Adipo-glial signaling mediates metabolic adaptation in peripheral nerve regeneration

The peripheral nervous system harbors a remarkable potential to regenerate after acute nerve trauma. Full functional recovery, however, is rare and critically depends on peripheral nerve Schwann cells that orchestrate breakdown and resynthesis of myelin and, at the same time, support axonal regrowth. How Schwann cells meet the high metabolic demand required for nerve repair remains poorly understood. We here report that nerve injury induces adipocyte to glial signaling and identify the adipokine leptin as an upstream regulator of glial metabolic adaptation in regeneration. Signal integration by leptin receptors in Schwann cells ensures efficient peripheral nerve repair by adjusting injury-specific catabolic processes in regenerating nerves, including myelin autophagy and mitochondrial respiration. Our findings propose a model according to which acute nerve injury triggers a therapeutically targetable intercellular crosstalk that modulates glial metabolism to provide sufficient energy for successful nerve repair.

Control of the Anterior Pituitary Cell Lineage Regulator POU1F1 by the Stem Cell Determinant Musashi

The adipokine leptin regulates energy homeostasis through ubiquitously expressed leptin receptors. Leptin has a number of major signaling targets in the brain, including cells of the anterior pituitary (AP). We have previously reported that mice lacking leptin receptors in AP somatotropes display growth hormone (GH) deficiency, metabolic dysfunction, and adult-onset obesity. Among other targets, leptin signaling promotes increased levels of the pituitary transcription factor POU1F1, which in turn regulates the specification of somatotrope, lactotrope, and thyrotrope cell lineages within the AP. Leptin's mechanism of action on somatotropes is sex dependent, with females demonstrating posttranscriptional control of Pou1f1 messenger RNA (mRNA) translation. Here, we report that the stem cell marker and mRNA translational control protein, Musashi1, exerts repression of the Pou1f1 mRNA. In female somatotropes, Msi1 mRNA and protein levels are increased in the mouse model that lacks leptin signaling (Gh-CRE Lepr-null), coincident with lack of POU1f1 protein, despite normal levels of Pou1f1 mRNA. Single-cell RNA sequencing of pituitary cells from control female animals indicates that both Msi1 and Pou1f1 mRNAs are expressed in Gh-expressing somatotropes, and immunocytochemistry confirms that Musashi1 protein is present in the somatotrope cell population. We demonstrate that Musashi interacts directly with the Pou1f1 mRNA 3' untranslated region and exerts translational repression of a Pou1f1 mRNA translation reporter in a leptin-sensitive manner. Musashi immunoprecipitation from whole pituitary reveals coassociated Pou1f1 mRNA. These findings suggest a mechanism in which leptin stimulation is required to reverse Musashi-mediated Pou1f1 mRNA translational control to coordinate AP somatotrope function with metabolic status.

OR24-3 Persistence of Progenitor Cell Markers Following the Selective Ablation of Musashi in Somatotropes

Metabolic and reproductive demands are met and coordinated through the complex control of hormone synthesis and secretion exerted by the anterior pituitary. While pituitary cells are known to possess remarkable plasticity to change their cell fate and alter hormone production in response to ever changing environmental cues, the underlying molecular control of this plasticity has not been established. Our lab has previously introduced the Musashi (MSI) family of RNA binding proteins as important players in control of pituitary hormone levels. Musashi typically governs stem cell fate and promotes self-renewal by repressing translation of target mRNAs needed for differentiation. However, we found that MSI1 is expressed in most differentiated cells of the adult pituitary and can specifically bind to the 3’ UTRs of Prl, Tsh, and Pou1f1 and exert translational control in reporter assays. Confirmation of the requirement for MSI was demonstrated through in vivo analyses where MSI1 and MSI2 were selectively ablated in somatotropes. The mutant animals were subfertile. The mutant males showed reduced serum and pituitary content of LH and FSH, and significant decreases in serum GH and PRL despite 2-fold increases in pituitary protein content of PRL and GH. To further assess the role and downstream pathways regulated by MSI in the pituitary, we collected somatotrope MSI-null pituitaries from males and females for qPCR analysis of common somatotrope target genes. In correlation with our previous findings described above (low serum GH and high pituitary GH content), we found that GHRHR mRNA levels were reduced by 2-fold in male mutants. RNA-seq analysis followed by qPCR validation shows that Prop1 mRNA was significantly increased in male mutants, with no significant change in Pou1f1 mRNA. These data suggest that MSI may be involved in the regulation of progenitor cells giving rise to the somatotrope lineage and that MSI ablation may have caused retention of these progenitors and/or a failure to fully differentiate somatotropes. Our studies of MSI-null somatotrope function support this interpretation because the mutant somatotropes clearly stored GH proteins, but could not secrete them, as demonstrated by the low serum GH values, and the 50% reduction in GHRHR mRNA. RNA-seq evaluation of males revealed a change in the expression of 720 genes between controls and mutants (FDR <0.05). When we examined female MSI-null somatotrope animals, a much smaller cohort of genes showed a change in expression (153 genes, FDR <0.05). Interestingly, 38 genes were altered in both mutant males and females suggesting shared regulation by MSI. Further characterization of the NGS datasets will elucidate additional downstream targets and effector pathways of MSI-dependent control of anterior pituitary cell differentiation and function.

