Myosin heavy chain isoform immunolabelling in diabetic rats with peripheral neuropathy

α-Motoneurons maintain biophysical heterogeneity in obesity and diabetes in Zucker rats

Small-diameter sensory dysfunction resulting from diabetes has received much attention in the literature, whereas the impact of diabetes on α-motoneurons (MN) has not. In addition, the chance of developing insulin resistance and diabetes is increased in obesity. No study has examined the impact of obesity or diabetes on the biophysical properties of MN. Lean Zucker rats and Zucker diabetic fatty (ZDF) rats were separated into lean, obese (ZDF fed standard chow), and diabetic (ZDF fed high-fat diet that led to diabetes) groups. Glass micropipettes recorded hindlimb MN properties from identified flexor and extensor MN. MN were separated within their groups on the basis of input conductance, which created high- and low-input conductance subpopulations for each. A significant shorter (20%) afterhyperpolarization half-decay (AHP_1/2) was found in low-conductance MN for the diabetic group only, whereas AHP½ tended to be shorter in the obese group (19%). Significant positive correlations were found among rheobase and input conductance for both lean and obese animals. No differences were found between the groups for afterhyperpolarization amplitude (AHP_amp), input conductance, rheobase, or any of the rhythmic firing properties (frequency-current slope and spike-frequency adaptation index). MN properties continue to be heterogeneous in obese and diabetic animals. Obesity does not seem to influence lumbar MN. Despite the resistance of MN to the impact of diabetes, the reduced AHP_1/2 decay and the tendency for a reduction in AHP_amp may be the first sign of change to MN function.NEW & NOTEWORTHY Knowledge about the impact of obesity and diabetes on the biophysical properties of motoneurons is lacking. We found that diabetes reduces the duration of the afterhyperpolarization and that motoneuron function is unchanged by obesity. A reduced afterhyperpolarization may impact discharge characteristics and may be the first sign of change to motoneuron function.

Striated muscle fiber size, composition, and capillary density in diabetes in relation to neuropathy and muscle strength

OBJECTIVE

Diabetic polyneuropathy (DPN) leads to progressive loss of muscle strength in the lower extremities due to muscular atrophy. Changes in vascularization occur in diabetic striated muscle; however, the relationship between these changes and DPN is as yet unexplored. The aim of the present study was to evaluate histologic properties and capillarization of diabetic skeletal muscle in relation to DPN and muscle strength.

METHODS

Twenty type 1 and 20 type 2 diabetic (T1D and T2D, respectively) patients underwent biopsy of the gastrocnemic muscle, isokinetic dynamometry at the ankle, electrophysiological studies, clinical examination, and quantitative sensory examinations. Muscle biopsies were stained immunohistochemically and muscle fiber diameter, fiber type distribution, and capillary density determined. Twenty control subjects were also included in the study.

RESULTS

No relationship was found between muscle fiber diameter, muscle fiber type distribution, or capillary density and degree of neuropathy or muscle strength for either patient group. Muscle fiber diameter and the proportion of Type II fibers were greater for T1D patients than both T2D patients and controls. The T2D patients had fewer capillaries per muscle fiber than T1D patients and controls.

CONCLUSIONS

Striated muscle fiber size, muscle fiber distribution, and vascularization are unrelated to DPN and muscle strength in diabetes.

Impaired macrophage and satellite cell infiltration occurs in a muscle-specific fashion following injury in diabetic skeletal muscle

BACKGROUND

Systemic elevations in PAI-1 suppress the fibrinolytic pathway leading to poor collagen remodelling and delayed regeneration of tibialis anterior (TA) muscles in type-1 diabetic Akita mice. However, how impaired collagen remodelling was specifically attenuating regeneration in Akita mice remained unknown. Furthermore, given intrinsic differences between muscle groups, it was unclear if the reparative responses between muscle groups were different.

PRINCIPAL FINDINGS

Here we reveal that diabetic Akita muscles display differential regenerative responses with the TA and gastrocnemius muscles exhibiting reduced regenerating myofiber area compared to wild-type mice, while soleus muscles displayed no difference between animal groups following injury. Collagen levels in TA and gastrocnemius, but not soleus, were significantly increased post-injury versus controls. At 5 days post-injury, when degenerating/necrotic regions were present in both animal groups, Akita TA and gastrocnemius muscles displayed reduced macrophage and satellite cell infiltration and poor myofiber formation. By 10 days post-injury, necrotic regions were absent in wild-type TA but persisted in Akita TA. In contrast, Akita soleus exhibited no impairment in any of these measures compared to wild-type soleus. In an effort to define how impaired collagen turnover was attenuating regeneration in Akita TA, a PAI-1 inhibitor (PAI-039) was orally administered to Akita mice following cardiotoxin injury. PAI-039 administration promoted macrophage and satellite cell infiltration into necrotic areas of the TA and gastrocnemius. Importantly, soleus muscles exhibit the highest inducible expression of MMP-9 following injury, providing a mechanism for normative collagen degradation and injury recovery in this muscle despite systemically elevated PAI-1.

CONCLUSIONS

Our findings suggest the mechanism underlying how impaired collagen remodelling in type-1 diabetes results in delayed regeneration is an impairment in macrophage infiltration and satellite cell recruitment to degenerating areas; a phenomena that occurs differentially between muscle groups.

