Growth hormone deficiency in 'little' mice results in aberrant body composition, reduced insulin-like growth factor-I and insulin-like growth factor-binding protein-3 (IGFBP-3), but does not affect IGFBP-2, -1 or -4

Characterization and Regulation of the Neonatal Growth Hormone Surge

High neonatal growth hormone (GH) secretion has been described in several species. However, the neuroendocrine mechanisms behind this surge remain unknown. Thus, the pattern of postnatal GH secretion was investigated in mice and rats. Blood GH levels were very high on postnatal day (P)1 and progressively decreased until near zero by P17 in C57BL/6 mice without sex differences. This pattern was similar to that observed in rats, except that female rats showed higher GH levels on P1 than males. In comparison, follicle-stimulating hormone exhibited higher secretion in females during the first 3 weeks of life. Hypothalamic Sst mRNA and somatostatin neuroendocrine terminals in the median eminence were higher in P20/P21 mice than in newborns. Knockout mice for GH-releasing hormone (GHRH) receptor showed no GH surge, whereas knockdown mice for the Sst gene displayed increased neonatal GH peak. Leptin deficiency caused only minor effects on early-life GH secretion. GH receptor ablation in neurons or the entire body did not affect neonatal GH secretion, but the subsequent reduction in blood GH levels was attenuated or prevented by these genetic manipulations, respectively. This phenotype was also observed in knockout mice for the insulin-like growth factor-1 (IGF-1) receptor in GHRH neurons. Moreover, glucose-induced hyperglycemia overstimulated GH secretion in neonatal mice. In conclusion, GH surge in the first days of life is not regulated by negative feedback loops. However, neonatal GH secretion requires GHRH receptor, and is modulated by somatostatin and blood glucose levels, suggesting that this surge is controlled by hypothalamic-pituitary communication.

Paracrine FGF1 signaling directs pituitary architecture and size

Organ architecture is established during development through intricate cell-cell communication mechanisms, yet the specific signals mediating these communications often remain elusive. Here, we used the anterior pituitary gland that harbors different interdigitated hormone-secreting homotypic cell networks to dissect cell-cell communication mechanisms operating during late development. We show that blocking differentiation of corticotrope cells leads to pituitary hypoplasia with a major effect on somatotrope cells that directly contact corticotropes. Gene knockout of the corticotrope-restricted transcription factor Tpit results in fewer somatotropes, with less secretory granules and a loss of cell polarity, resulting in systemic growth retardation. Single-cell transcriptomic analyses identified FGF1 as a corticotrope-specific Tpit dosage-dependent target gene responsible for these phenotypes. Consistently, genetic ablation of FGF1 in mice phenocopies pituitary hypoplasia and growth impairment observed in Tpit-deficient mice. These findings reveal FGF1 produced by the corticotrope cell network as an essential paracrine signaling molecule participating in pituitary architecture and size.

Common and uncommon mouse models of growth hormone deficiency

Mouse models of growth hormone deficiency (GHD) have provided important tools for uncovering the various actions of GH. Nearly 100 years of research using these mouse lines has greatly enhanced our knowledge of the GH/IGF-1 axis. Some of the shared phenotypes of the five "common" mouse models of GHD include reduced body size, delayed sexual maturation, decreased fertility, reduced muscle mass, increased adiposity, and enhanced insulin sensitivity. Since these common mouse lines outlive their normal-sized littermates - and have protection from age-associated disease - they have become important fixtures in the aging field. On the other hand, the twelve "uncommon" mouse models of GHD described herein have tremendously divergent health outcomes ranging from beneficial aging phenotypes (similar to those described for the common models) to extremely detrimental features (such as improper development of the CNS, numerous sensory organ defects, and embryonic lethality). Moreover, advancements in next generation sequencing technologies have led to the identification of an expanding array of genes that are recognized as causative agents to numerous rare syndromes with concomitant GHD. Accordingly, this review provides researchers with a comprehensive up-to-date collection of the common and uncommon mouse models of GHD that have been used to study various aspects of physiology and metabolism associated with multiple forms of GHD. For each mouse line presented, the closest comparable human syndromes are discussed providing important parallels to the clinic.

Increased GH Secretion and Body Growth in Mice Carrying Ablation of IGF-1 Receptor in GH-releasing Hormone Cells

Growth hormone (GH) secretion is controlled by short and long negative feedback loops. In this regard, both GH (short-loop feedback) and insulin-like growth factor 1 (IGF-1; long-loop feedback) can target somatotropic cells of the pituitary gland and neuroendocrine hypothalamic neurons to regulate the GH/IGF-1 axis. GH-releasing hormone (GHRH)-expressing neurons play a fundamental role in stimulating pituitary GH secretion. However, it is currently unknown whether IGF-1 action on GHRH-expressing cells is required for the control of the GH/IGF-1/growth axis. In the present study, we investigated the phenotype of male and female mice carrying ablation of IGF-1 receptor (IGF1R) exclusively in GHRH cells. After weaning, both male and female GHRHΔIGF1R mice exhibited increases in body weight, lean body mass, linear growth, and length of long bones (tibia, femur, humerus, and radius). In contrast, the percentage of body fat was similar between control and GHRHΔIGF1R mice. The higher body growth of GHRHΔIGF1R mice can be explained by increases in mean GH levels, GH pulse amplitude, and pulse frequency, calculated from 36 blood samples collected from each animal at 10-minute intervals. GHRHΔIGF1R mice also showed increased hypothalamic Ghrh mRNA levels, pituitary Gh mRNA expression, hepatic Igf1 expression, and serum IGF-1 levels compared with control animals. Furthermore, GHRHΔIGF1R mice displayed significant alterations in the sexually dimorphic hepatic gene expression profile, with a prevailing feminization in most genes analyzed. In conclusion, our findings indicate that GHRH neurons represent a key and necessary site for the long-loop negative feedback that controls the GH/IGF-1 axis and body growth.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

Growth hormone/IGF-I-dependent signaling restores decreased expression of the myokine SPARC in aged skeletal muscle

Skeletal muscle exerts many beneficial effects on the human body including the contraction-dependent secretion of peptides termed myokines. We have recently connected the myokine secreted protein acidic and rich in cysteine (SPARC) to the formation of intramuscular adipose tissue (IMAT) in skeletal muscle from aged mice and humans. Here, we searched for inducers of SPARC in order to uncover novel treatment approaches for IMAT. Endurance exercise in mice as well as forskolin treatment in vitro only modestly activated SPARC levels. However, through pharmacological treatments in vitro, we identified IGF-I as a potent inducer of SPARC expression in muscle cells, likely through a direct activation of its promoter via phosphatidylinositol 4,5-bisphospate 3-kinase (PI3K)-dependent signaling. We employed two different mouse models of growth hormone (GH)/IGF-I deficiency to solidify our understanding of the relationship between IGF-I and SPARC in vivo. GH administration robustly increased intramuscular SPARC levels (3.5-fold) in GH releasing hormone receptor-deficient mice and restored low intramuscular SPARC expression in skeletal muscle from aged mice. Intramuscular glycerol injections induced higher levels of adipocyte markers (adiponectin, perilipin) in aged compared to young mice, which was not prevented by GH treatment. Our study provides a roadmap for the study of myokine regulation during aging and demonstrates that the GH/IGF-I axis is critical for SPARC expression in skeletal muscle. Although GH treatment did not prevent IMAT formation in the glycerol model, targeting SPARC by exercise or by activation of IGF-I signaling might offer a novel therapeutic strategy against IMAT formation during aging. KEY MESSAGES : IGF-I regulates the myokine SPARC in muscle cells directly at the promoter level. GH/IGF-I is able to restore the decreased SPARC levels in aged skeletal muscle. The glycerol model induces higher adipocyte markers in aged compared to young muscle. GH treatment does not prevent IMAT formation in the glycerol model.

