Extracellular assembly and activation principles of oncogenic class III receptor tyrosine kinases

Mechanisms of Melanoma Progression and Treatment Resistance: Role of Cancer Stem-like Cells

Melanoma is the third most common type of skin cancer, characterized by its heterogeneity and propensity to metastasize to distant organs. Melanoma is a heterogeneous tumor, composed of genetically divergent subpopulations, including a small fraction of melanoma-initiating cancer stem-like cells (CSCs) and many non-cancer stem cells (non-CSCs). CSCs are characterized by their unique surface proteins associated with aberrant signaling pathways with a causal or consequential relationship with tumor progression, drug resistance, and recurrence. Melanomas also harbor significant alterations in functional genes (BRAF, CDKN2A, NRAS, TP53, and NF1). Of these, the most common are the BRAF and NRAS oncogenes, with 50% of melanomas demonstrating the BRAF mutation (BRAFV600E). While the successful targeting of BRAFV600E does improve overall survival, the long-term efficacy of available therapeutic options is limited due to adverse side effects and reduced clinical efficacy. Additionally, drug resistance develops rapidly via mechanisms involving fast feedback re-activation of MAPK signaling pathways. This article updates information relevant to the mechanisms of melanoma progression and resistance and particularly the mechanistic role of CSCs in melanoma progression, drug resistance, and recurrence.

Candidate Tracers for Imaging Colony-Stimulating Factor 1 Receptor in Neuroinflammation with Positron Emission Tomography: Issues and Progress

The tyrosine kinase, colony-stimulating factor 1 receptor (CSF1R), has attracted attention as a potential biomarker of neuroinflammation for imaging studies with positron emission tomography (PET), especially because of its location on microglia and its role in microglia proliferation. The development of an effective radiotracer for specifically imaging and quantifying brain CSF1R is highly challenging. Here we review the progress that has been made on PET tracer development and discuss issues that have arisen and which remain to be addressed and resolved.

Role of IL-34 and its receptors in inflammatory diseases

In recent years, IL-34 has been widely discussed as a novel cytokine. IL-34 is a pro-inflammatory cytokine binding four distinct receptors, namely CSF-1R, syndecan-1, PTP-ζ and TREM2. Previous studies have shown that IL-34 and its receptors play important roles in the development and progression of various inflammatory diseases. Therefore, IL-34 has the potential to be a biomarker and therapeutic target for inflammatory diseases. However, further study is still needed to identify the specific mechanism through which IL-34 contributes to illness. In this article, we review the recent advances in the biological roles of IL-34 and its receptors as well as their roles in the development and therapeutic application of inflammatory diseases.

Novel biomolecules in targeted cancer therapy: a new approach towards precision medicine

Cancer is a major threat to human life around the globe, and the discovery of novel biomolecules continue to be an urgent therapeutic need that is still unmet. Precision medicine relies on targeted therapeutic strategies. Researchers are better equipped to develop therapies that target proteins as they understand more about the genetic alterations and molecules that cause progression of cancer. There has been a recent diversification of the sorts of targets exploited in treatment. Therapeutic antibody and biotechnology advancements enabled curative treatments to reach previously inaccessible sites. New treatment strategies have been initiated for several undruggable targets. The application of tailored therapy has been proven to have efficient results in controlling cancer progression. Novel biomolecules like SMDCs, ADCs, mABs, and PROTACS has gained vast attention in the recent years. Several studies have shown that using these novel technology helps in reducing the drug dosage as well as to overcome drug resistance in different cancer types. Therefore, it is crucial to fully untangle the mechanism and collect evidence to understand the significance of these novel drug targets and strategies. This review article will be discussing the importance and role of these novel biomolecules in targeted cancer therapies.

Advances in the Xenopus immunome: Diversification, expansion, and contraction

Xenopus is a genus of African clawed frogs including two species, X. tropicalis and X. laevis that are extensively used in experimental biology, immunology, and biomedical studies. The availability of fully sequenced and annotated Xenopus genomes is strengthening genome-wide analyses of gene families and transgenesis to model human diseases. However, inaccuracies in genome annotation for genes involved in the immune system (i.e., immunome) hamper immunogenetic studies. Furthermore, advanced genome technologies (e.g., single-cell and RNA-Seq) rely on well-annotated genomes. The annotation problems of Xenopus immunome include a lack of established orthology across taxa, merged gene models, poor representation in gene pages on Xenbase, misannotated genes and missing gene IDs. The Xenopus Research Resource for Immunobiology in collaboration with Xenbase and a group of investigators are working to resolve these issues in the latest versions of genome browsers. In this review, we summarize the current problems of previously misannotated gene families that we have recently resolved. We also highlight the expansion, contraction, and diversification of previously misannotated gene families.

Cryo-EM analyses of KIT and oncogenic mutants reveal structural oncogenic plasticity and a target for therapeutic intervention

The receptor tyrosine kinase KIT and its ligand stem cell factor (SCF) are required for the development of hematopoietic stem cells, germ cells, and other cells. A variety of human cancers, such as acute myeloid leukemia, gastrointestinal stromal tumor, and mast cell leukemia, are driven by somatic gain-of-function KIT mutations. Here, we report cryo electron microscopy (cryo-EM) structural analyses of full-length wild-type and two oncogenic KIT mutants, which show that the overall symmetric arrangement of the extracellular domain of ligand-occupied KIT dimers contains asymmetric D5 homotypic contacts juxtaposing the plasma membrane. Mutational analysis of KIT reveals in D5 region an "Achilles heel" for therapeutic intervention. A ligand-sensitized oncogenic KIT mutant exhibits a more comprehensive and stable D5 asymmetric conformation. A constitutively active ligand-independent oncogenic KIT mutant adopts a V-shaped conformation solely held by D5-mediated contacts. Binding of SCF to this mutant fully restores the conformation of wild-type KIT dimers, including the formation of salt bridges responsible for D4 homotypic contacts and other hallmarks of SCF-induced KIT dimerization. These experiments reveal an unexpected structural plasticity of oncogenic KIT mutants and a therapeutic target in D5.

Discovery and optimization of natural-based nanomolar c-Kit inhibitors via in silico and in vitro studies

c-Kit is a receptor tyrosine kinase which is involved in intracellular signaling and mutations of c-Kit have been associated with various types of cancers. Investigations have shown that inhibition of c-Kit, using tyrosine kinase inhibitors, yielded promising results in cancer treatment marking it as a promising target for cancer therapy. However, the emerging resistance for the current therapy necessitates the development of more potent inhibitors which are not affected by these mutations. Herein, virtual screening of a library of natural-based compounds yielded three hits (2, 5 and 6) which possessed nanomolar inhibitory (2.02, 4.33 and 2.80 nM, respectively) activity when tested in vitro against c-Kit. Single point mutation docking studies showed the hits to be unaffected by the most common resistance mutation in imatinib-resistant cells, mutation of Val654. Although, the top hits exhibited around 3000 higher inhibitory potency toward c-Kit when compared to imatinib (5.4 µM), previous studies have shown that they are metabolically unstable. Fragment-based drug design approaches were then employed to enhance binding affinity of the top hit and make it more metabolically stable. Screening of the generated fragments yielded a new derivative, F1, which demonstrated stronger binding affinity, stability and binding free energy when compared to the hit compound 2.Communicated by Ramaswamy H. Sarma.

Silence of a dependence receptor CSF1R in colorectal cancer cells activates tumor-associated macrophages

BACKGROUND

Colony-stimulating factor 1 receptor (CSF1R), a classic tyrosine kinase receptor, has been identified as a proto-oncogene in multiple cancers. The CSF1/CSF1R axis is essential for the survival and differentiation of M2-phenotype tumor-associated macrophages (M2 TAMs). However, we found here that the CSF1R expression was abnormally down-regulated in colorectal cancer (CRC), and its biological functions and underlying mechanisms have become elusive in CRC progression.

