Decrease of muscle volume in chronic kidney disease: the role of mitochondria in skeletal muscle

Nutrient sensing, signaling transduction, and autophagy in podocyte injury: implications for kidney disease

Podocytes are terminally differentiated epithelial cells of the renal glomerular tuft and these highly specialized cells are essential for the integrity of the slit diaphragm. The biological function of podocytes is primarily based on a complex ramified structure that requires sufficient nutrients and a large supply of energy in support of their unique structure and function in the glomeruli. Of note, the dysregulation of nutrient signaling and energy metabolic pathways in podocytes has been associated with a range of kidney diseases i.e., diabetic nephropathy. Therefore, nutrient-related and energy metabolic signaling pathways are critical to maintaining podocyte homeostasis and the pathogenesis of podocyte injury. Recently, a growing body of evidence has indicated that nutrient starvation induces autophagy, which suggests crosstalk between nutritional signaling with the modulation of autophagy for podocytes to adapt to nutrient deprivation. In this review, the current knowledge and advancement in the understanding of nutrient sensing, signaling, and autophagy in the podocyte biology, injury, and pathogenesis of kidney diseases is summarized. Based on the existing findings, the implications and perspective to target these signaling pathways and autophagy in podocytes during the development of novel preventive and therapeutic strategies in patients with podocyte injury-associated kidney diseases are discussed.

Sarcopenia is associated with mild-to-moderate chronic kidney disease in Chinese community-dwelling older men but not in women

OBJECTIVE

To determine whether a relationship exists between sarcopenia, including its individual components (muscle mass, muscle strength and gait speed), and mild-to-moderate chronic kidney disease (CKD) in Chinese older adults.

METHODS

This cross-sectional study comprised participants aged ≥60 years from Tianjin and Shanghai, China, who joined a national free physical examination program between 2014 and 2019, and consented to study inclusion. Sarcopenia was defined according to the Asian Working Group for Sarcopenia (2019 version). Mild-to-moderate CKD was defined as estimated glomerular filtration rate (eGFR) between 45 ml/min/1.73 m² and 60 ml/min/1.73 m².

RESULTS

A total of 1627 participants were included (mean age, 69.32 ± 6.17 years; 43.8% male). Sarcopenia was significantly associated with mild-to-moderate CKD in men but not women. Among three physical performance components, slow gait speed (odds ratio 1.89, 95% confidence interval 1.38, 2.58) was associated with mild-to-moderate CKD in both men and women after adjusting for all other variables.

CONCLUSIONS

Sarcopenia was closely associated with mild-to-moderate CKD in older men, and slow gait speed was related to mild-to-moderate CKD in men and women. These findings may help guide better diagnosis and management of CKD in the context of slow gait speed, and facilitate earlier CKD detection and appropriate intervention in older adults.

Different stages of chronic kidney disease are associated with physical performance in adults over 60 years

Objective

The purpose of this study was to determine the association between different stages of chronic kidney disease (CKD) and sarcopenia and its components in the Chinese older population.

Methods

The study comprised of 2,213 participants aged ≥ 60 years (1,025 men; mean age: 70.7 years) recruited from Shanghai who were invited to participate in a comprehensive geriatric assessment. Sarcopenia was defined according to the AWGS 2019 consensus update on sarcopenia diagnosis criteria. The glomerular filtration rate (GFR) was estimated using the equation that originated from the CKD-EPI equation, the stages of CKD are classified according to the Kidney Disease-Improving Global Outcomes (KDIGO).

Results

The overall prevalence of sarcopenia was 19.0%, which increased with the severity of CKD. The prevalence of sarcopenia in patients with CKD 3-4 and kidney failure was significantly higher than that in normal and CKD 1-2 (p < 0.05). In logistic regression analysis model, compared with normal and CKD 1 patients, kidney failure was significantly associated with the increased risk of sarcopenia and low grip strength (p < 0.05); CKD 2, CKD 3-4 and kidney failure groups were significantly associated with an increased risk of low walking speed (p < 0.05), respectively; while the association between CKD and muscle mass was not shown.

