Podoplanin-positive periarteriolar stromal cells promote megakaryocyte growth and proplatelet formation in mice by CLEC-2

Megakaryopoiesis is a hierarchical differentiation process from hematopoietic stem cells to megakaryocytes, which culminates in platelet production.^1,2  Hematopoietic precursors are committed to megakaryocyte lineages via megakaryocyte-erythroid precursors. Megakaryocyte progenitors (burst forming unit–megakaryocyte, colony forming unit–megakaryocyte [CFU-MK], and megakaryoblasts) are immature cells that can expand to form megakaryocytic clusters, a process referred to as clonal expansion. Megakaryocyte progenitors in turn undergo a remarkable endoreplicative process known as endomitosis, resulting in cytoplasmic extension with polyploidization and abundant protein synthesis.^3,4  Fully matured megakaryocytes can produce numerous platelets through the elaboration of proplatelet formation (PPF).⁵  While differentiating from hematopoietic stem cells, megakaryocytes migrate between 2 distinct microenvironments: the endosteal niche and the vascular niche.^6,7  The endosteal niche provides a proliferative microenvironment for megakaryocyte progenitors. The vascular niche consists of sinusoids and perisinusoidal stromal cells that are present in the maturational microenvironment where platelets are produced and shed into bloodstream.

C-type lectin-like receptor 2 (CLEC-2) is a multifaceted platelet receptor that plays a role in lymph node (LN) homeostasis, maintenance of vasculature integrity, and thrombus formation.^8,9  CLEC-2 is not only highly expressed in platelets and megakaryocytes, but is also present at low levels in other hematopoietic cells, including neutrophils or dendritic cells.¹⁰  The endogenous ligand of CLEC-2, podoplanin (PDPN; also known as Aggrus or gp38), is highly expressed in lymphatic endothelial cells (LECs) and LN fibroblastic reticular cells (FRCs).¹¹  Within the LNs, PDPN on FRCs maintains high endothelial venule integrity by binding to CLEC-2 on extravasated platelets and sphingosine-1-phosphate released from activated platelets.¹²  Alternatively, dendritic cell–derived CLEC-2 modulates LN FRC network tension by binding to FRC PDPN, subsequently increasing dendritic cell motility.^13-15  This reciprocal interaction between dendritic cell CLEC-2 and FRC PDPN contributes to optimal immune responses in LNs. Moreover, platelet CLEC-2 facilitates the separation of blood/lymphatic vessels by binding PDPN on LECs during development,¹⁶  as mice deficient in CLEC-2 or PDPN show embryonic/neonatal lethality with blood-filled lymphatic vessels and severe edema due to blood-lymphatic vessel misconnections.^17-21  Several research groups, including ours, have found that surviving CLEC-2–knockout (KO) neonates or megakaryocyte/platelet-specific CLEC-2 conditional KO (cKO) mice exhibit thrombocytopenia (Figure 1A)^22,23 ; however, the role of CLEC-2 in thrombocytopenia pathogenesis remains to be elucidated.

In the present study, we demonstrated that immature megakaryocytes were reduced in the bone marrow (BM) of CLEC-2 cKO mice. In vitro experiments revealed that recombinant PDPN augmented megakaryocytic clonal expansion via binding to CLEC-2. We also identified PDPN-expressing periarteriolar pericytes in the BM, which were tentatively termed as BM fibroblastic reticular cells (BM FRC-like cells). In the BM, megakaryocytic colonies developed in the vicinity of arterioles by associating with BM FRC-like cells. In coculture of megakaryocytes with BM FRC-like cells, CLEC-2 cKO megakaryocyte expansion was significantly decreased when compared with CLEC-2 wild-type (WT) megakaryocytes, which indicated that BM FRC-like cells augmented megakaryocyte expansion via CLEC-2/PDPN interaction. In addition, CLEC-2 binding to BM FRC-like cells via PDPN induced the secretion of chemokine (C-C motif) ligand 5 (CCL5; regulated on activation normal T expressed and secreted [RANTES]), which potentiates megakaryocyte PPF. We propose a novel periarteriolar megakaryocytic microenvironment that involves reciprocal CLEC-2/PDPN interactions between megakaryocytes and BM FRC-like cells.