SAT-406 Deletion of Musashi in Gonadotropes Leads to Increased GnRHR Protein Levels and Gonadotrope Dysfunction

Leptin is a critical mediator of metabolic regulation of the hypothalamic-pituitary gonadal (HPG) axis. We have previously shown that leptin is responsible for the optimal expression of GnRHR, a rate-limiting component of the reproductive process. Female mice lacking leptin receptors (Lepr-null) specifically on gonadotropes are sub-fertile. Their reduced GnRHR proteins and normal Gnrhr mRNA levels suggest leptin’s actions on gonadotropes are post-transcriptional. A clue about a candidate translational regulator is seen in our studies showing that Lepr-null gonadotropes have increased expression of Musashi (MSI), which has binding sites within the Gnrhr 3’ UTR. Furthermore, leptin reduced MSI expression specifically in gonadotropes and increased GnRHR expression. We hypothesized that MSI may repress Gnrhr mRNA translation and that leptin alleviates this repression. To determine the effects of MSI on the HPG axis, we developed a gonadotrope-Msi1/2-null mouse line and compared mutant females (MUT) to their littermate controls (CTL) on the morning of diestrus, when the pituitary GnRHR protein levels should reach a peak. The levels of GnRHR proteins (measured by EIA) are significantly increased (1.6X) in the pituitary of the mutant females (CTL: 1.355 ± 0.11 ng/ml vs MUT: 2.202 ± 0.16 ng/ml, p=.0006), with no change in mRNA. The gonadotrope-Msi1/2-null females are subfertile, with litter sizes of 3 ± 0.4 pups, with the first litter at around day 40 and an average of 41 day delay between litters. To understand the downstream effect of the MSI knockout on gonadotropin levels, we measured pituitary and serum LH and FSH protein levels (Luminex EIA) and mRNA (qPCR). Serum FSH levels are decreased by >50% in mutant females (CTL: 0.947 ± 0.14 ng/mL vs MUT: 0.406 ± 0.05 ng/ml, p=.0049), but FSH stores and Fsh mRNAs are unchanged. Additionally, we observed a 2.2X increase in the pituitary LH protein content in mutant females (CTL: 0.328 ± 0.11 ng/ml vs MUT: 0.729 ± 0.09 ng/ml, p=.0174), but no changes in serum LH or Lh mRNA levels. These studies thus show that, as a repressor of GnRHR translation, Musashi can also regulate expression of gonadotropins. In the gonadotrope Lepr-null model, we hypothesized that leptin signals were needed to de-repress Musashi actions and allow GnRHR translation. In contrast, the gonadotrope-MSI-null mice over-express GnRHR, which confirms this role for MSI as a GnRHR regulator. We propose that the increased expression of GnRHR in the gonadotrope MSI-null animals causes the increased LH content, and disrupted FSH secretion, resulting in a higher serum LH:FSH ratio and subfertility. Collectively these data suggest that MSI regulation is necessary for optimal fertility in the adult female mouse. Future studies will determine the impact of loss of MSI on gonadotrope function throughout the estrous cycle.

