Cardiomyocyte-derived CXCL12 is not involved in cardiogenesis but plays a crucial role in myocardial infarction

One Endothelium-Targeted Combined Nucleic Acid Delivery System for Myocardial Infarction Therapy

Acute myocardial infarction (MI) and ischemic heart disease are the leading causes of heart failure and mortality. Currently, research on MI treatment is focused on angiogenic and anti-inflammatory therapies. Although endothelial cells (ECs) are critical for triggering inflammation and angiogenesis, no approach has targeted them for the treatment of MI. In this study, we proposed a nonviral combined nucleic acid delivery system consisting of an EC-specific polycation (CRPPR-grafted ethanolamine-modified poly(glycidyl methacrylate), CPC) that can efficiently codeliver siR-ICAM1 and pCXCL12 for the treatment of MI. Animals treated with the combination therapy exhibited better cardiac function than those treated with each nucleic acid alone. In particular, the combination therapy of CPC/siR-ICAM1 and CPC/pCXCL12 significantly improved cardiac systolic function, anti-inflammatory responses, and angiogenesis compared to the control group. In conclusion, CPC-based combined gene delivery systems show impressive performance in the treatment of MI and provide a programmed strategy for the development of codelivery systems for various EC-related diseases.

CXCL12/CXCR4: An amazing challenge and opportunity in the fight against fibrosis

Fibrosis is a pathological process caused by abnormal wound healing response, which often leads to excessive deposition of extracellular matrix, distortion of organ architecture, and loss of organ function. Aging is an important risk factor for the development of organ fibrosis. C-X-C receptor 4 (CXCR4) is the predominant chemokine receptor on fibrocytes, C-X-C motif ligand 12 (CXCL12) is the only ligand of CXCR4. Accumulated evidence have confirmed that CXCL12/CXCR4 can be involved in multiple pathological mechanisms in fibrosis, such as inflammation, immunity, epithelial-mesenchymal transition, and angiogenesis. In addition, CXCL12/CXCR4 have also been shown to improve fibrosis levels in many organs including the heart, liver, lung and kidney; thus, they are promising targets for anti-fibrotic therapy. Notably, inhibitors of CXCL12 or CXCR4 also play an important role in various fibrosis-related diseases. In summary, this review systematically summarizes the role of CXCL12/CXCR4 in fibrosis, and this information is of great significance for understanding CXCL12/CXCR4. This will also contribute to the design of further studies related to CXCL12/CXCR4 and fibrosis, and shed light on potential therapies for fibrosis.

Left ventricular assist device bioinformatics identify possible hubgenes and regulatory networks involved in the myocardium of patients with left ventricular assist device

Objective

The aim of this study was to clarify the changes of myocardial gene expression profile after left ventricular assist device (LVAD) implantation and the related molecular biological significance.

Methods

A thorough bioinformatic analysis to evaluate the changes in gene expression profile in patients pre-LVAD and post-LVAD was conducted. Four relevant gene expression datasets—GSE430, GSE974, GSE21610, and GSE52601 from Gene Expression Omnibus (GEO) database were downloaded. Analysis of GEO2R, Gene Ontology (GO), protein-protein interaction (PPI) were used to determine differentially expressed genes (DEGs) and their function, respectively.

Results

A total of 37 DEGs were identified, including 26 down-regulated and 11 up-regulated genes. The molecular function of DEGs were enriched in “cytokine activity,” “neurotransmitter binding,” “receptor ligand activity.” The gene set enrichment analysis (GSEA) revealed an overall marked increase of neutrophil degranulation signaling, closely correlated with the G protein coupled receptor (GPCR)—ligand binding process after LVAD assistance. 16 hubgenes in these DEGs were further selected and the biological process involved is mainly related to positive regulation of leukocyte chemotaxis mediated by chemokines.

Conclusion

Inflammatory signaling pathway is crucial for the pathophysiology after LVAD implantation. Chemokines mediate cardiac inflammatory response and tissue remodeling after LVAD implantation through GPCR—ligand binding.

