Bone marrow laminins influence hematopoietic stem and progenitor cell cycling and homing to the bone marrow

A bispecific nanosystem activates endogenous natural killer cells in the bone marrow for haematologic malignancies therapy

Haematologic malignancies commonly arise from the bone marrow lesion, yet there are currently no effective targeted therapies against tumour cells in this location. Here we constructed a bone-marrow-targeting nanosystem, CSF@E-Hn, which is based on haematopoietic-stem-cell-derived nanovesicles adorned with gripper ligands (aPD-L1 and aNKG2D) and encapsulated with colony-stimulating factor (CSF) for the treatment of haematologic malignancies. CSF@E-Hn targets the bone marrow and, thanks to the gripper ligands, pulls together tumour cells and natural killer cells, activating the latter for specific tumour cell targeting and elimination. The therapeutic efficacy was validated in mice bearing acute myeloid leukaemia and multiple myeloma. The comprehensive assessment of the post-treatment bone marrow microenvironment revealed that the integration of CSF into a bone-marrow-targeted nanosystem promoted haematopoietic stem cell differentiation, boosted memory T cell generation and maintained bone homoeostasis, with long-term prevention of relapse. Our nanosystem represents a promising strategy for the treatment of haematologic malignancies.

Involvement of BCR::ABL1 in laminin adhesion of Philadelphia chromosome-positive acute lymphoblastic leukemia through upregulation of integrin α6

BACKGROUND

Adhesion of cancer cells to extracellular matrix laminin through the integrin superfamily reportedly induces drug resistance. Heterodimers of integrin α6 (CD49f) with integrin β1 (CD29) or β4 (CD104) are major functional receptors for laminin. Higher CD49f expression is reportedly associated with a poorer response to induction therapy in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Moreover, a xenograft mouse model transplanted with primary BCP-ALL cells revealed that neutralized antibody against CD49f improved survival after chemotherapy.

AIMS

Considering the poor outcomes in Philadelphia chromosome (Ph)-positive ALL treated with conventional chemotherapy without tyrosine kinase inhibitors, we sought to investigate an involvement of the laminin adhesion.

METHODS AND RESULTS

Ph-positive ALL cell lines expressed the highest levels of CD49f among the BCP-ALL cell lines with representative translocations, while CD29 and CD104 were ubiquitously expressed in BCP-ALL cell lines. The association of Ph-positive ALL with high levels of CD49f gene expression was also confirmed in two databases of childhood ALL cohorts. Ph-positive ALL cell lines attached to laminin and their laminin-binding properties were disrupted by blocking antibodies against CD49f and CD29 but not CD104. The cell surface expression of CD49f, but not CD29 and CD104, was downregulated by imatinib treatment in Ph-positive ALL cell lines, but not in their T315I-acquired sublines. Consistently, the laminin-binding properties were disrupted by the imatinib pre-treatment in the Ph-positive ALL cell line, but not in its T315I-acquired subline.

CONCLUSION

BCR::ABL1 plays an essential role in the laminin adhesion of Ph-positive ALL cells through upregulation of CD49f.

Microenvironment in acute myeloid leukemia: focus on senescence mechanisms, therapeutic interactions, and future directions

Acute myeloid leukemia (AML) is a disease with a dismal prognosis, mainly affecting the elderly. In recent years, new drugs have improved life expectancy and quality of life, and a better understanding of the genetic-molecular nature of the disease has shed light on previously unknown aspects of leukemogenesis. In parallel, increasing attention has been attracted to the complex interactions between cells and soluble factors in the bone marrow (BM) environment, collectively known as the microenvironment. In this review, we discuss the central role of the microenvironment in physiologic and pathologic hematopoiesis and the mechanisms of senescence, considered a fundamental protective mechanism against the proliferation of damaged and pretumoral cells. The microenvironment also represents a fertile ground for the development of myeloid malignancies, and the leukemic niche significantly interacts with drugs commonly used in AML treatment. Finally, we focus on the role of the microenvironment in the engraftment and complications of allogeneic hematopoietic stem cell transplantation, the only curative option in a conspicuous proportion of patients.

Skin mesenchymal niches maintain and protect AML-initiating stem cells

Leukemia cutis or leukemic cell infiltration in skin is one of the common extramedullary manifestations of acute myeloid leukemia (AML) and signifies a poorer prognosis. However, its pathogenesis and maintenance remain understudied. Here, we report massive AML cell infiltration in the skin in a transplantation-induced MLL-AF9 AML mouse model. These AML cells could regenerate AML after transplantation. Prospective niche characterization revealed that skin harbored mesenchymal progenitor cells (MPCs) with a similar phenotype as BM mesenchymal stem cells. These skin MPCs protected AML-initiating stem cells (LSCs) from chemotherapy in vitro partially via mitochondrial transfer. Furthermore, Lama4 deletion in skin MPCs promoted AML LSC proliferation and chemoresistance. Importantly, more chemoresistant AML LSCs appeared to be retained in Lama4-/- mouse skin after cytarabine treatment. Our study reveals the characteristics and previously unrecognized roles of skin mesenchymal niches in maintaining and protecting AML LSCs during chemotherapy, meriting future exploration of their impact on AML relapse.

