Biology subjects

Mikkola, M. L.

Publications and source records attributed to Mikkola, M. L..

8 recordsLinked to original sources

bioRxiv2024

Wnt target gene Ascl4 is dispensable for skin appendage development

The development of skin appendages, including hair follicles, teeth and mammary glands is initiated through the formation of the placode - a local thickening of the epithelium. The Wnt/{beta}-catenin signaling cascade is an evolutionary conserved pathway with an essential role in placode morphogenesis, but its downstream targets and their exact functions remain ill defined. In this study, we identify Achaete-scute complex-like 4 (Ascl4) as a novel target of the Wnt/{beta}-catenin pathway and demonstrate its expression pattern in the signaling centers of developing hair follicles and teeth. Ascl transcription factors belong to the superfamily of basic helix-loop-helix transcriptional regulators involved in cell fate determination in many tissues. However, their specific role in the developing skin remains largely unknown. We report that Ascl4 null mice have no overt phenotype. Absence of Ascl4 did not impair hair follicle morphogenesis or hair shaft formation suggesting that it is non-essential for hair follicle development. No tooth or mammary gland abnormalities were detected either. We suggest that other transcription factors may functionally compensate for the absence of Ascl4, but further research is warranted to assess this possibility.

developmental biology↗

bioRxiv2023

Stabilization of epithelial β-catenin compromises mammary cell fate acquisition and branching morphogenesis

The Wnt/{beta}-catenin pathway plays a critical role in cell fate specification, morphogenesis, and stem cell activation across diverse tissues, including the skin. In mammals, the embryonic surface epithelium gives rise to the epidermis, as well as the associated appendages including hair follicles and mammary glands, both of which depend on epithelial Wnt/{beta}-catenin activity for initiation of their development. Later on, Wnts are thought to enhance mammary gland growth and branching while in hair follicles, they are essential for hair shaft formation. Here we report a strong downregulation of epithelial Wnt/{beta}-catenin activity as the mammary bud progresses to branching. We show that forced activation of epithelial {beta}-catenin severely compromises embryonic mammary gland branching. However, the phenotype of conditional Lef1 deficient embryos implies that a low level of Wnt/{beta}-catenin activity is necessary for mammary cell survival. Transcriptomic profiling suggests that sustained high {beta}-catenin activity leads to maintenance of mammary bud gene signature at the expense of outgrowth/branching signature. In addition, it leads to upregulation of epidermal differentiation genes. Strikingly, we find a partial switch to hair follicle fate early on upon stabilization of {beta}-catenin suggesting that the level of epithelial Wnt/{beta}-catenin signaling activity may contribute to the choice between skin appendage identities.

developmental biology↗

bioRxiv2023

Mesenchyme governs hair follicle induction

Tissue interactions are essential to guide organogenesis. The development of hair follicles is regulated by inductive signalling between the embryonic surface epithelium and the adjacent mesenchyme. Previous studies have established that the mesenchymal component of the hair follicle, the dermal papilla and its precursor dermal condensate, has the capacity for de novo hair follicle induction. However, studies prior to dermal condensate formation have been inconclusive, and therefore the source and the identity of the primary inductive signal have remained unknown. Here, we performed epithelial-mesenchymal tissue recombination experiments using hair-forming back skin and glabrous plantar skin from mouse embryos to unveil that the back skin mesenchyme is inductive even prior to dermal condensate formation. Moreover, the naive, unpatterned mesenchyme was sufficient to trigger hair follicle formation even in the oral epithelium. Considering the recognized role of Wnt signalling and Bmp activity inhibition in initiation of hair follicle development, we explored the hair-inductive ability of the Wnt agonist R-spondin-1 and a Bmp receptor inhibitor in embryonic skin explants. Although R-spondin-1 instigated precocious placode-specific transcriptional responses, it alone or in combination with the Bmp receptor inhibitor was insufficient for hair follicle induction. Our findings pave the way for identifying the hair follicle-inducing cue.

developmental biology↗

bioRxiv2023

On growth and form of the mammary gland: Epithelial-mesenchymal interactions in embryonic mammary gland development

Mammary gland is a unique organ that undergoes dynamic alterations throughout a females reproductive life, making it an ideal model for developmental, stem cell and cancer biology research. Mammary gland development begins in utero and proceeds via a quiescent bud stage before the initial outgrowth and subsequent branching morphogenesis. How mammary epithelial cells transit from quiescence to an actively proliferating and branching tissue during embryogenesis and, importantly, how the branch pattern is determined remain largely unknown. Here we provide evidence indicating that epithelial cell proliferation, segregation into basal and luminal lineages that characterize the postnatal mammary duct, and onset of branching are independent processes, yet partially coordinated by the Eda signaling pathway. By performing heterotypic and heterochronic epithelial-mesenchymal recombination experiments between mammary and salivary gland tissues and ex vivo live imaging, we demonstrate that unlike previously concluded, the mode of branching is an intrinsic property of the mammary epithelium while the growth pace and density of the mammary ductal tree are governed by the mesenchyme. Transcriptomic profiling and ex vivo and in vivo functional studies disclose that mesenchymal Wnt/{beta}-catenin signaling, and in particular IGF-1 downstream of it critically regulate mammary gland growth. These results underscore the general need to carefully decompose the different developmental processes producing branched organs.

