All 14 surface markers and receptors displayed on the LT-HSC plasma membrane, color-coded by molecular class. The bilayer cross-section shows each protein's topology — from GPI-anchored domains to 7-pass GPCRs and multi-domain RTKs.

Human LT-HSCs are identified by a precise combination of surface proteins that distinguish them from progenitors and mature blood cells. The gold-standard immunophenotype — Lin⁻ CD34⁺ CD38⁻ CD90⁺ CD45RA⁻ CD49f⁺ — enables isolation of cells with robust self-renewal and multilineage reconstitution potential.

CD34 is a type I transmembrane sialomucin glycoprotein expressed on hematopoietic stem cells, multipotent progenitors, early endothelial progenitors, and small vessel endothelium. It is a stemness-associated surface marker, not a lineage marker. Gene located on chromosome 1q32; encodes ~385 amino acids (varies by glycosylation).

Figure: Full-length vs. Truncated CD34 — showing the Mucin domain, Cysteine-containing globular domain, Stalk region, Transmembrane region, and intracellular phosphorylation motifs.

PDZ = PSD-95 / Dlg / ZO-1 — named after the three proteins where it was first discovered: PSD-95 (Postsynaptic Density protein 95), Dlg (Drosophila Discs Large tumor suppressor), ZO-1 (Zonula Occludens-1). PDZ domains are protein-interaction modules that bind short peptide sequences at the C-terminus of membrane proteins. In CD34, the PDZ-binding motif allows interaction with scaffold proteins that: anchor CD34 to cytoskeletal complexes · organize signaling platforms · influence cell adhesion and polarity.

CrkL = CT10 Regulator of Kinase-Like. Name origin: CT10 → avian sarcoma virus oncogene; Crk → CT10 regulator of kinase; CrkL → Crk-Like. CrkL is an adaptor protein connecting receptors to intracellular signaling cascades. Structure: SH2 domain (binds phosphorylated tyrosines) · SH3 domains (bind proline-rich regions). Role with CD34: when CD34 cytoplasmic tyrosines become phosphorylated, CrkL binds and activates pathways controlling cell adhesion, cytoskeletal rearrangement, and migration of HSCs — important in HSC homing to bone marrow.

PI3K = Phosphoinositide 3-Kinase. PI3K phosphorylates membrane lipids: PIP2 → PIP3, which recruits downstream signaling proteins. Major downstream cascade: PI3K → AKT (Protein Kinase B) → mTOR (Mechanistic Target Of Rapamycin). Role in CD34⁺ HSCs: regulates stem cell survival · quiescence vs proliferation · migration toward CXCL12 gradients. Interacts with CXCR4 signaling in stem-cell homing.

These are proteins controlling actin and microtubule dynamics.

These regulators control: cell shape · membrane protrusions · migration of HSCs.

Tyrosine (Y), Serine (S), or Threonine (T) residues that kinases can phosphorylate. Phosphorylation allows recruitment of proteins with SH2 (Src Homology 2) or PTB (Phosphotyrosine Binding) domains, creating temporary signaling complexes inside the cell.

Sialomucins are a class of heavily glycosylated cell-surface proteins rich in sialic acid–containing O-linked glycans. They form extended, highly hydrated structures on the cell surface and regulate cell adhesion, trafficking, and immune interactions. In hematopoietic stem cell biology, CD34 is the defining sialomucin — its expression marks LT-HSCs and early progenitors, and its molecular architecture directly explains its anti-adhesive, pro-migratory function.

Schematic of mucin glycoprotein structure — showing the core protein backbone, O-linked oligosaccharides, sialic acid residues (COO⁻), cysteine-rich domains, and key interaction types (electrostatic, hydrogen bonding, hydrophobic).

CD34 has critical clinical roles across hematology, oncology, transplantation, vascular biology, and regenerative medicine. Many therapies directly target or mobilize CD34⁺ cells, making it one of the most clinically actionable surface markers in medicine.

CD34 is the standard marker used to quantify hematopoietic stem cells for transplantation. Doctors measure CD34⁺ cells per kilogram of patient weight.

Higher CD34 counts correlate with faster neutrophil recovery, faster platelet recovery, and improved survival outcomes.

CD34 is frequently expressed on immature malignant blasts and is used in flow cytometry diagnostic panels.

Clinical implications: marker of immature leukemic cells; sometimes associated with poor prognosis; used in flow cytometry panels.

