Extracellular Vesicle-Encapsulated Adeno-Associated Viruses for Therapeutic Gene Delivery to the Heart

BACKGROUND

Adeno-associated virus (AAV) has emerged as one of the best tools for cardiac gene delivery due to its cardiotropism, long-term expression, and safety. However, a significant challenge to its successful clinical use is preexisting neutralizing antibodies (NAbs), which bind to free AAVs, prevent efficient gene transduction, and reduce or negate therapeutic effects. Here we describe extracellular vesicle-encapsulated AAVs (EV-AAVs), secreted naturally by AAV-producing cells, as a superior cardiac gene delivery vector that delivers more genes and offers higher NAb resistance.

METHODS

We developed a 2-step density-gradient ultracentrifugation method to isolate highly purified EV-AAVs. We compared the gene delivery and therapeutic efficacy of EV-AAVs with an equal titer of free AAVs in the presence of NAbs, both in vitro and in vivo. In addition, we investigated the mechanism of EV-AAV uptake in human left ventricular and human induced pluripotent stem cell-derived cardiomyocytes in vitro and mouse models in vivo using a combination of biochemical techniques, flow cytometry, and immunofluorescence imaging.

RESULTS

Using cardiotropic AAV serotypes 6 and 9 and several reporter constructs, we demonstrated that EV-AAVs deliver significantly higher quantities of genes than AAVs in the presence of NAbs, both to human left ventricular and human induced pluripotent stem cell-derived cardiomyocytes in vitro and to mouse hearts in vivo. Intramyocardial delivery of EV-AAV9-sarcoplasmic reticulum calcium ATPase 2a to infarcted hearts in preimmunized mice significantly improved ejection fraction and fractional shortening compared with AAV9-sarcoplasmic reticulum calcium ATPase 2a delivery. These data validated NAb evasion by and therapeutic efficacy of EV-AAV9 vectors. Trafficking studies using human induced pluripotent stem cell-derived cells in vitro and mouse hearts in vivo showed significantly higher expression of EV-AAV6/9-delivered genes in cardiomyocytes compared with noncardiomyocytes, even with comparable cellular uptake. Using cellular subfraction analyses and pH-sensitive dyes, we discovered that EV-AAVs were internalized into acidic endosomal compartments of cardiomyocytes for releasing and acidifying AAVs for their nuclear uptake.

CONCLUSIONS

Together, using 5 different in vitro and in vivo model systems, we demonstrate significantly higher potency and therapeutic efficacy of EV-AAV vectors compared with free AAVs in the presence of NAbs. These results establish the potential of EV-AAV vectors as a gene delivery tool to treat heart failure.

Therapeutic and Diagnostic Translation of Extracellular Vesicles in Cardiovascular Diseases: Roadmap to the Clinic

Exosomes are small membrane-bound vesicles of endocytic origin that are actively secreted. The potential of exosomes as effective communicators of biological signaling in myocardial function has previously been investigated, and a recent explosion in exosome research not only underscores their significance in cardiac physiology and pathology, but also draws attention to methodological limitations of studying these extracellular vesicles. In this review, we discuss recent advances and challenges in exosome research with an emphasis on scientific innovations in isolation, identification, and characterization methodologies, and we provide a comprehensive summary of web-based resources available in the field. Importantly, we focus on the biology and function of exosomes, highlighting their fundamental role in cardiovascular pathophysiology to further support potential applications of exosomes as biomarkers and therapeutics for cardiovascular diseases.

Abstract MP165: Exosome-mediated Encapsulation Alters AAV Antigenicity and Infectivity: Implications for Gene Delivery in the Heart