The potential role of circulating exosomes in protecting myocardial injury in acute myocardial infarction via regulating miR-190a-3p/CXCR4/CXCL12 pathway

Exosomes of different origins have been found to be protective against ischemic-induced myocardial injury. This study examined the protective effects of circulating exosomes in the mice model of acute myocardial infarction (AMI) and explored the underlying molecular mechanisms. The effects of exosomes on myocardial injury were assessed in the AMI mice model. The in vivo studies showed that circulating exosomes reduced the infarcted size, improved the morphology of heart tissues and also reduced apoptosis of the heart tissues. In addition, the model mice showed an increase in the CD34 + /VEGFR2 + cell population and CD31, CXCR4 and CXCL12 expression after exosomes treatment. MiR-190a-3p was significantly down-regulated in the exosomes derived from the culture medium of hypoxia-treated human cardiomyocytes (HCMs). Further analysis revealed that miR-190a-3p could physically interact with CXCR4/CXCL12 by targeting the respective 3'UTRs. These exosomes could up-regulated CXCR4 and CXCL12 expression in the EPCs; in addition, miR-190a-3p mimics repressed CXCR4/CXCL12 expression in EPCs, while its inhibitor had opposite effects. The in vitro functional assays showed that miR-190a-3p overexpression suppressed the cell viability, proliferation, migration, adhesion and tube formation of EPCs; while miR-190a-3p inhibitor had the opposite effects; exosomes derived from the culture medium of hypoxia-treated HCMs exhibited similar actions of miR-190a-3p inhibitor. Moreover, miR-190a-3p was down-regulated in exosomes from serum in the AMI group when compared to that from sham group. Treatment with exosomes from serum in the AMI group promoted cell proliferation, migration, adhesion and tube formation of EPCs when compared to that in the sham group. More importantly, IT1t attenuated the enhanced effects of miR-190a-3p inhibition on EPC proliferation, migration, adhesion and tube formation. In conclusion, circulating exosomes exerted protective effects on myocardial injury in the AMI mice model, and down-regulation of miR-190a-3p in the circulating exosomes may exert protective effects against myocardial injury. Hypoxia induced the downregulation of miR-190a-3p in the culture medium of HCMs, and the mechanistic investigations indicated that exosomes of hypoxia-conditioned HCM culture medium promoted the cell viability, proliferation, migration, adhesion and tube formation of EPCs via regulating miR-190a-3p/CXCR4/CXCL12 pathway.

The Role of CXC Chemokines in Cardiovascular Diseases

Cardiovascular disease (CVD) is a class of diseases with high disability and mortality rates. In the elderly population, the incidence of cardiovascular disease is increasing annually. Between 1990 and 2016, the age-standardised prevalence of CVD in China significantly increased by 14.7%, and the number of cardiovascular disease deaths increased from 2.51 million to 3.97 million. Much research has indicated that cardiovascular disease is closely related to inflammation, immunity, injury and repair. Chemokines, which induce directed chemotaxis of reactive cells, are divided into four subfamilies: CXC, CC, CX3C, and XC. As cytokines, CXC chemokines are similarly involved in inflammation, immunity, injury, and repair and play a role in many cardiovascular diseases, such as atherosclerosis, myocardial infarction, cardiac ischaemia-reperfusion injury, hypertension, aortic aneurysm, cardiac fibrosis, postcardiac rejection, and atrial fibrillation. Here, we explored the relationship between the chemokine CXC subset and cardiovascular disease and its mechanism of action with the goal of further understanding the onset of cardiovascular disease.

Resveratrol against Cardiac Fibrosis: Research Progress in Experimental Animal Models

Cardiac fibrosis is a heterogeneous disease, which is characterized by abundant proliferation of interstitial collagen, disordered arrangement, collagen network reconstruction, increased cardiac stiffness, and decreased systolic and diastolic functions, consequently developing into cardiac insufficiency. With several factors participating in and regulating the occurrence and development of cardiac fibrosis, a complex molecular mechanism underlies the disease. Moreover, cardiac fibrosis is closely related to hypertension, myocardial infarction, viral myocarditis, atherosclerosis, and diabetes, which can lead to serious complications such as heart failure, arrhythmia, and sudden cardiac death, thus seriously threatening human life and health. Resveratrol, with the chemical name 3,5,4'-trihydroxy-trans-stilbene, is a polyphenol abundantly present in grapes and red wine. It is known to prevent the occurrence and development of cardiovascular diseases. In addition, it may resist cardiac fibrosis through a variety of growth factors, cytokines, and several cell signaling pathways, thus exerting a protective effect on the heart.

