Chemokine receptor CXCR4-dependent internalization and resecretion of functional chemokine SDF-1 by bone marrow endothelial and stromal cells

SDF-1/CXCR4 axis participants in the pathophysiology of adult patients with moyamoya disease

BACKGROUND

Moyamoya disease (MMD) is characterized by an abundance of moyamoya vessels; however, the precise mechanism driving the spontaneous angiogenesis of these compensatory vessels remains unclear. Previous research has established a link between the stromal cell-derived factor-1 (SDF-1)/ CXC receptor 4 (CXCR4) axis and angiogenesis under hypoxic conditions. Nevertheless, the alterations in this axis within the cerebrospinal fluid, arachnoid membranes and vascular tissue of MMD patients have not been fully investigated.

METHODS

Our study enrolled 66 adult MMD patients and 61 patients with atherosclerotic vascular disease (ACVD). We investigated the SDF-1 concentration in cerebrospinal fluid (CSF) and CXCR4 expression level on the arachnoid membranes and vascular tissue. We utilized enzyme-linked immunosorbent assay and immunohistochemistr. Additionally, we cultured and stimulated human brain microvascular endothelial cells (HBMECs) and smooth muscle cells (SMCs) under oxygen and glucose deprivation (OGD) conditions followed by reoxygenation, to examine any changes in the SDF-1/CXCR4 axis.

RESULTS

The results demonstrated an elevation in the level of SDF-1 in CSF among MMD patients compared to those with ACVD. Moreover, the expression of CXCR4 in arachnoid membranes and vascular tissue showed a similar trend. Furthermore, the content of CXCR4 in HBMECs and SMCs increased with the duration of ischemia and hypoxia. However, it was observed that the expression of CXCR4 decreased at OGD/R 24h compared to OGD 24h. The temporal pattern of SDF-1 expression in HBMECs and SMCs mirrored that of CXCR4 expression.

CONCLUSION

These findings indicate a critical role for the SDF-1/CXCR4 axis in the angiogenesis of moyamoya disease.

Glucocorticoids paradoxically promote steroid resistance in B cell acute lymphoblastic leukemia through CXCR4/PLC signaling

Glucocorticoid (GC) resistance in childhood relapsed B-cell acute lymphoblastic leukemia (B-ALL) represents an important challenge. Despite decades of clinical use, the mechanisms underlying resistance remain poorly understood. Here, we report that in B-ALL, GC paradoxically induce their own resistance by activating a phospholipase C (PLC)-mediated cell survival pathway through the chemokine receptor, CXCR4. We identify PLC as aberrantly activated in GC-resistant B-ALL and its inhibition is able to induce cell death by compromising several transcriptional programs. Mechanistically, dexamethasone (Dex) provokes CXCR4 signaling, resulting in the activation of PLC-dependent Ca2+ and protein kinase C signaling pathways, which curtail anticancer activity. Treatment with a CXCR4 antagonist or a PLC inhibitor improves survival of Dex-treated NSG mice in vivo. CXCR4/PLC axis inhibition significantly reverses Dex resistance in B-ALL cell lines (in vitro and in vivo) and cells from Dex resistant ALL patients. Our study identifies how activation of the PLC signalosome in B-ALL by Dex limits the upfront efficacy of this chemotherapeutic agent.

Bone-Targeted Biomimetic Nanogels Re-Establish Osteoblast/Osteoclast Balance to Treat Postmenopausal Osteoporosis

Insufficient bone formation and excessive bone resorption caused by estrogen deficiency are the major factors resulting in the incidence of postmenopausal osteoporosis (PMOP). The existing drugs usually fail to re-establish the osteoblast/osteoclast balance from both sides and generate side-effects owing to the lack of bone-targeting ability. Here, engineered cell-membrane-coated nanogels PNG@mR&C capable of scavenging receptor activator of nuclear factor-κB ligand (RANKL) and responsively releasing therapeutic PTH 1-34 in the bone microenvironment are prepared from RANK and CXCR4 overexpressed bone mesenchymal stem cell (BMSC) membrane-coated chitosan biopolymers. The CXCR4 on the coated-membranes confer bone-targeting ability, and abundant RANK effectively absorb RANKL to inhibit osteoclastogenesis. Meanwhile, the release of PTH 1-34 triggered by osteoclast-mediated acid microenvironment promote osteogenesis. In addition, the dose and frequency are greatly reduced due to the smart release property, prolonged circulation time, and bone-specific accumulation. Thus, PNG@mR&C exhibits satisfactory therapeutic effects in the ovariectomized (OVX) mouse model. This study provides a new paradigm re-establishing the bone metabolic homeostasis from multitargets and shows great promise for the treatment of PMOP.

Peptide derived from stromal cell-derived factor 1δ enhances the in vitro expression of osteogenic proteins via bone marrow stromal cell differentiation and promotes bone formation in in vivo models

Mesenchymal stem cells (MSCs) rely on chemokines and chemokine receptors to execute their biological and physiological functions. Stromal cell-derived factor-1 (SDF-1) is upregulated in injury sites, where it acts as a chemotactic agent, attracting CXCR4-expressing MSCs, which play a pivotal role in the healing and regeneration of tissue throughout the body. Furthermore, SDF-1 expression has been observed in regions experiencing inflammation-induced bone destruction and fracture sites. In this study, we identified a novel peptide called bone-forming peptide-5 (BFP-5), derived from SDF-1δ, which can promote the osteogenesis of MSCs as well as bone formation and healing. Multipotent bone marrow stromal cells treated with BFP-5 showed enhanced alizarin red S staining and higher alkaline phosphatase (ALP) activity. Moreover, ALP and osterix proteins were more abundantly expressed when cells were treated with BFP-5 than SDF-1α. Histology and microcomputed tomography data at 12 weeks demonstrated that both rabbit and goat models transplanted with polycaprolactone (PCL) scaffolds coated with BFP-5 showed significantly greater bone formation than animals transplanted with PCL scaffolds alone. These findings suggest that BFP-5 could be useful in the development of related therapies for conditions associated with bones.

Young donor hematopoietic stem cells revitalize aged or damaged bone marrow niche by transdifferentiating into functional niche cells

The bone marrow niche maintains hematopoietic stem cell (HSC) homeostasis and declines in function in the physiologically aging population and in patients with hematological malignancies. A fundamental question is now whether and how HSCs are able to renew or repair their niche. Here, we show that disabling HSCs based on disrupting autophagy accelerated niche aging in mice, whereas transplantation of young, but not aged or impaired, donor HSCs normalized niche cell populations and restored niche factors in host mice carrying an artificially harassed niche and in physiologically aged host mice, as well as in leukemia patients. Mechanistically, HSCs, identified using a donor lineage fluorescence-tracing system, transdifferentiate in an autophagy-dependent manner into functional niche cells in the host that include mesenchymal stromal cells and endothelial cells, previously regarded as "nonhematopoietic" sources. Our findings thus identify young donor HSCs as a primary parental source of the niche, thereby suggesting a clinical solution to revitalizing aged or damaged bone marrow hematopoietic niche.

