaffold protein that functions as a regulator of the Hippo/SWH (Sav/Wts/Hpo) signaling pathway. This pathway plays a pivotal role in tumor suppression by restricting proliferation and promoting apoptosis . KIBRA functions in multiple cellular processes, including:

• Transcriptional coactivation of ESR1, essential for DYNLL1-mediated ESR1 transactivation

• Regulation of collagen-stimulated activation of the ERK/MAPK cascade

• Modulation of directional migration of podocytes

• Substrate for PRKCZ (Protein Kinase C zeta)

• Cognitive functions and memory performance

In molecular terms, KIBRA acts upstream of the Hippo pathway, interacting with MERLIN and LATS1/2 to inhibit the oncogenic transcriptional co-activators YAP/TAZ .

Why is Xenopus tropicalis preferred over Xenopus laevis for studying KIBRA?

Xenopus tropicalis offers several advantages over Xenopus laevis for genetic and functional studies of proteins like KIBRA:

• Xenopus tropicalis is the only known diploid species in the Xenopus genus, while X. laevis is allotetraploid

• The diploid genome structure of X. tropicalis is more likely to conserve gene function with mammalian species, providing a better comparative model

• X. tropicalis shows remarkable synteny with mammalian genomes, often in stretches of a hundred genes or more, far greater than that seen between fish and mammals

• The X. tropicalis genome has been fully sequenced, providing a comprehensive platform for genetic research

• Genetic manipulations are more straightforward in a diploid organism, avoiding the complications of redundant gene copies

These advantages make X. tropicalis particularly suitable for investigating conserved genes like KIBRA in contexts relevant to human biology.

What expression patterns of KIBRA have been documented during Xenopus development?

While specific expression patterns of KIBRA in Xenopus tropicalis development aren't detailed in the provided sources, its function can be inferred from its role in the Hippo signaling pathway. In vertebrates, KIBRA is likely expressed in tissues where Hippo signaling regulates organ size and tissue homeostasis. Given KIBRA's role in cognition and memory, expression in neural tissues would be expected. The expression pattern would be informative for researchers developing tissue-specific studies of KIBRA function.

To analyze KIBRA expression, researchers typically employ:

• RT-PCR to quantify mRNA levels

• In situ hybridization to visualize spatial expression patterns

• Immunohistochemistry to detect protein localization

• Western blotting to measure protein levels in different tissues

How can recombinant KIBRA protein be utilized in Xenopus research?

Recombinant Xenopus tropicalis Protein KIBRA (wwc1) can be utilized in multiple experimental applications:

• Protein-protein interaction studies: Purified recombinant KIBRA can be used in pull-down assays, co-immunoprecipitation, or BioID proximity labeling to identify novel interaction partners

• Functional domain analysis: Using recombinant KIBRA variants with specific domain deletions can help determine which domains are essential for particular functions

• Antibody production: Recombinant KIBRA can serve as an antigen for generating specific antibodies for Xenopus research

• In vitro enzymatic assays: As KIBRA interacts with kinases like LATS1/2, recombinant protein can be used in kinase assays

• Structure-function analysis: Purified protein can be used for structural studies using X-ray crystallography or cryo-EM

The table below shows available sources for recombinant Xenopus tropicalis KIBRA protein:

What methods are optimal for manipulating KIBRA expression in Xenopus tropicalis?

Several approaches can be employed to manipulate KIBRA expression in Xenopus tropicalis for functional studies:

• Morpholino oligonucleotides: For transient knockdown of KIBRA during early development

• CRISPR-Cas9 genome editing: For generating stable knockout or knock-in lines

• mRNA microinjection: For overexpression studies

• Stable cell lines: Generation of X. tropicalis cell lines with altered KIBRA expression using retroviral or lentiviral vectors

• Transient transfection: For in vitro studies using X. tropicalis-derived cell lines

• Tissue-specific manipulation: Using tissue-specific promoters for targeted expression or chimeric approaches combining mutant and wildtype tissues

For PCR-based analysis of KIBRA expression, researchers should design primers using Primer3 (http://bioinfo.ut.ee/primer3-0.4.0/) and normalize to appropriate reference genes like Gapdh, Hprt, and Rpl13a for mouse models or similar housekeeping genes for X. tropicalis .

How can researchers effectively validate KIBRA knockdown or overexpression in Xenopus models?

