Cre-ativity in the liver: transgenic approaches to targeting hepatic nonparenchymal cells

Abstract

Rapid evolution in transgenic (Tg) mouse technology now permits cell-specific and temporal control of fluorescent cell-labeling and gene inactivation. Here, we discuss the principal strategies that have been utilized to target, label, and manipulate hepatic nonparenchymal cells, with emphasis on the utility of constitutive and inducible Cre-lox systems. We summarize key findings of studies employing Tg technology to target hepatic stellate cells, myofibroblasts, liver sinusoidal endothelial cells, and macrophages to illustrate the power of these approaches in identifying cell-specific molecular mechanisms critical to the pathophysiology of liver disease. Increasing adoption of Tg techniques will help to answer fundamental questions regarding the pathogenesis of hepatic diseases and provide the mechanistic rationale to allow identification of novel drug targets, ultimately translating into effective therapies for patients with liver disease.

Figure 1

Schematic diagram of cell-specific Cre recombinase-induced gene inactivation and fluorescent reporter expression systems. (A) Cell-specific expression of Cre recombinase results in excision of loxP-flanked sequences leading to gene inactivation or fluorescent reporter expression. (B) Spatiotemporal regulation of Cre activity: tamoxifen (TAM) administration permits entry of the Cre-ER^T2 complex into the nucleus, allowing excision of loxP-flanked sequences.

Figure 2

Labeling of hepatic NPCs with fluorescent reporters. (A and B) Lrat-Cre-driven ZsGreen labeling of qHSCs and aHSCs in whole livers from untreated (A) and CCl₄-treated (B) mice. Adapted by permission from Macmillan Publishers Ltd: Nat Commun, © 2013. (C and D) Pdgfrb-Cre-driven membranous GFP labeling (green) of qHSCs and aHSCs in Pdgfrb-Cre;mTmG mice following olive oil (control, C) or chronic CCl₄ (D) administration. Adapted by permission from Macmillan Publishers Ltd: Nat Med, © 2013. (E) Tamoxifen-induced Cdh5-PAC-CreER^T2-driven TdTomato (red) labeling of LSECs. Nuclei (blue), α-SMA⁺ cells (green). Adapted from Supplementary Information to a previous work, by permission from the authors. (F) LysM-Cre-driven membranous GFP labeling (green) of macrophages in uninjured liver of LysM-Cre;mTmG mice (K.P.C. and N.C.H., unpublished data).

Figure 3

Tg approaches to targeting NPCs in the liver. qHSC: Lrat and Pdgfrb; Mac (macrophage): Csfr1 and LysM; LSEC: Cdh5; myofibroblast: α-SMA, Col-α1(I), Col-α2(I), Lrat, Pdgfrb, and vimentin.