Late Health Effects of Partial Body Irradiation Injury in a Minipig Model Are Associated with Changes in Systemic and Cardiac IGF-1 Signaling

Clinical, epidemiological, and experimental evidence demonstrate non-cancer, cardiovascular, and endocrine effects of ionizing radiation exposure including growth hormone deficiency, obesity, metabolic syndrome, diabetes, and hyperinsulinemia. Insulin-like growth factor-1 (IGF-1) signaling perturbations are implicated in development of cardiovascular disease and metabolic syndrome. The minipig is an emerging model for studying radiation effects given its high analogy to human anatomy and physiology. Here we use a minipig model to study late health effects of radiation by exposing male Göttingen minipigs to 1.9–2.0 Gy X-rays (lower limb tibias spared). Animals were monitored for 120 days following irradiation and blood counts, body weight, heart rate, clinical chemistry parameters, and circulating biomarkers were assessed longitudinally. Collagen deposition, histolopathology, IGF-1 signaling, and mRNA sequencing were evaluated in tissues. Our findings indicate a single exposure induced histopathological changes, attenuated circulating IGF-1, and disrupted cardiac IGF-1 signaling. Electrolytes, lipid profiles, liver and kidney markers, and heart rate and rhythm were also affected. In the heart, collagen deposition was significantly increased and transforming growth factor beta-1 (TGF-beta-1) was induced following irradiation; collagen deposition and fibrosis were also observed in the kidney of irradiated animals. Our findings show Göttingen minipigs are a suitable large animal model to study long-term effects of radiation exposure and radiation-induced inhibition of IGF-1 signaling may play a role in development of late organ injuries.

Mouse models of growth hormone deficiency

Nearly one century of research using growth hormone deficient (GHD) mouse lines has contributed greatly toward our knowledge of growth hormone (GH), a pituitary-derived hormone that binds and signals through the GH receptor and affects many metabolic processes throughout life. Although delayed sexual maturation, decreased fertility, reduced muscle mass, increased adiposity, small body size, and glucose intolerance appear to be among the negative characteristics of these GHD mouse lines, these mice still consistently outlive their normal sized littermates. Furthermore, the absence of GH action in these mouse lines leads to enhanced insulin sensitivity (likely due to the lack of GH's diabetogenic actions), delayed onset for a number of age-associated physiological declines (including cognition, cancer, and neuromusculoskeletal frailty), reduced cellular senescence, and ultimately, extended lifespan. In this review, we provide details about history, availability, growth, physiology, and aging of five commonly used GHD mouse lines.

Central Regulation of Metabolism by Growth Hormone

Growth hormone (GH) is secreted by the pituitary gland, and in addition to its classical functions of regulating height, protein synthesis, tissue growth, and cell proliferation, GH exerts profound effects on metabolism. In this regard, GH stimulates lipolysis in white adipose tissue and antagonizes insulin's effects on glycemic control. During the last decade, a wide distribution of GH-responsive neurons were identified in numerous brain areas, especially in hypothalamic nuclei, that control metabolism. The specific role of GH action in different neuronal populations is now starting to be uncovered, and so far, it indicates that the brain is an important target of GH for the regulation of food intake, energy expenditure, and glycemia and neuroendocrine changes, particularly in response to different forms of metabolic stress such as glucoprivation, food restriction, and physical exercise. The objective of the present review is to summarize the current knowledge about the potential role of GH action in the brain for the regulation of different metabolic aspects. The findings gathered here allow us to suggest that GH represents a hormonal factor that conveys homeostatic information to the brain to produce metabolic adjustments in order to promote energy homeostasis.

Optimization of Mouse Growth Hormone Plasmid DNA Electrotransfer into Tibialis Cranialis Muscle of "Little" Mice

Previous non-viral gene therapy was directed towards two animal models of dwarfism: Immunodeficient (lit/scid) and immunocompetent (lit/lit) dwarf mice. The former, based on hGH DNA administration into muscle, performed better, while the latter, a homologous model based on mGH DNA, was less efficient, though recommended as useful for pre-clinical assays. We have now improved the growth parameters aiming at a complete recovery of the lit/lit phenotype. Electrotransfer was based on three pulses of 375 V/cm of 25 ms each, after mGH-DNA administration into two sites of each non-exposed tibialis cranialis muscle. A 36-day bioassay, performed using 60-day old lit/lit mice, provided the highest GH circulatory levels we have ever obtained for GH non-viral gene therapy: 14.7 ± 3.7 ng mGH/mL. These levels, at the end of the experiment, were 8.5 ± 2.3 ng/mL, i.e., significantly higher than those of the positive control (4.5 ± 1.5 ng/mL). The catch-up growth reached 40.9% for body weight, 38.2% for body length and 82.6%–76.9% for femur length. The catch-up in terms of the mIGF-1 levels remained low, increasing from the previous value of 5.9% to the actual 8.5%. Although a complete phenotypic recovery was not obtained, it should be possible starting with much younger animals and/or increasing the number of injection sites.

Tyrosine Hydroxylase Neurons Regulate Growth Hormone Secretion via Short-Loop Negative Feedback

Classical studies suggest that growth hormone (GH) secretion is controlled by negative-feedback loops mediated by GH-releasing hormone (GHRH)- or somatostatin-expressing neurons. Catecholamines are known to alter GH secretion and neurons expressing TH are located in several brain areas containing GH-responsive cells. However, whether TH-expressing neurons are required to regulate GH secretion via negative-feedback mechanisms is unknown. In the present study, we showed that between 50% and 90% of TH-expressing neurons in the periventricular, paraventricular, and arcuate hypothalamic nuclei and locus ceruleus of mice exhibited STAT5 phosphorylation (pSTAT5) after an acute GH injection. Ablation of GH receptor (GHR) from TH cells or in the entire brain markedly increased GH pulse secretion and body growth in both male and female mice. In contrast, GHR ablation in cells that express the dopamine transporter (DAT) or dopamine β-hydroxylase (DBH; marker of noradrenergic/adrenergic cells) did not affect body growth. Nevertheless, less than 50% of TH-expressing neurons in the hypothalamus were found to express DAT. Ablation of GHR in TH cells increased the hypothalamic expression of Ghrh mRNA, although very few GHRH neurons were found to coexpress TH- and GH-induced pSTAT5. In summary, TH neurons that do not express DAT or DBH are required for the autoregulation of GH secretion via a negative-feedback loop. Our findings revealed a critical and previously unidentified group of catecholaminergic interneurons that are apt to sense changes in GH levels and regulate the somatotropic axis in mice.SIGNIFICANCE STATEMENT Textbooks indicate until now that the pulsatile pattern of growth hormone (GH) secretion is primarily controlled by GH-releasing hormone and somatostatin neurons. The regulation of GH secretion relies on the ability of these cells to sense changes in circulating GH levels to adjust pituitary GH secretion within a narrow physiological range. However, our study identifies a specific population of tyrosine hydroxylase-expressing neurons that is critical to autoregulate GH secretion via a negative-feedback loop. The lack of this mechanism in transgenic mice results in aberrant GH secretion and body growth. Since GH plays a key role in cell proliferation, body growth, and metabolism, our findings provide a major advance to understand how the brain regulates the somatotropic axis.

Rhythmic growth hormone secretion in physiological and pathological conditions: Lessons from rodent studies

Evolutionally conserved in all mammalians, the release of GH occurs in a rhythmic pattern, characterized by several dominant surges (pulsatile GH) with tonic low inter-pulse levels (tonic GH). Such pulsatile secretion pattern is essential for many physiological actions of GH on different tissues with defined gender dimorphism. Rhythmic release of pulsatile GH is tightly controlled by hypothalamic neurons as well as peripheral metabolic factors. Changes of GH pattern occur within a range of sophisticated physiological and pathological settings and significantly contribute to growth, ageing, survival and disease predispositions. Precise analysis of GH secretion pattern is vitally important for a comprehensive understanding of the function of GH and the components that regulate GH secretion pattern.