SAT-417 The Gonadotrope Leptin Signal Is Critical for the Early-Morning Estrus Rise in FSH in Female Mice

The importance of peripheral nutritional signals to the functioning of the reproductive axis is appreciated, but the mechanisms are not well understood. The three tiers of the hypothalamic-pituitary-gonadal axis are receptive to many of these nutritional signals, including leptin. We have previously shown that leptin communicates with pituitary gonadotropes to maintain numbers of gonadotropin-releasing-hormone receptors (GnRHrs), and that loss of the leptin signal (LEPR) in gonadotropes results in subfertility in females. Using our current model of gonadotrope leptin resistance (GnRHR-cre, floxed Lepr exon 1) which targets the deletion of all LEPRs specifically to gonadotropes expressing Gnrhr, we investigated the role of leptin in the follicle-stimulating hormone (FSH) surge in female mice. Pituitaries and serum from 2-3 month-old control (CTL) and gonadotrope-Lepr-null (MUT) females were collected at midnight between proestrus and estrus, and at 0400 and 0900 on the morning of estrus. The samples were assayed for LH and FSH, and pituitaries were also used to determine GnRHr levels (ELISA) and Lh, Fsh, and Gnrhr mRNA levels (qPCR). Our gonadotrope-Lepr-null females showed a dramatic decrease in serum FSH during the peak of FSH secretion (0400 estrus, CTL: 25.8 ng/mL ± 5.7, n=6, MUT: 6.9 ng/mL ± 0.8, n=5, p<0.03). This result is significant, despite the large natural variability in gonadotropin surges. MUT FSH secretion also trended down at midnight by about 40%, although this effect was not significant. Interestingly, stores of FSH at 0400 were significantly increased in MUT pituitaries compared to controls. Control FSH stores dropped by ~74% from midnight to 0400, whereas mutant stores only dropped about 14% in the same time period, suggesting that the mutants have insufficient FSH secretion beginning at midnight. Analyses of ovarian sections from diestrous or proestrous MUT females showed a significant 38-43% decline in the average number of antral follicles (ANOVA, p<0.05 n=7-8), which correlates with the low FSH secretion. We also discovered a major drop in Fsh mRNA in our MUT pituitaries at 0400 (CTL RQ: 0.68 ± 0.14, n=5; MUT RQ: 0.10 ± 0.04, n=6, p<0.02). MUT Fsh mRNA remained low at 0900 (NS), at about 40% of control mRNA. Importantly, despite finding GnRHR proteins decreased at other stages in the cycle, no differences were seen between controls and mutants with regards to GnRHR (protein or mRNA) during these specific FSH surge time points. Our study indicates that leptin signals to the reproductive axis through control of FSH secretion specifically during the time of the FSH surge in female mice. We hypothesize that leptin may also be able to regulate Fsh transcription. Future studies will test the ability of leptin to stimulate Fsh transcription and secretion from normal, early AM estrous pituitaries and will investigate the pathways that mediate these effects.

MON-482 Sex-Dependent Biphasic Response by Somatotropes to Fasting and Loss of Leptin Signaling

Fasting results in a well-established rise in serum growth hormone (GH) in mice and humans. In mice, this coincides with increases in Gh and growth hormone releasing hormone receptor (Ghrhr) mRNA, as well as serum ghrelin. However, fasting also results in a dramatic decline in serum leptin and we have shown that pituitary somatotropes are dependent on circulating leptin to maintain sufficient stores of GH and GHRHR proteins. This study was designed to determine the mechanisms by which GH increases in the presence of reduced serum leptin in fasted mice. We hypothesized that the rise in GH was due to ghrelin-stimulated pathways and therefore developed a fasting model to test this. However, a review of the literature revealed a study by Steyn et al. (2011) that demonstrated an acute decline in GH pulses in male mice after 12-18 hours of fasting, which suggest that somatotropes may display a biphasic response to fasting, with the early or acute phase driven by the absence of leptin signals. We tested this hypothesis in 3 groups of male and female mice: group 1 was fasted for 24 hours; group 2 was also fasted for 24 hours and received 10% glucose water, which is known to normalize serum leptin; and group 3 mice were fed ad libitum. Following a 24 h fast, leptin levels declined significantly from 5.5±2 ng/ml to 0.37±0.1 ng/ml (Student’s t; p=0.01) along with significant decreases in body weight, serum glucose and insulin. Fasted mice receiving glucose water showed a significant recovery in leptin to 6.9±2.7 ng/ml (Student’s t; p=0.03 n=8) along with a recovery in serum glucose and insulin. In agreement with Steyn et al. (2011), serum GH in male mice showed a significant decline from 10.7±3 ng/ml (fed) to 3±0.6 ng/ml after the 24 h fast (p=0.04 ANOVA). Glucose water and the recovery in serum glucose, leptin and insulin were unable to normalize GH levels in fasted male. In contrast, fasted female mice had a 2.2 fold (p= 0.0427) increase in serum GH and no change with glucose water. There was a significant increase in pituitary Ghrhr and Gh mRNA in fasted male mice but not in fasted female mice. Assays of serum or mRNA levels of most pituitary hormones showed no significant changes following a 24 h fast. These studies have thus uncovered an acute response to fasting by GH cells in male mice, which correlates to reduced serum GH resulting from a loss of leptin signals. The data extends findings reported previously showing that male somatotropes are more dependent on leptin signals than female somatotropes. Whereas the high Gh and Ghrhr mRNA support somatotrope stimulation, the somatotropes are unable to secrete normal levels of GH in the male, suggesting defects at the level of GH or GHRHR protein expression.