Conclusions

In our study, only decreased physical performance, as represented by walking speed, was significantly associated with increased CKD severity. This may improve the evidence for the prevention and intervention of sarcopenia in patients with CKD.

Impaired muscle mitochondrial energetics is associated with uremic metabolite accumulation in chronic kidney disease

Chronic kidney disease (CKD) causes progressive skeletal myopathy involving atrophy, weakness, and fatigue. Mitochondria have been thought to contribute to skeletal myopathy; however, the molecular mechanisms underlying muscle metabolism changes in CKD are unknown. We employed a comprehensive mitochondrial phenotyping platform to elucidate the mechanisms of skeletal muscle mitochondrial impairment in mice with adenine-induced CKD. CKD mice displayed significant reductions in mitochondrial oxidative phosphorylation (OXPHOS), which was strongly correlated with glomerular filtration rate, suggesting a link between kidney function and muscle mitochondrial health. Biochemical assays uncovered that OXPHOS dysfunction was driven by reduced activity of matrix dehydrogenases. Untargeted metabolomics analyses in skeletal muscle revealed a distinct metabolite profile in CKD muscle including accumulation of uremic toxins that strongly associated with the degree of mitochondrial impairment. Additional muscle phenotyping found CKD mice experienced muscle atrophy and increased muscle protein degradation, but only male CKD mice had lower maximal contractile force. CKD mice had morphological changes indicative of destabilization in the neuromuscular junction. This study provides the first comprehensive evaluation of mitochondrial health in murine CKD muscle to our knowledge and uncovers several unknown uremic metabolites that strongly associate with the degree of mitochondrial impairment.

Chronic kidney disease exacerbates ischemic limb myopathy in mice via altered mitochondrial energetics

Chronic kidney disease (CKD) substantially increases the severity of peripheral arterial disease (PAD) symptomology, however, the biological mechanisms remain unclear. The objective herein was to determine the impact of CKD on PAD pathology in mice. C57BL6/J mice were subjected to a diet-induced model of CKD by delivery of adenine for six weeks. CKD was confirmed by measurements of glomerular filtration rate, blood urea nitrogen, and kidney histopathology. Mice with CKD displayed lower muscle force production and greater ischemic lesions in the tibialis anterior muscle (78.1 ± 14.5% vs. 2.5 ± 0.5% in control mice, P < 0.0001, N = 5-10/group) and decreased myofiber size (1661 ± 134 μm2 vs. 2221 ± 100 μm2 in control mice, P < 0.01, N = 5-10/group). This skeletal myopathy occurred despite normal capillary density (516 ± 59 vs. 466 ± 45 capillaries/20x field of view) and limb perfusion. CKD mice displayed a ~50-65% reduction in muscle mitochondrial respiratory capacity in ischemic muscle, whereas control mice had normal mitochondrial function. Hydrogen peroxide emission was modestly higher in the ischemic muscle of CKD mice, which coincided with decreased oxidant buffering. Exposure of cultured myotubes to CKD serum resulted in myotube atrophy and elevated oxidative stress, which were attenuated by mitochondrial-targeted therapies. Taken together, these findings suggest that mitochondrial impairments caused by CKD contribute to the exacerbation of ischemic pathology.

Uremic metabolites impair skeletal muscle mitochondrial energetics through disruption of the electron transport system and matrix dehydrogenase activity