Smooth Muscle Specific Ablation of CXCL12 in Mice Downregulates CXCR7 Associated with Defective Coronary Arteries and Cardiac Hypertrophy

The chemokine CXCL12 plays a fundamental role in cardiovascular development, cell trafficking, and myocardial repair. Human genome-wide association studies even have identified novel loci downstream of the CXCL12 gene locus associated with coronary artery disease and myocardial infarction. Nevertheless, cell and tissue specific effects of CXCL12 are barely understood. Since we detected high expression of CXCL12 in smooth muscle (SM) cells, we generated a SM22-alpha-Cre driven mouse model to ablate CXCL12 (SM-CXCL12^−/−). SM-CXCL12^−/− mice revealed high embryonic lethality (50%) with developmental defects, including aberrant topology of coronary arteries. Postnatally, SM-CXCL12^−/− mice developed severe cardiac hypertrophy associated with fibrosis, apoptotic cell death, impaired heart function, and severe coronary vascular defects characterized by thinned and dilated arteries. Transcriptome analyses showed specific upregulation of pathways associated with hypertrophic cardiomyopathy, collagen protein network, heart-related proteoglycans, and downregulation of the M2 macrophage modulators. CXCL12 mutants showed endothelial downregulation of the CXCL12 co-receptor CXCR7. Treatment of SM-CXCL12^−/− mice with the CXCR7 agonist TC14012 attenuated cardiac hypertrophy associated with increased pERK signaling. Our data suggest a critical role of smooth muscle-specific CXCL12 in arterial development, vessel maturation, and cardiac hypertrophy. Pharmacological stimulation of CXCR7 might be a promising target to attenuate adverse hypertrophic remodeling.

Comparative Effect of MSC Secretome to MSC Co-culture on Cardiomyocyte Gene Expression Under Hypoxic Conditions in vitro

Introduction

Despite major leaps in regenerative medicine, the regeneration of cardiomyocytes after ischemic conditions remains to elucidate. It is crucial to understand hypoxia induced cellular mechanisms to provide advanced treatment options, including the use of stem cell paracrine factors for myocardial regeneration.

Materials and Methods

In this study, the regenerative potential of hypoxic human cardiomyocytes (group Hyp-CMC) in vitro was evaluated when co-cultured with human bone-marrow derived MSC (group Hyp-CMC-MSC) or stimulated with the secretome of MSC (group Hyp-CMC-S_MSC). The secretome of normoxic MSC and CMC, and the hypoxic CMC was analyzed with a cytokine panel. Gene expression changes of HIF-1α, proliferation marker Ki-67 and cytokinesis marker RhoA over different reoxygenation time periods of 4, 8, 24, 48, and 72 h were analyzed in comparison to normoxic CMC and MSC. Further, the proinflammatory cytokine IL-18 protein expression change, metabolic activity and proliferation was assessed in all experimental setups.

Results and Conclusion

HIF-1α was persistently overexpressed in Hyp-CMC-S_MSC as compared to Hyp-CMC (except at 72 h). Hyp-CMC-MSC showed a weaker HIF-1α expression than Hyp-CMC-S_MSC in most tested time points, except after 8 h. The Ki-67 expression showed the strongest upregulation in Hyp-CMC after 24 and 48 h incubation, then returned to baseline level, while a temporary increase in Ki-67 expression in Hyp-CMC-MSC at 4 and 8 h and at 48 h in Hyp-CMC-S_MSC could be observed. RhoA was increased in normoxic MSCs and in Hyp-CMC-S_MSC over time, but not in Hyp-CMC-MSC. A temporary increase in IL-18 protein expression was detected in Hyp-CMC-SMSC and Hyp-CMC. Our study demonstrates timely dynamic changes in expression of different ischemia and regeneration-related genes of CMCs, depending from the culture condition, with stronger expression of HIF-1α, RhoA and IL-18 if the hypoxic CMC were subjected to the secretome of MSCs.

