Liver Mouse Models: Advances in Human Liver Research

Studying human liver diseases and drug metabolism is challenging due to the organ’s complexity and ethical limitations on human experimentation. To address this, researchers have developed specialized mouse models incorporating human liver cells, enabling more accurate investigations into liver function, disease progression, and treatment responses.

These models provide a controlled setting for studying human-specific liver biology. Understanding their creation and unique features offers insight into their applications and limitations.

Constructing Humanized Liver Models

Developing effective humanized liver mouse models requires selecting appropriate human donor tissue, modifying the mouse host to support engraftment, and employing specialized transplantation techniques. Each step ensures the successful integration of human liver cells into a viable animal host.

Selection Of Human Donor Tissue

The success of a humanized liver model depends on the quality and origin of transplanted human hepatocytes. Primary human hepatocytes, derived from donor liver tissue, are preferred for maintaining native liver functions, including drug metabolism and protein synthesis. These cells are typically sourced from liver resections, transplant discards, or cadaveric donors, with viability assessed before transplantation.

Cryopreservation is commonly used for storage, though it can impact cell viability. A 2021 Hepatology study found that freshly isolated hepatocytes exhibit superior engraftment rates compared to cryopreserved cells, though optimized thawing protocols help mitigate differences. Induced pluripotent stem cell (iPSC)-derived hepatocytes offer a renewable alternative with patient-specific genetic backgrounds, but their immature metabolic profiles limit their use in drug metabolism research.

Engineering Mice For Lowered Immunity

To prevent rejection of human liver cells, recipient mice are genetically modified to tolerate xenografts. Commonly used strains include severe combined immunodeficient (SCID) mice and NOD-SCID gamma (NSG) mice, both lacking functional T and B lymphocytes. Further modifications, such as deletion of the interleukin-2 receptor gamma (IL2rg) gene, enhance immunodeficiency and improve human cell engraftment.

Some models also incorporate liver injury mechanisms to promote human hepatocyte repopulation. The FRG (Fah−/−, Rag2−/−, Il2rg−/−) mouse model, described in a 2020 Gastroenterology study, induces liver damage through fumarylacetoacetate hydrolase (FAH) deficiency. Administering protective agents like NTBC (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione) allows researchers to control liver injury and facilitate human cell integration.

Techniques For Tissue Engraftment

Successful engraftment of human hepatocytes requires precise delivery methods to ensure cell survival and integration. The most common technique is intrasplenic injection, where hepatocytes migrate to the liver via the portal circulation, ensuring even distribution while minimizing tissue damage.

Alternative methods include direct intrahepatic injection, which offers localized delivery but requires careful handling to prevent excessive disruption. A 2022 Cell Stem Cell study highlighted scaffold-based approaches, where hepatocytes are seeded onto biodegradable matrices before transplantation, enhancing attachment and survival. Co-transplantation with stromal cells or extracellular matrix components further improves engraftment efficiency by providing structural support and biochemical signals that promote hepatocyte function.

Refining these techniques remains an active research area aimed at improving long-term hepatocyte survival and integration.

Distinguishing Features In Grafted Liver Tissue

Grafted human liver tissue in chimeric mouse models exhibits distinct structural and functional characteristics. Immunohistochemical staining shows that engrafted human hepatocytes retain their polygonal shape and form bile canaliculi similar to human liver tissue, despite being surrounded by murine extracellular matrix components. This structural preservation is crucial for accurately modeling liver function.

Beyond morphology, grafted human hepatocytes display species-specific protein expression patterns. A 2021 Hepatology Communications study found that human albumin secretion in highly repopulated chimeric mice can exceed 5 mg/mL. Additionally, cytochrome P450 enzymes responsible for drug metabolism exhibit human-specific isoform expression, making these models more relevant for pharmacokinetic studies than conventional rodent liver systems.

Vascularization patterns also influence hepatocyte survival and function. While murine sinusoidal endothelial cells provide initial structural support, engrafted human hepatocytes secrete angiogenic factors like vascular endothelial growth factor (VEGF), promoting localized blood vessel remodeling. A 2022 American Journal of Pathology study highlighted this partial humanization of hepatic vasculature, which has implications for studying liver diseases such as fibrosis and cirrhosis.

Cellular Cross-Talk In Liver Microenvironments

Humanized liver mouse models offer insight into the complex cellular interactions that regulate liver function. Hepatocytes communicate with non-parenchymal cells like sinusoidal endothelial cells, hepatic stellate cells, and Kupffer-like macrophages, influencing metabolism and extracellular matrix remodeling. Understanding these interactions is essential for interpreting disease mechanisms and drug responses.

Engrafted hepatocytes must adapt to murine sinusoidal architecture, altering the expression of transporters such as organic anion-transporting polypeptides (OATPs), which mediate drug uptake. RNA sequencing studies show that human hepatocytes in chimeric livers exhibit modified transporter expression compared to in vitro cultures, emphasizing the role of cellular cross-talk in shaping hepatic function.

Hepatic stellate cells regulate extracellular matrix composition and respond to hepatocyte-derived signaling molecules like transforming growth factor-beta (TGF-β). In humanized liver models, murine stellate cells react to human hepatocyte-secreted cytokines, influencing fibrogenic pathways. Recent studies show that even in a murine-dominant extracellular matrix, human hepatocytes can modulate stellate cell activation, highlighting the bidirectional nature of these interactions.

Metabolic Enzyme Expression Patterns

Humanized liver mouse models are valuable for analyzing metabolic enzyme expression, particularly in drug metabolism and xenobiotic processing. Human hepatocytes express cytochrome P450 (CYP) enzymes responsible for phase I metabolism of pharmaceuticals, toxins, and endogenous compounds. Expression levels vary based on donor cell origin, host liver microenvironment, and the extent of human cell repopulation, factors critical for assessing these models’ relevance in pharmacokinetics and toxicology research.

CYP3A4, the most abundant drug-metabolizing enzyme in the human liver, exhibits significantly higher expression in humanized mice than in conventional murine models, where its orthologous enzyme, Cyp3a11, dominates. This allows for more accurate predictions of human drug clearance rates, as demonstrated in pharmacokinetic studies of midazolam and atorvastatin, both metabolized by CYP3A4. However, some CYP enzymes, such as CYP2D6 and CYP1A2, show variable expression depending on the donor hepatocyte source, reflecting interindividual differences observed in human populations.