METHODS

The expression of class III receptor tyrosine kinases in CRC and normal intestinal mucosa was accessed using The Cancer Genome Atlas and Gene Expression Omnibus datasets and was further validated by our tested cohort. CSF1R was reconstructed in CRC cells to identify its biological functions in vitro and in vivo. We compared CSF1R expression and methylation differences between CRC cells and macrophages. Furthermore, a co-culture system was used to mimic a competitive mechanism between CSF1R-overexpressed CRC cells and M2-like macrophages. We utilized a CSF1R inhibitor PLX3397 to ablate M2 TAMs and evaluated its efficacy on CRC treatment in animal models.

RESULTS

We found here that the CSF1R is silenced in CRC, and the reintroduced expression of the receptor in CRC cells can be cleaved by caspases and constrain tumor growth in vitro and in vivo, functioning as a tumor suppressor gene. We further identified CSF1R as a novel dependence receptor, which has the potential to act as either a tumor suppressor gene or an oncogene, depending on its activated state. In CRC tumors, CSF1R expression is enriched in TAMs, and its expression is associated with poor prognosis in patients ith CRC. In a co-culture system, CRC cells expressing CSF1R compete with M2-like macrophages for CSF1R ligands, resulting in a decrease in CSF1R activation and cell proliferation in macrophages. Blocking CSF1R by PLX3397 could deplete M2 TAMs and augments CD8⁺ T cell infiltration, effectively inhibiting tumor growth and metastasis and improving responses to chemotherapy and immunotherapy.

CONCLUSION

Our findings revealed that CSF1R is a novel identified dependence receptor silenced in CRC. The silence abalienates its ligands to stimulate CSF1R expressed on M2 TAMs, which is an appealing therapeutic target for M2 TAM depletion and CRC treatment.

Evolution and Potential Subfunctionalization of Duplicated fms-Related Class III Receptor Tyrosine Kinase flt3s and Their Ligands in the Allotetraploid Xenopus laevis

The fms-related tyrosine kinase 3 (Flt3) and its ligand (Flt3lg) are important regulators of hematopoiesis and dendritic cell (DC) homeostasis with unsettled coevolution. Gene synteny and deduced amino acid sequence analyses identified conserved flt3 gene orthologs across all jawed vertebrates. In contrast, flt3lg orthologs were not retrieved in ray-finned fish, and the gene locus exhibited more variability among species. Interestingly, duplicated flt3/flt3lg genes were maintained in the allotetraploid Xenopus laevis Comparison of modeled structures of X. laevis Flt3 and Flt3lg homoeologs with the related diploid Xenopus tropicalis and with humans indicated a higher conformational divergence between the homoeologous pairs than their respective counterparts. The distinctive developmental and tissue expression patterns of Flt3 and Flt3lg homoeologs in tadpoles and adult frogs suggest a subfunctionalization of these homoeologs. To characterize Flt3 cell surface expression, X. laevis-tagged rFlt3lg.S and rFlt3lg.L were produced. Both rFlt3lg.S and rFlt3lg.L bind in vitro Flt3.S and Flt3.L and can trigger Erk1/2 signaling, which is consistent with a partial overlapping function between homoeologs. In spleen, Flt3.S/L cell surface expression was detected on a fraction of B cells and a population of MHC class IIhigh/CD8+ leukocytes phenotypically similar to the recently described dual follicular/conventional DC-like XL cells. Our result suggests that 1) Flt3lg.S and Flt3lg.L are both involved in XL cell homeostasis and that 2) XL cells have hematopoietic origin. Furthermore, we detected surface expression of the macrophage/monocyte marker Csf1r.S on XL cells as in mammalian and chicken DCs, which points to a common evolutionary origin in vertebrate DCs.

High frequency of PDGFRA and MUC family gene mutations in diffuse hemispheric glioma, H3 G34-mutant: a glimmer of hope?

BACKGROUND

Diffuse hemispheric glioma H3 G34-mutant (G34-DHG) is a new type of pediatric-type diffuse high-grade glioma in the fifth edition of the WHO Classification of Tumors of the Central Nervous System. The current treatment for G34-DHG involves a combination of surgery and conventional radiotherapy or chemotherapy; however, the therapeutic efficacy of this approach is not satisfactory. In recent years, molecular targeted therapy and immunotherapy have achieved significant benefits in a variety of tumors. In-depth understanding of molecular changes and immune infiltration in G34-DHGs will help to establish personalized tumor treatment strategies. Here, we report the clinicopathological, molecular and immune infiltration characteristics of G34-DHG cases from our center along with cases from the HERBY Trial and the Chinese Glioma Genome Atlas database (CGGA).

METHODS

Hematoxylin-eosin (HE) and immunohistochemistry (IHC) staining were used to present the clinicopathological characteristics of 10 Chinese G34-DHG patients treated at our institution. To address the molecular characteristics of G34-DHG, we performed whole-exome sequencing (WES) and RNA sequencing (RNA-seq) analyses of 5 patients from our center and 3 Chinese patients from the Chinese Glioma Genome Atlas (CGGA) database. Additionally, 7 European G34-DHG patients from the HERBY Trail were also subjected to analyses, with 7 cases of WES data and 2 cases of RNA-seq data. Six G34-DHG patients from another organization were used as external validation.

RESULTS

WES showed a high frequency of PDGFRA mutation in G34-DHGs (12/15). We further identified frequent mutations in MUC family genes in G34-DHGs, including MUC16 (8/15) and MUC17 (8/15). Although no statistical difference was found, PDGFRA mutation tended to be an indicator for worse prognosis whereas MUC16/MUC17 mutation indicated a favorable prognosis in G34-DHGs. RNA sequencing results revealed that most G34-DHG are considered to be immune cold tumors. However, one patient in our cohort with MUC16 mutation showed significant immune infiltration, and the total overall survival of this patient reached 75 months.

CONCLUSIONS

Our results demonstrate that G34-DHG is a new high-grade glioma with high frequency of PDGFRA and MUC gene family mutations. PDGFRA may serve as an indicator of poor prognosis and an effective therapeutic target. Moreover, MUC16 tends to be a favorable prognostic factor and indicates high immune infiltration in certain patients, and these findings may provide a new direction for targeted therapy and immunotherapy of patients with G34-DHGs.

Synthesis and biological evaluation of novel 5,6-dihydrobenzo[h]quinazoline derivatives as FLT3 inhibitors

Fms-like tyrosine kinase 3 (FLT3) is widely expressed and often mutated in acute myeloid leukemia (AML), which makes it an important target for the treatment of AML. The structure-based synthesis and biological evaluation of 5,6-dihydrobenzo[h]quinazoline derivatives as FLT3 inhibitors have been studied in this paper. III-1a, III-1c, III-2a, III-2c, and III-4a displayed comparable inhibitory potency against FLT3-ITD and showed remarkable antiproliferative activities against MV4-11.

Discover Novel Covalent Inhibitors Targeting FLT3 through Hybrid Virtual Screening Strategy

FMS-like tyrosine kinase 3 (FLT3) plays a very important role in regulating the proliferation, differentiation and survival of normal hematopoietic stem cells. Internal tandem duplications of the FLT3 gene (FLT3-ITD) mutations are present in 25% of all acute myeloid leukemia (AML) patients and are frequently associated with adverse clinical outcomes. Therefore, FLT3-ITD is a promising target for the treatment of AML. The use of covalent virtual screenings has shown that efficient rational approaches for the rapid discovery of new drugs scaffold. Herein, we report a hybrid virtual screening strategy that led to the discovery of FLT3 inhibitors. Using the combination of non-covalent docking and covalent docking, 8 compounds were found to inhibit FLT3, and G856-8335, S346-0154 are also effective against mutant FLT3. These two compounds also show selectivity to receptor tyrosine kinase (C-KIT), which has the potential for optimization. And this work can be extended to the screening of other covalent inhibitors.