Validating KIBRA manipulation requires multiple complementary approaches:

• mRNA level validation:

  • RT-qPCR using KIBRA-specific primers

  • In situ hybridization to visualize spatial changes in expression

• Protein level validation:

  • Western blotting using KIBRA-specific antibodies

  • Immunohistochemistry to assess changes in protein localization

• Functional validation:

  • Analysis of Hippo pathway target gene expression (e.g., CYR61 and CTGF)

  • Assessment of YAP/TAZ nuclear localization by immunofluorescence

  • Evaluation of downstream phenotypes such as cell proliferation, migration, or apoptosis

• Rescue experiments:

  • Co-expression of exogenous KIBRA to rescue knockdown phenotypes

  • Expression of domain-specific mutants to identify critical functional regions

How does KIBRA regulate the Hippo signaling pathway in Xenopus and how does this compare to mammalian systems?

KIBRA functions as an upstream regulator of the Hippo tumor suppressor pathway in vertebrates, including Xenopus. The molecular mechanism appears conserved across species:

• KIBRA forms a complex with MERLIN and LATS1/2 kinases, promoting LATS1/2 phosphorylation and activation

• Activated LATS1/2 phosphorylates YAP/TAZ transcriptional co-activators

• Phosphorylated YAP/TAZ is sequestered in the cytoplasm, preventing its nuclear accumulation and transcriptional activity

• This inhibits expression of YAP/TAZ target genes involved in proliferation and anti-apoptotic functions

Studies in MDA-MB-231 cells showed that KIBRA expression severely diminished nuclear YAP/TAZ localization in response to extracellular matrix stiffness, an effect that was abrogated by deletion of the WW domains . This mechanotransduction-dependent regulation is likely conserved in Xenopus tropicalis.

In some contexts, KIBRA function may differ between species. For example, in MDA-MB-231 cells, KIBRA did not induce LATS1/2 auto-phosphorylation, possibly due to the absence of MERLIN . This highlights the importance of cellular context when studying KIBRA function across different model systems.

What is the significance of KIBRA's WW domains in its function and protein interactions?

The WW domains of KIBRA are critical for its function and protein-protein interactions:

• Functional significance: Deletion of the WW1/2 domains abolishes KIBRA's ability to:

  • Inhibit tumorsphere formation in cancer cells

  • Reduce expression of YAP/TAZ transcriptional targets (CYR61 and CTGF)

  • Prevent nuclear accumulation of YAP/TAZ in response to ECM stiffness

• Protein interactions: BioID proximity labeling identified PTPN14 (protein tyrosine phosphatase non-receptor 14) as a significant interactor with KIBRA that was lost when the WW domains were deleted . This interaction was validated by co-immunoprecipitation .

• Structural basis: WW domains typically recognize proline-rich motifs (often PPxY sequences) in interacting proteins. The WW domains of KIBRA likely mediate interactions with multiple proteins containing such motifs.

• Conservation: The importance of WW domains is likely conserved between Xenopus tropicalis and mammals, making X. tropicalis a valuable model for studying WW domain-dependent functions of KIBRA.

Understanding these domain-specific functions is crucial for designing targeted experimental approaches in Xenopus tropicalis.

How does KIBRA function as a metastasis suppressor and what implications does this have for cancer research?

KIBRA functions as a metastasis suppressor through several mechanisms:

• Inhibition of YAP/TAZ: By promoting LATS1/2-mediated phosphorylation of YAP/TAZ, KIBRA prevents the expression of genes driving cell proliferation and survival

• Regulation of cell invasion and migration: Studies showed that Kibra knockdown in mouse MMTV-Met tumor cells significantly increased:

  • The percentage of cysts displaying invasion into surrounding collagen matrix

  • Cell migration in two-dimensional assays

  • Metastatic capacity to lungs and lymph nodes in vivo

• Suppression of cancer stem cell properties: KIBRA expression reduced sphere-forming efficiency (SFE) in cancer cells, with this effect dependent on the WW domains

• Clinical relevance: Analysis of TCGA breast cancer data showed that claudin-low and basal tumors with KIBRA copy number loss displayed enrichment of a YAP/TAZ signature compared to tumors without KIBRA loss

These findings suggest that Xenopus tropicalis could serve as a valuable model for studying KIBRA's role in cancer progression and metastasis. The conservation of the Hippo pathway between X. tropicalis and mammals makes findings potentially translatable to human cancer biology.

What are the optimal methods for KIBRA mutagenesis studies in Xenopus tropicalis?

