Differential muscle hypertrophy is associated with satellite cell numbers and Akt pathway activation following activin type IIB receptor inhibition in Mtm1 p.R69C mice.

X-linked myotubular myopathy is a congenital myopathy caused by deficiency of myotubularin. Patients often present with severe perinatal weakness, requiring mechanical ventilation to prevent death from respiratory failure. We recently reported that an activin receptor type IIB inhibitor produced hypertrophy of type 2b myofibers and modest increases of strength and life span in the severely myopathic Mtm1δ4 mouse model of X-linked myotubular myopathy. We have now performed a similar study in the less severely symptomatic Mtm1 p.R69C mouse in hopes of finding greater treatment efficacy. Activin receptor type IIB inhibitor treatment of Mtm1 p.R69C animals produced behavioral and histological evidence of hypertrophy in gastrocnemius muscles but not in quadriceps or triceps. The ability of the muscles to respond to activin receptor type IIB inhibitor treatment correlated with treatment-induced increases in satellite cell number and several muscle-specific abnormalities of hypertrophic signaling. Treatment-responsive Mtm1 p.R69C gastrocnemius muscles displayed lower levels of phosphorylated ribosomal protein S6 and higher levels of phosphorylated eukaryotic elongation factor 2 kinase than were observed in Mtm1 p.R69C quadriceps muscle or in muscles from wild-type littermates. Hypertrophy in the Mtm1 p.R69C gastrocnemius muscle was associated with increased levels of phosphorylated ribosomal protein S6. Our findings indicate that muscle-, fiber type-, and mutation-specific factors affect the response to hypertrophic therapies that will be important to assess in future therapeutic trials.

G-protein signaling modulator 1 deficiency accelerates cystic disease in an orthologous mouse model of autosomal dominant polycystic kidney disease.

Polycystic kidney diseases are the most common genetic diseases that affect the kidney. There remains a paucity of information regarding mechanisms by which G proteins are regulated in the context of polycystic kidney disease to promote abnormal epithelial cell expansion and cystogenesis. In this study, we describe a functional role for the accessory protein, G-protein signaling modulator 1 (GPSM1), also known as activator of G-protein signaling 3, to act as a modulator of cyst progression in an orthologous mouse model of autosomal dominant polycystic kidney disease (ADPKD). A complete loss of Gpsm1 in the Pkd1(V/V) mouse model of ADPKD, which displays a hypomorphic phenotype of polycystin-1, demonstrated increased cyst progression and reduced renal function compared with age-matched cystic Gpsm1(+/+) and Gpsm1(+/-) mice. Electrophysiological studies identified a role by which GPSM1 increased heteromeric polycystin-1/polycystin-2 ion channel activity via Gßγ subunits. In summary, the present study demonstrates an important role for GPSM1 in controlling the dynamics of cyst progression in an orthologous mouse model of ADPKD and presents a therapeutic target for drug development in the treatment of this costly disease.

Increased susceptibility to kidney injury by transfer of genomic segment from SHR onto Dahl S genetic background.

The Dahl salt-sensitive (S) rat is a widely studied model of salt-sensitive hypertension and develops proteinuria, glomerulosclerosis, and renal interstitial fibrosis. An earlier genetic analysis using a population derived from the S and spontaneously hypertensive rat (SHR) identified eight genomic regions linked to renal injury in the S rat and one protective locus on chromosome 11. The "protective" locus in the S rat was replaced with the SHR genomic segment conferring "susceptibility" to kidney injury. The progression of kidney injury in the S.SHR(11) congenic strain was characterized in the present study. Groups of S and S.SHR(11) rats were followed for 12 wk on either a low-salt (0.3% NaCl) or high-salt (2% NaCl) diet. By week 12 (low-salt), S.SHR(11) demonstrated a significant decline in kidney function compared with the S. Blood pressure was significantly elevated in both strains on high salt. Despite similar blood pressure, the S.SHR(11) exhibited a more significant decline in kidney function compared with the S. The decline in S.SHR(11) kidney function was associated with more severe kidney injury including tubular loss, immune cell infiltration, and tubulointerstitial fibrosis compared with the S. Most prominently, the S.SHR(11) exhibited a high degree of medullary fibrosis and a significant increase in renal vascular medial hypertrophy. In summary, genetic modification of the S rat generated a model of accelerated renal disease that may provide a better system to study progression to renal failure as well as lead to the identification of genetic variants involved in kidney injury.

