A Biomimetic Approach Utilizing Pulsatile Perfusion Generates Contractile Vascular Grafts.

Surgical implantation of decellularized cadaveric arteries is routinely used to treat right-sided congenital cardiac lesions. These acellular conduits lack the capacity for somatic growth and are prone to stenosis and calcification, necessitating multiple operations throughout childhood. Islet-1+ cardiovascular progenitor cells (CPCs) have demonstrated the capacity for differentiation into all cell types of the heart and outflow tracts. We hypothesize that CPC seeding of decellularized pulmonary arteries and bioreactor culture under physiologic flow conditions will drive vascular differentiation of CPCs and result in a conduit more suitable for implantation and long-term growth. We began by decellularizing ovine pulmonary arteries and characterizing the composition of the extracellular matrix (ECM). Hemodynamic testing of decellularized vessels in a custom bioreactor was used to define the scaffold mechanical properties over a range of pressures and flow rates. Next, our expanded ovine CPCs were suspended in growth media and injected intramurally into decellularized pulmonary arteries that were subsequently cultured in either static or pulsatile cultures. A combination of immunohistochemistry, real-time polymerase chain reaction (PCR), and tissue bath contraction studies were used to evaluate the bioengineered arteries before transplantation. Pulmonary artery patches from the most favorable culture conditions were then implanted into juvenile sheep to provide proof of concept. Hematoxylin and eosin staining indicated complete removal of cell nuclei (n = 9), whereas double-stranded DNA isolation from tissue homogenates showed 99.1% DNA removal (p < 0.01, n = 4). Furthermore, trichrome and elastin staining verified maintenance of collagen and elastin. Immunohistochemistry and PCR analyses (n = 4 per group) confirmed contractile smooth muscle presence on only our 3-week pulsatile scaffolds via presence of calponin 1 and myosin heavy chain 11. Tissue bath studies demonstrated that smooth muscle contraction generated by our 3-week pulsatile scaffolds (2.23 ± 0.19 g, n = 4) is comparable with native tissue contraction strength (2.78 ± 0.06 g, n = 4). Ovine transplantation confirmed that our graft can be safely implanted, retains contractile smooth muscle cells, and recruits native endothelium. Longer duration of physiologic pulsatile culture drives differentiation of CPCs seeded on ECM conduits toward a mature, contractile phenotype that is maintained for several weeks in vivo. Longer term studies to assess somatic growth potential are needed. Impact statement The current field of vascular transplantation relies on cadaveric and synthetic grafts to treat right-sided congenital cardiac lesions. These grafts do not grow somatically with our patients. This results in multiple reoperations throughout childhood to increase the size of the graft. Our bioengineered alternative demonstrates successful implantation, contractile smooth muscle cells, and a native endothelial layer. This research demonstrates a pilot study confirming the viability of a bioengineered alternative to the current standard of care in the field of vascular transplantation.

Impact of transplant center volume on donor heart offer utilization rates in the United States.

BACKGROUND: We tested the hypothesis that transplant centers (TCs) with higher volumes have higher donor heart (DH) offer utilization rates. METHODS: Using the Annual Data reports of the US Organ Procurement and Transplantation Network and the Scientific Registry of Transplant Recipients (SRTR) we reviewed all adult heart transplant offers between July 1, 2016 and June 29, 2019. Unadjusted donor offer utilization rates and observed to expected (O/E) DH utilization ratios adjusted using the SRTR model were calculated for each TC for all DH offers and for the following sub-categories: DH with left ventricular ejection fraction <60%, DH >40 years, DH >500 miles from TC, "hard-to-place hearts" (defined as those offered to >50 TCs) and DH designated as increased infectious risk. Univariable linear regression was used to identify a relationship between average yearly center volume and DH utilization. RESULTS: During the study 118,841 total offers were made to 107 TCs and 8300 transplants were performed. The unadjusted utilization rate was not associated with TC volume for all donor offers (p = .517). However, among all subcategories other than DH >40 years, the unadjusted DH utilization rate was associated with TC volume (p < .05). In addition, using the adjusted SRTR O/E ratio, there was a significant impact of TC volume on utilization rate for all donor offers (for an increase TC volume of 10 transplants/year coefficient = 0.095, 95% confidence interval: 0.037-0.151, p = .001). This relationship persisted with an identifiable change for each of the subcategories (p ≤ .001). CONCLUSIONS: TC volume is significantly correlated to DH offer utilization rate.