Fingolimod Attenuates Lung Injury and Cardiac Dysfunction after Traumatic Brain Injury

Acute lung injury (ALI) and cardiac dysfunction are common in traumatic brain injury (TBI) patients and always indicate poor outcomes. Inflammatory responses play important roles in TBI-induced cardiac and pulmonary damage. Fingolimod, an immunomodulatory agent, alleviates brain edema, restores the integrity of the blood-brain barrier (BBB), and improves functional deficits by inhibiting multiple inflammatory responses. Fingolimod (1 mg/kg) was injected intraperitoneally at 2 h after the controlled cortical impact (CCI) model was established in adult male mice. The concentration of inflammatory cytokines in the lung and heart after TBI was measured with a cytokine array. The lung wet/dry weight ratio and Evans blue dye leakage were used to quantify pulmonary edema and capillary leakage. Immunofluorescence, electron microscopy, and echocardiographic examination were used to assess the pathology and functional deficits in hearts. We found that TBI caused significant heart and lung damage. The administration of fingolimod significantly reduced the elevated inflammatory cytokine production, neutrophil infiltration, the leakage of protein in bronchoalveolar lavage fluid (BALF), and the wet/dry weight ratio in lung tissue at 3 days after TBI. In addition, fingolimod treatment also alleviated the inflammatory response in the heart; decreased cardiac apoptosis, fibrosis, and histological microstructural changes; and improved cardiac function from 3 days after TBI and maintained it for 30 days after TBI as measured by echocardiography. These results suggest that TBI resulted in significant cardiac and pulmonary damage accompanied by significant inflammatory responses in heart and lung tissue. Fingolimod treatment reduced the inflammatory response and alleviated TBI-induced lung and heart injury.

Chemokines in Myocardial Infarction

In the infarcted myocardium, cardiomyocyte necrosis triggers an intense inflammatory reaction that not only is critical for cardiac repair, but also contributes to adverse remodeling and to the pathogenesis of heart failure. Both CC and CXC chemokines are markedly induced in the infarcted heart, bind to endothelial glycosaminoglycans, and regulate leukocyte trafficking and function. ELR+ CXC chemokines (such as CXCL8) control neutrophil infiltration, whereas CC chemokines (such as CCL2) mediate recruitment of mononuclear cells. Moreover, some members of the chemokine family (such as CXCL10 and CXCL12) may mediate leukocyte-independent actions, directly modulating fibroblast and vascular cell function. This review manuscript discusses our understanding of the role of the chemokines in regulation of injury, repair, and remodeling following myocardial infarction. Although several chemokines may be promising therapeutic targets in patients with myocardial infarction, clinical implementation of chemokine-based therapeutics is hampered by the broad effects of the chemokines in both injury and repair.

Platelet-derived β2m regulates age related monocyte/macrophage functions

Platelets have central roles in both immune responses and development. Stimulated platelets express leukocyte adhesion molecules and release numerous immune modulatory factors that recruit and activate leukocytes, both at the sites of activation and distantly. Monocytes are innate immune cells with dynamic immune modulatory functions that change during the aging process, a phenomenon termed “inflammaging”. We have previously shown that platelets are a major source of plasma beta-2 microglobulin (β2M) and that β2M induced a monocyte pro-inflammatory phenotype. Plasma β2M increases with age and is a pro-aging factor. We hypothesized that platelet derived β2M regulates monocyte phenotypes in the context of aging. Using wild-type (WT) and platelet specific β2M knockout mice (Plt-β2M^-/-) mice, we found that plasma β2M increased with age and correlated with increased circulating Ly6C^Hi monocytes. However, aged Plt-β2M^-/- mice had significantly fewer Ly6C^Hi monocytes compared to WT mice. Quantitative real-time PCR of circulating monocytes showed that WT mouse monocytes were more “pro-inflammatory” with age, while Plt-β2M^-/- derived monocytes adopted a “pro-reparative” phenotype. Older Plt-β2M^-/- mice had a significant decline in heart function compared to age matched WT mice, as well as increased cardiac fibrosis and pro-fibrotic markers. These data suggest that platelet-derived β2M regulates age associated monocyte polarization, and a loss of platelet derived β2M shifted monocytes and macrophages to a pro-reparative phenotype and increased pro-fibrotic cardiac responses. Platelet regulation of monocyte phenotypes via β2M may maintain a balance between inflammatory and reparative signals that affects age related physiologic outcomes.

Stromal cell-derived factor-1α signals via the endothelium to protect the heart against ischaemia-reperfusion injury

AIMS

The chemokine stromal derived factor-1α (SDF-1α) is known to protect the heart acutely from ischaemia-reperfusion injury via its cognate receptor, CXCR4. However, the timing and cellular location of this effect, remains controversial.

