Recombinant Human Leucine-rich repeat transmembrane protein FLRT2 (FLRT2)

What is the molecular structure of FLRT2?

FLRT2 is an 85 kDa type I transmembrane glycoprotein synthesized as a 660 amino acid precursor with a distinct domain organization:

• 35 amino acid signal sequence

• 506 amino acid extracellular domain (ECD)

• 21 amino acid transmembrane segment

• 98 amino acid cytoplasmic region

The extracellular domain contains 10 N-terminal leucine-rich repeats (LRRs) flanked by cysteine-rich areas, followed by a juxtamembrane fibronectin type III domain . The protein contains two dimerization motifs in its transmembrane helix, specifically involving Small-X3-Small motifs (G544-X3-G548 and G545-X3-G549), which facilitate dimerization in the cell membrane .

The human FLRT2 ECD shares high sequence conservation with other mammals: 97% identity with mouse, 96% with rat, 99% with equine, 96% with canine, and 95% with bovine FLRT2 ECD . Human FLRT1 and FLRT3 ECDs share approximately 47% amino acid identity with FLRT2 .

What are the primary physiological functions of FLRT2 in different tissues?

FLRT2 serves diverse functions across multiple biological systems:

Cellular adhesion and migration:

• Mediates cell-cell adhesion through homophilic binding via its LRR domains

• Functions in cell migration during development

• Forms interendothelial adhesions in blood vessels

Developmental processes:

• Interacts with FGF receptors via its fibronectin domain to regulate FGF signaling during development

• Required for normal cardiac basement membrane organization during embryogenesis

• Essential for normal embryonic epicardium and heart morphogenesis

• Contributes to vascular development and angiogenesis

Neuronal function:

• Plays roles in axon guidance via interaction with UNC5D

• Mediates axon growth cone collapse

• Contributes to cortical neuron migration during brain development

Vascular biology:

• Prevents endothelial cell senescence and vascular aging

• Forms tumor-specific interendothelial adhesions that enable abnormalized vessels to facilitate cancer progression

• Regulates venous-mediated angiogenic expansion

What is the tissue expression pattern of FLRT2 in normal physiology?

FLRT2 exhibits distinct expression patterns at different developmental stages and across various tissues:

Adult tissues:

• Highest expression in pancreas

• Also expressed in skeletal muscle, brain, and heart

• In the central nervous system, FLRT2 appears mainly in layer IV of the adult cerebral cortex and in the reticular thalamic nucleus

Embryonic and developmental expression:

• In mouse embryos, FLRT2 shows highest expression in a subset of the sclerotome in the brain

• Also expressed in the developing stomach and posterior to the developing heart

• Expression pattern distinct from FLRT1 and FLRT3

• At early postnatal stages, expression is largely restricted to several regions of the striatum and deep layers of the cerebral cortex

Vascular expression:

• Preferentially expressed in abnormalized vessels of advanced colorectal cancers

• Expression shows significant decrease in vascular tissues with aging

How should recombinant FLRT2 be handled and prepared for experiments?

Reconstitution protocol:

• Lyophilized FLRT2 should be reconstituted at 200 μg/mL in sterile PBS

• Reconstituted protein should be stored at recommended temperatures to maintain stability

Storage recommendations:

• Store immediately upon receipt at the recommended temperature

• Use a manual defrost freezer and avoid repeated freeze-thaw cycles to maintain protein integrity

Formulation considerations:

• Carrier-free (CF) recombinant protein lacks BSA as a carrier protein

• CF formulation is recommended for applications where the presence of BSA could interfere with experiments

• BSA-containing formulations enhance protein stability, increase shelf-life, and allow storage at more dilute concentrations

Applications guidance:

• For cell/tissue culture or ELISA standards, use recombinant protein with BSA

• For applications where BSA might interfere, use carrier-free protein

• Optimal dilutions should be determined by each laboratory for specific applications

What experimental approaches can be used to study FLRT2 function in different contexts?

