Rat skeletal muscle glycogen degradation pathways reveal differential association of glycogen-related proteins with glycogen granules

Skeletal muscle cell-specific differences in type 2 diabetes

Major stores of glucose are found as glycogen in skeletal muscle and liver. Skeletal muscle is a heterogenous tissue, with cellular metabolic and contractile distinctions dependent on whether the cell (fibre) is slow-twitch (Type I) or fast-twitch (Type II). We hypothesised that proteins important for glycogen metabolism would be differentially abundant between these diverse fibres. We further hypothesised that the cellular location of these proteins would be different in muscle samples between control (CON) and individuals with type 2 diabetes (T2D). We dissected individual muscle fibre segments from vastus lateralis skeletal muscle biopsy samples from CON and T2D and used cell-type-specific approaches to address muscle heterogeneity. We measured glycogen and glycogen-related proteins by immunoblotting techniques. A lower proportion of Type I fibres was found in muscle in T2D compared with CON. AMPK-β2, glycogen branching enzyme (GBE), glycogen debranching enzyme (GDE), and glycogen phosphorylase (GP) were differentially localized between fibre types and in fibres from CON and T2D individuals. A key novel finding was that the majority of glycogen is loosely bound or cytosolic in location in human skeletal muscle. The proportion of this diffusible pool of glycogen was significantly lower in Type I fibres in T2D compared to CON. A hyperinsulinaemic, euglycaemic clamp in people with type 2 diabetes had no effect on the proportion of diffusible glycogen. We identify cell-type as an important consideration when assessing glycogen metabolism in muscle. Our findings demonstrate varying glucose handling abilities in specific muscle fibre types in type 2 diabetes. A model is presented to provide an overview of the cell-specific differences in glycogen metabolism in type 2 diabetes.

Glycogen in the uterus and fallopian tubes is an important source of glucose during early pregnancy†

Pregnancy loss is common during the peri-implantation period in mammals when glucose is required for both embryonic development and decidualization of the endometrium. As the uterus cannot synthesize glucose, all glucose must come directly from maternal circulation as needed or transiently stored as the macromolecule glycogen. Glycogen acts as a glucose reservoir, storing up to 55 000 glucose moieties per molecule. Endometrial glycogen concentrations are correlated with fertility in humans, indicating that glycogen is an essential source of glucose during early pregnancy. In humans and primates, endometrial glycogen concentrations peak during the luteal phase due to progesterone. In contrast, in rats and mink, estradiol triggers an accumulation of uterine glycogen during proestrus and estrus. In mated rats, the glycogen content of the endometrium increases again after implantation due to high levels of glycogen stored in the decidua. In mink, endometrial glycogen reserves are localized in the uterine epithelia at estrus. These reserves are mobilized before implantation, suggesting they are used to support embryonic growth. Uterine glycogen concentrations continue to decrease after implantation in mink, probably due to a lack of decidualization. How ovarian steroids stimulate glycogenesis in the endometrium is unclear, but current evidence suggests that estradiol/progesterone interacts with insulin or insulin-like growth factor signaling. In summary, endometrial glycogen is an essential source of glucose during the peri-implantation period. More work is needed to characterize differences among species, elucidate the fate of the glucose liberated from glycogen, and understand how ovarian steroids regulate glycogen metabolism in the uterus.

Physiological and biochemical characteristics of skeletal muscles in sedentary and active rats

Laboratory rats are sedentary if housed in conditions where activity is limited. Changes in muscle characteristics with chronic inactivity were investigated by comparing sedentary rats with rats undertaking voluntary wheel running for either 6 or 12 weeks. EDL (type II fibers) and soleus (SOL) muscles (predominantly type I fibers) were examined. When measured within 1-2 h post-running, calcium sensitivity of the contractile apparatus was increased, but only in type II fibers. This increase disappeared when fibers were treated with DTT, indicative of oxidative regulation of the contractile apparatus, and was absent in fibers from rats that had ceased running 24 h prior to experiments. Specific force production was ~ 10 to 25% lower in muscle fibers of sedentary compared to active rats, and excitability of skinned fibers was decreased. Muscle glycogen content was ~ 30% lower and glycogen synthase content ~ 50% higher in SOL of sedentary rats, and in EDL glycogenin was 30% lower. Na⁺, K⁺-ATPase α1 subunit density was ~ 20% lower in both EDL and SOL in sedentary rats, and GAPDH content in SOL ~ 35% higher. There were no changes in content of the calcium handling proteins calsequestrin and SERCA, but the content of CSQ-like protein was increased in active rats (by ~ 20% in EDL and 60% in SOL). These findings show that voluntary exercise elicits an acute oxidation-induced increase in Ca²⁺ sensitivity in type II fibers, and also that there are substantial changes in skeletal muscle characteristics and biochemical processes in sedentary rats.

Exploring glycogen biosynthesis through Monte Carlo simulation

Glycogen, a complex branched polymer of glucose (average chain length ~10 monomer units), is the blood-sugar reservoir in humans and other animals. Certain aspects of its molecular structure relevant to its biological functions are currently unamenable to experimental exploration. Knowledge of these is needed to develop future models for quantitative data-fitting to obtain mechanistic understanding of the biosynthetic processes that give rise to glycogen structure. Monte Carlo simulations of the biosynthesis of this structure with realistic macromolecular parameters reveal how chain growth and stoppage (the latter assumed to be through both the action of glycogen branching enzyme and other degradative enzymes, and by hindrance) control structural features. The simulated chain-length, pair-distance and radial density distributions agree semi-quantitatively with the limited available data. The simulations indicate that a steady state in molecular structure and size is rapidly obtained, that molecular density reaches a maximum near the center of the particle (not at the periphery, as is the case with dendrimers), and that particle size is controlled by both enzyme activity and hindrance. This knowledge will aid in the understanding of diabetes (loss of blood-sugar control), which has been found to involve subtle differences in glycogen molecular structure.

