Differences in H3K4me3 and chromatin accessibility contribute to altered T-cell receptor signaling in neonatal naïve CD4 T cells

Neonatal CD4+ T cells have reduced or delayed T-cell receptor (TCR) signaling responses compared with adult cells, but the mechanisms underlying this are poorly understood. This study tested the hypothesis that human neonatal naïve CD4+ TCR signaling and activation deficits are related to differences in H3K4me3 patterning and chromatin accessibility. Following initiation of TCR signaling using anti-CD3/anti-CD28 beads, adult naïve CD4+ T cells demonstrated increased CD69, phospho-CD3ε and interleukin (IL)-2, tumor necrosis factor-α (TNF-α), interferon-γ and IL-17A compared with neonatal cells. By contrast, following TCR-independent activation using phorbol myristate acetate (PMA)/ionomycin, neonatal cells demonstrated increased expression of CD69, IL-2 and TNF-α and equivalent phospho-ERK compared with adult cells. H3K4me3 chromatin immunoprecipitation-sequencing (ChIP-seq) and assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) were performed on separate cohorts of naïve CD4+ T cells from term neonates and adults, and RNA-seq data from neonatal and adult naïve CD4+ T cells were obtained from the Blueprint Consortium. Adult cells demonstrated overall increased chromatin accessibility and a higher proportion of H3K4me3 sites associated with open chromatin and active gene transcription compared with neonatal cells. Adult cells demonstrated increased mRNA expression of the TCR-associated genes FYN, ITK, CD4, LCK and LAT, which was associated with increased H3K4me3 at the FYN and ITK gene loci and increased chromatin accessibility at the CD4, LCK and LAT loci. These findings indicate that neonatal TCR-dependent defects in activation are epigenetically regulated and provide a potentially targetable mechanism to enhance neonatal CD4+ T-cell responses.

IFN-κ is critical for normal wound repair and is decreased in diabetic wounds

Wound repair following acute injury requires a coordinated inflammatory response. Type I IFN signaling is important for regulating the inflammatory response after skin injury. IFN-κ, a type I IFN, has recently been found to drive skin inflammation in lupus and psoriasis; however, the role of IFN-κ in the context of normal or dysregulated wound healing is unclear. Here, we show that Ifnk expression is upregulated in keratinocytes early after injury and is essential for normal tissue repair. Under diabetic conditions, IFN-κ was decreased in wound keratinocytes, and early inflammation was impaired. Furthermore, we found that the histone methyltransferase mixed-lineage leukemia 1 (MLL1) is upregulated early following injury and regulates Ifnk expression in diabetic wound keratinocytes via an H3K4me3-mediated mechanism. Using a series of in vivo studies with a geneticall y engineered mouse model (Mll1fl/fl K14cre-) and human wound tissues from patients with T2D, we demonstrate that MLL1 controls wound keratinocyte-mediated Ifnk expression and that Mll1 expression is decreased in T2D keratinocytes. Importantly, we found the administration of IFN-κ early following injury improves diabetic tissue repair through increasing early inflammation, collagen deposition, and reepithelialization. These findings have significant implications for understanding the complex role type I IFNs play in keratinocytes in normal and diabetic wound healing. Additionally, they suggest that IFN may be a viable therapeutic target to improve diabetic wound repair.

Interferon kappa (IFNk) in keratinocytes is critical for normal wound repair and is decreased in diabetic wounds

Wound repair following acute injury requires a coordinated inflammatory response. Type I interferon (IFN) signaling is important for regulating the inflammatory response post- skin injury. IFN kappa (IFNk), a type I IFN, has recently been found to drive skin inflammation in lupus and psoriasis; however, the role of IFNk in the context of normal or dysregulated wound healing is unclear. Thus, this project explores the role of IFNk in wound repair. Here, we found that IFNk expression is upregulated in keratinocytes early post-injury and is essential for normal tissue repair. Under diabetic conditions, IFNk was decreased in wound keratinocytes, and early inflammation was impaired. Further, we found that the histone methyltransferase mixed lineage leukemia protein-1(MLL1) regulates IFNk expression in diabetic wound keratinocytes via an H3K4me3 mediated mechanism. Using a series of in vivo studies with a genetically engineered mouse model(Mll1fl/flK14cre−) and human wound tissues from patients with T2D, we demonstrate that MLL1 controls wound keratinocyte-mediated IFNk and MLL1 is decreased in T2D keratinocytes. Importantly, we find the administration of IFNk early following injury improves diabetic tissue repair. These findings have significant implications for understanding the complex role type I interferons play in keratinocytes in normal and diabetic wound healing. Additionally, they suggest IFNk may be a viable therapeutic target to improve diabetic wound repair.