Novel Multidisciplinary Management of Acute Kidney Injury After Infant Orthotopic Heart Transplantation.

Acute kidney injury following orthotopic heart transplantation in pediatric recipients is often multifactorial, requiring balance of immune suppression, nephrotoxic medication exposure, nutrition, and fluid status. Therapeutic options are often limited by patient size and hemodynamic stability. We describe a four-month, 4.9-kg female bridged by mechanical circulatory support to transplant after failed stage 1 palliation secondary to recurrent aortic stenosis and severe ventricular dysfunction. Posttransplant, kidney injury was managed by transcatheter relief of central obstruction from an anastomotic stricture and continuous renal replacement therapy, allowing uninterrupted immune suppression, medication, and nutrition delivery until sufficient recovery of renal function.

Primary Transplantation for Congenital Heart Disease in the Neonatal Period: Long-term Outcomes.

BACKGROUND: Primary transplantation was developed in the 1980s as an alternative therapy to palliative reconstruction of uncorrectable congenital heart disease. Although transplantation achieved more favorable results, its utilization has been limited by the availability of donor organs. This review examines the long-term outcomes of heart transplantation in neonates at our institution. METHODS: The institutional pediatric heart transplant database was queried for all neonatal heart transplants performed between 1985 and 2017. Follow-up was obtained from medical records and an annually administered questionnaire. Overall survival and time to development of complications were estimated using the Kaplan Meier method. Univariate and multivariate analyses were performed to identify independent predictors of survival. RESULTS: Heart transplantation was performed in 104 neonates. Median age was 17 days. Hypoplastic left heart syndrome (classic or variant) was the primary diagnosis in 77.8% of patients. Survival at 10 years and 25 years was 73.9% and 55.8%, respectively. At 20 years, freedom from allograft vasculopathy and lymphoproliferative disease was 72.0% and 81.9%, respectively. Freedom from re-transplantation was 81.4% at 20 years. Eight patients (7.6%) developed end-stage renal disease. By multivariate analysis, lower glomerular filtration rate and allograft vasculopathy were the only significant predictors of death. CONCLUSIONS: Neonatal heart transplantation remains a durable therapy with very acceptable long-term survival. Children transplanted in the newborn period have the potential to reach adulthood with minimal need for reintervention.

Interaction Between Ischemic Time and Donor Age on Adult Heart Transplant Outcomes in the Modern Era.

BACKGROUND: We examined the effect of cold ischemic interval on modern outcomes to determine whether advances in patient management have made an impact. METHODS: Using the United Network of Organ Sharing database, we reviewed adult heart transplants between January 2000 and March 2016. We divided donor age into terciles: younger than 18 years, 18 to 33 years, and 34 years and older. Within each tercile, transplants were divided by cold ischemic interval of less than 4 hours, 4 to 6 hours, and more than 6 hours. Survival curves were compared between cold ischemic interval categories within each tercile. Covariate-adjusted and donor age-stratified Cox proportional hazards regression models were used to estimate overall mortality and graft failure hazards ratios. RESULTS: Of 29,192 transplants, no significant differences between cold ischemic interval groups in survival or graft failure were apparent in the group aged younger than 18. For donors older than 18, significant differences were found for survival and graft failure with cold ischemic interval exceeding 4 hours in both univariate and multivariate analysis, and survival functions at different ischemic intervals continued to diverge beyond 1 year. The interaction effect between donor age and cold ischemic interval on overall mortality was not significant when analyzed as continuous variables, however younger donor age appeared to attenuate increase in overall mortality with longer cold ischemic intervals. CONCLUSIONS: Despite advances in perioperative management during the past 30 years, for donors older than 18 years, cold ischemic interval exceeding 4 hours is associated with gradual but significantly diminished survival that persists well beyond the perioperative period. Comparison to historical data suggests that advances in management have somewhat attenuated the hazard associated with longer ischemic times.

Long-term transplant outcomes of donor hearts with left ventricular dysfunction.

