Dual Cre and Dre recombinases mediate synchronized lineage tracing and cell subset ablation in vivo

Ductal or Ngn3+ cells do not contribute to adult pancreatic islet beta-cell neogenesis in homeostasis

The adult pancreatic ducts have long been proposed to contain rare progenitors, some of which expressing Ngn3, that generate new beta cells in endocrine-islet homeostasis. Due to their postulated rarity and the lack of definitive markers, the existence or absence of ductal endocrine progenitors remains unsettled despite many studies. Genetic lineage tracing of ductal cells or Ngn3+ cells with currently available CreER drivers has been complicated by off-target labeling of pre-existing beta cells. Here, using dual-recombinase-mediated intersectional genetic strategy and newly-derived Ngn3-2A-CreER and Hnf1b-2A-CreER knock-in drivers, we succeeded in specifically labeling Ngn3-positive cells and Hnf1b-positive ductal cells without marking pre-existing beta cells. These data revealed no evidence of de novo generation of insulin-producing beta cells from ductal cells or endogenous Ngn3-positive cells in the adult pancreas during homeostasis.

Dual genetic tracing demonstrates the heterogeneous differentiation and function of neuromesodermal progenitors in vivo

In recent decades, the traditional paradigm of three distinct germ layers formed during gastrulation has been revised with the identification of neuromesodermal progenitors (NMPs). These progenitors emerge during gastrulation and contribute to both the neural ectoderm, particularly the spinal cord, and the adjacent paraxial mesoderm [D. Henrique et al., Development 142, 2864-2875 (2015); R. J. Garriock et al., Development 142, 1628-1638 (2015); E. Tzouanacou et al., Dev. Cell 17, 365-376 (2009)]. However, effective genetic tools for lineage tracing and functional assessments of NMPs in vivo are currently lacking. Here, we developed a dual recombinase-mediated genetic system to specifically trace and ablate Brachyury+Sox2+ NMPs. Our genetic tracing and single-cell RNA sequencing analyses revealed that NMPs consist of three distinct unipotent and bipotent progenitor populations that progressively differentiate into neural and mesodermal fates. Genetic depletion of NMPs demonstrated their critical role in trunk and tail formation. This study provides in vivo genetic evidence supporting the heterogeneity of NMPs in terms of cell fate determination and their functional roles in the developing embryo.

Tumor-initiating cells escape tumor immunity via CCL8 from tumor-associated macrophages in mice

Tumor-initiating cells (TICs) play a key role in cancer progression and immune escape. However, how TICs evade immune elimination remains poorly characterized. Combining single-cell RNA-Seq (scRNA-Seq), dual-recombinase-based lineage tracing, and other approaches, we identified a WNT-activated subpopulation of malignant cells that act as TICs in vivo. We found intensive reciprocal interactions between TICs and immune-regulatory tumor-associated macrophages (Reg-TAMs) via growth arrest-specific 6/AXL receptor tyrosine kinase/MER proto-oncogene, tyrosine kinase (GAS6/AXL/MERTK) signaling pathways, which facilitated the immune escape of TICs. In this study, we used chemical inhibitors and Axl/Mertk conditional double-KO (cDKO) mice to demonstrate that inhibiting the interaction between TIC-derived GAS6 and AXL/MERTK in Reg-TAMs reactivated antitumor immune responses. We identified CCL8 as a critical mediator of the GAS6/AXL/MERTK pathway, primarily by inhibiting Treg infiltration into the tumor. Furthermore, the AXL/MERTK signaling blockade sensitized tumor cells to anti-programmed cell death 1 (anti-PD-1) treatment. Thus, we elucidated a detailed mechanism by which TICs evade tumor immunity, providing insights into strategies to eradicate TICs that escape conventional immunotherapy.