Multifunctional Elastin-Like Polypeptide Fusion Protein Coacervates Inhibit Receptor-Mediated Proinflammatory Signals and Promote Angiogenesis in Mouse Diabetic Wounds

Objective: Chronic skin wounds are one of the most devastating complications in diabetic patients due to the formation of advanced glycation end-products (AGEs) resulting from nonenzymatic glycation of proteins and lipids in hyperglycemia. AGEs, upon binding their receptors (RAGEs), trigger proinflammatory signals that impair wound healing in diabetes and contribute to the pathology of chronic skin wounds. Approach: We previously developed a recombinant fusion protein containing the binding domain of RAGE (vRAGE) linked to elastin-like polypeptides (ELPs) that acts as a competitive inhibitor of AGEs, and another ELP fusion protein containing stromal cell-derived factor 1 (SDF1) that promotes revascularization. In this study, we report the effects of protein coacervates incorporating both vRAGE-ELP and SDF1-ELP on wound healing in an in vitro diabetes-mimicking cell culture system, and in in vivo in full-thickness wounds on diabetic mice. Results: The combination of vRAGE-ELP and SDF1-ELP increased cell metabolic activity in AGE-stimulated endothelial cells, promoted in vitro tube formation and accelerated healing in an in vitro cell migration assay. When used in a single topical application on full-thickness excisional skin wounds in diabetic mice, wound closure in the combination groups reached almost 100% on postwounding day 35, compared to 62% and 85% on the same days in animals treated with fibrin gel control and vehicle control consisting of ELP alone. Innovation: To our knowledge, this is the first study that attempts to reverse the AGE-RAGE-mediated signaling as well as to promote cell proliferation and vascularization in one single treatment. Conclusion: The codelivery of vRAGE-ELP and SDF1-ELP has potential for the treatment of diabetic wounds.

Physical biomarkers for human hematopoietic stem and progenitor cells

Adhesion of hematopoietic stem and progenitor cells (HSPCs) to the bone marrow niche plays critical roles in the maintenance of the most primitive HSPCs. The interactions of HSPC-niche interactions are clinically relevant in acute myeloid leukemia (AML), because (i) leukemia-initiating cells adhered to the marrow niche are protected from the cytotoxic effect by chemotherapy and (ii) mobilization of HSPCs from healthy donors' bone marrow is crucial for the effective stem cell transplantation. However, although many clinical agents have been developed for the HSPC mobilization, the effects caused by the extrinsic molecular cues were traditionally evaluated based on phenomenological observations. This review highlights the recent interdisciplinary challenges of hematologists, biophysicists and cell biologists towards the design of defined in vitro niche models and the development of physical biomarkers for quantitative indexing of differential effects of clinical agents on human HSPCs.

Dapagliflozin-Loaded Exosome Mimetics Facilitate Diabetic Wound Healing by HIF-1α-Mediated Enhancement of Angiogenesis

Angiogenesis plays a critical role in diabetic wound healing. However, no effective strategies have been developed to target endothelial cells (ECs) to facilitate diabetic wound healing. Dapagliflozin (DA) as a sodium-glucose linked transporter 2 (SGLT2) inhibitor, may promote neovascularization in diabetic mice via HIF-1α-mediated enhancement of angiogenesis. Here, the bioinspired nanovesicles (NVs) prepared from induced pluripotent stem cells-derived ECs through an extrusion approach are reported, which can function as exosome mimetics to achieve targeted deliver of DA. Abundant membrane C-X-C motif chemokine receptor 4 conferred the EC-targeting ability of these NVs and the endothelial homology facilitated the accumulation in ECs. Furthermore, these DA-loaded induced pluripotent stem cells (iPSC)-EC NVs can facilitate angiogenesis and diabetic wound healing by HIF-1α/VEGFA pathway. Taken together, this study indicated that targeting ECs and regulating angiogenesis may be a promising strategy for the treatment of diabetic wound healing.

Mesenchymal stem cells in radiation-induced lung injury: From mechanisms to therapeutic potential

Radiotherapy (RT) is an effective treatment option for multiple thoracic malignant tumors, including lung cancers, thymic cancers, and tracheal cancers. Radiation-induced lung injury (RILI) is a serious complication of radiotherapy. Radiation causes damage to the pulmonary cells and tissues. Multiple factors contribute to the progression of Radiation-induced lung injury, including genetic alterations, oxidative stress, and inflammatory responses. Especially, radiation sources contribute to oxidative stress occurrence by direct excitation and ionization of water molecules, which leads to the decomposition of water molecules and the generation of reactive oxygen species (ROS), reactive nitrogen species (RNS). Subsequently, reactive oxygen species and reactive nitrogen species overproduction can induce oxidative DNA damage. Immune cells and multiple signaling molecules play a major role in the entire process. Mesenchymal stem cells (MSCs) are pluripotent stem cells with multiple differentiation potentials, which are under investigation to treat radiation-induced lung injury. Mesenchymal stem cells can protect normal pulmonary cells from injury by targeting multiple signaling molecules to regulate immune cells and to control balance between antioxidants and prooxidants, thereby inhibiting inflammation and fibrosis. Genetically modified mesenchymal stem cells can improve the natural function of mesenchymal stem cells, including cellular survival, tissue regeneration, and homing. These reprogrammed mesenchymal stem cells can produce the desired products, including cytokines, receptors, and enzymes, which can contribute to further advances in the therapeutic application of mesenchymal stem cells. Here, we review the molecular mechanisms of radiation-induced lung injury and discuss the potential of Mesenchymal stem cells for the prevention and treatment of radiation-induced lung injury. Clarification of these key issues will make mesenchymal stem cells a more fantastic novel therapeutic strategy for radiation-induced lung injury in clinics, and the readers can have a comprehensive understanding in this fields.

A biomimetic nanocomposite with enzyme-like activities and CXCR4 antagonism efficiently enhances the therapeutic efficacy of acute myeloid leukemia

Despite the progress made to improve therapeutic outcomes for acute myeloid leukemia (AML), many unmet clinical needs remain to be resolved. Unlike existing anti-AML strategies, here we developed a biomimetic nanocomposite to efficiently eliminate the leukemia cells in the bone marrow and prevent the homing of AML. To fulfill our design, the ultra-small nanozyme was conjugated onto the surface of an oxygen-carrying nanoparticle, which was further coated with bone marrow stromal cell membrane. After entering the blood, this biomimetic nanocomposite got actively internalized by the leukemia cells in the blood and released the loaded chemotherapeutics and nanozyme inside the leukemia cells to achieve a synergistic antitumor efficacy. Meanwhile, the adhesive properties of the stromal cell membrane enabled the nanocomposite to home to the bone marrow, where the nanocomposite effectively killed the retained leukemia cells. More importantly, the biomimetic cell membrane also acted as a CXCR4 antagonism to block the CXCR4/CXCL12-mediated homing of leukemia cells to the bone marrow and infiltration to other organs like the liver and spleen. In conclusion, this proof-of-concept study demonstrated that our designed platform effectively kills leukemia cells while preventing their infiltration, thus providing a promising prospect for resolving the clinical challenges in current AML treatment.