Impaired cerebral angiogenesis in the fetal lamb model of persistent pulmonary hypertension.

BACKGROUND: Persistent pulmonary hypertension of the newborn (PPHN) is associated with increased risk of neuro-developmental impairments. Whether relative fetal hypoxia during evolution of PPHN renders the fetal brain vulnerable to perinatal brain injury remains unclear. We hypothesized that in utero ductal constriction, which induces PPHN also impairs cerebral angiogenesis. METHODS: Fetal lambs with PPHN induced by prenatal ligation of the ductus arteriosus were compared to gestation matched twin controls. Freshly collected or fixed brain specimens were analyzed by immunohistochemistry, Western blot analysis, and RT-PCR. RESULTS: Cortical capillary density was decreased in PPHN lambs compared to controls (Glut-1, isolectin B-4 and factor VIII, n=6, p<0.05). Hypoxia inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) protein levels were decreased in cortical cell lysates of PPHN lambs. PPHN increased angiopoetin-1 (Ang-1) and tyrosine-protein kinase receptor (Tie-2) protein expression while angiopoetin-2 (Ang-2) protein levels were decreased (n=6, p<0.05). PPHN did not change mRNA levels of these proteins significantly (n=6). CONCLUSIONS: PPHN decreased cortical capillary density in fetal lamb brain. PPHN decreased the expression of proteins involved in angiogenesis. These findings suggest that PPHN is associated with impaired cortical angiogenesis.

Sucrose non-fermenting related kinase enzyme is essential for cardiac metabolism.

In this study, we have identified a novel member of the AMPK family, namely Sucrose non-fermenting related kinase (Snrk), that is responsible for maintaining cardiac metabolism in mammals. SNRK is expressed in the heart, and brain, and in cell types such as endothelial cells, smooth muscle cells and cardiomyocytes (CMs). Snrk knockout (KO) mice display enlarged hearts, and die at postnatal day 0. Microarray analysis of embryonic day 17.5 Snrk hearts, and blood profile of neonates display defect in lipid metabolic pathways. SNRK knockdown CMs showed altered phospho-acetyl-coA carboxylase and phospho-AMPK levels similar to global and endothelial conditional KO mouse. Finally, adult cardiac conditional KO mouse displays severe cardiac functional defects and lethality. Our results suggest that Snrk is essential for maintaining cardiac metabolic homeostasis, and shows an autonomous role for SNRK during mammalian development.

Prematurity in mice leads to reduction in nephron number, hypertension, and proteinuria.

The nephron number at birth is a quantitative trait that correlates inversely with the risk of hypertension and chronic kidney disease later in life. During kidney development, the nephron number is controlled by multiple factors including genetic, epigenetic, and environmental modifiers. Premature birth, which represents more than 12% of annual live births in the United States, has been linked to low nephron number and the development of hypertension later in life. In this report, we describe the development of a mouse model of prematurity-induced reduction of nephron number. Premature mice, delivered 1 and 2 days early, have 17.4 ± 2.3% (n = 6) and 23.6 ± 2% (n = 10) fewer nephrons, respectively, when compared with full-term animals (12,252 ± 571 nephrons/kidney, n = 10). After 5 weeks of age, the mice delivered 2 days premature show lower real-time glomerular filtration rate (GFR, 283 ± 13 vs 389 ± 26 µL/min). The premature mice also develop hypertension (mean arterial pressure [MAP], 134 ± 18 vs 120 ± 14 mm Hg) and albuminuria (286 ± 83 vs 176 ± 59 µg albumin/mg creatinine). This mouse model provides a proof of concept that prematurity leads to reduced nephron number and hypertension, and this model will be useful in studying the pathophysiology of prematurity-induced nephron number reductions and hypertension.