Supported by MNORC P30-DK089503 

Inhibition of macrophage histone demethylase JMJD3 protects against abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are a life-threatening disease for which there is a lack of effective therapy preventing aortic rupture. During AAA formation, pathological vascular remodeling is driven by macrophage infiltration, and the mechanisms regulating macrophage-mediated inflammation remain undefined. Recent evidence suggests that an epigenetic enzyme, JMJD3, plays a critical role in establishing macrophage phenotype. Using single-cell RNA sequencing of human AAA tissues, we identified increased JMJD3 in aortic monocyte/macrophages resulting in up-regulation of an inflammatory immune response. Mechanistically, we report that interferon-β regulates Jmjd3 expression via JAK/STAT and that JMJD3 induces NF-κB–mediated inflammatory gene transcription in infiltrating aortic macrophages. In vivo targeted inhibition of JMJD3 with myeloid-specific genetic depletion (JMJD3^f/fLyz2^Cre+) or pharmacological inhibition in the elastase or angiotensin II–induced AAA model preserved the repressive H3K27me3 on inflammatory gene promoters and markedly reduced AAA expansion and attenuated macrophage-mediated inflammation. Together, our findings suggest that cell-specific pharmacologic therapy targeting JMJD3 may be an effective intervention for AAA expansion.

Epigenetic regulation of the PGE2 pathway modulates macrophage phenotype in normal and pathologic wound repair

Macrophages are a primary immune cell involved in inflammation, and their cell plasticity allows for transition from an inflammatory to a reparative phenotype and is critical for normal tissue repair following injury. Evidence suggests that epigenetic alterations play a critical role in establishing macrophage phenotype and function during normal and pathologic wound repair. Here, we find in human and murine wound macrophages that cyclooxygenase 2/prostaglandin E2 (COX-2/PGE2) is elevated in diabetes and regulates downstream macrophage-mediated inflammation and host defense. Using single-cell RNA sequencing of human wound tissue, we identify increased NF-κB-mediated inflammation in diabetic wounds and show increased COX-2/PGE2 in diabetic macrophages. Further, we identify that COX-2/PGE2 production in wound macrophages requires epigenetic regulation of 2 key enzymes in the cytosolic phospholipase A2/COX-2/PGE2 (cPLA2/COX-2/PGE2) pathway. We demonstrate that TGF-β-induced miRNA29b increases COX-2/PGE2 production via inhibition of DNA methyltransferase 3b-mediated hypermethylation of the Cox-2 promoter. Further, we find mixed-lineage leukemia 1 (MLL1) upregulates cPLA2 expression and drives COX-2/PGE2. Inhibition of the COX-2/PGE2 pathway genetically (Cox2fl/fl Lyz2Cre+) or with a macrophage-specific nanotherapy targeting COX-2 in tissue macrophages reverses the inflammatory macrophage phenotype and improves diabetic tissue repair. Our results indicate the epigenetically regulated PGE2 pathway controls wound macrophage function, and cell-targeted manipulation of this pathway is feasible to improve diabetic wound repair.

Palmitate-TLR4 signaling regulates the histone demethylase, JMJD3, in macrophages and impairs diabetic wound healing

