Ciliary Hedgehog Signaling Restricts Injury-Induced Adipogenesis

Mechanism and Regulation of Centriole and Cilium Biogenesis

The centriole is an ancient microtubule-based organelle with a conserved nine-fold symmetry. Centrioles form the core of centrosomes, which organize the interphase microtubule cytoskeleton of most animal cells and form the poles of the mitotic spindle. Centrioles can also be modified to form basal bodies, which template the formation of cilia and play central roles in cellular signaling, fluid movement, and locomotion. In this review, we discuss developments in our understanding of the biogenesis of centrioles and cilia and the regulatory controls that govern their structure and number. We also discuss how defects in these processes contribute to a spectrum of human diseases and how new technologies have expanded our understanding of centriole and cilium biology, revealing exciting avenues for future exploration.

The cross-talk between TGF-β and PDGFRα signaling pathways regulates stromal fibro/adipogenic progenitors’ fate

Fibro/adipogenic progenitors (FAPs) are tissue-resident mesenchymal stromal cells (MSCs) required for proper skeletal muscle development, regeneration, and maintenance. However, FAPs are also responsible for fibro-fatty scar deposition following chronic damage. We aimed to study a functional cross-talk between TGF-β and PDGFRα signaling pathways in FAPs’ fate. Here, we show that the number of FAPs correlates with TGF-β levels and with extracellular matrix deposition during regeneration and repair. Interestingly, the expression of PDGFRα changed dynamically in the stromal/fibroblast lineage after injury. Furthermore, PDGFRα-dependent immediate early gene expression changed during regeneration and repair. We also found that TGF-β signaling reduces PDGFRα expression in FAPs, mouse dermal fibroblasts, and in two related mesenchymal/fibroblast cell lines. Moreover, TGF-β promotes myofibroblast differentiation of FAPs but inhibits their adipogenicity. Accordingly, TGF-β impairs the expression of PDGFRα-dependent immediate early genes in a TGF-BR1-dependent manner. Finally, pharmacological inhibition of PDGFRα activity with AG1296 impaired TGF-β-induced extracellular matrix remodeling, Smad2 signaling, myofibroblast differentiation, and migration of MSCs. Thus, our work establishes a functional cross-talk between TGF-β and PDGFRα signaling pathways that is involved in regulating the biology of FAPs/MSCs.

Sulforaphene Inhibition of Adipogenesis via Hedgehog Signaling in 3T3-L1 Adipocytes

Obesity is a risk factor for numerous metabolic disorders. In this study, we investigated the effects of the isothiocyanates sulforaphane (SA) and sulforaphene (SE) on adipogenesis in 3T3-L1 adipocytes. SE, a compound that is abundant in radish, inhibited adipogenesis by suppressing the adipogenic transcription factors peroxisome proliferator-activated receptor γ (PPARγ, 69.2 ± 2.4%, P < 0.05) and CCAAT/enhancer-binding protein α (C/EBPα, 36.1 ± 3.1%, P < 0.05), thereby reducing fat accumulation in 3T3-L1 adipocytes (45.6 ± 2.7%, P < 0.05); SA was less effective. SE exerted these activities through the activation of the Hedgehog (Hh) signaling pathway by restoring Smo ((2.1 ± 0.2)-fold, P < 0.05) and Gli1 ((2.8 ± 0.1)-fold, P < 0.05) expression, which was suppressed by adipogenic signals. These effects of SE were abrogated by treatment with the Hh inhibitor vismodegib. Thus, SE inhibits adipocyte differentiation via Hh signaling and may be an effective natural agent for preventing adipocyte hyperplasia and obesity.

Deficient primary cilia in obese adipose-derived mesenchymal stem cells: obesity, a secondary ciliopathy?

Obesity alters the composition, structure and function of adipose tissue, characterized by chronic inflammation, insulin resistance and metabolic dysfunction. Adipose-derived mesenchymal stem cells (ASCs) are responsible for cell renewal, spontaneous repair and immunomodulation in adipose tissue. Increasing evidence highlights that ASCs are deficient in obesity, and the underlying mechanisms are not well understood. We have recently shown that obese ASCs have defective primary cilia, which are shortened and unable to properly respond to stimuli. Impaired cilia compromise ASC functions. This work suggests an intertwined connection of obesity, defective cilia and dysfunctional ASCs. We have here discussed the current data regarding defective cilia in various cell types in obesity. Based on these observations, we hypothesize that obesity, a systemic chronic metainflammation, could impair cilia in diverse ciliated cells, like pancreatic islet cells, stem cells and hypothalamic neurons, making these critical cells dysfunctional by shutting down their signal sensors and transducers. In this context, obesity may represent a secondary form of ciliopathy induced by obesity-related inflammation and metabolic dysfunction. Reactivation of ciliated cells might be an alternative strategy to combat obesity and its associated diseases.