Decellularized Extracellular Matrix for Remodeling Bioengineering Organoid's Microenvironment

Over the past decade, stem cell- and tumor-derived organoids are the most promising models in developmental biology and disease modeling, respectively. The matrix is one of three main elements in the construction of an organoid and the most important module of its extracellular microenvironment. However, the source of the currently available commercial matrix, Matrigel, limits the application of organoids in clinical medicine. It is worth investigating whether the original decellularized extracellular matrix (dECM) can be exploited as the matrix of organoids and improving organoid construction are very important. In this review, tissue decellularization protocols and the characteristics of decellularization methods, the mechanical support and biological cues of extraccellular matrix (ECM), methods for construction of multifunctional dECM and responsive dECM hydrogel, and the potential applications of functional dECM are summarized. In addition, some expectations are provided for dECM as the matrix of organoids in clinical applications.

Mesenchymal stromal cell-associated migrasomes: a new source of chemoattractant for cells of hematopoietic origin

BACKGROUND

Multipotent mesenchymal stromal cells (MSCs) are precursors of various cell types. Through soluble factors, direct cell-cell interactions and other intercellular communication mechanisms such as extracellular vesicles and tunneling nanotubes, MSCs support tissue homeostasis. In the bone marrow microenvironment, they promote hematopoiesis. The interaction between MSCs and cancer cells enhances the cancer and metastatic potential. Here, we have demonstrated that plastic-adherent MSCs isolated from human bone marrow generate migrasomes, a newly discovered organelle playing a role in intercellular communication.

RESULTS

Migrasomes are forming a network with retraction fibers behind the migrating MSCs or surrounding them after membrane retraction. The MSC markers, CD44, CD73, CD90, CD105 and CD166 are present on the migrasome network, the latter being specific to migrasomes. Some migrasomes harbor the late endosomal GTPase Rab7 and exosomal marker CD63 indicating the presence of multivesicular bodies. Stromal cell-derived factor 1 (SDF-1) was detected in migrasomes, suggesting that they play a chemoattractant role. Co-cultures with KG-1a leukemic cells or primary CD34⁺ hematopoietic progenitors revealed that MSC-associated migrasomes attracted them, a process intercepted by the addition of AMD3100, a specific CXCR4 receptor inhibitor, or recombinant SDF-1. An antibody directed against CD166 reduced the association of hematopoietic cells and MSC-associated migrasomes. In contrast to primary CD34⁺ progenitors, leukemic cells can take up migrasomes.

CONCLUSION

Overall, we described a novel mechanism used by MSCs to communicate with cells of hematopoietic origin and further studies are needed to decipher all biological aspects of migrasomes in the healthy and transformed bone marrow microenvironment. Video Abstract.

Living Biomaterials to Engineer Hematopoietic Stem Cell Niches

Living biointerfaces are a new class of biomaterials combining living cells and polymeric matrices that can act as biologically active and instructive materials that host and provide signals to surrounding cells. Here, living biomaterials based on Lactococcus lactis to control hematopoietic stem cells in 2D surfaces and 3D hydrogels are introduced. L. lactis is modified to express C-X-C motif chemokine ligand 12 (CXCL12), thrombopoietin (TPO), vascular cell adhesion protein 1 (VCAM1), and the 7th-10th type III domains of human plasma fibronectin (FN III_7-10 ), in an attempt to mimic ex vivo the conditions of the human bone marrow. These results suggest that living biomaterials that incorporate bacteria expressing recombinant CXCL12, TPO, VCAM1, and FN in both 2D systems direct hematopoietic stem and progenitor cells (HSPCs)-bacteria interaction, and in 3D using hydrogels functionalized with full-length human plasma fibronectin allow for a notable expansion of the CD34⁺ /CD38^- /CD90⁺ HSPC population compared to the initial population. These results provide a strong evidence based on data that suggest the possibility of using living materials based on genetically engineered bacteria for the ex-vivo expansion of HSPC with eventual practical clinical applications in HSPCs transplantation for hematological disorders.