OBJECTIVE: Despite small single-center reports demonstrating acceptable outcomes using donor hearts with left ventricular dysfunction, 19% of potential donor hearts are currently unused exclusively because of left ventricular dysfunction. We investigated modern long-term survival of transplanted donor hearts with left ventricular dysfunction using a large, diverse cohort. METHODS: Using the United Network for Organ Sharing database, we reviewed all adult heart transplants between January 2000 and March 2016. Baseline and postoperative characteristics and Kaplan-Meier survival curves were compared. A covariates-adjusted Cox regression model was developed to estimate post-transplant mortality. To address observed variation in patient profile across donor ejection fraction, a propensity score was built using Cox predictors as covariates in a generalized multiple linear regression model. All the variables in the original Cox model were included. For each recipient, a predicted donor ejection fraction was generated and exported as a new balancing score that was used in a subsequent Cox model. Cubic spline analysis suggested that at most 3 and perhaps no ejection fraction categories were appropriate. Therefore, in 1 Cox model we added donor ejection fraction as a grouped variable (using the spline-directed categories) and in the other as a continuous variable. RESULTS: A total of 31,712 donor hearts were transplanted during the study period. A total of 742 donor hearts were excluded for no recorded left ventricular ejection fraction, and 20 donor hearts were excluded for left ventricular ejection fraction less than 20%. Donor hearts with reduced left ventricular ejection fraction were from younger donors, more commonly male donors, and donors with lower body mass index than normal donor hearts. Recipients of donor hearts with reduced left ventricular ejection fraction were more likely to be on mechanical ventilation. Kaplan-Meier curves revealed no significant differences in recipient survival up to 15 years of follow-up (P = .694 log-rank test). Cox regression analysis showed that after adjustment for propensity variation, transplant year, and region, ejection fraction had no statistically significant impact on mortality when analyzed as a categoric or continuous variable. Left ventricular ejection fraction at approximately 1 year after transplantation was normal for all groups. CONCLUSIONS: Carefully selected donor hearts with even markedly diminished left ventricular ejection fraction can be transplanted with long-term survival equivalent to normal donor hearts and therefore should not be excluded from consideration on the basis of depressed left ventricular ejection fraction alone. Functional recovery of even the most impaired donor hearts in this study suggests that studies of left ventricular function in the setting of brain death should be interpreted cautiously.

Composite scaffold provides a cell delivery platform for cardiovascular repair.

Control over cell engraftment, survival, and function remains critical for heart repair. We have established a tissue engineering platform for the delivery of human mesenchymal progenitor cells (MPCs) by a fully biological composite scaffold. Specifically, we developed a method for complete decellularization of human myocardium that leaves intact most elements of the extracellular matrix, as well as the underlying mechanical properties. A cell-matrix composite was constructed by applying fibrin hydrogel with suspended cells onto decellularized sheets of human myocardium. We then implanted this composite onto the infarct bed in a nude rat model of cardiac infarction. We next characterized the myogenic and vasculogenic potential of immunoselected human MPCs and demonstrated that in vitro conditioning with a low concentration of TGF-ß promoted an arteriogenic profile of gene expression. When implanted by composite scaffold, preconditioned MPCs greatly enhanced vascular network formation in the infarct bed by mechanisms involving the secretion of paracrine factors, such as SDF-1, and the migration of MPCs into ischemic myocardium, but not normal myocardium. Echocardiography demonstrated the recovery of baseline levels of left ventricular systolic dimensions and contractility when MPCs were delivered via composite scaffold. This adaptable platform could be readily extended to the delivery of other reparative cells of interest and used in quantitative studies of heart repair.

Downregulation of the CXC chemokine receptor 4/stromal cell-derived factor 1 pathway enhances myocardial neovascularization, cardiomyocyte survival, and functional recovery after myocardial infarction.

