Osteoblast — the bone-building niche cell whose expansion suppresses CML and makes MLL-AF9 AML worse
also called osteoblastic cell · SNO cell (spindle-shaped N-cadherin+ CD45- osteoblastic cell) · osteolineage cell · endosteal niche cell
Osteoblasts are the cells that build bone, and they line the inner surface of the marrow cavity where some blood stem cells sit. They were the first cells proposed to form the stem-cell niche — and, awkwardly for anyone hoping to treat leukaemia by manipulating them, increasing their number helps one kind of leukaemia and hurts another.
The same thing, for a scientist
Osteoblasts line the endosteum and were the first proposed HSC niche: osteoblast-specific activation of the PTH/PTHrP receptor increases osteoblast number, jagged-1 and HSC number with in vivo Notch1 activation, and PTH injection improves post-transplant survival [E1]; independently, BMPRIA-dependent expansion of spindle-shaped N-cadherin-positive CD45-negative (SNO) osteoblastic cells increases HSC number, with long-term HSCs attached to SNO cells and N-cadherin and beta-catenin asymmetrically localised at the contact [E2]. Conditional deletion later showed osteoblast-derived SCF is **dispensable** for HSC maintenance [E3], demoting the osteoblast from required niche cell to modulatory one. The decisive result for this graph is comparative and inconvenient: the same osteoblastic PTH-receptor activation **attenuated BCR-ABL1-driven CML-like disease (15-fold LSC reduction) but ENHANCED MLL-AF9-driven AML**, plausibly through opposing effects of increased niche TGF-beta1 on the two leukaemia stem cells [E4]. And in MLL::AF9 AML the CXCL12 that maintains leukaemia stem cells comes from Prx1+ mesenchymal cells and **not from mature osteolineage cells** [E5]. Both KMT2A-relevant results say the osteoblast is not the shelter.
Why an entry, when the cell-identity ledger rejected osteoblast
ana-cell-identity-rejected-candidates rejected osteoblast as a cell-identity candidate, correctly: the osteoblast x KMT2A intersection is 5 records and none describes a leukaemia of osteoblast lineage. That is the right disposition for the question that ledger was asking, which was 'which cell does this leukaemia come from'.
This entry asks a different question — 'which cells form the place the leukaemia lives' — and for that question three of those five records are directly load-bearing 4, 5 and the sympathetic-neuropathy paper's osteoblast-primed MSPC expansion. A cell can be irrelevant as a cell of origin and central as a niche component. The two dispositions are not in conflict, and this entry records the distinction so the next reader does not treat the earlier rejection as covering this ground.
The comparative result, and what it forbids
Krause and colleagues altered the marrow microenvironment by osteoblast-specific PTH-receptor activation and ran two leukaemias through it. In BCR-ABL1 CML-like disease the manipulation attenuated the disease, with a 15-fold decrease in LSCs in wild-type mice and reduced engraftment of immunodeficient mice by primary human CML cells. In MLL-AF9 AML the same manipulation enhanced the disease. The proposed explanation is opposing effects of increased microenvironmental TGF-beta1 on the two LSC types 4.
This forbids a specific and otherwise attractive line of thinking. PTH is a licensed drug; 'expand the osteoblastic niche to suppress leukaemia stem cells' is a real strategy with real clinical support in CML. In the genotype this graph exists for it is predicted to do harm. Any niche-directed proposal in this project has to be genotype-specific, and this is the cleanest demonstration of why.
The second KMT2A-relevant result points the same way from the other side. Viñado and colleagues asked which stromal cells supply the CXCL12 that maintains MLL::AF9 leukaemia stem cells and found it is Prx1+ mesenchymal cells, not mature osteolineage cells 5. The osteoblast is neither the source of the maintenance signal nor a safe lever to pull.
