A Novel Hemophilia a Mouse Model Shows Expression of a Fluorescent Reporter Under the Endogenous F8 promoter in Subsets of Liver and Kidney Endothelial Cells

Coagulation factor VIII (FVIII) is a protein (encoded by F8) essential for blood clotting, a deficiency of which underlies the bleeding disorder hemophilia A (HA). Much FVIII in healthy individuals comes from the liver, but, on the cellular level, FVIII originates from the liver sinusoidal endothelial cells (LSECs), not hepatocytes. However, gene therapy for HA currently induces ectopic FVIII expression in hepatocytes, which has been implicated in mild toxicities and FVIII decline observed in most recipients. Zones 1-3 of hepatic lobules consist of phenotypically distinct LSECs with potentially differential contributions to systemic FVIII levels. Extrahepatic production of FVIII has been controversial. A better understanding of intra- and extrahepatic FVIII production is needed to improve gene therapy for HA and inform the development of other therapeutic interventions.

We developed, in collaboration with Jackson Laboratory, a new strain of hemophilic mice on the C57BL/6J background by knocking in a transgene encoding the fluorescent protein mScarlet under the endogenous F8 promoter. Using the CRISPR/Cas9 gene editing system, we inserted mScarlet into exon 1 of F8 through homology-directed repair. The animals showed a bleeding phenotype consistent with other HA mouse strains.

We first evaluated mScarlet expression in hepatocytes and liver ECs of mScarlet mice (n=3) using flow cytometry. After in situ perfusion via the portal vein, the livers were collected and processed into single-cell suspensions. An anti-CD31 antibody was used to identify ECs. Hepatocytes were identified by exclusion of CD45.2⁺ cells. CD31⁺ cells from mScarlet mice but not from control C57BL/6J mice expressed mScarlet. Neither mouse strain had detectable mScarlet in hepatocytes. We next evaluated mScarlet expression in LSECs located in different hepatic lobule zones. CD32b^hiLSECs (which are located in zones 2 and 3, closest to the central capillary venule) showed the highest mScarlet expression, with ~30% of cells being mScarlet⁺. CD32b^-/low LSECs (zone 1) showed markedly lower expression with only a small fraction of cells (~1%) being mScarlet⁺. We also detected mScarlet expression in non-LSEC hepatic ECs.

For fluorescence microscopy, we employed an anti-mScarlet antibody for improved detection. The hepatic staining pattern was consistent with that of the CD31 antibody, whereby the cells were in a sinusoidal arrangement of hepatic capillaries running between rows of hepatocytes toward central venules. We found mScarlet expression primarily in zones 2 and 3, with weaker signals in zone 1, consistent with the flow cytometry findings. Additionally, liver sections were co-stained for mScarlet and von Willebrand factor (vWF), the hepatic expression of which is restricted to LSECs. We found that mScarlet and vWF had the same staining pattern in the livers of the reporter mice. Liver sections from control mice showed vWF expression only.

We next used flow cytometry to investigate mScarlet expression in extrahepatic tissues. Liver lymphoid, mesenteric and inguinal lymph node, and lung, kidney, testicular, and splenic endothelial and lymphoid tissues of mScarlet were collected, processed, and analyzed. Kidney ECs, except for renal peritubular endothelial cells (RPECs), showed expression of mScarlet. However, kidney ECs showed ~5-fold lower mScarlet expression than liver ECs. The other tissues showed no mScarlet expression.

Kidney sections were then stained with anti-mScarlet and CD31 or vWF antibodies. We found that mScarlet was expressed in the kidneys of mScarlet mice in the same pattern as CD31 and vWF. No mScarlet expression was seen in the kidney sections of C57BL/6J HA mice lacking the reporter gene, which only showed anti-CD31 or anti-VWF antibody binding.

We propose that FVIII is primarily produced in LSECs located in zones 2 and 3 of hepatic lobules in the mouse. Additionally, FVIII is expressed in kidney ECs, but not in RPECs, hepatocytes, hepatic leukocytes, mesenteric or inguinal lymph nodes, or lung, kidney, testicular, and splenic ECs or leukocytes. However, expression in kidney ECs is considerably lower than in LSEC. Our novel mouse model expressing a well-defined and readily detectable reporter protein overcomes the challenges of immunohistochemistry using anti-FVIII antibodies, affording a better insight into the cellular origins of FVIII