Portis J L, Fujisawa R, Mcatee F J
Laboratory of Persistent Viral Diseases, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, Hamilton, Montana 59840, USA.
Virology. 1996 Dec 15;226(2):384-92. doi: 10.1006/viro.1996.0666.
Neuroinvasiveness is a property of all neurovirulent murine retroviruses, although the factors which facilitate infection of the CNS are not understood. We previously showed that mutant MuLV which lack expression of an accessory protein, glycosylated gag, had lost neurovirulence, indicating that this protein may be involved in promoting CNS infection. The mutations were located in the "Kozak" consensus sequence of the initiation codon for this protein. Here it is shown that shortly after inoculation of mice with one of these mutant viruses, revertants emerged which had regained expression of glycosylated gag and had also regained the neuroinvasiveness and neurovirulence exhibited by the wild-type virus. The phenotypic revertants retained the mutations in the "Kozak" consensus sequence but exhibited a G-->A mutation 12 codons downstream from the mutated start site, creating a new initiation codon and a glycosylated gag protein, which was truncated at its N-terminus. Using antibodies specific for glycosylated gag it is shown that the frequency of splenic infectious centers expressing revertant virus increased progressively during the 2 months following inoculation of mutant virus until > or = 50% of the virus-producing cells in the spleen expressed revertant virus. In contrast, although phenotypic revertants were detectable at low frequency after cell-free passage in vitro in M. dunni fibroblasts, there was no evidence for selection. These results indicate that glycosylated gag facilitates virus spread within the spleen and to extra-splenic sites, such as the CNS, and suggest that the protein may function through its interaction with the host.
神经侵袭性是所有具有神经毒性的鼠逆转录病毒的一种特性，尽管目前尚不清楚促进中枢神经系统感染的因素。我们之前发现，缺乏辅助蛋白糖基化gag表达的突变型鼠白血病病毒（MuLV）失去了神经毒性，这表明该蛋白可能参与促进中枢神经系统感染。这些突变位于该蛋白起始密码子的“Kozak”共有序列中。本文表明，在用其中一种突变病毒接种小鼠后不久，就出现了回复突变体，它们恢复了糖基化gag的表达，同时也恢复了野生型病毒所具有的神经侵袭性和神经毒性。表型回复突变体在“Kozak”共有序列中保留了突变，但在突变起始位点下游12个密码子处出现了一个G→A突变，产生了一个新的起始密码子和一个糖基化gag蛋白，该蛋白在其N端被截断。使用针对糖基化gag的特异性抗体表明，在接种突变病毒后的2个月内，表达回复突变病毒的脾脏感染中心频率逐渐增加，直到脾脏中≥50%的病毒产生细胞表达回复突变病毒。相比之下，尽管在体外无细胞传代培养邓恩氏小鼠成纤维细胞后，可低频率检测到表型回复突变体，但没有选择的证据。这些结果表明，糖基化gag促进病毒在脾脏内以及向脾脏外部位（如中枢神经系统）传播，并提示该蛋白可能通过与宿主相互作用发挥功能。