OBJECTIVES: Although adequate numbers of hematopoietic progenitor cells reside in the human bone marrow, the extent of endogenous neovascularization after myocardial infarction remains insufficient. The aim of this study was to identify the role of the CXC chemokine receptor 4/stromal cell-derived factor 1 axis in the mobilization and homing of hematopoietic progenitor cells in the ischemic heart. METHODS: Human bone marrow-derived hematopoietic progenitor cells or saline were injected systemically into athymic nude rats 48 hours after myocardial infarction. Myocardial and bone marrow expression of stromal cell-derived factor 1 and chemotaxis of hematopoietic progenitor cells were measured in vitro in the presence or absence of stromal cell-derived factor 1. The role of the CXC chemokine receptor 4/stromal cell-derived factor 1 axis was investigated by means of antibody blockade or systemic administration of granulocyte colony-stimulating factor. Morphologic analysis included measurement of the infarct area, capillary density, and apoptosis, whereas left ventricular function was measured by means of echocardiographic analysis. RESULTS: Expression of postinfarct stromal cell-derived factor 1 was increased by 67% in the bone marrow and decreased by 43% in myocardium. Disruption of bone marrow stromal cell-derived factor 1/CXC chemokine receptor 4 interactions by antibody blockade resulted in a redirection of human hematopoietic progenitor cells from the bone marrow to the ischemic heart and augmented neovascularization and cardiomyocyte survival. Similarly, systemic administration of granulocyte colony-stimulating factor to block CXC chemokine receptor 4/stromal cell-derived factor 1 interaction resulted in increased mobilization and homing of hematopoietic progenitor cells to the ischemic heart, which translated to augmented myocardial neovascularization, prevention of apoptosis, and improved cardiac function. CONCLUSIONS: Bone marrow stromal cell-derived factor 1 upregulation after myocardial ischemia prevents mobilization of endogenous hematopoietic progenitor cells. We provide evidence that disruption of stromal cell-derived factor 1/CXC chemokine receptor 4 interactions allows redirection of hematopoietic progenitor cells to ischemic myocardium and enhances recovery of left ventricular function.

Challenges in cardiac tissue engineering.

Cardiac tissue engineering aims to create functional tissue constructs that can reestablish the structure and function of injured myocardium. Engineered constructs can also serve as high-fidelity models for studies of cardiac development and disease. In a general case, the biological potential of the cell-the actual "tissue engineer"-is mobilized by providing highly controllable three-dimensional environments that can mediate cell differentiation and functional assembly. For cardiac regeneration, some of the key requirements that need to be met are the selection of a human cell source, establishment of cardiac tissue matrix, electromechanical cell coupling, robust and stable contractile function, and functional vascularization. We review here the potential and challenges of cardiac tissue engineering for developing therapies that could prevent or reverse heart failure.

Naringenin inhibits neointimal hyperplasia following arterial reconstruction with interpositional vein graft.

Vessels respond to injury by a healing process that includes the development of neointima. Stenosis secondary to neointima formation is the main cause of failure following arterial reconstructions. Vessel wall homeostasis is regulated by proinflammatory cytokines that affect smooth muscle cell proliferation, growth, migration, and death. We assessed the hypothesis that naringenin, a flavinoid possessing anti-inflammatory, antioxidant, and antiproliferative activities, reduces neointimal hyperplasia (NIH) following vascular injury.Arterial injury was created by interposition grafting of autologous right superficial epigastric vein graft into the right femoral artery (FA) in 48 male Sprague-Dawley rats. Following injury, the rats were divided into 4 groups (n = 12). Two groups were treated with naringenin (100 mg/kg intraperitoneal q daily) for 2 and 4 weeks each while 2 control groups received normal saline for the same durations. For Sham group (n = 10), the FA and vein were isolated without any additional procedure. Rats were killed at the end of treatment regimen in all groups, and FAs were harvested. Thickness of intima was measured in histologic sections, and levels of platelet derived growth factor (PDGF)-BB, TNFalpha, and Ki67 labeling index (Ki67 LI) were quantified in immunohistochemical analyses to assess the amount of NIH and mechanisms underlying its formation.Although there was no significant difference between the groups at 2 weeks, neointima thickness was lower in the naringenin treated group at 4 weeks (23.7 +/- 2.3 vs. 35.6 +/- 2.6 microm in control group; P < 0.001). The levels of PDGF-BB, and TNFalpha were lower in naringenin treated groups at both 2 weeks (PDGF-BB [0.21% +/- 0.03% versus 0.39% +/- 0.05% in control group, P < 0.001), TNFalpha (21.2% +/- 0.8% vs. 36.1% +/- 1.9% in control group, P < 0.001]) and 4 weeks (PDGF-BB [0.25% +/- 0.03% vs. 0.57% +/- 0.09% in control group, P < 0.001], TNFalpha [25.5% +/- 1.8% vs. 45.0% +/- 2.9% in control group, P < 0.001]). Ki67 LI was lower in naringenin treated groups at 2 weeks (13.9% +/- 2.8% vs. 18.7% +/- 3.7% in control group, P < 0.05), and at 4 weeks (17.5% +/- 2.6% vs. 31.1% +/- 4.7% in control group, P < 0.001), indicating a lower level of cellular proliferation.Naringenin reduces NIH following arterial reconstruction. This may be mediated by a decrease in PDGF-BB and TNFalpha levels and the resulting down-regulation of smooth muscle cells' migration and proliferation.

