概述
RCAS（Replication-Competent Avian Sarcoma-Leukosis Virus Long Terminal Repeat with a Splice Acceptor）是一种源自具有复制能力的Rous肉瘤病毒（Rous Sarcoma Virus, RSV）的逆转录病毒载体。该载体具备剪切受体以及禽肉瘤-白血病病毒（ASLV）的长末端重复序列（LTR）。野生型RSV保留了所有必要的病毒基因以及原癌基因src。在RCAS载体里，src原癌基因被移除，并由目的基因替代，但仍保留了src的剪接位点，由此插入的目的基因能够在LTR的驱动下，通过剪接形成病毒转录本进而实现表达。RCAS载体在合适的禽类细胞中依旧具备复制能力，并且能够感染表达禽类病毒受体的哺乳动物细胞。
RCAS表达载体RCASBP(A)，是基于Bryan Polymerase（BP）株的RSV病毒基因组序列构建而成。该毒株属于A亚群，拥有能与TVA受体特异性结合的包膜蛋白。与普通RCAS相比，RCASBP株在鸡细胞中有更高的复制效率。其关键改造在于将野生型RSV的pol基因替换为来源于高滴度Bryan株的pol基因，从而显著提升病毒复制效率与产量。逆转录病毒感染细胞需借助病毒表面包膜糖蛋白与靶细胞表面相应受体（在本例中为TVA）之间的特异性相互作用来达成。在禽肉瘤白血病病毒（Avian Sarcoma-Leukosis Virus, ASLV）家族的五个主要亚群里，A亚群因具有低细胞毒性和高病毒滴度的特性而成为应用最广泛的类型。由于原代鸡胚成纤维细胞、永生化的鸡胚成纤维细胞（DF-1）以及鹌鹑成纤维细胞系（QT6或QT-35）天然表达TVA受体，所以能够被A亚群病毒感染。相反，哺乳动物细胞内源性不表达TVA，必须借助基因工程手段使其表达该受体，方能实现RCASBP(A)的感染。
RCASBP(A)在受体介导下进入细胞后，其RNA基因组发生逆转录，并将产生的前病毒DNA稳定整合到宿主细胞基因组中。由于其在允许性禽类细胞中具有复制能力，病毒产量可通过细胞间传播而得到放大，而无需辅助系统。这种特性使得该病毒相对适合在禽类胚胎模型中进行应用，可实现局部感染，从而在时空上精确控制基因表达，适用于研究神经图示和肢体发育等体内发育过程。在哺乳动物系统中，RCASBP(A)病毒载体通常与表达TVA的细胞或转基因小鼠模型联合使用，以实现高度特异性得的基因靶向递送。虽然在哺乳动物细胞中病毒的复制受到限制，但稳定整合的特性得以保留，仍可实现原癌基因、抑癌基因或报告基因的持续表达。基于其特性，RCAS-TVA系统已被广泛应用于肿瘤学诸如胶质瘤、肉瘤和上皮性肿瘤等研究，可用于在体内模拟肿瘤发生、进展以及遗传相互作用。通过将感染限制在特定细胞群内，使用RCASBP(A)能够在最大限度减少脱靶效应的同时实现精确的遗传操作。
关于RCAS表达载体的更多信息，请参考以下文献。
亮点
此载体系统经过优化，适用于在合适的禽类细胞以及表达TVA受体的工程化哺乳动物系统中进行稳定、高效的转基因递送，为功能研究、发育生物学以及肿瘤发生通路建模等领域的体外与体内实验提供了可靠而强大的工具。
试验验证
我们的RCAS载体经过验证，可以在TVA表达细胞中实现特异且高效的基因表达，见下方图1
图1 使用RCAS载体系统生成表达EGFP的HEK293T-TVA细胞。(A) 将编码EGFP的RCAS载体转染至DF-1细胞中。经扩增后，收集RCAS病毒颗粒，并用其分别转导野生型和表达TVA的HEK293T 细胞（MOI=30）。转导后第3天，使用流式细胞术定量EGFP的荧光强度。(B) 转导后第3天的典型荧光显微镜图像。曝光时间：明场=10 ms；EGFP=100 ms。放大倍数：100×。(C) 感染 RCAS-EGFP的HEK293T-TVA 细胞，其EGFP表达量较NC增加约72%
优势
载体DNA可永久整合：使用常规转染方式通常只能进行瞬时递送，这是因为宿主细胞随着时间将丢失转染的DNA。这种现象在快速分裂的细胞中尤为显著。相对地，逆转录病毒可将转基因永久整合宿主细胞基因组，实现转基因的持续表达。.