For KIBRA mutagenesis studies in Xenopus tropicalis, several approaches are recommended:

• Site-directed mutagenesis:

  • Q5 site-directed mutagenesis (New England Biolabs) has been successfully used to generate KIBRA mutants

  • This approach allows precise engineering of specific mutations in functional domains

• Domain deletion strategy:
  Key domains for targeted deletion include:

  • WW1/2 domains: Critical for protein-protein interactions, particularly with PTPN14

  • PDZ/aPKC binding regions: Involved in interactions with PDZ domain-containing proteins

  • Glu-rich regions: Potentially involved in protein-protein interactions

  • C2 domain: Involved in membrane association

• Expression systems:

  • For in vivo studies: pLVX lentiviral vector for N-terminally GFP-tagged wild-type and mutant KIBRA

  • For in vitro studies: pENTR11 vector for Gateway cloning compatibility

  • For BioID studies: pSTV2-BirA*-FLAG using Gateway LR clonase

• Functional validation assays:

  • Analysis of YAP/TAZ target gene expression by RT-qPCR

  • Assessment of nuclear YAP/TAZ localization by immunofluorescence

  • Evaluation of tumorsphere formation capacity

  • RHO-A activity assays and actin stress fiber scoring

These methodologies should be optimized for X. tropicalis, considering species-specific codon usage and expression requirements.

How can BioID proximity labeling be implemented to study KIBRA interactome in Xenopus models?

BioID proximity labeling is a powerful technique for identifying protein-protein interactions in vivo. For studying the KIBRA interactome in Xenopus models:

• Construct design:

  • Clone X. tropicalis KIBRA into a vector containing the promiscuous biotin ligase BirA* (e.g., pSTV2-BirA*-FLAG)

  • Generate domain-specific mutants (e.g., ΔWW1/2-KIBRA) to identify domain-dependent interactions

• Expression in Xenopus tropicalis:

  • Microinjection of mRNA encoding the BirA*-KIBRA fusion into embryos

  • For tissue-specific analysis, use tissue-specific promoters to drive expression

  • For cell culture studies, express in X. tropicalis-derived cell lines

• Biotinylation and isolation:

  • Supply biotin to the system (injection or media supplementation)

  • Allow BirA* to biotinylate proteins in proximity to KIBRA

  • Lyse tissues/cells and perform streptavidin pull-down to isolate biotinylated proteins

  • Analyze by mass spectrometry

• Data analysis:

  • Use significance analysis of interactome (SAINTexpress) to identify high-confidence interactors

  • Compare wild-type KIBRA interactome with domain mutants to identify domain-specific interactions

  • Validate key interactions by co-immunoprecipitation

This approach has successfully identified PTPN14 as a significant interactor with KIBRA dependent on the WW domains , and could be adapted to identify tissue-specific or developmental stage-specific interactions in Xenopus models.

What considerations are important when transitioning from in vitro KIBRA studies to in vivo Xenopus models?

When transitioning from in vitro to in vivo Xenopus studies of KIBRA, researchers should consider:

• Developmental timing:

  • Determine the appropriate developmental stages for manipulation and analysis

  • Consider stage-specific expression patterns of KIBRA and interacting partners

  • Plan interventions to coincide with relevant developmental processes

• Tissue specificity:

  • Target manipulations to relevant tissues (e.g., neural tissues for cognition studies)

  • Consider using tissue-specific promoters for targeted expression

  • Design tissue-specific phenotypic readouts

• Dosage and expression levels:

  • Calibrate protein or mRNA amounts for physiologically relevant expression

  • Consider potential dominant-negative effects of overexpression

  • Include appropriate controls for expression levels

• Functional readouts:

  • Design assays to measure relevant phenotypes (e.g., cell migration, proliferation)

  • Include molecular readouts of Hippo pathway activity

  • Consider behavioral assays for cognitive function studies

• Technical adaptations:

  • Optimize antibodies and PCR primers for X. tropicalis-specific sequences

  • Adjust protocols for embryo and tissue handling

  • Consider the advantages of X. tropicalis for live imaging studies

• Comparative approach:

  • Compare findings with mammalian models to assess conservation

  • Consider evolutionary differences that might impact interpretation

Xenopus tropicalis offers unique advantages as described by researchers: "This model may more closely capture important aspects of the pathology under investigation" compared to in vitro systems .

How conserved is the KIBRA gene and protein structure between Xenopus tropicalis and humans?

The conservation of KIBRA between Xenopus tropicalis and humans can be analyzed at multiple levels:

• Genomic conservation:

  • X. tropicalis shows remarkable synteny with mammalian genomes, with conserved gene organization often extending to hundreds of genes

  • This suggests conservation of genomic context around the KIBRA locus

• Protein domain structure:

  • Key functional domains of KIBRA (WW domains, C2 domain, PDZ-binding motif) are likely conserved

  • The high degree of conservation enables cross-species functional studies

• Signaling pathway integration:

  • KIBRA's role in the Hippo pathway appears conserved across vertebrates

  • Its interactions with core Hippo components like LATS1/2 are likely preserved

• Functional conservation:

  • KIBRA's roles in tumor suppression, regulation of YAP/TAZ, and potentially memory formation appear conserved

  • This functional conservation makes X. tropicalis studies relevant to human health

The high degree of conservation makes X. tropicalis an excellent model for studying KIBRA functions relevant to human biology and disease.