CXCL13-CXCR5 axis: Regulation in inflammatory diseases and cancer

Chemokine C-X-C motif ligand 13 (CXCL13), originally identified as a B-cell chemokine, plays an important role in the immune system. The interaction between CXCL13 and its receptor, the G-protein coupled receptor (GPCR) CXCR5, builds a signaling network that regulates not only normal organisms but also the development of many diseases. However, the precise action mechanism remains unclear. In this review, we discussed the functional mechanisms of the CXCL13-CXCR5 axis under normal conditions, with special focus on its association with diseases. For certain refractory diseases, we emphasize the diagnostic and therapeutic role of CXCL13-CXCR5 axis.

Bioinspired Nanovesicles Convert the Skeletal Endothelium-Associated Secretory Phenotype to Treat Osteoporosis

Recently, bone marrow endothelial cells (BMECs) were found to play an important role in regulating bone homeostasis. However, few studies utilized BMECs to treat bone metabolic diseases including osteoporosis. Here, we reported bioinspired nanovesicles (BNVs) prepared from human induced pluripotent stem cells-derived endothelial cells under hypoxia culture through an extrusion approach. Abundant membrane C-X-C motif chemokine receptor 4 conferred these BNVs bone-targeting ability and the endothelial homology facilitated the BMEC tropism. Due to their unique endogenous miRNA cargos, these BNVs re-educated BMECs to secret cytokines favoring osteogenesis and anti-inflammation. Owing to the conversion of secretory phenotype, the osteogenic differentiation of bone mesenchymal stem cells was facilitated, and the M1-macrophage-dominant pro-inflammatory microenvironment was ameliorated in osteoporotic bones. Taken together, this study proposed BMEC-targeting nanovesicles treating osteoporosis via converting the skeletal endothelium-associated secretory phenotype.

Niche Regulation of Hematopoiesis: The Environment Is "Micro," but the Influence Is Large

Immune cell production is governed by a process known as hematopoiesis, where hematopoietic stem cells (HSCs) differentiate through progenitor cells and ultimately to the mature blood and immune cells found in circulation. While HSCs are capable of cell-autonomous regulation, they also rely on extrinsic factors to balance their state of quiescence and activation. These cues can, in part, be derived from the niche in which HSCs are found. Under steady-state conditions, HSCs are found in the bone marrow. This niche is designed to support HSCs but also to respond to external factors, which allows hematopoiesis to be a finely tuned and coordinated process. However, the niche, and its regulation, can become dysregulated to potentiate inflammation during disease. This review will highlight the architecture of the bone marrow and key regulators of hematopoiesis within this niche. Emphasis will be placed on how these mechanisms go awry to exacerbate hematopoietic contributions that drive cardiovascular disease.

The Bone Marrow Microenvironment in B-Cell Development and Malignancy

B lymphopoiesis is characterized by progressive loss of multipotent potential in hematopoietic stem cells, followed by commitment to differentiate into B cells, which mediate the humoral response of the adaptive immune system. This process is tightly regulated by spatially distinct bone marrow niches where cells, including mesenchymal stem and progenitor cells, endothelial cells, osteoblasts, osteoclasts, and adipocytes, interact with B-cell progenitors to direct their proliferation and differentiation. Recently, the B-cell niche has been implicated in initiating and facilitating B-cell precursor acute lymphoblastic leukemia. Leukemic cells are also capable of remodeling the B-cell niche to promote their growth and survival and evade treatment. Here, we discuss the major cellular components of bone marrow niches for B lymphopoiesis and the role of the malignant B-cell niche in disease development, treatment resistance and relapse. Further understanding of the crosstalk between leukemic cells and bone marrow niche cells will enable development of additional therapeutic strategies that target the niches in order to hinder leukemia progression.

Silica Nanoparticle Internalization Improves Chemotactic Behaviour of Human Mesenchymal Stem Cells Acting on the SDF1α/CXCR4 Axis

Human mesenchymal stem cell (hMSC)-based therapy is an emerging resource in regenerative medicine. Despite the innate ability of hMSCs to migrate to sites of injury, homing of infused hMSCs to the target tissue is inefficient. It was shown that silica nanoparticles (SiO₂-NPs), previously developed to track the stem cells after transplantation, accumulated in lysosomes leading to a transient blockage of the autophagic flux. Since CXCR4 turnover is mainly regulated by autophagy, we tested the effect of SiO₂-NPs on chemotactic migration of hMSCs along the SDF1α/CXCR4 axis that plays a pivotal role in directing MSC homing to sites of injury. Our results showed that SiO₂-NP internalization augmented CXCR4 surface levels. We demonstrated that SiO₂-NP-dependent CXCR4 increase was transient, and it reversed at the same time as lysosomal compartment normalization. Furthermore, the autophagy inhibitor Bafilomycin-A1 reproduced CXCR4 overexpression in control hMSCs confirming the direct effect of the autophagic degradation blockage on CXCR4 expression. Chemotaxis assays showed that SiO₂-NPs increased hMSC migration toward SDF1α. In contrast, migration improvement was not observed in TNFα/TNFR axis, due to the proteasome-dependent TNFR regulation. Overall, our findings demonstrated that SiO₂-NP internalization increases the chemotactic behaviour of hMSCs acting on the SDF1α/CXCR4 axis, unmasking a high potential to improve hMSC migration to sites of injury and therapeutic efficacy upon cell injection in vivo.

"Attractive/adhesion force" dual-regulatory nanogels capable of CXCR4 antagonism and autophagy inhibition for the treatment of metastatic breast cancer

Metastasis is refractory systemic disease resulting in low survival rate of breast cancer patients, especially in the late stage. The processes of metastasis are mainly initiated by strong "attractive force" from distant organs and deteriorated by weak "adhesion force" in primary tumor. Here, we reported "attractive/adhesion force" dual-regulatory nanogels (CQ-HF/PTX) for the precise treatment of both primary and metastasis of metastatic breast cancer. Hydroxychloroquine (HCQ) and hydrophobic Fmoc were grafted on hydrophilic hydroxyethyl starch (HES) to obtain amphiphilic CQ-HF polymer, which was assembly with chemotherapy drug paclitaxel (PTX) to form the nanogels for anti-primary tumor. Meanwhile, CQ-HF/PTX nanogels play two roles in anti-metastasis: i) For reducing the "attractive force", it could block the CXCR4/SDF-1 pathway, preventing tumor cells metastasis to the lung; ii) For reinforcing "adhesion force", it could inhibit the excessive autophagy for hindering the degradation of paxillin and enhancing the cell adhesion. As a result, dual-regulatory CQ-HF/PTX nanogels dramatically inhibited tumor and the lung metastasis of mouse breast cancer. Therefore, the fabricating of synergetic dual-regulatory nanogels uncovered the explicit mechanism and provided an efficient strategy for combating malignant metastatic tumors.

Overcoming ischemia in the diabetic foot: Minimally invasive treatment options

As the global burden of diabetes is rapidly increasing, the incidence of diabetic foot ulcers is continuously increasing as the mean age of the world population increases and the obesity epidemic advances. A significant percentage of diabetic foot ulcers are caused by mixed micro and macro-vascular dysfunction leading to impaired perfusion of foot tissue. Left untreated, chronic limb-threatening ischemia has a poor prognosis and is correlated with limb loss and increased mortality; prompt treatment is required. In this review, the diagnostic challenges in diabetic foot disease are discussed and available data on minimally invasive treatment options such as endovascular revascularization, stem cells, and gene therapy are examined.

