Immunity keeping p53 in check!

In a 2012 study on the topology of the human and mouse m6A RNA methylomes, Gene Ontology (GO) analysis of differentially expressed genes (DEG's) indicated a noteworthy enrichment of the p53 signaling pathway: 22/23 genes had differentially expressed splice variants, of which 18 were methylated. Moreover, 15 other members of the signaling pathway, which were not significant DEG's, exhibited significant differential isoform expressions. For example, isoforms of MDM4, needed for p53 inactivation were downregulated. Similar pro-apoptotic effects were observed in other pathway genes including MDM2, FAS and BAX. Higher apoptosis rate in HaCaT-T cells resulted with knockdown of m6A subunit METTL3, which also reversed a significant decrease in p53 activity. Modulation of p53 signaling through splicing may be relevant to induction of apoptosis by silencing of METTL3. 

Then, in 2019 a study of arsenite-induced human keratinocyte transformation demonstrated that knockdown of METTL3 significantly decreased m6A level, restored p53 activation and inhibited cellular transformation phenotypes in the-transformed cells. Further, m6A downregulated the expression of the positive p53 regulator, PRDM2, through the YTHDF2-promoted decay of PRDM2 mRNAs. m6A also upregulated expression of negative p53 regulator, YY1 and MDM2 through YTHDF1-stimulated translation of YY1 and MDM2 mRNA. Taken together, the study revealed the novel role of m6A in mediating human keratinocyte transformation by suppressing p53 activation and sheds light on the mechanisms of arsenic carcinogenesis via RNA epigenetics.

Finally in 2021 a discovery that YTHDF2 is upregulated in NK cells upon activation by cytokines, tumors, and cytomegalovirus infection. Ythdf2 deficiency in NK cells impaired its anti-tumor and anti-viral activity in vivo. YTHDF2 maintains NK cell homeostasis and terminal maturation, correlating with modulating NK cell trafficking and regulating Eomes, respectively. It promotes NK cell effector function and is required for IL-15-mediated NK cell survival and proliferation by forming a STAT5-YTHDF2 positive feedback loop. Analysis showed significant enrichment in cell cycle, division, and division-related processes, including mitotic cytokinesis, chromosome segregation, spindle, nucleosome, midbody, and chromosome. This data supports roles of YTHDF2 in regulating NK proliferation, survival, and effector functions. Transcriptome-wide screening identified Tardbp (TDP-43) to be involved in cell proliferation or survival as a YTHDF2-binding target in NK cells.

Downregulation of METTL3, which in spinal cord contributes with YTHDF2 to modulate inflammatory pain may upregulate differentially expressed p53 network splice variants that oppose YTHDF2 induced downregulation of p53, via PRDM2 leading to apoptotic or diseased cells. In diseased environments cytokines may upregulate YTHDF2 in NK cells leading to downregulation of p53 and cytoskeletal transformation that may be sufficient, at an immune synapse to advance cytolysis.

p53 signals may inform selections of cells and tissue that prime NK cells for advanced, personalized immune therapy. 

Not Only A Killer A System for Killing!

The next time you're out exercising, spare a thought for your busy mitochondria. NASA scientists just reported mitochondria as the key to health problems in space.

Natural killer (NK) cells can extend membrane probes into cells or pathogens. These are loaded with granulysin (GNLY) to penetrate and perforin (PFN) to kill intracellular bacteria or protozoa and can lyse entire cells. The probes can also transfer healthy mitochondria to apoptotic cardiomyocytes (and other cells) in need of mitochondrial transfer. Uterine NK cells of the decidua send probes into trophoblasts to selectively kill intracellular Listeria monocytogenes without killing the trophoblast host. Stressed cells, moving toward apoptosis can behave similarly, but in reverse shooting out nanoprobes to proximal cells seeking cooperation and urgent mitochondrial transfers including to cancer cells.

A meta-analysis of gene expression signatures for blood pressure and hypertension in 7017 individuals from 6 international studies found of 7717 genes, 34 were most differentialy expressed including GNLY. Enrichment analysis for the diastolic and systolic gene group's associated strongly with NK cell mediated cytotoxicity and 13 other pathways including antigen processing and inflammatory response.

Formation of membrane probes or tubes, in which mitochondria travel and establishment of intracellular mitochondrial networks in the peripheral zone of cells require Kinesin-1 heavy chain (KIF5B). KIF5B is also required for female meiosis (oogenesis) and proper chromosomal segregation in mitotic cells and modulates central spindle organization in late-stage cytokinesis in chondrocytes.

A study of centromere heterochromatin (connected with central spindle) surprisingly showed that distant euchromatic regions, enriched in repressed methylated genes also interacted with the hierarchical organization of centromeric DNA. These 3D spatial interactions (at a distance) are likely mediated by liquid-like fusion events and can influence the health of individuals. Repressed gene's were identified as transposable elements, sequences often associated with pathogenic DNA insertions that have been persistently retained.  

KIF5B is an interaction partner of ADP-ribosylation factor-like 8b (Arl8b), which is required for NK cell–mediated cytotoxicity that drives polarization of lytic granules and microtubule-organizing centers (MTOCs) toward the immune synapse between NK and target cells. Silencing experiments that led to failure of MTOC-lytic granule polarization suggest Arl8b and KIF5B together control the critical step in NK cell cytotoxicity. 

KIF5B is also a critical transporter of p53 and c-Myc to the cytoplasm for degradation. However, subcellular localization of Arl8b and p53-dependent cell death was shown to occur through knockdown of acetylation subunit NatC. As a consequence, p53 is stabilized, phosphorylated and significantly activates transcription of downstream proapoptotic genes. In the absence KIF5B, or presence of  mutants p53 and c-Myc aggregate in the nucleus where they signal DNA damage-induced apoptosis through the control of p53 by endogenous c-Myc (in vivo).

Finely tuned, frequently used KIF5B in NK cells expressing GNLY may induce effects on local tissue blood pressure, as was discovered by expression of Renin-Angiotensin vasoactive proteins AT1, AT2, and ANP in pregnancy-induced uterine NK cells. Inflammation signaling, via tissue bound NK cells may result from stretch-mediated release of angiotensin II, which is coupled with p53 acetylation apoptosis and activation of p53. This may prolong upregulation of the local renin-angiotensin system, increase susceptibility of target cells to apoptosis and signal adaptive immune cells. 

Somewhere in the balance between NatC knockdown induced apoptosis and angiotensin II induced apoptosis p53 may direct traffic to keep your cells healthy!