CD34 is widely used by pathologists to identify vascular endothelial cells and diagnose specific tumors.

CD34⁺ cells include endothelial progenitor cells and hematopoietic stem cells. Experimental therapies investigate CD34⁺ cells for: myocardial infarction repair · peripheral artery disease · stroke recovery. Clinical trials transplant autologous CD34⁺ cells to improve blood vessel formation in ischemic tissues.

CD34 is expressed on endothelial cells and endothelial progenitor cells. CD34⁺ progenitors contribute to angiogenesis in ischemic tissues. Research investigates their role in: diabetic vascular disease · wound healing · tumor angiogenesis.

CD34-positive fibroblast populations exist in many tissues. Loss of CD34 expression has been observed in fibrotic conditions.

CD38 is a type II transmembrane glycoprotein with ecto-enzymatic activity. Unlike CD34, it is absent on LT-HSCs — its expression marks the transition away from deep stemness toward commitment and activation. Gene located on chromosome 4p15; encodes ~300 amino acids.

Note: CD38 appearance = loss of deep stemness

Figure: Types of integral membrane proteins — CD38 is a Type II transmembrane protein (N-terminus intracellular, C-terminus extracellular), in contrast to CD34 which is Type I.

Figure: CD38 dual function — ecto-enzymatic NAD⁺/NAADP metabolism and interaction with CD31 (PECAM-1), with downstream signaling into cytoplasm and nucleus.

Figure: CD38 molecular mechanism — showing intracellular and extracellular β-NAD⁺ pathways, cADPR production by soluble CD38, Ca²⁺ release via Ryanodine receptors and TRP channels, NMN/NMN-NR recycling via Slc12a8, CD39/CD73 ectonucleotidase axis generating adenosine, and downstream purinergic and adenosine receptor signaling.

Daratumumab is a human IgG1κ monoclonal antibody that targets CD38 — a surface glycoprotein highly expressed on plasma cells and myeloma cells. It is the direct clinical application of CD38 biology: because LT-HSCs are CD38⁻, the drug selectively destroys malignant plasma cells while sparing the stem cell compartment.

CD38 is a transmembrane ectoenzyme involved in NAD⁺ metabolism, calcium signaling, cell adhesion, and immune regulation.

Figure: Daratumumab mechanisms: CDC, ADCC, ADCP, direct apoptosis via cross-linking, and modulation of CD38 enzymatic activity.

Daratumumab also depletes immunosuppressive CD38⁺ cells: regulatory T cells (Tregs) · regulatory B cells · myeloid-derived suppressor cells. Result: ↑ cytotoxic T cell activity → enhanced anti-tumor immune response.

Main indication: Multiple Myeloma — both newly diagnosed and relapsed/refractory disease.

Common combinations:

Brand names: Darzalex (IV) · Darzalex Faspro (subcutaneous)

New research directions: AL amyloidosis · NK/T-cell lymphomas · autoimmune diseases · post-transplant relapse.

Beyond multiple myeloma, CD38 is implicated in a wide spectrum of diseases — from hematologic malignancies and autoimmune disorders to metabolic aging, neurodegeneration, and transplant rejection. Its dual role as an ectoenzyme and signaling receptor makes it one of the most clinically versatile surface markers in medicine.

CD38 regulates immune cell activation. Anti-CD38 therapies are now studied in:

CD38 expression increases during immune activation. In HIV infection, CD38 on CD8 T cells is a marker of disease progression. High CD38 expression correlates with: high viral load · immune exhaustion. Clinically used as a prognostic biomarker.

CD38 is a major NAD⁺ consumer. It degrades NAD⁺ through: NAD⁺ → ADP-ribose and NAD⁺ → cyclic ADP-ribose, which regulate Ca²⁺ signaling.

Aging: CD38 activity increases with age → NAD⁺ depletion → mitochondrial dysfunction · metabolic disease · neurodegeneration.

CD38 is involved in: oxytocin release · synaptic signaling · neuroinflammation. Associated conditions: autism spectrum disorders · Alzheimer's disease · Parkinson's disease. Mouse studies show: CD38 knockout → impaired social behavior.

CD38 regulates insulin secretion through Ca²⁺ signaling. Mechanism: CD38 → cyclic ADP-ribose → Ca²⁺ release → insulin secretion. CD38 dysfunction may contribute to type 2 diabetes and pancreatic β-cell failure.