Integrins, anchors and signal transducers of hematopoietic stem cells during development and in adulthood

Hematopoietic stem cells (HSCs), the apex of the hierarchically organized blood cell production system, are generated in the yolk sac, aorta-gonad-mesonephros region and placenta of the developing embryo. To maintain life-long hematopoiesis, HSCs emigrate from their site of origin and seed in distinct microenvironments, called niches, of fetal liver and bone marrow where they receive supportive signals for self-renewal, expansion and production of hematopoietic progenitor cells (HPCs), which in turn orchestrate the production of the hematopoietic effector cells. The interactions of hematopoietic stem and progenitor cells (HSPCs) with niche components are to a large part mediated by the integrin superfamily of adhesion molecules. Here, we summarize the current knowledge regarding the functional properties of integrins and their activators, Talin-1 and Kindlin-3, for HSPC generation, function and fate decisions during development and in adulthood. In addition, we discuss integrin-mediated mechanosensing for HSC-niche interactions, ex vivo protocols aimed at expanding HSCs for therapeutic use, and recent approaches targeting the integrin-mediated adhesion in leukemia-inducing HSCs in their protecting, malignant niches.

Critical role of Lama4 for hematopoiesis regeneration and acute myeloid leukemia progression

Impairment of normal hematopoiesis and leukemia progression are 2 well-linked processes during leukemia development and are controlled by the bone marrow (BM) niche. Extracellular matrix proteins, including laminin, are important BM niche components. However, their role in hematopoiesis regeneration and leukemia is unknown. Laminin α4 (Lama4), a major receptor-binding chain of several laminins, is altered in BM niches in mice with acute myeloid leukemia (AML). So far, the impact of Lama4 on leukemia progression remains unknown. We here report that Lama4 deletion in mice resulted in impaired hematopoiesis regeneration following irradiation-induced stress, which is accompanied by altered BM niche composition and inflammation. Importantly, in a transplantation-induced MLL-AF9 AML mouse model, we demonstrate accelerated AML progression and relapse in Lama4-/- mice. Upon AML exposure, Lama4-/- mesenchymal stem cells (MSCs) exhibited dramatic molecular alterations, including upregulation of inflammatory cytokines that favor AML growth. Lama4-/- MSCs displayed increased antioxidant activities and promoted AML stem cell proliferation and chemoresistance to cytarabine, which was accompanied by increased mitochondrial transfer from the MSCs to AML cells and reduced reactive oxygen species in AML cells in vitro. Similarly, we detected lower levels of reactive oxygen species in AML cells from Lama4-/- mice post-cytarabine treatment. Notably, LAMA4 inhibition or knockdown in human MSCs promoted human AML cell proliferation and chemoprotection. Together, our study for the first time demonstrates the critical role of Lama4 in impeding AML progression and chemoresistance. Targeting Lama4 signaling pathways may offer potential new therapeutic options for AML.

The extracellular matrix of hematopoietic stem cell niches

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Comprehensive overview of different classes of ECM molecules in the HSC niche.

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Overview of current knowledge on role of biophysics of the HSC niche.

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Description of approaches to create artificial stem cell niches for several application.

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Importance of considering ECM in drug development and testing.

Hematopoietic stem cells (HSCs) are the life-long source of all types of blood cells. Their function is controlled by their direct microenvironment, the HSC niche in the bone marrow. Although the importance of the extracellular matrix (ECM) in the niche by orchestrating niche architecture and cellular function is widely acknowledged, it is still underexplored. In this review, we provide a comprehensive overview of the ECM in HSC niches. For this purpose, we first briefly outline HSC niche biology and then review the role of the different classes of ECM molecules in the niche one by one and how they are perceived by cells. Matrix remodeling and the emerging importance of biophysics in HSC niche function are discussed. Finally, the application of the current knowledge of ECM in the niche in form of artificial HSC niches for HSC expansion or targeted differentiation as well as drug testing is reviewed.

The Laminin Receptors Basal Cell Adhesion Molecule/Lutheran and Integrin α7β1 on Human Hematopoietic Stem Cells

In the adult organism, hematopoietic stem and progenitor cells (HSPC) reside in the bone marrow (BM) in specialized hematopoietic stem cell niches of which the extracellular matrix (ECM) is an integral component. Laminins (LM) are a family of heterotrimeric ECM molecules of which mainly family members containing an α4 or α5 chain are expressed in cells from BM niches and involved in HSPC homing and proliferation. Various integrin and non-integrin laminin receptors have been identified and characterized. Among these, the integrins α6β1 and α3β1 were reported to be strongly expressed on human and mouse HSPC. In the present study, we focus on two further specific laminin receptors, namely integrin α7β1 and basal cell adhesion molecule/Lutheran (BCAM/Lu). Using RT-PCR analyses, immunofluorescence staining, immunoblotting and flow cytometry, we show that both are strongly expressed by human lineage-negative CD34 + HSPC. Treatment with function-blocking antibodies against BCAM/Lu neither inhibits the strong adhesive interaction of CD34 + HSPC with LM-511/LM-521 nor the LM-511/LM-521 mediated changes in CD34 + HSPC proliferation, but however, influences the cytokine-induced differentiation of HSPC in colony formation assays. In addition, stromal-derived factor (SDF) 1α-mediated transmigration of CD34 + HSPC through an endothelial cell layer was effectively diminished by BCAM/Lu antibodies, suggesting a direct involvement of BCAM/Lu in the migration process. This study indicates that both laminin receptors newly identified on human CD34 + HSPC should be taken into consideration in future studies.

