Generation of complex bone marrow organoids from human induced pluripotent stem cells

The human bone marrow (BM) niche sustains hematopoiesis throughout life. We present a method for generating complex BM-like organoids (BMOs) from human induced pluripotent stem cells (iPSCs). BMOs consist of key cell types that self-organize into spatially defined three-dimensional structures mimicking cellular, structural and molecular characteristics of the hematopoietic microenvironment. Functional properties of BMOs include the presence of an in vivo-like vascular network, the presence of multipotent mesenchymal stem/progenitor cells, the support of neutrophil differentiation and responsiveness to inflammatory stimuli. Single-cell RNA sequencing revealed a heterocellular composition including the presence of a hematopoietic stem/progenitor (HSPC) cluster expressing genes of fetal HSCs. BMO-derived HSPCs also exhibited lymphoid potential and a subset demonstrated transient engraftment potential upon xenotransplantation in mice. We show that the BMOs could enable the modeling of hematopoietic developmental aspects and inborn errors of hematopoiesis, as shown for human VPS45 deficiency. Thus, iPSC-derived BMOs serve as a physiologically relevant in vitro model of the human BM microenvironment to study hematopoietic development and BM diseases.

Blood Vessel Organoids for Development and Disease

Despite enormous advances, cardiovascular disorders are still a major threat to global health and are responsible for one-third of deaths worldwide. Research for new therapeutics and the investigation of their effects on vascular parameters is often limited by species-specific pathways and a lack of high-throughput methods. The complex 3-dimensional environment of blood vessels, intricate cellular crosstalks, and organ-specific architectures further complicate the quest for a faithful human in vitro model. The development of novel organoid models of various tissues such as brain, gut, and kidney signified a leap for the field of personalized medicine and disease research. By utilizing either embryonic- or patient-derived stem cells, different developmental and pathological mechanisms can be modeled and investigated in a controlled in vitro environment. We have recently developed self-organizing human capillary blood vessel organoids that recapitulate key processes of vasculogenesis, angiogenesis, and diabetic vasculopathy. Since then, this organoid system has been utilized as a model for other disease processes, refined, and adapted for organ specificity. In this review, we will discuss novel and alternative approaches to blood vessel engineering and explore the cellular identity of engineered blood vessels in comparison to in vivo vasculature. Future perspectives and the therapeutic potential of blood vessel organoids will be discussed.

Ronin governs the metabolic capacity of the embryonic lineage for post-implantation development

During implantation, the murine embryo transitions from a “quiet” into an active metabolic/proliferative state, which kick‐starts the growth and morphogenesis of the post‐implantation conceptus. Such transition is also required for embryonic stem cells to be established from mouse blastocysts, but the factors regulating this process are poorly understood. Here, we show that Ronin plays a critical role in the process by enabling active energy production, and the loss of Ronin results in the establishment of a reversible quiescent state in which naïve pluripotency is promoted. In addition, Ronin fine‐tunes the expression of genes that encode ribosomal proteins and is required for proper tissue‐scale organisation of the pluripotent lineage during the transition from blastocyst to egg cylinder stage. Thus, Ronin function is essential for governing the metabolic capacity so that it can support the pluripotent lineage’s high‐energy demands for cell proliferation and morphogenesis.

The spatio-temporal organization of mitochondrial F1FO ATP synthase in cristae depends on its activity mode

F₁F_O ATP synthase, also known as complex V, is a key enzyme of mitochondrial energy metabolism that can synthesize and hydrolyze ATP. It is not known whether the ATP synthase and ATPase function are correlated with a different spatio-temporal organisation of the enzyme. In order to analyze this, we tracked and localized single ATP synthase molecules in situ using live cell microscopy. Under normal conditions, complex V was mainly restricted to cristae indicated by orthogonal trajectories along the cristae membranes. In addition confined trajectories that are quasi immobile exist. By inhibiting glycolysis with 2-DG, the activity and mobility of complex V was altered. The distinct cristae-related orthogonal trajectories of complex V were obliterated. Moreover, a mobile subpopulation of complex V was found in the inner boundary membrane. The observed changes in the ratio of dimeric/monomeric complex V, respectively less mobile/more mobile complex V and its activity changes were reversible. In IF1-KO cells, in which ATP hydrolysis is not inhibited by IF1, complex V was more mobile, while inhibition of ATP hydrolysis by BMS-199264 reduced the mobility of complex V. Taken together, these data support the existence of different subpopulations of complex V, ATP synthase and ATP hydrolase, the latter with higher mobility and probably not prevailing at the cristae edges. Obviously, complex V reacts quickly and reversibly to metabolic conditions, not only by functional, but also by spatial and structural reorganization.