Plasma cells produce donor-specific antibodies causing antibody-mediated rejection. Anti-CD38 drugs (daratumumab, isatuximab) are being investigated in kidney transplant rejection and refractory antibody-mediated rejection.

This marker combination is critical in: stem cell research · bone marrow transplantation · leukemia diagnosis.

CD90 (Thy-1) is a GPI-anchored glycoprotein belonging to the Immunoglobulin (Ig) superfamily. It is expressed on LT-HSCs (human), mesenchymal stem cells, activated T cells, neurons, and fibroblasts. In HSC biology, CD90 positivity strengthens long-term repopulation capacity. Gene located on chromosome 11q23; encodes ~160 amino acids.

Figure: Representatives of the Immunoglobulin superfamily of cell adhesion molecules — CD90 belongs to this superfamily, featuring a single Ig-like V-type domain.

Note: CD90 positivity = strengthened long-term repopulation capacity

Figure: Representatives of the Immunoglobulin superfamily of cell adhesion molecules — CD90 belongs to this superfamily, featuring a single Ig-like V-type domain.

Figure: CD90 (Thy-1) protein structure — showing the single Ig-like V-type domain with RLD (Arg-Leu-Asp) and HBD (Heparin-Binding Domain) motifs, attached to the outer membrane leaflet via a GPI anchor (hexagonal symbol at membrane).

Figure: GPI anchor biochemical structure — Phosphoethanolamine linker → Man₃GlcN oligosaccharide core → Phosphatidylinositol embedded in the outer membrane leaflet. PLC-PI cleavage site shown. CD90 is attached via this structure with no cytoplasmic tail.

Figure: Comparison of Ig superfamily membrane proteins — Thy-1 (CD90) shown as a GPI-anchored single-domain protein on the outer cell membrane, contrasted with transmembrane proteins like CD4, CD8, CD28, MHC proteins, and T-cell receptor.

Integrins are cell adhesion receptors that connect the extracellular matrix → integrins → actin cytoskeleton.

Integrins interacting with CD90

CD90 binds integrins in trans or cis. Example: CD90 on stem cell → binds αvβ3 integrin → activates integrin signaling → focal adhesion formation · actin polymerization · traction force generation.

Figure: CD90 (Thy-1) signaling at the cell membrane — showing interactions with integrins, FAK, Src, Rac1, RhoA, and downstream cytoskeletal effectors including ROCK, MLC, and Cofilin. Note the Cbp/Csk axis regulating p190GAP → RhoA → axonal/dendrite contraction.

CD90 signaling alters RhoA activity — this is where CD90 becomes particularly interesting biologically.

CD90 (Thy-1) → recruits Cbp (Csk-binding protein) → activates Csk → phosphorylates and inhibits Src → Src inhibition → p190RhoGAP becomes less active → RhoA-GTP remains active → ROCK activation → phosphorylates MLC and Cofilin → actin stress fiber formation and cell contraction.

Alternatively, when Src IS active: Src → phosphorylates p190RhoGAP → inactivates RhoA-GTP → Rac1 becomes dominant → actin polymerization at leading edge → cell migration.

Figure: RhoA/Rac1 balance downstream of CD90 (Thy-1) — Panel A: intracellular signaling via Cbp/Csk/Src/p190RhoGAP axis altering actin dynamics and neurite retraction. Panel B: trans interaction with integrins on adjacent cells (e.g., astrocyte-neuron) modulating the same RhoA/Rac1 switch.

CD90 (Thy-1) is a GPI-anchored glycoprotein involved in cell adhesion, cell–cell interaction, cytoskeleton signaling, and stem cell niche regulation. Because of these functions, CD90 appears across multiple clinical fields — from hematopoietic transplantation and fibrosis to cancer biology, neuroscience, and regenerative medicine.

CD90 is one of the most reliable markers of true hematopoietic stem cells. Classic LT-HSC phenotype: CD34⁺ CD38⁻ CD90⁺ CD49f⁺. Stem cell transplant labs use CD90 sorting to isolate pure HSC populations. Benefits: higher engraftment efficiency · fewer contaminating progenitors · better long-term hematopoiesis.

CD90 is one of the core markers of MSCs. MSC phenotype: CD90⁺ CD73⁺ CD105⁺ CD45⁻ CD34⁻. MSCs are used in: regenerative medicine · cartilage repair · immune modulation therapies. CD90 helps identify true MSC populations for clinical use.

