Caspase-3 Regulates YAP-Dependent Cell Proliferation and Organ Size.

Apoptosis culminates in the activation of caspase-3, which plays an important role in implementing the cell death program. Here, we reveal a non-apoptotic role of caspase-3 as a key regulator of cell proliferation and organ size. Caspase-3 is specifically activated in the proliferating cells of the sebaceous gland, but does not instruct cell elimination. Deletion or chemical inhibition of caspase-3 diminishes cell proliferation, decreases cell number and reduces sebaceous gland size in vivo. Exploring the underlying mechanism, we demonstrate that α-catenin is cleaved by caspase-3, thus facilitating the activation and nuclear translocation of yes-associated protein (YAP), a vital regulator of organ size. Accordingly, activation of caspase-3 leads to YAP-dependent organ size augmentation. Finally, we show that X-linked inhibitor of apoptosis protein (XIAP) serves as an endogenous feedback antagonist for the caspase-3/YAP signaling module. Taken together, we report here a molecular mechanism wherein the apoptotic machinery is refocused to regulate cell proliferation and orchestrate organ size.

Apoptotic dysregulation mediates stem cell competition and tissue regeneration.

Since adult stem cells are responsible for replenishing tissues throughout life, it is vital to understand how failure to undergo apoptosis can dictate stem cell behavior both intrinsically and non-autonomously. Here, we report that depletion of pro-apoptotic Bax protein bestows hair follicle stem cells with the capacity to eliminate viable neighboring cells by sequestration of TNFα in their membrane. This in turn induces apoptosis in "loser" cells in a contact-dependent manner. Examining the underlying mechanism, we find that Bax loss-of-function competitive phenotype is mediated by the intrinsic activation of NFκB. Notably, winner stem cells differentially respond to TNFα, owing to their elevated expression of TNFR2. Finally, we report that in vivo depletion of Bax results in an increased stem cell pool, accelerating wound-repair and de novo hair follicle regeneration. Collectively, we establish a mechanism of mammalian cell competition, which can have broad therapeutic implications for tissue regeneration and tumorigenesis.

Altered ubiquitin signaling induces Alzheimer's disease-like hallmarks in a three-dimensional human neural cell culture model.

Alzheimer's disease (AD) is characterized by toxic protein accumulation in the brain. Ubiquitination is essential for protein clearance in cells, making altered ubiquitin signaling crucial in AD development. A defective variant, ubiquitin B + 1 (UBB+1), created by a non-hereditary RNA frameshift mutation, is found in all AD patient brains post-mortem. We now detect UBB+1 in human brains during early AD stages. Our study employs a 3D neural culture platform derived from human neural progenitors, demonstrating that UBB+1 alone induces extracellular amyloid-ß (Aß) deposits and insoluble hyperphosphorylated tau aggregates. UBB+1 competes with ubiquitin for binding to the deubiquitinating enzyme UCHL1, leading to elevated levels of amyloid precursor protein (APP), secreted Aß peptides, and Aß build-up. Crucially, silencing UBB+1 expression impedes the emergence of AD hallmarks in this model system. Our findings highlight the significance of ubiquitin signalling as a variable contributing to AD pathology and present a nonclinical platform for testing potential therapeutics.

May the best wound WIHN: the hallmarks of wound-induced hair neogenesis.

The hair follicle is a unique mini organ that undergoes continuous cycles of replenishment. While hair follicle formation was long thought to occur strictly during embryogenesis, it is now becoming increasingly clear that hair follicles can regenerate from the wound bed. Here, we provide an overview of the recent advancements in the field of Wound Induced Hair Neogenesis (WIHN) in mice. We briefly outline the hair follicle morphogenic process and discuss the major features of adult hair follicle regeneration. We examine the role of distinct cell types and review the contribution of specific signaling pathways to the WIHN phenotype. The phenomenon of neogenic hair regeneration provides an important platform, which may offer new insights into mammalian regeneration in the adult setting.

THY1-mediated mechanisms converge to drive YAP activation in skin homeostasis and repair.

Anchored cells of the basal epidermis constantly undergo proliferation in an overcrowded environment. An important regulator of epidermal proliferation is YAP, which can be controlled by both cell-matrix and cell-cell interactions. Here, we report that THY1, a GPI-anchored protein, inhibits epidermal YAP activity through converging molecular mechanisms. THY1 deficiency leads to increased adhesion by activating the integrin-ß1-SRC module. Notably, regardless of high cellular densities, the absence of THY1 leads to the dissociation of an adherens junction complex that enables the release and translocation of YAP. Due to increased YAP-dependent proliferation, Thy1-/- mice display enhanced wound repair and hair follicle regeneration. Taken together, our work reveals THY1 as a crucial regulator of cell-matrix and cell-cell interactions that controls YAP activity in skin homeostasis and regeneration.