Combined Bone Marrow Aspirate and Platelet-Rich Plasma for Cartilage Repair: Two-Year Clinical Results

PURPOSE

To evaluate the clinical and biological outcome of combined bone marrow aspirate concentrate (BMAC) and platelet-rich plasma (PRP) on a collagen scaffold for treating cartilage lesions in the knee.

METHODS AND MATERIALS

Ten patients (mean age 29.4 years, range 18-36) suffering from large full-thickness cartilage in the knee were treated with BMAC and PRP from January 2015 to December 2016. In a 1-step procedure autologous BMAC and PRP was seeded onto a collagen scaffold and sutured into the debrided defect. Patients were evaluated by clinical outcome scores (IKDC [International Knee Documentation Committee Subjective Knee Form], KOOS [Knee Injury and Osteoarthritis Outcome Score], and pain score using the Numeric Rating Scale [NRS]) preoperatively, after 3 months, and after 1 and 2 years. Second-look arthroscopies were performed (n = 7) with biopsies of the repair tissue for histology. All patients had magnetic resonance imaging (MRI) preoperatively, after 1 year, and after 2 to 3.5 years with MOCART (magnetic resonance observation of cartilage repair tissue) scores evaluating cartilage repair.

RESULTS

After 1 year significant improvements were found in IKDC, KOOS symptoms, KOOS ADL (Activities of Daily Living), KOOS QOL (Quality of Life), and pain at activity. At the latest follow-up significant improvements were seen in IKDC, KOOS symptoms, KOOS QOL, pain at rest, and pain at activity. MRI MOCART score for cartilage repair improved significantly from baseline to 1-year follow-up. Histomorphometry of repair tissue demonstrated a mixture of fibrous tissue (58%) and fibrocartilage (40%).

CONCLUSION

Treatment of cartilage injuries using combined BMAC and PRP improved subjective clinical outcome scores and pain scores at 1 and 2 years postoperatively. MRI and histology indicated repair tissue inferior to the native hyaline cartilage.

Therapeutic approach of adipose-derived mesenchymal stem cells in refractory peptic ulcer

Peptic ulcer is one of the most common gastrointestinal tract disorders worldwide, associated with challenges such as refractory morbidity, bleeding, interference with use of anticoagulants, and potential side effects associated with long-term use of proton pump inhibitors. A peptic ulcer is a defect in gastric or duodenal mucosa extending from muscularis mucosa to deeper layers of the stomach wall. In most cases, ulcers respond to standard treatments. However, in some people, peptic ulcer becomes resistant to conventional treatment or recurs after initially successful therapy. Therefore, new and safe treatments, including the use of stem cells, are highly favored for these patients. Adipose-derived mesenchymal stem cells are readily available in large quantities with minimal invasive intervention, and isolation of adipose-derived mesenchymal stromal stem cells (ASC) produces large amounts of stem cells, which are essential for cell-based and restorative therapies. These cells have high flexibility and can differentiate into several types of cells in vitro. This article will investigate the effects and possible mechanisms and signaling pathways of adipose tissue-derived mesenchymal stem cells in patients with refractory peptic ulcers.

Nonoperative and Operative Soft-Tissue and Cartilage Regeneration and Orthopaedic Biologics of the Knee: An Orthoregeneration Network (ON) Foundation Review

Orthoregeneration is defined as a solution for orthopedic conditions that harnesses the benefits of biology to improve healing, reduce pain, improve function, and optimally, provide an environment for tissue regeneration. Options include: drugs, surgical intervention, scaffolds, biologics as a product of cells, and physical and electro-magnetic stimuli. The goal of regenerative medicine is to enhance the healing of tissue after musculoskeletal injuries as both isolated treatment and adjunct to surgical management, using novel therapies to improve recovery and outcomes. Various orthopaedic biologics (orthobiologics) have been investigated for the treatment of pathology involving the knee, including symptomatic osteoarthritis and chondral injuries, as well as injuries to tendon, meniscus, and ligament, including the anterior cruciate ligament. Promising and established treatment modalities include hyaluronic acid (HA) in liquid or scaffold form; platelet-rich plasma (PRP); bone marrow aspirate (BMA) comprising mesenchymal stromal cells (MSCs), hematopoietic stem cells, endothelial progenitor cells, and growth factors; connective tissue progenitor cells (CTPs) including adipose-derived mesenchymal stem cells (AD-MSCs) and tendon-derived stem cells (TDSCs); matrix cell-based therapy including autologous chondrocytes or allograft; vitamin D; and fibrin clot. Future investigations should standardize solution preparations, because inconsistent results reported may be due to heterogeneity of HA, PRP, BMAC, or MSC preparations and regimens, which may inhibit meaningful comparison between studies to determine the true efficacy and safety for each treatment.

Niches that regulate stem cells and hematopoiesis in adult bone marrow

In mammals, hematopoietic stem cells (HSCs) engage in hematopoiesis throughout adult life within the bone marrow, where they produce the mature cells necessary to maintain blood cell counts and immune function. In the bone marrow and spleen, HSCs are sustained in perivascular niches (microenvironments) associated with sinusoidal blood vessels-specialized veins found only in hematopoietic tissues. Endothelial cells and perivascular leptin receptor⁺ stromal cells produce the known factors required to maintain HSCs and many restricted progenitors in the bone marrow. Various other cells synthesize factors that maintain other restricted progenitors or modulate HSC or niche function. Recent studies identified new markers that resolve some of the heterogeneity among stromal cells and refine the localization of restricted progenitor niches. Other recent studies identified ways in which niches regulate HSC function and hematopoiesis beyond growth factors. We summarize the current understanding of hematopoietic niches, review recent progress, and identify important unresolved questions.

Biased action of the CXCR4-targeting drug plerixafor is essential for its superior hematopoietic stem cell mobilization

Following the FDA-approval of the hematopoietic stem cell (HSC) mobilizer plerixafor, orally available and potent CXCR4 antagonists were pursued. One such proposition was AMD11070, which was orally active and had superior antagonism in vitro; however, it did not appear as effective for HSC mobilization in vivo. Here we show that while AMD11070 acts as a full antagonist, plerixafor acts biased by stimulating β-arrestin recruitment while fully antagonizing G protein. Consequently, while AMD11070 prevents the constitutive receptor internalization, plerixafor allows it and thereby decreases receptor expression. These findings are confirmed by the successful transfer of both ligands' binding sites and action to the related CXCR3 receptor. In vivo, plerixafor exhibits superior HSC mobilization associated with a dramatic reversal of the CXCL12 gradient across the bone marrow endothelium, which is not seen for AMD11070. We propose that the biased action of plerixafor is central for its superior therapeutic effect in HSC mobilization.

