Postnatal Maintenance Research Articles

Interplay of the Norrin and Wnt7a/Wnt7b signaling systems in blood–brain barrier and blood–retina barrier development and maintenance

β-Catenin signaling controls the development and maintenance of the blood-brain barrier (BBB) and the blood-retina barrier (BRB), but the division of labor and degree of redundancy between the two principal ligand-receptor systems-the Norrin and Wnt7a/Wnt7b systems-are incompletely defined. Here, we present a loss-of-function genetic analysis of postnatal BBB and BRB maintenance in mice that shows striking threshold and partial redundancy effects. In particular, the combined loss of Wnt7a and Norrin or Wnt7a and Frizzled4 (Fz4) leads to anatomically localized BBB defects that are far more severe than observed with loss of Wnt7a, Norrin, or Fz4 alone. In the cerebellum, selective loss of Wnt7a in glia combined with ubiquitous loss of Norrin recapitulates the phenotype observed with ubiquitous loss of both Wnt7a and Norrin, implying that glia are the source of Wnt7a in the cerebellum. Tspan12, a coactivator of Norrin signaling in the retina, is also active in BBB maintenance but is less potent than Norrin, consistent with a model in which Tspan12 enhances the amplitude of the Norrin signal in vascular endothelial cells. Finally, in the context of a partially impaired Norrin system, the retina reveals a small contribution to BRB development from the Wnt7a/Wnt7b system. Taken together, these experiments define the extent of CNS region-specific cooperation for several components of the Norrin and Wnt7a/Wnt7b systems, and they reveal substantial regional heterogeneity in the extent to which partially redundant ligands, receptors, and coactivators maintain the BBB and BRB.

Role of Sonic Hedgehog Signaling Pathway in Intervertebral Disc Formation and Maintenance.

The intervertebral discs (IVD) are an essential component of the spine. Degeneration of the discs, commonly due to age or injury, is a leading cause of chronic lower back pain. Despite its high prevalence, there is no effective treatment for disc disease due to limited understanding of disc at the cellular and molecular level. Recent research has demonstrated the importance of the intracellular developmental pathway sonic hedgehog (Shh) during the formation and postnatal maintenance of the IVD. Recent studies corroborate that the down-regulation of SHH expression is associated with pathological changes in the IVDs and demonstrate the reactivation of the hedgehog pathway as a promising avenue for rescuing health disc structure and function. Understanding the role of developmental signaling pathways that regulate disc formation and maintenance may help develop strategies to recapitulate the same mechanism for disc treatment and hence improve the quality and longevity of patient lives.

MANF Is Required for the Postnatal Expansion and Maintenance of Pancreatic β-Cell Mass in Mice.

Global lack of mesencephalic astrocyte-derived neurotropic factor (MANF) leads to progressive postnatal loss of β-cell mass and insulin-dependent diabetes in mice. Similar to Manf-/- mice, embryonic ablation of MANF specifically from the pancreas results in diabetes. In this study, we assessed the importance of MANF for the postnatal expansion of pancreatic β-cell mass and for adult β-cell maintenance in mice. Detailed analysis of Pdx-1Cre+/- ::Manffl/fl mice revealed mosaic MANF expression in postnatal pancreata and a significant correlation between the number of MANF-positive β-cells and β-cell mass in individual mice. In vitro, recombinant MANF induced β-cell proliferation in islets from aged mice and protected from hyperglycemia-induced endoplasmic reticulum (ER) stress. Consequently, excision of MANF from β-cells of adult MIP-1CreERT::Manffl/fl mice resulted in reduced β-cell mass and diabetes caused largely by β-cell ER stress and apoptosis, possibly accompanied by β-cell dedifferentiation and reduced rates of β-cell proliferation. Thus, MANF expression in adult mouse β-cells is needed for their maintenance in vivo. We also revealed a mechanistic link between ER stress and inflammatory signaling pathways leading to β-cell death in the absence of MANF. Hence, MANF might be a potential target for regenerative therapy in diabetes.