Systemic growth hormone deficiency causes mechanical and thermal hypersensitivity during early postnatal development

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Systemic GHD causes behavioral hypersensitivity at P7 and P14, but not P21.

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Primary afferent sensitization is observed in GHRHr KOs.

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Knockout of GHRHr changes DRG gene expression that is observed throughout development.

Injury during early postnatal life causes acute alterations in afferent function and DRG gene expression, which in addition to producing short-term sensitivity has the potential to influence nociceptive responses in adulthood. We recently discovered that growth hormone (GH) is a key regulator of afferent sensitization and pain-related behaviors during developmental inflammation of the skin. Peripheral injury caused a significant reduction in cutaneous GH levels, which corresponded with the observed hypersensitivity. However, it has yet to be determined whether GH deficiency (GHD) is sufficient to drive peripheral sensitization in uninjured animals. Here, we found that systemic GHD, induced by knockout of the GH release hormone receptor (GHRHr), was able to induce behavioral and afferent hypersensitivity to peripheral stimuli specifically during early developmental stages. GHD also produced an upregulation of many receptors and channels linked to nociceptive processing in the DRGs at these early postnatal ages (P7 and P14). Surprisingly, P21 GHRHr knockouts also displayed significant alterations in DRG gene expression even though behavioral and afferent hypersensitivity resolved. These data support previous findings that GH is a key modulator of neonatal hypersensitivity. Results may provide insight into whether GH treatment may be a therapeutic strategy for pediatric pain.

Plausible Links Between Metabolic Networks, Stem Cells, and Longevity

Aging is an inevitable consequence of life, and all multicellular organisms undergo a decline in tissue and organ functions as they age. Several well-known risk factors, such as obesity, diabetes, and lack of physical activity that lead to the cardiovascular system, decline and impede the function of vital organs, ultimately limit overall life span. Over recent years, aging research has experienced an unparalleled growth, particularly with the discovery and recognition of genetic pathways and biochemical processes that control to some extent the rate of aging.In this chapter, we focus on several aspects of stem cell biology and aging, beginning with major cellular hallmarks of aging, endocrine regulation of aging and its impact on stem cell compartment, and mechanisms of increased longevity. We then discuss the role of epigenetic modifications associated with aging and provide an overview on a most recent search of antiaging modalities.

Growth hormone facilitates 5'-azacytidine-induced myogenic but inhibits 5'-azacytidine-induced adipogenic commitment in C3H10T1/2 mesenchymal stem cells

The C3H10T1/2 cells are considered mesenchymal stem cells (MSCs) because they can be induced to become the progenitor cells for myocytes, adipocytes, osteoblasts, and chondrocytes by the DNA methyltransferase inhibitor 5'-azacytidine. In this study, we determined the effect of growth hormone (GH) on the myogenic and adipogenic lineage commitment in C3H10T1/2 cells. The C3H10T1/2 cells were treated with recombinant bovine GH in the presence or absence of 5'-azacytidine for 4 days. The myogenic commitment in C3H10T1/2 cells was assessed by immunostaining them for MyoD, the marker for myoblasts, and by determining their capacity to differentiate into the multinucleated myotubes. The adipogenic commitment in C3H10T1/2 cells was assessed by determining their ability to differentiate into adipocytes. Myotubes and adipocyteswere identified by immunocytochemistry and Oil Red O staining, respectively. C3H10T1/2 cells treated with 5'-azacytidine and GH for 4 days contained a greater percentage of MyoD-positive cells than those treated with 5'-axacytidine alone (P < 0.05). The former generated more myotubes than the latter upon induced myoblast differentiation (P < 0.05). However, C3H10T1/2 cells treated with GH alone did not form any myotubes. C3H10T1/2 cells treated with 5'-azacytidine formed adipocytes upon adipocyte differentiation induction, whereas C3H10T1/2 cells treated with GH alone did not form any adipocytes. C3H10T1/2 cells treated with both 5'-azacytidine and GH formed fewer adipocytes than those treated with 5'-azacytidine alone (P < 0.05). Both GHR and IGF-I mRNA expression in C3H10T1/2 cells were increased by 5'-azacytidine (P < 0.05), but neither was affected by GH. Overall, this study showed that GH enhanced 5'-azacytidine-induced commitment in C3H10T1/2 cells to myoblasts but inhibited 5'-azacytidine-induced commitment to preadipocytes. These results support the possibility that GH stimulates skeletal muscle growth and inhibits adipose tissue growth in part by stimulating the myogenic commitment and inhibiting the adipogenic commitment, respectively, in mesenchymal stem cells.

Burly1 is a mouse QTL for lean body mass that maps to a 0.8-Mb region of chromosome 2

To fine map a mouse QTL for lean body mass (Burly1), we used information from intercross, backcross, consomic, and congenic mice derived from the C57BL/6ByJ (host) and 129P3/J (donor) strains. The results from these mapping populations were concordant and showed that Burly1 is located between 151.9 and 152.7 Mb (rs33197365 to rs3700604) on mouse chromosome 2. The congenic region harboring Burly1 contains 26 protein-coding genes, 11 noncoding RNA elements (e.g., lncRNA), and 4 pseudogenes, with 1949 predicted functional variants. Of the protein-coding genes, 7 have missense variants, including genes that may contribute to lean body weight, such as Angpt41, Slc52c3, and Rem1. Lean body mass was increased by the B6-derived variant relative to the 129-derived allele. Burly1 influenced lean body weight at all ages but not food intake or locomotor activity. However, congenic mice with the B6 allele produced more heat per kilogram of lean body weight than did controls, pointing to a genotype effect on lean mass metabolism. These results show the value of integrating information from several mapping populations to refine the map location of body composition QTLs and to identify a short list of candidate genes.

Experimental repetitive mild traumatic brain injury induces deficits in trabecular bone microarchitecture and strength in mice

To evaluate the long-term consequence of repetitive mild traumatic brain injury (mTBI) on bone, mTBI was induced in 10-week-old female C57BL/6J mice using a weight drop model, once per day for 4 consecutive days at different drop heights (0.5, 1 and 1.5 m) and the skeletal phenotype was evaluated at different time points after the impact. In vivo micro-CT (μ-CT) analysis of the tibial metaphysis at 2, 8 and 12 weeks after the impact revealed a 5%-32% reduction in trabecular bone mass. Histomorphometric analyses showed a reduced bone formation rate in the secondary spongiosa of 1.5 m impacted mice at 12 weeks post impact. Apparent modulus (bone strength), was reduced by 30% (P<0.05) at the proximal tibial metaphysis in the 1.5 m drop height group at 2 and 8 weeks post impact. Ex vivo μ-CT analysis of the fifth lumbar vertebra revealed a significant reduction in trabecular bone mass at 12 weeks of age in all three drop height groups. Serum levels of osteocalcin were decreased by 22%, 15%, and 19% in the 0.5, 1.0 and 1.5 m drop height groups, respectively, at 2 weeks post impact. Serum IGF-I levels were reduced by 18%-32% in mTBI mice compared to contro1 mice at 2 weeks post impact. Serum osteocalcin and IGF-I levels correlated with trabecular BV/TV (r² =0.14 and 0.16, P<0.05). In conclusion, repetitive mTBI exerts significant negative effects on the trabecular bone microarchitecture and bone mechanical properties by influencing osteoblast function via reduced endocrine IGF-I actions.