Automated rodent in situ muscle contraction assay and myofiber organization analysis in sarcopenia animal models

Age-related sarcopenia results in frailty and decreased mobility, which are associated with increased falls and long-term disability in the elderly. Given the global increase in lifespan, sarcopenia is a growing, unmet medical need. This report aims to systematically characterize muscle aging in preclinical models, which may facilitate the development of sarcopenia therapies. Naïve rats and mice were subjected to noninvasive micro X-ray computed tomography (micro-CT) imaging, terminal in situ muscle function characterizations, and ATPase-based myofiber analysis. We developed a Definiens (Parsippany, NJ)-based algorithm to automate micro-CT image analysis, which facilitates longitudinal in vivo muscle mass analysis. We report development and characterization of translational in situ skeletal muscle performance assay systems in rat and mouse. The systems incorporate a custom-designed animal assay stage, resulting in enhanced force measurement precision, and LabVIEW (National Instruments, Austin, TX)-based algorithms to support automated data acquisition and data analysis. We used ATPase-staining techniques for myofibers to characterize fiber subtypes and distribution. Major parameters contributing to muscle performance were identified using data mining and integration, enabled by Labmatrix (BioFortis, Columbia, MD). These technologies enabled the systemic and accurate monitoring of muscle aging from a large number of animals. The data indicated that longitudinal muscle cross-sectional area measurement effectively monitors change of muscle mass and function during aging. Furthermore, the data showed that muscle performance during aging is also modulated by myofiber remodeling factors, such as changes in myofiber distribution patterns and changes in fiber shape, which affect myofiber interaction. This in vivo muscle assay platform has been applied to support identification and validation of novel targets for the treatment of sarcopenia.

Influence of insulin and muscle fiber type in nepsilon-(carboxymethyl)-lysine accumulation in soleus muscle of rats with streptozotocin-induced diabetes mellitus

BACKGROUND

Nepsilon-(carboxymethyl)-lysine (CML) is an advanced glycation end product (AGE), the accumulation of which has been implicated in the etiology of diabetes complications. Skeletal muscle in diabetes demonstrates altered function, and increased accumulation of CML has been found in several fast-twitch muscles of diabetic animals.

OBJECTIVE

This study aims to explore the accumulation of CML in soleus (a slow muscle) in diabetic animals, with and without insulin therapy.

METHODS

Twenty-one rats were randomly divided into control and diabetes groups (DNI: diabetes without insulin; DI: diabetes with insulin; C: control). Diabetes was induced by intravenous administration of streptozotocin. At the end of the 12-week experimental period the soleus muscle was excised and snap frozen in liquid nitrogen. Muscle cross-sections were immunolabeled for CML. The number of CML-labeled muscle fibers was quantified; fibers were also evaluated for fiber types and cross-sectional areas.

RESULTS

The percentage of myofibers immunolabeling for CML was highest in the DNI group (13.8 +/- 2.5%), lower in the DI group (5.4 +/- 1.1%) and lowest in the C group (2.1 +/- 0.6%). Statistical analysis revealed that AGE accumulation was significantly greater in the DNI group than in both C and DI groups (p = 0.0002). There was no significant difference between C and DI groups. In the DNI animals, AGE-positive myofibers showed a higher percentage of fast fiber types than did the AGE-negative fibers (49.5 +/- 6.9 vs. 13.7 +/- 1.5%, p = 0.002). No differences existed in cross-sectional areas between AGE-positive and AGE-negative fibers within any group.

CONCLUSION

The greatest accumulation of AGE was in the soleus of the DNI group, and was significantly less in the DI group. These findings may be linked to disordered glucose metabolism, increased oxidative stress and/or fiber type transformation in these muscles.

Advanced Glycation End Product in Diabetic Rat Skeletal Muscle in vivo

BACKGROUND

Advanced glycation end products (AGEs) are implicated in the etiology of diabetic complications in the kidney, nerve and eye. Skeletal muscle contractile parameters have also been found to be altered in diabetes. Glycation has not been extensively studied in skeletal muscle, but AGE-modified proteins may influence contractility.

OBJECTIVE AND METHODS

The aim of this study was to use immunohistochemistry to identify distribution patterns of the AGE Nepsilon-(carboxymethyl)-lysine in plantaris muscle of diabetic rats.

RESULTS

Results revealed the presence of Nepsilon-(carboxymethyl)-lysine intracellularly and also at sites along the myofiber periphery. The number of myofibers immunolabeling for AGE in animals with diabetes was more than 4-fold greater than in control animals. Additionally, there was a greater proportion of slow + fast myosin heavy chain coexpression in the AGE-positive cells from diabetic animals than in AGE-positive fibers from control animals. No significant difference was present between cross-sectional areas of AGE-positive fibers and AGE-negative fibers within the respective experimental groups.

CONCLUSIONS

AGE accumulation is greater in skeletal muscle in vivo from diabetic animals than in control animals. This AGE accumulation appears to be associated with fiber-type transformation rather than with myofiber size. Further study is needed to determine the identity of these AGE-modified proteins and to determine how they influence skeletal muscle function in diabetes.