高特异性：我们的载体编码了与靶细胞TVA受体相结合的包膜糖蛋白，可实现精确的基因递送，同时最小化脱靶效应。
体外与体内高效性：我们的载体可应用于在体外的禽类细胞及其他表达TVA的细胞中，以及在鸡胚及其他表达TVA的动物模型中高效递送并表达转基因。
技术简易： 该载体可通过标准的禽类细胞系中高效生产，无需辅助系统或者特殊设备，从而显著降低整体生产成本。
安全性： 与野生型RSV不同，该载体已去除src原癌基因，从而显著降低了致瘤风险，同时仍保留了在禽类细胞中复制所需的必需病毒基因。
不足之处
装载能力非常有限：RCASBP(A)载体基因组约为10 kb，其中仅约2.5 kb可用于容纳外源序列。如果目的基因开放阅读框（ORF）超过这一大小限制，病毒滴度将显著下降。
在禽类宿主细胞中具有复制能力：RCASBP(A)载体含有所有必需的病毒复制基因，一旦感染禽类细胞，即可自然复制并传播至邻近细胞。
哺乳动物细胞应用需过表达TVA：由于哺乳动物细胞不表达RCASBP(A)受体，必须人工表达TVA才能允许病毒进入。虽然RCASBP(A)可感染表达TVA的哺乳动物细胞，但由于受到多种宿主水平限制，其在哺乳动物细胞中的复制效率极低，在标准哺乳动物系统中实际上是非复制性的。
关键元件
RSV LTR: Rous sarcoma virus (RSV) long terminal repeat. The LTRs reside on two ends of the viral genome and point in the same direction, with both promoter and polyadenylation function. The LTR upstream of the viral genome acts as a promoter to drive transcription while the downstream LTR acts as a polyadenylation signal to terminate the upstream transcript.
Gag-Pol-Env: Codes for the viral structural proteins (Gag), enzymes (Pol), and envelope glycoprotein (Env) required for viral particle assembly, replication, and TVA receptor tropism, respectively. In our vector, the pol region is derived from the Bryan high-titer strain of RSV.
Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest because it is believed to facilitate translation initiation in eukaryotes.
ORF: The open reading frame of your gene of interest is placed here. Its expression is driven by the ubiquitous promoter function in the 5' LTR.
Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.
pBR322 ori: pBR322 origin of replication. Allows plasmid replication in E.coli.
Rop: Repressor of primer. Regulates plasmid copy number by controlling replication initiation at the pBR3322 ori.
Overview
The RCAS (Replication-Competent Avian Sarcoma-Leukosis Virus long terminal repeat with a Splice acceptor) retroviral vectors are derived from the Rous sarcoma virus (RSV). Wildtype RSV retains all essential viral genes and the oncogene src. In RCAS vectors, the src oncogene is deleted and replaced with a transgene of interest while maintaining the src splice sites, enabling expression of the inserted gene from a spliced viral transcript driven by the viral long terminal repeat (LTR). As a result, RCAS vectors remain replication-competent in appropriate avian cells and can infect mammalian cells engineered to express the appropriate viral receptor.
Our RCAS expression vector, RCASBP(A), is a derivative based on the Bryan Polymerase (BP) strain with envelope specificity to subgroup A (TVA receptor). The RCASBP strain has been shown to have higher efficiency replication in chicken cells compared to RCAS. In this vector, the wildtype RSV pol gene is replaced with the pol gene from the high-titer Bryan strain, resulting in enhanced viral production. Retroviral infection generally requires a specific interaction between the envelope glycoprotein on the viral surface and its corresponding receptor (TVA) on the target cell surface. Among the five primary envelope subgroups of the Avian Sarcoma-Leukosis Virus (ASLV) family, subgroup A is most frequently used due to its low cytotoxicity and high viral titers. Primary chicken embryo fibroblasts, immortalized chicken embryo fibroblasts (DF-1), and quail fibroblast cell lines (QT6 or QT-35) naturally express TVA and can be infected with subgroup A viruses. In contrast, mammalian cells lack endogenous TVA and must be genetically engineered to express this receptor to enable RCASBP(A) infection.
Following receptor-mediated entry, RCASBP(A) undergoes reverse transcription of its RNA genome and stable integration of the resulting proviral transgene DNA into the host cell genome. Due to its replication-competent nature in permissive avian cells, viral production is amplified through cell-to-cell spread without the need for helper systems. This is particularly powerful in avian embryo models, where localized infection enables spatially and temporally controlled gene expression for the in vivo study of developmental processes such as neural patterning and limb development.