Gene therapy is a promising approach for the treatment of cardiovascular disease. Current strategies for myocardial gene transfer include the use of adeno-associated virus (AAV) vectors. However, AAVs may not be ideal for gene therapy vectors owing to pre-existing AAV capsid immunity in the human population that may reduce transduction efficacy and hinder preclinical-to-clinical translation. Interestingly, recent studies suggest that exosome-mediated encapsulation may protect viruses from neutralizing antibodies (NAbs) against the capsid and promote viral infectivity. Here, we describe the ability of exosome-enveloped AAVs, i.e. exosomal AAVs (eAAVs), to evade NAbs and serve as a highly efficient gene delivery tool for cardiovascular therapeutics. We have developed a method to purifiy eAAVs from AAV-producing HEK-293T cells, and used electron/confocal microscopy, qPCR, immunoblotting, dynamic light scattering and interferometric imaging measurements to characterize eAAV morphology, contents and mechanism of action. We confirmed eAAVs represent vesicular fractions that exhibit common exosome phenotype, along with the presence of virus particles, and demonstrated that eAAV infectious entry potentially involves trafficking via endocytic compartments. Using flow cytometry, Langendorff perfusion system and bioluminescence imaging, we then evaluated efficiency of heart targeting for eAAV9/eAAV6 and standard AAV9/AAV6 encoding for mCherry or firefly luciferase in human cardiomyocytes in vitro and in mouse model in vivo . Regardless of the presence or absence of NAbs, we showed that eAAVs are more efficient in transduction in the same titer ranges as compared to standard AAVs. To test therapeutic efficacy, we intramyocardially injected eAAV9 or AAV9 vectors encoding for SERCA2a in NAb+ post-myocardial infarction mice and further evaluated cardiac function using echocardiography. Remarkably, eAAV9-SERCA2a outperformed standard AAVs significantly improving cardiac function in the presence of NAbs (%EF 55.14 ± 3.50 compared to 27.31 ± 1.63 at 6 weeks, respectively). In summary, delivery of AAVs protected by carrier exosomes (i.e. eAAVs) may retain the clinical benefits of AAVs while addressing one of its major challenges.

FTO-Dependent N6-Methyladenosine Regulates Cardiac Function During Remodeling and Repair

BACKGROUND

Despite its functional importance in various fundamental bioprocesses, studies of N⁶-methyladenosine (m6A) in the heart are lacking. Here, we show that the FTO (fat mass and obesity-associated protein), an m6A demethylase, plays a critical role in cardiac contractile function during homeostasis, remodeling, and regeneration.

METHODS

We used clinical human samples, preclinical pig and mouse models, and primary cardiomyocyte cell cultures to study the functional role of m6A and FTO in the heart and in cardiomyocytes. We modulated expression of FTO by using adeno-associated virus serotype 9 (in vivo), adenovirus (both in vivo and in vitro), and small interfering RNAs (in vitro) to study its function in regulating cardiomyocyte m6A, calcium dynamics and contractility, and cardiac function postischemia. We performed methylated (m6A) RNA immunoprecipitation sequencing to map transcriptome-wide m6A, and methylated (m6A) RNA immunoprecipitation quantitative polymerase chain reaction assays to map and validate m6A in individual transcripts, in healthy and failing hearts, and in myocytes.

RESULTS

We discovered that FTO has decreased expression in failing mammalian hearts and hypoxic cardiomyocytes, thereby increasing m6A in RNA and decreasing cardiomyocyte contractile function. Improving expression of FTO in failing mouse hearts attenuated the ischemia-induced increase in m6A and decrease in cardiac contractile function. This is performed by the demethylation activity of FTO, which selectively demethylates cardiac contractile transcripts, thus preventing their degradation and improving their protein expression under ischemia. In addition, we demonstrate that FTO overexpression in mouse models of myocardial infarction decreased fibrosis and enhanced angiogenesis.

CONCLUSIONS

Collectively, our study demonstrates the functional importance of the FTO-dependent cardiac m6A methylome in cardiac contraction during heart failure and provides a novel mechanistic insight into the therapeutic mechanisms of FTO.

Abstract 326: FTO-mediated mRNA Demethylation Regulates Cardiac Contractile Protein Expression and Function

Background: Exciting new discoveries in RNA biology underscore the importance of post-transcriptional chemical modifications to mRNAs (epitranscriptome) in regulating RNA stability, nuclear export, cellular compartmentalization, splicing, translation and degradation. The most abundant and functionally relevant modification in RNA, N6-methyladenosine (m6A) is reversibly demethylated by one of the m6A demethylases, fat mass and obesity-associated protein (FTO) whose function in the mammalian heart remains incompletely understood.

Materials and Methods: We used clinical human samples, preclinical pig and mouse models and primary cardiomyocytes to study m6A and FTO in the heart and in cardiomyocytes. We modulated FTO expression using AAV9 (in vivo), adenovirus (in vivo and in vitro) and siRNAs (in vitro). We investigated m6A-induced changes to contractile protein expression using m6A RNA immunoprecipitation sequencing (MeRIP-seq) and stable isotope labeling of amino acids in cell culture (SILAC).