Menin in osteoblasts, which is a separate story already in the graph
This graph already holds mechanism-menin-has-opposite-jobs-at-two-stages-of-the-osteoblast and risk-menin-loss-in-osteoblast-lineage-lowers-bone-mass, which concern menin's cell-intrinsic role in osteoblast differentiation and the skeletal toxicity risk of menin inhibition in a growing child. Those are about what menin does inside the osteoblast. This entry is about what the osteoblast does to a haematopoietic cell sitting next to it. They should be read together — a menin inhibitor given to an infant acts on both — but they are different things and are deliberately kept as different entries.
| ontology | CL:0000062 (osteoblast) |
|---|---|
| location | endosteum, lining the inner bone surface of the marrow cavity |
| upstream progenitor | cell-type-car-cell (CAR/LEPR+ adipo-osteogenic progenitor) |
markers
[ "RUNX2", "SP7/Osterix", "COL1A1", "ALPL", "N-cadherin (SNO subset)", "CD45 negative" ]
secreted factors
[ "jagged-1 (NOTCH ligand)", "osteopontin/SPP1", "ANGPT1", "CXCL12 (dispensable for MLL::AF9 LSC maintenance)", "TGF-beta1 (released during bone remodelling)" ]
kmt2a relevance
{
"pth_experiment": "osteoblast-specific PTH-receptor activation attenuates BCR-ABL1 CML (15-fold LSC reduction) but ENHANCES MLL-AF9 AML",
"cxcl12_source": "mature osteolineage CXCL12 is not required for MLL::AF9 LSC maintenance; Prx1+ mesenchymal CXCL12 is",
"remodelling": "MLL-AF9 AML expands osteoblast-primed MSPCs at the expense of NG2+ periarteriolar cells",
"cell_identity_ledger_note": "osteoblast x KMT2A intersection is 5 PubMed records; correctly rejected as a cell-of-origin candidate in ana-cell-identity-rejected-candidates, and retained here as a niche component, which is a different question"
}related menin entries
[ "mechanism-menin-has-opposite-jobs-at-two-stages-of-the-osteoblast", "risk-menin-loss-in-osteoblast-lineage-lowers-bone-mass" ]
atlas refs
[
{
"label": "Osteoblastic regulation of the HSC niche via PTH receptor and jagged-1",
"pmid": "14574413",
"title": "Osteoblastic cells regulate the haematopoietic stem cell niche.",
"authors": "Calvi LM, Adams GB, Weibrecht KW, et al.",
"venue": "Nature",
"year": 2003,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
},
{
"label": "Osteoblastic niche expansion suppresses CML but enhances MLL-AF9 AML",
"pmid": "24162813",
"title": "Differential regulation of myeloid leukemias by the bone marrow microenvironment.",
"authors": "Krause DS, Fulzele K, Catic A, et al.",
"venue": "Nat Med",
"year": 2013,
"title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
}
]- PTH and other bone-anabolic agents would be predicted to WORSEN KMT2A-rearranged myeloid leukaemia while improving CML, so a niche-directed strategy borrowed from the CML literature would be actively harmful here.
- Menin inhibitors already carry an osteoblast-lineage liability in a growing child (`risk-menin-loss-in-osteoblast-lineage-lowers-bone-mass`); adding a second osteolineage-directed agent compounds a skeletal risk in a patient who has decades of growth ahead.
- The endosteal compartment is where leukaemia-initiating cells were imaged homing and dividing symmetrically, so disrupting it could scatter rather than eliminate the initiating population.
- Why does osteoblastic expansion suppress CML and enhance MLL-AF9 AML? The TGF-beta1 explanation is a hypothesis in the source paper and has not been tested by blocking TGF-beta1 in the AML arm.
- Does the same divergence hold for KMT2A-rearranged ALL, which is the disease this graph is actually about, or is it a property of myeloid leukaemia stem cells?
- Infants are in rapid skeletal growth with a far more active osteoblast compartment than an adult mouse. Does a growing skeleton constitute a systematically more MLL-AF9-permissive niche, and could that contribute to the age distribution of KMT2A-rearranged disease?
- A menin inhibitor acts on the osteoblast as well as on the blast. Does menin inhibition change the osteoblastic niche in a way that alters leukaemia behaviour, independently of its effect inside the leukaemic cell?