developmental biology↗

bioRxiv2022

Mechanical forces across compartments coordinate cell shape and fate transitions to generate tissue architecture

Morphogenesis and cell state transitions must be coordinated in time and space to produce a functional tissue. An excellent paradigm to understand the coupling of these processes is mammalian hair follicle development, initiated by the formation of an epithelial invagination - termed placode - that coincides with the emergence of a designated hair follicle stem cell population. The mechanisms directing the deformation of the epithelium, cell state transitions, and physical compartmentalization of the placode are unknown. Here, we identify a key role for coordinated mechanical forces stemming from contractile, proliferative, and proteolytic activities across the epithelial and mesenchymal compartments in generating the placode structure. A ring of fibroblast cells gradually wraps around the placode cells to generate centripetal contractile forces, which in collaboration with polarized epithelial myosin activity promote elongation and local tissue thickening. These mechanical stresses further enhance and compartmentalize Sox9 expression to promote stem cell positioning. Subsequently, proteolytic remodeling locally softens the basement membrane to facilitate release of pressure on the placode, enabling localized cell divisions, tissue fluidification, and epithelial invagination into the underlying mesenchyme. Together, our experiments and modeling identify dynamic cell shape transformations and tissue-scale mechanical co-operation as key factors for orchestrating organ formation.

developmental biology↗

bioRxiv2022

Transcriptomic landscape of early hair follicle and epidermal development

Morphogenesis of ectodermal organs, such as hair, tooth, and mammary gland, starts with the formation of local epithelial thickenings, or placodes, but it remains to be determined how distinct cell types and differentiation programs are established during ontogeny. Here, we use bulk and single-cell transcriptomics and pseudotime modelling to address these questions in developing hair follicles and epidermis, and produce a comprehensive transcriptomic profile of cellular populations in the hair placode and interplacodal epithelium. We report previously unknown cell populations and marker genes, including early suprabasal and genuine interfollicular basal markers, and propose the identity of suprabasal progenitors. By uncovering four different hair placode cell populations organized in three spatially distinct areas, with fine gene expression gradients between them, we posit early establishment of cell fates. This work is accompanied by a readily accessible online tool to stimulate further research on skin appendages and their progenitors.

developmental biology↗

bioRxiv2022

Spatially coordinated cell cycle activity and motility govern mammary ductal growth and tip bifurcation

Branching morphogenesis is the common evolutionary solution of multiple organs to combine maximal epithelial function with compact organ size. It involves successive rounds of branch elongation and branch point generation to generate a branched epithelial network. Branch points form most commonly at the tips of branches as they split into two. However, it is unclear how epithelial cells in tips drive both elongation and branching. In this study, we used ex vivo live imaging to identify these fundamental cellular mechanisms in the embryonic mammary gland. Our 4D analyses show that tips of branches are driven forward by directional cell migration, while elongation of the subtending duct is supported by cell proliferation that feeds a retrograde flow of lagging cells into the duct, established upon differential cell motility. Tip bifurcation involved localized repression of both cell cycle and cell motility at the branch point. Cells in the nascent daughter tips remained proliferative, but changed the direction of their movement to elongate new branches. Our study also reports the fundamental importance of the contractile actin cytoskeleton in regulating branch point generation in the mammary epithelium.

cell biology↗

bioRxiv2022

Unraveling the principles of mammary gland branching morphogenesis

Branching morphogenesis is a characteristic feature of many essential organs such as the lung, kidney, and most glands, and the net result of two tissue behaviors: branch point initiation and elongation. Each branched organ has a distinct architecture customized to its physiological function, but how patterning occurs in these ramified tubular structures is a fundamental problem of development. Here we use quantitative 3D morphometrics, time-lapse imaging, manipulation of ex vivo cultured embryonic organs, and mice deficient in the planar cell polarity component Vangl2 to address this question in the developing mammary gland. Our results show that the embryonic epithelial trees are highly complex in topology owing to the flexible use of two distinct modes of branch point initiation: lateral branching and tip bifurcation. This non-stereotypy was contrasted by the remarkably constant average branch frequency indicating a ductal growth-invariant, yet stochastic propensity to branch. The probability to branch was malleable and could be tuned by manipulating the Fgf10 and Tgf-{beta}1 pathways. Finally, our in vivo and ex vivo time-lapse imaging suggested the involvement of tissue rearrangements in mammary branch elongation.

developmental biology↗