Results: We discovered in human heart failure that reduced FTO expression is associated with aberrant increase in m6A mRNA methylation, which is conserved in swine and mouse models of myocardial ischemia (MI). AAV9-mediated FTO gene delivery in mouse MI attenuated m6A increase and improved cardiac function with enhanced contractility, angiogenesis and reduced fibrosis. At the molecular level, FTO-mediated mRNA demethylation serves to increase contractile protein expression in mouse hearts as well as in isolated primary cardiomyocytes. By comparing human and mouse transcriptome-wide m6A maps with SILAC proteomic profiling from cardiomyocytes, we identified FTO-mediated m6A demethylation is transcript-specific and leads to altered protein expression of several key contractile, angiogenic and regenerative proteins.

Conclusion: Using new RNA-based investigations, we uncovered a novel regulatory layer beyond the genome working at the level of epitranscriptome governing cardiac function. Our findings on the dynamic nature of the cardiac m6A-epitranscriptome will lead to deeper understanding of the mechanism of cardiac remodeling on one hand and innovative therapeutic interventions on the other.

Abstract 170: Exosomal AAV-mediated SERCA2a Gene Transfer Improves Cardiac Function in a Mouse Model of Heart Failure

Adeno-associated viruses (AAVs) are promising therapeutic tools for gene delivery to the heart. However, pre-existing antibodies (NAbs) to many cardiotropic AAV serotypes naturally present in humans pose a critical challenge for the translation of gene therapies to clinical applications. Here, we describe the use of exosomal AAVs (eAAV) as a robust heart gene delivery system that improves transduction efficiency while protecting from pre-existing immunity to the viral capsid. To obtain eAAV specimens from conditioned medium from AAV-producing HEK-293T cells, we have developed a state-of-the-art multi-step ultracentrifugation strategy. We demonstrated through electron microscopy-based visualization, size distribution measurements and distribution of AAV genomes in post-centrifugation iodixanol gradients, that our purification process enables isolation of eAAVs with high purity and minimal contamination with standard AAVs. Efficiency of heart targeting was then evaluated for eAAV9 or eAAV6 and standard AAV9 or AAV6 in human cardiomyocytes (hCMs) in vitro and in passive immunity nude mouse model in vivo . Regardless of the presence or absence of NAbs, we demonstrated that eAAVs are more efficient in transduction of cells in the same titer ranges as standard AAVs. To test the therapeutic efficacy, eAAV9-SERCA2a or AAV9-SERCA2a were injected intramyocardially in post-myocardial infarction (MI) mice preinjected with NAbs. Remarkably, eAAV9-SERCA2a outperformed standard AAVs 6 weeks post-MI, significantly improving cardiac function in the presence of NAbs (%EF 55.14 ± 3.50 vs. 27.31 ± 1.63, respectively). Additionally, we demonstrated in vivo that eAAV9-mediated gene delivery is more specific to CMs than to other cell types present in the heart, which suggests that eAAVs preserve cardiotropic properties of AAV9 serotype. With examination of colocalization of eAAVs and markers specific for endosomes (Rab5 and Rab7) in hCMs in vitro , our preliminary data indicated that eAAV infectious entry potentially involves trafficking via endocytic compartments. In conclusion, these results underline the therapeutic potential of eAAVs to evade NAbs, and to facilitate the clinical translation of AAV-based gene therapies to a larger human population.

Abstract 584: FTO-Dependent m6A Regulates Cardiomyocyte and Cardiac Function During Remodeling and Repair

Background: Despite its functional importance in various fundamental bioprocesses, the studies of N6-methyladenosine (m6A) in the heart are lacking. In this study, we investigated the role of fat mass and obesity-associated (FTO), an m6A demethylase, in cardiac contractile function during homeostasis and remodeling.

Methods: We used clinical human samples, preclinical pig and mouse models and primary cardiomyocyte cell cultures to study the functional role of m6A and FTO in the heart and in cardiomyocytes. We modulated expression of FTO using AAV9 (in vivo), adenovirus (both in vivo and in vitro) and siRNAs (in vitro). We performed methylated (m6A) RNA immunoprecipitation sequencing (MeRIP-seq) to map transcriptome-wide m6A, and MeRIP qPCR assays to map and validate m6A in individual transcripts, in healthy and failing hearts and myocytes. We performed proteomics analysis using stable isotope labeling with amino acids in cell culture to study m6A role in mRNA to protein translation.