Proliferation and migration of label-retaining cells of the kidney papilla.

The kidney papilla contains a population of cells with several characteristics of adult stem cells, including the retention of proliferation markers during long chase periods (i.e., they are label-retaining cells [LRCs]). To determine whether the papillary LRCs generate new cells in the normal adult kidney, we examined cell proliferation throughout the kidney and found that the upper papilla is a site of enhanced cell cycling. Using genetically modified mice that conditionally expressed green fluorescence protein fused to histone 2B, we observed that the LRCs of the papilla proliferated only in its upper part, where they associate with "chains" of cycling cells. The papillary LRCs decreased in number with age, suggesting that the cells migrated to the upper papilla before entering the cell cycle. To test this directly, we marked papillary cells with vital dyes in vivo and found that some cells in the kidney papilla, including LRCs, migrated toward other parts of the kidney. Acute kidney injury enhanced both cell migration and proliferation. These results suggest that during normal homeostasis, LRCs of the kidney papilla (or their immediate progeny) migrate to the upper papilla and form a compartment of rapidly proliferating cells, which may play a role in repair after ischemic injury.

Mesenchymal cell transplantation and myocardial remodeling after myocardial infarction.

BACKGROUND: Targeted delivery of mesenchymal precursor cells (MPCs) can modify left ventricular (LV) cellular and extracellular remodeling after myocardial infarction (MI). However, whether and to what degree LV remodeling may be affected by MPC injection post-MI, and whether these effects are concentration-dependent, remain unknown. METHODS AND RESULTS: Allogeneic MPCs were expanded from sheep bone marrow, and direct intramyocardial injection was performed within the borderzone region 1 hour after MI induction (coronary ligation) in sheep at the following concentrations: 25x10(6) (25 M, n=7), 75x10(6) (75 M, n=7), 225x10(6) (225 M, n=10), 450x10(6) (450 M, n=8), and MPC free media only (MI Only, n=14). LV end diastolic volume increased in all groups but was attenuated in the 25 and 75 M groups. Collagen content within the borderzone region was increased in the MI Only, 225, and 450 M groups, whereas plasma ICTP, an index of collagen degradation, was highest in the 25 M group. Within the borderzone region matrix metalloproteinases (MMPs) and MMP tissue inhibitors (TIMPs) also changed in a MPC concentration-dependent manner. For example, borderzone levels of MMP-9 were highest in the 25 M group when compared to the MI Only and other MPC treatment group values. CONCLUSIONS: MPC injection altered collagen dynamics, MMP, and TIMP levels in a concentration-dependent manner, and thereby influenced indices of post-MI LV remodeling. However, the greatest effects with respect to post-MI remodeling were identified at lower MPC concentrations, thus suggesting a therapeutic threshold exists for this particular cell therapy.

Percutaneous cell delivery into the heart using hydrogels polymerizing in situ.

Heart disease is the leading cause of death in the US. Following an acute myocardial infarction, a fibrous, noncontractile scar develops, and results in congestive heart failure in more than 500,000 patients in the US each year. Muscle regeneration and the induction of new vascular growth to treat ischemic disorders of the heart can have significant therapeutic implications. Early studies in patients with chronic ischemic systolic left ventricular dysfunction (SLVD) using skeletal myoblasts or bone marrow-derived cells report improvement in left ventricular ejection function (LVEF) and clinical status, without notable safety issues. Nonetheless, the efficacy of cell transfer for cardiovascular disease is not established, in part due to a lack of control over cell retention, survival, and function following delivery. We studied the use of biocompatible hydrogels polymerizable in situ as a cell delivery vehicle, to improve cell retention, survival, and function following delivery into the ischemic myocardium. The study was conducted using human bone marrow-derived mesenchymal stem cells and fibrin glue, but the methods are applicable to any human stem cells (adult or embryonic) and a wide range of hydrogels. We first evaluated the utility of several commercially available percutaneous catheters for delivery of viscous cell/hydrogel suspensions. Next we characterized the polymerization kinetics of fibrin glue solutions to define the ranges of concentrations compatible with catheter delivery. We then demonstrate the in vivo effectiveness of this preparation and its ability to increase cell retention and survival in a nude rat model of myocardial infarction.