Hematopoietic Cell Kinase (HCK) Is a Player of the Crosstalk Between Hematopoietic Cells and Bone Marrow Niche Through CXCL12/CXCR4 Axis

The crosstalk between hematopoietic stem/progenitor cells (HSC), both normal and leukemic, and their neighboring bone marrow (BM) microenvironment (niche) creates a reciprocal dependency, a master regulator of biological process, and chemotherapy resistance. In acute myeloid leukemia (AML), leukemic stem/progenitor cells (LSC) anchored in the protective BM microenvironment, reprogram and transform this niche into a leukemia-supporting and chemoprotective environment. One most important player involved in this crosstalk are CXCL12, produced by the BM mesenchymal stromal cells, and its receptor CXCR4, present onto HSC. The downstream molecular mechanisms involved in CXCL12/CXCR4 axis have many targets, including the Src family members of non-receptor tyrosine kinase (SFK). We herein study the role of one SFK member, the Hematopoietic Cell Kinase (HCK), in CXCL12/CXCR4 pathway and its contribution to the AML pathogenesis. We verified that the inhibition of HCK severely impaired CXCL12-induced migration of leukemic cell lines and CD34 positive cells from AML patients bone marrow, through a disruption of the activation of CXCL12/CXCR4/PI3K/AKT and CXCL12/CXCR4/MAPK/ERK signaling, and by a decreased cytoskeleton dynamic through a lower rate of actin polymerization. We provide new insights into the key role of HCK in conferring a migratory advantage to leukemic cells thought CXCL12/CXCR4 axis. HCK represents an important protein of the main pathway involved in the crosstalk between HSC, and their surrounding milieu. Thus, HCK inhibition could represent a novel approach for the treatment of the acute myeloid leukemia.

Targeting mesenchymal stem cell therapy for severe pneumonia patients

Pneumonia is the inflammation of the lungs and it is the world's leading cause of death for children under 5 years of age. The latest coronavirus disease 2019 (COVID-19) virus is a prominent culprit to severe pneumonia. With the pandemic running rampant for the past year, more than 1590000 deaths has occurred worldwide up to December 2020 and are substantially attributable to severe pneumonia and induced cytokine storm. Effective therapeutic approaches in addition to the vaccines and drugs under development are hence greatly sought after. Therapies harnessing stem cells and their derivatives have been established by basic research for their versatile capacity to specifically inhibit inflammation due to pneumonia and prevent alveolar/pulmonary fibrosis while enhancing antibacterial/antiviral immunity, thus significantly alleviating the severe clinical conditions of pneumonia. In recent clinical trials, mesenchymal stem cells have shown effectiveness in reducing COVID-19-associated pneumonia morbidity and mortality; positioning these cells as worthy candidates for combating one of the greatest challenges of our time and shedding light on their prospects as a next-generation therapy to counter future challenges.

Mapping Distinct Bone Marrow Niche Populations and Their Differentiation Paths

The bone marrow microenvironment is composed of heterogeneous cell populations of non-hematopoietic cells with complex phenotypes and undefined trajectories of maturation. Among them, mesenchymal cells maintain the production of stromal, bone, fat, and cartilage cells. Resolving these unique cellular subsets within the bone marrow remains challenging. Here, we used single-cell RNA sequencing of non-hematopoietic bone marrow cells to define specific subpopulations. Furthermore, by combining computational prediction of the cell state hierarchy with the known expression of key transcription factors, we mapped differentiation paths to the osteocyte, chondrocyte, and adipocyte lineages. Finally, we validated our findings using lineage-specific reporter strains and targeted knockdowns. Our analysis reveals differentiation hierarchies for maturing stromal cells, determines key transcription factors along these trajectories, and provides an understanding of the complexity of the bone marrow microenvironment.

Local SDF-1α application enhances the therapeutic efficacy of BMSCs transplantation in osteoporotic bone healing

Bone defect healing is markedly impaired in osteoporotic patient due to poor bone regeneration ability. Stromal cell derived factor-1α (SDF-1α) plays a pivotal role in the repair of various injured tissues including bone. Here, we definite that SDF-1α hydrogels potentiates in vivo osteogenesis of bone marrow-derived stromal stem cells (BMSCs) in osteoporosis. The characteristics of rat primary BMSCs including superficial markers by flow cytometry and multi-lineage differentiation by induction were determined. At different time intervals, the release media from the SDF-1α-releasing hydrogels were collected to identificate SDF-1α exhibited a sustained release profile and maintained its bioactivity after release from the hydrogels to stimulate chemotaxis of BMSCs in a time dependent manner. Bilateral alveolar defects were operated in ovariectomized (OVX) rats and repaired with systemic BMSCs transplantation with or without the hydrogels. Local administration of SDF-1α significantly enhanced BMSCs recruitment and promoted more bone regeneration as well as the expression of OCN and Runx2 compared with the effect of BMSCs transplantation alone. Moreover, after BMSCs transplantation with SDF-1α delivery, macrophage polarization was promoted toward the M2 phenotype, that is identified as an important symbol in tissue regeneration process. Taken together, local SDF-1α application enhances the efficacy of BMSCs transplantation therapy in osteoporotic bone healing, suggesting clinical potential of SDF-1α to serve as a therapeutic drug target for osteoporosis treatment.

MicroRNA-141-3p Negatively Modulates SDF-1 Expression in Age-Dependent Pathophysiology of Human and Murine Bone Marrow Stromal Cells

Stromal cell-derived factor-1 (SDF-1 or CXCL12) is a cytokine secreted by cells including bone marrow stromal cells (BMSCs). SDF-1 plays a vital role in BMSC migration, survival, and differentiation. Our group previously reported the role of SDF-1 in osteogenic differentiation in vitro and bone formation in vivo; however, our understanding of the post-transcriptional regulatory mechanism of SDF-1 remains poor. MicroRNAs are small noncoding RNAs that post-transcriptionally regulate the messenger RNAs (mRNAs) of protein-coding genes. In this study, we aimed to investigate the impact of miR-141-3p on SDF-1 expression in BMSCs and its importance in the aging bone marrow (BM) microenvironment. Our data demonstrated that murine and human BMSCs expressed miR-141-3p that repressed SDF-1 gene expression at the functional level (luciferase reporter assay) by targeting the 3'-untranslated region of mRNA. We also found that transfection of miR-141-3p decreased osteogenic markers in human BMSCs. Our results demonstrate that miR-141-3p expression increases with age, while SDF-1 decreases in both the human and mouse BM niche. Taken together, these results support that miR-141-3p is a novel regulator of SDF-1 in bone cells and plays an important role in the age-dependent pathophysiology of murine and human BM niche.

The uterine-chemokine-brain axis: menstrual cycle-associated symptoms (MCAS) are in part mediated by CCL2, CCL5, CCL11, CXCL8 and CXCL10

OBJECTIVE

To examine associations between chemokines and menstrual cycle associated symptoms (MCAS).

METHODS

Forty-one women completed the Daily Record of Severity of Problems (DRSP) rating scale during 28 consecutive days of the menstrual cycle. MCAS is diagnosed when the total daily DRSP score during the menstrual cycle is > 0.666 percentile. We assayed plasma CCL2, CCL5, CCL11, CXCL8, CXCL10, EGF, IGF-1, and PAI-1 at days 7, 14, 21 and 28 of the menstrual cycle.