Results: We discovered that FTO has decreased expression in failing mammalian hearts and hypoxic cardiomyocytes, thereby increasing m6A in RNA and decreasing cardiomyocyte contractile function. Improving expression of FTO in failing mouse hearts attenuated the ischemia-induced increase in m6A and decrease in cardiac contractile function. This is carried out by the demethylation activity of FTO, which selectively demethylates cardiac contractile transcripts, thus preventing their degradation and improving their protein expression under ischemia.

Conclusion: Collectively, our study demonstrates the functional importance of FTO-dependent cardiac m6A methylome in cardiac contraction during heart failure and provides a novel mechanistic insight into the therapeutic mechanisms of FTO.

Abstract 104: AAV-Exosomes: A Novel Platform for Myocardial Gene Delivery for Cardioprotection

Introduction: Adeno-associated viruses (AAVs) are viral vectors of choice for delivering genes for long-term expression due to their safety in clinics. However, pre-existing immunity to AAVs from naturally present neutralizing antibodies (NAbs, present in between 60% and 90% of population) poses a significant challenge for AAV-mediated gene delivery. NAbs prevent AAVs from infecting target cells, greatly reducing transduction efficiency and, therefore, the clinical efficacy. Thus, it is essential to develop novel AAV-based vectors that circumvent the effect of NAbs.

Objectives: We aimed to investigate the ability of exosome-encapsulated AAVs (AAVExo) to evade NAbs and serve as a highly efficient gene transfer tool for cardiovascular therapeutics.

Methods and Results: We developed a multi-step purification strategy using iodixanol density gradient to isolate AAVExo with minimal contamination from free AAVs (AAV1, 6 or 9). Biochemical assays, flow cytometry, IVIS Spectrum in vivo optical imaging and echocardiography were used to detect AAV-mediated gene delivery and evaluate cardiac function. AAV6Exo-mCherry and AAV9Exo-FLuc were resistant to NAbs and significantly improved expression of mCherry and firefly luciferase (FLuc) both in mouse and human (iPSC-derived) cardiomyocytes in vitro , and in murine hearts in vivo (in nude mice preinjected with NAbs), compared to free AAVs. To test the therapeutic efficacy of AAVExo-mediated gene delivery in the presence of NAbs, we injected AAV9-SERCA2a or AAV9Exo-SERCA2a into post-MI hearts of nude mice preinjected with IVIg (human intravenous immunoglobulin) preparation. Hearts treated with AAV9Exo-SERCA2a had significantly improved cardiac function compared to those treated with free AAV9-SERCA2a (%EF 62.6 ± 9.8 vs. 28.4 ± 5.3, respectively; 2 weeks after surgery).

Conclusion: Delivery of AAVs protected by carrier exosomes is a promising approach to circumvent the issue of NAbs in AAV-based gene therapy, which can be used in the entire population of patients and may result in higher gene delivery efficacy.

Abstract 301: An m6A Demethylase, FTO Mediates Post-transcriptional mRNA Modifications to Regulate Cardiac and Cardiomyocyte Function

Background: Post-transcriptional modifications in the form of m6A (N6-methyladenosine) regulate mRNA fate and translation, miRNA biogenesis, lncRNA function and several cellular processes. However, m6A mechanisms in the mature post-mitotic tissues such as the mammalian heart remain unexplored. Here, we investigated the role of transcriptome-wide m6A in cardiac protein expression as well as lncRNA function.

Methods: Using biochemical assays, we investigated the spatiotemporal gene expression patterns of m6A regulators in non-failing, failing (ischemic) human hearts, in mouse and pig MI models. In mouse heart, we mapped transcriptome-wide m6A by developing state-of-the-art MeRIP-seq coupled with novel bioinformatics analysis. To investigate the functional relevance of m6A to cardiac proteome, we used SILAC-LC-MS. By silencing (siRNA) m6A-reader proteins, we investigated the role of m6A in cardiomyocyte mRNA stability, decay and nuclear export. For in vivo myocardial gene delivery, we used AAV9 and adenovirus vectors. Finally, we performed immunohistology in cardiomyocytes and mouse heart tissues to study nuclei size, fibrosis and angiogenesis.

