FLT3 - fms related receptor tyrosine kinase 3 | Elisa - Clia - Antibody - Protein

Background

FLT3 (Fms-like tyrosine kinase 3), also known as CD135, is a cell-surface receptor primarily expressed on hematopoietic progenitor cells in the bone marrow and on certain immune cells, such as dendritic cells. Classified as a receptor tyrosine kinase (RTK), FLT3 belongs to the type III RTK family, which also includes the KIT, PDGFR, and CSF1R proteins. FLT3 plays a central role in the regulation of hematopoiesis by promoting the survival, proliferation, and differentiation of hematopoietic stem and progenitor cells (HSPCs).

The FLT3 ligand (FLT3L) is the primary growth factor that binds to and activates FLT3, resulting in downstream signaling essential for immune cell development. FLT3 has attracted significant scientific interest due to its implication in hematologic malignancies, especially acute myeloid leukemia (AML). Mutations in the FLT3 gene, particularly internal tandem duplications (ITDs) and point mutations in the tyrosine kinase domain (TKD), are common in AML and are associated with a poor prognosis. Consequently, FLT3 is a prominent target in therapeutic research, with several FLT3 inhibitors under development or in clinical use for the treatment of FLT3-mutated AML.

Protein Structure

FLT3 is a type I transmembrane protein with a complex structure that is typical of receptor tyrosine kinases in the type III subclass. It has distinct extracellular, transmembrane, and intracellular regions, each critical for its function and ligand interaction:

Extracellular Domain:

• The extracellular domain of FLT3 contains five immunoglobulin-like (Ig-like) domains that are responsible for ligand binding. This region allows FLT3 to interact specifically with FLT3L, initiating the dimerization and activation of the receptor.
• The FLT3L binding to these Ig-like domains induces a conformational change that brings two FLT3 molecules close together, a process known as dimerization. This dimerization is essential for receptor activation and subsequent signal transduction.
• This domain also contains several glycosylation sites that are important for protein stability, trafficking to the cell surface, and receptor-ligand interaction.

Transmembrane Domain:

• Following the extracellular domain, the FLT3 transmembrane domain consists of a short hydrophobic sequence that spans the cell membrane. This domain anchors the receptor in the membrane and supports the structural integrity necessary for ligand-induced dimerization and signal transduction.

Intracellular (Cytoplasmic) Domain:

• The intracellular region of FLT3 comprises two major domains: a juxtamembrane domain and a split tyrosine kinase domain.
• The juxtamembrane domain has an autoinhibitory function, maintaining FLT3 in an inactive conformation in the absence of FLT3L. Mutations in this region, particularly internal tandem duplications (ITDs), disrupt this autoinhibition, resulting in constitutive FLT3 activation, which drives cell proliferation and survival in AML.
• The kinase domain is split into an N-lobe and C-lobe, with a cleft between them that serves as the ATP binding site. Activation of FLT3 following ligand binding leads to autophosphorylation of tyrosine residues within the kinase domain, initiating downstream signaling cascades.
• Key autophosphorylation sites within the kinase domain enable the recruitment of adaptor proteins and activation of signaling pathways such as PI3K-Akt, MAPK, and JAK-STAT, which promote cell survival, proliferation, and differentiation.

Classification and Subtypes

FLT3 is classified as part of the type III receptor tyrosine kinase (RTK) family, which is characterized by a ligand-binding extracellular region with five immunoglobulin-like domains, a single transmembrane domain, and a cytoplasmic split kinase domain. Type III RTKs are structurally distinct from other RTK classes due to this split kinase domain and their unique regulatory mechanisms.

FLT3 mutations are broadly divided into two types based on their location and mechanism of action:

1. Internal Tandem Duplication (ITD): Occurs in the juxtamembrane domain of the FLT3 gene, leading to constitutive activation of the receptor. ITD mutations are the most common FLT3 alterations in AML.
2. Tyrosine Kinase Domain (TKD) Mutations: Point mutations in the kinase domain, commonly affecting residues like D835, disrupt kinase activity regulation, resulting in unregulated cell proliferation.

Function and Biological Significance

FLT3 is integral to the development and function of hematopoietic cells. Its primary biological functions include:

Regulation of Hematopoiesis:

• FLT3 signaling, in the presence of FLT3L, is crucial for the maintenance and proliferation of HSPCs. FLT3L is expressed by bone marrow stromal cells and other immune cells, providing a niche environment that supports hematopoiesis.
• FLT3 expression is highest in progenitor cells, including those that give rise to dendritic cells, B cells, and natural killer (NK) cells, indicating its role in the differentiation of the immune system’s components.

Immune System Modulation:

• FLT3 signaling promotes the differentiation of dendritic cells, especially plasmacytoid dendritic cells (pDCs), which are vital for antigen presentation and initiating immune responses. It also supports the expansion of other immune cell types, contributing to adaptive and innate immunity.

Downstream Signaling Pathways:

FLT3 activation triggers several downstream signaling pathways, including:

• PI3K-Akt Pathway: Promotes cell survival and growth.
• MAPK Pathway: Involved in cell proliferation and differentiation.
• JAK-STAT Pathway: Essential for immune cell development and cytokine signaling.

These signaling pathways collectively ensure the survival, proliferation, and differentiation of hematopoietic cells in response to FLT3 activation, particularly during immune responses or bone marrow stress.

Clinical Issues

Mutations and dysregulation of FLT3 signaling are strongly associated with hematologic malignancies, particularly AML. Major clinical issues related to FLT3 include:

Acute Myeloid Leukemia (AML):

• FLT3 mutations are present in approximately 30% of AML cases, with ITDs being the most common. FLT3-ITD mutations are associated with a poor prognosis due to increased cell proliferation, resistance to apoptosis, and higher relapse rates.
• The identification of FLT3 mutations has driven the development of targeted therapies, particularly FLT3 inhibitors such as midostaurin, gilteritinib, and quizartinib. These inhibitors block the ATP binding site within the kinase domain, thereby preventing downstream signaling and reducing leukemia cell survival.

Potential in Other Hematologic Disorders:

• While most commonly associated with AML, FLT3 mutations or overexpression may contribute to other myeloid malignancies, such as myelodysplastic syndromes (MDS) and acute lymphoblastic leukemia (ALL), though at lower frequencies.

Drug Resistance:

• FLT3-targeted therapies have improved AML treatment, but resistance often develops. Secondary mutations in the kinase domain or compensatory activation of other signaling pathways can confer resistance to FLT3 inhibitors, challenging long-term treatment efficacy.
• Combination therapies targeting multiple pathways or sequential administration of different FLT3 inhibitors are being investigated to mitigate resistance.

Summary

FLT3 (CD135) is a receptor tyrosine kinase in the type III RTK family, essential for hematopoietic cell development and differentiation. Structurally, FLT3 consists of an extracellular ligand-binding domain with Ig-like domains, a transmembrane segment, and a split kinase domain crucial for signal transduction. Through its interaction with FLT3L, FLT3 supports the proliferation and survival of hematopoietic progenitor cells, especially those involved in immune responses.

FLT3 mutations, particularly ITDs and TKD mutations, are frequently observed in AML and confer poor prognoses due to their role in enhancing proliferation and reducing apoptosis. Consequently, FLT3 is a prominent therapeutic target, with several FLT3 inhibitors developed and used in clinical settings. Despite their efficacy, drug resistance remains a significant hurdle, necessitating combination strategies to improve long-term outcomes