The Actin Nucleator Cobl Is Critical for Centriolar Positioning, Postnatal Planar Cell Polarity Refinement, and Function of the Cochlea

Proper cochlear hair cell array development and sensory apparatus positioning are achieved by planar cell polarity signaling. Effectors executing proper tissue development and maturation programs are largely unknown. We show that the actin nucleator Cobl is an important effector in postnatal refinement and maintenance of planar cell polarity. During the critical time of hearing onset, these polarity defects coincided with reduced F-actin beneath the sensory apparatus and with premature kinocilium retraction. These defects were accompanied by organizational defects of the pericentriolar scaffold that coincided with basal body and centriolar mispositionings. Importantly, the pericentriolar defects observed in Cobl KO mice were demonstrated to be actin polymerization dependent and calcium/calmodulin signaling dependent. Because Cobl KO phenotypes manifested postnatally, planar cell polarity is not solely an important developmental process. The Cobl-dependent planar cell polarity maintenance and refinement processes we describe here seem critical for hearing, as Cobl KO mice show deficient cochlear amplification.

Tgfbr2 is required in Acan-expressing cells for maintenance of the intervertebral and sternocostal joints.

BackgroundMembers of the transforming growth factor beta (TGF‐β) family are secreted proteins that regulate skeletal development. TGF‐β signaling is critical in embryonic development of the annulus fibrosus (AF) of the intervertebral disc (IVD). To address the question of the role of TGF‐β signaling in postnatal development and maintenance of the skeleton, we generated mice in which Tgfbr2 was deleted at 2‐weeks of age in Aggrecan (Acan)‐expressing cells using inducible Cre/LoxP recombination.MethodsLocalization of Cre recombination was visualized by crossing Acantm1(cre/ERT2)Crm mice to fluorescent mTmG reporter mice. Acantm1(cre/ERT2)Crm mice were mated to Tgfbr2 LoxP/LoxP mice and Cre recombinase was activated by tamoxifen injection at 2‐weeks postnatally. Following tamoxifen injection, mice were aged to 3, 6, and 12‐months and control mice were compared to the experimental (cKO) group. Mice were initially analyzed using X‐ray and skeletal preparations. Sternocostal joints and IVD tissues were further analyzed histologically by hematoxylin and eosin (H&E), Safranin O, and Picrosirius Red staining as well as Col10 immunostaining.ResultsCre recombination was observed in the IVD and sternocostal joints. X‐ray analysis revealed osteophyte formation within the disc space of 12‐month‐old cKO mice. Skeletal preparations confirmed calcification within the IVD and the sternocostal joints in cKO mice. H&E staining of cKO IVD revealed disorganized growth plates, delay in the formation of the bony endplate, and Col10 staining in the AF indicative of ectopic endochondral bone formation. Furthermore, proteoglycan loss was observed and collagen bundles within the inner AF were thinner and less organized. Alterations in the IVD were apparent beginning at 3 months and were progressively more visible at 6 and 12 months. Similarly, histological analysis of cKO sternocostal joints revealed joint calcification, proteoglycan loss, and disorganization of the collagen architecture at 12 months of age.ConclusionsTGF‐β signaling is important for postnatal development and maintenance of fibrocartilaginous IVD and sternocostal joints.

The actin-organizing formin protein Fhod3 is required for postnatal development and functional maintenance of the adult heart in mice

Cardiac development and function require actin-myosin interactions in the sarcomere, a highly organized contractile structure. Sarcomere assembly mediated by formin homology 2 domain-containing 3 (Fhod3), a member of formins that directs formation of straight actin filaments, is essential for embryonic cardiogenesis. However, the role of Fhod3 in the neonatal and adult stages has remained unknown. Here, we generated floxed Fhod3 mice to bypass the embryonic lethality of an Fhod3 knockout (KO). Perinatal KO of Fhod3 in the heart caused juvenile lethality at around day 10 after birth with enlarged hearts composed of severely impaired myofibrils, indicating that Fhod3 is crucial for postnatal heart development. Tamoxifen-induced conditional KO of Fhod3 in the adult heart neither led to lethal effects nor did it affect sarcomere structure and localization of sarcomere components. However, adult Fhod3-deleted mice exhibited a slight cardiomegaly and mild impairment of cardiac function, conditions that were sustained over 1 year without compensation during aging. In addition to these age-related changes, systemic stimulation with the α1-adrenergic receptor agonist phenylephrine, which induces sustained hypertension and hypertrophy development, induced expression of fetal cardiac genes that was more pronounced in adult Fhod3-deleted mice than in the control mice, suggesting that Fhod3 modulates hypertrophic changes in the adult heart. We conclude that Fhod3 plays a crucial role in both postnatal cardiac development and functional maintenance of the adult heart.