Melanoma Targeted Therapies beyond BRAF-Mutant Melanoma: Potential Druggable Mutations and Novel Treatment Approaches

Melanomas exhibit the highest rate of somatic mutations among all different types of cancers (with the exception of BCC and SCC). The accumulation of a multimode of mutations in the driver oncogenes are responsible for the proliferative, invasive, and aggressive nature of melanomas. High-resolution and high-throughput technology has led to the identification of distinct mutational signatures and their downstream alterations in several key pathways that contribute to melanomagenesis. This has enabled the development of individualized treatments by targeting specific molecular alterations that are vital for cancer cell survival, which has resulted in improved outcomes in several cancers, including melanomas. To date, BRAF and MEK inhibitors remain the only approved targeted therapy with a high level of evidence in BRAFV600E/K mutant melanomas. The lack of approved precision drugs in melanomas, relative to other cancers, despite harboring one of the highest rates of somatic mutations, advocates for further research to unveil effective therapeutics. In this review, we will discuss potential druggable mutations and the ongoing research of novel individualized treatment approaches targeting non-BRAF mutations in melanomas.

CD40 Pathway and IL-2 Expression Mediate the Differential Outcome of Colorectal Cancer Patients with Different CSF1R c.1085 Genotypes

Colony-stimulating factor 1 receptor (CSF-1R) acts as the receptor for colony stimulating factor 1, a cytokine that controls the production, differentiation, and function of macrophages. Prior studies showed cancer patients harboring germline CSF1R c.1085A>G genetic variant had better survival. Here, primary tumor samples from a stage III colorectal cancer (CRC) cohort were analyzed by a targeted gene expression assay containing 395 immune-related genes to study the immune mechanism underlying the different outcomes. CRC patients with CSF1R c.1085 genotype A_G had a better disease-free and overall survival than those with CSF1R genotype A_A. Compared to the group of patients without CSF1R variant, higher CD40LG expression, a surface marker of T cells, was found in the tumor tissues of patients with CSF1R c.1085 variant. In parallel with the higher CD40LG gene expression, immunofluorescent staining also showed more CD3⁺CD40L⁺ T cell infiltrates in tumors with CSF1R c.1085 genotype A_G. Moreover, higher IL-2 expression, known to be regulated by CD40 pathway, was also observed in tumors with CSF1R c.1085 genotype A_G than genotype A_A. Higher IL-2 expression generated by the interaction of CD40 ligand and CD40 between T cells and macrophages with CSF1R c.1085A>G variant is the potential mechanism explaining the different outcomes.

Tyrosine kinase inhibitor prodrug-loaded liposomes for controlled release at tumor microenvironment

Tyrosine kinase inhibitors (TKIs) represent one of the most advanced class of therapeutics for cancer treatment. Most of them are also cytochrome P450 (CYP) inhibitors and/or substrates thereof. Accordingly, their efficacy and/or toxicity can be affected by CYP-mediated metabolism and by metabolism-derived drug-drug interactions. In order to enhance the therapeutic performance of these drugs, we developed a prodrug (Pro962) of our TKI TK962 specifically designed for liposome loading and pH-controlled release in the tumor. A cholesterol moiety was linked to TK962 through pH-sensitive hydrazone bond for anchoring to the liposome phospholipid bilayer to prevent leakage of the prodrug from the nanocarrier. Bioactivity studies performed on isolated target kinases showed that the prodrug maintains only partial activity against them and the release of TK962 is required. Biopharmaceutical studies carried out with prodrug loaded liposomes showed that the prodrug was firmly associated with the vesicles and the drug release was prevented under blood-mimicking conditions. Conversely, conventional liposome loaded with TK962 readily released the drug. Flow cytometric studies showed that liposomes efficiently provided for intracellular prodrug delivery. The use of the hydrazone linker yielded a pH-controlled drug release, which resulted in about 50% drug release at pH 4 and 5 in 2 h. Prodrug, prodrug loaded liposomes and active lead compound have been tested against cancer cell lines in either 2D or 3D models. The liposome formulation showed higher cytotoxicity than the unformulated lead TK962 in both 2D and 3D models. The stability of prodrug, prodrug loaded liposomes and active lead compound in human serum and against human, mouse, and rat microsomes was also assessed, demonstrating that liposome formulations impair the metabolic reactions and protect the loaded compounds from catabolism. The results suggest that the liposomal formulation of pH releasable TKI prodrugs is a promising strategy to improve the metabolic stability, intracellular cancer cell delivery and release, and in turn the efficacy of this class of anticancer drugs.

Engineering and crystal structure of a monomeric FLT3 ligand variant

The overarching paradigm for the activation of class III and V receptor tyrosine kinases (RTKs) prescribes cytokine-mediated dimerization of the receptor ectodomains and homotypic receptor-receptor interactions. However, structural studies have shown that the hematopoietic receptor FLT3, a class III RTK, does not appear to engage in such receptor-receptor contacts, despite its efficient dimerization by dimeric FLT3 ligand (FL). As part of efforts to better understand the intricacies of FLT3 activation, we sought to engineer a monomeric FL. It was found that a Leu27Asp substitution at the dimer interface of the cytokine led to a stable monomeric cytokine (FLL27D) without abrogation of receptor binding. The crystal structure of FLL27D at 1.65 Å resolution revealed that the introduced point mutation led to shielding of the hydrophobic footprint of the dimerization interface in wild-type FL without affecting the conformation of the FLT3 binding site. Thus, FLL27D can serve as a monomeric FL variant to further interrogate the assembly mechanism of extracellular complexes of FLT3 in physiology and disease.

PDGF receptor mutations in human diseases

PDGFRA and PDGFRB are classical proto-oncogenes that encode receptor tyrosine kinases responding to platelet-derived growth factor (PDGF). PDGFRA mutations are found in gastrointestinal stromal tumors (GISTs), inflammatory fibroid polyps and gliomas, and PDGFRB mutations drive myofibroma development. In addition, chromosomal rearrangement of either gene causes myeloid neoplasms associated with hypereosinophilia. Recently, mutations in PDGFRB were linked to several noncancerous diseases. Germline heterozygous variants that reduce receptor activity have been identified in primary familial brain calcification, whereas gain-of-function mutants are present in patients with fusiform aneurysms, Kosaki overgrowth syndrome or Penttinen premature aging syndrome. Functional analysis of these variants has led to the preclinical validation of tyrosine kinase inhibitors targeting PDGF receptors, such as imatinib, as a treatment for some of these conditions. This review summarizes the rapidly expanding knowledge in this field.

Homogeneously N-glycosylated proteins derived from the GlycoDelete HEK293 cell line enable diffraction-quality crystallogenesis

Structural studies of glycoproteins and their complexes provide critical insights into their roles in normal physiology and disease. Most glycoproteins contain N-linked glycosylation, a key post-translation modification that critically affects protein folding and stability and the binding kinetics underlying protein interactions. However, N-linked glycosylation is often an impediment to yielding homogeneous protein preparations for structure determination by X-ray crystallography or other methods. In particular, obtaining diffraction-quality crystals of such proteins and their complexes often requires modification of both the type of glycosylation patterns and their extent. Here, we demonstrate the benefits of producing target glycoproteins in the GlycoDelete human embryonic kidney 293 cell line that has been engineered to produce N-glycans as short glycan stumps comprising N-acetylglucosamine, galactose and sialic acid. Protein fragments of human Down syndrome cell-adhesion molecule and colony-stimulating factor 1 receptor were obtained from the GlycoDelete cell line for crystallization. The ensuing reduction in the extent and complexity of N-glycosylation in both protein molecules compared with alternative glycoengineering approaches enabled their productive deployment in structural studies by X-ray crystallography. Furthermore, a third successful implementation of the GlycoDelete technology focusing on murine IL-12B is shown to lead to N-glycosylation featuring an immature glycan in diffraction-quality crystals. It is proposed that the GlycoDelete cell line could serve as a valuable go-to option for the production of homogeneous glycoproteins and their complexes for structural studies by X-ray crystallography and cryo-electron microscopy.