Results: We discovered that the expression of m6A demethylase, FTO is decreased in ischemic myocardium and cardiomyocytes, thus in vivo FTO gene delivery resulted in attenuation of ischemia-induced increase in m6A and decrease in cardiac contractile function post-MI. FTO overexpression in mouse heart and human cardiomyocytes revealed selective demethylation (MeRIP-seq) of cardiac contractile transcripts resulting in induced contractile protein expression (SILAC-LC-MS) thus rescuing heart function post-MI. Mechanistically, we demonstrate that the cardioprotective mechanism of FTO is mediated by selective demethylation of cardiac contractile transcripts under ischemia, which prevents mRNA degradation as well as enhanced nuclear compaction. Finally, we demonstrate that FTO overexpression in mouse models of MI resulted in decreased fibrosis and enhanced angiogenesis.

Conclusion: Our study provides the first description of a cardiac active m6A demethylase working at post-transcriptional level as a critical regulator of cardiac contractile function, fibrosis and angiogenesis.

Induced Pluripotent Stem Cell (iPSC)-Derived Extracellular Vesicles Are Safer and More Effective for Cardiac Repair Than iPSCs

RATIONALE

Extracellular vesicles (EVs) are tiny membrane-enclosed droplets released by cells through membrane budding or exocytosis. The myocardial reparative abilities of EVs derived from induced pluripotent stem cells (iPSCs) have not been directly compared with the source iPSCs.

OBJECTIVE

To examine whether iPSC-derived EVs can influence the biological functions of cardiac cells in vitro and to compare the safety and efficacy of iPSC-derived EVs (iPSC-EVs) and iPSCs for cardiac repair in vivo.

METHODS AND RESULTS

Murine iPSCs were generated, and EVs isolated from culture supernatants by sequential centrifugation. Atomic force microscopy, high-resolution flow cytometry, real-time quantitative RT-PCR, and mass spectrometry were used to characterize EV morphology and contents. iPSC-EVs were enriched in miRNAs and proteins with proangiogenic and cytoprotective properties. iPSC-EVs enhanced angiogenic, migratory, and antiapoptotic properties of murine cardiac endothelial cells in vitro. To compare the cardiac reparative capacities in vivo, vehicle, iPSCs, and iPSC-EVs were injected intramyocardially at 48 hours after a reperfused myocardial infarction in mice. Compared with vehicle-injected mice, both iPSC- and iPSC-EV-treated mice exhibited improved left ventricular function at 35 d after myocardial infarction, albeit iPSC-EVs rendered greater improvement. iPSC-EV injection also resulted in reduction in left ventricular mass and superior perfusion in the infarct zone. Both iPSCs and iPSC-EVs preserved viable myocardium in the infarct zone, whereas reduction in apoptosis was significant with iPSC-EVs. iPSC injection resulted in teratoma formation, whereas iPSC-EV injection was safe.

CONCLUSIONS

iPSC-derived EVs impart cytoprotective properties to cardiac cells in vitro and induce superior cardiac repair in vivo with regard to left ventricular function, vascularization, and amelioration of apoptosis and hypertrophy. Because of their acellular nature, iPSC-EVs represent a safer alternative for potential therapeutic applications in patients with ischemic myocardial damage.

Polylactide- and polycaprolactone-based substrates enhance angiogenic potential of human umbilical cord-derived mesenchymal stem cells in vitro - implications for cardiovascular repair

Recent approaches in tissue regeneration focus on combining innovative achievements of stem cell biology and biomaterial sciences to develop novel therapeutic strategies for patients. Growing recent evidence indicates that mesenchymal stem cells harvested from human umbilical cord Wharton's jelly (hUC-MSCs) are a new valuable source of cells for autologous as well as allogeneic therapies in humans. hUC-MSCs are multipotent, highly proliferating cells with prominent immunoregulatory activity. In this study, we evaluated the impact of widely used FDA approved poly(α-esters) including polylactide (PLA) and polycaprolactone (PCL) on selected biological properties of hUC-MSCs in vitro. We found that both polymers can be used as non-toxic substrates for ex vivo propagation of hUC-MSCs as shown by no major impact on cell proliferation or viability. Moreover, PCL significantly enhanced the migratory capacity of hUC-MSCs. Importantly, genetic analysis indicated that both polymers promoted the angiogenic differentiation potential of hUC-MSCs with no additional chemical stimulation. These results indicate that PLA and PCL enhance selected biological properties of hUC-MSCs essential for their regenerative capacity including migratory and proangiogenic potential, which are required for effective vascular repair in vivo. Thus, PLA and PCL-based scaffolds combined with hUC-MSCs may be potentially employed as future novel grafts in tissue regeneration such as blood vessel reconstruction.