Genetic manipulation approaches:

• Conditional knockout using Cre-lox systems (e.g., Flrt2fl/fl crossed with tissue-specific Cre lines)

• siRNA-mediated knockdown for transient reduction of FLRT2 expression

• Plasmid-based overexpression systems for gain-of-function studies

Functional assays:

• Cell proliferation assays to assess effects on growth (colony formation, dye-based proliferation)

• Migration assays to evaluate effects on cell motility

• Adhesion assays on different substrates (e.g., collagen I-coated plates)

• Flow cytometry for apoptosis measurements

• Single Molecule Tracking (SMT) microscopy for studying FLRT2 clustering and dynamics

Protein interaction studies:

• Co-immunoprecipitation to identify binding partners (e.g., with ITGB4, VE-cadherin)

• Molecular dynamics simulations for transmembrane domain interactions

• X-ray crystallography for structural characterization of binding interfaces

In vivo models:

• Mouse models with tissue-specific FLRT2 deletion

• Ischemic retinopathy models to study vascular abnormalization

• Spinal cord injury models to examine FLRT2 expression in response to injury

How does FLRT2 function as a tumor suppressor in breast cancer?

FLRT2 demonstrates tumor suppressor properties in breast cancer through several mechanisms:

Epigenetic regulation:

• FLRT2 is hypermethylated in breast cancer tissues compared to normal breast tissues

• Hypermethylation correlates with downregulation of FLRT2 expression

• Treatment with 5-Aza-2′-deoxycytidine (a demethylating agent) restores FLRT2 expression in most cancer cell lines, confirming methylation as a key regulatory mechanism

Effects on cellular processes:

• Downregulation of FLRT2 increases cell proliferation and migration

• Overexpression of FLRT2 reduces proliferation and migration

• FLRT2 significantly decreases cell adhesion to collagen I-coated surfaces

• Knockdown of FLRT2 in MCF-10A cells decreases apoptotic rate by 19%, supporting an anti-proliferation role

Signaling pathway regulation:

• FLRT2 manipulation affects the "Cancer, cellular movement, and tumor morphology" network

• EGFR and focal adhesion kinase (FAK) are upregulated when FLRT2 is downregulated

• Expression of proliferation-enhancing genes (TLR3, IRS2) increases when FLRT2 is knocked down

• Expression of cell proliferation-inhibiting genes (PTEN, PPARG) decreases when FLRT2 is downregulated

This data collectively supports FLRT2 as a novel tumor suppressor in breast cancer, which is inactivated by hypermethylation during tumor development .

What is the role of FLRT2 in tumor blood vessel abnormalization?

FLRT2 plays a complex role in tumor vasculature with seemingly contradictory effects:

Expression pattern in tumor vessels:

• FLRT2 is expressed preferentially in abnormalized vessels of advanced colorectal cancers in humans

• Expression correlates negatively with long-term survival in colorectal cancer patients

• High expression of endothelial FLRT2 is an independent risk factor for recurrence-free survival, regardless of tumor stage

Functional impact on tumor vasculature:

• FLRT2 forms noncanonical interendothelial adhesions through homophilic binding

• These adhesions help safeguard against oxidative stress

• Endothelial cell-specific deletion of Flrt2 in mice selectively prunes abnormalized vessels

• This pruning creates a unique metabolic state termed "oxygen-glucose uncoupling," which suppresses tumor metastasis

• Flrt2 deletion also increases the number of mature vessels, enhancing the efficacy of immune checkpoint blockers

Regulatory mechanisms:

• FLRT2 expression in endothelial cells is triggered by elevated reactive oxygen species (ROS)

• Treatment with the antioxidant N-acetyl-L-cysteine suppresses FLRT2 expression

• FLRT2 is abundant in aberrantly expanding neovessels exposed to high oxidative stress

These findings suggest FLRT2 enables abnormalized vessels to facilitate cancer aggressiveness, and targeting this adhesion complex could represent a therapeutic strategy to suppress cancer progression .

What experimental approaches can be used to target FLRT2 in cancer therapy development?