CD90 was originally discovered in neurons. It is abundant in axons and synapses. Functions: neurite outgrowth · synaptic plasticity · neuron–glia interaction. Abnormal CD90 signaling may be involved in neurodegeneration and nerve regeneration disorders.

CD90 is expressed on cardiac fibroblasts and vascular endothelial cells. Roles: vascular remodeling · angiogenesis · fibrosis after myocardial infarction. CD90⁺ fibroblasts are involved in cardiac scar formation.

CD90 participates in T-cell activation and adhesion. It interacts with integrins and Src family kinases. CD90 signaling can influence: immune synapse formation · leukocyte migration.

RhoA is a ~21 kDa small GTP-binding protein that acts as a molecular switch downstream of CD90 and integrin signaling. It is the central cytoskeletal regulator determining whether an LT-HSC remains anchored in the niche (quiescent) or detaches and mobilizes.

The Rho GTPase signaling cycle — GEF activates Rho by exchanging GDP for GTP; GAP inactivates Rho by hydrolyzing GTP to GDP; GDI sequesters inactive Rho-GDP in the cytoplasm. Active Rho-GTP engages effectors controlling cytoskeletal remodeling, transcription, migration, adhesion, and cell survival.

The most important effector is ROCK (Rho-associated kinase). Full signaling cascade:

CD90 → Integrin signaling → RhoA → ROCK → Myosin light chain (MLC) phosphorylation → Actomyosin contraction

RhoA–GTP → ROK (ROCK) → actin stress fiber formation and focal adhesion maturation. The Src/FAK/Paxillin complex at focal adhesions integrates RhoA signaling with ECM attachment.

In bone marrow: CD90 + integrin β1 → RhoA activation → cytoskeletal tension → anchoring in niche.

If RhoA signaling drops: stem cell detaches → enters circulation. This mechanism is critical for: stem cell mobilization · transplantation protocols · G-CSF/Plerixafor mobilization strategies.

c-Kit (CD117) is the receptor for Stem Cell Factor (SCF), also called Steel factor or KIT ligand (KITL). It belongs to the Type III Receptor Tyrosine Kinase (RTK) family — the same family as FLT3, PDGFR, and CSF1R. Gene located on chromosome 4q12; ~145 kDa protein. c-Kit is the first true signaling receptor in our HSC marker series — unlike CD34, CD38, or CD90, it has intrinsic enzymatic (kinase) activity.

Figure: c-Kit (CD117) receptor structure — 5 IgG-like extracellular domains (SCF dimer binds domains 1–3), single transmembrane domain, juxtamembrane (JM) regulatory domain, split tyrosine kinase domain (TK1 ATP-binding n-lobe + kinase insert + TK2 phosphotransferase c-lobe). Downstream: STAT←JAK, PI3K→Akt, Ras→Raf→Mek→Erk.

Figure: RTK activation mechanism — (1) Ligand binds extracellular domain → (2) Receptor monomers dimerize, enabling transphosphorylation of A-loop tyrosines → (3) A-loop transphosphorylation activates kinase allosterically, leading to secondary phosphorylation of kinase insert and other regions → (4) Secondary transphosphorylation recruits intracellular substrates, initiating downstream signaling cascade.

Figure: c-Kit signal transduction pathways — SCF homodimer activates c-Kit → Src, Grb2, PI3K, PLCγ, JAK2 recruitment → downstream: Akt (cell survival), Rac1/JNK (proliferation), RAS→RAF→MEK→ERK1/2→p38 (gene transcription/chemotaxis), STAT1α/3/5A/5B (differentiation), adhesion remodeling.

Figure: SCF/c-Kit signaling cascade — SCF binding → c-Kit dimerization → Grb2/Shc/Sos recruitment → Ras-GTP activation (NF1 as negative regulator) → parallel pathways: Raf→MEK→ERK1/2 (proliferation), p38→Paks (migration), PI3K→Rac→Paks, PI3K→Akt (survival).

c-Kit is one of the most clinically important receptor tyrosine kinases in medicine. Its signaling controls hematopoietic stem cell survival, mast cell development, melanocyte migration, and germ cell biology. Activating mutations drive some of the most well-characterized cancers in oncology, making KIT a paradigm for targeted therapy.