Hematological parameters in Polish mixed breed rabbits with addition of meat breed blood in the annual cycle

In the paper we studied haematologic values, such as haemoglobin concentration, haematocrit value, thrombocytes, leucocytes: lymphocytes, neutrophils, basophils, eosinophils and monocytes in the pheral blood in Polish mixed-breeds with addition of meat breed blood in order to obtain the reference values which are until now not available for this animals. In studying this indices we took into consideration the impact of the season (spring, summer, autumn, winter), and sex of the animals. The studies have shown a high impact of the season of the year on those rabbits, but only in spring and summer. Moreover we observed that the sex has mean impact on the studied values of haematological parameters in those rabbits. According to our knowledge, this is the first paper on haematologic values in this widely used group of rabbits, so they may serve as reference values.

Human Induced Pluripotent Stem Cell-Derived Microvesicles Transmit RNAs and Proteins to Recipient Mature Heart Cells Modulating Cell Fate and Behavior

Microvesicles (MVs) are membrane-enclosed cytoplasmic fragments released by normal and activated cells that have been described as important mediators of cell-to-cell communication. Although the ability of human induced pluripotent stem cells (hiPSCs) to participate in tissue repair is being increasingly recognized, the use of hiPSC-derived MVs (hiPSC-MVs) in this regard remains unknown. Accordingly, we investigated the ability of hiPSC-MVs to transfer bioactive molecules including mRNA, microRNA (miRNA), and proteins to mature target cells such as cardiac mesenchymal stromal cells (cMSCs), and we next analyzed effects of hiPSC-MVs on fate and behavior of such target cells. The results show that hiPSC-MVs derived from integration-free hiPSCs cultured under serum-free and feeder-free conditions are rich in mRNA, miRNA, and proteins originated from parent cells; however, the levels of expression vary between donor cells and MVs. Importantly, we found that transfer of hiPSC components by hiPSC-MVs impacted on transcriptome and proteomic profiles of target cells as well as exerted proliferative and protective effects on cMSCs, and enhanced their cardiac and endothelial differentiation potential. hiPSC-MVs also transferred exogenous transcripts from genetically modified hiPSCs that opens new perspectives for future strategies to enhance MV content. We conclude that hiPSC-MVs are effective vehicles for transferring iPSC attributes to adult somatic cells, and hiPSC-MV-mediated horizontal transfer of RNAs and proteins to injured tissues may be used for therapeutic tissue repair. In this study, for the first time, we propose a new concept of use of hiPSCs as a source of safe acellular bioactive derivatives for tissue regeneration.

Natural immunity factors in Polish mixed breed rabbits

Mixed-breed rabbits in Poland are widely used for diagnostic and scientific research and as utility animals, therefore there is a need to know their immunological status, as well as their haematological status. In this study natural immunity factors were analyzed in Polish mixed-breed rabbits and Polish mixed-breed rabbits with addition of blood of meet-breed, considering the impact of sex and season of the year (spring, summer, autumn, winter) using measurement of non-specific cellular and humoral immunity parameters in peripheral blood. The study has revealed that there is a variety between the two commonly used mixed-breed types of rabbits, especially when sex and season is concerned, which is crucial for using these animals in experiments.

Membrane lipid rafts, master regulators of hematopoietic stem cell retention in bone marrow and their trafficking

Cell outer membranes contain glycosphingolipids and protein receptors, which are integrated into glycoprotein microdomains, known as lipid rafts, which float freely in the membrane bilayer. These structures have an important role in assembling signaling molecules (e.g., Rac-1, RhoH and Lyn) together with surface receptors, such as the CXCR4 receptor for α-chemokine stromal-derived factor-1, the α4β1-integrin receptor (VLA-4) for vascular cell adhesion molecule-1 and the c-kit receptor for stem cell factor, which together regulate several aspects of hematopoietic stem/progenitor cell (HSPC) biology. Here, we discuss the role of lipid raft integrity in the retention and quiescence of normal HSPCs in bone marrow niches as well as in regulating HSPC mobilization and homing. We will also discuss the pathological consequences of the defect in lipid raft integrity seen in paroxysmal nocturnal hemoglobinuria and the emerging evidence for the involvement of lipid rafts in hematological malignancies.