METHODS AND RESULTS

Wild type male and female mice were subjected to 40 min LAD territory ischaemia in vivo and injected with either saline (control) or SDF-1α prior to 2 h reperfusion. Infarct size as a proportion of area at risk was assessed histologically using Evans blue and triphenyltetrazolium chloride. Our results confirm the cardioprotective effect of exogenous SDF-1α in mouse ischaemia-reperfusion injury and, for the first time, show protection when SDF-1α is delivered just prior to reperfusion, which has important therapeutic implications. The role of cell type was examined using the same in vivo ischaemia-reperfusion protocol in cardiomyocyte- and endothelial-specific CXCR4-null mice, and by Western blot analysis of endothelial cells treated in vitro. These experiments demonstrated that the acute infarct-sparing effect is mediated by endothelial cells, possibly via the signalling kinases Erk1/2 and PI3K/Akt. Unexpectedly, cardiomyocyte-specific deletion of CXCR4 was found to be cardioprotective per se. RNAseq analysis indicated altered expression of the mitochondrial protein co-enzyme Q10b in these mice.

CONCLUSIONS

Administration of SDF-1α is cardioprotective when administered prior to reperfusion and may, therefore, have clinical utility. SDF-1α-CXCR4-mediated cardioprotection from ischaemia-reperfusion injury is contingent on the cellular location of CXCR4 activation. Specifically, cardioprotection is mediated by endothelial signalling, while cardiomyocyte-specific deletion of CXCR4 has an infarct-sparing effect per se.

Exosomal Expression of CXCR4 Targets Cardioprotective Vesicles to Myocardial Infarction and Improves Outcome after Systemic Administration

Cell therapy has been evaluated to enhance heart function after injury. Delivered cells mostly act via paracrine mechanisms, including secreted growth factors, cytokines, and vesicles, such as exosomes (Exo). Intramyocardial injection of cardiac-resident progenitor cells (CPC)-derived Exo reduced scarring and improved cardiac function after myocardial infarction in rats. Here, we explore a clinically relevant approach to enhance the homing process to cardiomyocytes (CM), which is crucial for therapeutic efficacy upon systemic delivery of Exo. By overexpressing exosomal CXCR4, we increased the efficacy of plasmatic injection of cardioprotective Exo-CPC by increasing their bioavailability to ischemic hearts. Intravenous injection of Exo^CXCR4 significantly reduced infarct size and improved left ventricle ejection fraction at 4 weeks compared to Exo^CTRL (p < 0.01). Hemodynamic measurements showed that Exo^CXCR4 improved dp/dt min, as compared to Exo^CTRL and PBS group. In vitro, Exo^CXCR4 was more bioactive than Exo^CTRL in preventing CM death. This in vitro effect was independent from SDF-1α, as shown by using AMD3100 as specific CXCR4 antagonist. We showed, for the first time, that systemic administration of Exo derived from CXCR4-overexpressing CPC improves heart function in a rat model of ischemia reperfusion injury These data represent a substantial step toward clinical application of Exo-based therapeutics in cardiovascular disease.

CXCL12 in Patients with Chronic Kidney Disease and Healthy Controls: Relationships to Ambulatory 24-Hour Blood Pressure and Echocardiographic Measures

BACKGROUND/AIMS

Chronic kidney disease is a pro-inflammatory condition where the interplay between different regulatory pathways and immune cells mediates an unfavorable remodeling of the vascular wall and myocardial hypertrophy. These mechanisms include the action of CXCL12. The aim of this study is to evaluate the association between serum CXCL12 with left ventricular hypertrophy (LVH) and blood pressure control in chronic kidney disease (CKD) patients.

METHODS

This single-center observational study involved 90 stable CKD stage 1-5 patients (including 33 renal transplant recipients) and 25 healthy age- and sex-matched control subjects. CXCL12 was quantified by ELISA. 24-h ambulatory blood pressure monitoring was performed in 90 patients and 25 healthy controls. Left ventricular mass index (LVMI) was calculated based on the transthoracic echocardiography measurements in 27 patients out of the CKD population and in the whole control group.