RESULTS

CCL2, CCL5, CCL11 and EGF are significantly higher in women with MCAS than in those without. Increased CCL2, CXCL10, CXCL8, CCL11 and CCL5 levels are significantly associated with DRSP scores while CCL2 is the most significant predictor explaining 39.6% of the variance. The sum of the neurotoxic chemokines CCL2, CCL11 and CCL5 is significantly associated with the DRSP score and depression, physiosomatic, breast-craving and anxiety symptoms. The impact of chemokines on MCAS symptoms differ between consecutive weeks of the menstrual cycle with CCL2 being the most important predictor of increased DRSP levels during the first two weeks, and CXCL10 or a combination of CCL2, CCL11 and CCL5 being the best predictors during week 3 and 4, respectively.

DISCUSSION

The novel case definition "MCAS" is externally validated by increased levels of uterus-associated chemokines and EGF. Those chemokines are involved in MCAS and are regulated by sex hormones and modulate endometrium functions and brain neuro-immune responses, which may underpin MCAS symptoms. As such, uterine-related chemokines may link the uterus with brain functions via a putative uterine-chemokine-brain axis.

Engineered niches support the development of human dendritic cells in humanized mice

Classical dendritic cells (cDCs) are rare sentinel cells specialized in the regulation of adaptive immunity. Modeling cDC development is crucial to study cDCs and harness their therapeutic potential. Here we address whether cDCs could differentiate in response to trophic cues delivered by mesenchymal components of the hematopoietic niche. We find that mesenchymal stromal cells engineered to express membrane-bound FLT3L and stem cell factor (SCF) together with CXCL12 induce the specification of human cDCs from CD34⁺ hematopoietic stem and progenitor cells (HSPCs). Engraftment of engineered mesenchymal stromal cells (eMSCs) together with CD34⁺ HSPCs creates an in vivo synthetic niche in the dermis of immunodeficient mice driving the differentiation of cDCs and CD123⁺AXL⁺CD327⁺ pre/AS-DCs. cDC2s generated in vivo display higher levels of resemblance with human blood cDCs unattained by in vitro-generated subsets. Altogether, eMSCs provide a unique platform recapitulating the full spectrum of cDC subsets enabling their functional characterization in vivo.

Hematopoiesis and Cardiovascular Disease

A central feature of atherosclerosis, the most prevalent chronic vascular disease and root cause of myocardial infarction and stroke, is leukocyte accumulation in the arterial wall. These crucial immune cells are produced in specialized niches in the bone marrow, where a complex cell network orchestrates their production and release. A growing body of clinical studies has documented a correlation between leukocyte numbers and cardiovascular disease risk. Understanding how leukocytes are produced and how they contribute to atherosclerosis and its complications is, therefore, critical to understanding and treating the disease. In this review, we focus on the key cells and products that regulate hematopoiesis under homeostatic conditions, during atherosclerosis and after myocardial infarction.

In Vitro Dynamic Phenotyping for Testing Novel Mobilizing Agents

A new method to quantify the influence of mobilization agents on the dynamics of human hematopoietic stem and progenitor cells (HSPC) is introduced. Different from the microscopy-based high-content screening relying on multiple staining, machine learning, and molecular-level perturbation, the proposed method sheds light on the "dynamics" of HSPC in the presence of extrinsic factors, including SDF1α and mobilization agents. A well-defined model of the bone marrow niche is fabricated by the deposition of planar lipid membranes on glass slides (called supported membranes) displaying ligand molecules at precisely controlled surface densities. The dynamics of human HSPC, CD34⁺ cells from umbilical cord blood or peripheral blood, are monitored by time-lapse, live cell imaging with a standard phase-contrast microscopy or a specially designed microinterferometry in the absence or presence of mobilization agents. After extracting the contour of each cell, one can analyze the dynamics of cell "shapes" step-by-step, yielding various levels of information ranging from the principal mode of deformation, the persistence of deformation patterns, and the energy consumption by HSPC in the absence and presence of mobilization agents. Moreover, by tracking the migration trajectories of HSPC, one can gain insight how mobilization agents influence the "motion" of HSPC. As these readouts can be connected to a theoretical model, this strategy enables one to classify the influence of not only mobilization agents but also target-specific inhibitors or other treatments in quantitative indices.

Concepts of GPCR-controlled navigation in the immune system

G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.

Chronic Intermittent Hypobaric Hypoxia Enhances Bone Fracture Healing

The effect of chronic intermittent hypobaric hypoxia (CIHH) on bone fracture healing is not elucidated. The present study aimed to investigate the role of CIHH on bone fracture healing and the mechanism. The Sprague-Dawley rats were randomly divided into the CIHH group and control group and monitored for 2, 4, or 8 weeks after femoral fracture surgery. Bone healing efficiency was significantly increased in the CIHH group as evidenced by higher high-density bone volume fractions, higher bone mineral density, higher maximum force, and higher stiffness. Histologically, the CIHH group exhibited superior bone formation, endochondral ossification, and angiogenic ability compared with the control group. The expression of HIF-1α and its downstream signaling proteins VEGF, SDF-1/CXCR4 axis were increased by the CIHH treatment. Moreover, the expression of RUNX2, osterix, and type I collagen in the callus tissues were also up-regulated in the CIHH group. In conclusion, our study demonstrated that CIHH treatment improves fracture healing, increases bone mineral density, and increases bone strength via the activation of HIF-1α and bone production-related genes.

Effects of Stromal Cell-Derived Factor-1α Secreted in Degenerative Intervertebral Disc on Activation and Recruitment of Nucleus Pulposus-Derived Stem Cells

Stromal cell-derived factor-1α (SDF-1α) plays a significant role in mobilizing and recruiting mesenchymal stem cells (MSCs) to the sites of injury. This study investigated the potential of SDF-1α released in the degenerative intervertebral disc (IVD) to activate and recruit endogenous nucleus pulposus-derived stem cells (NPSCs) for regeneration in situ. We found SDF-1α was highly expressed and secreted by the native disc cells when cultured in the proinflammatory mediators in vitro mimicking the degenerative settings. Immunohistochemical staining also showed that the expression level of SDF-1α was significantly higher in the degenerative group compared to that in the normal group. In addition to enhancement of viability, SDF-1α significantly increased the number of NPSCs migrating into the center of the nucleotomized bovine IVD ex vivo. After the systemic delivery of exogenous PKH26-labelled NPSCs into the rats in vivo, there was a significant difference in the distribution of the migrated cells between the normal and the degenerative IVDs, which might be caused by the different expression levels of SDF-1α. However, blocking CXC chemokine receptor 4 (CXCR4) with AMD3100 effectively abrogated SDF-1α-stimulated proliferation and migration. Taken together, SDF-1α may be a key chemoattractant that is highly produced in response to the degenerative changes, which can be used to enhance the proliferation and recruitment of endogenous stem cells into the IVDs. These findings may be of importance for understanding IVD regenerative mechanisms and development of regenerative strategies in situ for IVD degeneration.

Noninvasive application of mesenchymal stem cell spheres derived from hESC accelerates wound healing in a CXCL12-CXCR4 axis-dependent manner

Mesenchymal stem cells (MSC) derived from adult tissues effectively promote wound healing. However, MSC quality varies, and the quantity of MSC is limited, as MSC are acquired through donations. Moreover, the survival and functioning of dissociated MSC delivered to an inflammatory lesion are subject to challenges.