Thy1 marks a distinct population of slow-cycling stem cells in the mouse epidermis.

The presence of distinct stem cells that maintain the interfollicular epidermis is highly debated. Here, we report a population of keratinocytes, marked by Thy1, in the basal layer of the interfollicular epidermis. We find that epidermal cells expressing differential levels of Thy1 display distinct transcriptional signatures. Thy1+ keratinocytes do not express T cell markers, express a unique transcriptional profile, cycle significantly slower than basal epidermal progenitors and display significant expansion potential in vitro. Multicolor lineage tracing analyses and mathematical modeling reveal that Thy1+ basal keratinocytes do not compete neutrally alike interfollicular progenitors and contribute long-term to both epidermal replenishment and wound repair. Importantly, ablation of Thy1+ cells strongly impairs these processes, thus indicating the non-redundant function of Thy1+ stem cells in the epidermis. Collectively, these results reveal a distinct stem cell population that plays a critical role in epidermal homeostasis and repair.

Apoptotic cells represent a dynamic stem cell niche governing proliferation and tissue regeneration.

Stem cells (SCs) play a key role in homeostasis and repair. While many studies have focused on SC self-renewal and differentiation, little is known regarding the molecular mechanism regulating SC elimination and compensation upon loss. Here, we report that Caspase-9 deletion in hair follicle SCs (HFSCs) attenuates the apoptotic cascade, resulting in significant temporal delays. Surprisingly, Casp9-deficient HFSCs accumulate high levels of cleaved caspase-3 and are improperly cleared due to an essential caspase-3/caspase-9 feedforward loop. These SCs are retained in an apoptotic-engaged state, serving as mitogenic signaling centers by continuously releasing Wnt3 and instructing proliferation. Investigating the underlying mechanism, we reveal a caspase-3/Dusp8/p38 module responsible for Wnt3 induction, which operates in both normal and Casp9-deleted HFSCs. Notably, Casp9-deleted mice display accelerated wound repair and de novo hair follicle regeneration. Taken together, we demonstrate that apoptotic cells represent a dynamic SC niche, from which emanating signals drive SC proliferation and tissue regeneration.

Ex Vivo Expanded 3D Human Kidney Spheres Engraft Long Term and Repair Chronic Renal Injury in Mice.

End-stage renal disease is a worldwide epidemic requiring renal replacement therapy. Harvesting tissue from failing kidneys and autotransplantation of tissue progenitors could theoretically delay the need for dialysis. Here we use healthy and end-stage human adult kidneys to robustly expand proliferative kidney epithelial cells and establish 3D kidney epithelial cultures termed "nephrospheres." Formation of nephrospheres reestablishes renal identity and function in primary cultures. Transplantation into NOD/SCID mice shows that nephrospheres restore self-organogenetic properties lost in monolayer cultures, allowing long-term engraftment as tubular structures, potentially adding nephron segments and demonstrating self-organization as critical to survival. Furthermore, long-term tubular engraftment of nephrospheres is functionally beneficial in murine models of chronic kidney disease. Remarkably, nephrospheres inhibit pro-fibrotic collagen production in cultured fibroblasts via paracrine modulation, while transplanted nephrospheres induce transcriptional signatures of proliferation and release from quiescence, suggesting re-activation of endogenous repair. These data support the use of human nephrospheres for renal cell therapy.

More than One Way to Skin a Wound.

Large cutaneous and non-healing ulcers exhibit impaired epithelialization, which represents a major therapeutic challenge. Recently in Nature, Kurita et al. (2018) reported a surprising potential source of epithelial cells. The authors demonstrate that wound-resident mesenchymal cells can be reprogrammed to generate expandable epithelial tissues and facilitate wound closure.

ARTS mediates apoptosis and regeneration of the intestinal stem cell niche.

Stem cells (SCs) play a pivotal role in fueling homeostasis and regeneration. While much focus has been given to self-renewal and differentiation pathways regulating SC fate, little is known regarding the specific mechanisms utilized for their elimination. Here, we report that the pro-apoptotic protein ARTS (a Septin4 isoform) is highly expressed in cells comprising the intestinal SC niche and that its deletion protects Lgr5+ and Paneth cells from undergoing apoptotic cell death. As a result, the Sept4/ARTS-/- crypt displays augmented proliferation and, in culture, generates massive cystic-like organoids due to enhanced Wnt/ß-catenin signaling. Importantly, Sept4/ARTS-/- mice exhibit resistance against intestinal damage in a manner dependent upon Lgr5+ SCs. Finally, we show that ARTS interacts with XIAP in intestinal crypt cells and that deletion of XIAP can abrogate Sept4/ARTS-/--dependent phenotypes. Our results indicate that intestinal SCs utilize specific apoptotic proteins for their elimination, representing a unique target for regenerative medicine.