In LT-HSCs, c-Kit maintains survival signals from the bone marrow niche. LT-HSC phenotype includes: CD34⁺ CD38⁻ CD90⁺ CD49f⁺ c-Kit⁺. SCF is produced by: bone marrow stromal cells · endothelial cells · osteoblasts. SCF–KIT signaling ensures stem cell maintenance and proliferation within the niche.

The most famous KIT-related cancer. GIST arises from interstitial cells of Cajal — KIT-positive pacemaker cells in the GI tract. Most GIST tumors have activating KIT mutations causing constitutive kinase activation → continuous growth signaling → tumor proliferation.

Pharmacologic treatment — targeted therapy revolutionized GIST management. Drugs that block the ATP-binding pocket of KIT kinase: Imatinib · Sunitinib · Regorafenib · Avapritinib.

c-Kit is essential for mast cell development. The KIT D816V mutation causes constitutive activation → uncontrolled mast cell proliferation → tissue infiltration. Symptoms: flushing · anaphylaxis · bone pain · organ infiltration. Targeted treatments: Midostaurin · Avapritinib.

KIT mutations occur particularly in core-binding factor AML, involving exon 8 and exon 17. These mutations increase relapse risk and aggressive disease behavior. KIT expression (CD117) is also used in flow cytometry panels for AML blast classification.

CD117 (KIT) immunostaining is widely used in pathology labs.

In cancer therapy, tumors may develop secondary resistance mutations. In GIST: secondary mutations in exon 13 and exon 17 alter the kinase structure and prevent drug binding. New-generation inhibitors (e.g., Avapritinib, Ripretinib) are designed to overcome this resistance.

The Homing Receptor of HSCs — Bone Marrow Retention Axis

CXCR4 (C-X-C chemokine receptor type 4) is a Class A GPCR and the principal navigation system of hematopoietic stem cells. Its ligand is CXCL12 (SDF-1: Stromal Derived Factor-1), produced by bone marrow stromal cells, osteoblasts, and endothelial niche cells. This axis controls homing, retention, migration, and survival. Gene located on chromosome 2q22; ~352 amino acids.

Figure: GPCR serpentine structure — 7 transmembrane α-helices (H1–H7), 4 extracellular loops (E1–E4, N-terminus NH₃⁺), 4 intracellular loops (C1–C4, C-terminus COO⁻). Classic Class A GPCR topology shared by CXCR4.

Figure: CXCR4 receptor topology — NH2 extracellular N-terminus, 7 transmembrane domains (7TM), intracellular loops ICL1, ICL2, ICL3, and COOH cytoplasmic C-terminal tail with phosphorylation sites. Lower panel shows ICL mutants used in functional studies.

Figure: CXCL12–CXCR4 signaling network — CXCL12 binding activates Gαi (cAMP↓ via adenylyl cyclase, PKA→stemness/survival) and βγ (PI3K→Akt→survival/migration/stemness; Rac/Rho/Cdc42→proliferation/migration; Src→NFkB/FOXO→survival/stemness). PLC→PIP2→IP3/DAG→Ca²⁺ release (migration/survival). RTK/FAK/Ras→Raf→ERK1/2 (chemotaxis/proliferation). GRK/Arrestin→endocytosis→recycling or degradation. AMD3100 shown as CXCR4 antagonist.

Figure: CXCR4 context-dependent signaling — (A) Cell-autonomous CXCR4/ACKR3 signaling: CXCL12 activates MAPK, PI3K, PLC → Ca²⁺ mobilization, proliferation, differentiation, migration, β-arrestin recruitment; CXCR4 activity and CXCL12 levels regulated by ACKR3. (B) T cell-specific: CXCR4 co-signals with TCR via ZAP70/Rac1/ERK → chemotaxis, cytokine secretion. (C) Tumor-macrophage interaction: CD47/CXCR4/ACKR3 co-internalization → tumor phagocytosis.

CXCR4 signaling activates key Rho GTPases, which orchestrate the dynamic remodeling of the cytoskeleton for precise cell movement:

This coordinated action leads to rapid actin polymerization, lamellipodia formation, and directed cell migration along CXCL12 gradients.

β-arrestins (ARRB1 and ARRB2) are multifunctional adaptor proteins that regulate both GPCR signaling termination and alternative downstream signaling. They create a second parallel signaling system alongside classical G-protein signaling.

Figure: β-arrestin–mediated GPCR regulation — Class A GPCRs (top): β-arrestin recruits clathrin for receptor internalization, then dissociates; receptor is recycled. Class B GPCRs (middle): β-arrestin remains bound through endosomal trafficking; receptor may be degraded. β-adrenergic receptor (right): β-arrestin targets to clathrin-coated structures and activates MAPK signaling independently.

