How to Build a Cortex: Coordinated Assembly of Cortical Septins and Actomyosin in the Leader Bleb

In confined spaces, migrating cells can undergo mesenchymal-to-amoeboid transitions by altering their cortical dynamics and adhesion with the environment. Septins frequently associate with cortical actin and non-muscle myosin (NMII), but the functional nature of these interactions remains unclear. Upon non-adhesive confinement and NMII enrichment, fibroblasts can switch to a fast, leader bleb-based mode of motility, characterized by the absence of adhesions and stress fibers and formation of a single, elongated leader bleb. During this transition, cortical actin remodeling and polarized NMII contractility drive leader bleb stabilization by generating long-range cortical flows, in coordination with changes in septin localization and assembly dynamics. Meanwhile, septin depletion increases global NMII expression, promoting cellular rounding and transient blebbing under non-adhesive confinement. These findings demonstrate the plasticity of fibroblast migration behavior, mediated by cortical septin-actomyosin remodeling and, further, open the door for future studies on the functional relationship between septins and NMII at the cortex

Functional cis-regulatory modules encoded by mouse-specific endogenous retrovirus

Cis-regulatory modules contain multiple transcription factor (TF)-binding sites and integrate the effects of each TF to control gene expression in specific cellular contexts. Transposable elements (TEs) are uniquely equipped to deposit their regulatory sequences across a genome, which could also contain cis-regulatory modules that coordinate the control of multiple genes with the same regulatory logic. We provide the first evidence of mouse-specific TEs that encode a module of TF-binding sites in mouse embryonic stem cells (ESCs). The majority (77%) of the individual TEs tested exhibited enhancer activity in mouse ESCs. By mutating individual TF-binding sites within the TE, we identified a module of TF-binding motifs that cooperatively enhanced gene expression. Interestingly, we also observed the same motif module in the in silico constructed ancestral TE that also acted cooperatively to enhance gene expression. Our results suggest that ancestral TE insertions might have brought in cis-regulatory modules into the mouse genome

Transposable Elements Containing Binding Sites For Pluripotency Transcription Factors Function As Enhancers In Mouse Embryonic Stems Cells

Transposable elements (TEs) make up nearly half of the human genome, but until the 1950’s were considered “junk DNA.” Barbara McClintock’s seminal maize experiments established TEs as key regulatory components due to their ability to move about the genome. More recent studies demonstrate that TEs are enriched for binding sites for cell-specific transcription factors (TFs). This study examines the extent to which a TE with binding sites for multiple pluripotency TFs regulates differential gene expression in mouse embryonic stem (ES) cells. Three potential regulatory regions consisting of a TE with multiple binding sites, a non-TE region with multiple binding sites, and a TE without binding sites were cloned from the mouse genome and inserted into luciferase reporter vectors. These vectors were transfected into cells and luciferase assays were conducted to measure the regulatory potential of these three regions, along with positive and negative controls. The results of these assays indicate that a TE bound with multiple TFs enhances cell-specific expression of a nearby gene to a greater extent than a TE without TF binding sites or a non-TE region bound with multiple TFs. In conclusion, these findings highlight the role of TEs in enhancing differential gene expression in conjunction with cell-specific TFs. However, further research is needed to investigate role of TEs bound by cell-specific TFs in gene regulatory networks