Pitx2 maintains mitochondrial function during regeneration to prevent myocardial fat deposition

Loss of the paired-like homeodomain transcription factor 2 (Pitx2) in cardiomyocytes predisposes mice to atrial fibrillation and compromises neonatal regenerative capacity. In addition, Pitx2 gain-of-function protects mature cardiomyocytes from ischemic injury and promotes heart repair. Here, we characterized the long-term myocardial phenotype following myocardial infarction (MI) in Pitx2 conditional-knockout (Pitx2 CKO) mice. We found adipose-like tissue in Pitx2 CKO hearts 60 days after MI induced surgically at postnatal day 2 but not at day 8. Molecular and cellular analyses showed the onset of adipogenic signaling in mutant hearts after MI. Lineage tracing experiments showed a non-cardiomyocyte origin of the de novo adipose-like tissue. Interestingly, we found that Pitx2 promotes mitochondrial function through its gene regulatory network, and that the knockdown of a key mitochondrial Pitx2 target gene, Cox7c, also leads to the accumulation of myocardial fat tissue. Single-nuclei RNA-seq revealed that Pitx2-deficient hearts were oxidatively stressed. Our findings reveal a role for Pitx2 in maintaining proper cardiac cellular composition during heart regeneration via the maintenance of proper mitochondrial structure and function.

Adipose Organ Development and Remodeling

During the last decades, research on adipose tissues has spread in parallel with the extension of obesity. Several observations converged on the idea that adipose tissues are organized in a large organ with endocrine and plastic properties. Two parenchymal components: white (WATs) and brown adipose tissues (BATs) are contained in subcutaneous and visceral compartments. Although both have endocrine properties, their function differs: WAT store lipids to allow intervals between meals, BAT burns lipids for thermogenesis. In spite of these opposite functions, they share the ability for reciprocal reversible transdifferentiation to tackle special physiologic needs. Thus, chronic need for thermogenesis induces browning and chronic positive energy balance induce whitening. Lineage tracing and data from explant studies strongly suggest other remodeling properties of this organ. During pregnancy and lactation breast WAT transdifferentiates into milk-secreting glands, composed by cells with abundant cytoplasmic lipids (pink adipocytes) and in the postlactation period pink adipocytes transdifferentiate back into WAT and BAT. The plastic properties of mature adipocytes are supported also by a liposecretion process in vitro where adult cell in culture transdifferentiate to differentiated fibroblast-like elements able to give rise to different phenotypes (rainbow adipocytes). In addition, the inflammasome system is activated in stressed adipocytes from obese adipose tissue. These adipocytes die and debris are reabsorbed by macrophages inducing a chronic low-grade inflammation, potentially contributing to insulin resistance and T2 diabetes. Thus, the plastic properties of this organ could open new therapeutic perspectives in the obesity-related metabolic disease and in breast pathologies. © 2018 American Physiological Society. Compr Physiol 8:1357-1431, 2018.

Signaling Mechanisms of Myofibroblastic Activation: Outside-in and Inside-Out

Myofibroblasts are central mediators of fibrosis. Typically derived from resident fibroblasts, myofibroblasts represent a heterogeneous population of cells that are principally defined by acquired contractile function and high synthetic ability to produce extracellular matrix (ECM). Current literature sheds new light on the critical role of ECM signaling coupled with mechanotransduction in driving myofibroblastic activation. In particular, transforming growth factor β1 (TGF-β1) and extra domain A containing fibronectin (EDA-FN) are thought to be the primary ECM signaling mediators that form and also induce positive feedback loops. The outside-in and inside-out signaling circuits are transmitted and integrated by TGF-β receptors and integrins at the cell membrane, ultimately perpetuating the abundance and activities of TGF-β1 and EDA-FN in the ECM. In this review, we highlight these conceptual advances in understanding myofibroblastic activation, in hope of revealing its therapeutic anti-fibrotic implications.