Structural Perspectives on Extracellular Recognition and Conformational Changes of Several Type-I Transmembrane Receptors

Type-I transmembrane proteins represent a large group of 1,412 proteins in humans with a multitude of functions in cells and tissues. They are characterized by an extracellular, or luminal, N-terminus followed by a single transmembrane helix and a cytosolic C-terminus. The domain composition and structures of the extracellular and intercellular segments differ substantially amongst its members. Most of the type-I transmembrane proteins have roles in cell signaling processes, as ligands or receptors, and in cellular adhesion. The extracellular segment often determines specificity and can control signaling and adhesion. Here we focus on recent structural understanding on how the extracellular segments of several diverse type-I transmembrane proteins engage in interactions and can undergo conformational changes for their function. Interactions at the extracellular side by proteins on the same cell or between cells are enhanced by the transmembrane setting. Extracellular conformational domain rearrangement and structural changes within domains alter the properties of the proteins and are used to regulate signaling events. The combination of structural properties and interactions can support the formation of larger-order assemblies on the membrane surface that are important for cellular adhesion and intercellular signaling.

Structural Analysis Reveals that the Cytokine IL-17F Forms a Homodimeric Complex with Receptor IL-17RC to Drive IL-17RA-Independent Signaling

Interleukin-17A (IL-17A), IL-17F, and IL-17A/F heterodimers are key cytokines of the innate and adaptive immune response. Dysregulation of the IL-17 pathway contributes to immune pathology, and it is therefore important to elucidate the molecular mechanisms that govern IL-17 recognition and signaling. The receptor IL-17RC is thought to act in concert with IL-17RA to transduce IL-17A-, IL-17F-, and IL-17A/F-mediated signals. We report the crystal structure of the extracellular domain of human IL-17RC in complex with IL-17F. In contrast to the expected model, we found that IL-17RC formed a symmetrical 2:1 complex with IL-17F, thus competing with IL-17RA for cytokine binding. Using biophysical techniques, we showed that IL-17A and IL-17A/F also form 2:1 complexes with IL-17RC, suggesting the possibility of IL-17RA-independent IL-17 signaling pathways. The crystal structure of the IL-17RC:IL-17F complex provides a structural basis for IL-17F signaling through IL-17RC, with potential therapeutic applications for respiratory allergy and inflammatory bowel diseases.

Melanoma stem cell maintenance and chemo-resistance are mediated by CD133 signal to PI3K-dependent pathways

Melanoma stem cells (MSCs) are characterized by their unique cell surface proteins and aberrant signaling pathways. These stemness properties are either in a causal or consequential relationship to melanoma progression, treatment resistance and recurrence. The functional analysis of CD133⁺ and CD133^- cells in vitro and in vivo revealed that melanoma progression and treatment resistance are the consequences of CD133 signal to PI3K pathway. CD133 signal to PI3K pathway drives two downstream pathways, the PI3K/Akt/MDM2 and the PI3K/Akt/MKP-1 pathways. Activation of PI3K/Akt/MDM2 pathway results in the destabilization of p53 protein, while the activation of PI3K/Akt/MKP-1 pathway results in the inhibition of mitogen-activated protein kinases (MAPKs) JNK and p38. Activation of both pathways leads to the inhibition of fotemustine-induced apoptosis. Thus, the disruption of CD133 signal to PI3K pathway is essential to overcome Melanoma resistance to fotemustine. The pre-clinical verification of in vitro data using xenograft mouse model of MSCs confirmed the clinical relevance of CD133 signal as a therapeutic target for melanoma treatment. In conclusion, our study provides an insight into the mechanisms regulating MSCs growth and chemo-resistance and suggested a clinically relevant approach for melanoma treatment.

Wild-type FLT3 and FLT3 ITD exhibit similar ligand-induced internalization characteristics

Class III receptor tyrosine kinases control the development of hematopoietic stem cells. Constitutive activation of FLT3 by internal tandem duplications (ITD) in the juxtamembrane domain has been causally linked to acute myeloid leukaemia. Oncogenic FLT3 ITD is partially retained in compartments of the biosynthetic route and aberrantly activates STAT5, thereby promoting cellular transformation. The pool of FLT3 ITD molecules in the plasma membrane efficiently activates RAS and AKT, which is likewise essential for cell transformation. Little is known about features and mechanisms of FLT3 ligand (FL)-dependent internalization of surface-bound FLT3 or FLT3 ITD. We have addressed this issue by internalization experiments using human RS4-11 and MV4-11 cells with endogenous wild-type FLT3 or FLT3 ITD expression, respectively, and surface biotinylation. Further, FLT3 wild-type, or FLT3 ITD-GFP hybrid proteins were stably expressed and characterized in 32D cells, and internalization and stability were assessed by flow cytometry, imaging flow cytometry, and immunoblotting. FL-stimulated surface-exposed FLT3 WT or FLT3 ITD protein showed similar endocytosis and degradation characteristics. Kinase inactivation by mutation or FLT3 inhibitor treatment strongly promoted FLT3 ITD surface localization, and attenuated but did not abrogate FL-induced internalization. Experiments with the dynamin inhibitor dynasore suggest that active FLT3 as well as FLT3 ITD is largely endocytosed via clathrin-dependent endocytosis. Internalization of kinase-inactivated molecules occurred through a different yet unidentified mechanism. Our data demonstrate that FLT3 WT and constitutively active FLT3 ITD receptor follow, despite very different biogenesis kinetics, similar internalization and degradation routes.

Antitumor Activity of a Novel Tyrosine Kinase Inhibitor AIU2001 Due to Abrogation of the DNA Damage Repair in Non-Small Cell Lung Cancer Cells

Class III receptor tyrosine kinase (RTK) inhibitors targeting mainly FLT3 or c-KIT have not been well studied in lung cancer. To identify a small molecule potentially targeting class III RTK, we synthesized novel small molecule compounds and identified 5-(4-bromophenyl)-N-(naphthalen-1-yl) oxazol-2-amine (AIU2001) as a novel class III RKT inhibitor. In an in vitro kinase profiling assay, AIU2001 inhibited the activities of FLT3, mutated FLT3, FLT4, and c-KIT of class III RTK, and the proliferation of NSCLC cells in vitro and in vivo. AIU2001 induced DNA damage, reactive oxygen species (ROS) generation, and cell cycle arrest in the G2/M phase. Furthermore, AIU2001 suppressed the DNA damage repair genes, resulting in the ‘BRCAness’/‘DNA-PKness’ phenotype. The mRNA expression level of STAT5 was downregulated by AIU2001 treatment and knockdown of STAT5 inhibited the DNA repair genes. Our results show that compared to either drug alone, the combination of AIU2001 with a poly (ADP-ribose) polymerase (PARP) inhibitor olaparib or irradiation showed synergistic efficacy in H1299 and A549 cells. Hence, our findings demonstrate that AIU2001 is a candidate therapeutic agent for NSCLC and combination therapies with AIU2001 and a PARP inhibitor or radiotherapy may be used to increase the therapeutic efficacy of AIU2001 due to inhibition of DNA damage repair.

PTPσ inhibitors promote hematopoietic stem cell regeneration

Receptor type protein tyrosine phosphatase-sigma (PTPσ) is primarily expressed by adult neurons and regulates neural regeneration. We recently discovered that PTPσ is also expressed by hematopoietic stem cells (HSCs). Here, we describe small molecule inhibitors of PTPσ that promote HSC regeneration in vivo. Systemic administration of the PTPσ inhibitor, DJ001, or its analog, to irradiated mice promotes HSC regeneration, accelerates hematologic recovery, and improves survival. Similarly, DJ001 administration accelerates hematologic recovery in mice treated with 5-fluorouracil chemotherapy. DJ001 displays high specificity for PTPσ and antagonizes PTPσ via unique non-competitive, allosteric binding. Mechanistically, DJ001 suppresses radiation-induced HSC apoptosis via activation of the RhoGTPase, RAC1, and induction of BCL-XL. Furthermore, treatment of irradiated human HSCs with DJ001 promotes the regeneration of human HSCs capable of multilineage in vivo repopulation. These studies demonstrate the therapeutic potential of selective, small-molecule PTPσ inhibitors for human hematopoietic regeneration.