What are the advantages of using Xenopus tropicalis to model human diseases related to KIBRA dysfunction?

Xenopus tropicalis offers several advantages for modeling human diseases related to KIBRA dysfunction:

• Genetic tractability:

  • Diploid genome simplifies genetic manipulation and analysis

  • High conservation with human genome enables modeling of human mutations

• Developmental biology advantages:

  • External development allows direct visualization of developmental processes

  • Large embryo size facilitates microinjection and manipulation

  • Transparent embryos enable live imaging of cellular processes

• Experimental versatility:

  • Ability to generate large numbers of synchronously developing embryos

  • Feasibility of high-throughput screening approaches

  • Capability for tissue-specific and temporal manipulation

• Relevance to human disease:

  • "Xenopus: Driving the Discovery of Novel Genes in Patient Disease and Development" highlights the value of Xenopus for disease modeling

  • "Frog model organisms... [are] poised to serve as a high throughput vertebrate organism to model patient-driven genetic diseases"

  • Facilitates "investigation of effects of patient mutations on specific organs and signaling pathways"

• Translational potential:

  • "This approach promises a rapid investigation into novel mechanisms that disrupt normal organ morphology and function"

  • Provides an in vivo alternative to in vitro cell culture models that may better capture pathological mechanisms

These advantages make X. tropicalis particularly valuable for studying KIBRA's roles in cancer progression, developmental disorders, and potentially neurological conditions related to its role in cognition.

What are the most promising areas for future research on KIBRA function in Xenopus tropicalis?

Several promising research directions for KIBRA studies in Xenopus tropicalis include:

• Developmental roles:

  • Investigating KIBRA's function in organ size control during X. tropicalis development

  • Exploring tissue-specific requirements during embryogenesis

  • Analyzing potential roles in neural development related to its cognitive functions

• Cancer and metastasis models:

  • Developing X. tropicalis tumor models to study KIBRA's metastasis suppressor function

  • Investigating interactions between KIBRA and other cancer-related pathways

  • Testing pharmacological interventions targeting the Hippo pathway

• Mechanotransduction studies:

  • Exploring KIBRA's role in mechanosensing during development

  • Investigating how mechanical forces influence Hippo signaling in X. tropicalis tissues

  • Analyzing ECM stiffness effects on YAP/TAZ localization in the presence/absence of KIBRA

• Comparative genomics:

  • Detailed comparison of KIBRA function between X. tropicalis and X. laevis

  • Investigating potential subspecialization of duplicated KIBRA genes in X. laevis

  • Cross-species functional rescue experiments

• Novel interaction partners:

  • Comprehensive interactome studies using BioID or similar approaches

  • Identification of tissue-specific KIBRA interactors

  • Validation of evolutionarily conserved interaction networks

These research directions would leverage the unique advantages of X. tropicalis to provide insights into KIBRA function relevant to human health and disease.

How might novel techniques in genome editing enhance KIBRA functional studies in Xenopus?

Emerging genome editing technologies offer new opportunities for KIBRA functional studies in Xenopus tropicalis:

• Precise gene editing:

  • CRISPR-Cas9 with homology-directed repair (HDR) to introduce specific patient mutations

  • Base editing for introducing point mutations without double-strand breaks

  • Prime editing for precise insertions, deletions, or base changes

• Spatiotemporal control:

  • Optogenetic or chemically inducible Cas systems for temporal control of editing

  • Tissue-specific promoters driving Cas9 expression for spatial control

  • Split-Cas9 systems requiring multiple inputs for combinatorial regulation

• Multiplexed approaches:

  • Simultaneous editing of KIBRA and interacting partners

  • CRISPR interference (CRISPRi) or activation (CRISPRa) for modulating expression

  • Screening approaches using CRISPR libraries targeting Hippo pathway components

• Functional genomics integration:

  • Combination with single-cell RNA-seq to analyze cell-type-specific effects

  • Integration with ChIP-seq to identify YAP/TAZ binding sites affected by KIBRA

  • Proteomics approaches to identify post-translational modifications

• In vivo reporters:

  • Knock-in fluorescent tags for visualizing endogenous KIBRA localization

  • YAP/TAZ activity reporters to monitor Hippo pathway output

  • Live imaging of KIBRA dynamics during developmental processes

These advanced techniques would enhance our understanding of KIBRA function in normal development and disease contexts, leveraging the experimental advantages of Xenopus tropicalis.