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Osteoblastic cells are a regulatory component of the HSC niche in vivo: osteoblast-specific expression of an activated PTH/PTHrP receptor increases osteoblast number and jagged-1 and supports increased HSC number with in vivo Notch1 activation; PTH increases osteoblasts in stromal culture and augments primitive haematopoietic cell growth in a manner abrogated by gamma-secretase inhibition; PTH injection into wild-type mice increases stem-cell number and markedly improves survival after bone marrow transplantation.
Osteoblastic cells regulate the haematopoietic stem cell niche · Calvi LM, Adams GB, Weibrecht KW, Weber JM, Olson DP, Knight MC, et al. · Nature 2003What this evidence cannot show
Mouse gain-of-function genetics plus pharmacology; establishes sufficiency, not necessity. PTH has systemic effects on calcium and on many tissues, so the in vivo arm is not osteoblast-specific. Read: abstract only.
Evidence that argues against this (1)
- Conditional deletion showed that SCF from osteoblasts is dispensable for HSC maintenance while SCF from endothelium and LEPR+ perivascular cells is required, so osteoblasts can modulate the niche without being a necessary part of it. Read: abstract only. Ding L et al. 2012
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Conditional inactivation of BMP receptor type IA increases the number of spindle-shaped N-cadherin-positive CD45-negative (SNO) osteoblastic cells, correlating with an increase in HSC number; long-term HSCs are found attached to SNO cells, with N-cadherin and beta-catenin asymmetrically localised between the SNO cell and the HSC, indicating that bone-lining SNO cells are a key niche component and that BMP signalling controls HSC number by regulating niche size.
Identification of the haematopoietic stem cell niche and control of the niche size · Zhang J, Niu C, Ye L, Huang H, He X, Tong WG, et al. · Nature 2003What this evidence cannot show
Mouse conditional genetics; the SNO cell has proven hard to identify prospectively and the requirement for HSC N-cadherin was subsequently disputed by several groups using N-cadherin-deletion and reporter approaches. The HSC-SNO attachment is a histological observation. Read: abstract only.
Evidence that argues against this (1)
- The functional centre of gravity of the niche moved away from the endosteum within a decade: conditional deletion of Scf from osteoblasts left HSCs unaffected, while deletion from endothelium or LEPR+ perivascular cells depleted them. The osteoblastic-niche literature of 2003 has not been retracted but it no longer describes the required cell. Read: abstract only. Ding L et al. 2012
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
Conditional deletion of Scf from osteoblasts does not affect HSC frequency or function, whereas deletion from endothelial cells or leptin-receptor-expressing perivascular stromal cells depletes HSCs.
Endothelial and perivascular cells maintain haematopoietic stem cells · Ding L, Saunders TL, Enikolopov G, Morrison SJ · Nature 2012What this evidence cannot show
A negative result for one factor in one cre line. Osteoblasts produce other candidate niche factors (jagged-1, osteopontin, ANGPT1) that this experiment does not test. Read: abstract only.
Evidence that argues against this (1)
- Osteoblastic manipulation demonstrably changes leukaemia outcomes in vivo in both directions, so the compartment is functionally live even where SCF deletion is silent: PTH-receptor activation in osteoblasts reduced CML LSCs 15-fold and enhanced MLL-AF9 AML. A dispensable SCF source is not a dispensable cell. Read: abstract only. Krause DS et al. 2013
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
Alteration of the bone-marrow microenvironment by osteoblastic-cell-specific activation of the parathyroid hormone receptor attenuates BCR-ABL1-induced CML-like myeloproliferative neoplasia but ENHANCES MLL-AF9-induced AML in mouse transplantation models, possibly through opposing effects of increased TGF-beta1 on the respective leukaemia stem cells; PTH treatment caused a 15-fold decrease in LSCs in wild-type mice with CML-like disease and reduced engraftment of immunodeficient mice with primary human CML cells.