Elevated GLUT4 and glycogenin protein abundance correspond to increased glycogen content in the soleus muscle of mdx mice with no benefit associated with taurine supplementation

Duchenne muscular dystrophy (DMD) patients and the dystrophic mdx mouse have an elevated demand for ATP requiring processes, including Ca2+ regulation and skeletal muscle regeneration. As a key substrate for cellular ATP production, altered glycogen metabolism may contribute significantly to dystrophic pathology and explain reports of mild glucose intolerance. We compare the soleus and extensor digitorum longus (EDL) muscles of the mdx mouse during active muscle necrosis (at 28 days) and at 70 days where pathology is stable. We further investigate the impact of taurine (tau) on dystrophic glycogen metabolism to identify if the benefit seen with tau in a previous study (Barker et al. ) was in part owed to altered glycogen handling. The soleus muscle of 28- and 70-day-old mdx mice had elevated glucose transporter type 4 (GLUT4), glycogenin protein abundances and glycogen content compared to WT (C57BL10/ScSn) controls. Mdx tau mice exhibited modestly reduced glycogen compared to their respective mdx group. The EDL muscle of 28 days mdx tau mice had a ~70% increase in glycogenin protein abundance compared to the mdx but 50% less glycogen content. A twofold greater phosphorylated glycogen synthase (p-GS) and glycogen phosphorylase (p-GP) protein abundance was observed in the 70-day-old mdx soleus muscle than in the 28-day-old mdx soleus muscle. Glycogen debranching enzyme (GDE) protein abundance was elevated in both 28- and 70-day-old mdx soleus muscles compared to WT controls. We identified an increase in proteins associated with glucose uptake and utilization specific to the predominantly slow-twitch soleus muscle of mdx mice regardless of age and that taurine affords no obvious benefit to glycogen metabolism in the mdx mouse.

Changes in contractile and metabolic parameters of skeletal muscle as rats age from 3 to 12 months

Laboratory rats are considered mature at 3 months despite that musculoskeletal growth is still occurring. Changes in muscle physiological and biochemical characteristics during development from 3 months, however, are not well understood. Whole muscles and single skinned fibres from fast-twitch extensor digitorum longus (EDL) and predominantly slow-twitch soleus (SOL) muscles were examined from male Sprague-Dawley rats (3, 6, 9, 12 months). Ca²⁺ sensitivity of contractile apparatus decreased with age in both fast- (~ 0.04 pCa units) and slow-twitch (~ 0.07 pCa units) muscle fibres, and specific force increased (by ~ 50% and ~ 25%, respectively). Myosin heavy chain composition of EDL and SOL muscles altered to a small extent with age (decrease in MHCIIa proportion after 3 months). Glycogen content increased with age (~ 80% in EDL and 25% in SOL) and GLUT4 protein density decreased (~ 35 and 20%, respectively), whereas the glycogen-related enzymes were little changed. GAPDH protein content was relatively constant in both muscle types, but COXIV protein decreased ~ 40% in SOL muscle. Calsequestrin (CSQ) and SERCA densities remained relatively constant with age, whereas there was a progressive ~ 2-3 fold increase in CSQ-like proteins, though their role and importance remain unclear. There was also ~ 40% decrease in the density of the Na⁺, K⁺-ATPase (NKA) α1 subunit in EDL and the α2 subunit in SOL. These findings emphasise there are substantial changes in skeletal muscle function and the density of key proteins during early to mid-adulthood in rats, which need to be considered in the design and interpretation of experiments.

When phosphorylated at Thr148, the β2-subunit of AMP-activated kinase does not associate with glycogen in skeletal muscle

The 5′-AMP-activated protein kinase (AMPK), a heterotrimeric complex that functions as an intracellular fuel sensor that affects metabolism, is activated in skeletal muscle in response to exercise and utilization of stored energy. The diffusibility properties of α- and β-AMPK were examined in isolated skeletal muscle fiber segments dissected from rat fast-twitch extensor digitorum longus and oxidative soleus muscles from which the surface membranes were removed by mechanical dissection. After the muscle segments were washed for 1 and 10 min, ∼60% and 75%, respectively, of the total AMPK pools were found in the diffusible fraction. After in vitro stimulation of the muscle, which resulted in an ∼80% decline in maximal force, 20% of the diffusible pool became bound in the fiber. This bound pool was not associated with glycogen, as determined by addition of a wash step containing amylase. Stimulation of extensor digitorum longus muscles resulted in 28% glycogen utilization and a 40% increase in phosphorylation of the downstream AMPK target acetyl carboxylase-CoA. This, however, had no effect on the proportion of total β2-AMPK that was phosphorylated in whole muscle homogenates measured by immunoprecipitation. These findings suggest that, in rat skeletal muscle, β2-AMPK is not associated with glycogen and that activation of AMPK by muscle contraction does not dephosphorylate β2-AMPK. These findings question the physiological relevance of the carbohydrate-binding function of β2-AMPK in skeletal muscle.