Loss-of-function of SHARPIN causes an osteopenic phenotype in mice

Downregulation of the Autism Spectrum Disorder Gene Shank2 Decreases Bone Mass in Male Mice

Mutations of the postsynaptic scaffold protein Shank2 lead to autism spectrum disorders (ASD). These patients frequently suffer from higher fracture risk. Here, we investigated whether Shank2 directly regulates bone mass. We show that Shank2 is expressed in bone and that Shank2 levels are increased during osteoblastogenesis. Knockdown of Shank2 by siRNA targeting the encoding regions for PDZ and SAM domain inhibits osteoblastogenesis of primary murine calvarial osteoblasts. Shank2 knockout mice (Shank2 ^-/-) have a decreased bone mass due to reduced osteoblastogenesis and bone formation, whereas bone resorption remains unaffected. Induced pluripotent stem cells (iPSCs)-derived osteoblasts from a loss-of-function Shank2 mutation in a patient showed a significantly reduced osteoblast differentiation potential. Moreover, silencing of known Shank2 interacting proteins revealed that a majority of them promote osteoblast differentiation. From this we conclude that Shank2 and interacting proteins known from the central nervous system are decisive regulators in osteoblast differentiation. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

SHARPIN: Role in Finding NEMO and in Amyloid-Beta Clearance and Degradation (ABCD) Pathway in Alzheimer's Disease?

SHANK- associated RH domain-interacting protein (SHARPIN) is a multifunctional protein associated with numerous physiological functions and many diseases. The primary role of the protein as a LUBAC-dependent component in regulating the activation of the transcription factor NF-κB accounts to its role in inflammation and antiapoptosis. Hence, an alteration of SHARPIN expression or genetic mutations or polymorphisms leads to the alteration of the above-mentioned primary physiological functions contributing to inflammation-associated diseases and cancer, respectively. However, there are complications of targeting SHARPIN as a therapeutic approach, which arises from the wide-range of LUBAC-independent functions and yet unknown roles of SHARPIN including neuronal functions. The identification of SHARPIN as a postsynaptic protein and the emerging studies indicating its role in several neurodegenerative diseases including Alzheimer's disease suggests a strong role of SHARPIN in neuronal functioning. This review summarizes the functional roles of SHARPIN in normal physiology and disease pathogenesis and strongly suggests a need for concentrating more studies on identifying the unknown neuronal functions of SHARPIN and hence its role in neurodegenerative diseases.

Reduced femoral bone mass in both diet-induced and genetic hyperlipidemia mice

Growing evidence argues for a relationship between lipid and bone metabolisms with inconsistent conclusions. Sphingosine-1-phosphate (S1P) has been recognized as a suitable candidate for possible link between lipid metabolism and bone metabolism. This study was designed to investigate the effects of hyperlipidemia on bone metabolism using diet-induced and genetic-induced hyperlipidemia animal models and to explore whether S1P is involved. Wild-type mice and low-density lipoprotein receptor gene deficient (LDLR^-/-) mice at age of 8weeks were placed on either control diet or high-fat diet (HFD) for 12weeks. Bone structural parameters were determined using microCT. Cross-linked type I collagen (CTx) and S1P levels in plasma were measured by ELISA methods. Bone marrow cells from wild type and LDLR^-/- mice were induced to differentiate into osteoblasts, osteoclasts and adipocytes respectively. Gene expressions in distal femur metaphyses and cultured cells were studied by qRT-PCR. Moderate hypercholesterolemia was found in HFD-feeding mice; severe hypercholesterolemia and moderate hypertriglyceridemia were present in LDLR^-/- mice. Femoral trabecular bone mass was reduced in both diet-induced and genetic hyperlipidemia mice. Mice feeding on HFD showed higher CTx levels, and mice with hyperlipidemia had elevated S1P levels. Correlation analysis found a positive correlation between CTx and S1P levels. Lower Runx2 expression and higher TRAP expression were found in both diet-induced and genetic hyperlipidemia mice, indicating decreased osteoblastic functions and increased osteoclastic functions in these mice. Bone marrow cells from LDLR^-/- mice also showed increased adipogenesis and inhibited osteogenesis accompanied by enhanced PPARγ expression. In conclusion, our study found decreased bone mass in both diet-induced and genetic hyperlipidemia mice.