In mammalian systems, RCASBP(A) vectors are widely used in combination with TVA-expressing cells or transgenic mouse models to achieve highly targeted gene delivery. Although viral replication is limited in mammalian cells, stable integration allows sustained expression of oncogenes, tumor suppressors, or reporter genes. The RCAS–TVA system has been extensively applied in oncology research to model tumor initiation, progression, and genetic interactions in vivo, including studies of glioma, sarcoma, and epithelial cancers. By restricting infection to defined cell populations, RCASBP(A) enables precise genetic manipulation while minimizing off-target effects.
For further information about this vector system, please refer to the papers below:
Highlights
This vector system is optimized for stable and efficient transgene delivery into appropriate avian and TVA-engineered mammalian systems, providing a robust and reliable system for in vitro and in vivo applications in functional studies, developmental biology, and modeling of oncogenic pathways.
Experimental validation
Our RCAS vector has been validated for specific and efficient gene expression in TVA-expressing cells as shown in Figure 1 below.
Figure 1. Generation of EGFP-expressing HEK293T-TVA cells using the RCAS vector system. (A) An RCAS vector encoding for EGFP was transfected in DF-1 cells. Upon amplification, RCAS virions were collected and used to transduce wildtype or TVA-expressing HEK293T cells (MOI = 30). Fluorescence intensity of EGFP was quantified using flow cytometry three days post-transduction. (B) Representative fluorescence microscopy images three days post-transduction. Exposure: brightfield=10 ms; EGFP=100 ms. Magnification: 100x. (C) HEK293T-TVA cells infected with RCAS-EGFP showed a ~72% increase in EGFP expression.
Advantages
Permanent integration of vector DNA: Conventional transfection results in almost entirely transient delivery of DNA into host cells due to the loss of DNA over time. This problem is especially prominent in rapidly dividing cells. In contrast, retroviral transduction can deliver genes permanently into host cells due to integration of the cargo into the host genome.
High specificity: Our vector encodes for the envelope glycoprotein that specifically infects cells expressing TVA, enabling precise transgene delivery and minimizing off-target effects.
In vitro and in vivo efficacy: Our vectors are designed to efficiently deliver and express transgenes in cultured avian and other TVA-positive cells as well as in chicken embryos and other TVA-positive animal models, making them effective tools for both in vitro and in vivo studies.
Technical simplicity: These vectors can be produced efficiently in standard avian cell lines without the need for helper systems or specialized equipment, which reduces overall cost.
Safety: Unlike wildtype RSV, our vector lacks the src oncogene, thereby significantly reducing oncogenic risk while still retaining essential viral genes required for replication in avian cells.
Disadvantages
Very limited cargo capacity: The RCASBP(A) vector genome is ~10 kb, which leaves only ~2.5 kb to accommodate the user's DNA of interest. If a large ORF exceeds this size limit, viral titer can be severely reduced.
Replication-competent in avian host cells: The RCASBP(A) vector contains all essential viral genes for replication and, upon infection of avian cells, can naturally replicate and spread to neighboring cells. While RCASBP(A) can infect TVA-expressing mammalian cells, replication is inefficient due to multiple host-level restrictions, rendering it effectively non-replicative in standard mammalian systems.
TVA overexpression required for mammalian cell applications: As mammalian cells do not express the RCASBP(A) receptor, TVA expression is necessary to allow viral entry. RCAS vectors can efficiently infect and deliver transgenes to TVA-positive mammalian cells, although replication remains limited due to host-specific restrictions.
Key components of our vector
RSV LTR: Rous sarcoma virus (RSV) long terminal repeat. The LTRs reside on two ends of the viral genome and point in the same direction, with both promoter and polyadenylation function. The LTR upstream of the viral genome acts as a promoter to drive transcription while the downstream LTR acts as a polyadenylation signal to terminate the upstream transcript.
Gag-Pol-Env: Codes for the viral structural proteins (Gag), enzymes (Pol), and envelope glycoprotein (Env) required for viral particle assembly, replication, and TVA receptor tropism, respectively. In our vector, the pol region is derived from the Bryan high-titer strain of RSV.
Kozak: Kozak consensus sequence. It is placed in front of the start codon of the ORF of interest because it is believed to facilitate translation initiation in eukaryotes.
ORF: The open reading frame of your gene of interest is placed here. Its expression is driven by the ubiquitous promoter function in the 5' LTR.
Ampicillin: Ampicillin resistance gene. It allows the plasmid to be maintained by ampicillin selection in E. coli.
pBR322 ori: pBR322 origin of replication. Allows plasmid replication in E.coli.
Rop: Repressor of primer. Regulates plasmid copy number by controlling replication initiation at the pBR3322 ori.
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