Multistep allelic conversion in mouse pre-implantation embryos by AAV vectors

Site-specific recombinases (SSRs) are critical for achieving precise spatiotemporal control of engineered alleles. These enzymes play a key role in facilitating the deletion or inversion of loci flanked by recombination sites, resulting in the activation or repression of endogenous genes, selection markers or reporter elements. However, multiple recombination in complex alleles can be laborious. To address this, a new and efficient method using AAV vectors has been developed to simplify the conversion of systems based on Cre, FLP, Dre and Vika recombinases. In this study, we present an effective method for ex vivo allele conversion using Cre, FLP (flippase), Dre, and Vika recombinases, employing adeno-associated viruses (AAV) as delivery vectors. AAVs enable efficient allele conversion with minimal toxicity in a reporter mouse line. Moreover, AAVs facilitate sequential allele conversion, essential for fully converting alleles with multiple recombination sites, typically found in conditional knockout mouse models. While simple allele conversions show a 100% efficiency rate, complex multiple conversions consistently achieve an 80% conversion rate. Overall, this strategy markedly reduces the need for animals and significantly speeds up the process of allele conversion, representing a significant improvement in genome engineering techniques.

A knock-in allele of Hand2 expressing Dre recombinase

HAND2 is a basic helix-loop-helix transcription factor with diverse functions during development. To facilitate the investigation of genetic and functional diversity among Hand2-expressing cells in the mouse, we have generated Hand2Dre, a knock-in allele expressing Dre recombinase. To avoid disrupting Hand2 function, the Dre cDNA is inserted at the 3' end of the Hand2 coding sequence following a viral 2A peptide. Hand2Dre homozygotes can therefore be used in complex crosses to increase the proportion of useful genotypes among offspring. Dre expression in mid-gestation Hand2Dre embryos is indistinguishable from wild-type Hand2 expression, and HandDre efficiently recombines rox target sites in vivo. In combination with existing Cre and Flp mouse lines, Hand2Dre will therefore extend the ability to perform genetic intersectional labeling, fate mapping, and functional manipulation of subpopulations of cells characterized by developmental expression of Hand2.

Current perspectives on the multiple roles of osteoclasts: Mechanisms of osteoclast-osteoblast communication and potential clinical implications

Bone remodeling is a complex process involving the coordinated actions of osteoblasts and osteoclasts to maintain bone homeostasis. While the influence of osteoblasts on osteoclast differentiation is well established, the reciprocal regulation of osteoblasts by osteoclasts has long remained enigmatic. In the past few years, a fascinating new role for osteoclasts has been unveiled in promoting bone formation and facilitating osteoblast migration to the remodeling sites through a number of different mechanisms, including the release of factors from the bone matrix following bone resorption and direct cell-cell interactions. Additionally, considerable evidence has shown that osteoclasts can secrete coupling factors known as clastokines, emphasizing the crucial role of these cells in maintaining bone homeostasis. Due to their osteoprotective function, clastokines hold great promise as potential therapeutic targets for bone diseases. However, despite long-standing work to uncover new clastokines and their effect in vivo, more substantial efforts are still required to decipher the mechanisms and pathways behind their activity in order to translate them into therapies. This comprehensive review provides insights into our evolving understanding of the osteoclast function, highlights the significance of clastokines in bone remodeling, and explores their potential as treatments for bone diseases suggesting future directions for the field.

From Chaos to Opportunity: Decoding Cancer Heterogeneity for Enhanced Treatment Strategies

Cancer manifests as a multifaceted disease, characterized by aberrant cellular proliferation, survival, migration, and invasion. Tumors exhibit variances across diverse dimensions, encompassing genetic, epigenetic, and transcriptional realms. This heterogeneity poses significant challenges in prognosis and treatment, affording tumors advantages through an increased propensity to accumulate mutations linked to immune system evasion and drug resistance. In this review, we offer insights into tumor heterogeneity as a crucial characteristic of cancer, exploring the difficulties associated with measuring and quantifying such heterogeneity from clinical and biological perspectives. By emphasizing the critical nature of understanding tumor heterogeneity, this work contributes to raising awareness about the importance of developing effective cancer therapies that target this distinct and elusive trait of cancer.