Effect of left ventricular assist device infection on post-transplant outcomes.

BACKGROUND: In this study, we sought to confirm which types of device-related infections impact bridge-to-transplant rates. We also aimed to determine the effect of device-related infections on post-transplant survival and post-transplant infection. METHODS: We retrospectively reviewed paper and electronic medical records for 149 patients undergoing left ventricular assist device (LVAD) implantation as a bridge to transplantation at the Columbia Presbyterian Medical Center between 2000 and 2006. The primary outcome measures were survival to transplantation, post-transplant infection and post-transplant survival. RESULTS: Patients with sepsis were less likely to be successfully bridged to cardiac transplantation (7 of 22 vs 103 of 127, 31.8% vs 81.1%, p = 0.01). However, if transplanted, their survival rates at 1 year were not decreased (6 of 7 vs 85 of 103, 85.7% vs 82.5%, p = 1.00). No other pre-transplant device-related infection affected post-transplant survival at 1 year (22 of 27 vs 69 of 83, 81.5% vs 83.1%, p = 1.00). Pre-transplant drive-line infections predicted post-transplant infection in former drive-line or pocket sites (11 of 16 vs 14 of 94, 68.8% vs 14.9%, p = 0.01) and increased overall post-transplant hospital length of stay (16 vs 19 days, p = 0.04). They did not, however, affect post-transplant survival at 1 year (22 of 25 vs 69 of 85, 88% vs 81.2%, p = 0.56). CONCLUSIONS: Although survival to transplantation was diminished in LVAD patients with sepsis, if successfully transplanted, post-transplant survival was unaffected. Patients with local device infections and signs of early sepsis may warrant evaluation for urgent transplantation. A pre-transplant drive-line infection was associated with post-transplant infection in either the former pocket or drive-line site, and increased overall length of stay, but it did not decrease post-transplant survival.

Allogeneic mesenchymal precursor cell therapy to limit remodeling after myocardial infarction: the effect of cell dosage.

BACKGROUND: This experiment assessed the dose-dependent effect of a unique allogeneic STRO-3-positive mesenchymal precursor cell (MPC) on postinfarction left ventricular (LV) remodeling. The MPCs were administered in a manner that would simulate an off-the-self, early postinfarction, preventative approach to cardiac cell therapy in a sheep transmural myocardial infarct (MI) model. METHODS: Allogeneic MPCs were isolated from male crossbred sheep. Forty-six female sheep underwent coronary ligation to produce a transmural LV anteroapical infarction. One hour after infarction, the borderzone myocardium received an injection of 25, 75, 225, or 450 x 10(6) MPCs, or cell medium. Echocardiography was performed at 4 and 8 weeks after MI to quantify LV end-diastolic (LVEDV) and end-systolic volumes (LVESV), ejection fraction (EF), and infarct expansion. CD31 and smooth muscle actin (SMA) immunohistochemical staining was performed on infarct and borderzone specimens to quantify vascular density. RESULTS: Compared with controls, low-dose (25 and 75 x 10(6) cells) MPC treatment significantly attenuated infarct expansion and increases in LVEDV and LVESV. EF was improved at all cell doses. CD31 and SMA immunohistochemical staining demonstrated increased vascular density in the borderzone only at the lower cell doses. There was no evidence of myocardial regeneration within the infarct. CONCLUSION: Allogeneic STRO-3 positive MPCs attenuate the remodeling response to transmural MI in a clinically relevant large-animal model. This effect is associated with vasculogenesis and arteriogenesis within the borderzone and infarct and is most pronounced at lower cell doses.

Biomimetic approach to tissue engineering.

The overall goal of tissue engineering is to create functional tissue grafts that can regenerate or replace our defective or worn out tissues and organs. Examples of grafts that are now in pre-clinical studies or clinical use include engineered skin, cartilage, bone, blood vessels, skeletal muscle, bladder, trachea, and myocardium. Engineered tissues are also finding applications as platforms for pharmacological and physiological studies in vitro. To fully mobilize the cell's biological potential, a new generation of tissue engineering systems is now being developed to more closely recapitulate the native developmental milieu, and mimic the physiologic mechanisms of transport and signaling. We discuss the interactions between regenerative biology and engineering, in the context of (i) creation of functional tissue grafts for regenerative medicine (where biological input is critical), and (ii) studies of stem cells, development and disease (where engineered tissues can serve as advanced 3D models).