Researchers investigating FLRT2 as a therapeutic target can employ several experimental approaches:

For targeting hypermethylation in tumors where FLRT2 acts as a tumor suppressor:

• Demethylating agents (e.g., 5-Aza-2′-deoxycytidine) to restore FLRT2 expression

• Epigenetic modifier screening to identify compounds that specifically restore FLRT2 expression

• Development of targeted demethylation approaches using CRISPR-based systems

For targeting FLRT2-mediated abnormal vessel formation in tumors:

• Antibody-based blocking of FLRT2 homophilic interactions to disrupt interendothelial adhesions

• Design of peptide inhibitors targeting the LRR domains responsible for homophilic binding

• Combination therapy approaches testing FLRT2 inhibitors with immune checkpoint blockers

Experimental models for therapeutic testing:

• Patient-derived xenografts to evaluate the impact of FLRT2 targeting in human tumors

• Endothelial cell-specific knockout mouse models for studying vascular normalization effects

• In vitro co-culture systems of endothelial cells with cancer cells to assess tumor-endothelial interactions

Biomarker development:

• Use of FLRT2 expression as a prognostic marker in colorectal cancer

• Development of imaging agents targeting FLRT2 for visualization of abnormal tumor vessels

• Monitoring circulating FLRT2 levels as potential liquid biopsy biomarkers

How does FLRT2 dimerization occur and what are its functional implications?

FLRT2 dimerization involves specific structural motifs with important functional consequences:

Dimerization mechanisms:

• FLRT2 dimerizes in cis via dual transmembrane helix interactions

• Two dimerization motifs in the FLRT2 transmembrane helix have been identified

• These motifs are Small-X3-Small motifs: G544-X3-G548 and G545-X3-G549

• Molecular dynamics simulations reveal a dynamical equilibrium between conformations involving these two successive motifs

Experimental validation:

• Single particle tracking (SMT) experiments confirmed dimerization on live cells

• Mutating glycine residues to isoleucine or valine disrupted dimer formation

• The duration of co-localization events (τon) was used to characterize the stability of receptor interactions

Functional significance:

• The Small-X3-Small motifs are conserved in all three FLRT human homologues (FLRT1-3) and across different species

• Cancer-related mutations (A544V and G545V) targeting the TM domain of FLRT2 map to these motifs and may affect function

• Lipid environment can modulate transmembrane association of FLRT2

• Dimerization likely influences FLRT2's adhesive properties and signaling capabilities

Structural data availability:

• Three main conformations (RH1, RH2, and LH) in both coarse-grained and atomistic representations have been made available for further research (https://github.com/MChavent/FLRT)[5]

How does FLRT2 prevent endothelial cell senescence and vascular aging?

FLRT2 plays a critical role in preventing endothelial senescence through specific signaling pathways:

Expression patterns in senescence and aging:

• FLRT2 expression decreases in replicatively senescent endothelial cells (HUVECs, ECFCs, and HMVECs)

• Expression levels decline in aortas of old rats (24 months) compared to young rats (6 months)

• In human vascular tissues, FLRT2 levels decrease with age, especially in people over 50 years old

Molecular mechanism:

• FLRT2 directly associates with integrin subunit beta 4 (ITGB4)

• This association promotes ITGB4 phosphorylation

• FLRT2 mediates endothelial cell senescence via the mTOR complex 2 (mTORC2), AKT, and p53 signaling pathway

• Inhibition of ITGB4 substantially mitigates the senescence triggered by FLRT2 depletion

• FLRT2 silencing increases AKT phosphorylation at S473

• Double siRNA experiments showed that silencing both FLRT2 and AKT abrogated the rise in p53 and p21 seen after FLRT2 silencing alone

In vivo validation:

• FLRT2 silencing in mice promotes vascular aging

• Overexpression of FLRT2 rescues premature vascular aging phenotype

These findings identify a novel function of FLRT2 in preventing endothelial cell senescence and vascular aging, suggesting potential therapeutic applications for age-related vascular diseases .

What is the role of FLRT2 in neural development and regeneration?