Methods: Here, spheres (EMSC_Sp) generated from human embryonic stem cell-derived MSC (EMSC) were directly dropped onto excised wounds in mice; the effects of EMSC_Sp were compared to those of dissociated EMSC (EMSC_Diss). Following transplantation, we measured the extent of wound closure, dissected the histological features of the wounds, determined transcriptomic changes in cells isolated from the treated and control wounds, and evaluated the molecular mechanism of the effects of EMSC.

Results: The application of EMSC_Sp onto murine dermal wounds substantially increased survival and efficacy of EMSC compared to the topical application of EMSC_Diss. RNA sequencing (RNA-Seq) of cells isolated from the wounds highlighted the involvement of CXCL12-CXCR4 signaling in the effects of EMSC_Sp, which was verified in EMSC via CXCL12 knockdown and in target cells (vascular endothelial cells, epithelial keratinocytes, and macrophages) via CXCR4 inhibition. Finally, we enhanced the biosafety of EMSC_Sp by engineering cells with an inducible suicide gene.

Conclusions: Together, these data suggest the topical application of EMSC_Sp as an unlimited, quality-assured, safe, and noninvasive therapy for wound healing and the CXCL12-CXCR4 axis as a key player in this treatment.

Structural and Biological Characterizations of Novel High-Affinity Fluorescent Probes with Overlapped and Distinctive Binding Regions on CXCR4

CXC-type chemokine receptor 4 (CXCR4) is well known as a co-receptor for cellular entry and infection of human immunodeficiency virus type 1 (HIV-1). As an important member of the G protein-coupled receptor (GPCR) family, CXCR4 also mediates a variety of cellular processes and functions, such as cell chemotaxis, proliferation, and calcium signal transductions. Identification and characterization of molecular ligands or probes of CXCR4 have been an intensive area of investigations as such ligands or probes are of significant clinical values for the studies and treatments of HIV-1 infection and other human diseases mediated by the receptor. The crystal structures of CXCR4 in complex with different ligands have revealed two distinctive binding regions or subpockets. Thus, understanding the interactions of diverse ligands with these distinctive CXCR4 binding regions has become vital for elucidating the relationship between binding modes and biological mechanisms of ligand actions. Peptidic CVX15 is the only ligand that has been validated to bind one of these distinctive binding regions (or so called the major subpocket) of CXCR4. Therefore, in this study, we developed an efficient probe system including two high-affinity peptidic fluorescent probes, designated as FITC-CVX15 and FITC-DV1, with the aim of targeting distinctive CXCR4 subpockets. We conducted rational design and chemical characterization of the two CXCR4-specific probes and examined their application in biological experiments including competitive binding assays, flow cytometry analysis, and confocal imaging. Especially these two probes were applied in parallel CXCR4 competitive binding assays to detect and analyze potential binding modes of diverse CXCR4 ligands, together with molecular docking and simulations. Our results have indicated that these peptidic fluorescent probe systems provide novel ligand detecting tools, as well as present a new approach for analyzing distinctive binding modes of diverse CXCR4 ligands.

Bone Marrow-Derived CD44+ Cells Migrate to Tissue-Engineered Constructs via SDF-1/CXCR4-JNK Pathway and Aid Bone Repair

Background and Aims

Host-derived cells play crucial roles in the regeneration process of tissue-engineered constructs (TECs) during the treatment of large segmental bone defects (LSBDs). However, their identity, source, and cell recruitment mechanisms remain elusive.

Methods

A complex model was created using mice by combining methods of GFP⁺ bone marrow transplantation (GFP-BMT), parabiosis (GFP⁺-BMT and wild-type mice), and femoral LSBD, followed by implantation of TECs or DBM scaffolds. Postoperatively, the migration of host BM cells was detected by animal imaging and immunofluorescent staining. Bone repair was evaluated by micro-CT. Signaling pathway repressors including AMD3100 and SP600125 associated with the migration of BM CD44⁺ cells were further investigated. In vitro, transwell migration and western-blotting assays were performed to verify the related signaling pathway. In vivo, the importance of the SDF-1/CXCR4-JNK pathway was validated by ELISA, fluorescence-activated cell sorting (FACS), immunofluorescent staining, and RT-PCR.

Results

First, we found that host cells recruited to facilitate TEC-mediated bone repair were derived from bone marrow and most of them express CD44, indicating the significance of CD44 in the migration of bone marrow cells towards donor MSCs. Then, the predominant roles of SDF-1/CXCR4 and downstream JNK in the migration of BM CD44⁺ cells towards TECs were demonstrated.

Conclusion

Together, we demonstrated that during bone repair promoted by TECs, BM-derived CD44⁺ cells were essential and their migration towards TECs could be regulated by the SDF-1/CXCR4-JNK signaling pathway.

Clathrin Heavy Chain Knockdown Impacts CXCR4 Signaling and Post-translational Modification

Recent research has implicated endocytic pathways as important regulators of receptor signaling. However, the role of endocytosis in regulating chemokine CXC receptor 4 (CXCR4) signaling remains largely unknown. In the present work we systematically investigate the impact of clathrin knockdown on CXCR4 internalization, signaling, and receptor post-translational modification. Inhibition of clathrin-mediated endocytosis (CME) significantly reduced CXCR4 internalization. In contrast to other receptors, clathrin knockdown increased CXCL12-dependent ERK1/2 signaling. Simultaneous inhibition of CME and lipid raft disruption abrogated this increase in ERK1/2 phosphorylation suggesting that endocytic pathway compensation can influence signaling outcomes. Interestingly, using an antibody sensitive to CXCR4 post-translational modification, we also found that our ability to detect CXCR4 was drastically reduced upon clathrin knockdown. We hypothesize that this effect was due to differences in receptor post-translational modification as total CXCR4 protein and mRNA levels were unchanged. Lastly, we show that clathrin knockdown reduced CXCL12-dependent cell migration irrespective of an observed increase in ERK1/2 phosphorylation. Altogether, this work supports a complex model by which modulation of endocytosis affects not only receptor signaling and internalization but also receptor post-translational modification.

Mechanisms of extramedullary relapse in acute lymphoblastic leukemia: Reconciling biological concepts and clinical issues

Long-term survival rates in childhood acute lymphoblastic leukemia (ALL) are currently above 85% due to huge improvements in treatment. However, 15-20% of children still experience relapses. Relapses can either occur in the bone marrow or at extramedullary sites, such as gonads or the central nervous system (CNS), formerly referred to as ALL-blast sanctuaries. The reason why ALL cells migrate to and stay in these sites is still unclear. In this review, we have attempted to assemble the evidence concerning the microenvironmental factors that could explain why ALL cells reside in such sites. We present criteria that make extramedullary leukemia niches and solid tumor metastatic niches comparable. Indeed, considering extramedullary leukemias as metastases could be a useful approach for proposing more effective treatments. In this context, we conclude with several examples of potential niche-based therapies which could be successfully added to current treatments of ALL.