Interplay between Follistatin, Activin A and Bmp4 Signaling Regulates Postnatal Thymic Epithelial Progenitor Cell Differentiation During Aging

One of the key features of immune functional impairment with age is the progressive involution of the thymic tissue responsible for naive T cell production. In this study, we identify two major phases of thymic epithelial cell (TEC) loss during aging; a block in mature TEC differentiation from the pool of immature precursors, occurring at the onset of puberty, followed by impaired bipotent TEC progenitor differentiation, with depletion of Sca-1lo cTEC and mTEC lineage-specific precursors. We reveal the increased production of follistatin by aging TECs contributes to their own demise. This TEC loss occurs primarily through antagonism of activin A signaling, which we show is required for TEC maturation and acts in dissonance to Bmp4. Together, these results indicate that an imbalance of activin A and Bmp4 signaling underpins degeneration of postnatal TEC maintenance during aging, and its reversal enables transient replenishment of mature TECs.

Virgin Beta Cells Persist throughout Life at a Neogenic Niche within Pancreatic Islets

Postnatal maintenance or regeneration of pancreatic beta cells is considered to occur exclusively via the replication of existing beta cells, but clinically meaningful restoration of human beta cell mass by proliferation has never been achieved. We discovered a population of immature beta cells that is present throughout life and forms from non-beta precursors at a specialized micro-environment or "neogenic niche" at the islet periphery. These cells express insulin, but lack other key beta cell markers, and are transcriptionally immature, incapable of sensing glucose, and unable to support calcium influx. They constitute an intermediate stage in the transdifferentiation of alpha cells to cells that are functionally indistinguishable from conventional beta cells. We thus identified a lifelong source of new beta cells at a specialized site within healthy islets. By comparing co-existing immature and mature beta cells within healthy islets, we stand to learn how to mature insulin-expressing cells into functional beta cells.

Type A monoamine oxidase and serotonin are coordinately involved in depressive disorders: from neurotransmitter imbalance to impaired neurogenesis.

Type A monoamine oxidase (MAOA) catabolizes monoamine transmitters, serotonin, norepinephrine and dopamine, and plays a major role in the onset, progression and therapy of neuropsychiatric disorders. In depressive disorders, increase in MAOA expression and decrease in brain levels of serotonin and norepinephrine are proposed as the major pathogenic factors. The functional polymorphism of MAOA gene and genes in serotonin signal pathway are associated with depression. This review presents recent advance in studies on the role of MAOA in major depressive disorder and related emotional disorders. MAOA and serotonin regulate the prenatal development and postnatal maintenance of brain architecture and neurocircuit, as shown by MAOA-deficient humans and MAO knockout animal models. Impaired neurogenesis in the mature hippocampus has been proposed as "adult neurogenesis" hypothesis of depression. MAOA modulates the sensitivity to stress in the stages of brain development and maturation, and the interaction of gene-environmental factors in the early stage regulates the onset of depressive behaviors in adulthood. Vice versa environmental factors affect MAOA expression by epigenetic regulation. MAO inhibitors not only restore compromised neurotransmitters, but also protect neurons from cell death in depression through induction of anti-apoptotic Bcl-2 and prosurvival neurotrophic factors, especially brain-derived neurotrophic factor, the deficiency of which is detected in depression. This review discusses novel role of MAOA and serotonin in the pathogenesis and therapy of depressive disorders.

Parent and Clinician Perspectives on Sustained Behavior Change after a Prenatal Obesity Program: A Qualitative Study.

