Twisting of the zebrafish heart tube during cardiac looping is a tbx5-dependent and tissue-intrinsic process

Organ laterality refers to the left-right asymmetry in disposition and conformation of internal organs and is established during embryogenesis. The heart is the first organ to display visible left-right asymmetries through its left-sided positioning and rightward looping. Here, we present a new zebrafish loss-of-function allele for tbx5a, which displays defective rightward cardiac looping morphogenesis. By mapping individual cardiomyocyte behavior during cardiac looping, we establish that ventricular and atrial cardiomyocytes rearrange in distinct directions. As a consequence, the cardiac chambers twist around the atrioventricular canal resulting in torsion of the heart tube, which is compromised in tbx5a mutants. Pharmacological treatment and ex vivo culture establishes that the cardiac twisting depends on intrinsic mechanisms and is independent from cardiac growth. Furthermore, genetic experiments indicate that looping requires proper tissue patterning. We conclude that cardiac looping involves twisting of the chambers around the atrioventricular canal, which requires correct tissue patterning by Tbx5a.

Bone morphogenetic protein 2-induced cellular chemotaxis drives tissue patterning during critical-sized bone defect healing: an in silico study

Critical-sized bone defects are critical healing conditions that, if left untreated, often lead to non-unions. To reduce the risk, critical-sized bone defects are often treated with recombinant human BMP-2. Although enhanced bone tissue formation is observed when BMP-2 is administered locally to the defect, spatial and temporal distribution of callus tissue often differs from that found during regular bone healing or in defects treated differently. How this altered tissue patterning due to BMP-2 treatment is linked to mechano-biological principles at the cellular scale remains largely unknown. In this study, the mechano-biological regulation of BMP-2-treated critical-sized bone defect healing was investigated using a multiphysics multiscale in silico approach. Finite element and agent-based modeling techniques were combined to simulate healing within a critical-sized bone defect (5 mm) in a rat femur. Computer model predictions were compared to in vivo microCT data outcome of bone tissue patterning at 2, 4, and 6 weeks postoperation. In vivo, BMP-2 treatment led to complete healing through periosteal bone bridging already after 2 weeks postoperation. Computer model simulations showed that the BMP-2 specific tissue patterning can be explained by the migration of mesenchymal stromal cells to regions with a specific concentration of BMP-2 (chemotaxis). This study shows how computational modeling can help us to further understand the mechanisms behind treatment effects on compromised healing conditions as well as to optimize future treatment strategies.

Niche-guided tissue patterning by chemomechanical flow lithography

Pluripotent-stem-cell-derived tissue-models have been established with increasingly physiological shape, size and function1–3. However, the histogenic and morphogenetic processes present in these models proceed stochastically. This reflects an absence of technologies able to produce complex supportive cell niches that can reproducibly guide tissue patterning and generate well-defined tissue structures. To address this, we have developed chemomechanical flow lithography (CMFL), a printing technology that delivers orthogonally programmable chemical and mechanical properties to microstructured niches that drive the differentiation of selective cell types and spatial emplacement of these cells in a micropattern. We print microstructured niches conjugated with peptides, proteins and macromolecular morphogens across a range of Young’s moduli. Using such niches, we generated tissue patterned constructs from a single cell source with regionalised cell differentiation, including a bone-fat osteoid from stromal mesenchyme, and a patterned assembly of germ-layer tissues derived from pluripotent stem cells. Thus, CMFL is a valuable tool for generating orthogonally and chemomechanically defined niches able to guide spatially defined tissue patterns. This approach enables studies to better understand how extrinsic niche factors regulate histogenic and morphogenetic processes, towards engineering complex structured tissue and organ systems with higher-level emergent function.

SABRE populates ER domains essential for cell plate maturation and cell expansion influencing cell and tissue patterning

The SABRE protein, originally identified in plants, is found throughout eukaryotes. In plants, SABRE has been implicated in cell expansion, division plane orientation and planar polarity. However, how SABRE mediates these processes remains an open question. Here, we have taken advantage of the fact that the bryophyte Physcomitrium patens has a single copy of SABRE, is an excellent model for cell biology and is readily amenable to precise genetic alterations to investigate SABRE’s mechanism of action. We discovered that SABRE null mutants were stunted in both polarized growing and diffusely growing tissues, similar to reported phenotypes in seed plants. However, in polarized growing cells, we observed significant delays in cell plate formation and sometimes catastrophic failures in cell division. We generated a functional SABRE fluorescent fusion protein and determined that it forms dynamic puncta on regions of the endoplasmic reticulum (ER) both in the cytoplasm during interphase and at the new cell plate during division. In the absence of SABRE, ER morphology was severely compromised with large aggregates accumulating in the cytoplasm and abnormal buckling along the developing cell plate late in cytokinesis. In fact, SABRE and the ER maximally accumulated on the developing plate specifically during cell plate maturation, coincident with the timing of the onset of failures in cell plate formation in cells lacking SABRE. Further we discovered that callose deposition is delayed in Δsabre cells, and in cells that failed to divide, abnormal callose accumulations formed at the cell plate. Our findings demonstrated that SABRE functions by influencing the ER and callose deposition, revealing a surprising and essential role for the ER in cell plate maturation. Given that SABRE is conserved, understanding how SABRE influences cell and tissue patterning has profound significance across eukaryotes.