The ability to transiently modulate cell identity in vivo using non-integrating mRNA technology presents a transformative opportunity for regenerative medicine [[1], [2], [3], [4]]. Unlike viral vectors that pose risks of insertional mutagenesis and uncontrolled transgene expression, synthetic mRNA delivery enables precise, time-limited control of transcriptional programs with a favorable safety profile [5,6]. While mRNA has shown clinical utility in vaccination and gene editing [[7], [8], [9], [10]], its broader therapeutic potential for in vivo cell-fate programming remains largely unexplored. In particular, leveraging mRNA-based reprogramming to modulate hepatocyte identity and function represents an attractive strategy to restore liver homeostasis in fibrotic disease.

Liver fibrosis is a progressive and often irreversible pathology resulting from chronic liver injury caused by viral hepatitis, alcohol abuse, metabolic dysfunction, or autoimmunity. It represents a major global healthcare burden, yet without effective antifibrotic therapies been approved so far [[11], [12], [13], [14]]. Hepatocyte dysfunction plays a central role in disease progression by releasing profibrogenic signals that activate hepatic stellate cells (HSCs), driving excessive extracellular matrix (ECM) deposition and architectural distortion [[15], [16], [17]]. While current strategies primarily focus on HSC inactivation, their inability to restore hepatocyte regenerative capacity constitutes a major barrier to fibrosis reversal [18,19].

mRNA delivery via lipid nanoparticles (LNPs) has shown promise in promoting hepatocyte-driven regeneration in genetic liver disease models [2,20,21]. Partial cellular reprogramming with Yamanaka factors (Oct4, Sox2, Klf4, c-Myc) can enhance tissue plasticity and repair [[22], [23], [24], [25]]. However, clinical application of Yamanaka factors faces challenges, particularly due to the safety concerns associated with c-Myc [26,27]. Studies have shown that a combination of Oct4, Sox2, and KLF4 (hereafter referred to as OSK) demonstrates regenerative potential while avoiding the oncogenic risk associated with c-Myc [25]. Another critical challenge is the need for precise spatial restriction and temporal control of OSK expression to prevent adverse effects, such as loss of cell identity, organ dysfunction, and tumorigenesis, while still promoting tissue regeneration [28]. Despite advancements in in vivo reprogramming, virus-based platforms rely on sustained transgene activation, carrying risks of insertional mutagenesis and uncontrolled reprogramming [25]. Inducible promoter-based systems, while providing temporal control, lack the cell type specificity necessary for safe and effective reprogramming [22,28]. Thus, despite substantial progress, achieving safe, tissue- or cell type–specific delivery of OSK mRNA in vivo remains a critical challenge for the therapeutic application of cellular reprogramming.

Here, we address this challenge by developing a hepatocyte-targeted LNP platform for the transient delivery of OSK mRNA to induce partial reprogramming and treat liver fibrosis (Scheme 1). Our system is based on a chemically defined class of unsaturated ionizable lipids, incorporating natural fatty alcohol-derived tails to enhance delivery efficiency. Unsaturated lipids are abundant in nature and play important roles in maintaining membrane stability and fluidity [[29], [30], [31], [32]]. Building on the natural properties and their proven ability to enhance mRNA delivery [[30], [31], [32]], we rationally designed a chemically diverse library of unsaturated ionizable lipids. This involved incorporating natural unsaturated fatty alcohol fragments (farnesol, geraniol, citronellol) into hydrophobic tails, which were combinatorially reacted with selected amine heads via Michael addition reaction. In vivo screening identified H4T3, a lead ionizable lipid with mRNA delivery efficacy comparable to the clinical benchmark SM102. Further formulation optimization yielded the highly selective three-component H4T3_F6 LNP system. Using this system, we show that transient expression of OSK mRNA induces partial hepatocyte reprogramming, promotes tissue regeneration, and attenuates fibrosis in a CCl4-induced mouse model of chronic liver injury. This study establishes a minimally invasive LNP-mRNA strategy for in vivo cellular reprogramming and functional tissue repair, and highlights the power of rational ionizable lipid design in enabling cell-type-specific programming interventions.