The selective homing of eGFP+ cells in our model to the site of maternal cardiac injury with lack of such homing to non-injured tissues points to the presence of precise signals sensed by cells of fetal origin that enable them to target diseased myocardium specifically, and to differentiate into diverse cardiac lineages (). Most notable is their differentiation into functional cardiomyocytes that are able to beat in syncytium with neighboring cardiomyocytes (Online Movie I, IA
and Movie Still Image IB
), thus potentially uncovering an evolutionary mechanism whereby the fetus assists in protecting the mother’s heart during and after pregnancy. These studies were inspired by the recovery noted in peripartum cardiomyopathy, whereby a remarkable 50% of women recover from heart failure spontaneously 37-39
. Peripartum cardiomyopathy has the highest rate of recovery amongst all etiologies of heart failure 18
, and the reasons for this high rate of recovery are not understood. In fact, it was this very observation that prompted us to hypothesize that there might be a fetal or placental contribution to counteract maternal cardiac injury. Our mouse injury model is not a precise representation of peripartum cardiomyopathy, rather, it is a model system of murine fetomaternal microchimerism that can help identify appropriate cell types for cardiac regeneration.
Model depicting trafficking of cells from fetus across placenta into maternal circulation to injury and peri-injury zones of the maternal heart
To this end, a far greater spectrum of potential applications to the field of heart disease emerges from these studies. The challenge of cardiovascular regenerative medicine is to develop novel therapeutic strategies to facilitate regeneration of normally functioning cardiomyocytes in the diseased heart. Thus, many investigators have explored a myriad of approaches in the last decade, many of which we have recently reviewed 40
. Despite investigations with a wide variety of cell types as candidates to attain this goal, the results of stem cell transplantation are somewhat ambiguous and the ideal cell type has yet to be established. The use of bone marrow cells to regenerate infarcted myocardium has been investigated in numerous studies since the initial findings of Orlic et al.
. Currently, however, a consensus has emerged that the ability of bone marrow-derived stem cells to differentiate into cardiomyocytes is questionable. Less controversy surrounds evidence from several groups demonstrating that ES cells 42-45
and endogenous populations of cardiac stem cells 28, 30, 31, 46, 47 have replicative and potentially regenerative capacities. Despite promising results with ES cells, there are ethical issues regarding the use of embryonic material as well as the tendency of ES cells to form teratomas 42
. Native cardiac progenitors, left in their natural milieu at their naturally occurring frequency, are clearly inadequate in reversing the downward spiral of events culminating in heart failure. Many of these progenitor types have not been reported to differentiate to functional beating cardiomyocytes when tested ex vivo. Utilizing live imaging, we have demonstrated that fetal cells differentiate into spontaneously beating cardiomyocytes after homing to the heart. The demonstration of spontaneous beating ex vivo has been a major stumbling block in the field. Coculture with neonatal cardiomyocytes was necessary in our study to induce beating, but we did not find any examples of nuclear fusion amongst the cardiomyocytes that were also GFP-positive. We cannot rule out ‘transient cell fusion’ as described by Dimmeler and colleagues 48
, but they noted that the nanotubular structures underlying these intercellular connections had declined by 48 hrs after coculture. We did not observe any beating GFP+ cells until at least 4 weeks after coculture, implying that true differentiation took place. Further studies and perhaps novel methods are needed to surmount these challenges in ascertaining true differentiation.
Our identification of Cdx2
as a unique and highly prevalent marker expressed on fetal cells in the maternal myocardium offers a new perspective regarding the appropriate cell type that might achieve these aims. The Cdx family of transcription factors consist of three mouse homologues (Cdx 1, 2
, and 4
) of the Drosophila
caudal homeobox genes, which are involved in specifying cell position along the anteroposterior axis, with similar functions in the later developmental stages of the mouse embryo 20, 49
as well as morphological specification of murine gut endoderm 50, 51
is also required for trophectoderm fate commitment in the developing blastocyst 19, 20, 52
. The trophectoderm gives rise to the trophoblast stem cells which have previously been associated solely with differentiation to the placenta lineage 53, 54
Bianchi and colleagues found that fetal cells that traffic to maternal blood and organs comprise a mixed population of progenitor and differentiated cells, with different relative proportions in different maternal organs 3
in a study that was performed in the non-injured state. In accordance with prior studies demonstrating a variety of different phenotypes in fetal microchimeric cells, our results also point towards the transfer of several populations of progenitor cells, but our finding of Cdx2
cells of fetal or placental origin in the heart may have uncovered a novel cell type that is capable of cardiac differentiation under injury conditions that can be readily isolated from placenta.