Interaction of Recombinant Gallus gallus SEPT5 and Brain Proteins of H5N1-Avian Influenza Virus-Infected Chickens

Unearthing the role of septins in viral infections

Septin proteins are a subfamily of closely related GTP-binding proteins conserved in all species except for higher plants and perform essential biological processes. Septins self-assemble into heptameric or octameric complexes and form higher-order structures such as filaments, rings, or gauzes by end-to-end binding. Their close association with cell membrane components makes them central in regulating critical cellular processes. Due to their organisation and properties, septins function as diffusion barriers and are integral in providing scaffolding to support the membrane's curvature and stability of its components. Septins are also involved in vesicle transport and exocytosis through the plasma membrane by co-localising with exocyst protein complexes. Recently, there have been emerging reports of several human and animal diseases linked to septins and abnormalities in their functions. Most of our understanding of the significance of septins during microbial diseases mainly pertains to their roles in bacterial infections but not viruses. This present review focuses on the known roles of septins in host-viral interactions as detailed by various studies.

SEPTIN2 suppresses an IFN-γ-independent, proinflammatory macrophage activation pathway

Interferon-gamma (IFN-γ) signaling is necessary for the proinflammatory activation of macrophages but IFN-γ-independent pathways, for which the initiating stimuli and downstream mechanisms are lesser known, also contribute. Here we identify, by high-content screening, SEPTIN2 (SEPT2) as a negative regulation of IFN-γ-independent macrophage autoactivation. Mechanistically, endoplasmic reticulum (ER) stress induces the expression of SEPT2, which balances the competition between acetylation and ubiquitination of heat shock protein 5 at position Lysine 327, thereby alleviating ER stress and constraining M1-like polarization and proinflammatory cytokine release. Disruption of this negative feedback regulation leads to the accumulation of unfolded proteins, resulting in accelerated M1-like polarization, excessive inflammation and tissue damage. Our study thus uncovers an IFN-γ-independent macrophage proinflammatory autoactivation pathway and suggests that SEPT2 may play a role in the prevention or resolution of inflammation during infection.

Chicken Hypothalamic and Ovarian DNA Methylome Alteration in Response to Forced Molting

Epigenetic modifications play an important role in regulating animal adaptation to external stress. To explore how DNA methylation regulates the expression levels of related genes during forced molting (FM) of laying hens, the hypothalamus and ovary tissues were analyzed at five periods using Whole-Genome Bisulfite Sequencing. The results show that methylation levels fluctuated differently in the exon, intron, 5′UTR, 3′UTR, promoter, and intergenic regions of the genome during FM. In addition, 16 differentially methylated genes (DMGs) regulating cell aging, immunity, and development were identified in the two reversible processes of starvation and redevelopment during FM. Comparing DMGs with differentially expressed genes (DEGs) obtained in the same periods, five hypermethylated DMGs (DSTYK, NKTR, SMOC1, SCAMP3, and ATOH8) that inhibited the expression of DEGs were found. Therefore, DMGs epigenetically modify the DEGs during the FM process of chickens, leading to the rapid closure and restart of their reproductive function and a re-increase in the egg-laying rate. Therefore, this study further confirmed that epigenetic modifications could regulate gene expression during FM and provides theoretical support for the subsequent optimization of FM technology.

Septins in Infections: Focus on Viruses

Human septins comprise a family of 13 genes that encode conserved GTP-binding proteins. They form nonpolar complexes, which assemble into higher-order structures, such as bundles, scaffolding structures, or rings. Septins are counted among the cytoskeletal elements. They interact with the actin and microtubule networks and can bind to membranes. Many cellular functions with septin participation have been described in the literature, including cytokinesis, motility, forming of scaffolding platforms or lateral diffusion barriers, vesicle transport, exocytosis, and recognition of micron-scale curvature. Septin dysfunction has been implicated in diverse human pathologies, including neurodegeneration and tumorigenesis. Moreover, septins are thought to affect the outcome of host–microbe interactions. Implication of septins has been demonstrated in fungal, bacterial, and viral infections. Knowledge on the precise function of a particular septin in the different steps of the virus infection and replication cycle is still limited. Published data for vaccinia virus (VACV), hepatitis C virus (HCV), influenza A virus (H1N1 and H5N1), human herpesvirus 8 (HHV-8), and Zika virus (ZIKV), all of major concern for public health, will be discussed here.

Role of WW domain E3 ubiquitin protein ligase 2 in modulating ubiquitination and Degradation of Septin4 in oxidative stress endothelial injury

Oxidative stress-associated endothelial injury is the initial event and major cause of multiple cardiovascular diseases such as atherosclerosis and hypertensive angiopathy. A protein homeostasis imbalance is a critical cause of endothelial injury, and homologous to E6AP C-terminus (HECT)-type E3 ubiquitin ligases are the core factors controlling protein homeostasis. Although HECT-type E3 ubiquitin ligases are involved in the regulation of cardiac development and diseases, their roles in endothelial injury remain largely unknown. This study aimed to identify which HECT-type E3 ubiquitin ligase is involved in endothelial injury and clarify the mechanisms at molecular, cellular, and organism levels. We revealed a novel role of the HECT-type E3 ubiquitin ligase WWP2 in regulating endothelial injury and vascular remodeling after endothelial injury. Endothelial/myeloid-specific WWP2 knockout in mice significantly aggravated angiotensin II/oxidative stress-induced endothelial injury and vascular remodeling after endothelial injury. The same results were obtained from in vitro experiments. Mechanistically, the endothelial injury factor Septin4 was identified as a novel physiological substrate of WWP2. In addition, WWP2 interacted with the GTPase domain of Septin4, ubiquitinating Septin4-K174 to degrade Septin4 through the ubiquitin-proteasome system, which inhibited the Septin4-PARP1 endothelial damage complex. These results identified the first endothelial injury-associated physiological pathway regulated by HECT-type E3 ubiquitin ligases in vivo as well as a unique proteolytic mechanism through which WWP2 controls endothelial injury and vascular remodeling after endothelial injury. These findings might provide a novel treatment strategy for oxidative stress-associated atherosclerosis and hypertensive vascular diseases.

Turning it inside out: The organization of human septin heterooligomers

Septin family proteins are quite similar to each other both within and between eukaryotic species. Typically, multiple discrete septins co-assemble into linear heterooligomers (usually hexameric or octameric rods) with a variety of cellular functions. We know little about how incorporation of different septins confers different properties to such complexes. This issue is especially acute in human cells where 13 separate septin gene products (often produced in multiple forms arising from alternative start codons and differential splicing) are expressed in a tissue-specific manner. Based on sequence alignments and phylogenetic criteria, human septins fall into four distinct groups predictive of their interactions, that is, members of the same group appear to occupy the same position within oligomeric septin protomers, which are "palindromic" (have twofold rotational symmetry about a central homodimeric pair). Many such protomers are capable of end-to-end polymerization, generating filaments. Over a decade ago, a study using X-ray crystallography and single-particle electron microscopy deduced the arrangement within recombinant heterohexamers comprising representatives of three human septin groups-SEPT2, SEPT6, and SEPT7. This model greatly influenced subsequent studies of human and other septin complexes, including how incorporating a septin from a fourth group forms heterooctamers, as first observed in budding yeast. Two recent studies, including one in this issue of Cytoskeleton, provide clear evidence that, in fact, the organization of subunits within human septin heterohexamers and heterooctamers is inverted relative to the original model. These findings are discussed here in a broader context, including possible causes for the initial confusion.