Regulation of mRNA degradation is critical in maintaining normal gene expression and cellular function. We are interested in understanding how cells regulate gene expression by the control of mRNA stability with an emphasis on the role of the 5´-end cap and its removal (decapping) in modulating mRNA turnover. Identification and characterization of critical components in the decapping pathway have enabled the lab to uncover (1) all known mammalian decapping enzymes (Wang and Kiledjian, 2001; Wang et al., 2002; Liu et al., 2002; Song et al., 2010; Song et al., 2013; Grudzien-Nogalska et al., 2016 RNA); (2) novel mRNA quality control mechanisms (Xiang et al., 2009; Jiao et al., 2010; Chang et al., 2012; Jiao et al., 2013); (3) new classes of 5´-end RNA caps (Jiao et al. 2017; Mauer et al., 2017); and (4) important links between mRNA decapping and human disorders (Li et al., 2012; Ahmed et al., 2015; Castellanos-Rubio et al., 2016).
• Canonical mRNA m7G-cap Decapping
We have been at the forefront of identifying mammalian decapping enzymes, characterizing their functional significance in the modulation of gene expression, and their physiological significance in cells. These range from identification of Dcp2 as the first mRNA decapping enzyme to Nudt3 as the most recent and their respective roles in the regulation of innate immunity and cell migration (Grudzien-Nogalska and Kiledjian, 2017). We are currently pursuing the functional role of these decapping enzymes as well as additional decapping enzymes in various cellular functions.
• 5´-end Nicotinamide Adenine Diphosphate (NAD) cap in mammalian cells.
The most recent exiting finding from the lab is the demonstration that mammalian RNA can possess an alternative cap consisting of NAD instead of the m7G-cap at the 5´ end that can be added by a novel NAD cap addition (NADding) mechanism (Jiao et al. 2017). Furthermore, the fungal and mammalian DXO proteins are potent “deNADding” enzymes that remove the NAD cap to facilitate RNA degradation. Unlike the canonical cap, the NAD cap does not support mRNA translation and surprisingly, promotes rapid mRNA decay. Moreover, NAD cap levels are altered following cellular exposure to environmental stress indicating an important correlation between stress and NAD capping. These findings present a transformative mode of 5´ end epitranscriptomic RNA modification and a new modulatory network potentially linking RNA metabolism to cellular energetics and stress (Kiledjian, 2018) that is being pursued.
• Scavenger Decapping in Cognitive Disability
The lab also identified a distinct decapping protein, DcpS, that functions as a scavenger decapping enzyme in the 3´-end decay of RNA. DcpS is implicated in a range of functions including as a mediator of human cognitive function where individuals with a disruption of DcpS decapping activity exhibit cognitive impairment (Ahmed et al., 2015). Using induced pluripotent stem cells (iPSCs) generated from individuals with homozygous disruption of the DcpS gene, we are currently focused on delineating the molecular mechanism by which DcpS decapping contributes to neural function and human cognition.
• 5´-end Cap Quality Control (5´QC).
We identified a novel 5´ end capping quality control (5´QC) mechanism in eukaryotic cells that degrades mRNAs with incomplete 5´ends (Kiledjian et al., 2012). This finding revealed mRNA capping, long thought to be an unregulated default process, is a modulated process. We demonstrated that transcripts lacking a complete 5´ end cap can be generated in a regulated manner and further identified a novel class of non-canonical decapping enzymes termed DXO family of proteins, that clear these incomplete caps (Xiang et al., 2009 Nature; Jiao et al., 2010 Nature; Chang et al., 2012 NSMB; Jiao et al., 2013 Mol. Cell). Collectively, these findings establish a new paradigm in post-transcriptional gene expression whereby addition of the 5´ cap is a regulated process and a heretofore unknown 5´QC mechanism exists to ensure the integrity of the cap. The molecular mechanism of 5´QC and its physiological consequence in cells are being pursued.