Liver sinusoids are lined by liver sinusoidal endothelial cells (LSEC), which represent approximately 15 to 20% of the liver cells, but only 3% of the total liver volume. LSEC have unique functions, such as fluid filtration, blood vessel tone modulation, blood clotting, inflammatory cell recruitment, and metabolite and hormone trafficking. Different subtypes of liver endothelial cells are also known to control liver zonation and hepatocyte function. Here, we have reviewed the origin of LSEC, the different subtypes identified in the liver, as well as their renewal during homeostasis. The liver has the exceptional ability to regenerate from small remnants. The past decades have seen increasing awareness in the role of non-parenchymal cells in liver regeneration despite not being the most represented population. While a lot of knowledge has emerged, clarification is needed regarding the role of LSEC in sensing shear stress and on their participation in the inductive phase of regeneration by priming the hepatocytes and delivering mitogenic factors. It is also unclear if bone marrow-derived LSEC participate in the proliferative phase of liver regeneration. Similarly, data are scarce as to LSEC having a role in the termination phase of the regeneration process. Here, we review what is known about the interaction between LSEC and other liver cells during the different phases of liver regeneration. We next explain extended hepatectomy and small liver transplantation, which lead to “small for size syndrome” (SFSS), a lethal liver failure. SFSS is linked to endothelial denudation, necrosis, and lobular disturbance. Using the knowledge learned from partial hepatectomy studies on LSEC, we expose several techniques that are, or could be, used to avoid the “small for size syndrome” after extended hepatectomy or small liver transplantation.

Surgical resection remains the gold standard for liver tumor treatment, yet the emergence of post-operative liver failure, known as the small for size syndrome (SFSS), poses a significant challenge. The activation of hypoxia sensors in a SFSS liver remnant initiated early angiogenesis, improving vascular architecture, safeguarding against liver failure and reducing mortality. The study aimed to elucidate vascular remodeling mechanisms in SFSS, its impact on hepatocyte function and subsequent liver failure. Mice underwent extended partial hepatectomy to induce SFSS, with a subset exposed to hypoxia immediately after surgery. Hypoxia bolstered post-hepatectomy survival rates. Early proliferation of liver sinusoidal cells, coupled with recruitment of putative endothelial progenitor cells (EPC), increased vascular density, improved lobular perfusion, and limited hemorrhagic events in the regenerating liver under hypoxia. Administration of granulocyte colony stimulating factor (G-CSF) in hepatectomized mice mimicked the effects of hypoxia on vascular remodeling and EPC recruitment, but failed to rescue survival. Compared to normoxia, hypoxia favored hepatocyte function over proliferation, promoting functional preservation in the regenerating remnant. Injection of AAV8-TBG-HNF4α virus for hepatocyte-specific overexpression of HNF4α, the master regulator of hepatocyte function, enforced functionality in proliferating hepatocytes but did not rescue survival. The combination of HNF4α overexpression and G-CSF treatment rescued survival after SFSS-setting hepatectomy. In summary, SFSS arises from an imbalance and desynchronized interplay between functional regeneration and vascular restructuring. To improve survival following SFSS-hepatectomy, it is essential to adopt a two-pronged strategy aimed at preserving the function of proliferating parenchymal cells and simultaneously attenuating vascular damage. Export