Mice were bred and maintained under standard conditions. This study was approved by the animal care and use committee at the University of Yamanashi. Megakaryocyte and platelet-specific CLEC-2–cKO mice (Pf4-Cre, Clec1b^flox/flox) were generated as described.^18,19  C57BL/6 mice were purchased from Japan CLEA (Tokyo, Japan) or Japan Charles River (Kanagawa, Japan).

BM cells were harvested from femurs and tibias of young mice (∼7-12 weeks old). Flushed BM was suspended with cold Iscove modified Dulbecco medium (IMDM; Life Technologies) supplemented with antibiotics and mixed well by pipetting. After passing the cells through a 40-μm strainer (BD Biosciences), megakaryocyte progenitors were separated by Lin⁺ depletion, as previously described.²⁴  Briefly, BM cells were stained with specific antibodies for hematopoietic markers (CD4, CD8, B220, Thy1.2, TER-119, Ly-6G, CD11b, F4/80, anti-neutrophil antibody, and CD71). Stained Lin⁺ cells were depleted using sheep anti-rat immunoglobulin G (IgG) polyclonal antibody (pAb)-conjugated magnetic beads (Dynabeads; Life Technologies). Live cell numbers were counted at least twice using a TC20 automatic cell counter (Bio-Rad, Hercules, CA) with trypan blue staining.

BM stromal cells were separated as previously described.²⁵  Briefly, flushed BM cells were suspended in IMDM containing 10% fetal calf serum (FCS) and antibiotics and seeded into 12-well plates or cell-culture dishes. Culture media was daily changed to remove nonadhesive cells for 5 to 7 days until adhesive BM stromal cells formed a monolayer.

BM progenitors or embryonic day 13.5 fetal liver cells were cultured in IMDM supplemented with 10% FCS, antibiotics, and 50 ng/mL thrombopoietin (TPO) for 5 days to induce megakaryocyte differentiation. At culture day 3, GM6001 (50 μM final concentration; Millipore, Billerica, MA), a broad-range matrix metalloproteinase inhibitor, was added to the culture medium to suppress the shedding of surface proteins from megakaryocytes. For the coculturing experiments, BM progenitors (1.0 × 10⁵ cells per mL) were cultured on a BM stromal cell layer in 12-well plates.

BM megakaryocyte progenitors (2.2 × 10⁶/mL) were applied to MegaCult-C (Stem Cell Technologies, Vancouver, BC, Canada) supplemented with 50 ng/mL TPO, 10 ng/mL interleukin-3 (IL-3), and 20 ng/mL IL-6 in double-chamber slides (Nunc, Rochester, NY). Megakaryocytic colonies (CFU-MK) were detected by acetylcholinesterase staining. Counter staining was performed with Harris’ hematoxylin (Sigma-Aldrich). We defined CFU-MK as colonies tightly contacting at least 3 acetylcholinesterase-positive megakaryocytes according to the manufacturer’s instructions.

Before enucleation of femurs, we perfused 8-week-old mice with 4% paraformaldehyde (PFA). Long bones were then fixed with 4% PFA for 3 hours and decalcified overnight with K-CX solution (FALMA, Tokyo, Japan). After washing with diluted water, long bones were cut into longitudinal or transverse pieces. Bone pieces were subsequently incubated in 30% sucrose for cryoprotection. Treated tissues were embedded in OCT (Tissue-Tek, Sakura, Japan) at −80°C. Frozen sections were prepared as previously described.²⁶  Frozen BM sections sliced at 10 μm were blocked with phosphate-buffered saline (PBS) containing 3% bovine serum albumin and 2% goat serum. Anti-CLEC-2 (2A2B10, 1/200 dilution), anti-CD41 (MWReg30, 1/500 dilution), anti-PDPN (8.1.1, 1/200 dilution) were diluted with blocking reagent and incubated for 1 hour at room temperature. Anti-Syrian hamster Alexa 488 conjugate (for PDPN, 1/1000 dilution) and anti-rat Alexa 546 conjugate (for CD41, 1/2000 dilution) were used as secondary antibodies. 4,6 Diamidino-2-phenylindole (DAPI) was used to counterstain the nucleus. After inclusion with Mounting PermaFluor (Thermo Fisher, Waltham, MA), sections were observed using an inverted fluorescence microscope (IX71; Olympus) or inverted confocal fluorescence microscope (FV1000; Olympus).