Inflammatory Diseases and Growth: Effects on the GH-IGF Axis and on Growth Plate

This review briefly describes the most common chronic inflammatory diseases in childhood, such as cystic fibrosis (CF), inflammatory bowel diseases (IBDs), juvenile idiopathic arthritis (JIA), and intrauterine growth restriction (IUGR) that can be considered, as such, for the changes reported in the placenta and cord blood of these subjects. Changes in growth hormone (GH) secretion, GH resistance, and changes in the insulin-like growth factor (IGF) system are described mainly in relationship with the increase in nuclear factor-κB (NF-κB) and pro-inflammatory cytokines. Changes in the growth plate are also reported as well as a potential role for microRNAs (miRNAs) and thus epigenetic changes in chronic inflammation. Many mechanisms leading to growth failure are currently known; however, it is clear that further research in the field is still warranted.

Impact of Growth Hormone on Regulation of Adipose Tissue

Increasing prevalence of obesity and obesity-related conditions worldwide has necessitated a more thorough understanding of adipose tissue (AT) and expanded the scope of research in this field. AT is now understood to be far more complex and dynamic than previously thought, which has also fueled research to reevaluate how hormones, such as growth hormone (GH), alter the tissue. In this review, we will introduce properties of AT important for understanding how GH alters the tissue, such as anatomical location of depots and adipokine output. We will provide an overview of GH structure and function and define several human conditions and cognate mouse lines with extremes in GH action that have helped shape our understanding of GH and AT. A detailed discussion of the GH/AT relationship will be included that addresses adipokine production, immune cell populations, lipid metabolism, senescence, differentiation, and fibrosis, as well as brown AT and beiging of white AT. A brief overview of how GH levels are altered in an obese state, and the efficacy of GH as a therapeutic option to manage obesity will be given. As we will reveal, the effects of GH on AT are numerous, dynamic and depot-dependent. © 2017 American Physiological Society. Compr Physiol 7:819-840, 2017.

Further investigation of phenotypes and confounding factors of progressive ratio performance and feeding behavior in the BACHD rat model of Huntington disease

Huntington disease is an inherited neurodegenerative disorder characterized by motor, cognitive, psychiatric and metabolic symptoms. We recently published a study describing that the BACHD rat model of HD shows an obesity phenotype, which might affect their motivation to perform food-based behavioral tests. Further, we argued that using a food restriction protocol based on matching BACHD and wild type rats' food consumption rates might resolve these motivational differences. In the current study, we followed up on these ideas in a longitudinal study of the rats' performance in a progressive ratio test. We also investigated the phenotype of reduced food consumption rate, which is typically seen in food-restricted BACHD rats, in greater detail. In line with our previous study, the BACHD rats were less motivated to perform the progressive ratio test compared to their wild type littermates, although the phenotype was no longer present when the rats' food consumption rates had been matched. However, video analysis of food consumption tests suggested that the reduced consumption rate found in the BACHD rats was not entirely based on differences in hunger, but likely involved motoric impairments. Thus, restriction protocols based on food consumption rates are not appropriate when working with BACHD rats. As an alternative, we suggest that studies where BACHD rats are used should investigate how the readouts of interest are affected by motivational differences, and use appropriate control tests to avoid misleading results. In addition, we show that BACHD rats display distinct behavioral changes in their progressive ratio performance, which might be indicative of striatal dysfunction.

Models of GH deficiency in animal studies

Growth hormone (GH) is a peptide hormone released from pituitary somatotrope cells that promotes growth, cell division and regeneration by acting directly through the GH receptor (GHR), or indirectly via hepatic insulin-like growth factor 1 (IGF1) production. GH deficiency (GHD) can cause severe consequences, such as growth failure, changes in body composition and altered insulin sensitivity, depending of the origin, time of onset (childhood or adulthood) or duration of GHD. The highly variable clinical phenotypes of GHD can now be better understood through research on transgenic and naturally-occurring animal models, which are widely employed to investigate the origin, phenotype, and consequences of GHD, and particularly the underlying mechanisms of metabolic disorders associated to GHD. Here, we reviewed the most salient aspects of GH biology, from somatotrope development to GH actions, linked to certain GHD types, as well as the animal models employed to reproduce these GHD-associated alterations.

Partial correction of the dwarf phenotype by non-viral transfer of the growth hormone gene in mice: Treatment age is critical

Non-viral transfer of the growth hormone gene to different muscles of immunodeficient dwarf (lit/scid) mice is under study with the objective of improving phenotypic correction via this particular gene therapy approach. Plasmid DNA was administered into the exposed quadriceps or non-exposed tibialis cranialis muscle of lit/scid mice followed by electroporation, monitoring several growth parameters. In a 6-month bioassay, 50μg DNA were injected three times into the quadriceps muscle of 80-day old mice. A 50% weight increase, with a catch-up growth of 21%, together with a 16% increase for nose-to-tail and tail lengths (catch-up=19-21%) and a 24-28% increase for femur length (catch-up=53-60%), were obtained. mIGF1 serum levels were ~7-fold higher than the basal levels for untreated mice, but still ~2-fold lower than in non-dwarf scid mice. Since treatment age was found to be particularly important in a second bioassay utilizing 40-day old mice, these pubertal mice were compared in a third bioassay with adult (80-day old) mice, all treated twice with 50μg DNA injected into each tibialis cranialis muscle, via a less invasive approach. mIGF1 concentrations at the same level as co-aged scid mice were obtained 15days after administration in pubertal mice. Catch-up growth, based on femur length (77%), nose-to-tail (36%) and tail length (39%) increases was 40 to 95% higher than those obtained upon treating adult mice. These data pave the way for the development of more effective pre-clinical assays in pubertal dwarf mice for the treatment of GH deficiency via plasmid-DNA muscular administration.

Signal transducer and activator of transcription 5B (STAT5B) modulates adipocyte differentiation via MOF

The role and mechanism of signal transducer and activator of transcription 5B (STAT5B) in adipogenesis remain unclear. In this study, our data showed that Males absent on the first (MOF) protein expression was increased during 3 T3-L1 preadipocytes differentiation accompanied with STAT5B expression increasing. Over-expression STAT5B enhanced MOF promoter trans-activation in HeLa cells. Mutagenesis assay and ChIP analysis exhibited that STAT5B was able to bind MOF promoter. Knocking-down STAT5B in 3 T3-L1 preadipocytes led to decreased expression of MOF, but resulted in increased expression of peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer-binding protein α (C/EBPα) and fatty acid-binding protein 4 (Fabp4), which were important factors or enzymes for adipogenesis. We also found that knocking-down MOF in 3 T3-L1 preadipocytes resulted in increased expression of PPARγ, C/EBPα and Fabp4, which was in the same trend as STAT5B knocking-down. Over-expression MOF resulted in reduced promoter trans-activation activity of C/EBPα. These results suggest that STAT5B and MOF work as negative regulators in adipogenesis, and STAT5B modulates preadipocytes differentiation partially by regulating MOF expression.