Chronic kidney disease (CKD) leads to increased skeletal muscle fatigue, weakness, and atrophy. Previous work has implicated mitochondria within the skeletal muscle as a mediator of muscle dysfunction in CKD; however, the mechanisms underlying mitochondrial dysfunction in CKD are not entirely known. The purpose of this study was to define the impact of uremic metabolites on mitochondrial energetics. Skeletal muscle mitochondria were isolated from C57BL/6N mice and exposed to vehicle (DMSO) or varying concentrations of uremic metabolites: indoxyl sulfate, indole-3-acetic-acid, l-kynurenine, and kynurenic acid. A comprehensive mitochondrial phenotyping platform that included assessments of mitochondrial oxidative phosphorylation (OXPHOS) conductance and respiratory capacity, hydrogen peroxide production (JH2O2), matrix dehydrogenase activity, electron transport system enzyme activity, and ATP synthase activity was employed. Uremic metabolite exposure resulted in a ~25-40% decrease in OXPHOS conductance across multiple substrate conditions (P < 0.05, n = 5-6/condition), as well as decreased ADP-stimulated and uncoupled respiratory capacity. ATP synthase activity was not impacted by uremic metabolites; however, a screen of matrix dehydrogenases indicated that malate and glutamate dehydrogenases were impaired by some, but not all, uremic metabolites. Assessments of electron transport system enzymes indicated that uremic metabolites significantly impair complex III and IV. Uremic metabolites resulted in increased JH2O2 under glutamate/malate, pyruvate/malate, and succinate conditions across multiple levels of energy demand (all P < 0.05, n = 4/group). Disruption of mitochondrial OXPHOS was confirmed by decreased respiratory capacity and elevated superoxide production in cultured myotubes. These findings provide direct evidence that uremic metabolites negatively impact skeletal muscle mitochondrial energetics, resulting in decreased energy transfer, impaired complex III and IV enzyme activity, and elevated oxidant production.

Association of renal function with ambulation in mild acute stroke patients

Background: Renal dysfunction has affected the functional outcome after stroke. However, the association of renal function with walking endurance after stroke is poorly understood. Objectives: This study aimed to investigate the relationship between renal function and walking endurance and speed in mild acute stroke patients. Methods: Eighty-nine patients with mild acute stroke were enrolled. Walking endurance and speed were assessed by the 6-minute walk test (6MWT) and comfortable and maximal 10-meter walk tests (10MWT) within 7 days of hospital admission. Stroke severity was assessed using the National Institutes of Health stroke scale (NIHSS) on admission. The estimated glomerular filtration rate (eGFR) was calculated based on creatinine levels as a renal function. Pearson's correlation coefficients were calculated between eGFR and walking ability. Multivariate regression analysis was used to investigate the relationship between eGFR and walking ability in mild acute stroke patients. Results: The 6MWT distance was significantly correlated with eGFR (r = 0.212, p = .046). On multivariate regression analysis, the 6MWT was significantly associated with age (p = .029), body mass index (p = .020), NIHSS score (p = .016), and eGFR (p = .028), whereas the comfortable 10MWT was significantly associated with the NIHSS score alone (p = .009) and the maximal 10MWT was significantly associated with age (p = .032) and NIHSS score (p = .007). Conclusion: The eGFR based on creatinine levels of acute stroke patients may be important factor to predict the walking endurance in mild acute stroke patients.

Kidney function, gait pattern and fall in the general population: a cohort study

Background

Gait disturbance is proposed as a mechanism for higher risk of fall in kidney disease patients. We investigated the association of kidney function with gait pattern in the general population and tested whether the association between impaired kidney function and fall is more pronounced in subjects with lower gait function.

Methods

We included 1430 participants (mean age: 60 years) from the Rotterdam Study. Kidney function was assessed using estimated glomerular filtration rate (eGFR) and albumin-to-creatinine ratio (ACR). We assessed global gait, gait velocity and seven independent gait domains: Rhythm, Phases, Variability, Pace, Tandem, Turning and Base of Support. Regression models adjusted for cardiometabolic and neurological factors were used. We evaluated whether participants with impaired kidney function and impaired gait fell more in the previous year.

Results

The study population had a median (interquartile range) ACR of 3.6 (2.5-6.2) mg/g and mean ± SD eGFR of 87.6 ± 15 mL/min/1.73 m2. Higher ACR and lower eGFR were associated with lower global gait score [per doubling of ACR: -0.10, 95% confidence interval (CI): -0.14 to -0.06, and per SD eGFR:-0.09, 95% CI: -0.14 to -0.03] and slower gait speed (ACR: -1.44 cm/s, CI: -2.12 to -0.76; eGFR: -1.55 cm/s, CI: -2.43 to -0.67). Worse kidney function was associated with lower scores in Variability domain. The association between impaired kidney function and history of fall was present only in participants with lower gait scores [odds ratio (95% CI): ACR: 1.34 (1.09-1.65); eGFR: 1.58 (1.07-2.33)].

