Cardiac fibroblasts: A key player in the regenerative response to pressure overload in neonatal mice?

Introduction: Heart disease is the most common cause of death worldwide. That is due to insufficient regenerative capacity of the adult myocardium. Proliferation and differentiation of cardiac fibroblasts (cFB), leads to fibrosis, myocardium stiffness and reduced contractility of the heart, which cannot provide the circulation demand of the body. Unlike adult mammalian hearts, neonatal mice at postnatal day 0 (P0) have the ability to regenerate the heart but this capacity is lost shortly after birth at P7. A study from our group has shown that neonatal heart at P0 have also the ability to adapt to pressure overload (POL), which is one of the detrimental stimuli inducing heart failure in adults. This adaptive response of P0 mice is characterised by enhanced cardiomyocyte (CM) proliferation and angiogenesis, no interstitial fibrosis and preserved cardiac function. In contrast, POL in P7 mice induces CM hypertrophy, interstitial fibrosis, and leads to reduced cardiac function as early as 7 days after injury. Given the differences observed in interstitial fibrosis upon POL in P0 compared to P7 mice we speculated that P0 and P7 cFB have different features and respond differently to POL. Therefore, in this project we aimed to gain an understating into the molecular and cellular properties of P0 and P7 cFB and their response to POL to discover their contribution in the adaptive response of P0 mice and lack of it in P7.

Methods: POL was induced in wild type (CD1) and transgenic mouse line (TCF21^MCM x mTmG) by performing neonatal transverse aortic constriction (nTAC) at P0 or P7. Cardiac function was assessed by echocardiography 7 and 14 days after nTAC. Heart weight (HW) and body weight (BW) were measured at P14 or P21. Employing TCF21^MCM mice enabled us to perform genetic fate mapping of cFBs upon tamoxifen injection to identity their fate, and response upon nTAC at P0 and P7. Bulk RNAseq of sorted TCF21⁺cFBs upon nTAC at P0 and P7 revealed their transcriptomic changes in response to POL. Furthermore, RNAseq data were validated using protein expression analysis by western blot as well as immunohistological stainings upon nTAC at P0 and P7.

Results: 14 days of POL in P0 increased HW/BW ratio and LV wall thickness without inducing any changes in cardiac function. However, one and two weeks of POL induced at P7 led to impaired heart function. Histological analysis revealed augmented cFB proliferation and cFB/CM ratio in P7 but not in P0 operated mice, which could explain the underlying reason of observed interstitial fibrosis in P7 and lack of it in P0. RNAseq analysis of sorted TCF21⁺cFBs from P0 operated mice revealed overexpression of cardioprotective genes and downregulaion of genes associated with CM hypertrophy, fibrosis and apoptosis. However, cFBs from P7 operated mice showed overexpression of apoptosis associated genes. Additionally, western blot as well as immunohistological analysis at P14 confirmed RNAseq data and also demonstrated preserved number of periostin⁺cFBs from P0-P14 upon POL in P0 but not P7.

Conclusion: Our study revealed cardioprotective features of P0 cFBs, which was extended up to P14 in response to POL. P7 cFBs, however, responded differently to POL and showed a cardio-detrimental effect upon POL. To further understand the molecular mechanisms and their direct effect we will investigate cFB cross-talk with CMs and endothelial cells in vitro using secreted factors and conditioned medium.