Sex-specific changes in postnatal GH and PRL secretion in somatotrope LEPR-null mice

The developing pituitary is a rapidly changing environment that is constantly meeting the physiological demands of the growing organism. During early postnatal development, the anterior pituitary is refining patterns of anterior hormone secretion in response to numerous genetic factors. Our laboratory previously developed a somatotrope leptin receptor (LEPR) deletion mouse model that had decreased lean body mass, disrupted metabolism, decreased GH stores and was GH deficient as an adult. To understand how deletion of LEPR in somatotropes altered GH, we turned our attention to postnatal development. The current study examines GH, PRL, TSH, ACTH, LH and FSH secretion during postnatal days 4, 5, 8, 10 and 15 and compares age and sex differences. The LEPR mutants have dysregulation of GH (P < 0.03) and a reduced developmental prolactin peak in males (P < 0.04) and females (P < 0.002). There were no differences in weight between groups, and the postnatal leptin surge appeared to be normal. Percentages of immunolabeled GH cells were reduced in mutants compared with controls in all age groups by 35-61% in males and 41-44% in females. In addition, we measured pituitary expression of pituitary transcription factors, POU1F1 and PROP1. POU1F1 was reduced in mutant females at PND 10 (P < 0.009) and PND 15 (P < 0.02) but increased in males at PND 10 (P < 0.01). PROP1 was unchanged in female mutants but showed developmental increases at PND 5 (P < 0.02) and PND 15 (P < 0.01). These studies show that the dysfunction caused by LEPR deletion in somatotropes begins as early as neonatal development and involves developing GH and prolactin cells (somatolactotropes).

The War on the Placenta: The Differing Battles of High-Fat Diet and Obesity

Commentary on Mahany et al., “Obesity and High-Fat Diet Induce Distinct Changes in Placental Gene Expression and Pregnancy Outcome.”

Sex Differences in Somatotrope Dependency on Leptin Receptors in Young Mice: Ablation of LEPR Causes Severe Growth Hormone Deficiency and Abdominal Obesity in Males

Leptin receptor (LEPR) signaling controls appetite and energy expenditure. Somatotrope-specific deletion of the LEPRb signaling isoform causes GH deficiency and obesity. The present study selectively ablated Lepr exon 1 in somatotropes, which removes the signal peptide, causing the loss of all isoforms of LEPR. Excision of Lepr exon 1 was restricted to the pituitary, and mutant somatotropes failed to respond to leptin. Young (2-3 mo) males showed a severe 84% reduction in serum GH levels and more than 60% reduction in immunolabeled GH cells compared with 41%-42% reductions in GH and GH cells in mutant females. Mutant males (35 d) and females (45 d) weighed less than controls and males had lower lean body mass. Image analysis of adipose tissue by magnetic resonance imaging showed that young males had a 2-fold increase in abdominal fat mass and increased adipose tissue density. Young females had only an overall increase in adipose tissue. Both males and females showed lower energy expenditure and higher respiratory quotient, indicating preferential carbohydrate burning. Young mutant males slept less and were more restless during the dark phase, whereas the opposite was true of females. The effects of a Cre-bearing sire on his non-Cre-recombinase bearing progeny are seen by increased respiratory quotient and reduced litter sizes. These studies elucidate clear sex differences in the extent to which somatotropes are dependent on all isoforms of LEPR. These results, which were not seen with the ablation of Lepr exon 17, highlight the severe consequences of ablation of LEPR in male somatotropes.

