mBio. 2014 Apr 1;5(2):e00941-14. doi: 10.1128/mBio.00941-14.
Passage in mice of opportunistic pathogens such as Cryptococcus neoformans is known to increase virulence, but little is known about the molecular mechanisms involved in virulence adaptation. Serial mouse passage of nine environmental strains of serotype A C. neoformans identified two highly adapted virulent strains that showed a 4-fold reduction in time to death after four passages. Transcriptome sequencing expression studies demonstrated increased expression of a FRE3-encoded iron reductase in the two strains but not in a control strain that did not demonstrate increased virulence during mouse passage. FRE3 was shown to express an iron reductase activity and to play a role in iron-dependent growth of C. neoformans. Overexpression of FRE3 in the two original environmental strains increased growth in the macrophage cell line J774.16 and increased virulence. These data demonstrate a role for FRE3 in the virulence of C. neoformans and demonstrate how the increased expression of such a "virulence acquisition gene" during the environment-to-mammal transition, can optimize the virulence of environmental strains in mammalian hosts. IMPORTANCE Cryptococcus neoformans is a significant global fungal pathogen that also resides in the environment. Recent studies have suggested that the organism may undergo microevolution in the host. However, little is known about the permitted genetic changes facilitating the adaptation of environmental strains to mammalian hosts. The present studies subjected environmental strains isolated from several metropolitan areas of the United States to serial passages in mice. Transcriptome sequencing expression studies identified the increased expression of an iron reductase gene, FRE3, in two strains that adapted in mice to become highly virulent, and overexpression of FRE3 recapitulated the increased virulence after mouse passage. Iron reductase in yeast is important to iron uptake in a large number of microbial pathogens. These studies demonstrate the capacity of C. neoformans to show reproducible changes in the expression levels of small numbers of genes termed "virulence adaptation genes" to effectively increase pathogenicity during the environment-to-mammal transition.
在老鼠中，机遇性病原体（如新型隐球菌）的定植会增加其毒力，但关于参与毒力适应的分子机制知之甚少。对九株环境分离的 A 型新型隐球菌血清型菌株进行连续小鼠传代，发现两株高度适应和毒力增强的菌株在经过四轮传代后，死亡时间缩短了 4 倍。转录组测序表达研究表明，在这两株菌中，FRE3 编码的铁还原酶表达增加，但在另一株在小鼠传代过程中未表现出毒力增强的对照株中没有增加。FRE3 表现出铁还原酶活性，并在新型隐球菌的铁依赖性生长中发挥作用。在两株原始环境株中过表达 FRE3 增加了巨噬细胞系 J774.16 的生长，并增加了毒力。这些数据表明 FRE3 在新型隐球菌的毒力中起作用，并表明在环境到哺乳动物的转变过程中，这种“毒力获得基因”的表达增加如何使环境株在哺乳动物宿主中的毒力得到优化。重要性新型隐球菌是一种重要的全球性真菌病原体，也存在于环境中。最近的研究表明，该生物体可能在宿主中发生微进化。然而，对于促进环境株适应哺乳动物宿主的允许遗传变化知之甚少。本研究将从美国几个大都市区分离的环境株进行连续小鼠传代。转录组测序表达研究发现，在两株适应小鼠并变得高度毒力的菌株中，铁还原酶基因 FRE3 的表达增加，过表达 FRE3 可重现小鼠传代后的毒力增加。酵母中的铁还原酶对许多微生物病原体的铁摄取很重要。这些研究表明，新型隐球菌具有重现性地改变少数基因（称为“毒力适应基因”）表达水平的能力，以在环境到哺乳动物的转变过程中有效地增加致病性。