Differential regulation of myeloid leukemias by the bone marrow microenvironment · Krause DS, Fulzele K, Catic A, Sun CC, Dombkowski D, Hurley MP, et al. · Nature Medicine 2013What this evidence cannot show
Mouse retroviral transplantation models of both leukaemias; MLL-AF9 is myeloid and murine and is not infant KMT2A::AFF1 ALL. The TGF-beta1 explanation is offered as a possibility by the authors, not demonstrated. The quantified 15-fold effect is on the CML side; the magnitude of the AML enhancement is not given in the abstract. Read: abstract only.
Evidence that argues against this (1)
- Osteoblastic expansion is not the only way the osteolineage compartment moves in this disease, and the direction may differ by context: in MLL-AF9 AML the mesenchymal compartment expands towards osteoblast-primed progenitors spontaneously, as part of leukaemia-induced remodelling, which would already be supplying whatever the PTH manipulation adds. Whether PTH enhances AML by amplifying a change the leukaemia makes anyway, or by a separate route, is not resolved. Read: abstract only. Hanoun M et al. 2014
- E5strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In an MLL::AF9-induced AML model, CXCL12 produced by Prx1-positive mesenchymal cells but NOT by mature osteolineage cells provides the cues required for maintenance of leukaemia stem cells in the marrow.
The bone marrow niche regulates redox and energy balance in MLL::AF9 leukemia stem cells · Vinado AC, Calvo IA, Cenzano I, Olaverri D, Cocera M, San Martin-Uriz P, et al. · Leukemia 2022What this evidence cannot show
Mouse; the negative osteolineage result depends on the specificity of the osteolineage cre used, which the abstract does not name. Read: abstract only. **Resolved, and it withdraws the result.** The driver is **Prx1-Cre**, not an osteoblast-restricted Cre: GSE189928 carries `genotype: Prx1-Cxcl12-mutant` and `Prx1-Cxcl12-wild type` across 18 of 18 Mus musculus samples, bone marrow, leukaemia stem cells, C57BL/6JRj. Prx1 is a limb-bud mesenchymal driver whose domain includes but is far wider than osteoblasts, and the leukaemia is murine MLL::AF9. **This negative result therefore cannot be read as an osteoblast negative at all.** Verified by the coordinator against the GEO SOFT records. Resolved 2026-09-19 by the Q18b unstated-systems sweep from the study's deposition, not from its text; bounded as a statement about the deposited data rather than about the whole study.
Evidence that argues against this (2)
- Osteolineage cells being a dispensable CXCL12 source does not make the endosteal compartment irrelevant to this leukaemia. Metabolic imaging in the same MLL-AF9 driver reports that the most leukemogenic glycolytic leukaemia-initiating cells preferentially home to and divide symmetrically in an endosteal niche, which is the osteoblast's territory. The compartment may matter through something other than CXCL12. Read: abstract only. Hao X et al. 2019
- The entry asked which osteolineage Cre was used, expecting the answer to be decisive, and it was - in the direction that withdraws the finding. A Prx1-Cre deletion failing to change leukaemia says nothing specific about osteoblasts, because the deletion is not osteoblast-specific; it also removes the gene from chondrocytes and the wider limb-bud mesenchyme, so a null result is compatible with a real osteoblast effect masked or compensated elsewhere in that domain. NCBI Gene Expression Omnibus 2026
endosteal compartment; first-proposed niche cell
osteoblastic PTH-receptor activation ENHANCES MLL-AF9 AML while attenuating BCR-ABL1 CML
the comparator arm: the same osteoblastic manipulation reduced CML LSCs 15-fold
CAR/LEPR+ cells are the adipo-osteogenic progenitor of marrow osteoblasts
menin acts cell-intrinsically in this cell; this entry covers its niche function instead
principal cellular constituent
UniProt: specifically expressed in osteoblasts; master regulator of osteoblast maturation and ossification
- ← expressed inCXCL12 (SDF-1) - the marrow retention chemokine this graph already had a mechanism entry for and no entry for the protein
osteoblast CXCL12 maintains early lymphoid progenitors - the compartment that matters for a B-lineage disease
classical osteoblastic/endosteal source, untested in KMT2A-rearranged disease
the unnamed driver is Prx1-Cre, a limb-bud mesenchymal driver, so the negative is not an osteoblast negative