CD146 Delineates an Interfascicular Cell Sub-Population in Tendon That Is Recruited during Injury through Its Ligand Laminin-α4

The interfascicular matrix (IFM) binds tendon fascicles and contains a population of morphologically distinct cells. However, the role of IFM-localised cell populations in tendon repair remains to be determined. The basement membrane protein laminin-α4 also localises to the IFM. Laminin-α4 is a ligand for several cell surface receptors, including CD146, a marker of pericyte and progenitor cells. We used a needle injury model in the rat Achilles tendon to test the hypothesis that the IFM is a niche for CD146+ cells that are mobilised in response to tendon damage. We also aimed to establish how expression patterns of circulating non-coding RNAs alter with tendon injury and identify potential RNA-based markers of tendon disease. The results demonstrate the formation of a focal lesion at the injury site, which increased in size and cellularity for up to 21 days post injury. In healthy tendon, CD146+ cells localised to the IFM, compared with injury, where CD146+ cells migrated towards the lesion at days 4 and 7, and populated the lesion 21 days post injury. This was accompanied by increased laminin-α4, suggesting that laminin-α4 facilitates CD146+ cell recruitment at injury sites. We also identified a panel of circulating microRNAs that are dysregulated with tendon injury. We propose that the IFM cell niche mediates the intrinsic response to injury, whereby an injury stimulus induces CD146+ cell migration. Further work is required to fully characterise CD146+ subpopulations within the IFM and establish their precise roles during tendon healing.

Autophagic Mediators in Bone Marrow Niche Homeostasis

The bone marrow serves as a reservoir for a multifunctional assortment of stem, progenitor, and mature cells, located in functional anatomical micro-areas termed niches. Within the niche, hematopoietic and mesenchymal progenies establish a symbiotic relationship characterized by interdependency and interconnectedness. The fine-tuned physical and molecular interactions that occur in the niches guarantee physiological bone turnover, blood cell maturation and egression, and moderation of inflammatory and oxidative intramural stressful conditions. The disruption of bone marrow niche integrity causes severe local and systemic pathological settings, and thus bone marrow inhabitants have been the object of extensive study. In this context, research has revealed the importance of the autophagic apparatus for niche homeostatic maintenance. Archetypal autophagic players such as the p62 and the Atg family proteins have been found to exert a variety of actions, some autophagy-related and others not; they moderate the essential features of mesenchymal and hematopoietic stem cells and switch their operational schedules. This chapter focuses on our current understanding of bone marrow functionality and the role of the executive autophagic apparatus in the niche framework. Autophagic mediators such as p62 and Atg7 are currently considered the most important orchestrators of stem and mature cell dynamics in the bone marrow.

Leukemia stem cell-bone marrow microenvironment interplay in acute myeloid leukemia development

Despite the advances in intensive chemotherapy regimens and targeted therapies, overall survival (OS) of acute myeloid leukemia (AML) remains unfavorable due to inevitable chemotherapy resistance and high relapse rate, which mainly caused by the persistence existence of leukemia stem cells (LSCs). Bone marrow microenvironment (BMM), the home of hematopoiesis, has been considered to play a crucial role in both hematopoiesis and leukemogenesis. When interrupted by the AML cells, a malignant BMM formed and thus provided a refuge for LSCs and protecting them from the cytotoxic effects of chemotherapy. In this review, we summarized the alterations in the bidirectional interplay between hematopoietic cells and BMM in the normal/AML hematopoietic environment, and pointed out the key role of these alterations in pathogenesis and chemotherapy resistance of AML. Finally, we focused on the current potential BMM-targeted strategies together with future prospects and challenges. Accordingly, while further research is necessary to elucidate the underlying mechanisms behind LSC–BMM interaction, targeting the interaction is perceived as a potential therapeutic strategy to eradicate LSCs and ultimately improve the outcome of AML.

Tonsil-derived mesenchymal stem cells enhance allogeneic bone marrow engraftment via collagen IV degradation

BACKGROUND

Co-transplantation of bone marrow cells (BMCs) and mesenchymal stem cells (MSCs) is used as a strategy to improve the outcomes of bone marrow transplantation. Tonsil-derived MSCs (TMSCs) are a promising source of MSCs for co-transplantation. Previous studies have shown that TMSCs or conditioned media from TMSCs (TMSC-CM) enhance BMC engraftment. However, the factors in TMSCs that promote better engraftment have not yet been identified.