More than the sum of the parts: Toward full-length receptor tyrosine kinase structures

Intercellular communication governs complex physiological processes ranging from growth and development to the maintenance of cellular and organ homeostasis. In nearly all metazoans, receptor tyrosine kinases (RTKs) are central players in these diverse and fundamental signaling processes. Aberrant RTK signaling is at the root of many developmental diseases and cancers and it remains a key focus of targeted therapies, several of which have achieved considerable success in patients. These therapeutic advances in targeting RTKs have been propelled by numerous genetic, biochemical, and structural studies detailing the functions and molecular mechanisms of regulation and activation of RTKs. The latter in particular have proven to be instrumental for the development of new drugs, selective targeting of mutant forms of RTKs found in disease, and counteracting ensuing drug resistance. However, to this day, such studies have not yet yielded high-resolution structures of intact RTKs that encompass the extracellular and intracellular domains and the connecting membrane-spanning transmembrane domain. Technically challenging to obtain, these structures are instrumental to complete our understanding of the mechanisms by which RTKs are activated by extracellular ligands and of the effect of pathological mutations that do not directly reside in the catalytic sites of tyrosine kinase domains. In this review, we focus on the recent progress toward obtaining such structures and the insights already gained by structural studies of the subdomains of the receptors that belong to the epidermal growth factor receptor, insulin receptor, and platelet-derived growth factor receptor RTK families. © 2019 IUBMB Life, 71(6):706-720, 2019.

Interleukin-34 as a promising clinical biomarker and therapeutic target for inflammatory arthritis

Interleukin-34 (IL-34), recently identified as a novel inflammatory cytokine and the second ligand for colony-stimulating factor-1 receptor, is known to play regulatory roles in the development, maintenance, and function of mononuclear phagocyte lineage cells - especially osteoclasts. Regarding its primary effect on osteoclasts, IL-34 has been shown to stimulate formation and activation of osteoclasts, which in turn magnifies osteoclasts-resorbing activity. In addition to its role in osteoclastogenesis, IL-34 has been implicated in inflammation of synovium via augmenting production of inflammatory mediators, in which altered IL-34 expression is regulated by pro-inflammatory cytokines responsible for cartilage degradation. Indeed, IL-34 has been documented to be highly expressed in inflamed synovium of rheumatoid arthritis (RA) and knee osteoarthritis (OA) patients, which are recognized as inflammatory arthritis. Furthermore, a number of clinical studies demonstrated that IL-34 levels were significantly increased in the circulation and synovial fluid of patients with RA and knee OA. Its levels were also found to be positively associated with disease severity - especially radiographic severity of both RA and knee OA patients. Interestingly, emerging evidence has accumulated that functional blockage of IL-34 with specific antibody can alleviate the severity of inflammatory arthritis. It is therefore reasonable to speculate that IL-34 may be developed as a potential biomarker and a new therapeutic candidate for inflammatory arthritis. To date, there are numerous studies showing IL-34 involvement and association with many aspects of inflammatory arthritis. Herein, this review aimed to summarize the recent findings regarding regulatory role of IL-34 in synovial inflammation-mediated cartilage destruction and update the current comprehensive knowledge on usefulness of IL-34-based treatment in inflammatory arthritis - particularly RA and knee OA.

Synthesis and Initial In Vivo Evaluation of [11C]AZ683-A Novel PET Radiotracer for Colony Stimulating Factor 1 Receptor (CSF1R)

Positron emission tomography (PET) imaging of Colony Stimulating Factor 1 Receptor (CSF1R) is a new strategy for quantifying both neuroinflammation and inflammation in the periphery since CSF1R is expressed on microglia and macrophages. AZ683 has high affinity for CSF1R (Ki = 8 nM; IC50 = 6 nM) and >250-fold selectivity over 95 other kinases. In this paper, we report the radiosynthesis of [11C]AZ683 and initial evaluation of its use in CSF1R PET. [11C]AZ683 was synthesized by 11C-methylation of the desmethyl precursor with [11C]MeOTf in 3.0% non-corrected activity yield (based upon [11C]MeOTf), >99% radiochemical purity and high molar activity. Preliminary PET imaging with [11C]AZ683 revealed low brain uptake in rodents and nonhuman primates, suggesting that imaging neuroinflammation could be challenging but that the radiopharmaceutical could still be useful for peripheral imaging of inflammation.

Identification of compound D2923 as a novel anti-tumor agent targeting CSF1R

Colony-stimulating factor 1 receptor (CSF1R) plays a critical role in promoting tumor progression in various types of tumors. Here, we identified D2923 as a novel and selective inhibitor of CSF1R and explored its antitumor activity both in vitro and in vivo. D2923 potently inhibited CSF1R in vitro kinase activity with an IC50 value of 0.3 nM. It exhibited 10- to 300-fold less potency against a panel of kinases tested. D2923 markedly blocked CSF-1-induced activation of CSF1R and its downstream signaling transduction in THP-1 and RAW264.7 macrophages and thus inhibited the in vitro growth of macrophages. Moreover, D2923 dose-dependently attenuated the proliferation of a small panel of myeloid leukemia cells, mainly by arresting the cells at G1 phase as well as inducing apoptosis in the cells. The results of the in vivo experiments further demonstrated that D2923 displayed potent antitumor activity against M-NFS-60 xenografts, with tumor growth inhibition rates of 50% and 88% at doses of 40 and 80 mg/kg, respectively. Additionally, D2923 was well tolerated with no significant body-weight loss observed in the treatment groups compared with the control. Furthermore, a western blot analysis and the immunohistochemistry results confirmed that the phosphorylation of CSF1R in tumor tissue was dramatically reduced after D2923 treatment, and this was accompanied by the depletion of macrophages in the tumor. Meanwhile, the expression of the proliferation marker Ki67 was also markedly decreased in the D2923 treatment group compared with the control group. Taken together, we identified D2923 as a novel and effective CSF1R inhibitor, which deserves further investigation.

Targeting Oncogenic Signaling in Mutant FLT3 Acute Myeloid Leukemia: The Path to Least Resistance

The identification of recurrent driver mutations in genes encoding tyrosine kinases has resulted in the development of molecularly-targeted treatment strategies designed to improve outcomes for patients diagnosed with acute myeloid leukemia (AML). The receptor tyrosine kinase FLT3 is the most commonly mutated gene in AML, with internal tandem duplications within the juxtamembrane domain (FLT3-ITD) or missense mutations in the tyrosine kinase domain (FLT3-TKD) present in 30–35% of AML patients at diagnosis. An established driver mutation and marker of poor prognosis, the FLT3 tyrosine kinase has emerged as an attractive therapeutic target, and thus, encouraged the development of FLT3 tyrosine kinase inhibitors (TKIs). However, the therapeutic benefit of FLT3 inhibition, particularly as a monotherapy, frequently results in the development of treatment resistance and disease relapse. Commonly, FLT3 inhibitor resistance occurs by the emergence of secondary lesions in the FLT3 gene, particularly in the second tyrosine kinase domain (TKD) at residue Asp835 (D835) to form a ‘dual mutation’ (ITD-D835). Individual FLT3-ITD and FLT3-TKD mutations influence independent signaling cascades; however, little is known about which divergent signaling pathways are controlled by each of the FLT3 specific mutations, particularly in the context of patients harboring dual ITD-D835 mutations. This review provides a comprehensive analysis of the known discrete and cooperative signaling pathways deregulated by each of the FLT3 specific mutations, as well as the therapeutic approaches that hold the most promise of more durable and personalized therapeutic approaches to improve treatments of FLT3 mutant AML.

A history of exploring cancer in context

The concept that progression of cancer is regulated by interactions of cancer cells with their microenvironment was postulated by Stephen Paget over a century ago. Contemporary tumour microenvironment (TME) research focuses on the identification of tumour-interacting microenvironmental constituents, such as resident or infiltrating non-tumour cells, soluble factors and extracellular matrix components, and the large variety of mechanisms by which these constituents regulate and shape the malignant phenotype of tumour cells. In this Timeline article, we review the developmental phases of the TME paradigm since its initial description. While illuminating controversies, we discuss the importance of interactions between various microenvironmental components and tumour cells and provide an overview and assessment of therapeutic opportunities and modalities by which the TME can be targeted.