Odd skipped-related 1 (Osr1) identifies muscle-interstitial fibro-adipogenic progenitors (FAPs) activated by acute injury

Fibro-adipogenic progenitors (FAPs) are resident mesenchymal progenitors in adult skeletal muscle that support muscle repair, but also give rise to fibrous and adipose infiltration in response to disease and chronic injury. FAPs are identified using cell surface markers that do not distinguish between quiescent FAPs and FAPs actively engaged in the regenerative process. We have shown previously that FAPs are derived from cells that express the transcription factor Osr1 during development. Here we show that adult FAPs express Osr1 at low levels and frequency, however upon acute injury FAPs reactivate Osr1 expression in the injured tissue. Osr1⁺ FAPs are enriched in proliferating and apoptotic cells demonstrating that Osr1 identifies activated FAPs. In vivo genetic lineage tracing shows that Osr1⁺ activated FAPs return to the resident FAP pool after regeneration as well as contribute to adipocytes after glycerol-induced fatty degeneration. In conclusion, reporter LacZ or eGFP-CreERt2 expression from the endogenous Osr1 locus serves as marker for FACS isolation and tamoxifen-induced manipulation of activated FAPs.

Interleukin-15 facilitates muscle regeneration through modulation of fibro/adipogenic progenitors

Background

Chronic muscle injury is characteristics of fatty infiltration and fibrosis. Recently, fibro/adipogenic progenitors (FAPs) were found to be indispensable for muscular regeneration while were also responsible for fibrosis and fatty infiltration in muscle injury. Many myokines have been proven to regulate the adipose or cell proliferation. Because the fate of FAPs is largely dependent on microenvironment and the regulation of myokines on FAPs is still unclear. We screened the potential myokines and found Interleukin-15 (IL-15) may regulate the fatty infiltration in muscle injury. In this study, we investigated how IL-15 regulated FAPs in muscle injury and the effect on muscle regeneration.

Methods

Cell proliferation assay, western blots, qRT-PCR, immunohistochemistry, flow cytometric analysis were performed to investigate the effect of IL-15 on proliferation and adipogensis of FAPs. Acute muscle injury was induced by injection of glycerol or cardiotoxin to analyze how IL-15 effected on FAPs in vivo and its function on fatty infiltration or muscle regeneration.

Results

We identified that the expression of IL-15 in injured muscle was negatively associated with fatty infiltration. IL-15 can stimulate the proliferation of FAPs and prevent the adipogenesis of FAPs in vitro and in vivo. The growth of FAPs caused by IL-15 was mediated through JAK-STAT pathway. In addition, desert hedgehog pathway may participate in IL-15 inhibiting adipogenesis of FAPs. Our study showed IL-15 can cause the fibrosis after muscle damage and promote the myofiber regeneration. Finally, the expression of IL-15 was positively associated with severity of fibrosis and number of FAPs in patients with chronic rotator cuff tear.

Conclusions

These findings supported the potential role of IL-15 as a modulator on fate of FAPs in injured muscle and as a novel therapy for chronic muscle injury.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0251-0) contains supplementary material, which is available to authorized users.

Ciliary parathyroid hormone signaling activates transforming growth factor-β to maintain intervertebral disc homeostasis during aging

Degenerative disc disease (DDD) is associated with intervertebral disc degeneration of spinal instability. Here, we report that the cilia of nucleus pulposus (NP) cells mediate mechanotransduction to maintain anabolic activity in the discs. We found that mechanical stress promotes transport of parathyroid hormone 1 receptor (PTH1R) to the cilia and enhances parathyroid hormone (PTH) signaling in NP cells. PTH induces transcription of integrin α_vβ₆ to activate the transforming growth factor (TGF)-β-connective tissue growth factor (CCN2)-matrix proteins signaling cascade. Intermittent injection of PTH (iPTH) effectively attenuates disc degeneration of aged mice by direct signaling through NP cells, specifically improving intervertebral disc height and volume by increasing levels of TGF-β activity, CCN2, and aggrecan. PTH1R is expressed in both mouse and human NP cells. Importantly, knockout PTH1R or cilia in the NP cells results in significant disc degeneration and blunts the effect of PTH on attenuation of aged discs. Thus, mechanical stress-induced transport of PTH1R to the cilia enhances PTH signaling, which helps maintain intervertebral disc homeostasis, particularly during aging, indicating therapeutic potential of iPTH for DDD.