Growth hormone replacement therapy regulates microRNA-29a and targets involved in insulin resistance

Abstract

Replacement of growth hormone (GH) in patients suffering from GH deficiency (GHD) offers clinical benefits on body composition, exercise capacity, and skeletal integrity. However, GH replacement therapy (GHRT) is also associated with insulin resistance, but the mechanisms are incompletely understood. We demonstrate that in GH-deficient mice (growth hormone-releasing hormone receptor (Ghrhr)^lit/lit), insulin resistance after GHRT involves the upregulation of the extracellular matrix (ECM) and the downregulation of microRNA miR-29a in skeletal muscle. Based on RNA deep sequencing of skeletal muscle from GH-treated Ghrhr^lit/lit mice, we identified several upregulated genes as predicted miR-29a targets that are negative regulators of insulin signaling or profibrotic/proinflammatory components of the ECM. Using gain- and loss-of-function studies, five of these genes were confirmed as endogenous targets of miR-29a in human myotubes (PTEN, COL3A1, FSTL1, SERPINH1, SPARC). In addition, in human myotubes, IGF1, but not GH, downregulated miR-29a expression and upregulated COL3A1. These results were confirmed in a group of GH-deficient patients after 4 months of GHRT. Serum IGF1 increased, skeletal muscle miR-29a decreased, and miR-29a targets were upregulated in patients with a reduced insulin response (homeostatic model assessment of insulin resistance (HOMA-IR)) after GHRT. We conclude that miR-29a could contribute to the metabolic response of muscle tissue to GHRT by regulating ECM components and PTEN. miR-29a and its targets might be valuable biomarkers for muscle metabolism following GH replacement.

Key messages

GHRT most significantly affects the ECM cluster in skeletal muscle from mice.

GHRT downregulates miR-29a and upregulates miR-29a targets in skeletal muscle from mice.

PTEN, COL3A1, FSTL1, SERPINH1, and SPARC are endogenous miR-29a targets in human myotubes.

IGF1 decreases miR-29a levels in human myotubes.

miR-29a and its targets are regulated during GHRT in skeletal muscle from humans.

Electronic supplementary material

The online version of this article (doi:10.1007/s00109-015-1322-y) contains supplementary material, which is available to authorized users.

Effects of growth hormone-releasing hormone on sleep and brain interstitial fluid amyloid-β in an APP transgenic mouse model

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by impairment of cognitive function, extracellular amyloid plaques, intracellular neurofibrillary tangles, and synaptic and neuronal loss. There is substantial evidence that the aggregation of amyloid β (Aβ) in the brain plays a key role in the pathogenesis of AD and that Aβ aggregation is a concentration dependent process. Recently, it was found that Aβ levels in the brain interstitial fluid (ISF) are regulated by the sleep-wake cycle in both humans and mice; ISF Aβ is higher during wakefulness and lower during sleep. Intracerebroventricular infusion of orexin increased wakefulness and ISF Aβ levels, and chronic sleep deprivation significantly increased Aβ plaque formation in amyloid precursor protein transgenic (APP) mice. Growth hormone-releasing hormone (GHRH) is a well-documented sleep regulatory substance which promotes non-rapid eye movement sleep. GHRHR(lit/lit) mice that lack functional GHRH receptor have shorter sleep duration and longer wakefulness during light periods. The current study was undertaken to determine whether manipulating sleep by interfering with GHRH signaling affects brain ISF Aβ levels in APPswe/PS1ΔE9 (PS1APP) transgenic mice that overexpress mutant forms of APP and PSEN1 that cause autosomal dominant AD. We found that intraperitoneal injection of GHRH at dark onset increased sleep and decreased ISF Aβ and that delivery of a GHRH antagonist via reverse-microdialysis suppressed sleep and increased ISF Aβ. The diurnal fluctuation of ISF Aβ in PS1APP/GHRHR(lit/lit) mice was significantly smaller than that in PS1APP/GHRHR(lit/+) mice. However despite decreased sleep in GHRHR deficient mice, this was not associated with an increase in Aβ accumulation later in life. One of several possibilities for the finding is the fact that GHRHR deficient mice have GHRH-dependent but sleep-independent factors which protect against Aβ deposition.

Minireview: The Complexities of IGF/Insulin Signaling in Aging: Why Flies and Worms Are Not Humans

A remarkable plasticity in life span has been uncovered in recent years, offering hope that the basic mechanisms of aging and interventions that delay aging may be identified in the coming decades. Life span extension has been achieved by genetic manipulation in multiple organisms including Sarcomyces cervisae, Caenorhabditis elegans, and Drosophila melanogaster, resulting in more than a doubling of life span in some cases. Typically, a reduction in function has been the most effective approach to extending life span, and a reduction in the insulin/IGF-1 signaling pathway appears to provide the most robust increase in life span. This highly conserved pathway integrates growth/survival signals with nutrient status. In mammals, it comprises part of the neuroendocrine axis, a critical regulator of growth and development. Reduced functionality of the neuroendocrine axis itself promotes life span extension in mammals; however, reduced activity of the IGF-1 signaling pathway specifically leads to less robust increases in life span. This review examines the differences in the insulin/IGF-1 axis between invertebrate and mammalian systems and discusses implications of these differences in terms of life span modulation.

Hepatic oxidative stress promotes insulin-STAT-5 signaling and obesity by inactivating protein tyrosine phosphatase N2

Hepatic insulin resistance is a key contributor to the pathogenesis of obesity and type 2 diabetes (T2D). Paradoxically, the development of insulin resistance in the liver is not universal, but pathway selective, such that insulin fails to suppress gluconeogenesis but promotes lipogenesis, contributing to the hyperglycemia, steatosis, and hypertriglyceridemia that underpin the deteriorating glucose control and microvascular complications in T2D. The molecular basis for the pathway-specific insulin resistance remains unknown. Here we report that oxidative stress accompanying obesity inactivates protein-tyrosine phosphatases (PTPs) in the liver to activate select signaling pathways that exacerbate disease progression. In obese mice, hepatic PTPN2 (TCPTP) inactivation promoted lipogenesis and steatosis and insulin-STAT-5 signaling. The enhanced STAT-5 signaling increased hepatic IGF-1 production, which suppressed central growth hormone release and exacerbated the development of obesity and T2D. Our studies define a mechanism for the development of selective insulin resistance with wide-ranging implications for diseases characterized by oxidative stress.

Effects of isolated GH deficiency on adipose tissue, feeding and adipokines in mice

OBJECTIVE

Growth hormone deficiency (GHD) leads to growth failure and changes in body composition including increased fat accumulation and reduced lean body mass in both humans and rodents. The aim of this study was to characterize the consequences of isolated GHD (IGHD) on adiposity, total body weight (TBW), and food intake in a mouse model of autosomal recessive IGHD due to targeted ablation of the GH-releasing hormone (GHRH) gene [GHRH knockout (GHRHKO)]. Animals were also analyzed with respect to leptin, adiponectin and visfatin circulating levels and gene expression in both intra-abdominal and subcutaneous fat.

DESIGN

We studied 8 male mice homozygous for GHRHKO allele (-/-) and 8 heterozygous (+/-) animals as controls. Feeding and TBW data were collected weekly from 3 through 5 months of age. Animals were then euthanized for measurement of body length and intra-abdominal (epididymal and retroperitoneal) and subcutaneous (interscapular, axillary, gluteal and inguinal) fat weights, and for blood collection for leptin, adiponectin and visfatin measurement. Gene expression of leptin, adiponectin and visfatin in adipose tissue was evaluated by real-time reverse transcription polymerase chain reaction.

RESULTS

GHRHKO mice had significantly increased relative intra-abdominal (P<0.01) and subcutaneous (P<0.0001) fat, accompanied by significantly increased food intake per TBW (P<0.01), whereas - despite 40% higher food consumption--TBW change was not different from controls over the 2 month period. Adiponectin and visfatin mRNA levels were decreased in both intra-abdominal (P<0.001) and subcutaneous fat (P<0.0001), while leptin mRNA levels were not different from controls. Conversely, serum adiponectin levels were higher in GHRHKO mice (P<0.0001), whereas serum visfatin and leptin did not significantly differ from controls.

CONCLUSIONS

IGHD due to targeted ablation of the GHRH gene in mice is associated with increased relative subcutaneous and intra-abdominal fat mass and higher food consumption which is not related to changes in circulating leptin.