Cells were plated and cultured onto φ15-mm growth cover glasses (Thermo) in 12-well culture plates. Cells were fixed with final 4% PFA. For megakaryocytes and stromal cell cocultures, we carefully pipetted drops of fixative into the wells and fixed cells for 1 hour to preserve cell-cell contacts. Cells were then permeabilized with 0.1% Triton X-100 in PBS for 10 minutes if necessary. After washing with PBS, cells were blocked with PBS containing 3% bovine serum albumin and 2% goat serum, and incubated with antibodies of anti-PDPN (8.1.1, 1/200 dilution), anti-CD41 (MWReg30, 1/500 dilution), anti-CLEC-2 (2A2B10, 1/200 dilution), anti-CD45 (30-F11, 1/200 dilution), anti-osteocalcin rabbit pAb (1/1000 dilution), or anti-N-cadherin rabbit pAb (1/400 dilution). The secondary antibodies used were anti-Syrian hamster Alexa 488 conjugate (for PDPN, 1/1000 dilution), anti-rat Alexa 546 conjugate (for CD41, 1/2000 dilution), and anti-rabbit Alexa 546 conjugate (for N-cadherin, 1/2000 dilution). DAPI was used as a nuclear counterstain. After mounting the sections with Mounting PermaFluor (Thermo Fisher), they were observed using an inverted fluorescence microscope (IX71; Olympus). Acquired images were quantitatively analyzed by Image J 1.46r software (http://rsb.info.nih.gov/ij/).

PPF assays were performed as previously described.²⁷  In brief, mouse fetal liver cells were harvested from embryonic day 13.5 embryos and cultured in IMDM supplemented with 10% FCS, antibiotics, and 50 ng/mL TPO. The next day, supernatant hematopoietic cells were transferred to new wells to remove adherent cells. At culture day 4, proplatelet-forming megakaryocytes were counted using an IX71 inverted microscope (Olympus).

In proplatelet formation assay with conditioned medium, conditioned medium was added at culture day 2. To prepare conditioned medium, 20 μg/mL recombinant rFC2 (control protein) or mouse CLEC-2 rFC2 fusion protein was added to BM stromal cell layers containing BM FRC-like cells. After 48 hours, the culture supernatant was centrifuged at 1000g for 5 minutes, and the harvested supernatant preserved at −80°C until analysis.

The concentrations of cytokines and chemokines in conditioned medium were analyzed using the Proteome Profiler Mouse Cytokine Array Panel A (ARY006; R&D Systems).²⁸  Conditioned medium (1 mL) was applied to the array membrane according to the manufacturer’s instructions. We used ECL Prime (GE Healthcare, Little Chalfont, United Kingdom) as a horseradish peroxidase substrate, and chemiluminescence signals were detected with an LAS4000 Mini (GE Healthcare). Quantification analysis was performed using Image J 1.46r software.

All quantitative data are shown as the mean ± standard error of the mean (SEM). Representative data from at least 3 independent experiments are shown in the immunohistochemistry (IHC) and immunocytochemistry (ICC) photographs. Comparisons between 2 groups were analyzed using unpaired the Student t test or Mann-Whitney U test. Comparisons in multiple groups were examined by Tukey 1-way analysis of variance. Independence in a contiguity table was examined by χ² test. Statistical analyses were carried out in GraphPad Prism 5.

