Development of abnormal tissue architecture in transplanted neonatal rat myocytes

Collagenase treatment reduces the anisotropy of ultrasonic backscatter in rat myocardium by reducing collagen crosslinks

Dysregulation of collagen deposition, degradation, and crosslinking in the heart occur in response to increased physiological stress. Collagen content has been associated with ultrasonic backscatter (brightness), and we have shown that the anisotropy of backscatter can be used to measure myofiber alignment, that is, variation in the brightness of a left ventricular short-axis ultrasound. This study investigated collagen's role in anisotropy of ultrasonic backscatter; female Sprague-Dawley rat hearts were treated with a collagenase-containing solution, for either 10 or 30 min, or control solution for 30 min. Serial ultrasound images were acquired at 2.5-min intervals throughout collagenase treatment. Ultrasonic backscatter was assessed from anterior and posterior walls, where collagen fibrils are predominately aligned perpendicular to the angle of insonification, and the lateral and septal walls, where collagen is predominately aligned parallel to the angle of insonification. Collagenase digestion reduced backscatter anisotropy within the myocardium. Collagen remains present in the myocardium throughout collagenase treatment, but crosslinking is altered within 10 min. These data suggest that crosslinking of collagen modulates the anisotropy of ultrasonic backscatter. An Anisotropy Index, derived from differences in backscatter from parallel and perpendicularly aligned fibers, may provide a noninvasive index to monitor the progression and state of myocardial fibrosis.

Experience from experimental cell transplantation therapy of myocardial infarction: what have we learned?

During the past 15 years, our research group has transplanted fetal/neonatal cardiomyocytes, mesenchymal stem cells, and embryonic stem cell-derived cardiomyocytes into infarcted myocardium in a rat myocardial infarction model. Our experimental data demonstrated that cell transplantation therapy provides a potential approach for the treatment of injured myocardium after myocardial infarction based on the reported positive effects upon histological appearance and left ventricular function. However, the underlying mechanisms of the benefits from cell transplantation therapy remain unclear and may involve replacement of scar tissue by transplanted cells, induced neoangiogenesis and paracrine effects of factors released by the transplanted cells. In this review, we summarize our experiences from experimental cell transplantation therapy in a rat myocardial infarction model and discuss the controversies and questions that need to be addressed in future studies.

3-dimensional structures to enhance cell therapy and engineer contractile tissue

Experimental studies in animals and recent human clinical trials have revealed the current limitations of cellular transplantation, which include poor cell survival, lack of cell engraftment, and poor differentiation. Evidence in animals suggests that use of a 3-dimensional scaffold may enhance cell therapy and engineer myocardial tissue by improving initial cell retention, survival, differentiation, and integration. Several scaffolds of synthetic or natural origin are under development. Until now, contractility has been demonstrated in vitro only in biological scaffolds prepared from decellularized organs or tissue, or in collagenic porous scaffold obtained by crosslinking collagen fibers. While contractility of a cellularized collagen construct is poor, it can be greatly enhanced by tumor basement membrane extract. Recent advances in biochemistry have shown improved cell-matrix interactions by coupling adhesion molecules to achieve an efficient and safe bioartificial myocardium with no tumoral component. Fixation of adhesion molecules may also be a way to enhance cell homing and/or differentiation to increase local angiogenesis. Whatever the clinically successful combination ultimately proves to be, it is likely that cell therapy will require providing a supportive biochemical, physical, and spatial environment that will allow the cells to optimally differentiate and integrate within the target myocardial tissue.

Nuclear morphology and deformation in engineered cardiac myocytes and tissues

Cardiac tissue engineering requires finely-tuned manipulation of the extracellular matrix (ECM) microenvironment to optimize internal myocardial organization. The myocyte nucleus is mechanically connected to the cell membrane via cytoskeletal elements, making it a target for the cellular response to perturbation of the ECM. However, the role of ECM spatial configuration and myocyte shape on nuclear location and morphology is unknown. In this study, printed ECM proteins were used to configure the geometry of cultured neonatal rat ventricular myocytes. Engineered one- and two-dimensional tissue constructs and single myocyte islands were assayed using live fluorescence imaging to examine nuclear position, morphology and motion as a function of the imposed ECM geometry during diastolic relaxation and systolic contraction. Image analysis showed that anisotropic tissue constructs cultured on microfabricated ECM lines possessed a high degree of nuclear alignment similar to that found in vivo; nuclei in isotropic tissues were polymorphic in shape with an apparently random orientation. Nuclear eccentricity was also increased for the anisotropic tissues, suggesting that intracellular forces deform the nucleus as the cell is spatially confined. During systole, nuclei experienced increasing spatial confinement in magnitude and direction of displacement as tissue anisotropy increased, yielding anisotropic deformation. Thus, the nature of nuclear displacement and deformation during systole appears to rely on a combination of the passive myofibril spatial organization and the active stress fields induced by contraction. Such findings have implications in understanding the genomic consequences and functional response of cardiac myocytes to their ECM surroundings under conditions of disease.