Comparative inhibition of the GH/IGF-I axis obtained with either the targeted secretion inhibitor SXN101959 or the somatostatin analog octreotide in growing male rats

Abnormally high GH/IGF-I levels, most often caused by adenomas arising from pituitary somatotrophs, generate deleterious effects. We recently described a targeted secretion inhibitor (SXN101742) comprising a GHRH domain and the endopeptidase domain of botulinum toxin serotype D (GHRH-light chain endopeptidase type D domain [LC/D] associated to a heavy chain translocation domain [HN]) able to down-regulate the GH/IGF-I axis. In the present study, we compared the effect of a single iv bolus of a related molecule developed for clinical studies (SXN101959, 1 mg/kg) with a sc infusion of the somatostatin analog octreotide (SMS201-995, 10 μg/kg · h) to lower GH/IGF-I activity in growing male rats. Ten days after administration of SXN101959 or initiation of the octreotide infusion, body and pituitary weights, body length, GH peaks, and IGF-I production were reduced by both treatments but to a greater extent with SXN101959. In contrast to unaltered GH gene expression and increased GH storage in pituitaries from octreotide-treated rats, the inhibition of GH secretion was associated with a collapse of both GH mRNA and protein level in pituitaries from SXN101959-treated rats, in line with a specific decrease in hypothalamic GHRH production, not observed with octreotide. SXN101959 did not induce major apoptotic events in anterior pituitary and exhibited a reversible mode of action with full recovery of somatotroph cell functionality 30 days after treatment. Octreotide infusion permanently decreased ghrelin levels, whereas SXN101959 only transiently attenuated ghrelinemia. Both treatments limited bone mass acquisition and altered specifically tissues development. In conclusion, SXN101959 exerts a powerful and reversible inhibitory action on the somatotropic axis. Specific features of SXN101959, including long duration of action coupled to a strong inhibition of pituitary GH synthesis, represent advantages when treating overproduction of GH.

Farnesoid X receptor directly regulates xenobiotic detoxification genes in the long-lived Little mice

Activation of xenobiotic metabolism pathways has been linked to lifespan extension in different models of aging. However, the mechanisms underlying activation of xenobiotic genes remain largely unknown. Here we showed that although farnesoid X receptor (FXR, Nr1h4) mRNA levels do not change significantly, FXR protein levels are elevated in the livers of the long-lived Little mice, leading to increased DNA binding activity of FXR. Hepatic FXR expression is sex-dependent in wild-type mice but not in Little mice, implying that up-regulation of FXR might be dependent on the reduction of growth hormone in Little mice. Growth hormone treatment decreased hepatic expression of FXR and xenobiotic genes Abcb1a, Fmo3 and Gsta2 in both wild-type and Little mice, suggesting an association between FXR and xenobiotic gene expression. We found that Abcb1a is transactivated by FXR via direct binding of FXR/retinoid X receptor α (RXRα) heterodimer to a response element at the proximal promoter. FXR also positively controls Fmo3 and Gsta2 expression through direct interaction with the response elements in these genes. Our study demonstrates that xenobiotic genes are direct transcriptional targets of FXR and suggests that FXR signaling may play a critical role in the lifespan extension observed in Little mice.

Disruption of JAK2 in adipocytes impairs lipolysis and improves fatty liver in mice with elevated GH

Nonalcoholic fatty liver disease (NAFLD) is considered the hepatic expression of the metabolic syndrome, and its prevalence is increasing. The factors that influence the development of fatty liver and its progression to steatohepatitis and cirrhosis are not well understood. The pleiotropic hormone, GH, has been associated with an increased risk of NAFLD in humans and mice. GH is known to have diverse effects on lipid metabolism including decreasing body fat in vivo, presumably through stimulation of lipolysis via an undefined mechanism. Previously we described mice with hepatocyte-specific deletion of the GH signaling mediator, Janus kinase 2 (JAK2L). JAK2L animals have elevated serum GH, reduced body fat, high liver triglyceride content, and increased serum markers of hepatocyte injury (alanine transaminase and aspartate transaminase). We aimed to determine whether the elevation of GH in JAK2L mice contributed to fatty liver by promoting lipolysis directly in adipocytes. We generated mice with adipocyte-specific disruption of JAK2 (JAK2A) and found that GH resistance in adipocytes reduced lipolysis and increased body fat. JAK2A mice were then crossed to JAK2L mice, and the resultant JAK2L/A animals had increased body fat and decreased lipolysis, despite elevated circulating GH. Furthermore, the increased triglyceride content, serum alanine transaminase, and serum aspartate transaminase observed in JAK2L mice were nearly normalized with the additional disruption of JAK2 in adipocytes (JAK2L/A mice). Our results offer novel mechanistic insights into the long-recognized effects of GH on lipid flux and suggest that GH signaling may play an important regulatory role in the development of NAFLD.

The GH/IGF-1 axis in ageing and longevity

Secretion of growth hormone (GH), and consequently that of insulin-like growth factor 1 (IGF-1), declines over time until only low levels can be detected in individuals aged ≥60 years. This phenomenon, which is known as the 'somatopause', has led to recombinant human GH being widely promoted and abused as an antiageing drug, despite lack of evidence of efficacy. By contrast, several mutations that decrease the tone of the GH/IGF-1 axis are associated with extended longevity in mice. In humans, corresponding or similar mutations have been identified, but whether these mutations alter longevity has yet to be established. The powerful effect of reduced GH activity on lifespan extension in mice has generated the hypothesis that pharmaceutically inhibiting, rather than increasing, GH action might delay ageing. Moreover, mice as well as humans with reduced activity of the GH/IGF-1 axis are protected from cancer and diabetes mellitus, two major ageing-related morbidities. Here, we review data on mouse strains with alterations in the GH/IGF-1 axis and their effects on lifespan. The outcome of corresponding or similar mutations in humans is described, as well as the potential mechanisms underlying increased longevity and the therapeutic benefits and risks of medical disruption of the GH/IGF-1 axis in humans.

The GH/IGF-1 axis in obesity: pathophysiology and therapeutic considerations

Obesity has become one of the most common medical problems in developed countries, and this disorder is associated with high incidences of hypertension, dyslipidaemia, cardiovascular disease, type 2 diabetes mellitus and specific cancers. Growth hormone (GH) stimulates the production of insulin-like growth factor 1 in most tissues, and together GH and insulin-like growth factor 1 exert powerful collective actions on fat, protein and glucose metabolism. Clinical trials assessing the effects of GH treatment in patients with obesity have shown consistent reductions in total adipose tissue mass, in particular abdominal and visceral adipose tissue depots. Moreover, studies in patients with abdominal obesity demonstrate a marked effect of GH therapy on body composition and on lipid and glucose homeostasis. Therefore, administration of recombinant human GH or activation of endogenous GH production has great potential to influence the onset and metabolic consequences of obesity. However, the clinical use of GH is not without controversy, given conflicting results regarding its effects on glucose metabolism. This Review provides an introduction to the role of GH in obesity and summarizes clinical and preclinical data that describe how GH can influence the obese state.

A dwarf mouse model with decreased GH/IGF-1 activity that does not experience life-span extension: potential impact of increased adiposity, leptin, and insulin with advancing age

Reduced growth hormone (GH) action is associated with extended longevity in many vertebrate species. GH receptor (GHR) null (GHR(-)(/-)) mice, which have a disruption in the GHR gene, are a well-studied example of mice that are insulin sensitive and long lived yet obese. However, unlike other mouse lines with reduced GH action, GH receptor antagonist (GHA) transgenic mice have reduced GH action yet exhibit a normal, not extended, life span. Understanding why GHA mice do not have extended life span though they share many physiological attributes with GHR(-)(/-) mice will help provide clues about how GH influences aging. For this study, we examined age- and sex-related changes in body composition, glucose homeostasis, circulating adipokines, and tissue weights in GHA mice and littermate controls. Compared with previous studies with GHR(-)(/-) mice, GHA mice had more significant increases in fat mass with advancing age. The increased obesity resulted in significant adipokine changes. Euglycemia was maintained in GHA mice; however, hyperinsulinemia developed in older male GHA mice. Overall, GHA mice experience a more substantial, generalized obesity accompanied by altered adipokine levels and glucose homeostasis than GHR(-)(/-) mice, which becomes more exaggerated with advancing age and which likely contributes to the lack of life-span extension in these mice.

