RNA-binding proteins direct myogenic cell fate decisions

RNA-binding proteins (RBPs), essential for skeletal muscle regeneration, cause muscle degeneration and neuromuscular disease when mutated. Why mutations in these ubiquitously expressed RBPs orchestrate complex tissue regeneration and direct cell fate decisions in skeletal muscle remains poorly understood. Single-cell RNA-sequencing of regenerating Mus musculus skeletal muscle reveals that RBP expression, including the expression of many neuromuscular disease-associated RBPs, is temporally regulated in skeletal muscle stem cells and correlates with specific stages of myogenic differentiation. By combining machine learning with RBP engagement scoring, we discovered that the neuromuscular disease-associated RBP Hnrnpa2b1 is a differentiation-specifying regulator of myogenesis that controls myogenic cell fate transitions during terminal differentiation in mice. The timing of RBP expression specifies cell fate transitions by providing post-transcriptional regulation of messenger RNAs that coordinate stem cell fate decisions during tissue regeneration.

Injury-mediated stiffening persistently activates muscle stem cells through YAP and TAZ mechanotransduction

The skeletal muscle microenvironment transiently remodels and stiffens after exercise and injury, as muscle ages, and in myopathic muscle; however, how these changes in stiffness affect resident muscle stem cells (MuSCs) remains understudied. Following muscle injury, muscle stiffness remained elevated after morphological regeneration was complete, accompanied by activated and proliferative MuSCs. To isolate the role of stiffness on MuSC behavior and determine the underlying mechanotransduction pathways, we cultured MuSCs on strain-promoted azide-alkyne cycloaddition hydrogels capable of in situ stiffening by secondary photocrosslinking of excess cyclooctynes. Using pre- to post-injury stiffness hydrogels, we found that elevated stiffness enhances migration and MuSC proliferation by localizing yes-associated protein 1 (YAP) and WW domain-containing transcription regulator 1 (WWTR1; TAZ) to the nucleus. Ablating YAP and TAZ in vivo promotes MuSC quiescence in postinjury muscle and prevents myofiber hypertrophy, demonstrating that persistent exposure to elevated stiffness activates mechanotransduction signaling maintaining activated and proliferating MuSCs.

Fibrinolysis Shutdown Correlation with Thromboembolic Events in Severe COVID-19 Infection

Background

COVID-19 predisposes patients to a prothrombotic state with demonstrated microvascular involvement. The degree of hypercoagulability appears to correlate with outcomes; however, optimal criteria to assess for the highest-risk patients for thrombotic events remain unclear; we hypothesized that deranged thromboelastography measurements of coagulation would correlate with thromboembolic events.

Study Design

Patients admitted to an ICU with COVID-19 diagnoses who had thromboelastography analyses performed were studied. Conventional coagulation assays, d-dimer levels, and viscoelastic measurements were analyzed using a receiver operating characteristic curve to predict thromboembolic outcomes and new-onset renal failure.

Results

Forty-four patients with COVID-19 were included in the analysis. Derangements in coagulation laboratory values, including elevated d-dimer, fibrinogen, prothrombin time, and partial thromboplastin time, were confirmed; viscoelastic measurements showed an elevated maximum amplitude and low lysis of clot at 30 minutes. A complete lack of lysis of clot at 30 minutes was seen in 57% of patients and predicted venous thromboembolic events with an area under the receiver operating characteristic curve of 0.742 (p = 0.021). A d-dimer cutoff of 2,600 ng/mL predicted need for dialysis with an area under the receiver operating characteristic curve of 0.779 (p = 0.005). Overall, patients with no lysis of clot at 30 minutes and a d-dimer > 2,600 ng/mL had a venous thromboembolic event rate of 50% compared with 0% for patients with neither risk factor (p = 0.008), and had a hemodialysis rate of 80% compared with 14% (p = 0.004).

Conclusions

Fibrinolysis shutdown, as evidenced by elevated d-dimer and complete failure of clot lysis at 30 minutes on thromboelastography predicts thromboembolic events and need for hemodialysis in critically ill patients with COVID-19. Additional clinical trials are required to ascertain the need for early therapeutic anticoagulation or fibrinolytic therapy to address this state of fibrinolysis shutdown.

Structured inquiry-based learning: Drosophila GAL4 enhancer trap characterization in an undergraduate laboratory course

This education article describes a modular laboratory exercise in which undergraduates use fruit flies to generate novel experimental data while learning to perform advanced molecular techniques.

We have developed and tested two linked but separable structured inquiry exercises using a set of Drosophila melanogaster GAL4 enhancer trap strains for an upper-level undergraduate laboratory methods course at Bucknell University. In the first, students learn to perform inverse PCR to identify the genomic location of the GAL4 insertion, using FlyBase to identify flanking sequences and the primary literature to synthesize current knowledge regarding the nearest gene. In the second, we cross each GAL4 strain to a UAS-CD8-GFP reporter strain, and students perform whole mount CNS dissection, immunohistochemistry, confocal imaging, and analysis of developmental expression patterns. We have found these exercises to be very effective in teaching the uses and limitations of PCR and antibody-based techniques as well as critical reading of the primary literature and scientific writing. Students appreciate the opportunity to apply what they learn by generating novel data of use to the wider research community.

Satb1 regulates the self-renewal of hematopoietic stem cells by promoting quiescence and repressing differentiation commitment

How hematopoietic stem cells coordinate the regulation of opposing cellular mechanisms like self-renewal and differentiation commitment remains unclear. Here, we identified the transcription factor and chromatin remodeler Satb1 as a critical regulator of the hematopoietic stem cell (HSC) fate. HSCs lacking Satb1 displayed defective self-renewal, less quiescence and accelerated lineage commitment, resulting in progressive depletion of functional HSCs. Increased commitment was caused by reduced symmetric self-renewal and increased symmetric differentiation divisions of Satb1-deficient HSCs. Satb1 simultaneously repressed gene sets involved in HSC activation and cellular polarity, including Numb and Myc, two key factors for stem cell fate specification. Thus, Satb1 is a regulator that promotes HSC quiescence and represses lineage commitment.