Infants of obese women are at a high risk for development of obesity. Prenatal interventions targeting gestational weight gain among obese women have not demonstrated consistent benefits for infant growth trajectories. To better understand why such programs may not influence infant growth, qualitative semi-structured interviews were conducted with 19 mothers who participated in a prenatal nutrition intervention for women with BMI 30 kg/m2 or greater, and with 19 clinicians (13 pediatric, 6 obstetrical). Interviews were transcribed and coded with themes emerging inductively from the data, using a grounded theory approach. Mothers were interviewed a mean of 18 months postpartum and reported successful postnatal maintenance of behaviors that were relevant to the family food environment (Theme 1). Ambivalence around the importance of postnatal behavior maintenance (Theme 2) and enhanced postnatal healthcare (Theme 3) emerged as explanations for the failure of prenatal interventions to influence child growth. Mothers acknowledged their importance as role models for their children's behavior, but they often believed that body habitus was beyond their control. Though mothers attributed prenatal behavior change, in part, to additional support during pregnancy, clinicians had hesitations about providing children of obese parents with additional services postnatally. Both mothers and clinicians perceived a lack of interest or concern about infant growth during pediatric visits (Theme 4). Prenatal interventions may better influence childhood growth if paired with improved communication regarding long-term modifiable risks for children. The healthcare community should clarify a package of enhanced preventive services for children with increased risk of developing obesity.

A Cascade of Wnt, Eda, and Shh Signaling Is Essential for Touch Dome Merkel Cell Development.

The Sonic hedgehog (Shh) signaling pathway regulates developmental, homeostatic, and repair processes throughout the body. In the skin, touch domes develop in tandem with primary hair follicles and contain sensory Merkel cells. The developmental signaling requirements for touch dome specification are largely unknown. We found dermal Wnt signaling and subsequent epidermal Eda/Edar signaling promoted Merkel cell morphogenesis by inducing Shh expression in early follicles. Lineage-specific gene deletions revealed intraepithelial Shh signaling was necessary for Merkel cell specification. Additionally, a Shh signaling agonist was sufficient to rescue Merkel cell differentiation in Edar-deficient skin. Moreover, Merkel cells formed in Fgf20 mutant skin where primary hair formation was defective but Shh production was preserved. Although developmentally associated with hair follicles, fate mapping demonstrated Merkel cells primarily originated outside the hair follicle lineage. These findings suggest that touch dome development requires Wnt-dependent mesenchymal signals to establish reciprocal signaling within the developing ectoderm, including Eda signaling to primary hair placodes and ultimately Shh signaling from primary follicles to extrafollicular Merkel cell progenitors. Shh signaling often demonstrates pleiotropic effects within a structure over time. In postnatal skin, Shh is known to regulate the self-renewal, but not the differentiation, of touch dome stem cells. Our findings relate the varied effects of Shh in the touch dome to the ligand source, with locally produced Shh acting as a morphogen essential for lineage specification during development and neural Shh regulating postnatal touch dome stem cell maintenance.

Severe muscle wasting and denervation in mice lacking the RNA-binding protein ZFP106

Innervation of skeletal muscle by motor neurons occurs through the neuromuscular junction, a cholinergic synapse essential for normal muscle growth and function. Defects in nerve-muscle signaling cause a variety of neuromuscular disorders with features of ataxia, paralysis, skeletal muscle wasting, and degeneration. Here we show that the nuclear zinc finger protein ZFP106 is highly enriched in skeletal muscle and is required for postnatal maintenance of myofiber innervation by motor neurons. Genetic disruption of Zfp106 in mice results in progressive ataxia and hindlimb paralysis associated with motor neuron degeneration, severe muscle wasting, and premature death by 6 mo of age. We show that ZFP106 is an RNA-binding protein that associates with the core splicing factor RNA binding motif protein 39 (RBM39) and localizes to nuclear speckles adjacent to spliceosomes. Upon inhibition of pre-mRNA synthesis, ZFP106 translocates with other splicing factors to the nucleolus. Muscle and spinal cord of Zfp106 knockout mice displayed a gene expression signature of neuromuscular degeneration. Strikingly, altered splicing of the Nogo (Rtn4) gene locus in skeletal muscle of Zfp106 knockout mice resulted in ectopic expression of NOGO-A, the neurite outgrowth factor that inhibits nerve regeneration and destabilizes neuromuscular junctions. These findings reveal a central role for Zfp106 in the maintenance of nerve-muscle signaling, and highlight the involvement of aberrant RNA processing in neuromuscular disease pathogenesis.