Heterogeneity and Functional Analysis of Cardiac Fibroblasts in Heart Development

Background

As one of the major cell types in the heart, fibroblasts play critical roles in multiple biological processes. Cardiac fibroblasts are known to develop from multiple sources, but their transcriptional profiles have not been systematically compared. Furthermore, while the function of a few genes in cardiac fibroblasts has been studied, the overall function of fibroblasts as a cell type remains uninvestigated.

Methods

Single-cell mRNA sequencing (scRNA-seq) and bioinformatics approaches were used to analyze the genome-wide genes expression and extracellular matrix genes expression in fibroblasts, as well as the ligand-receptor interactions between fibroblasts and cardiomyocytes. Single molecular in situ hybridization was employed to analyze the expression pattern of fibroblast subpopulation-specific genes. The Diphtheria toxin fragment A (DTA) system was utilized to ablate fibroblasts at each developmental phase.

Results

Using RNA staining of Col1a1 at different stages, we grouped cardiac fibroblasts into four developmental phases. Through the analysis of scRNA-seq profiles of fibroblasts at 18 stages from two mouse strains, we identified significant heterogeneity, preserving lineage gene expression in their precursor cells. Within the main fibroblast population, we found differential expressions of Wt1, Tbx18, and Aldh1a2 genes in various cell clusters. Lineage tracing studies showed Wt1- and Tbx18-positive fibroblasts originated from respective epicardial cells. Furthermore, using a conditional DTA system-based elimination, we identified the crucial role of fibroblasts in early embryonic and heart growth, but not in neonatal heart growth. Additionally, we identified the zone- and stage-associated expression of extracellular matrix genes and fibroblast-cardiomyocyte ligand-receptor interactions. This comprehensive understanding sheds light on fibroblast function in heart development.

Conclusion

We observed cardiac fibroblast heterogeneity at embryonic and neonatal stages, with preserved lineage gene expression. Ablation studies revealed their distinct roles during development, likely influenced by varying extracellular matrix genes and ligand-receptor interactions at different stages.

A novel lineage of osteoprogenitor cells with dual epithelial and mesenchymal properties govern maxillofacial bone homeostasis and regeneration after MSFL

Bone regeneration originates from proliferation and differentiation of osteoprogenitors via either endochondral or intramembranous ossification; and the regeneration capacities decline with age and estrogen loss. Maxillary sinus floor lifting (MSFL) is a commonly used surgical procedure for guiding bone regeneration in maxilla. Radiographic analysis of 1210 clinical cases of maxilla bone regeneration after MSFL revealed that the intrasinus osteogenic efficacy was independent of age and gender, however; and this might be related to the Schneiderian membrane that lines the sinus cavity. In view of the particularity of this biological process, our present study aimed to elucidate the underlying mechanism of MSFL-induced bone regeneration. We first established a murine model to simulate the clinical MSFL. By single-cell RNA-sequencing and flow cytometry-based bulk RNA-sequencing, we identified a novel Krt14⁺Ctsk⁺ subset of cells that display both epithelial and mesenchymal properties and the transcriptomic feature of osteoprogenitors. Dual recombinases-mediated lineage tracing and loss-of-function analyses showed that these Krt14⁺Ctsk⁺ progenitors contribute to both MSFL-induced osteogenesis and physiological bone homeostasis by differentiating into Krt14^-Ctsk⁺ descendants which show robust osteogenic capacity. In addition, we detected a similar population of Krt14⁺Ctsk⁺ cells in human samples of Schneiderian membrane, which show a highly similar osteogenic potential and transcriptomic feature to the corresponding cells in mice. The identification of this Krt14⁺Ctsk⁺ population, featured by osteoprogenitor characteristics and dual epithelial-mesenchymal properties, provides new insight into the understanding of bone regeneration and may open more possibilities for clinical applications.