Splicing serves as a mechanism to regulate gene expression in development and differentiation, and “regulators” have been identified for tissue-specific splicing (22
). Although little is known about the role of splicing in regulation of gene expression in acute response to stimuli, it is clear that splicing is involved in gene regulation during cellular activation. For example, alternative splicing of mRNA of certain proteins occurs in response to activation signals (20
). This study presents the first clear evidence of a critical role of activation-induced splicing in gene expression after T cell activation. In naive T cells, splicing of TNF pre-mRNA is an inefficient process that results in the accumulation of TNF pre-mRNA in the nucleus. The splicing of all introns of TNF pre-mRNA is triggered rapidly by TCR engagement, and this activation-induced splicing is followed by TNF protein production. Thus, splicing of nuclear pre-mRNA can be a regulating factor in induced gene expression. In addition, activation-induced splicing only occurred for a short time after TCR engagement (possibly as little as 12 h), and activation-induced splicing was apparently reversible. The reversible process and different signal requirements distinguish activation-induced splicing of TNF pre-mRNA from alternative splicing. The mechanism(s) of activation-induced splicing remains unclear. However, in human T cells, splicing of TNF pre-mRNA, but not other cytokine mRNAs, can be specifically inhibited by 2-aminopurine (25
), suggesting an mRNA-specific splicing regulator. In naive T cells, this regulator may block the nuclear processing of TNF pre-mRNA. When T cells are activated, the regulator may be released from TNF pre-mRNA allowing the pre-mRNA to be quickly processed. In the later stages of activation, the regulator molecules may again accumulate, bind to TNF pre-mRNA, and stop the splicing of TNF pre-mRNA. Whether such an mRNA-specific splicing regulator can be identified remains an interesting question. It will also be interesting to study whether activation-induced splicing is involved in the regulation of expression of other cytokines, presumably using different regulator(s).
Transcription and splicing are highly coordinated for the expression of many genes. However, transcriptional activity without splicing of pre-mRNA has been observed in the expression of both IL
-1 and IL
-2 genes (15
). Here we provide evidence that splicing of TNF pre-mRNA in naive CD4+
T cells occurs after an activation signal(s) in the absence of transcription. The coordination and dissociation of transcription and splicing may be controlled by different mechanisms to allow genes to be constitutively active or for genes for which function is transiently required. The mature mRNA of cytokines degrades rapidly due to a conserved AU sequence in the 3′ untranslated region (26
). The dissociation of transcription and splicing may result in nuclear accumulation of the cytokine pre-mRNA that is relatively stable and can be released as functionally mature mRNA after appropriate activation stimuli. The mechanism of activation-induced splicing may thus facilitate an immediate response to activation stimuli.
TNF is a cytokine with powerful inflammatory effects involving many autoimmune diseases in animal models (28
), thus expression of TNF has to be tightly controlled. However, TNF is one of the major players in hyperacute responses (31
), mounts the first line of host defense, and triggers a cascade of immune response (32
). Particularly in T cells, TNF is the first cytokine produced after activation, followed by many others (34
). Thus, whereas TCR-mediated activation-induced splicing of TNF pre-mRNA allows and may enhance the immediate alarm signal after T cell receptor engagement and multiple layers of regulation, including transcription, splicing, RNA stability, translation, and posttranslational events, ensure the accurate regulation of TNF expression.