The Multidrug Resistance 1 Gene Abcb1 in Brain and Placenta: Comparative Analysis in Human and Guinea Pig

The Multidrug Resistance 1 (MDR1; alternatively ABCB1) gene product P-glycoprotein (P-gp), an ATP binding cassette transporter, extrudes multiple endogenous and exogenous substrates from the cell, playing an important role in normal physiology and xenobiotic distribution and bioavailability. To date, the predominant animal models used to investigate the role of P-gp have been the mouse and rat, which have two distinct genes, Abcb1a and Abcb1b. In contrast, the human has a single gene, ABCB1, for which only a single isoform has been validated. We and others have previously shown important differences between Abcb1a and Abcb1b, limiting the extrapolation from rodent findings to the human. Since the guinea pig has a relatively long gestation, hemomonochorial placentation and neuroanatomically mature offspring, it is more similar to the human, and may provide a more comparable model for investigating the regulation of P-gp in the brain and placenta, however, to date, the Abcb1 gene in the guinea pig remains to be characterized. The placenta and fetal brain are barrier sites that express P-gp and that play a critical role of protection of the fetus and the fetal brain from maternally administered drugs and other xenobiotics. Using RNA sequencing (RNA-seq), reverse transcription-polymerase chain reaction (RT-PCR) and quantitative PCR (QPCR) to sequence the expressed isoforms of guinea pig Abcb1, we demonstrate that like the human, the guinea pig genome contains one gene for Abcb1 but that it is expressed as at least three different isoforms via alternative splicing and alternate exon usage. Further, we demonstrate that these isoforms are more closely related to human than to rat or mouse isoforms. This striking, overall similarity and evolutionary relatedness between guinea pig Abcb1 and human ABCB1 indicate that the guinea pig represents a relevant animal model for investigating the function and regulation of P-gp in the placenta and brain.多药耐药基因1（Multidrug Resistance 1, MDR1；别名ABCB1）编码的P-糖蛋白（P-glycoprotein, P-gp）属于ATP结合盒转运体（ATP binding cassette transporter），可将多种内源性与外源性底物排出细胞，在正常生理过程、外源性物质分布及生物利用度调控中发挥关键作用。截至目前，用于探究P-gp功能的主流动物模型为小鼠与大鼠，二者均拥有两个独立基因Abcb1a与Abcb1b。与之相对，人类仅携带单个ABCB1基因，且仅验证得到一种同工型（isoform）。此前本团队与其他研究组均已证实Abcb1a与Abcb1b间存在显著差异，这限制了啮齿类动物研究结果向人类的外推。豚鼠妊娠期相对较长，具备血绒毛膜胎盘（hemomonochorial placentation）结构，子代神经解剖发育成熟，与人类更为相似，或可成为研究脑与胎盘组织中P-gp调控机制的更合适模型，但截至目前豚鼠的Abcb1基因仍未完成表征。胎盘与胎儿脑组织均为表达P-gp的屏障位点，在保护胎儿及胎儿脑组织免受母体给药药物与其他外源性物质侵害方面发挥核心作用。本研究通过RNA测序（RNA sequencing, RNA-seq）、逆转录聚合酶链反应（reverse transcription-polymerase chain reaction, RT-PCR）与定量PCR（quantitative PCR, QPCR）对豚鼠Abcb1的表达同工型进行测序分析，结果显示：与人类一致，豚鼠基因组仅包含一个Abcb1基因，但该基因可通过可变剪接（alternative splicing）与可变外显子使用（alternate exon usage）方式表达至少三种不同的同工型。进一步实验证实，这些同工型与人类ABCB1的进化亲缘关系较大鼠或小鼠同工型更为接近。豚鼠Abcb1与人类ABCB1整体存在显著的相似性与进化亲缘关系，表明豚鼠可作为研究胎盘与脑组织中P-gp功能及调控机制的相关动物模型。