To first verify the presence of the megakaryocyte-specific genetic deletion of CLEC-2 in the BM, we assessed its expression in the femur BM of CLEC-2 WT and cKO mice. As expected, histologic analysis demonstrated that CLEC-2 cKO mice were completely devoid of CLEC-2 expression within the BM compartment (supplemental Figure 1, available on the Blood Web site). Moreover, we also measured CLEC-2 expression during megakaryocyte differentiation from the early to the late stages (Figure 1B). CLEC-2 expression was detected from CD41⁺ mononuclear small megakaryocytes to mature large polyploidic megakaryocytes. These findings indicated that megakaryocytes expressed CLEC-2 during the early phase of differentiation.

Interestingly, an analysis of BM megakaryocyte numbers in WT and CLEC-2 cKO mice revealed that CLEC-2 cKO mice exhibited a marked reduction in megakaryocyte-lineage numbers compared with their WT counterparts, particularly in immature megakaryocytes (CD41⁺CD42b⁻; Figure 1C). These observations suggested that CLEC-2 is likely involved in the clonal expansion of megakaryocyte progenitors in the BM; however, we found no differences in the in vitro autonomous clonal expansion of megakaryocyte progenitors between CLEC-2 WT and cKO mice (Figure 1D). We next investigated the effect of recombinant PDPN (PDPN-rFC2) treatment on megakaryocyte progenitor expansion and found that PDPN-rFC2 enhanced the expansion of WT megakaryocytes, but not that of CLEC-2 cKO megakaryocytes (Figure 1E). These data indicated that the CLEC-2/PDPN interaction accelerates megakaryocytic clonal expansion.

Next, we investigated whether PDPN-expressing stromal cells existed within the BM compartment. A histologic analysis revealed the presence of PDPN along vascular structures (Figure 2A-B; supplemental Figure 2). The BM vasculature is typically divided into 3 subtypes: arterioles (CD31⁺Sca-1⁺), sinusoids (CD31⁺Sca-1⁻), and central veins.²⁹  The central vein can be easily identified by its large structure, and it is PDPN⁻. Next, we focused on other BM sinusoids and arterioles. IHC showed that PDPN⁺ cells surrounded the CD31⁺Sca-1⁺ vasculature (Figure 2C-E), indicating that PDPN⁺ stromal cells were analogous to BM arteriole pericytes. Furthermore, we found that mature large megakaryocytes and immature megakaryocytic colonies (CD41⁺ clusters) were adjacent to PDPN⁺ stromal cells in the WT BM (Figure 2F). In contrast, in the CLEC-2 cKO BM, megakaryocytes or immature megakaryocytic colonies were rarely detected in the periarteriolar space. We also calculated the percentage of megakaryocytes and megakaryocytic colonies associated with PDPN⁺ stromal cells at periarteriolar sites (Figure 2G). CD41⁺ nucleated cells and colonies localized within 10 μm from PDPN⁺ stromal cells were defined as megakaryocytes or immature megakaryocytic colonies associated with arterioles. In the BM of WT and CLEC-2 cKO mice, the percentages of megakaryocytes and related colonies localized at the periarteriolar space were 26.09% and 3.00%, respectively (P < .0001, χ² test). These observations suggest that PDPN-expressing stromal cells contribute to megakaryopoiesis within the periarteriolar space.

To further characterize the BM PDPN⁺ stromal cells, we separated and cultured BM adhesive stromal cells and found that they displayed fibroblastic morphology (Figure 3A). In general, vascular pericytes lack PDPN expression. However, FRCs that surround high endothelial venules in LNs are PDPN⁺ fibroblastic stromal cells.^11-14  Because BM arteriolar pericytes, which appear to support megakaryocyte expansion, resemble LN FRCs in terms of morphology and PDPN expression, we compared the expression of several surface antigens between these 2 cell types. Immunophenotyping revealed that the BM PDPN⁺ stromal cells had a surface antigen pattern quite similar to that of LN FRCs (Figure 3B-C; supplemental Figure 3A). In addition, BM PDPN⁺ stromal cells were found to be negative for the osteoblastic lineage markers osteocalcin and N-cadherin (supplemental Figure 3B-C), indicating that PDPN⁺ stromal cells were distinct from other endosteal niche cells, such as spindle-shaped N-cadherin⁺CD45⁻ osteoblastic cells.³⁰  chemokine (C-X-C motif) ligand 12 (CXCL12)-abundant reticular (CAR) cells are widely distributed among sinusoids in the BM.³¹  CAR cells are niche cells that serve not only to maintain hematopoietic cells, but also to regulate erythrocyte and B-cell differentiation.³²  We next investigated whether BM PDPN⁺ stromal cells were identical to CAR cells by evaluating the expression of CD51, a surface marker of CAR cells.³²  However, fluorescence-activated cell sorter (FACS) analysis showed that BM PDPN⁺ stromal cells were CD51⁻ (Figure 3B). From these observations, we tentatively termed these BM PDPN⁺ periarteriolar stromal cells as “BM FRC-like cells” based on their morphology and surface marker expression. To the best of our knowledge, this is the first report to suggest that arteriolar pericytes are PDPN⁺ in BM.