RESULTS

CXCL12 correlated significantly with LVMI by multivariate regression analysis (coefficient B = 0.33, p = 0.02) together with age (B = 0.30, p = 0.03) and gender (B = 0.41, p = 0.003). A positive correlation was observed between CXCL12 and average 24-h systolic blood pressure (SBP) (rho = 0.35, p = 0.001), daytime SBP (rho = 0.35, p = 0.001), and nocturnal SBP (rho = 0.30, p = 0.002). Nocturnal hypertension was frequent (46% of CKD patients).

CONCLUSIONS

The results of our study point towards a link between CXCL12 and LVH as well as blood pressure control among patients with CKD, supporting the thesis that CXCL12 may be regarded as a new potential uremic toxin.

Do DPP-4 Inhibitors Cause Heart Failure Events by Promoting Adrenergically Mediated Cardiotoxicity? Clues From Laboratory Models and Clinical Trials

RATIONALE

DPP-4 (dipeptidyl peptidase-4) inhibitors have increased the risk of heart failure events in both randomized clinical trials and observational studies, but the mechanisms that underlie their deleterious effect remain to be elucidated. Previous work has implicated a role of these drugs to promote cardiac fibrosis.

OBJECTIVE

This article postulates that DPP-4 inhibitors increase the risk of heart failure events by activating the sympathetic nervous system to stimulate cardiomyocyte cell death, and it crystallizes the findings from both experimental studies and clinical trials that support the hypothesis.

METHODS AND RESULTS

Inhibition of DPP-4 not only potentiates the actions of GLP-1 (glucagon-like peptide-1; which can increase myocardial cAMP) but also potentiates the actions of SDF-1 (stromal cell-derived factor 1), NPY (neuropeptide Y), and substance P to activate the sympathetic nervous system and stimulate β-adrenergic receptors to cause cardiomyocyte apoptosis, presumably through a CaMKII (Ca++/calmodulin-dependent protein kinase II) pathway. An action of SDF-1 to interfere with cAMP and protein kinase A signaling may account for the absence of a clinically overt positive chronotropic effect. This conceptual framework is supported by the apparent ability of β-blocking drugs to attenuate the increased risk of DPP-4 inhibitors in a large-scale clinical trial.

CONCLUSIONS

Sympathetic activation may explain the increased risk of heart failure produced by DPP-4 inhibitors. The proposed mechanism has major implications for clinical care because in the treatment of patients with type 2 diabetes mellitus, DPP-4 inhibitors are widely prescribed, but β-blockers are underutilized because of fears that they might mask hypoglycemia.

Have dipeptidyl peptidase-4 inhibitors ameliorated the vascular complications of type 2 diabetes in large-scale trials? The potential confounding effect of stem-cell chemokines

Drugs that inhibit dipeptidyl peptidase-4 (DPP-4) are conventionally regarded as incretin-based agents that signal through the glucagon-like peptide-1 (GLP-1) receptor. However, inhibition of DPP-4 also potentiates the stem cell chemokine, stromal cell-derived factor-1 (SDF-1), which can promote inflammation, proliferative responses and neovascularization. In large-scale cardiovascular outcome trials, enhanced GLP-1 signaling has reduced the risk of atherosclerotic ischemic events, potentially because GLP-1 retards the growth and increases the stability of atherosclerotic plaques. However, DPP-4 inhibitors have not reduced the risk of major adverse cardiovascular events, possibly because potentiation of SDF-1 enhances plaque growth and instability, activates deleterious neurohormonal mechanisms, and promotes cardiac inflammation and fibrosis. Similarly, trials with GLP-1 agonists and sodium-glucose cotransporter 2 inhibitors have reported favorable effects on renal function, even after only 3-4 years of treatment. In contrast, no benefits on the rate of decline in glomerular filtration rate have been seen in trials of DPP-4 inhibitors, perhaps because the renal actions of DPP-4 inhibitors are primarily mediated by potentiation of SDF-1, not GLP-1. Experimentally, SDF-1 can promote podocyte injury and glomerulosclerosis. Furthermore, the natriuretic action of SDF-1 occurs primarily in the distal tubules, where it cannot utilize tubuloglomerular feedback to modulate the deleterious effects of glomerular hyperfiltration. Potentiation of SDF-1 in experimental models may also exacerbate both retinopathy and neuropathy. Therefore, although DPP-4 inhibitors have attractive clinical features, the benefits that might be expected from GLP-1 signaling may be undermined by their actions to enhance SDF-1.