Recent advances in colony stimulating factor-1 receptor/c-FMS as an emerging target for various therapeutic implications

Colony stimulating factor-1 (CSF-1) is one of the most common proinflammatory cytokine responsible for various inflammatory disorders. It has a remarkable role in the development and progression of osteoarthritis, cancer and other autoimmune disease conditions. The CSF-1 acts by binding to the receptor, called colony stimulating factor-1 receptor (CSF-1R) also known as c-FMS resulting in the cascade of signalling pathway causing cell proliferation and differentiation. Interleukin-34 (IL-34), recently identified as another ligand for CSF-IR, is a cytokine protein. Both, CSF-1 and IL-34, although two distinct cytokines, follow the similar signalling pathway on binding to the same receptor, CSF-1R. Like CSF-1, IL-34 promotes the differentiation and survival of monocyte, macrophages and osteoclasts. This CSF-1R/c-FMS is over expressed in many cancers and on tumour associated macrophages, consequently, have been exploited as a drug target for promising treatment for cancer and inflammatory diseases. Some CSF-1R/c-FMS inhibitors such as ABT-869, Imatinib, AG013736, JNJ-40346527, PLX3397, DCC-3014 and Ki20227 have been successfully used in these disease conditions. Many c-FMS inhibitors have been the candidates of clinical trials, but suffer from some side effects like cardiotoxicity, vomiting, swollen eyes, diarrhoea, etc. If selectivity of cFMS inhibition is achieved successfully, side effects can be overruled and this approach may become a novel therapy for treatment of various therapeutic interventions. Thus, successful targeting of c-FMS may result in multifunctional therapy. With this background of information, the present review focuses on the recent developments in the area of CSF-1R/c-FMS inhibitors with emphasis on crystal structure, mechanism of action and various therapeutic implications in which c-FMS plays a pivotal role. The review on structure activity relationship of various compounds acting as the inhibitors of c-FMS which gives the selection criteria for the development of novel molecules is also being presented.

Altered compensatory cytokine signaling underlies the discrepancy between Flt3-/- and Flt3l-/- mice

The receptor Flt3 and its ligand Flt3L are both critical for dendritic cell (DC) development, but DC deficiency is more severe in Flt3l^-/- mice than in Flt3^-/- mice. This has led to speculation that Flt3L binds to another receptor that also supports DC development. However, we found that Flt3L administration does not generate DCs in Flt3^-/- mice, arguing against a second receptor. Instead, Flt3^-/- DC progenitors matured in response to macrophage colony-stimulating factor (M-CSF) or stem cell factor, and deletion of Csf1r in Flt3^-/- mice further reduced DC development, indicating that these cytokines could compensate for Flt3. Surprisingly, Flt3^-/- DC progenitors displayed enhanced M-CSF signaling, suggesting that loss of Flt3 increased responsiveness to other cytokines. In agreement, deletion of Flt3 in Flt3l^-/- mice paradoxically rescued their severe DC deficiency. Thus, multiple cytokines can support DC development, and the discrepancy between Flt3^-/- and Flt3l^-/- mice results from the increased sensitivity of Flt3^-/- progenitors to these cytokines.

Inhibition of neuronal FLT3 receptor tyrosine kinase alleviates peripheral neuropathic pain in mice

Peripheral neuropathic pain (PNP) is a debilitating and intractable chronic disease, for which sensitization of somatosensory neurons present in dorsal root ganglia that project to the dorsal spinal cord is a key physiopathological process. Here, we show that hematopoietic cells present at the nerve injury site express the cytokine FL, the ligand of fms-like tyrosine kinase 3 receptor (FLT3). FLT3 activation by intra-sciatic nerve injection of FL is sufficient to produce pain hypersensitivity, activate PNP-associated gene expression and generate short-term and long-term sensitization of sensory neurons. Nerve injury-induced PNP symptoms and associated-molecular changes were strongly altered in Flt3-deficient mice or reversed after neuronal FLT3 downregulation in wild-type mice. A first-in-class FLT3 negative allosteric modulator, discovered by structure-based in silico screening, strongly reduced nerve injury-induced sensory hypersensitivity, but had no effect on nociception in non-injured animals. Collectively, our data suggest a new and specific therapeutic approach for PNP.

Sensitisation of dorsal root ganglia neurons contributes to neuropathic pain. Here the authors demonstrate the cytokine FL contributes to sensitisation of DRGs via its receptor FLT3 expressed on neurons, and identify a novel FLT3 inhibitor that attenuates neuropathic pain in mice.

Germline CDKN2A/P16INK4A mutations contribute to genetic determinism of sarcoma

BACKGROUND

Sarcomas are rare mesenchymal malignancies whose pathogenesis is poorly understood; both environmental and genetic risk factors could contribute to their aetiology.

METHODS AND RESULTS

We performed whole-exome sequencing (WES) in a familial aggregation of three individuals affected with soft-tissue sarcoma (STS) without TP53 mutation (Li-Fraumeni-like, LFL) and found a shared pathogenic mutation in CDKN2A tumour suppressor gene. We searched for individuals with sarcoma among 474 melanoma-prone families with a CDKN2A-/+ genotype and for CDKN2A mutations in 190 TP53-negative LFL families where the index case was a sarcoma. Including the initial family, eight independent sarcoma cases carried a germline mutation in the CDKN2A/p16INK4A gene. In five out of seven formalin-fixed paraffin-embedded sarcomas, heterozygosity was lost at germline CDKN2A mutations sites demonstrating complete loss of function. As sarcomas are rare in CDKN2A/p16INK4A carriers, we searched in constitutional WES of nine carriers for potential modifying rare variants and identified three in platelet-derived growth factor receptor (PDGFRA) gene. Molecular modelling showed that two never-described variants could impact the PDGFRA extracellular domain structure.

CONCLUSION

Germline mutations in CDKN2A/P16INK4A, a gene known to predispose to hereditary melanoma, pancreatic cancer and tobacco-related cancers, account also for a subset of hereditary sarcoma. In addition, we identified PDGFRA as a candidate modifier gene.

The stem cell factor (SCF)/c-KIT signalling in testis and prostate cancer

The stem cell factor (SCF) is a cytokine that specifically binds the tyrosine kinase receptor c-KIT. The SCF/c-KIT interaction leads to receptor dimerization, activation of kinase activity and initiation of several signal transduction pathways that control cell proliferation, apoptosis, differentiation and migration in several tissues. The activity of SCF/c-KIT system is linked with the phosphatidylinositol 3-kinase (PI3-K), the Src, the Janus kinase/signal transducers and activators of transcription (JAK/STAT), the phospholipase-C (PLC-γ) and the mitogen-activated protein kinase (MAPK) pathways. Moreover, it has been reported that cancer cases display an overactivation of c-KIT due to the presence of gain-of-function mutations or receptor overexpression, which renders c-KIT a tempting target for cancer treatment. In the case of male cancers the most documented activated pathways are the PI3-K and Src, both enhancing abnormal cell proliferation. It is also known that the Src activity in prostate cancer cases depends on the presence of tr-KIT, the cytoplasmic truncated variant of c-KIT that is specifically expressed in tumour tissues and, thus, a very interesting target for drug development. The present review provides an overview of the signalling pathways activated by SCF/c-KIT and discusses the potential application of c-KIT inhibitors for treatment of testicular and prostatic cancers.

Aminoisoquinoline benzamides, FLT3 and Src-family kinase inhibitors, potently inhibit proliferation of acute myeloid leukemia cell lines

AIM

Mutated or overexpressed FLT3 drives about 30% of reported acute myeloid leukemia (AML). Currently, FLT3 inhibitors have shown durable clinical responses but a complete remission of AML with FLT3 inhibitors remains elusive due to mutation-driven resistance mechanisms. The development of FLT3 inhibitors that also target other downstream oncogenic kinases may combat the resistance mechanism.

RESULTS

4-substituted aminoisoquinoline benzamides potently inhibit Src-family kinases and FLT3, including secondary mutations, such as FLT3D835. Modifications of aminoisoquinoline benzamide to aminoquinoline or aminoquinazoline abrogated FLT3 and Src-family kinase binding.

CONCLUSION

The lead aminoisoquinolines potently inhibited FLT3-driven AML cell lines, MV4-11 and MOLM-14. These aminoisoquinoline benzamides represent new kinase scaffolds with high potential to be translated into anticancer agents.