Figure: Step-by-step arrestin-mediated desensitization and internalization of GPCRs — (1) Agonist binds receptor; (2) G-protein activation; (3) GRK phosphorylates receptor; (4) Arrestin binds phosphorylated receptor; (5) G-protein dissociation; (6) Clathrin/AP2 recruitment → receptor internalization into clathrin-coated pits.

Systems view: c-Kit = growth signal | CD38 = activation switch | CXCR4 = positioning system | CD90 = structural stability | CD34 = mobility modulation

CXCR4 is one of the most clinically actionable surface receptors in medicine. Its role in HSC retention, immune cell trafficking, viral entry, and tumor metastasis makes it a target across hematology, oncology, infectious disease, and regenerative medicine. The drug plerixafor (a CXCR4 antagonist) is the clearest example of translating CXCR4 biology directly into therapy.

Cause: Gain-of-function mutations in the C-terminal cytoplasmic tail of CXCR4 → loss of phosphorylation sites → β-arrestin cannot bind → receptor fails to internalize → persistent CXCR4 signaling → excess CXCL12 retention → neutrophils trapped in bone marrow (myelokathexis) → neutropenia in peripheral blood.

Bone marrow finding: hypercellular marrow with mature neutrophils that cannot exit. CXCL12 gradient: bone marrow high → blood low. Overactive CXCR4 prevents neutrophil detachment from niche.

HIV entry requires two receptors: CD4 + co-receptor (CCR5 or CXCR4). Process: gp120 binds CD4 → conformational change → gp120 binds CXCR4 → gp41 exposes fusion peptide → viral membrane fuses with host membrane.

CXCR4-tropic HIV is associated with rapid CD4 decline and more aggressive disease. CXCR4 is strongly expressed on naive CD4 T cells.

CXCR4 is one of the most important metastasis receptors. Many tumors overexpress CXCR4 and follow CXCL12 gradients to metastatic sites. Organs producing CXCL12: bone marrow · liver · lung · lymph nodes.

Mechanism: CXCL12 binds CXCR4 → Gαi signaling → PI3K/AKT · MAPK · RhoA/Rac → cytoskeleton rearrangement · ↑ integrin activation · ↑ migration · ↑ invasion. β-arrestin signaling also activated → survival signaling.

In bone marrow: stromal cells produce CXCL12 → binds CXCR4 on HSCs and progenitors → stem cells retained in niche. Mobilization strategy: Plerixafor blocks CXCR4 → stem cells detach → enter bloodstream → blood apheresis → collect CD34⁺ cells. Used for: lymphoma · multiple myeloma · stem cell transplant donors.

Standard protocol: G-CSF for 4 days + Plerixafor → massive HSC release into peripheral blood.

CXCL12 released during tissue injury recruits endothelial progenitor cells and stem cells → angiogenesis and vascular repair. Studied for: myocardial infarction · ischemic limb disease · tissue regeneration. Strategies: CXCL12 delivery · CXCR4 agonists.

The Quiescence Receptor — Vascular Niche Stability Axis

Tie2 (TEK / CD202b) is a single-pass receptor tyrosine kinase (RTK) that links hematopoietic stem cells to the vascular niche. Its primary activating ligand is Angiopoietin-1 (Ang1), produced constitutively by osteoblasts, perivascular stromal cells, and endothelial niche cells. Ang2 acts as a context-dependent antagonist/agonist. This pathway regulates stem cell quiescence, niche adhesion, and vascular stability. TEK gene located on chromosome 9p21; ~1124 amino acids.

Figure: Tie2 domain organization — Extracellular ligand-binding domain (IG domain → EGF repeats → IG domain → FN3 repeats), single transmembrane domain, intracellular kinase domain.

Figure: (A) Tie2 receptor — Ig1-like (tip), Ig2-like, EGF-like repeats, Ig3-like, Fibronectin type III domains → transmembrane → intracellular tyrosine kinase. (B) Angiopoietin structure — Superclustering domain (SCD), Coiled-coil domain (CCD), Fibrinogen-like domain (FReD, receptor-binding).