Sharpin Controls Osteogenic Differentiation of Mesenchymal Bone Marrow Cells

The cytosolic protein Sharpin is a component of the linear ubiquitin chain assembly complex, which regulates NF-κB signaling in response to specific ligands, such as TNF-α. Its inactivating mutation in chronic proliferative dermatitis mutation (Cpdm) mice causes multiorgan inflammation, yet this phenotype is not transferable into wild-type mice by hematopoietic stem cell transfer. Recent evidence demonstrated that Cpdm mice additionally display low bone mass, and that this osteopenia is corrected by Tnf deletion. Because the cellular mechanism underlying this pathology, however, was still undefined, we performed a thorough skeletal phenotyping of Cpdm mice on the basis of nondecalcified histology and cellular and dynamic histomorphometry. We show that the trabecular and cortical osteopenia in Cpdm mice is solely explained by impaired bone formation, whereas osteoclastogenesis is unaffected. Consistently, Cpdm primary calvarial cells display reduced osteogenic capacity ex vivo, and the same was observed with CD11b(-) bone marrow cells. Unexpectedly, short-term treatment of these cultures with TNF-α did not reveal an impaired molecular response in the absence of Sharpin. Instead, genome-wide and gene-specific expression analyses revealed that Cpdm mesenchymal cells display increased responsiveness toward TNF-α-induced expression of specific cytokines, such as CXCL5, IL-1β, and IL-6. Therefore, our data not only demonstrate that the skeletal defects of Cpdm mice are specifically caused by impaired differentiation of osteoprogenitor cells, they also suggest that increased cytokine expression in mesenchymal bone marrow cells contributes to the inflammatory phenotype of Cpdm mice.

MicroRNA-17-92 cluster regulates osteoblast proliferation and differentiation

MicroRNAs (miRNAs) have been identified to play important functions during osteoblast proliferation, differentiation, and apoptosis. The miR-17~92 cluster is highly conserved in all vertebrates. Loss-of-function of the miR-17-92 cluster results in smaller embryos and immediate postnatal death of all animals. Germline hemizygous deletions of MIR17HG are accounted for microcephaly, short stature, and digital abnormalities in a few cases of Feingold syndrome. These reports indicate that miR-17~92 may play important function in skeletal development and mature. To determine the functional roles of miR-17~92 in bone metabolism as well as osteoblast proliferation and differentiation. Murine embryonic stem cells D3 and osteoprogenitor cell line MC3T3-E1 were induced to differentiate into osteoblasts; the expression of miR-17-92 was assayed by quantitative real-time RT-PCR. The skeletal phenotypes were assayed in mice heterozygous for miR-17~92 (miR-17~92 (+/Δ) ). To determine the possibly direct function of miR-17~92 in bone cells, osteoblasts from miR-17~92 (+/Δ) mice were investigated by ex vivo cell culture. miR-17, miR-92a, and miR-20a within miR-17-92 cluster were expressed at high level in bone tissue and osteoblasts. The expression of miR-17-92 was down-regulated along with osteoblast differentiation, the lowest level was found in mature osteoblasts. Compared to wildtype controls, miR-17-92 (+/Δ) mice showed significantly lower trabecular and cortical bone mineral density, bone volume and trabecular number at 10 weeks old. mRNA expression of Runx2 and type I collagen was significantly lower in bone from miR-17-92 (+/Δ) mice. Osteoblasts from miR-17-92 (+/Δ) mice showed lower proliferation rate, ALP activity and less calcification. Our research suggests that the miR-17-92 cluster critically regulates bone metabolism, and this regulation is mostly through its function in osteoblasts.