Structure and antagonism of the receptor complex mediated by human TSLP in allergy and asthma

The pro-inflammatory cytokine thymic stromal lymphopoietin (TSLP) is pivotal to the pathophysiology of widespread allergic diseases mediated by type 2 helper T cell (Th2) responses, including asthma and atopic dermatitis. The emergence of human TSLP as a clinical target against asthma calls for maximally harnessing its therapeutic potential via structural and mechanistic considerations. Here we employ an integrative experimental approach focusing on productive and antagonized TSLP complexes and free cytokine. We reveal how cognate receptor TSLPR allosterically activates TSLP to potentiate the recruitment of the shared interleukin 7 receptor α-chain (IL-7Rα) by leveraging the flexibility, conformational heterogeneity and electrostatics of the cytokine. We further show that the monoclonal antibody Tezepelumab partly exploits these principles to neutralize TSLP activity. Finally, we introduce a fusion protein comprising a tandem of the TSLPR and IL-7Rα extracellular domains, which harnesses the mechanistic intricacies of the TSLP-driven receptor complex to manifest high antagonistic potency.

The pro-inflammatory cytokine thymic stromal lymphopoietin (TSLP) is a promising therapeutic target. Here the authors characterize the assembly mechanism of the receptor complex driven by human TSLP, and its antagonism by the monoclonal antibody Tezepelumab and a fusion protein comprising the TSLP receptors.

Distinct signaling programs control human hematopoietic stem cell survival and proliferation

Several growth factors (GFs) that together promote quiescent human hematopoietic stem cell (HSC) expansion ex vivo have been identified; however, the molecular mechanisms by which these GFs regulate the survival, proliferation. and differentiation of human HSCs remain poorly understood. We now describe experiments in which we used mass cytometry to simultaneously measure multiple surface markers, transcription factors, active signaling intermediates, viability, and cell-cycle indicators in single CD34⁺ cord blood cells before and up to 2 hours after their stimulation with stem cell factor, Fms-like tyrosine kinase 3 ligand, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor (5 GFs) either alone or combined. Cells with a CD34⁺CD38^-CD45RA^-CD90⁺CD49f⁺ (CD49f⁺) phenotype (∼10% HSCs with >6-month repopulating activity in immunodeficient mice) displayed rapid increases in activated STAT1/3/5, extracellular signal-regulated kinase 1/2, AKT, CREB, and S6 by 1 or more of these GFs, and β-catenin only when the 5 GFs were combined. Certain minority subsets within the CD49f⁺ compartment were poorly GF-responsive and, among the more GF-responsive subsets of CD49f⁺ cells, different signaling intermediates correlated with the levels of the myeloid- and lymphoid-associated transcription factors measured. Phenotypically similar, but CD90^-CD49f^- cells (MPPs) contained lower baseline levels of multiple signaling intermediates than the CD90⁺CD49f⁺ cells, but showed similar response amplitudes to the same GFs. Importantly, we found activation or inhibition of AKT and β-catenin directly altered immediate CD49f⁺ cell survival and proliferation. These findings identify rapid signaling events that 5 GFs elicit directly in the most primitive human hematopoietic cell types to promote their survival and proliferation.

Functions of interleukin-34 and its emerging association with rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic, synovial inflammation affecting multiple joints, finally leading to extra-articular lesions for which limited effective treatment options are currently available. Interleukin-34 (IL-34), recently discovered as the second colony-stimulating factor-1 receptor (CSF-1R) ligand, is a newly discovered cytokine. Accumulating evidence has disclosed crucial roles of IL-34 in the proliferation and differentiation of mononuclear phagocyte lineage cells, osteoclastogenesis and inflammation. Recently, IL-34 was detected at high levels in patients with active RA and in experimental models of inflammatory arthritis. Blockade of functional IL-34 with a specific monoclonal antibody can reduce the severity of inflammatory arthritis, suggesting that targeting IL-34 or its receptors may constitute a novel therapeutic strategy for autoimmune diseases such as RA. Here, we have comprehensively discussed the structure and biological functions of IL-34, and reviewed recent advances in our understanding of the emerging role of IL-34 in the development of RA as well as its potential utility as a therapeutic target.

Receptor-Targeted Nipah Virus Glycoproteins Improve Cell-Type Selective Gene Delivery and Reveal a Preference for Membrane-Proximal Cell Attachment

Receptor-targeted lentiviral vectors (LVs) can be an effective tool for selective transfer of genes into distinct cell types of choice. Moreover, they can be used to determine the molecular properties that cell surface proteins must fulfill to act as receptors for viral glycoproteins. Here we show that LVs pseudotyped with receptor-targeted Nipah virus (NiV) glycoproteins effectively enter into cells when they use cell surface proteins as receptors that bring them closely enough to the cell membrane (less than 100 Å distance). Then, they were flexible in receptor usage as demonstrated by successful targeting of EpCAM, CD20, and CD8, and as selective as LVs pseudotyped with receptor-targeted measles virus (MV) glycoproteins, the current standard for cell-type specific gene delivery. Remarkably, NiV-LVs could be produced at up to two orders of magnitude higher titers compared to their MV-based counterparts and were at least 10,000-fold less effectively neutralized than MV glycoprotein pseudotyped LVs by pooled human intravenous immunoglobulin. An important finding for NiV-LVs targeted to Her2/neu was an about 100-fold higher gene transfer activity when particles were targeted to membrane-proximal regions as compared to particles binding to a more membrane-distal epitope. Likewise, the low gene transfer activity mediated by NiV-LV particles bound to the membrane distal domains of CD117 or the glutamate receptor subunit 4 (GluA4) was substantially enhanced by reducing receptor size to below 100 Å. Overall, the data suggest that the NiV glycoproteins are optimally suited for cell-type specific gene delivery with LVs and, in addition, for the first time define which parts of a cell surface protein should be targeted to achieve optimal gene transfer rates with receptor-targeted LVs.

Pseudotyping of lentiviral vectors (LVs) with glycoproteins from other enveloped viruses has not only often been revealing in mechanistic studies of particle assembly and entry, but is also of practical importance for gene delivery. LVs pseudotyped with engineered glycoproteins allowing free choice of receptor usage are expected to overcome current limitations in cell-type selectivity of gene transfer. Here we describe for the first time receptor-targeted Nipah virus glycoproteins as important step towards this goal. LV particles carrying the engineered Nipah virus glycoproteins were substantially more efficient in gene delivery than their state-of-the-art measles virus-based counterparts, now making the production of receptor-targeted LVs for clinical applications possible. Moreover, the data define for the first time the molecular requirements for membrane fusion with respect to the position of the receptor binding site relative to the cell membrane, a finding with implications for the molecular evolution of paramyxoviruses using proteinaceous receptors for cell entry.

Structure of Full-Length Human PDGFRβ Bound to Its Activating Ligand PDGF-B as Determined by Negative-Stain Electron Microscopy

Members of the receptor tyrosine kinases (RTKs) regulate important cellular functions such as cell growth and migration, which are key steps in angiogenesis, in organ morphogenesis and in the unregulated states, cancer formation. One long-standing puzzle regarding RTKs centers on how the extracellular domain (ECD), which detects and binds to growth factors, is coupled with the intracellular domain kinase activation. While extensive structural works on the soluble portions of RTKs have provided critical insights into RTK structures and functions, lack of a full-length receptor structure has hindered a comprehensive overview of RTK activation. In this study, we successfully purified and determined a 27-Å-resolution structure of PDGFRβ [a full-length human platelet-derived growth factor receptor], in complex with its ligand PDGF-B. In the ligand-stimulated complex, two PDGFRβs assemble into a dimer via an extensive interface essentially running along the full-length of the receptor, suggesting that the membrane-proximal region, the transmembrane helix and the kinase domain of PDGFRβ are involved in dimerization. Major structural differences are seen between the full-length and soluble ECD structures, rationalizing previous experimental data on how membrane-proximal domains modulate receptor ligand-binding affinity and dimerization efficiency. Also, in contrast to the 2-fold symmetry of the ECD, the intracellular kinase domains adopt an asymmetric dimer arrangement, in agreement with prior observations for the closely related KIT receptor. In essence, the structure provides a first glimpse into how platelet-derived growth factor receptor ECD, upon ligand stimulation, is coupled to its intracellular domain kinase activation.