Sensory structures found in the jelly-like space between spinal discs release a hormone that helps preserve back health in aging mice. Xu Cao from Johns Hopkins University in Baltimore, Maryland, USA, and colleagues observed that levels of a critical growth factor declined in the space between adjacent vertebrae as mice aged, and that injecting a naturally occurring hormone that activates this growth factor could attenuate disc degeneration in older animals. The researchers showed, in response to mechanical stresses, receptor proteins that respond to this hormone relocate themselves to particular sensory organelles known as cilia that found within cells of the intervertebral core. That results in elevated hormone signaling—and drugs designed to have the same effect could help treat degenerative disc disease, one of the most common causes of chronic back pain.

Emerging nonmetabolic functions of skin fat

Although the major white adipose depots evolved primarily to store energy, secrete hormones and thermo-insulate the body, multiple secondary depots developed additional specialized and unconventional functions. Unlike any other fat tissue, dermal white adipose tissue (dWAT) evolved a large repertoire of novel features that are central to skin physiology, which we discuss in this Review. dWAT exists in close proximity to hair follicles, the principal appendages of the skin that periodically grow new hairs. Responding to multiple hair-derived signals, dWAT becomes closely connected to cycling hair follicles and periodically cycles itself. At the onset of new hair growth, hair follicles secrete activators of adipogenesis, while at the end of hair growth, a reduction in the secretion of activators or potentially, an increase in the secretion of inhibitors of adipogenesis, results in fat lipolysis. Hair-driven cycles of dWAT remodelling are uncoupled from size changes in other adipose depots that are controlled instead by systemic metabolic demands. Rich in growth factors, dWAT reciprocally signals to hair follicles, altering the activation state of their stem cells and modulating the pace of hair regrowth. dWAT cells also facilitate skin repair following injury and infection. In response to wounding, adipose progenitors secrete repair-inducing activators, while bacteria-sensing adipocytes produce antimicrobial peptides, thus aiding innate immune responses in the skin.

The primary cilium is necessary for the differentiation and the maintenance of human adipose progenitors into myofibroblasts

The primary cilium is an organelle, present at the cell surface, with various biological functions. We, and others, have shown that it plays a role in the differentiation of adipose progenitors (APs) into adipocytes. APs can also differentiate into myofibroblasts when treated with TGF-β1. Several components of the TGF-β1 pathway are located within the cilium suggesting a function for this organelle in AP myofibrogenesis. We studied differentiation of APs into myofibroblasts in two human models: APs of the adipose tissue (aAPs) and APs resident in the skeletal muscles (mAPs). We showed that, in vivo, myofibroblasts within muscles of patients with Duchenne Muscular Dystrophy were ciliated. In vitro, myofibroblasts derived from APs maintained a functional primary cilium. Using HPI4, a small molecule that inhibits ciliogenesis, and siRNA against Kif-3A, we provide evidence that the primary cilium is necessary both for the differentiation of APs into myofibroblasts and the maintenance of the phenotype. Disruption of the primary cilium inhibited TGF-β1-signalisation providing a molecular mechanism by which the cilium controls myofibroblast differentiation. These data suggest that myofibroblasts from various origins are controlled differently by their primary cilium.

Cilia Control Fat Deposition during Tissue Repair

Fibro/adipogenic progenitors (FAPs) are emerging as crucial regulators of fibrous and fat deposits during skeletal muscle regeneration. In a recent issue of Cell, Kopinke et al. (2017) report that primary cilia induce the adipogenic fate of FAPs in injured and diseased muscle by restraining Hedgehog signaling.

Contributions of Noncanonical Smoothened Signaling During Embryonic Development

The Sonic Hedgehog (Shh) signaling pathway is active during embryonic development in metazoans, and provides instructional cues necessary for proper tissue patterning. The pathway signal transducing component, Smoothened (Smo), is a G protein-coupled receptor (GPCR) that has been demonstrated to signal through at least two effector routes. The first is a G protein–independent canonical route that signals to Gli transcriptional effectors to establish transcriptional programs specifying cell fate during early embryonic development. The second, commonly referred to as the noncanonical Smo signal, induces rapid, transcription-independent responses that are essential for establishing and maintaining distinct cell behaviors during development. Herein, we discuss contributions of this noncanonical route during embryonic development. We also highlight important open questions regarding noncanonical Smo signal route selection during development, and consider implications of noncanonical signal corruption in disease.