Linear ubiquitination-mediated NF-κB regulation and its related disorders

Ubiquitination is a post-translational modification involved in the regulation of a broad variety of cellular functions, such as protein degradation and signal transduction, including nuclear factor-κB (NF-κB) signalling. NF-κB is crucial for inflammatory and immune responses, and aberrant NF-κB signalling is implicated in multiple disorders. We found that linear ubiquitin chain assembly complex (LUBAC), composed of HOIL-1L, HOIP and SHARPIN, generates a novel type of Met1 (M1)-linked linear polyubiquitin chain and specifically regulates the canonical NF-κB pathway. Moreover, specific deubiquitinases, such as CYLD, A20 (TNFAIP3) and OTULIN/gumby, inhibit LUBAC-induced NF-κB activation by different molecular mechanisms, and several M1-linked ubiquitin-specific binding domains have been structurally defined. LUBAC and these linear ubiquitination-regulating factors contribute to immune and inflammatory processes and apoptosis. Functional impairments of these factors are correlated with multiple disorders, including autoinflammation, immunodeficiencies, dermatitis, B-cell lymphomas and Parkinson's disease. This review summarizes the molecular basis and the pathophysiological implications of the linear ubiquitination-mediated NF-κB activation pathway regulation by LUBAC.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Specific recognition of linear polyubiquitin by A20 zinc finger 7 is involved in NF-κB regulation

LUBAC (linear ubiquitin chain assembly complex) activates the canonical NF-κB pathway through linear polyubiquitination of NEMO (NF-κB essential modulator, also known as IKKγ) and RIP1. However, the regulatory mechanism of LUBAC-mediated NF-κB activation remains elusive. Here, we show that A20 suppresses LUBAC-mediated NF-κB activation by binding linear polyubiquitin via the C-terminal seventh zinc finger (ZF7), whereas CYLD suppresses it through deubiquitinase (DUB) activity. We determined the crystal structures of A20 ZF7 in complex with linear diubiquitin at 1.70-1.98 Å resolutions. The crystal structures revealed that A20 ZF7 simultaneously recognizes the Met1-linked proximal and distal ubiquitins, and that genetic mutations associated with B cell lymphomas map to the ubiquitin-binding sites. Our functional analysis indicated that the binding of A20 ZF7 to linear polyubiquitin contributes to the recruitment of A20 into a TNF receptor (TNFR) signalling complex containing LUBAC and IκB kinase (IKK), which results in NF-κB suppression. These findings provide new insight into the regulation of immune and inflammatory responses.

Negative regulators of integrin activity

Integrins are heterodimeric transmembrane adhesion receptors composed of α- and β-subunits. They are ubiquitously expressed and have key roles in a number of important biological processes, such as development, maintenance of tissue homeostasis and immunological responses. The activity of integrins, which indicates their affinity towards their ligands, is tightly regulated such that signals inside the cell cruicially regulate the switching between active and inactive states. An impaired ability to activate integrins is associated with many human diseases, including bleeding disorders and immune deficiencies, whereas inappropriate integrin activation has been linked to inflammatory disorders and cancer. In recent years, the molecular details of integrin 'inside-out' activation have been actively investigated. Binding of cytoplasmic proteins, such as talins and kindlins, to the cytoplasmic tail of β-integrins is widely accepted as being the crucial step in integrin activation. By contrast, much less is known with regard to the counteracting mechanism involved in switching integrins into an inactive conformation. In this Commentary, we aim to discuss the known mechanisms of integrin inactivation and the molecules involved.