Chronic macrophage inflammation is a hallmark of type 2 diabetes (T2D) and linked to the development of secondary diabetic complications. T2D is characterized by excess concentrations of saturated fatty acids (SFA) that activate innate immune inflammatory responses, however, mechanism(s) by which SFAs control inflammation is unknown. Using monocyte-macrophages isolated from human blood and murine models, we demonstrate that palmitate (C16:0), the most abundant circulating SFA in T2D, increases expression of the histone demethylase, Jmjd3. Upregulation of Jmjd3 results in removal of the repressive histone methylation (H3K27me3) mark on NFκB-mediated inflammatory gene promoters driving macrophage-mediated inflammation. We identify that the effects of palmitate are fatty acid specific, as laurate (C12:0) does not regulate Jmjd3 and the associated inflammatory profile. Further, palmitate-induced Jmjd3 expression is controlled via TLR4/MyD88-dependent signaling mechanism, where genetic depletion of TLR4 (Tlr4^-/- ) or MyD88 (MyD88^-/- ) negated the palmitate-induced changes in Jmjd3 and downstream NFκB-induced inflammation. Pharmacological inhibition of Jmjd3 using a small molecule inhibitor (GSK-J4) reduced macrophage inflammation and improved diabetic wound healing. Together, we conclude that palmitate contributes to the chronic Jmjd3-mediated activation of macrophages in diabetic peripheral tissue and a histone demethylase inhibitor-based therapy may represent a novel treatment for nonhealing diabetic wounds.

Epigenetic Regulation of TLR4 in Diabetic Macrophages Modulates Immunometabolism and Wound Repair

Macrophages are critical for the initiation and resolution of the inflammatory phase of wound healing. In diabetes, macrophages display a prolonged inflammatory phenotype preventing tissue repair. TLRs, particularly TLR4, have been shown to regulate myeloid-mediated inflammation in wounds. We examined macrophages isolated from wounds of patients afflicted with diabetes and healthy controls as well as a murine diabetic model demonstrating dynamic expression of TLR4 results in altered metabolic pathways in diabetic macrophages. Further, using a myeloid-specific mixed-lineage leukemia 1 (MLL1) knockout (Mll1^f/fLyz2^Cre+ ), we determined that MLL1 drives Tlr4 expression in diabetic macrophages by regulating levels of histone H3 lysine 4 trimethylation on the Tlr4 promoter. Mechanistically, MLL1-mediated epigenetic alterations influence diabetic macrophage responsiveness to TLR4 stimulation and inhibit tissue repair. Pharmacological inhibition of the TLR4 pathway using a small molecule inhibitor (TAK-242) as well as genetic depletion of either Tlr4 (Tlr4^-/- ) or myeloid-specific Tlr4 (Tlr4^f/fLyz2^Cre+) resulted in improved diabetic wound healing. These results define an important role for MLL1-mediated epigenetic regulation of TLR4 in pathologic diabetic wound repair and suggest a target for therapeutic manipulation.

Sepsis Induces Prolonged Epigenetic Modifications in Bone Marrow and Peripheral Macrophages Impairing Inflammation and Wound Healing

OBJECTIVE

Sepsis represents an acute life-threatening disorder resulting from a dysregulated host response. For patients who survive sepsis, there remains long-term consequences, including impaired inflammation, as a result of profound immunosuppression. The mechanisms involved in this long-lasting deficient immune response are poorly defined. Approach and Results: Sepsis was induced using the murine model of cecal ligation and puncture. Following a full recovery period from sepsis physiology, mice were subjected to our wound healing model and wound macrophages (CD11b+, CD3-, CD19-, Ly6G-) were sorted. Post-sepsis mice demonstrated impaired wound healing and decreased reepithelization in comparison to controls. Further, post-sepsis bone marrow-derived macrophages and wound macrophages exhibited decreased expression of inflammatory cytokines vital for wound repair (IL [interleukin]-1β, IL-12, and IL-23). To evaluate if decreased inflammatory gene expression was secondary to epigenetic modification, we conducted chromatin immunoprecipitation on post-sepsis bone marrow-derived macrophages and wound macrophages. This demonstrated decreased expression of Mll1, an epigenetic enzyme, and impaired histone 3 lysine 4 trimethylation (activation mark) at NFκB (nuclear factor kappa-light-chain-enhancer of activated B cells)-binding sites on inflammatory gene promoters in bone marrow-derived macrophages and wound macrophages from postcecal ligation and puncture mice. Bone marrow transplantation studies demonstrated epigenetic modifications initiate in bone marrow progenitor/stem cells following sepsis resulting in lasting impairment in peripheral macrophage function. Importantly, human peripheral blood leukocytes from post-septic patients demonstrate a significant reduction in MLL1 compared with nonseptic controls.

CONCLUSIONS

These data demonstrate that severe sepsis induces stable mixed-lineage leukemia 1-mediated epigenetic modifications in the bone marrow, which are passed to peripheral macrophages resulting in impaired macrophage function and deficient wound healing persisting long after sepsis recovery.

