The testis isoform of the phosphorylase kinase catalytic subunit (PhK-gammaT) plays a critical role in regulation of glycogen mobilization in developing lung

Analysis of a lung defect in autophagy-deficient mouse strains

Yeast Atg1 initiates autophagy in response to nutrient limitation. The Ulk gene family encompasses the mammalian orthologs of yeast ATG1. We created mice deficient for both Ulk1 and Ulk2 and found that the mice die within 24 h of birth. When found alive, pups exhibited signs of respiratory distress. Histological sections of lungs of the Ulk1/2 DKO pups showed reduced airspaces with thickened septae. A similar defect was seen in Atg5-deficient pups as both Ulk1/2 DKO and Atg5 KO lungs show numerous glycogen-laden alveolar type II cells by electron microscopy, PAS staining, and increased levels of glycogen in lung homogenates. No abnormalities were noted in expression of genes encoding surfactant proteins but the ability to incorporate exogenous choline into phosphatidylcholine, the major phospholipid component of surfactant, was increased in comparison to controls. Despite this, there was a trend for total phospholipid levels in lung tissue to be lower in Ulk1/2 DKO and Atg5 KO compared with controls. Autophagy was abundant in lung epithelial cells from wild-type mice, but lacking in Atg5 KO and Ulk1/2 DKO mice at P1. Analysis of the autophagy signaling pathway showed the existence of a negative feedback loop between the ULK1 and 2 and MTORC1 and 2, in lung tissue. In the absence of autophagy, alveolar epithelial cells are unable to mobilize internal glycogen stores independently of surfactant maturation. Together, the data suggested that autophagy plays a vital role in lung structural maturation in support of perinatal adaptation to air breathing.

Delayed lung maturation of foetus of diabetic mother rats develop with a diminish, but without changes in the proportion of type I and II pneumocytes, and decreased expression of protein D-associated surfactant factor

Newborn children of diabetic mothers have an increased morbidity and mortality because of respiratory distress syndrome. We study lung histogenesis during intrauterine development of offspring of diabetic Sprague-Dawley rats at 18, 19 and 21 days of gestation (DG). Pregnant rats were grouped into diabetic (streptozotocin-induced), citrate, and control groups; five female and five male offspring were selected randomly from each group at 18, 19 and 21 DG, and a biopsy of the lung was taken and processed in paraffin for histological examination. The biopsy for the transmission electron microscopy (TEM) analysis was taken at 21 days. A delay in alveolization of the offspring at 18, 19 and 21 days of the diabetic group was observed, which was confirmed at TEM level, and also less quantity of protein D associated to surfactant in diabetic group was detected (P < 0.001). The foetuses of the diabetic group presented a delay in lung histogenesis and in differentiation of the type II pneumocytes cells, but conserved the proportion with a decrease in 50% of pneumocytes, accompanied by a diminish of protein D associated to surfactant factor.

In Silico characterization of phosphorylase kinase: evidence for an alternate intronic polyadenylation site in PHKG1

Phosphorylase kinase (PhK), the key enzyme that regulates glycogenolysis, has traditionally been thought to be expressed predominantly in muscle and liver. In this study, we show by two different database searches (Expressed Sequence Tag and UniGene) that PhK gene expression occurs in at least 28-36 different tissues, and that the genes encoding the alpha, beta, and gamma subunits of PhK undergo extensive transcriptional processing. In particular, we have identified exon 6 of PHKG1 as a 3' composite terminal exon due to the presence of a weak polyadenylation and cleavage site in intron 6. We have verified biochemically that transcriptional processing of PHKG1 does occur in vivo; mRNA corresponding to the alternate variant is expressed in skeletal muscle, brain, heart, and tongue. In silico translation of this mRNA yields a PhK gamma subunit that contains the first 181 residues of the protein, followed by an additional 21 amino acids. The implication of this alternate processing is discussed within the context of gamma catalysis and regulation.

Specific features of glycogen metabolism in the liver

Although the general pathways of glycogen synthesis and glycogenolysis are identical in all tissues, the enzymes involved are uniquely adapted to the specific role of glycogen in different cell types. In liver, where glycogen is stored as a reserve of glucose for extrahepatic tissues, the glycogen-metabolizing enzymes have properties that enable the liver to act as a sensor of blood glucose and to store or mobilize glycogen according to the peripheral needs. The prime effector of hepatic glycogen deposition is glucose, which blocks glycogenolysis and promotes glycogen synthesis in various ways. Other glycogenic stimuli for the liver are insulin, glucocorticoids, parasympathetic (vagus) nerve impulses and gluconeogenic precursors such as fructose and amino acids. The phosphorolysis of glycogen is mainly mediated by glucagon and by the orthosympathetic neurotransmitters noradrenaline and ATP. Many glycogenolytic stimuli, e.g. adenosine, nucleotides and NO, also act indirectly, via secretion of eicosanoids from non-parenchymal cells. Effectors often initiate glycogenolysis cooperatively through different mechanisms.

Mutations in the testis/liver isoform of the phosphorylase kinase gamma subunit (PHKG2) cause autosomal liver glycogenosis in the gsd rat and in humans

Heritable deficiency of phosphorylase kinase (Phk), a regulatory enzyme of glycogen metabolism, is responsible for 25% of all cases of glycogen storage disease and occurs with a frequency of -1 in 100,000 births. It is genetically and clinically heterogeneous, occurring in X-linked and autosomal-recessive forms and exhibiting various patterns of principally affected tissues (liver only, muscle only, liver and muscle, liver and kidney, heart only). This heterogeneity is thought to reflect the enzyme's structural complexity [subunit composition, (alpha beta gamma delta)4] and isoform diversity. Two isoforms encoded by separate genes are known for the subunits alpha (muscle [alpha M] and liver [alpha L isoforms) and gamma (muscle [gamma M] and testis [gamma T] isoforms), whereas only one gene appears to exist for the subunit beta. The subunit delta is calmodulin; identical calmodulins are expressed from three different human genes. Additional isoform diversity arises by differential mRNA splicing of the alpha M, alpha L and beta subunits. Mutations responsible for the various forms of Phk deficiency are sought in those subunit/isoform genes with a matching chromosomal location and tissue-specificity of expression. We report here that autosomal liver-specific Phk deficiency is associated with mutations in the gene encoding the testis/liver isoform of the catalytic gamma subunit (PHKG2). We found homozygous PHKG2 mutations in three human patients of consanguineous parentage and in the gsd (glycogen storage disease) rat strain, which is thus identified as an animal model for the human disorder. One human mutation is a single base-pair insertion in codon 89 that causes a frameshift and premature chain termination. The three other mutations result in non-conservative replacements of amino acid residues (V106E, G189E, D215N) that are highly conserved within the catalytic core regions of all protein kinases. These are the first mutations to be reported for an autosomal form of Phk deficiency. The findings suggest that the PHKG2 gene product is the predominant isoform of the catalytic gamma subunit of Phk not only in testis but also in liver, erythrocytes and, possibly, other non-muscle tissues.