The use of three-dimensional printing to produce in vitro slice chambers

BACKGROUND

In recent years, 3D printing technology has become inexpensive and simple enough that any lab can own and use one of these printers.

NEW METHOD

We explored the potential use of 3D printers for quickly and easily producing in vitro slice chambers for patch clamp electrophysiology. Slice chambers were produced using five available plastics: ABS, PLA, Nylon 618, Nylon 680, and T-glase. These "lab-made" chambers were also made using stereolithography through a professional printing service (Shapeways). This study measured intrinsic membrane properties of neurons in the brain stem pedunculopontine nucleus (PPN) and layer V pyramidal neurons in retrosplenial cortex.

RESULTS

Nylon 680 and T-glase significantly hyperpolarized PPN neurons. ABS increased input resistance, decreased action potential amplitude, and increased firing frequency in pyramidal cortical neurons. To test long term exposure to each plastic, human neuroblastoma SHSY5Y cell cultures were exposed to each plastic for 1 week. ABS decreased cell counts while Nylon 618 and Shapeways plastics eliminated cells. Primary mouse pituitary cultures were also tested for 24-h exposure. ABS decreased cell counts while Nylon 618 and Shapeways plastics dramatically decreased cell counts.

COMPARISON TO EXISTING METHODS

Chambers can be quickly and inexpensively printed in the lab. ABS, PLA, Nylon 680, and T-glase plastics would suffice for many experiments instead of commercially produced slice chambers.

CONCLUSIONS

While these technologies are still in their infancy, they represent a powerful addition to the lab environment. With careful selection of print material, slice chambers can be quickly and inexpensively manufactured in the lab.

The somatotrope as a metabolic sensor: deletion of leptin receptors causes obesity

Leptin, the product of the Lep gene, reports levels of adiposity to the hypothalamus and other regulatory cells, including pituitary somatotropes, which secrete GH. Leptin deficiency is associated with a decline in somatotrope numbers and function, suggesting that leptin may be important in their maintenance. This hypothesis was tested in a new animal model in which exon 17 of the leptin receptor (Lepr) protein was selectively deleted in somatotropes by Cre-loxP technology. Organ genotyping confirmed the recombination of the floxed LepR allele only in the pituitary. Deletion mutant mice showed a 72% reduction in pituitary cells bearing leptin receptor (LEPR)-b, a 43% reduction in LEPR proteins and a 60% reduction in percentages of immunopositive GH cells, which correlated with reduced serum GH. In mutants, LEPR expression by other pituitary cells was like that of normal animals. Leptin stimulated phosphorylated Signal transducer and activator of transcription 3 expression in somatotropes from normal animals but not from mutants. Pituitary weights, cell numbers, IGF-I, and the timing of puberty were not different from control values. Growth curves were normal during the first 3 months. Deletion mutant mice became approximately 30-46% heavier than controls with age, which was attributed to an increase in fat mass. Serum leptin levels were either normal in younger animals or reflected the level of obesity in older animals. The specific ablation of the Lepr exon 17 gene in somatotropes resulted in GH deficiency with a consequential reduction in lipolytic activity normally maintained by GH and increased adiposity.

Modafinil increases arousal determined by P13 potential amplitude: an effect blocked by gap junction antagonists

STUDY OBJECTIVES

We recorded the effects of administration of the stimulant modafinil on the amplitude of the sleep state-dependent auditory P13 evoked potential in freely moving rats, a measure of arousal thought to be generated by the cholinergic arm of the reticular activating system, specifically the pedunculopontine nucleus (PPN).

DESIGN

Groups of rats were implanted for recording auditory evoked responses and the effects on P13 potential amplitude of intracranial injections into the PPN of neuroactive agents determined.

MEASUREMENTS AND RESULTS

The effects of intracranial injections into the PPN of modafinil showed that P13 potential amplitude increased in a dose-dependent manner at doses of 100, 200, and 300 microM. The effect was blocked by pretreatment with either of the gap junction antagonists carbenoxolone (300 microM) or mefloquine (25 microM), which by themselves slightly decreased P13 potential amplitude.

CONCLUSIONS

These results suggest that modafinil increases arousal levels as determined by the amplitude of the P13 potential, an effect blocked by gap junction antagonists, suggesting that one mechanism by which modafinil increases arousal may be by increasing electrical coupling.