Histone Methylation Directs Myeloid TLR4 Expression and Regulates Wound Healing following Cutaneous Tissue Injury

Myeloid cells are critical for orchestrating regulated inflammation during wound healing. TLRs, particularly TLR4, and its downstream-signaling MyD88 pathway play an important role in regulating myeloid-mediated inflammation. Because an initial inflammatory phase is vital for tissue repair, we investigated the role of TLR4-regulated, myeloid-mediated inflammation in wound healing. In a cutaneous tissue injury murine model, we found that TLR4 expression is dynamic in wound myeloid cells during the course of normal wound healing. We identified that changes in myeloid TLR4 during tissue repair correlated with increased expression of the histone methyltransferase, mixed-lineage leukemia 1 (MLL1), which specifically trimethylates the histone 3 lysine 4 (H3K4me3) position of the TLR4 promoter. Furthermore, we used a myeloid-specific Mll1 knockout (Mll1^f/fLyz2^Cre+ ) to determine MLL1 drives Tlr4 expression during wound healing. To understand the critical role of myeloid-specific TLR4 signaling, we used mice deficient in Tlr4 (Tlr4^-/- ), Myd88 (Myd88 ^-/-), and myeloid-specific Tlr4 (Tlr4^f/fLyz2^Cre+) to demonstrate delayed wound healing at early time points postinjury. Furthermore, in vivo wound myeloid cells isolated from Tlr4^-/- and Myd88 ^-/- wounds demonstrated decreased inflammatory cytokine production. Importantly, adoptive transfer of monocyte/macrophages from wild-type mice trafficked to wounds with restoration of normal healing and myeloid cell function in Tlr4-deficient mice. These results define a role for myeloid-specific, MyD88-dependent TLR4 signaling in the inflammatory response following cutaneous tissue injury and suggest that MLL1 regulates TLR4 expression in wound myeloid cells.

Chorioamnionitis exposure remodels the unique histone modification landscape of neonatal monocytes and alters the expression of immune pathway genes

Chorioamnionitis is an intrauterine infection involving inflammation of the chorion, amnion, and placenta. It leads to a fetal systemic inflammatory response that can alter the transcription of neonatal immune genes. We have previously shown that neonatal monocytes gain the activating histone tail modification H3K4me3 at promoter sites of immunologically important genes as development progresses from preterm neonate to adult. In this study, we applied ChIP-seq and RNA-seq to evaluate the impact of chorioamnionitis on the neonatal monocyte H3K4me3 histone modification landscape over the course of fetal and neonatal immune system development. Chorioamnionitis exposure in neonatal monocytes resulted in a net increase in total monocyte H3K4me3, primarily in introns and intergenic regions. Immune gene expression was decreased in chorioamnionitis-exposed monocytes, with the majority of enriched transcripts falling into pathways that are not linked to the immune system. Over half of all neonatal monocyte H3K4me3 peaks, independent of their location, were associated with active gene transcription. Overall, chorioamnionitis exposure resulted in the global remodeling of the neonatal monocyte H3K4me3 landscape and changes in the expression of known immune genes. These changes resulted in a less robust inflammatory response upon exposure to a secondary challenge, which may explain why chorioamnionitis-exposed neonates have an increased risk of sepsis. DATABASE: ChIP-seq data for U30/O30/Term: GEO GSE81957 ChIP-seq data for U30C/O30C/TermC: GEO GSE111873 RNA-seq data for U/L/CU/CL: GEO GSE111927.

Murine macrophage chemokine receptor CCR2 plays a crucial role in macrophage recruitment and regulated inflammation in wound healing

Macrophages play a critical role in the establishment of a regulated inflammatory response following tissue injury. Following injury, CCR2⁺ monocytes are recruited from peripheral blood to wound tissue, and direct the initiation and resolution of inflammation that is essential for tissue repair. In pathologic states where chronic inflammation prevents healing, macrophages fail to transition to a reparative phenotype. Using a murine model of cutaneous wound healing, we found that CCR2-deficient mice (CCR2^-/- ) demonstrate significantly impaired wound healing at all time points postinjury. Flow cytometry analysis of wounds from CCR2^-/- and WT mice revealed a significant decrease in inflammatory, Ly6C^Hi recruited monocyte/macrophages in CCR2^-/- wounds. We further show that wound macrophage inflammatory cytokine production is decreased in CCR2^-/- wounds. Adoptive transfer of mT/mG monocyte/macrophages into CCR2^+/+ and CCR2^-/- mice demonstrated that labeled cells on days 2 and 4 traveled to wounds in both CCR2^+/+ and CCR2^-/- mice. Further, adoptive transfer of monocyte/macrophages from WT mice restored normal healing, likely through a restored inflammatory response in the CCR2-deficient mice. Taken together, these data suggest that CCR2 plays a critical role in the recruitment and inflammatory response following injury, and that wound repair may be therapeutically manipulated through modulation of CCR2.

