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.

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.

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.

Glycine decarboxylase maintains mitochondrial protein lipoylation to support tumor growth.

The folic acid cycle mediates the transfer of one-carbon (1C) units to support nucleotide biosynthesis. While the importance of serine as a mitochondrial and cytosolic donor of folate-mediated 1C units in cancer cells has been thoroughly investigated, a potential role of glycine oxidation remains unclear. We developed an approach for quantifying mitochondrial glycine cleavage system (GCS) flux by combining stable and radioactive isotope tracing with computational flux decomposition. We find high GCS flux in hepatocellular carcinoma (HCC), supporting nucleotide biosynthesis. Surprisingly, other than supplying 1C units, we found that GCS is important for maintaining protein lipoylation and mitochondrial activity. Genetic silencing of glycine decarboxylase inhibits the lipoylation and activity of pyruvate dehydrogenase and impairs tumor growth, suggesting a novel drug target for HCC. Considering the physiological role of liver glycine cleavage, our results support the notion that tissue of origin plays an important role in tumor-specific metabolic rewiring.

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.

Blimp1+ cells generate functional mouse sebaceous gland organoids in vitro.

Most studies on the skin focus primarily on the hair follicle and interfollicular epidermis, whereas little is known regarding the homeostasis of the sebaceous gland (SG). The SG has been proposed to be replenished by different pools of hair follicle stem cells and cells that resides in the SG base, marked by Blimp1. Here, we demonstrate that single Blimp1+ cells isolated from mice have the potential to generate SG organoids in vitro. Mimicking SG homeostasis, the outer layer of these organoids is composed of proliferating cells that migrate inward, undergo terminal differentiation and generating lipid-filled sebocytes. Performing confocal microscopy and mass-spectrometry, we report that these organoids exhibit known markers and a lipidomic profile similar to SGs in vivo. Furthermore, we identify a role for c-Myc in sebocyte proliferation and differentiation, and determine that SG organoids can serve as a platform for studying initial stages of acne vulgaris, making this a useful platform to identify potential therapeutic targets.

Sensitivity of pituitary gonadotropes to hyperglycemia leads to epigenetic aberrations and reduced follicle-stimulating hormone levels.

The connection between metabolism and reproductive function is well recognized, and we hypothesized that the pituitary gonadotropes, which produce luteinizing hormone and follicle-stimulating hormone (FSH), mediate some of the effects directly via insulin-independent glucose transporters, which allow continued glucose metabolism during hyperglycemia. We found that glucose transporter 1 is the predominant glucose transporter in primary gonadotropes and a gonadotrope precursor-derived cell line, and both are responsive to culture in high glucose; moreover, metabolite levels were altered in the cell line. Several of the affected metabolites are cofactors for chromatin-modifying enzymes, and in the gonadotrope precursor-derived cell line, we recorded global changes in histone acetylation and methylation, decreased DNA methylation, and increased hydroxymethylation, some of which did not revert to basal levels after cells were returned to normal glucose. Despite this weakening of epigenetic-mediated repression seen in the model cell line, FSH ß-subunit ( Fshb) mRNA levels in primary gonadotropes were significantly reduced, apparently due in part to increased autocrine/paracrine effects of inhibin. However, unlike thioredoxin interacting protein and inhibin subunit α, Fshb mRNA levels did not recover after the return of cells to normal glucose. The effect on Fshb expression was also seen in 2 hyperglycemic mouse models, and levels of circulating FSH, required for follicle growth and development, were reduced. Thus, hyperglycemia seems to target the pituitary gonadotropes directly, and the likely extensive epigenetic changes are sensed acutely by Fshb. This scenario would explain clinical findings in which, even after restoration of optimal blood glucose levels, fertility often remains adversely affected. However, the relative accessibility of the pituitary provides a possible target for treatment, particularly crucial in the young in which hyperglycemia is increasingly common and fertility most relevant.-Feldman, A., Saleh, A., Pnueli, L., Qiao, S., Shlomi, T., Boehm, U., Melamed, P. Sensitivity of pituitary gonadotropes to hyperglycemia leads to epigenetic aberrations and reduced follicle-stimulating hormone levels.

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.

GnRH Induces ERK-Dependent Bleb Formation in Gonadotrope Cells, Involving Recruitment of Members of a GnRH Receptor-Associated Signalosome to the Blebs.

We have previously described a signaling complex (signalosome) associated with the GnRH receptor (GnRHR). We now report that GnRH induces bleb formation in the gonadotrope-derived LßT2 cells. The blebs appear within ~2 min at a turnover rate of ~2-3 blebs/min and last for at least 90 min. Formation of the blebs requires active ERK1/2 and RhoA-ROCK but not active c-Src. Although the following ligands stimulate ERK1/2 in LßT2 cells: EGF > GnRH > PMA > cyclic adenosine monophosphate (cAMP), they produced little or no effect on bleb formation as compared to the robust effect of GnRH (GnRH > PMA > cAMP > EGF), indicating that ERK1/2 is required but not sufficient for bleb formation possibly due to compartmentalization. Members of the above mentioned signalosome are recruited to the blebs, some during bleb formation (GnRHR, c-Src, ERK1/2, focal adhesion kinase, paxillin, and tubulin), and some during bleb retraction (vinculin), while F-actin decorates the blebs during retraction. Fluorescence intensity measurements for the above proteins across the cells showed higher intensity in the blebs vs. intracellular area. Moreover, GnRH induces blebs in primary cultures of rat pituitary cells and isolated mouse gonadotropes in an ERK1/2-dependent manner. The novel signalosome-bleb pathway suggests that as with the signalosome, the blebs are apparently involved in cell migration. Hence, we have extended the potential candidates which are involved in the blebs life cycle in general and for the GnRHR in particular.

Gonadotropin gene transcription is activated by menin-mediated effects on the chromatin.

The genes encoding luteinizing hormone and follicle stimulating hormone are activated by gonadotropin-releasing hormone (GnRH), and we hypothesized that this involves GnRH-induction of various histone modifications. At basal conditions in an immature gonadotrope-derived cell line, the hormone-specific ß-subunit gene promoters are densely packed with histones, and contain low levels of H3K4 trimethylation (H3K4me3). GnRH both induces this modification and causes histone loss, creating a more active chromatin state. The H3K4me3 appears to be mediated by menin and possibly catalyzed by the menin-mixed-lineage leukemia (MLL) 1/2 methyl transferase complex, as inhibition of MLL recruitment or menin knockdown reduced gene expression and the levels of H3K4me3 on all three promoters. Menin recruitment to the ß-subunit gene promoters is increased by GnRH, possibly involving transcription factors such as estrogen receptor α and/or steroidogenic factor 1, with which menin interacts. Menin also interacts with ring finger protein 20, which ubiquitylates H2BK120 (H2BK120ub), which was reported to be a pre-requisite for H3K4me3 at various gene promoters. Although levels of H2BK120ub are increased by GnRH in the coding regions of these genes, levels at the promoters do not correlate with those of H3K4me3, nor with gene expression, suggesting that H3K4me3 is not coupled to H2BK120ub in transcriptional activation of these genes.