piRNA-mediated RNA oxidation as a new route to heart regeneration

This editorial refers to ‘MCPPIR promotes cardiomyocyte proliferation and cardiac repair via o8G oxidation of POC1B mRNA’, by T. Wang et al., https://doi.org/10.1093/cvr/cvaf265. The negligible regenerative capacity of the human heart is one of the main obstacles to successful recovery after a myocardial infarction or stroke. The adult human heart has limited ability to replace lost cardiomyocytes (CMs), leading to permanent structural damage and progressive heart failure. By contrast, lower vertebrates such as zebrafish and neonatal mice exhibit remarkable cardiac regenerative abilities, which are triggered by endogenous CM proliferation.1–3 However, this potential is rapidly lost in mammals after birth.1 Despite extensive research over the past decade, the molecular mechanisms underlying the loss of regenerative potential in the adult heart remain incompletely understood. A complex network of signalling pathways, including YAP, WNT, AKT, thyroid hormones, immune modulators, hypoxia, and telomerase, has been implicated in regulating CM cell-cycle re-entry and proliferation.2,4 Furthermore, emerging studies have identified non-coding RNAs (such as microRNAs, long non-coding RNAs, and circular RNAs) as crucial regulators of gene expression that play important roles in maintaining cardiac function, promoting cardiomyocyte proliferation, and facilitating cardiac regeneration.5,6 Of the various families of non-coding RNAs, PIWI-interacting RNAs (piRNAs) are a largely unexplored frontier in the field of cardiovascular biology. Traditionally studied in the context of germline genome defence, piRNAs play a critical role in transposon silencing and the preservation of genomic integrity.7 However, their function in somatic tissues is far less understood, as is their potential contribution to cardiac development, disease, or regeneration. The presence and function of piRNAs in somatic tissues, particularly in the heart and the epigenetic and post-transcriptional mechanisms driven by PIWI proteins remain largely unexplored. Only a few preliminary studies have suggested that piRNAs may be involved in the proliferation, differentiation, or stress responses of somatic cells,7–9 raising important but unanswered questions about their broader biological roles. In this context, the study by Wang et al. is both timely and significant.10 By identifying a novel piRNA that directly regulates cardiomyocyte proliferation and myocardial repair, the authors broaden our understanding of piRNA biology, revealing an unexpected mechanistic link between piRNAs, epigenetic modifications and cardiac regeneration. Wang et al. identified a novel piRNA, termed MCPPIR (myocardial cell proliferation-promoting piRNA), as a potent regulator of cardiomyocyte proliferation and cardiac repair. Through gain- and loss-of-function experiments in mice, the authors demonstrate that silencing MCPPIR compromises cardiomyocyte proliferation and post-injury recovery. Conversely, MCPPIR overexpression enhances regeneration, reduces fibrosis, and consequently improves cardiac function. MCPPIR promotes cardiomyocyte proliferation via an epigenetic post-transcriptional mechanism involving 8-oxoguanine (o8G) modification of POC1B mRNA, which stabilizes the transcript and sustains expression of POC1B protein expression. POC1B is an essential structural component of the centriole. MCPPIR binds to and inhibits HNRNPH1 association with POCB1 mRNA, thereby preventing the degradation of o8G-modified POC1B transcripts and preserving centrosome integrity to enable sustained cardiomyocyte proliferation (see also Figure 1). Summary figure. In their study, Wang et al. reveal an unexpected and conceptually significant connection between piRNA biology and cardiac regeneration. This study provides novel insight into the emerging role of piRNAs in cardiac biology, extending their functional relevance into the field of regeneration and repair of diseased hearts. Demonstrating that MCPPIR operates through oxidative RNA modification unveils a previously unrecognized mechanism of post-transcriptional regulation with broad implications for cellular plasticity and tissue regeneration. PIWI proteins, which were originally characterized for their role in transposon silencing and genome integrity in germ cells, are now emerging as multifaceted regulators of gene expression in somatic tissues. Their interaction with RNAs creates a flexible regulatory system that can influence RNA stability, translation and epigenetic states. In the cardiac context, such regulation could influence key processes, including cell-cycle re-entry, stress responses and structural remodelling. In addition to the mechanistic discoveries presented by Wang et al., the study addresses several broader conceptual gaps in the field. Firstly, it resolves the long-standing ambiguity surrounding the functional relevance of piRNA-like molecules identified in somatic tissues, including those in the heart. Although previous transcriptomic datasets had suggested the presence of piRNAs in CMs, their biological function remained entirely speculative. The identification of MCPPIR as a regulator of cardiomyocyte proliferation demonstrates that piRNAs can play a vital role in maintaining the balance and regeneration of somatic tissues. A second major conceptual contribution lies in elucidating the complex MCPPIR–HNRNPH1–POC1B axis, which encompasses piRNA biology, oxidative RNA modifications, centrosome integrity and cardiac regeneration. This work shows that 8-oxoguanine RNA modification, which is usually linked to oxidative stress and RNA damage, can act as a regulatory signal to stabilize particular transcripts. In this case, it is POC1B that is stabilized. This finding expands the functional repertoire of RNA epitranscriptomics, suggesting that oxidative RNA marks may operate as molecular switches during tissue regeneration. Furthermore, the implications for centrosome biology are equally profound. Centrosome disassembly is a defining event associated with the post-natal loss of regenerative capacity in mammalian CMs. However, by preserving POC1B expression and maintaining centrosome cohesion, MCPPIR appears to counteract this post-natal shutdown mechanism, effectively restoring an essential structural prerequisite for cardiomyocyte proliferation. This places MCPPIR at the intersection of structural cell-cycle control and RNA-based epigenetic regulation. Further research is needed to determine how MCPPIR expression is regulated under physiological and pathological conditions. While the piRNA–PIWI system is broadly conserved across species, piRNA sequences vary considerably, raising questions about the existence of analogous MCPPIR–HNRNPH1–POC1B regulatory circuits in human CMs and the safety and specificity of modulating the PIWI-piRNA machinery in the adult heart. Addressing these questions will be critical for translating these findings into MCPPIR-based therapeutic strategies. HNRNPH1 is the key mediator of o8G modification of POC1B; however, its broad expression and pivotal role in RNA processing make it challenging to target directly. Conversely, MCPPIR mimics could offer a cardiomyocyte-selective approach to modulating this pathway, providing a potentially safer method of promoting myocardial regeneration. C.B. received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), SFB TRR267. The figure accompanying this editorial was created in BioRender. BioRender.com/v86k884.