PDGFRB mutants found in patients with familial infantile myofibromatosis or overgrowth syndrome are oncogenic and sensitive to imatinib

Recently, germline and somatic heterozygous mutations in the platelet-derived growth factor receptor β (PDGFRB) have been associated with familial infantile myofibromatosis (IM), which is characterized by soft tissue tumors, and overgrowth syndrome, a disease that predisposes to cancer. These mutations have not been functionally characterized. In the present study, the activity of three PDGFRB mutants associated with familial IM (R561C, P660T and N666K) and one PDGFRB mutant found in patients with overgrowth syndrome (P584R) was tested in various models. The P660T mutant showed no difference with the wild-type receptor, suggesting that it might represent a polymorphic variant unrelated to the disease. By contrast, the three other mutants were constitutively active and able to transform NIH3T3 and Ba/F3 cells to different extents. In particular, the germline mutant identified in overgrowth syndrome, P584R, was a stronger oncogene than the germline R561C mutant associated with myofibromatosis. The distinct phenotypes associated with these two mutations could be related to this difference of potency. Importantly, all activated mutants were sensitive to tyrosine kinase inhibitors such as imatinib, nilotinib and ponatinib. In conclusion, the PDGFRB mutations previously identified in familial IM and overgrowth syndrome activate the receptor in the absence of ligand, supporting the hypothesis that these mutations cause the diseases. Moreover, imatinib seems to be a promising treatment for patients carrying these mutations. To our knowledge, these are the first confirmed gain-of-function point mutations of PDGFRB in human cancer.

The Phosphatases STS1 and STS2 Regulate Hematopoietic Stem and Progenitor Cell Fitness

FLT3 and c-KIT are crucial regulators of hematopoietic stem and progenitor cells. We investigated the role of STS1 and STS2 on FLT3 and c-KIT phosphorylation, activity, and function in normal and stress-induced hematopoiesis. STS1/STS2-deficient mice show a profound expansion of multipotent progenitor and lymphoid primed multipotent progenitor cells with elevated colony-forming capacity. Although long-term hematopoietic stem cells are not increased in numbers, lack of STS1 and STS2 significantly promotes long-term repopulation activity, demonstrating a pivotal role of STS1/STS2 in regulating hematopoietic stem and progenitor cell fitness. Biochemical analysis identified STS1/STS2 as direct phosphatases of FLT3 and c-KIT. Loss of STS1/STS2 induces hyperphosphorylation of FLT3, enhances AKT signaling, and confers a strong proliferative advantage. Therefore, our study reveals that STS1 and STS2 may serve as novel pharmaceutical targets to improve hematopoietic recovery after bone marrow transplantation.

Class III Receptor Tyrosine Kinases in Acute Leukemia - Biological Functions and Modern Laboratory Analysis

Acute myeloid leukemia (AML) is a complex disease caused by deregulation of multiple signaling pathways. Mutations in class III receptor tyrosine kinases (RTKs) have been implicated in alteration of cell signals concerning the growth and differentiation of leukemic cells. Point mutations, insertions, or deletions of RTKs as well as chromosomal translocations induce constitutive activation of the receptor, leading to uncontrolled proliferation of undifferentiated myeloid blasts. Aberrations can occur in all domains of RTKs causing either the ligand-independent activation or mimicking the activated conformation. The World Health Organization recommended including RTK mutations in the AML classification since their detection in routine laboratory diagnostics is a major factor for prognostic stratification of patients. Polymerase chain reaction (PCR)-based methods are well-validated for the detection of fms-related tyrosine kinase 3 (FLT3) mutations and can easily be applied for other RTKs. However, when methodological limitations are reached, accessory techniques can be applied. For a higher resolution and more quantitative approach compared to agarose gel electrophoresis, PCR fragments can be separated by capillary electrophoresis. Furthermore, high-resolution melting and denaturing high-pressure liquid chromatography are reliable presequencing screening methods that reduce the sample amount for Sanger sequencing. Because traditional DNA sequencing is time-consuming, next-generation sequencing (NGS) is an innovative modern possibility to analyze a high amount of samples simultaneously in a short period of time. At present, standardized procedures for NGS are not established, but when this barrier is resolved, it will provide a new platform for rapid and reliable laboratory diagnostic of RTK mutations in patients with AML. In this article, the biological and physiological role of RTK mutations in AML as well as possible laboratory methods for their detection will be reviewed.

Structure and Assembly Mechanism of the Signaling Complex Mediated by Human CSF-1

Human colony-stimulating factor 1 receptor (hCSF-1R) is unique among the hematopoietic receptors because it is activated by two distinct cytokines, CSF-1 and interleukin-34 (IL-34). Despite ever-growing insights into the central role of hCSF-1R signaling in innate and adaptive immunity, inflammatory diseases, and cancer, the structural basis of the functional dichotomy of hCSF-1R has remained elusive. Here, we report crystal structures of ternary complexes between hCSF-1 and hCSF-1R, including their complete extracellular assembly, and propose a mechanism for the cooperative human CSF-1:CSF-1R complex that relies on the adoption by dimeric hCSF-1 of an active conformational state and homotypic receptor interactions. Furthermore, we trace the cytokine-binding duality of hCSF-1R to a limited set of conserved interactions mediated by functionally equivalent residues on CSF-1 and IL-34 that play into the geometric requirements of hCSF-1R activation, and map the possible mechanistic consequences of somatic mutations in hCSF-1R associated with cancer.

Protein tyrosine phosphatase-σ regulates hematopoietic stem cell-repopulating capacity

Hematopoietic stem cell (HSC) function is regulated by activation of receptor tyrosine kinases (RTKs). Receptor protein tyrosine phosphatases (PTPs) counterbalance RTK signaling; however, the functions of receptor PTPs in HSCs remain incompletely understood. We found that a receptor PTP, PTPσ, was substantially overexpressed in mouse and human HSCs compared with more mature hematopoietic cells. Competitive transplantation of bone marrow cells from PTPσ-deficient mice revealed that the loss of PTPσ substantially increased long-term HSC-repopulating capacity compared with BM cells from control mice. While HSCs from PTPσ-deficient mice had no apparent alterations in cell-cycle status, apoptosis, or homing capacity, these HSCs exhibited increased levels of activated RAC1, a RhoGTPase that regulates HSC engraftment capacity. shRNA-mediated silencing of PTPσ also increased activated RAC1 levels in wild-type HSCs. Functionally, PTPσ-deficient BM cells displayed increased cobblestone area-forming cell (CAFC) capacity and augmented transendothelial migration capacity, which was abrogated by RAC inhibition. Specific selection of human cord blood CD34⁺CD38⁻CD45RA⁻lin⁻ PTPσ⁻ cells substantially increased the repopulating capacity of human HSCs compared with CD34⁺CD38⁻CD45RA⁻lin⁻ cells and CD34⁺CD38⁻CD45RA⁻lin⁻PTPσ⁺ cells. Our results demonstrate that PTPσ regulates HSC functional capacity via RAC1 inhibition and suggest that selecting for PTPσ-negative human HSCs may be an effective strategy for enriching human HSCs for transplantation.

Robust expansion of dendritic cells in vivo by hydrodynamic FLT3L-FC gene transfer

Due to low numbers of endogenous dendritic cells (DCs) in vivo, exogenous DC-poietin Fms-like tyrosine kinase 3-ligand (FLT3L) is routinely used to generate DC for subsequent studies. We engineered a novel FLT3L-FC DNA construct that, when combined with hydrodynamic gene transfer (HDT), induced robust DC expansion in mice. DC generated in vivo by FLT3L-FC HDT produced cytokines in response to stimulation by an array of TLR agonists and promoted T cell proliferation. The FLT3L-FC protein produced in vivo spontaneously homodimerized to enable effective FLT signaling and the FC-domain enhanced its plasma half-life, providing an improved reagent and method to boost DC numbers.