To mimic in vivo conditions where megakaryocyte lineages form expanded colonies at periarteriolar sites that associate with BM FRC-like cells, we cocultured megakaryocyte progenitors on a heterogeneous BM stromal cell layer in the presence of TPO (Figure 4A). The BM stromal cell layer contained ∼10% BM FRC-like cells, and all PDPN⁺ stromal cells were BM FRC-like cells (Figure 3A). After coculturing megakaryocyte progenitors with BM stromal cells, we found that CLEC-2 WT megakaryocyte colonies, but not CLEC-2 cKO megakaryocytes, associated with the BM FRC-like cells (Figure 4B). Quantitative analysis of ICC images revealed that the numbers of CLEC-2 cKO megakaryocytes were significantly reduced as compared with CLEC-2 WT megakaryocytes during coculture with BM stromal cells (Figure 4C-D). These results were consistent with the results of our colony-formation assay (Figure 1E). To confirm that this expansion was dependent on the CLEC-2/PDPN axis, we blocked the CLEC-2/PDPN interaction using a PDPN-neutralizing monoclonal antibody. As expected, abrogation of CELC-2/PDPN binding significantly suppressed the expansion of WT megakaryocyte lineages (Figure 4E-F), but had no effect on CLEC-2 cKO megakaryocytes (Figure 4G-H). It has been known that CLEC-2 is released from activated platelet as a soluble form by shedding.³³  Therefore, we also investigated the possibility that soluble CLEC-2 is involved in a megakaryocyte expansion associated with BM FRC-like cells. We compared the megakaryocyte expansion in coculture with BM FRC-like cells in the presence or absence of GM6001 and GM6001. The GM6001 compound is a broad-range inhibitor of matrix metalloprotease that blocks membrane protein shedding. The GM6001 compound did not affect a megakaryocyte expansion associated with BM FRC-like cells (supplemental Figure 4A), suggesting that shed soluble CLEC-2 was not involved in the expansion. These results suggested that BM FRC-like cells stimulate megakaryocyte expansion via interactions between PDPN and CLEC-2 expressed on the respective cell surfaces.

To investigate the mechanism by which the CLEC-2/PDPN interaction promotes megakaryocyte expansion, we analyzed the phosphorylation of signaling molecules in megakaryocytes stimulated with PDPN-rFC2. Moroi and Watson³⁴  and Manne et al³⁵  reported that CLEC-2 activation induced Akt, p44/42 (extracellular signal-regulated kinase 1/2), and p38 phosphorylation in platelets. Thus, we analyzed the phosphorylation levels of these signaling molecules in PDPN-rFC2–stimulated megakaryocytes. In the stimulated megakaryocytes, Akt phosphorylation levels significantly increased (Figure 5A), whereas those of p44/42 and p38 were not significantly altered (Figure 5B-C). These results suggested that PDPN binding to CLEC-2 positively regulates megakaryocyte expansion through the Akt pathway.