METHODS

Mice were subjected to a myeloablative regimen of busulfan and cyclophosphamide, and the mRNA expression in the bone marrow was analyzed using an extracellular matrix (ECM) and adhesion molecule-targeted polymerase chain reaction (PCR) array. Nano-liquid chromatography with tandem mass spectrometry, real-time quantitative PCR, western blots, and enzyme-linked immunosorbent assays were used to compare the expression levels of metalloproteinase 3 (MMP3) in MSCs derived from various tissues, including the tonsils, bone marrow, adipose tissue, and umbilical cord. Recipient mice were conditioned with busulfan and cyclophosphamide, and BMCs, either as a sole population or with control or MMP3-knockdown TMSCs, were co-transplanted into these mice. The effects of TMSC-expressed MMP3 were investigated. Additionally, Enzchek collagenase and Transwell migration assays were used to confirm that the collagenase activity of TMSC-expressed MMP3 enhanced BMC migration.

RESULTS

Mice subjected to the myeloablative regimen exhibited increased mRNA expression of collagen type IV alpha 1/2 (Col4a1 and Col4a2). Among the various extracellular matrix-modulating proteins secreted by TMSCs, MMP3 was expressed at higher levels in TMSCs than in other MSCs. Mice co-transplanted with BMCs and control TMSCs exhibited a higher survival rate, weight recovery, and bone marrow cellularity compared with mice co-transplanted with BMCs and MMP3-knockdown TMSCs. Control TMSC-CM possessed higher collagenase activity against collagen IV than MMP3-knockdown TMSC-CM. TMSC-CM also accelerated BMC migration by degrading collagen IV in vitro.

CONCLUSIONS

Collectively, these results indicate that TMSCs enhance BMC engraftment by the secretion of MMP3 for the modulation of the bone marrow extracellular matrix.

Whole-Exome Sequencing Reveals Rare Germline Mutations in Patients With Hemifacial Microsomia

Hemifacial microsomia (HFM) is a rare congenital disease characterized by a spectrum of craniomaxillofacial malformations, including unilateral hypoplasia of the mandible and surrounding structures. Genetic predisposition for HFM is evident but the causative genes have not been fully understood. Thus, in the present study, we used whole-exome sequencing to screen 52 patients with HFM for rare germline mutations. We revealed 3,341 rare germline mutations in this patient cohort, including those in 13 genes previously shown to be associated with HFM. Among these HFM-related genes, NID2 was most frequently mutated (in 3/52 patients). PED4DIP, which has not been previously associated with HFM, exhibited rare variants most frequently (in 7/52 patients). Pathway enrichment analysis of genes that were mutated in >2 patients predicted the "laminin interactions" pathway to be most significantly disrupted, predominantly by mutations in ITGB4, NID2, or LAMA5. In summary, this study is the first to identify rare germline mutations in HFM. The likely disruptions in the signaling pathways due to the mutations reported here may be considered potential causes of HFM.

Extracellular laminin regulates hematopoietic potential of pluripotent stem cells through integrin β1-ILK-β-catenin-JUN axis

Recombinant matrices have enabled feeder cell-free maintenance cultures of human pluripotent stem cells (hPSCs), with laminin 511-E8 fragment (LM511-E8) being widely used. However, we herein report that hPSCs maintained on LM511-E8 resist differentiating to multipotent hematopoietic progenitor cells (HPCs), unlike hPSCs maintained on LM421-E8 or LM121-E8. The latter two LM-E8s bound weakly to hPSCs compared with LM511-E8 and activated the canonical Wnt/β-catenin signaling pathway. Moreover, the extracellular LM-E8-dependent preferential hematopoiesis was associated with a higher expression of integrin β1 (ITGB1) and downstream integrin-linked protein kinase (ILK), β-catenin and phosphorylated JUN. Accordingly, the lower coating concentration of LM511-E8 or addition of a Wnt/β-catenin signaling activator, CHIR99021, facilitated higher HPC yield. In contrast, the inhibition of ILK, Wnt or JNK by inhibitors or mRNA knockdown suppressed the HPC yield. These findings suggest that extracellular laminin scaffolds modulate the hematopoietic differentiation potential of hPSCs by activating the ITGB1-ILK-β-catenin-JUN axis at the undifferentiated stage. Finally, the combination of low-concentrated LM511-E8 and a revised hPSC-sac method, which adds bFGF, SB431542 and heparin to the conventional method, enabled a higher yield of HPCs and higher rate for definitive hematopoiesis, suggesting a useful protocol for obtaining differentiated hematopoietic cells from hPSCs in general.