Conclusions

We observed a graded association between lower kidney function and impaired gait suggesting that individuals with decreased kidney function, even at an early stage, need to be evaluated for gait abnormalities and might benefit from fall prevention programmes.

Characterising skeletal muscle haemoglobin saturation during exercise using near-infrared spectroscopy in chronic kidney disease

Background

Chronic kidney disease (CKD) patients have reduced exercise capacity. Possible contributing factors may include impaired muscle O₂ utilisation through reduced mitochondria number and/or function slowing the restoration of muscle ATP concentrations via oxidative phosphorylation. Using near-infrared spectroscopy (NIRS), we explored changes in skeletal muscle haemoglobin/myoglobin O₂ saturation (SMO₂%) during exercise.

Methods

24 CKD patients [58.3 (± 16.5) years, eGFR 56.4 (± 22.3) ml/min/1.73 m²] completed the incremental shuttle walk test (ISWT) as a marker of exercise capacity. Using NIRS, SMO₂% was measured continuously before, during, and after (recovery) exercise. Exploratory differences were investigated between exercise capacity tertiles in CKD, and compared with six healthy controls.

Results

We identified two discrete phases; a decline in SMO₂% during incremental exercise, followed by rapid increase upon cessation (recovery). Compared to patients with low exercise capacity [distance walked during ISWT, 269.0 (± 35.9) m], patients with a higher exercise capacity [727.1 (± 38.1) m] took 45% longer to reach their minimum SMO₂% (P = .038) and recovered (half-time recovery) 79% faster (P = .046). Compared to controls, CKD patients took significantly 56% longer to recover (i.e., restore SMO₂% to baseline, full recovery) (P = .014).

Conclusions

Using NIRS, we have determined for the first time in CKD, that favourable SMO₂% kinetics (slower deoxygenation rate, quicker recovery) are associated with greater exercise capacity. These dysfunctional kinetics may indicate reduced mitochondria capacity to perform oxidative phosphorylation—a process essential for carrying out even simple activities of daily living. Accordingly, NIRS may provide a simple, low cost, and non-invasive means to evaluate muscle O₂ kinetics in CKD.

A Chinese herbal formula, Jian-Pi-Yi-Shen decoction, improves muscle atrophy via regulating mitochondrial quality control process in 5/6 nephrectomised rats

Muscle atrophy is one of the serious complications of chronic kidney disease (CKD). Dysregulation of mitochondrial quality control (MQC) process, including decrease mitochondrial biogenesis, impair mitochondrial dynamics and induce activation of mitophagy, play an important role in mediating muscle wasting. This study aimed to observe effects of Jian-Pi-Yi-Shen (JPYS) decoction on muscle atrophy in CKD rats and explore its possible mechanism on regulation of MQC processes. The 5/6 nephrectomised rats were randomly allocated into 2 groups: CKD group and JPYS group. Besides, a sham-operated rats as sham group. All rats were treated for 6 weeks. Results showed that administration of JPYS decoction prevented body weight loss, muscle loss, muscle fiber size decrease, muscle protein degradation, and increased muscle protein systhesis. In addition, JPYS decoction increased the mitochondrial content and biogenesis proteins, and down-regulated the autophagy and mitophagy proteins. Furthermore, JPYS decoction increased mitochondrial fusion proteins, while decreased mitochondrial fission proteins. In conclusion, JPYS decoction increased mitochondrial content and biogenesis, restore the balance between fission and fusion, and inhibited autophagy-lysosome pathway (mitophagy). Collectively, our data showed that JPYS decoction to be beneficial to muscle atrophy in CKD, which might be associated with the modulation of MQC process.