Linear ubiquitination: a novel NF-κB regulatory mechanism for inflammatory and immune responses by the LUBAC ubiquitin ligase complex

The NF-κB pathway is a central signaling pathway for inflammatory and immune responses, and aberrant NF-κB signaling is implicated multiple disorders, such as cancer and autoimmune, chronic inflammatory and metabolic diseases. NF-κB is regulated by various post-translational modifications, including phosphorylation and multiple ubiquitinations. We determined that LUBAC (linear ubiquitin chain assembly complex), composed of SHARPIN, HOIL-IL and HOIP, generates a novel type of Met1-linked linear polyubiquitin chain and specifically regulates the canonical NF-κB pathway via the linear ubiquitination of NEMO and RIP1. In the absence of LUBAC components, NF-κB signaling was attenuated and induced apoptosis and inflammation. Many studies on the pathophysiological functions of LUBAC, such as in B cell development, innate immune response, carcinogenesis, and osteogenesis, have been performed recently. This review summarizes these new findings on LUBAC- and linear ubiquitination-mediated NF-κB regulation and their implications in disorders.

SHARPIN negatively associates with TRAF2-mediated NFκB activation

NFκB is an inducible transcriptional factor controlled by two principal signaling cascades and plays pivotal roles in diverse physiological processes including inflammation, apoptosis, oncogenesis, immunity, and development. Activation of NFκB signaling was detected in skin of SHAPRIN-deficient mice and can be diminished by an NFκB inhibitor. However, in vitro studies demonstrated that SHARPIN activates NFκB signaling by forming a linear ubiquitin chain assembly complex with RNF31 (HOIP) and RBCK1 (HOIL1). The inconsistency between in vivo and in vitro findings about SHARPIN's function on NFκB activation could be partially due to SHARPIN's potential interactions with downstream molecules of NFκB pathway. In this study, 17 anti-flag immunoprecipitated proteins, including TRAF2, were identified by mass spectrum analysis among Sharpin-Flag transfected mouse fibroblasts, B lymphocytes, and BALB/c LN stroma 12 cells suggesting their interaction with SHARPIN. Interaction between SHARPIN and TRAF2 confirmed previous yeast two hybridization reports that SHARPIN was one TRAF2's partners. Furthermore, luciferase-based NFκB reporter assays demonstrated that SHARPIN negatively associates with NFκB activation, which can be partly compensated by over-expression of TRAF2. These data suggested that other than activating NFκB signaling by forming ubiquitin ligase complex with RNF31 and RBCK1, SHARPIN may also negatively associate with NFκB activation via interactions with other NFκB members, such as TRAF2.

Chronic Proliferative Dermatitis in Mice: NFκB Activation Autoinflammatory Disease

Autoinflammatory diseases are a heterogeneous group of congenital diseases characterized by the presence of recurrent inflammation, in the absence of infectious agents, detectable autoantibodies or antigen-specific autoreactive T-cells. SHARPIN deficient mice presents multiorgan chronic inflammation without known autoantibodies or autoreactive T-cells, designated Sharpin(cpdm). Histological studies demonstrated epidermal hyperproliferation, Th-2 inflammation, and keratinocyte apoptosis in this mutant. The mutant mice have decreased behavioral mobility, slower growth, and loss of body weight. Epidermal thickness and mitotic epidermal cells increase along with disease development. K5/K14 expression is distributed through all layers of epidermis, along with K6 expression in interfollicular epidermis, suggesting epidermal hyperproliferation. K1/K10 is only detectable in outer layers of spinosum epidermis, reflecting accelerated keratinocyte migration. Alpha smooth muscle actin is overexpressed in skin blood vessels, which may release the elevated white blood cells to dermis. CD3(+)CD45(+) cells and granulocytes, especially eosinophils and mast cells, aggregate in the mutant skin. TUNEL assay, together with Annexin-V/propidium iodide FACS analysis, confirmed the increase of apoptotic keratinocytes in skin. These data validate and provide new lines of evidence of the proliferation-inflammation-apoptosis triad in Sharpin(cpdm) mice, an NFκB activation autoinflammatory disease.