TP63 basal cells are indispensable during endoderm differentiation into proximal airway cells on acellular lung scaffolds

The use of decellularized whole-organ scaffolds for bioengineering of organs is a promising avenue to circumvent the shortage of donor organs for transplantation. However, recellularization of acellular scaffolds from multicellular organs like the lung with a variety of different cell types remains a challenge. Multipotent cells could be an ideal cell source for recellularization. Here we investigated the hierarchical differentiation process of multipotent ES-derived endoderm cells into proximal airway epithelial cells on acellular lung scaffolds. The first cells to emerge on the scaffolds were TP63⁺ cells, followed by TP63⁺/KRT5⁺ basal cells, and finally multi-ciliated and secretory airway epithelial cells. TP63⁺/KRT5⁺ basal cells on the scaffolds simultaneously expressed KRT14, like basal cells involved in airway repair after injury. Removal of TP63 by CRISPR/Cas9 in the ES cells halted basal and airway cell differentiation on the scaffolds. These findings suggest that differentiation of ES-derived endoderm cells into airway cells on decellularized lung scaffolds proceeds via TP63⁺ basal cell progenitors and tracks a regenerative repair pathway. Understanding the process of differentiation is key for choosing the cell source for repopulation of a decellularized organ scaffold. Our data support the use of airway basal cells for repopulating the airway side of an acellular lung scaffold.

Integrin α6 mediates the drug resistance of acute lymphoblastic B-cell leukemia

Resistance to multimodal chemotherapy continues to limit the prognosis of acute lymphoblastic leukemia (ALL). This occurs in part through a process called adhesion-mediated drug resistance, which depends on ALL cell adhesion to the stroma through adhesion molecules, including integrins. Integrin α6 has been implicated in minimal residual disease in ALL and in the migration of ALL cells to the central nervous system. However, it has not been evaluated in the context of chemotherapeutic resistance. Here, we show that the anti-human α6-blocking Ab P5G10 induces apoptosis in primary ALL cells in vitro and sensitizes primary ALL cells to chemotherapy or tyrosine kinase inhibition in vitro and in vivo. We further analyzed the underlying mechanism of α6-associated apoptosis using a conditional knockout model of α6 in murine BCR-ABL1+ B-cell ALL cells and showed that α6-deficient ALL cells underwent apoptosis. In vivo deletion of α6 in combination with tyrosine kinase inhibitor (TKI) treatment was more effective in eradicating ALL than treatment with a TKI (nilotinib) alone. Proteomic analysis revealed that α6 deletion in murine ALL was associated with changes in Src signaling, including the upregulation of phosphorylated Lyn (pTyr507) and Fyn (pTyr530). Thus, our data support α6 as a novel therapeutic target for ALL.

"Caught in the net": the extracellular matrix of the bone marrow in normal hematopoiesis and leukemia

The influence of the bone marrow microenvironment on normal hematopoiesis, but also leukemia, has largely been accepted. However, the focus has been predominantly on the role of various cell types or cytokines maintaining hematopoietic stem cells or protecting leukemia stem cells from different therapies. A frequently overlooked component of the bone marrow microenvironment is the extracellular matrix, which not only provides a mechanical scaffold, but also serves as a source of growth factors. We discuss here how extracellular matrix proteins directly or indirectly modulate hematopoietic stem cell physiology and influence leukemia progression. It is hoped that existing and future studies on this topic may propel forward the possibility of augmenting normal hematopoiesis and improving therapies for leukemia, for instance, by targeting of the extracellular matrix in the bone marrow.

Stem cell homing: From physiology to therapeutics

Stem cell homing is a multistep endogenous physiologic process that is also used by exogenously administered hematopoietic stem and progenitor cells (HSPCs). This multistep process involves cell migration and is essential for hematopoietic stem cell transplantation. The process can be manipulated to enhance ultimate engraftment potential, and understanding stem cell homing is also important to the understanding of stem cell mobilization. Homing is also of potential importance in the recruitment of marrow mesenchymal stem and stromal cells (MSCs) to sites of injury and regeneration. This process is less understood but assumes importance when these cells are used for repair purposes. In this review, the process of HSPC and MSC homing is examined, as are methods to enhance this process.

Bimodal Whole-Mount Imaging of Tendon Using Confocal Microscopy and X-ray Micro-Computed Tomography

Background

Three-dimensional imaging modalities for optically dense connective tissues such as tendons are limited and typically have a single imaging methodological endpoint. Here, we have developed a bimodal procedure utilising fluorescence-based confocal microscopy and x-ray micro-computed tomography for the imaging of adult tendons to visualise and analyse extracellular sub-structure and cellular composition in small and large animal species.

Results

Using fluorescent immunolabelling and optical clearing, we visualised the expression of the novel cross-species marker of tendon basement membrane, laminin-α4 in 3D throughout whole rat Achilles tendons and equine superficial digital flexor tendon 5 mm segments. This revealed a complex network of laminin-α4 within the tendon core that predominantly localises to the interfascicular matrix compartment. Furthermore, we implemented a chemical drying process capable of creating contrast densities enabling visualisation and quantification of both fascicular and interfascicular matrix volume and thickness by x-ray micro-computed tomography. We also demonstrated that both modalities can be combined using reverse clarification of fluorescently labelled tissues prior to chemical drying to enable bimodal imaging of a single sample.