Young little mice express a premature cardiovascular aging phenotype

To investigate the effect of growth hormone and insulin-like growth factor 1 deficiency on the aging mouse arterial system, we compared the hemodynamics in young (4 months) and old (30 months) growth hormone-releasing hormone receptor null dwarf (Little) mice and their wild-type littermates. Young Little mice had significantly lower peak and mean aortic velocity and significantly higher aortic impedance than young wild-type mice. However, unlike the wild-type mice, there were no significant changes in arterial function with age in the Little mice. Aortic pulse wave velocity estimated using characteristic impedance increased with age in the wild-type mice, but it changed minimally in the Little mouse. We therefore conclude that arterial function in Little mice expresses a premature aging phenotype at young age and may neither enhance nor reduce their longevity.

Growth hormone and aging

The potential usefulness of growth hormone (GH) as an anti-aging therapy is of considerable current interest. Secretion of GH normally declines during aging and administration of GH can reverse age-related changes in body composition. However, mutant dwarf mice with congenital GH deficiency and GH resistant GH-R-KO mice live much longer than their normal siblings, while a pathological elevation of GH levels reduces life expectancy in both mice and men. We propose that the actions of GH on growth, development, and adult body size may serve as important determinants of aging and life span, while the age-related decline in GH levels contributes to some of the symptoms of aging.

Somatotropic signaling: trade-offs between growth, reproductive development, and longevity

Growth hormone (GH) is a key determinant of postnatal growth and plays an important role in the control of metabolism and body composition. Surprisingly, deficiency in GH signaling delays aging and remarkably extends longevity in laboratory mice. In GH-deficient and GH-resistant animals, the "healthspan" is also extended with delays in cognitive decline and in the onset of age-related disease. The role of hormones homologous to insulin-like growth factor (IGF, an important mediator of GH actions) in the control of aging and lifespan is evolutionarily conserved from worms to mammals with some homologies extending to unicellular yeast. The combination of reduced GH, IGF-I, and insulin signaling likely contributes to extended longevity in GH or GH receptor-deficient organisms. Diminutive body size and reduced fecundity of GH-deficient and GH-resistant mice can be viewed as trade-offs for extended longevity. Mechanisms responsible for delayed aging of GH-related mutants include enhanced stress resistance and xenobiotic metabolism, reduced inflammation, improved insulin signaling, and various metabolic adjustments. Pathological excess of GH reduces life expectancy in men as well as in mice, and GH resistance or deficiency provides protection from major age-related diseases, including diabetes and cancer, in both species. However, there is yet no evidence of increased longevity in GH-resistant or GH-deficient humans, possibly due to non-age-related deaths. Results obtained in GH-related mutant mice provide striking examples of mutations of a single gene delaying aging, reducing age-related disease, and extending lifespan in a mammal and providing novel experimental systems for the study of mechanisms of aging.

The Human Obesity Gene Map: The 2003 Update

This is the tenth update of the human obesity gene map, incorporating published results up to the end of October 2003 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. Transgenic and knockout murine models relevant to obesity are also incorporated (N = 55). As of October 2003, 41 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. QTLs reported from animal models currently number 183. There are 208 human QTLs for obesity phenotypes from genome-wide scans and candidate regions in targeted studies. A total of 35 genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 272 studies reporting positive associations with 90 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, more than 430 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

Deletion of β-adrenergic receptor 1, 2, or both leads to different bone phenotypes and response to mechanical stimulation

As they age, mice deficient for the β2-adrenergic receptor (Adrb2(-/-) ) maintain greater trabecular bone microarchitecture, as a result of lower bone resorption and increased bone formation. The role of β1-adrenergic receptor signaling and its interaction with β2-adrenergic receptor on bone mass regulation, however, remains poorly understood. We first investigated the skeletal response to mechanical stimulation in mice deficient for β1-adrenergic receptors and/or β2-adrenergic receptors. Upon axial compression loading of the tibia, bone density, cancellous and cortical microarchitecture, as well as histomorphometric bone forming indices, were increased in both Adrb2(-/-) and wild-type (WT) mice, but not in Adrb1(-/-) nor in Adrb1b2(-/-) mice. Moreover, in the unstimulated femur and vertebra, bone mass and microarchitecture were increased in Adrb2(-/-) mice, whereas in Adrb1(-/-) and Adrb1b2(-/-) double knockout mice, femur bone mineral density (BMD), cancellous bone volume/total volume (BV/TV), cortical size, and cortical thickness were lower compared to WT. Bone histomorphometry and biochemical markers showed markedly decreased bone formation in Adrb1b2(-/-) mice during growth, which paralleled a significant decline in circulating insulin-like growth factor 1 (IGF-1) and IGF-binding protein 3 (IGF-BP3). Finally, administration of the β-adrenergic agonist isoproterenol increased bone resorption and receptor activator of NF-κB ligand (RANKL) and decreased bone mass and microarchitecture in WT but not in Adrb1b2(-/-) mice. Altogether, these results demonstrate that β1- and β2-adrenergic signaling exert opposite effects on bone, with β1 exerting a predominant anabolic stimulus in response to mechanical stimulation and during growth, whereas β2-adrenergic receptor signaling mainly regulates bone resorption during aging.

Growth hormone and adipose tissue: beyond the adipocyte

The last two decades have seen resurgence in research focused on adipose tissue. In part, the enhanced interest stems from an alarming increase in obesity rates worldwide. However, an understanding that this once simple tissue is significantly more intricate and interactive than previously realized has fostered additional attention. While few would argue that growth hormone (GH) radically alters fat mass, newer findings revealing the complexity of adipose tissue requires that GH's influence on this tissue be reexamined. Therefore, the objective of this review is to describe the more recent understanding of adipose tissue and to summarize our current knowledge of how GH may influence and contribute to these newer complexities of this tissue with special focus on the available data from mice with altered GH action.

Activation of the maternal immune system induces endocrine changes in the placenta via IL-6

Activation of the maternal immune system in rodent models sets in motion a cascade of molecular pathways that ultimately result in autism- and schizophrenia-related behaviors in offspring. The finding that interleukin-6 (IL-6) is a crucial mediator of these effects led us to examine the mechanism by which this cytokine influences fetal development in vivo. Here we focus on the placenta as the site of direct interaction between mother and fetus and as a principal modulator of fetal development. We find that maternal immune activation (MIA) with a viral mimic, synthetic double-stranded RNA (poly(I:C)), increases IL-6 mRNA as well as maternally-derived IL-6 protein in the placenta. Placentas from MIA mothers exhibit increases in CD69+ decidual macrophages, granulocytes and uterine NK cells, indicating elevated early immune activation. Maternally-derived IL-6 mediates activation of the JAK/STAT3 pathway specifically in the spongiotrophoblast layer of the placenta, which results in expression of acute phase genes. Importantly, this parallels an IL-6-dependent disruption of the growth hormone-insulin-like growth factor (GH-IGF) axis that is characterized by decreased GH, IGFI and IGFBP3 levels. In addition, we observe an IL-6-dependent induction in pro-lactin-like protein-K (PLP-K) expression as well as MIA-related alterations in other placental endocrine factors. Together, these IL-6-mediated effects of MIA on the placenta represent an indirect mechanism by which MIA can alter fetal development.