Nocturnin regulates circadian trafficking of dietary lipid in intestinal enterocytes.

BACKGROUND: Efficient metabolic function in mammals depends on the circadian clock, which drives temporal regulation of metabolic processes. Nocturnin is a clock-regulated deadenylase that controls its target mRNA expression posttranscriptionally through poly(A) tail removal. Mice lacking nocturnin (Noc(-/-) mice) are resistant to diet-induced obesity and hepatic steatosis yet are not hyperactive or hypophagic. RESULTS: Here we show that nocturnin is expressed rhythmically in the small intestine and is induced by olive oil gavage and that the Noc(-/-) mice have reduced chylomicron transit into the plasma following the ingestion of dietary lipids. Genes involved in triglyceride synthesis and storage and chylomicron formation have altered expression, and large cytoplasmic lipid droplets accumulate in the apical domains of the Noc(-/-) enterocytes. The physiological significance of this deficit in absorption is clear because maintenance of Noc(-/-) mice on diets that challenge the chylomicron synthesis pathway result in significant reductions in body weight, whereas diets that bypass this pathway do not. CONCLUSIONS: Therefore, we propose that nocturnin plays an important role in the trafficking of dietary lipid in the intestinal enterocytes by optimizing efficient absorption of lipids.

Light chain 1 of microtubule-associated protein 1B can negatively regulate the action of Pes1.

Pes1 was first identified as the locus affected in the zebrafish mutant pescadillo, which exhibits severe defects in gut and liver development. It has since been demonstrated that loss of Pes1 expression in mammals and yeast affects ribosome biogenesis, resulting in a block in cell proliferation. Pes1 contains a BRCA1 C-terminal domain, a structural motif that has been shown to facilitate protein-protein interactions, suggesting that Pes1 has binding partners. We used a yeast two-hybrid screen to identify putative interacting proteins. We found that light chain 1 of the microtubule-associated protein 1B (Mtap1b-LC1) could partner with Pes1, and deletion analyses revealed a specific interaction of Mtap1b-LC1 with the Pes1 BRCA1 C-terminal domain. We confirmed the integrity of the interaction between Pes1 and Mtap1b-LC1 by co-immunoprecipitation experiments. Protein localization studies in NIH3T3 cells revealed that exogenously expressed Pes1 was typically restricted to nuclei and nucleoli. However, exogenous Pes1 was found predominantly in the cytoplasm in cells that were forced to express Mtap1b-LC1. We also observed that the expression of endogenous Pes1 protein was significantly reduced or undetectable in nuclei when Mtap1b-LC1 was overexpressed, implying that a dynamic interaction exists between the two proteins and that Mtap1b-LC1 has the potential to negatively impact Pes1 function. Finally, we demonstrated that, as is the case when Pes1 expression is depleted by shRNA, overexpression of Mtap1b-LC1 resulted in diminished proliferation of NIH3T3 cells, suggesting that Mtap1b-LC1 has the potential to repress cell proliferation by modulating the nucleolar levels of Pes1.

Pescadillo is essential for nucleolar assembly, ribosome biogenesis, and mammalian cell proliferation.

Mutation of the zebrafish pescadillo gene blocks expansion of a number of tissues in the developing embryo, suggesting roles for its gene product in controlling cell proliferation. We report that levels of the pescadillo protein increase in rodent hepatocytes as they enter the cell cycle. Pescadillo protein localizes to distinct substructures of the interphase nucleus including nucleoli, the site of ribosome biogenesis. During mitosis pescadillo closely associates with the periphery of metaphase chromosomes and by late anaphase is associated with nucleolus-derived foci and prenucleolar bodies. Blastomeres in mouse embryos lacking pescadillo arrest at morula stages of development, the nucleoli fail to differentiate and accumulation of ribosomes is inhibited. We propose that in mammalian cells pescadillo is essential for ribosome biogenesis and nucleologenesis and that disruption to its function results in cell cycle arrest.