Conclusions

Whole-mount imaging of tendon allowed us to identify the presence of an extensive network of laminin-α4 within tendon, the complexity of which cannot be appreciated using traditional 2D imaging techniques. Creating contrast for x-ray micro-computed tomography imaging of tendon using chemical drying is not only simple and rapid, but also markedly improves on previously published methods. Combining these methods provides the ability to gain spatio-temporal information and quantify tendon substructures to elucidate the relationship between morphology and function.

Limitations of recellularized biological scaffolds for human transplantation

A shortage of donor organs for transplantation and the dependence of the recipients on immunosuppressive therapy have motivated researchers to consider alternative regenerative approaches. The answer may reside in acellular scaffolds generated from cadaveric human and animal tissues. Acellular scaffolds are expected to preserve the architectural and mechanical properties of the original organ, permitting cell attachment, growth, and differentiation. Although theoretically, the use of acellular scaffolds for transplantation should pose no threat to the recipient's immune system, experimental data have revealed significant immune responses to allogeneic and xenogeneic transplanted scaffolds. Herein, we review the various factors of the scaffold that could trigger an inflammatory and/or immune response, thereby compromising its use for human transplant therapy. In addition, we provide an overview of the major cell types that have been considered for recellularization of the scaffold and their potential contribution to triggering an immune response.

Laminin-Inspired Cell-Instructive Microenvironments for Neural Stem Cells

Laminin is a heterotrimeric glycoprotein with a key role in the formation and maintenance of the basement membrane architecture and properties, as well as on the modulation of several biological functions, including cell adhesion, migration, differentiation and matrix-mediated signaling. In the central nervous system (CNS), laminin is differentially expressed during development and homeostasis, with an impact on the modulation of cell function and fate. Within neurogenic niches, laminin is one of the most important and well described extracellular matrix (ECM) proteins. Specifically, efforts have been made to understand laminin assembly, domain architecture, and interaction of its different bioactive domains with cell surface receptors, soluble signaling molecules, and ECM proteins, to gain insight into the role of this ECM protein and its receptors on the modulation of neurogenesis, both in homeostasis and during repair. This is also expected to provide a rational basis for the design of biomaterial-based matrices mirroring the biological properties of the basement membrane of neural stem cell niches, for application in neural tissue repair and cell transplantation. This review provides a general overview of laminin structure and domain architecture, as well as the main biological functions mediated by this heterotrimeric glycoprotein. The expression and distribution of laminin in the CNS and, more specifically, its role within adult neural stem cell niches is summarized. Additionally, a detailed overview on the use of full-length laminin and laminin derived peptide/recombinant laminin fragments for the development of hydrogels for mimicking the neurogenic niche microenvironment is given. Finally, the main challenges associated with the development of laminin-inspired hydrogels and the hurdles to overcome for these to progress from bench to bedside are discussed.

The physical microenvironment of hematopoietic stem cells and its emerging roles in engineering applications

Stem cells are considered the fundamental underpinnings of tissue biology. The stem cell microenvironment provides factors and elements that play significant roles in controlling the cell fate direction. The bone marrow is an important environment for functional hematopoietic stem cells in adults. Remarkable progress has been achieved in the area of hematopoietic stem cell fate modulation based on the recognition of biochemical factors provided by bone marrow niches. In this review, we focus on emerging evidence that hematopoietic stem cell fate is altered in response to a variety of microenvironmental physical cues, such as geometric properties, matrix stiffness, and mechanical forces. Based on knowledge of these biophysical cues, recent developments in harnessing hematopoietic stem cell niches ex vivo are also discussed. A comprehensive understanding of cell microenvironments helps provide mechanistic insights into pathophysiological mechanisms and underlies biomaterial-based hematopoietic stem cell engineering.

Deconstructed Microfluidic Bone Marrow On-A-Chip to Study Normal and Malignant Hemopoietic Cell-Niche Interactions

Human hematopoietic niches are complex specialized microenvironments that maintain and regulate hematopoietic stem and progenitor cells (HSPC). Thus far, most of the studies performed investigating alterations of HSPC-niche dynamic interactions are conducted in animal models. Herein, organ microengineering with microfluidics is combined to develop a human bone marrow (BM)-on-a-chip with an integrated recirculating perfusion system that consolidates a variety of important parameters such as 3D architecture, cell-cell/cell-matrix interactions, and circulation, allowing a better mimicry of in vivo conditions. The complex BM environment is deconvoluted to 4 major distinct, but integrated, tissue-engineered 3D niche constructs housed within a single, closed, recirculating microfluidic device system, and equipped with cell tracking technology. It is shown that this technology successfully enables the identification and quantification of preferential interactions-homing and retention-of circulating normal and malignant HSPC with distinct niches.