Ndel1 and Reelin Maintain Postnatal CA1 Hippocampus Integrity.

How the integrity of laminar structures in the postnatal brain is maintained impacts neuronal functions. Ndel1, the mammalian homolog of NuDE from the filamentous fungus Aspergillus nidulans, is an atypical microtubule (MT)-associated protein that was initially investigated in the contexts of neurogenesis and neuronal migration. Constitutive knock-out mice for Ndel1 are embryonic lethal, thereby necessitating the creation a conditional knock-out to probe the roles of Ndel1 in postnatal brains. Here we report that CA1 pyramidal neurons from mice postnatally lacking Ndel1 (Ndel1 conditional knock-out) exhibit fragmented MTs, dendritic/synaptic pathologies, are intrinsically hyperexcitable and undergo dispersion independently of neuronal migration defect. Secondary to the pyramidal cell changes is the decreased inhibitory drive onto pyramidal cells from interneurons. Levels of the glycoprotein Reelin that regulates MTs, neuronal plasticity, and cell compaction are significantly reduced in hippocampus of mutant mice. Strikingly, a single injection of Reelin into the hippocampus of Ndel1 conditional knock-out mice ameliorates ultrastructural, cellular, morphological, and anatomical CA1 defects. Thus, Ndel1 and Reelin contribute to maintain postnatal CA1 integrity. The significance of this study rests in the elucidation of a role for Nde1l and Reelin in postnatal CA1 integrity using a new conditional knock-out mouse model for the cytoskeletal protein Ndel1, one that circumvents the defects associated with neuronal migration and embryonic lethality. Our study serves as a basis for understanding the mechanisms underlying postnatal hippocampal maintenance and function, and the significance of decreased levels of Ndel1 and Reelin observed in patients with neurological disorders.

AGPAT2 is essential for postnatal development and maintenance of white and brown adipose tissue.

ObjectiveCharacterize the cellular and molecular events responsible for lipodystrophy in AGPAT2 deficient mice.MethodsAdipose tissue and differentiated MEF were assessed using light and electron microscopy, followed by protein (immunoblots) and mRNA analysis (qPCR). Phospholipid profiling was determined by electrospray ionization tandem mass spectrometry (ESI-MS/MS).ResultsIn contrast to adult Agpat2−/− mice, fetuses and newborn Agpat2−/− mice have normal mass of white and brown adipose tissue. Loss of both the adipose tissue depots occurs during the first week of postnatal life as a consequence of adipocyte death and inflammatory infiltration of the adipose tissue. At the ultrastructural level, adipose tissue of newborn Agpat2−/− mice is virtually devoid of caveolae and has abnormal mitochondria and lipid droplets. Autophagic structures are also abundant. Consistent with these findings, differentiated Agpat2−/− mouse embryonic fibroblasts (MEFs) also have impaired adipogenesis, characterized by a lower number of lipid-laden cells and ultrastructural abnormalities in lipid droplets, mitochondria and plasma membrane. Overexpression of PPARγ, the master regulator of adipogenesis, increased the number of Agpat2−/− MEFs that differentiated into adipocyte-like cells but did not prevent morphological abnormalities and cell death. Furthermore, differentiated Agpat2−/− MEFs have abnormal phospholipid compositions with 3-fold increased levels of phosphatidic acid.ConclusionWe conclude that lipodystrophy in Agpat2−/− mice results from postnatal cell death of adipose tissue in association with acute local inflammation. It is possible that AGPAT2 deficient adipocytes have an altered lipid filling or a reduced capacity to adapt the massive lipid availability associated with postnatal feeding.