Abstract 255: Fatty Acid Binding Protein 4, FABP4, Causes Impaired Wound Healing in Diabetes

Wound healing in diabetes is impaired due to failed resolution of inflammation. Macrophages play a significant role in the establishment of a regulated inflammatory response during wounding. Macrophage function is dictated by metabolism, which alters gene expression. Recent studies suggest that a fatty acid binding protein, FABP4, may control macrophage function in diabetes by altering metabolism.

Thus, we examined whether FABP4 controls macrophage function and hence inflammation in diabetic wound healing. To investigate this, C57BL/6 mice were fed either a normal (12% saturated fat) diet or a high-fat (60% saturated fat) diet (HFD) for 12 weeks to induce physiologic “pre-diabetes.”

Wounds were created and CD3-CD19-NK1.1-CD11b+ cells (macrophages) were isolated each day following injury and FABP4 expression was quantified by qPCR and Western blot. We found that HFD wound macrophages demonstrated a significant increase in FABP4 gene expression and protein production on day 3 post-injury compared with controls.

To determine if FABP4 alters inflammatory gene expression in wound macrophages, we isolated wound macrophages with an FABP4 inhibitor, treated them, and analyzed for IL-1β and TNFα expression.

IL1β and TNFα gene expression were significantly reduced (P<0.01) in diabetic wound macrophages treated with the FABP4 inhibitor, suggesting that inflammatory gene expression can be controlled in diabetic wound macrophages through FABP4 modulation. As we have previously identified, epigenetic mechanisms often dictate gene expression during wound healing, thus we examined whether FABP4 expression in diabetic macrophages was regulated by histone modifications.

Chromatin immunoprecipitation (ChIP) analysis of the FABP4 promoter in wound macrophages revealed a significant increase in H3K4 trimethylation, an activating mark, on the FABP4 promoter in diabetic wound macrophages suggesting that epigenetic regulation may play an important role in the differential expression of FABP4 in diabetic wounds.

In conclusion, FABP4 appears to be upregulated in diabetic wound macrophages and contributes to increased macrophage inflammation. Modulation of FABP4 or its expression may help resolve inflammation in diabetic wounds and promote healing.

Modulation of Xanthine metabolism ameliorates inflammation and accelerates diabetic wound healing

Wound healing in diabetic patients is impaired because of dysregulated innate immune response that results in a chronic inflammatory state. It has been shown by our group and others that metabolites can contribute to a state of chronic inflammation in diabetic tissues. In a murine model of Type-2 diabetes (diet-induced obese mice; DIO), we assessed role of uric acid in driving macrophage-mediated inflammation and delayed wound healing. Using liquid chromatography-mass spectrometry, we show that macrophages from DIO mice have reduced levels of xanthine, a precursor in the uric acid pathway. Xanthine is converted to uric acid via Xanthine oxidase/dehydrogenase (XOR) encoded by the Xdh gene. DIO macrophages express higher levels of Xdh and increased levels of uric acid, suggesting increased XOR activity. Further, exogenous addition of uric acid led to increased production of IL-1β by macrophages. Importantly, pharmacologic inhibition of XOR with Allopurinol resulted in reduced levels of IL-1β expression by macrophages. In order to examine this in vivo, DIO mice were treated with Allopurinol leading to significantly improved wound healing. In summary, our data suggest that the xanthine metabolic pathway can be targeted therapeutically to accelerate wound healing in diabetic patients.

Ly6CHi Blood Monocyte/Macrophage Drive Chronic Inflammation and Impair Wound Healing in Diabetes Mellitus

OBJECTIVE

Wound monocyte-derived macrophage plasticity controls the initiation and resolution of inflammation that is critical for proper healing, however, in diabetes mellitus, the resolution of inflammation fails to occur. In diabetic wounds, the kinetics of blood monocyte recruitment and the mechanisms that control in vivo monocyte/macrophage differentiation remain unknown.