Cell therapy enhances function of remote non-infarcted myocardium

Cell transplantation improves cardiac function after myocardial infarction; however, the underlying mechanisms are not well-understood. Therefore, the goals of this study were to determine if neonatal rat cardiomyocytes transplanted into adult rat hearts 1 week after infarction would, after 8-10 weeks: 1) improve global myocardial function, 2) contract in a Ca2+ dependent manner, 3) influence mechanical properties of remote uninjured myocardium and 4) alter passive mechanical properties of infarct regions. The cardiomyocytes formed small grafts of ultrastructurally maturing myocardium that enhanced fractional shortening compared to non-treated infarcted hearts. Chemically demembranated tissue strips of cardiomyocyte grafts produced force when activated by Ca2+, whereas scar tissue did not. Furthermore, the Ca2+ sensitivity of force was greater in cardiomyocyte grafts compared to control myocardium. Surprisingly, cardiomyocytes grafts isolated in the infarct zone increased Ca2+ sensitivity of remote uninjured myocardium to levels greater than either remote myocardium from non-treated infarcted hearts or sham-operated controls. Enhanced calcium sensitivity was associated with decreased phosphorylation of cTnT, tropomyosin and MLC2, but not changes in myosin or troponin isoforms. Passive compliance of grafts resembled normal myocardium, while infarct tissue distant from grafts had compliance typical of scar. Thus, cardiomyocyte grafts are contractile, improve local tissue compliance and enhance calcium sensitivity of remote myocardium. Because the volume of remote myocardium greatly exceeds that of the grafts, this enhanced calcium sensitivity may be a major contributor to global improvements in ventricular function after cell transplantation.

Macrophage Colony-Stimulating Factor Treatment After Myocardial Infarction Attenuates Left Ventricular Dysfunction by Accelerating Infarct Repair

OBJECTIVES

We aimed to determine the effects of macrophage colony-stimulating factor (M-CSF) and granulocyte colony-stimulating factor (G-CSF) treatment on both the repair process and ventricular function after myocardial infarction (MI).

BACKGROUND

The M-CSF and G-CSF have multiple potential effects on cells involved in wound repair.

METHODS

Myocardial infarction was induced by 45- or 90-min coronary occlusion and reperfusion in rats with or without subsequent injection of M-CSF (10(6) IU/kg/day) or G-CSF (50 microg/kg/day) for five days. We examined histology and messenger ribonucleic acid (mRNA), and assessed left ventricular function in situ using a conductance catheter.

RESULTS

Five days after MI, M-CSF increased the number of ED-1-positive cells, mRNA levels of transforming growth factor-beta-1, collagen I and III, and collagen fibers within the infarct. Fourteen days after MI, induced by 45-min ischemia, left ventricular end-systolic elastance (Ees) was reduced (1,191 +/- 87 mm Hg/ml vs. 1,812 +/- 150 mm Hg/ml) and both isovolumic relaxation time constant (tau) (11.9 +/- 0.9 ms vs. 8.5 +/- 0.4 ms) and left ventricular end-diastolic volume (LVEDV) (0.225 +/- 0.014 ml vs. 0.172 +/- 0.011 ml) increased versus sham-operated rats. These alterations after MI were attenuated by M-CSF (Ees = 1,650 +/- 146, tau = 9.7 +/- 0.7, LVEDV = 0.199 +/- 0.012) but not by G-CSF. This beneficial effect of M-CSF on Ees was also detected in hearts with MI induced by 90-min ischemia. Furthermore, M-CSF increased collagen content within infarcts and reduced the proportion of thin collagen fibers 14 days after MI. The Ees significantly correlated with infarct collagen content. Nevertheless, neither M-CSF nor G-CSF modified infarct size.

CONCLUSIONS

The M-CSF treatment attenuates deterioration of left ventricular function after MI by accelerating infarct repair.

Matrix metalloproteinase-13/collagenase-3 deletion promotes collagen accumulation and organization in mouse atherosclerotic plaques

BACKGROUND

Interstitial collagen plays a crucial structural role in arteries. Matrix metalloproteinases (MMPs), including MMP-13/collagenase-3, likely contribute to collagen catabolism in atherosclerotic plaques.

METHODS AND RESULTS

To test the hypothesis that a specific MMP-collagenase influences the development and structure of atherosclerotic plaques, this study used atherosclerosis-susceptible apolipoprotein E-deficient mice that lack MMP-13/collagenase-3 (Mmp-13(-/-)/apoE(-/-)) or express wild-type MMP-13/collagenase-3 (Mmp-13(+/+)/apoE(-/-)). Both groups consumed an atherogenic diet for 5 (n=8) or 10 weeks (n=9). Histological analyses of the aortic root of both groups revealed similar plaque size and accumulation of smooth muscle cells (a collagen-producing cell type) and macrophages (a major source of plaque collagenases) after 5 and 10 weeks of atherogenic diet. By 10 weeks, the plaques of Mmp-13(-/-)/apoE(-/-) mice contained significantly more interstitial collagen than those of Mmp-13(+/+)/apoE(-/-) mice (P<0.01). Furthermore, quantitative optical analyses revealed thinner and less aligned periluminal collagen fibers within the plaques of Mmp-13(+/+)/apoE(-/-) mice versus those from Mmp-13(-/-)/apoE(-/-) mice.

CONCLUSIONS

These data support the hypothesis that MMP-13/collagenase-3 plays a vital role in the regulation and organization of collagen in atherosclerotic plaques.