With regard to megakaryocyte maturation, we also found that a number of CLEC-2 WT megakaryocytes displayed proplatelet-like morphologies, with enlarged cytoplasm and a protrusion of pseudopods observed in cocultures with BM FRC-like cells; however, the proplatelet formation of CLEC-2 cKO megakaryocytes was reduced (Figure 6A). We therefore investigated the effect of CLEC-2 for PPF using fetal liver PPF assays.²⁷  Notably, when megakaryocytes were cultured in the absence of BM FRC-like cells, the ratio of PPF in WT megakaryocytes to that in CLEC-2 cKO megakaryocytes was not significantly different (Figure 6B), suggesting that CLEC-2 deletion did not affect autonomous PPF. Moreover, the addition of PDPN-rFC2 into the culture medium also failed to promote PPF in either the CLEC-2 WT or cKO megakaryocytes (Figure 6C), suggesting that the CLEC-2/PDPN interaction does not directly facilitate PPF. We therefore hypothesized that the CLEC-2/PDPN axis may stimulate BM FRC-like cells to secrete unknown factors that facilitate PPF. To test this hypothesis, we used conditioned medium in our fetal liver PPF assays (Figure 6D) and found that CLEC-2–rFC2-conditioned medium significantly enhanced PPF when compared with rFC2-conditioned control medium (Figure 6E). This finding suggests that CLEC-2–rFC2-stimulated BM FRC-like cells secrete 1 or more PPF-promoting factors.

To identify PPF-promoting factors, we performed cytokine/chemokine arrays. Among the 40 proteins analyzed, the levels of CXCL10 (interferon-γ–inducible protein 10 [IP-10]), CXCL2 (macrophage inflammatory protein-2 [MIP-2]), and CCL5 (RANTES) increased more than fivefold in CLEC-2–rFC2-conditioned medium, compared with rFC2-control medium (Figure 7A-B; supplemental Figure 5). To determine the effect of these factors on PPF, we tested the recombinant candidate chemokines individually in fetal liver PPF assays, which revealed that only CCL5 promoted a significant increase in PPF (Figure 7C). Moreover, CCL5 neutralizing antibody markedly suppressed this effect in cultures with CLEC-2–rFC2-conditioned medium (Figure 7D). However, CCL5 did not enhance megakaryocyte colony expansion (supplemental Figure 6), suggesting that CCL5 specifically promotes megakaryocyte differentiation. These data suggested that the binding of megakaryocyte CLEC-2 to PDPN present on BM FRC-like cells results in CCL5 secretion from BM FRC-like cells, which then potentiates PPF in megakaryocytes.

In this study, we demonstrate that (1) BM FRC-like cells are periarteriolar stromal cells that express PDPN in the BM, (2) PDPN binding to CLEC-2 positively regulates megakaryocyte expansion, and (3) CLEC-2/PDPN binding stimulates BM FRC-like cells to secrete CCL5 to promote PPF in megakaryocytes. The CLEC-2/PDPN axis between megakaryocytes and BM FRC-like cells constitutes a reciprocal interaction that generates the megakaryopoietic microenvironment at periarteriolar sites in the BM (supplemental Figure 7). These data support our hypothesis that BM FRC-like cells provide a CLEC-2/PDPN-dependent niche that potentiates megakaryocyte expansion and CCL5-mediated PPF.

In megakaryopoiesis, phosphatidylinositol 3-kinase/Akt signaling promotes the proliferation of megakaryocyte progenitors,^36,37  and p44/42 signaling regulates megakaryocytic differentiation.³⁸  Interestingly, p38 does not appear to be involved in megakaryopoietic processes.³⁹  In this study, we found that PDPN binding to CLEC-2 increases Akt phosphorylation in megakaryocytes (Figure 5). Because the PDPN interaction with CLEC-2 in megakaryocytes led to megakaryocyte proliferation, the Akt pathway appears to be related to megakaryocytic expansion. Next, we investigated the role of CCL5/chemokine (C-C motif) receptor 5 (CCR5)^40,41 -related signaling in megakaryocyte differentiation. We evaluated the phosphorylation levels signaling molecules downstream of CCR5,⁴²  including myosin light chain, cofilin, and p44/42. However, no significant alterations in the phosphorylation levels of these molecules were observed (supplemental Figure 8). Thus, there may be an unknown signaling pathway(s) downstream of CCL5/CCR5 involved in megakaryocyte maturation.