APPROACH AND RESULTS

Here, we characterized the kinetics and function of Ly6C^Hi [Lin^- (CD3^-CD19^-NK1.1^-Ter-119^-) Ly6G^-CD11b⁺] and Ly6C^Lo [Lin^- (CD3^-CD19^-NK1.1^-Ter-119^-) Ly6G^-CD11b⁺] monocyte/macrophage subsets in normal and diabetic wounds. Using flow-sorted tdTomato-labeled Ly6C^Hi monocyte/macrophages, we show Ly6C^Hi cells transition to a Ly6C^Lo phenotype in normal wounds, whereas in diabetic wounds, there is a late, second influx of Ly6C^Hi cells that fail transition to Ly6C^Lo. The second wave of Ly6C^Hi cells in diabetic wounds corresponded to a spike in MCP-1 (monocyte chemoattractant protein-1) and selective administration of anti-MCP-1 reversed the second Ly6C^Hi influx and improved wound healing. To examine the in vivo phenotype of wound monocyte/macrophages, RNA-seq-based transcriptome profiling was performed on flow-sorted Ly6C^Hi [Lin^-Ly6G^-CD11b⁺] and Ly6C^Lo [Lin^-Ly6G^-CD11b⁺] cells from normal and diabetic wounds. Gene transcriptome profiling of diabetic wound Ly6C^Hi cells demonstrated differences in proinflammatory and profibrotic genes compared with controls.

CONCLUSIONS

Collectively, these data identify kinetic and functional differences in diabetic wound monocyte/macrophages and demonstrate that selective targeting of CD11b⁺Ly6C^Hi monocyte/macrophages is a viable therapeutic strategy for inflammation in diabetic wounds.

MLL1 and MLL1 fusion proteins have distinct functions in regulating leukemic transcription program

Mixed lineage leukemia protein-1 (MLL1) has a critical role in human MLL1 rearranged leukemia (MLLr) and is a validated therapeutic target. However, its role in regulating global gene expression in MLLr cells, as well as its interplay with MLL1 fusion proteins remains unclear. Here we show that despite shared DNA-binding and cofactor interacting domains at the N terminus, MLL1 and MLL-AF9 are recruited to distinct chromatin regions and have divergent functions in regulating the leukemic transcription program. We demonstrate that MLL1, probably through C-terminal interaction with WDR5, is recruited to regulatory enhancers that are enriched for binding sites of E-twenty-six (ETS) family transcription factors, whereas MLL-AF9 binds to chromatin regions that have no H3K4me1 enrichment. Transcriptome-wide changes induced by different small molecule inhibitors also highlight the distinct functions of MLL1 and MLL-AF9. Taken together, our studies provide novel insights on how MLL1 and MLL fusion proteins contribute to leukemic gene expression, which have implications for developing effective therapies in the future.

Permeability of fructose-1,6-bisphosphate in liposomes and cardiac myocytes

Fructose-1,6-bisphosphate (FBP) helps preserve heart and other organs under ischemic conditions. Previous studies indicated that it can be taken up by various cell types. Here we extended observations from our group that FBP could penetrate artificial lipid bilayers and be taken up by cardiac myocytes, comparing the uptake of FBP to that of L-glucose. Using liposomes prepared by the freeze-thaw method, FBP entered about 200-fold slower than L-glucose. For liposomes of either soybean or egg lipids, 50 mM FBP enhanced the permeability of FBP itself, with little effect on general permeability (measured by uptake of L-glucose). In experiments with isolated cardiac myocytes at 21 degrees C, FBP uptake exceeded the uptake of L-glucose by several fold and appeared to equilibrate by 60 min. There was both a saturable component at micromolar levels and a nonsaturable component which dominated at millimolar levels. The saturable component was inhibited by Pi and by other phosphorylated sugars, though with lower affinity than FBP. Both saturable and nonsaturable uptakes were also observed at 3 degrees C. The results indicate that FBP enters myocytes not by simple penetration through the lipid bilayer, but via at least two distinct protein-dependent processes. The uptake could lead to intracellular effects important in hypothermic heart preservation.