Hypoxia-induced PIM kinase and laminin-activated integrin α6 mediate resistance to PI3K inhibitors in bone-metastatic CRPC

Bone-metastatic castration-resistant prostate cancer (CRPC) is lethal due to inherent resistance to androgen deprivation therapy, chemotherapy, and targeted therapies. Despite the fact that a majority of CRPC patients (approximately 70%) harbor a constitutively active PI3K survival pathway, targeting the PI3K/mTOR pathway has failed to increase overall survival in clinical trials. Here, we identified two separate and independent survival pathways induced by the bone tumor microenvironment that are hyperactivated in CRPC and confer resistance to PI3K inhibitors. The first pathway involves integrin α6β1-mediated adhesion to laminin and the second involves hypoxia-induced expression of PIM kinases. In vitro and in vivo models demonstrate that these pathways transduce parallel but independent signals that promote survival by reducing oxidative stress and preventing cell death. We further demonstrate that both pathways drive resistance to PI3K inhibitors in PTEN-negative tumors. These results provide preclinical evidence that combined inhibition of integrin α6β1 and PIM kinase in CRPC is required to overcome tumor microenvironment-mediated resistance to PI3K inhibitors in PTEN-negative tumors within the hypoxic and laminin-rich bone microenvironment.

Prion protein deficiency impairs hematopoietic stem cell determination and sensitizes myeloid progenitors to irradiation

Highly conserved among species and expressed in various types of cells, numerous roles have been attributed to the cellular prion protein (PrPC). In hematopoiesis, PrPC regulates hematopoietic stem cell self-renewal but the mechanisms involved in this regulation are unknown. Here we show that PrPC regulates hematopoietic stem cell number during aging and their determination towards myeloid progenitors. Furthermore, PrPC protects myeloid progenitors against the cytotoxic effects of total body irradiation. This radioprotective effect was associated with increased cellular prion mRNA level and with stimulation of the DNA repair activity of the Apurinic/pyrimidinic endonuclease 1, a key enzyme of the base excision repair pathway. Altogether, these results show a previously unappreciated role of PrPC in adult hematopoiesis, and indicate that PrPC-mediated stimulation of BER activity might protect hematopoietic progenitors from the cytotoxic effects of total body irradiation.

Understanding the Extracellular Matrix to Enhance Stem Cell-Based Tissue Regeneration

The extracellular matrix is a biologically critical entity that has historically been poorly understood. Here we discuss how new tools for characterizing matrix composition and function enable us to design and deliver advanced matrices in vitro, to optimize regeneration, and in vivo, within a variety of tissues and organs.

Diverse functions of the thrombopoietin receptor agonist romiplostim rescue individuals exposed to lethal radiation

In cases of radiological accidents, especially victims exposed to high-dose ionizing radiation, the administration of appropriate approved pharmaceutical drugs is the most rapid medical treatment. However, currently, there are no suitable candidates. The thrombopoietin receptor (TPOR) agonist romiplostim (RP) is a therapeutic agent for immune thrombocytopenia and has potential to respond to such victims. Here, we show that RP administration in mice exposed to lethal-dose radiation leads not only to the promotion of haematopoiesis in multiple organs, including the lungs but also a reduction in damage to organs and cells. RP also causes a rapid increase in the number of mesenchymal stem cells in the spleen. In addition, RP suppresses the expression of several miRNAs involved in radiation-induced leukemogenesis, suggesting the presence of targets other than TPOR. Among the currently approved pharmaceutical drugs, RP is the most suitable candidate for victims exposed to high-dose ionizing radiation.

Metabolic underpinnings of leukemia pathology and treatment

BACKGROUND

Carcinogenic transformation of white blood cells during hematopoiesis leads to the development of leukemia, a cancer characterized by incompetent immune cells and a disruption of normal bone marrow function. Leukemias are diverse in type, affected population, prognosis, and treatment regimen, yet a common theme in leukemia is the dysregulated metabolism of leukemic cells and leukemic stem cells with respect to their noncancerous counterparts.

RECENT FINDINGS

In this review, we highlight current findings that elucidate metabolic traits unique to the four major types of leukemia, which confer carcinogenic survival but can be potentially exploited for therapeutic intervention. These metabolic features can work in conjunction with or be independent of unique aspects of the bone marrow microenvironment that can also influence cell survival and proliferation, thus sustaining carcinogenesis.

CONCLUSION

Deepening our understanding of the interactions of leukemias with their niche environments in vivo will inform future treatments for leukemia, particularly for those that are refractive to tyrosine kinase inhibitors and other therapeutic mainstays.