This study also represents the first report that PDPN signaling associated with CLEC-2 binding induces chemokine secretion (Figures 5-6). Although it has been reported previously that CLEC-2 binding to PDPN induces morphologic changes in LN FRCs by modulating actomyosin contractility,^13,14  it has not been demonstrated that PDPN signaling associated with CLEC-2 binding induces soluble factor secretion. In LECs, the binding of CLEC-2 to PDPN induces PDPN clustering, which elicits downstream signaling to modulate ezrin/radixin/moesin proteins.⁴³  By analogy, we assume that megakaryocyte CLEC-2–mediated clustering of PDPN on BM FRC-like cells elicits intracellular signaling in BM FRC-like cells, which results in CCL5 secretion.

It is well documented and widely accepted that megakaryocyte progenitor expansion mainly occurs in proliferative endosteal niches and it is assumed that the expanded megakaryocyte population migrates toward the sinusoidal vasculatures (vascular niches) following a CXCL12 (stromal cell-derived factor 1) chemoattractant gradient secreted from sinusoidal endothelial cells.^6,44  At vascular niches, megakaryocytes produce platelets and release them into the blood circulation.^44,45  Our present observations raise the possibility that BM arterioles partly contribute to the formation of both proliferative and maturational microenvironments for megakaryopoiesis (supplemental Figure 9), which may not require megakaryocyte migration toward different niches. The BM matrix is densely packed with sinusoids, many of which appear to be juxtaposed to arterioles.⁴⁶  We observed that mature megakaryocytes interacted with both PDPN⁺ arterioles and PDPN⁻ sinusoids (Figure 2F). Thus, although the migration of mature megakaryocytes to sinusoids may indeed be necessary, it is possible that the close proximity of arterioles and sinusoids allows megakaryocytes to remain stationary during cell expansion, maturation, and proplatelet release. However, this hypothesis should be further evaluated. To completely demonstrate a role for the CLEC-2/PDPN-dependent megakaryopoietic niche in the BM, it will be necessary to generate cKO mice podoplanin deleted specifically in BM FRC-like cells, which is currently under investigation in our laboratory.

In summary, we propose a novel BM periarteriolar megakaryocytic niche that provides a proliferative and maturational microenvironment for megakaryocytes. A reciprocal CLEC-2/PDPN interaction between megakaryocytes and BM FRC-like cells accelerates megakaryopoiesis, resulting from the direct promotion of megakaryocytic clonal expansion and indirect facilitation of megakaryocyte PPF. This concept contributes to the understanding of a complex regulatory process for platelet production in the BM.

The authors are grateful to Dr Shinji Kunishima for his technical advice on performing the PPF assays. They are also grateful to Hisaichiro Nakazawa, Uina Fukuda, and Junko Nakagomi for their technical assistance.

This work was supported in part by a Grant-in-Aid for Japan Society for the Promotion of Science fellows (25⋅4623), Young Scientist (B) (26860723), and the Funding Program for Next Generation World-leading Researchers (NEXT Program) (LS052). This work was also supported in part by grants from the SENSHIN Medical Research Foundation.

Contribution: S.T. performed the majority of experiments, designed the study, analyzed data, and wrote the manuscript; K.S.-I. designed the study, analyzed data, wrote the manuscript, and supervised the study; N.T., T. Shirai, and T. Sasaki performed the experiments and statistical analysis; M.O. and K.S. analyzed the data; Y.O. designed the study, analyzed data, wrote the manuscript, and supervised the study; and all authors discussed the results and contributed to manuscript preparation.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

The current affiliation for S.T. is Department of Pathophysiological Laboratory Sciences, Nagoya University Graduate School of Medicine, Nagoya, Japan.

Correspondence: Katsue Suzuki-Inoue, Department of Clinical and Laboratory Medicine, Faculty of Medicine, University of Yamanashi, 1110 Shimokato, Chuo, Yamanashi 409-3898, Japan; e-mail: katsuei@yamanashi.ac.jp.