Abstract 329: The Role of TIE1 in Flow Mediated Lymphatic Vessel Remodeling and Valvulogenesis

Recently, it has been shown that the mechanical stimulus of turbulent shear stress caused by onset of lymph flow is required for lymphatic remodeling, maturation, and lymphatic valve (LV) development. Homeostasis of the adult lymphatic vasculature also relies on flow-mediated signal transduction. However, the cellular machinery responsible for transducing mechanosensory signals required for lymphatic network formation and maintenance is unknown. Our laboratory has previously shown that TIE1 is at least partially responsible for mechanotransduction of turbulent flow required for initiation and maintenance of atherosclerotic plaque formation at the branch points of systemic vasculature in the adult animal. Moreover, TIE1 is expressed throughout lymphatic vasculature during mouse embryogenesis into adulthood, with enrichment in LVs. To circumvent the embryonic lethality caused by global Tie1 disruption, we conditionally deleted Tie1 using Nfatc1Cre. Nfatc1Cre drives recombination in lymphatic endothelial cells, with strong expression in the LVs. Nfatc1Cre:Tie1fl/fl mutants survive to birth but accumulate chyle in the peritoneal and pleural cavities by postnatal day 2. The lymphatic vessels in the mutants are dilated and tortuous, and do not undergo normal hierarchical remodeling. The constrictions that normally indicate intraluminal valve development are lacking in the mutant lymphatic vessels. Underlying these defects in the Nfatc1Cre:Tie1fl/fl mutants is loss of the normal molecular landscape associated with lymphatic patterning and valvulogenesis. Therefore, we hypothesize that Tie1 orchestrates the mechanotransduction necessary for intraluminal LV development and postnatal maintenance.

Semaphorin 3A promotes activation of Pax7, Myf5, and MyoD through inhibition of emerin expression in activated satellite cells.

We previously showed that Semaphorin 3A (Sema3A) expression was induced when quiescent muscle satellite cells were stimulated by hepatocyte growth factor and became activated satellite cells (ASCs). However, how Sema3A regulates genes in the early phase of ASCs remains unclear. In this study, we investigated whether Sema3A signaling can regulate the early phase of ASCs, an important satellite cell stage for postnatal growth, repair, and maintenance of skeletal muscle. We showed that expression of the myogenic proliferation regulatory factors Pax7 and Myf5 was decreased in myoblasts transfected with Sema3A siRNA. These cells failed to activate expression MyoD, another myogenic proliferation regulatory factor, during differentiation. Interestingly, some of the Sema3A‐depleted cells did not express Pax7 and MyoD and had enlarged nuclei and very large cytoplasmic areas. We also observed that Pax7 and Myf5 expression was increased in Myc‐Sema3A overexpressing myoblasts. BrdU analysis indicated that Sema3A regulated proliferation of ASCs. These findings suggest that Sema3A signaling can modulate expression of Pax7, Myf5, and MyoD. Moreover, we found that expression of emerin, an inner nuclear membrane protein, was regulated by Sema3A signaling. Emerin was identified by positional cloning as the gene responsible for the X‐linked form of Emery–Dreifuss muscular dystrophy (X‐EDMD). In conclusion, our results support a role for Sema3A in maintaining ASCs through regulation, via emerin, of Pax7, Myf5, and MyoD expression.

Postnatal knockdown of dok-7 gene expression in mice causes structural defects in neuromuscular synapses and myasthenic pathology.

The neuromuscular junction (NMJ) is a synapse between a motor neuron and skeletal muscle and is required for muscle contraction. The formation and maintenance of NMJs are governed by the muscle-specific receptor tyrosine kinase MuSK. We previously showed that the muscle cytoplasmic protein Dok-7 is an essential activator of MuSK. Indeed, mice lacking either Dok-7 or MuSK form no NMJs, and defects in the human DOK7 gene underlie a congenital myasthenic syndrome (an NMJ disorder). However, it remains unproven whether Dok-7 is required for the postnatal maintenance of NMJs. In this study, we generated recombinant adeno-associated virus (AAV) vectors encoding short hairpin RNAs targeting the mouse dok-7 gene (AAV-shD7). Systemic administration of AAV-shD7 into 2-week-old mice down-regulated dok-7 expression in muscle and induced myasthenic symptoms including reduction in body weight and motor function. Moreover, AAV-shD7 treatment suppressed MuSK-dependent gene expression of NMJ components and reduced the size of NMJs. These results demonstrate that correct, physiological levels of dok-7 expression are required for the postnatal maintenance of NMJs.

