Legionella pneumophila is a Gram-negative bacterium ubiquitously present in freshwater environments and causes a serious type of pneumonia called Legionnaires' disease. During infections, L. pneumophila releases >300 effector proteins into host cells through an Icm/Dot type IV secretion system to manipulate the host defense system for survival within the hosts. Notably, certain effector proteins mediate posttranslational modifications (PTMs), serving as useful approaches exploited by L. pneumophila to modify host proteins. Some effectors catalyze the addition of host protein PTMs, while others mediate the removal of PTMs of host proteins. In this review, we summarize L. pneumophila effector-mediated PTMs of host proteins, including phosphorylation, ubiquitination, glycosylation, AMPylation, phosphocholination, methylation, ADP-ribosylation, as well as dephosphorylation, deubiquitination, deAMPylation, deADP-ribosylation, dephosphocholination, and delipidation. We describe their molecular mechanisms and biological functions in the regulation of bacterial growth and Legionella-containing vacuole biosynthesis and in the disruption of host immune and defense machinery.

Circular RNAs (circRNAs) are unique single-stranded RNA molecules that are covalently closed and expressed in a tissue- and cell-specific manner. Most circRNAs are formed by back-splicing of pre-mRNA and have a wide range of functions in cells. As they lack a 5'-cap and a 3'-poly(A) tail, they are classified as noncoding RNAs that act as sponges for miRNAs and RNA-binding proteins. However, recent studies have found that some circRNAs can undergo cap-independent translation, making them capable of encoding proteins through alternative translation initiation mechanisms. CircRNAs are more stable than linear mRNAs due to their unique circular structure. With mRNA-based drugs receiving significant attention in the past two years, mRNA's instability and immunogenicity are major obstacles to its widespread use. Since circRNA is more stable than mRNA and less immunogenic and can be translated in a tissue-specific manner, it is a promising modality for RNA therapy. In this review, we will provide an overview of the biological functions and potential applications of circRNA.

To facilitate its survival, replication, and dissemination, the intracellular pathogen Legionella pneumophila relies on its unique Type IVB secretion system (T4SS) to deliver over 330 effectors to hijack host cell pathways in a spatiotemporal manner. The underlying mechanisms of the effectors and their host targets are largely unexplored due to their little sequence identity to known proteins and functional redundancy. The T4SS effector SidN (Lpg1083) is secreted into host cells during the late infection period. However, to the best of our knowledge, the molecular characterization of SidN has not been previously studied. Herein, we identified SidN as a nuclear envelope-localized effector. Its structure adopts a novel fold and the N-terminal domain is crucial for its specific subcellular localization. Furthermore, we found that SidN is transported by the eukaryotic karyopherin Importin-13 into the nucleus, where it attaches to the N-terminal region of Lamin-B2 to interfere with the integrity of the nuclear envelope, causing nuclear membrane disruption and eventually cell death. Our work provides new insights into the structure and function of an L. pneumophila effector protein, and suggests a potential strategy that the pathogen utilizes to promote cell death and then benefit bacterial escape from the host for secondary infection.

Hyperglycaemia-induced oxidative stress may disrupt insulin secretion and β-cell survival in diabetes mellitus by overproducing reactive oxygen species. Human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) exhibit anti-oxidant properties. However, the mechanisms by which hUC-MSCs protect β-cells from high glucose-induced oxidative stress remain underexplored. In this study, we showed that intravenous injection of hUC-MSCs engrafted into the injured pancreas and promoted pancreatic β-cell function in a mouse model of type 1 diabetes mellitus. In vitro study revealed that hUC-MSCs attenuated high glucose-induced oxidative stress and prevented β-cell impairment via the Nrf2/HO-1 signaling pathway. Nrf2 knockdown partially blocked the anti-oxidative effect of hUC-MSCs, resulting in β-cell decompensation in a high glucose environment. Overall, these findings provide novel insights into how hUC-MSCs protect β-cells from high glucose-induced oxidative stress.

AFP is the most widely used biomarker for the diagnosis of hepatocellular carcinoma. However, a substantial proportion of HCC patients have either normal or marginally increased AFP levels in serum, and the underlying mechanisms are not fully understood. In the present study, we provided in vitro as well as in vivo evidence that heat shock protein gp96 promoted AFP expression at the transcriptional level in HCC. NR5A2 was identified as a key transcription factor regulated by AFP and its stability was enhanced by gp96. A further mechanistic study by CO-IP, GST-pull down and molecular docking showed the competitive binding of gp96 and SUMO E3 ligase RanBP2 to NR5A2 at the sites spanning from aa 507 to 539. The binding of gp96 inhibited SUMOylating, ubiquitination, and subsequent degradation of NR5A2. In addition, clinical analysis of HCC patients indicated that gp96 expression was positively correlated to serum AFP levels in tumors. Therefore, our study uncovered the novel regulatory mechanism of gp96 on the stability of its client proteins by directly affecting their SUMOylation and ubiquitination. These findings will help in designing more accurate AFP-based HCC diagnosis and progression monitoring approaches.

Brain-specific serine/threonine-protein kinase 2 (BRSK2) plays critical roles in insulin secretion and β-cell biology. Whether BRSK2 is associated with human type 2 diabetes mellitus (T2DM) is never appreciated. Here, we report that BRSK2 genetic variants are closely related to worsening glucose metabolism due to hyperinsulinemia and insulin resistance in Chinese population. The BRSK2 protein levels are significantly accumulated in β cells from T2DM patients and high-fat-diet (HFD)-fed mice due to enhanced protein stability. Mice with inducible loss-of-function Brsk2 (βKO) are metabolic normal with high potential of insulin secretion under chow-diet condition. Moreover, βKO mice prevent from HFD-induced hyperinsulinemia, obesity, insulin resistance, and glucose intolerance. Conversely, gain-of-function Brsk2 in mature β cells reversibly triggers hyperglycemia due to β-cell hypersecretion-coupled insulin resistance. Mechanistically, BRSK2 senses lipid signals and induces basal insulin secretion in a kinase-dependent manner. The enhanced basal insulin secretion drives insulin resistance and β-cell exhaustion, and thus the onset of T2DM in mice with HFD feeding or β-cell gain-of-function BRSK2. These findings reveal that BRSK2 links hyperinsulinemia to systematic insulin resistance via interplaying between β cells and insulin-sensitive tissues in human genetic variant population or under nutrient-overload conditions.