FLRT2 serves multiple functions in neural development and shows potential involvement in regeneration:

Expression patterns in neural tissue:

• At early postnatal stages, FLRT2 expression is largely restricted to several regions of the striatum and deep layers of the cerebral cortex

• In adult mouse brain, FLRT2-expressing cells appear mainly in layer IV, which contains spiny stellate cells

• FLRT2 expression patterns change during development, indicating stage-specific roles

• In the spinal cord, FLRT2 expression decreases during development

Response to injury:

• FLRT2 is highly upregulated around lesion sites 7 days after thoracic spinal cord injury

• Weak expression is maintained until day 14 but disappears by day 28 post-injury

• FLRT2 is strongly expressed on GFAP+ reactive astrocytes after injury

• Induction of GFAP after spinal cord injury was decreased in FLRT2 conditional knockout mice

Functional mechanisms:

• FLRT2 may function as both a repulsive guidance cue and an adhesive molecule in cortical development

• It may play a role in fine-tuning cortical circuits during early postnatal development

• FLRT2 expression in layer IV of adult cortex suggests involvement in thalamocortical connections

• In spinal cord injury, FLRT2 may contribute to glial scar formation as an adhesive molecule

• It may also inhibit axonal regeneration as a repulsive molecule after spinal cord injury

Therapeutic implications:

• Inhibiting FLRT2 function using neutralizing antibodies may ameliorate scar formation after spinal cord injury

• This approach could potentially promote CNS regeneration

How does FLRT2 regulate vascular development through VE-cadherin interactions?

Recent research has revealed FLRT2's crucial role in vascular development through specific cellular mechanisms:

Vascular development regulation:

• FLRT2 is crucial for central nervous system (CNS) vascular development in mice

• Early postnatal FLRT2 deletion causes specific defects in retinal veins

• These defects impact endothelial cell proliferation, sprouting, and polarity

• FLRT2 deletion reduces tip cells at the vascular front

Molecular interactions:

• FLRT2 interacts with VE-cadherin in the vascular endothelium

• Together with the endocytic adaptor protein Numb, FLRT2 contributes to modulating adherens junction morphology in both retina and cerebral cortex

• Expansion microscopy has visualized the altered dynamic distribution of VE-cadherin in tissue of FLRT2 endothelial mutants

• In cortical vessels, FLRT2 regulates the crosstalk between adherens and tight junctions

Functional specificity:

• FLRT2 appears to be a vein-specific regulator of CNS vascular development

• This specificity is significant as veins are the origin of all other endothelial cell subtypes needed for vascular network expansion

• Veins drive the formation of capillary and arterial networks during development of tissues including the heart, brain, and retina

This research positions FLRT2 as a key regulator in the expanding field of vein-specific developmental biology and angiogenesis .

What are the implications of FLRT2 in immune cell function and differentiation?

Emerging research suggests FLRT2 plays roles in immune cell biology:

Monocyte/macrophage differentiation:

• FLRT2 has been implicated in driving monocyte differentiation processes

• Transcriptome sequencing data showed significantly higher FLRT2 expression in alveolar macrophages of tumor-bearing mice compared to normal mice

Osteoclast development:

• FLRT2 is involved in fine-tuning osteoclast multinucleation

• FLRT2 expression is induced during osteoclast differentiation from bone marrow-derived monocytes

• Expression peaks on day one after RANKL stimulation during osteoclastogenesis

These findings suggest broader roles for FLRT2 in immune cell biology and bone homeostasis, opening new research directions beyond its established functions in neural and vascular systems .

What structural features determine FLRT2's binding specificity with different partners?

FLRT2 interactions with various binding partners depend on distinct structural domains:

LRR domain interactions:

• The leucine-rich repeat (LRR) domains mediate homophilic FLRT-FLRT interactions

• These domains are responsible for the localization of FLRTs in areas of cell contact

• LRR domains also mediate interactions with UNC5D and possibly other UNC-5 family members

• X-ray crystallography has revealed the structural basis for FLRT-mediated cell adhesion and repulsion in neurons

Fibronectin domain interactions:

• The fibronectin type III domain of FLRT2 binds to FGF receptors

• This interaction regulates FGF signaling during development

• FLRT2 also interacts with fibronectin itself through this domain

Transmembrane domain:

• The transmembrane domain contains conserved Small-X3-Small motifs that facilitate dimerization

• Dimerization likely affects the presentation of extracellular domains for interaction with binding partners

Novel interactions:

• FLRT2 directly associates with integrin subunit beta 4 (ITGB4)

• FLRT2 interacts with VE-cadherin in vascular endothelium

• FLRT2 has been reported to interact with ROBO1, LPHN3 (ADGRL3), and UNC5

Understanding these domain-specific interactions is crucial for designing targeted interventions that could selectively modulate specific FLRT2 functions while preserving others .