The role of GH/IGF-I-mediated mechanisms in sex differences in cortical bone size in mice

Cortical bone dimensions are important determinants of bone strength. Gender differences in cortical bone size caused by greater periosteal expansion in males than in females during the pubertal growth spurt are well established both in humans and in experimental animal models. However, the mechanism by which gender influences cortical bone size is still a matter of investigation. The role of androgens and estrogen in pubertal bone growth has been examined in human disorders as well as animal models, such as gonadectomized or sex steroid receptor knockout mice. Based on the findings that growth hormone (GH) and insulin-like growth factor I (IGF-I) are major regulators of postnatal skeletal growth, we and others have predicted that sex hormones interact with the GH/IGF-I axis to regulate cortical bone size. However, studies conflict as to whether estrogen and androgens impact cortical bone size through the canonical pathway, through GH without IGF-I mediation, through IGF-I without GH stimulation, or independent of GH/IGF-I. We review recent data on the impact of sex steroids and components of the GH/IGF axis on sexual dimorphism in bone size. While the GH/IGF-I axis is a major player in regulating peak bone size, the relative contribution of GH/IGF-dependent mechanisms to sex differences in cortical bone size remains to be established.

Longevity and lifespan control in mammals: lessons from the mouse

Aging, which affects all organ systems, is one of the most complex phenotypes. Recent discoveries in long-lived mutant mice have revealed molecular mechanisms of longevity in mammals which may contribute to our understanding of why humans age. These mutations include naturally occurring spontaneous mutations, and those of mice genetically modified by modern genomic technologies. It is generally believed that the most fundamental mechanisms of aging are evolutionarily conserved across species. The following types of longevity mechanisms have been intensively studied: suppression of the somatotropic (growth hormone/insulin-like growth factor 1) axis, decreased metabolism and increased resistance of oxidative stress, reduced insulin secretion and increased insulin sensitivity, and delayed reproductive maturation and reduced fertility. In addition, many of the mutations have a sex-dependent effect on lifespan, and when present in different genetic backgrounds, the effects of the same gene mutation can vary considerably. The present review discusses these phenotypic variations as well as describing the known longevity genes in long-lived mutant mice and the molecular mechanisms specifying longevity. We anticipate that these mouse studies will ultimately provide clues about how to delay the aging and prolong lifespan, and help to develop therapies for healthier human aging.

Targeted deletion of somatotroph insulin-like growth factor-I signaling in a cell-specific knockout mouse model

The role of IGF-I in the negative regulation of GH expression and release is demonstrated by in vitro and in vivo models; however, the targets and mechanisms of IGF-I remain unclear. We have developed a cell-specific knockout mouse in which the IGF-I receptor was ablated from the somatotroph in order to validate and characterize IGF-I negative regulation; we termed this the somatotroph IGF-I receptor knockout (SIGFRKO) mouse. The SIGFRKO mice demonstrated increased GH gene expression and secretion as well as increased serum IGF-I. Compensatory changes were noted with decreased GHRH and increased somatostatin mRNA expression levels. SIGFRKO mice had normal linear growth, but by 14 wk of age weighed significantly less than controls. Furthermore, metabolic studies revealed SIGFRKO mice had significantly less fat mass and body percent fat. These data support somatotroph IGF-I negative regulation and suggest that hypothalamic feedback limits the extent of GH release. The SIGFRKO mouse is a model delineating the mechanisms of IGF-I regulation in the hypothalamic-pituitary axis and demonstrates compensatory mechanisms that mediate growth and metabolic function in mammals.

A novel spontaneous mutation of Irs1 in mice results in hyperinsulinemia, reduced growth, low bone mass and impaired adipogenesis

A spontaneous mouse mutant, designated 'small' (sml), was recognized by reduced body size suggesting a defect in the IGF1/GH axis. The mutation was mapped to the chromosome 1 region containing Irs1, a viable candidate gene whose sequence revealed a single nucleotide deletion resulting in a premature stop codon. Despite normal mRNA levels in mutant and control littermate livers, western blot analysis revealed no detectable protein in mutant liver lysates. When compared with the control littermates, Irs1(sml)/Irs1(sml) (Irs1(sml/sml)) mice were small, lean, hearing impaired; had 20% less serum IGF1; were hyperinsulinemic; and were mildly insulin resistant. Irs1(sml/sml) mice had low bone mineral density, reduced trabecular and cortical thicknesses, and low bone formation rates, while osteoblast and osteoclast numbers were increased in the females but not different in the males compared with the Irs1(+/+) controls. In vitro, Irs1(sml/sml) bone marrow stromal cell cultures showed decreased alkaline phosphatase-positive colony forming units (pre-osteoblasts; CFU-AP+) and normal numbers of tartrate-resistant acid phosphatase-positive osteoclasts. Irs1(sml/sml) stromal cells treated with IGF1 exhibited a 50% decrease in AKT phosphorylation, indicative of defective downstream signaling. Similarities between engineered knockouts and the spontaneous mutation of Irs1(sml) were identified as well as significant differences with respect to heterozygosity and gender. In sum, we have identified a spontaneous mutation in the Irs1 gene associated with a major skeletal phenotype. Changes in the heterozygous Irs1(+)(/sml) mice raise the possibility that similar mutations in humans are associated with short stature or osteoporosis.

Restricted growth and insulin-like growth factor-1 deficiency in mice lacking presenilin-1 in the neural crest cell lineage

Presenilin-1 (PS1) is a transmembrane protein that is in many cases responsible for the development of early-onset familial Alzheimer's disease. PS1 is essential for neurogenesis, somitogenesis, angiogenesis, and cardiac morphogenesis. We report here that PS1 is also required for maturation and/or maintenance of the pituitary gland. We generated PS1-conditional knockout (PS1-cKO) mice by crossing floxed PS1 and Wnt1-cre mice, in which PS1 was lacking in the neural crest-derived cell lineage. Although the PS1-cKO mice exhibited no obvious phenotypic abnormalities for several days after birth, reduced body weight in the mutant was evident by the age of 3-5 weeks. Pituitary weight and serum insulin-like growth factor (IGF)-1 level were also reduced in the mutant. Histologic analysis revealed severe atrophy of the cytosol in the anterior and intermediate pituitary lobes of the mutant. Immunohistochemistry did not reveal clear differences in the expression levels of thyroid-stimulating hormone, adrenocorticotropic hormone, or prolactin in the mutant pituitary. In contrast, growth hormone expression levels were reduced in the anterior lobe of the mutant. PS1 was defective in the posterior lobe, but not the anterior or intermediate lobes, in the mutant pituitary. These findings suggest that PS1 indirectly mediates the development and/or maintenance of the anterior and intermediate lobes in the pituitary gland via actions in other regions, such as the posterior lobe.

Two-year body composition analyses of long-lived GHR null mice

Growth hormone receptor gene-disrupted (GHR-/-) mice exhibit increased life span and adipose tissue mass. Although this obese phenotype has been reported extensively for young adult male GHR-/- mice, data for females and for other ages in either gender are lacking. Thus, the purpose of this study was to evaluate body composition longitudinally in both male and female GHR-/- mice. Results show that GHR-/- mice have a greater percent fat mass with no significant difference in absolute fat mass throughout life. Lean mass shows an opposite trend with percent lean mass not significantly different between genotypes but absolute mass reduced in GHR-/- mice. Differences in body composition are more pronounced in male than in female mice, and both genders of GHR-/- mice show specific enlargement of the subcutaneous adipose depot. Along with previously published data, these results suggest a consistent and intriguing protective effect of excess fat mass in the subcutaneous region.