Figure: TIE2 primary structure and mutation sites — Extracellular: Ig2, Ig1, EGF, Ig3, FN III domains. Intracellular: N-terminal kinase domain (824–915) with NBL region, Kinase Insert Domain (KID, 916–936), C-terminal kinase domain (TK2, 937–1096) with CL and YIA activation loop (975–977), C-terminal tail (1097–1124). Known gain-of-function mutations shown (venous malformations, leukemia).

Figure: Angiopoietin–Tie2 signaling model — Ang1 (constitutively released by pericytes/stroma) activates Tie2 → PI3K/Akt (survival), NFkB (anti-inflammatory), Rho kinase (cell junction stabilization, Ca²⁺ regulation) → blood vessel stabilization, leukocyte recruitment suppression. Ang2 (released from Weibel-Palade bodies upon inflammatory stimulus) antagonizes Ang1, promoting vascular destabilization and inflammatory activation.

Tie2 (TEK) is an endothelial receptor tyrosine kinase mainly expressed on:

Activating mutations in TEK (Tie2) lead to continuous signaling.

Tie2 plays a role in tumor blood vessel formation.

Seen in: breast cancer, glioblastoma, hepatocellular carcinoma, colorectal cancer

One of the most clinically relevant Tie2 pathways.

During systemic inflammation:

Ang2 is now considered a biomarker of sepsis severity.

Current treatment is mainly supportive, but Tie2-targeting drugs are being investigated.

Inflammation causes:

Hyperglycemia causes:

Ischemia causes:

Current therapy focuses on stroke treatment, but Tie2 is a target for future drugs.

Important for HSC Biology

Tie2 is expressed on long-term hematopoietic stem cells (LT-HSCs).

Tie2 signaling keeps HSCs:

Tie2 signaling affects:

Experimental approaches include:

Tie2 inhibition → reduces HSC adhesion → releases HSCs into blood

Used with agents like:

CD133 (Prominin-1) is a pentaspan transmembrane glycoprotein belonging to the Prominin family. Unlike classical receptors, it does not bind ligands — instead it organizes membrane protrusions and lipid microdomains, influencing stem cell polarity, asymmetric division, and membrane signaling architecture. PROM1 gene located on chromosome 4p15 (4p15.32); ~865 amino acids.

Figure: CD133 (Prominin-1) gene and protein structure — PROM1 gene at chromosome 4p15.32. Protein topology: NH2 intracellular N-terminus → M1 (TM1) → E1 (extracellular loop 1) → M2 (TM2) → C1 (cytoplasmic loop 1) → M3 (TM3) → E2 (large extracellular loop 2) → M4 (TM4) → C2 (cytoplasmic loop 2) → M5 (TM5) → E3 (extracellular loop 3) → C3 → COOH cytoplasmic C-terminus. Five transmembrane helices with 3 extracellular loops (E1, E2, E3) and 3 cytoplasmic loops (C1, C2, C3).

Figure: CD133 membrane topology — Five transmembrane domains (M1–M5), three extracellular loops (E1, E2, E3), three cytoplasmic loops (C1, C2, C3). Yellow disks = N-glycosylation sites on E2 and E3. AC-133 antibody epitope on E3 (glycosylation-dependent). Inset: CD133 localizes to GM1 ganglioside-rich lipid raft microdomains (GM1 shown in orange), interacts with PI3K (P1, P2 subunits) and IP (phosphoinositides) in the cytoplasmic face. Nter = N-terminus.

Figure: CD133 functional mechanisms — (a) Microvillus control: CD133 senses GM₁ gangliosides and interacts with PI3K (p85/p110) and Arp2/3 complex via Y828 phosphorylation → controls microvillus structure (normal vs branched/knob-like in 2M mutant vs short in Y828F mutant). (b) Primary cilium control: CD133 interacts with Arl13b and HDAC6 via K138 → controls ciliary assembly/length and EV release (K138Q mutant = short cilia). (c) Stem cell–transit amplifying cell axis: CD133 orchestrates ciliary dynamics to control stem cell vs transit amplifying cell balance.

Figure: Prominin-1 membrane microdomain organization — Wild-type CD133 forms normal microvilli with prominin-1-containing membrane microdomains. 2M mutant (increased Y828-P): branched microvilli, increased EV release, Arp2/3 complex and PI3K interactions altered. 2M+Y828F mutant: short microvilli, reduced EV release. Y819/Y828F mutant: short microvilli, terminal structures. Bottom: electron microscopy of MDCK cells expressing wild-type vs mutant human prominin-1.