How can contradictory findings about FLRT2's role in cancer be reconciled?

The published literature presents seemingly contradictory roles for FLRT2 in cancer that require careful interpretation:

Conflicting observations:

Reconciliation approaches:

• Context-dependent functions:

  • FLRT2 may function differently depending on cell type (cancer cells vs. endothelial cells)

  • In breast cancer cells, FLRT2 acts as a direct tumor suppressor

  • In tumor vasculature, FLRT2 promotes abnormalized vessels that facilitate cancer progression

• Tissue-specific effects:

  • Different tumor microenvironments may influence FLRT2 function

  • Varying expression patterns of FLRT2 binding partners across tissue types could determine outcome

• Methodological considerations:

  • Study different aspects of cancer biology (direct tumor growth vs. metastasis)

  • Examine whole tumor vs. specific cell types within tumor

  • In vitro vs. in vivo models may yield different results

• Therapeutic implications:

  • For breast cancer: strategies to restore FLRT2 expression in tumor cells may be beneficial

  • For colorectal cancer: targeting FLRT2 in tumor vessels may normalize vasculature and improve outcomes

Research focused on understanding these context-dependent roles will be essential for developing effective FLRT2-targeting therapeutic strategies.

What are the most promising applications of FLRT2 research in therapeutic development?

Based on current knowledge, several therapeutic applications show promise:

Vascular aging and endothelial senescence:

• FLRT2 overexpression could potentially prevent or reverse vascular aging

• Targeting the FLRT2-ITGB4-mTORC2-AKT pathway might provide new approaches for treating age-related vascular diseases

Cancer therapy:

• For breast cancer: demethylating agents to restore FLRT2 expression could be explored

• For colorectal cancer: antibodies blocking FLRT2-mediated interendothelial adhesions might normalize tumor vasculature

• Combination approaches with immune checkpoint inhibitors show potential, as FLRT2 deletion increased their efficacy in animal models

Neurodegenerative conditions:

• Neutralizing antibodies against FLRT2 might reduce glial scar formation after spinal cord injury

• Modulating FLRT2 function could potentially promote CNS regeneration

Vascular development disorders:

• Targeting FLRT2-VE-cadherin interactions might offer therapeutic avenues for blood-brain barrier disorders

• FLRT2's role in venous development could be leveraged in conditions requiring vascular regeneration

What key methodological advances would accelerate FLRT2 research?

Several methodological advances could significantly enhance FLRT2 research:

Structural biology approaches:

• Complete structural determination of full-length FLRT2, including transmembrane and cytoplasmic domains

• Cryo-EM studies of FLRT2 complexes with binding partners like UNC5D, FGFR, and ITGB4

• Improved computational modeling of FLRT2 transmembrane interactions in different lipid environments

Advanced imaging techniques:

• Super-resolution microscopy to visualize FLRT2 clustering and interactions in live cells

• Expansion microscopy application to study FLRT2 distribution and co-localization with partners in complex tissues

• Intravital imaging to monitor FLRT2 dynamics during development and disease progression

Genetic engineering tools:

• Development of inducible, tissue-specific FLRT2 knockout and knock-in models

• CRISPR-based approaches for domain-specific mutations to dissect function

• Reporter systems for real-time visualization of FLRT2 expression in vivo

Therapeutic development platforms:

• High-throughput screening systems to identify small molecule modulators of FLRT2 function

• Development of domain-specific blocking antibodies or peptide inhibitors

• Targeted delivery systems for FLRT2-modulating therapeutics to specific tissues

These methodological advances would facilitate deeper understanding of FLRT2 biology and accelerate translation to clinical applications.