Medullary thymic epithelial stem cells: role in thymic epithelial cell maintenance and thymic involution.

The thymus consists of two distinct anatomical regions, the cortex and the medulla; medullary thymic epithelial cells (mTECs) play a crucial role in establishing central T-cell tolerance for self-antigens. Although the understanding of mTEC development in thymic organogenesis as well as the regulation of their differentiation and maturation has improved, the mechanisms of postnatal maintenance remain poorly understood. This issue has a central importance in immune homeostasis and physiological thymic involution as well as autoimmune disorders in various clinicopathological settings. Recently, several reports have demonstrated the existence of TEC stem or progenitor cells in the postnatal thymus, which are either bipotent or unipotent. We identified stem cells specified for mTEC-lineage that are generated in the thymic ontogeny and may sustain mTEC regeneration and lifelong central T-cell self-tolerance. This finding suggested that the thymic medulla is maintained autonomously by its own stem cells. Although several issues, including the relationship with other putative TEC stem/progenitors, remain unclear, further examination of mTEC stem cells (mTECSCs) and their regulatory mechanisms may contribute to the understanding of postnatal immune homeostasis. Possible relationships between decline of mTECSC activity and early thymic involution as well as various autoimmune disorders are discussed.

Growth/differentiation factor-15 deficiency compromises dopaminergic neuron survival and microglial response in the 6-hydroxydopamine mouse model of Parkinson's disease

Growth/differentiation factor-15 (Gdf-15) is a member of the TGF-β superfamily and a pleiotropic, widely distributed cytokine, which has been shown to play roles in various pathologies, including inflammation. Analysis of Gdf-15−/− mice has revealed that it serves the postnatal maintenance of spinal cord motor neurons and sensory neurons. In a previous study, exogenous Gdf-15 rescued 6-hydroxydopamine (6-OHDA) lesioned Gdf-15+/+ nigrostriatal dopaminergic (DAergic) neurons in vitro and in vivo. Whether endogenous Gdf-15 serves the physiological maintenance of nigrostriatal DAergic neurons in health and disease is not known and was addressed in the present study. Stereotactic injection of 6-OHDA into the medial forebrain bundle (MFB) led to a significant decline in the numbers of DAergic neurons in both Gdf-15+/+ and Gdf-15−/− mice over a time-period of 14days. However, this decrease was exacerbated in the Gdf-15−/− mice, with only 5.5% surviving neurons as compared to 24% in the Gdf-15+/+ mice. Furthermore, the microglial response to the 6-OHDA lesion was reduced in Gdf-15−/− mice, with significantly lower numbers of total and activated microglia and a differential cytokine expression as compared to the Gdf-15+/+ mice. Using in vitro models, we could demonstrate the importance of endogenous Gdf-15 in promoting DAergic neuron survival thus highlighting its relevance in a direct neurotrophic supportive role. Taken together, these results indicate the importance of Gdf-15 in promoting survival of DAergic neurons and regulating the inflammatory response post 6-OHDA lesion.

Functional Overload Enhances Satellite Cell Properties in Skeletal Muscle.

Skeletal muscle represents a plentiful and accessible source of adult stem cells. Skeletal-muscle-derived stem cells, termed satellite cells, play essential roles in postnatal growth, maintenance, repair, and regeneration of skeletal muscle. Although it is well known that the number of satellite cells increases following physical exercise, functional alterations in satellite cells such as proliferative capacity and differentiation efficiency following exercise and their molecular mechanisms remain unclear. Here, we found that functional overload, which is widely used to model resistance exercise, causes skeletal muscle hypertrophy and converts satellite cells from quiescent state to activated state. Our analysis showed that functional overload induces the expression of MyoD in satellite cells and enhances the proliferative capacity and differentiation potential of these cells. The changes in satellite cell properties coincided with the inactivation of Notch signaling and the activation of Wnt signaling and likely involve modulation by transcription factors of the Sox family. These results indicate the effects of resistance exercise on the regulation of satellite cells and provide insight into the molecular mechanism of satellite cell activation following physical exercise.