Limiting tumor seeding as a therapeutic approach for metastatic disease

Here we propose that therapeutic targeting of circulating tumor cells (CTCs), which are widely understood to be the seeds of metastasis, would represent an effective strategy towards limiting numerical expansion of secondary lesions and containing overall tumor burden in cancer patients. However, the molecular mediators of tumor seeding have not been well characterized. This is in part due to the limited number of pre-clinical in vivo approaches that appropriately interrogate the mechanisms by which cancer cells home to arresting organs. It is critical that we continue to investigate the mediators of tumor seeding as it is evident that the ability of CTCs to colonize in distant sites is what drives disease progression even after the primary tumor has been ablated by local modalities. In addition to slowing disease progression, containing metastatic spread by impeding tumor cell seeding may also provide a clinical benefit by increasing the duration of the residence of CTCs in systemic circulation thereby increasing their exposure to pharmacological agents commonly used in the treatment of patients such as chemotherapy and immunotherapies. In this review we will examine the current state of knowledge about the mechanisms of tumor cells seeding as well as explore how targeting this stage of metastatic spreading may provide therapeutic benefit to patients with advanced disease.

The Role of Orthobiologics in the Management of Osteoarthritis and Focal Cartilage Defects

Individuals with osteoarthritis have a diminished quality of life, and the condition is a major cause of disability. Newer biologic treatments have been developed that are believed to modify disease progression. These predominantly include hyaluronic acid, platelet-rich plasma, bone marrow aspirate concentrate, and adipose-derived mesenchymal stem cells. There is conflicting evidence regarding the use of orthobiologics for osteoarthritis and for focal chondral defects, although most studies indicate that injections of biologics are safe and without significant adverse effects. [Orthopedics. 2019; 42(2):66-73.].

CXCR4-Overexpressing Umbilical Cord Mesenchymal Stem Cells Enhance Protection against Radiation-Induced Lung Injury

Less quantity of transplanted mesenchymal stem cells (MSCs) influences the therapeutic effects on radiation-induced lung injury (RILI). Previous studies have demonstrated that MSCs overexpressing Chemokine (C-X-C motif) receptor 4 (CXCR4) could increase the quantity of transplanted cells to local tissues. In the present study, we conducted overexpressing CXCR4 human umbilical cord mesenchymal stem cell (HUMSC) therapy for RILI. C57BL mice received single dose of thoracic irradiation with 13 Gy of X-rays and then were administered saline, control HUMSCs, or CXCR4-overexpressing HUMSCs via tail vein. Transfection with CXCR4 enhanced the quantity of transplanted HUMSCs in the radiation-induced injured lung tissues. CXCR4-overexpressing HUMSCs not only improved histopathological changes but also decreased the radiation-induced expression of SDF-1, TGF-β1, α-SMA, and collagen I and inhibited the radiation-induced decreased expression of E-cadherin. Transplanted CXCR4-overexpressing HUMSCs also could express pro-SP-C, indicated adopting the feature of ATII. These finding suggests that CXCR4-overexpressing HUMSCs enhance the protection against RILI and may be a promising strategy for RILI treatment.

Modelling of the SDF-1/CXCR4 regulated in vivo homing of therapeutic mesenchymal stem/stromal cells in mice

Background

Mesenchymal stem/stromal cells (MSCs) are a promising tool for cell-based therapies in the treatment of tissue injury. The stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) axis plays a significant role in directing MSC homing to sites of injury. However in vivo MSC distribution following intravenous transplantation remains poorly understood, potentially hampering the precise prediction and evaluation of therapeutic efficacy.

Methods

A murine model of partial ischemia/reperfusion (I/R) is used to induce liver injury, increase the hepatic levels of SDF-1, and study in vivo MSC distribution. Hypoxia-preconditioning increases the expression of CXCR4 in human bone marrow-derived MSCs. Quantitative assays for human DNA using droplet digital PCR (ddPCR) allow us to examine the in vivo kinetics of intravenously infused human MSCs in mouse blood and liver. A mathematical model-based system is developed to characterize in vivo homing of human MSCs in mouse models with SDF-1 levels in liver and CXCR4 expression on the transfused MSCs. The model is calibrated to experimental data to provide novel estimates of relevant parameter values.

Results

Images of immunohistochemistry for SDF-1 in the mouse liver with I/R injury show a significantly higher SDF-1 level in the I/R injured liver than that in the control. Correspondingly, the ddPCR results illustrate a higher MSC concentration in the I/R injured liver than the normal liver. CXCR4 is overexpressed in hypoxia-preconditioned MSCs. An increased number of hypoxia-preconditioned MSCs in the I/R injured liver is observed from the ddPCR results. The model simulations align with the experimental data of control and hypoxia-preconditioned human MSC distribution in normal and injured mouse livers, and accurately predict the experimental outcomes with different MSC doses.

Discussion

The modelling results suggest that SDF-1 in organs is an effective in vivo attractant for MSCs through the SDF-1/CXCR4 axis and reveal the significance of the SDF-1/CXCR4 chemotaxis on in vivo homing of MSCs. This in vivo modelling approach allows qualitative characterization and prediction of the MSC homing to normal and injured organs on the basis of clinically accessible variables, such as the MSC dose and SDF-1 concentration in blood. This model could also be adapted to abnormal conditions and/or other types of circulating cells to predict in vivo homing patterns.

MiR-9 Controls Chemotactic Activity of Cord Blood CD34⁺ Cells by Repressing CXCR4 Expression

Improved approaches for promoting umbilical cord blood (CB) hematopoietic stem cell (HSC) homing are clinically important to enhance engraftment of CB-HSCs. Clinical transplantation of CB-HSCs is used to treat a wide range of disorders. However, an improved understanding of HSC chemotaxis is needed for facilitation of the engraftment process. We found that ectopic overexpression of miR-9 and antisense-miR-9 respectively down- and up-regulated C-X-C chemokine receptor type 4 (CXCR4) expression in CB-CD34⁺ cells as well as in 293T and TF-1 cell lines. Since CXCR4 is a specific receptor for the stromal cell derived factor-1 (SDF-1) chemotactic factor, we investigated whether sense miR-9 and antisense miR-9 influenced CXCR4-mediated chemotactic mobility of primary CB CD34⁺ cells and TF-1 cells. Ectopic overexpression of sense miR-9 and antisense miR-9 respectively down- and up-regulated SDF-1-mediated chemotactic cell mobility. To our knowledge, this study is the first to report that miR-9 may play a role in regulating CXCR4 expression and SDF-1-mediated chemotactic activity of CB CD34⁺ cells.

CXCR4, the master regulator of neutrophil trafficking in homeostasis and disease

BACKGROUND

Chemokines play a critical role in orchestrating the distribution and trafficking of neutrophils in homeostasis and disease.

RESULTS

The CXCR4/CXCL12 chemokine axis has been identified as a central regulator of these processes.

CONCLUSION

In this review, we focus on the role of CXCR4/CXCL12 chemokine axis in regulating neutrophil release from the bone marrow and the trafficking of senescent neutrophils back to the bone marrow for clearance under homeostasis and disease. We also discuss the role of CXCR4 in fine-tuning neutrophil responses in the context of inflammation.