Biodegradable fibrous scaffolds with tunable properties formed from photo-cross-linkable poly(glycerol sebacate).

It is becoming increasingly apparent that the architecture and mechanical properties of scaffolds, particularly with respect to mimicking features of natural tissues, are important for tissue engineering applications. Acrylated poly(glycerol sebacate) (Acr-PGS) is a material that can be cross-linked upon exposure to ultraviolet light, leading to networks with tunable mechanical and degradation properties through simple changes during Acr-PGS synthesis. For example, the number of acrylate functional groups on the macromer dictates the concentration of cross-links formed in the resulting network. Three macromers were synthesized that form networks that vary dramatically with respect to their tensile modulus ( approximately 30 kPa to 6.6 MPa) and degradation behavior ( approximately 20-100% mass loss at 12 weeks) based on the extent of acrylation ( approximately 1-24%). These macromers were processed into biodegradable fibrous scaffolds using electrospinning, with gelatin as a carrier polymer to facilitate fiber formation and cell adhesion. The resulting scaffolds were also diverse with respect to their mechanics (tensile modulus ranging from approximately 60 kPa to 1 MPa) and degradation ( approximately 45-70% mass loss by 12 weeks). Mesenchymal stem cell adhesion and proliferation on all fibrous scaffolds was indistinguishable from those of controls. The scaffolds showed similar diversity when implanted on the surface of hearts in a rat model of acute myocardial infarction and demonstrated a dependence on the scaffold thickness and chemistry in the host response. In summary, these diverse scaffolds with tailorable chemical, structural, mechanical, and degradation properties are potentially useful for the engineering of a wide range of soft tissues.

Engineered microenvironments for human stem cells.

Regulation of cell differentiation and assembly remains a fundamental question in developmental biology. During development, tissues emerge from coordinated sequences of the renewal, differentiation, and assembly of stem cells. Likewise, regeneration of an adult tissue is driven by the migration and differentiation of repair cells. The fields of stem cells and regenerative medicine are starting to realize how important is the entire context of the cell environment, with the presence of other cells, three-dimensional matrices, and sequences of molecular and physical morphogens. The premise is that to unlock the full potential of stem cells, at least some aspects of the dynamic environments normally present in vivo need to be reconstructed in experimental systems used in vitro. We review here some recent work that utilized engineered environments for guiding the embryonic and adult human stem cells, and focus on vasculogenesis as a critical and universally important aspect of tissue development and regeneration. Birth Defects Research (Part C) 84:335-347, 2008. (c) 2008 Wiley-Liss, Inc.

Simvastatin reverses cardiac hypertrophy caused by disruption of the bradykinin 2 receptor.

Bradykinin 2 receptor (B2R) deficiency predisposes to cardiac hypertrophy and hypertension. The pathways mediating these effects are not known. Two-month-old B2R knockout (KO) and wild-type (WT) mice were assigned to 4 treatment groups (n = 12-14/group): control (vehicle); nitro-L-arginine methyl ester (L-NAME) an NO synthase inhibitor; simvastatin (SIM), an NO synthase activator; and SIM+L-NAME. Serial echocardiography was performed and blood pressure (BP) at 6 weeks was recorded using a micromanometer. Myocardial eNOS and mitogen-activated protein kinase (MAPK, including ERK, p38, and JNK) protein expression were measured. Results showed that (i) B2RKO mice had significantly lower ejection fraction than did WT mice (61% +/- 1% vs. 73% +/- 1%), lower myocardial eNOS and phospho-eNOS, normal systolic BP, and higher LV mass, phospho-p38, and JNK; (ii) L-NAME increased systolic BP in KO mice (117 +/- 19 mm Hg) but not in WT mice and exacerbated LV hypertrophy and dysfunction; and (iii) in KO mice, SIM decreased hypertrophy, p38, and JNK, improved function, increased capillary eNOS and phospho-eNOS, and prevented L-NAME-induced LV hypertrophy without lowering BP. We conclude that disruption of the B2R causes maladaptive cardiac hypertrophy with myocardial eNOS downregulation and MAPK upregulation. SIM reverses these abnormalities and prevents the development of primary cardiac hypertrophy as well as hypertrophy secondary to L-NAME-induced hypertension.