In chronic liver diseases (CLD), when native hepatocytes enter replicative senescence, new hepatocytes are produced via differentiation of reactive cholangiocytes, also known as ductular reaction - DR - cells. However, this mechanism alone is insufficient to prevent liver failure, emphasizing the need for a deeper understanding to enhance its therapeutic potential. We hypothesize that signals driving DR differentiation into hepatocytes reside in the local niche, particularly extracellular matrix (ECM) composition and neighboring cells. We modeled CLD in mice using CCl₄ injections (3x/week for 6 weeks). Using Opn-iCreERT2; Rosa26RYFP mice, we tracked DR fate, and with Cdh5-iCreERT2; Rosa26RmT/mG mice, we studied LSEC origins. Mouse liver tissues were analyzed using immunohistochemistry (IHC), flow cytometry, RT-qPCR and RESOLVE molecular cartography for spatial transcriptomic. Double-spheroids were used to study ECM effect in vitro. We examined human liver tissues with double IHC and publicly available scRNA-seq datasets were used. We identified two populations of ductular reaction (DR) cells: those surrounded by laminin (Lam-DRs) and those without laminin (NoLam-DRs). Lam-DRs retained a biliary phenotype, while NoLam-DRs showed increased Hnf4α expression. In vitro, a laminin-rich environment maintained cholangiocyte-specific gene expression, supporting laminin's role in preserving the biliary phenotype. Lam-DRs were closely associated with capillarized CD34+ LSECs, which expressed profibrotic and laminin-related genes. Preventing LSEC capillarization reduced laminin deposition and enhanced DR-to-hepatocyte differentiation in chronically injured mice. The CD34+ LSECs-Laminin-DR complex was also present in human liver disease samples. Capillarized CD34+ LSECs maintain the biliary phenotype of DR cells by preventing laminin degradation, thereby limiting their differentiation into hepatocytes. Targeting this axis could enhance liver regeneration and improve outcomes in chronic liver disease. Chronic liver disease progression is characterized by impaired hepatocyte regeneration, making alternative regenerative pathways such as DR-driven hepatocyte formation of particular interest. This study provides mechanistic insight into how the local microenvironment specifically capillarized CD34⁺ LSECs and laminin deposition restricts DR plasticity by stabilizing a biliary fate, thereby limiting endogenous regeneration. These findings are relevant for researchers investigating liver regeneration, fibrosis, and cholangiopathies, and for clinicians seeking to understand why regenerative responses remain inefficient in advanced disease. While acknowledging that DR-derived hepatocytes contribute modestly to overall parenchymal renewal, our results suggest that therapeutically modulating LSEC capillarization or ECM remodeling could represent a strategy to enhance regenerative competence in chronic liver disease, warranting further preclinical and translational investigation.

Surgical resection remains the gold standard for liver tumor treatment, yet the emergence of post-operative liver failure, known as the small for size syndrome (SFSS), poses a substantial challenge. The activation of hypoxia sensors in a SFSS liver remnant initiated early angiogenesis, improving vascular architecture, safeguarding against liver failure and reducing mortality. The study aimed to elucidate vascular remodeling mechanisms in SFSS, its impact on hepatocyte function and subsequent liver failure. Mice underwent extended partial hepatectomy to induce SFSS, a subset were exposed to hypoxia immediately after surgery. Hypoxia bolstered post- hepatectomy survival rates. Early proliferation of liver sinusoidal cells coupled with augmented recruitment of endothelial progenitor cells (EPC) via the VEGF/SDF-1α pathway resulted in heightened vascular density, improved lobular perfusion, and limited hemorrhagic events in the regenerating liver under hypoxia. The administration of G-CSF mimicked the effects of hypoxia on vascular remodeling and EPC recruitment, though it failed to rescue survival. Compared to normoxia, hypoxia restrained hepatocyte proliferation yet improved the function of the regenerating remnant, favoring functional preservation in the liver remnant. Injection of AAV8- TBG-HNF4α virus for hepatocyte-specific overexpression of HNF4α, the master regulator of hepatocyte function, enforced functionality in proliferating hepatocytes. The combination, only, of HNF4α overexpression and G-CSF treatment rescued survival post-SFSS-setting hepatectomy. In summary, SFSS arises due to imbalance and desynchronized interplay between functional regeneration and vascular restructuring. To enhance survival following SFSS-hepatectomy, a two- pronged strategy is essential, addressing the preservation of function in the proliferating parenchymal cells alongside the simultaneous mitigation of vascular harm. One Sentence Summary Combined treatment with G-CSF and HNF4α overexpression rescues vascular damage and function to improve survival after extended hepatectomy in mice.