Background/Objectives: Peroxisome proliferator-activated receptor gamma (PPARγ) is a fatty acid-binding transcription activator of the adipokine chemerin. The key role of PPARγ in adipogenesis was established by reports on adipose tissue-resident macrophages that express PPARγ. The present study examined PPARγ⁺ macrophages in human skeletal muscle tissues, their response to fatty acid (FA) species, and their correlations with age, obesity, adipokine expression, and an abundance of other macrophage phenotypes. Methods: An ex vivo human skeletal muscle model with surgical specimens that were maintained without or with FAs for up to 11 days was utilized. Immunofluorescence analysis was used to detect macrophage phenotypes and mitochondrial activity. Preconfigured arrays were used to detect the expression of 34 different adipokines and chemokines. Results: Data from 14 adults revealed that PPARγ⁺ macrophages exclusively reside in intermuscular adipose tissue (IMAT), and their abundance correlates with the metabolic status of surrounding adipocytes during tissue maintenance in vitro for 9–11 days. Elevated fatty acid levels lead to significant increases in PPARγ⁺ populations, which are correlated with the donor’s body mass index (BMI). Conclusions: PPARγ⁺ macrophages represent a distinctly specialized population of regulatory cells that reside within human IMATs in accordance with their metabolic status. Thus, future in-depth studies on IMAT-resident PPARγ⁺ macrophage action mechanisms will elucidate the role of skeletal muscle in the pathogenesis of human metabolic dysfunction.

Neuromuscular junctions (NMJs) are highly specialized synapses that enable efficient communication between motor neurons and skeletal muscle fibers. Impaired formation or maintenance of NMJs is implicated in the pathogenesis of multiple neuromuscular disorders and contributes to age-related declines in skeletal muscle mass and strength. NMJ functionality is governed by complex regulatory crosstalk among different cells and is mediated by a diverse network of proteins. Moreover, immune cells often reside at NMJs and exhibit phenotypically different characteristics depending on the regenerative state of the muscle. These complex interfaces have posed a significant challenge for elucidating pathogenic mechanisms and developing biomarkers or effective targeted treatments. Many animal models have been developed to address this challenge by characterizing the fundamental structural features of neuromuscular junctions (NMJs) and their transmission capacity under both healthy and disease conditions. In contrast, studies of human NMJs remain limited, although emerging evidence is increasingly revealing substantial morphological and functional differences from animal NMJs. This review provides an overview of animal research on NMJs over the past decades, highlighting interspecies differences and key advances in our understanding of human NMJs.

Chronic inflammation and elevated cytokine levels are closely associated with the progression of chronic kidney disease (CKD), which is responsible for the manifestation of numerous complications and mortality. In addition to conventional CKD therapies, the possibility of using natural compounds with anti-inflammatory potential has attracted widespread attention in scientific research. This study aimed to study the potential anti-inflammatory effects of a natural oil compound, farnesol, in primary human renal proximal tubule epithelial cell (RPTEC) culture. Farnesol was encapsulated in lipid-based small unilamellar vesicles (SUVs) to overcome its insolubility in cell culture medium. The cell attachment of empty vesicles (SUVs) and farnesol-loaded vesicles (farnesol-SUVs) was examined using BODIPY, a fluorescent dye with hydrophobic properties. Next, we used multiple protein, RNA, and protein phosphorylation arrays to investigate the impact of farnesol on inflammatory signaling in RPTECs. The results indicated that farnesol inhibits TNF-α/IL-1β-induced phosphorylation of the PI3 kinase p85 subunit and subsequent transcriptional activation of the inflammatory genes TNFRSF9, CD27, TNFRSF8, DR6, FAS, IL-7, and CCL2. Therefore, farnesol may be a promising natural compound for treating CKD.

Phenotypically heterogeneous populations of tissue-resident macrophages and stem cells play important roles in the regeneration of the skeletal muscle tissue. Previous studies using animal and cell culture models implied a beneficial effect of fatty acid (FA) species on tissue regeneration. Here, we applied a human experimental model using excised muscle tissues from reconstructive surgeries to study the effects of FAs on resident macrophages and stem cells in the natural environment of human skeletal muscle tissue. Muscle tissue samples from 20 donors were included in this study. The expression of 34 cytokines/chemokines was determined, using multiplex protein analysis. The phenotypes of macrophages and stem cells were determined immunohistochemically. The numbers of CD80⁺ macrophages correlated with the expression levels of IL-1α, IL-1RA, IL-8, IL-17A, and MCP-1, while the PAX7⁺ and MyoD⁺ stem cell counts were positively correlated with the expression level of CXCL12α, a recognized chemoattractant for muscle stem cells. Treatment of additional tissue sections with FAs revealed that CD80⁺ or MARCO⁺ macrophages- and PAX7⁺ or MyoD⁺ stem cells were simultaneously increased by unsaturated long-chain FAs. Taken together, this is the first experimental demonstration of a coordinated activation of macrophages and stem cells in human skeletal muscle tissue.

Human skeletal muscle contains different types of tissues with skeletal muscle fibers (SMFs) and intermuscular adipose tissues (IMATs) as the main components. We maintained human skeletal muscle tissues from 12 study participants under native conditions in vitro for 11 days to investigate the dynamics of macrophages that reside in adjacent IMATs and SMFs simultaneously. The samples were subjected to immunohistochemical analysis for macrophage phenotyping and mitochondrial mass assessment before and after maintenance in vitro. Multiplex protein analysis was used to determine cytokine/chemokine expression in tissue extracts. The results revealed significant correlations between donor age or body mass index (BMI) and distinct phenotypes of resident macrophages in SMFs and IMATs. The dynamics of SMF- and IMAT-resident macrophages differed significantly in vitro and exhibited inverse correlations with chemokine/cytokine expression levels and mitochondrial activity. Moreover, the responses of macrophages to saturated and unsaturated fatty acids (FAs) differed substantially between SMFs and IMATs. These findings showed the functional diversity of phenotypically identical macrophages in adjacent niches. Thus, the currently available macrophage markers cannot capture the functional diversity of human tissue-resident macrophages. The model used in the present study may help elucidate how macrophages affect muscle homeostasis and disease in humans.

Neuromuscular junctions (NMJs) connect motor neurons to muscle fibers, enabling electrical-to-chemical signal transmission and sensing of mechanical forces. We employed a previously introduced human skeletal muscle tissue model to study the NMJ presynaptic terminal (PT). Using immunohistochemical approaches, we analyzed PT formation in samples from 12 participants, either immediately after surgery or following 11 days of in vitro maintenance. In immediate postsurgical samples, the number, size, and integrated immunofluorescence intensity of detectable PTs increased with participant age, accompanied by elevated expression of neurotrophic and inflammatory proteins. Moreover, CD80⁺ and CD206⁺ macrophages were detected near PTs, suggesting a potential regulatory role in PT formation. Prolonged in vitro maintenance of muscle samples in a quiescent state for 11 days reproduced age-associated PT features and further increased PT spatial expansion and fragmentation. Notably, increased PT fragmentation positively correlated with participant body mass index (BMI), suggesting a possible link between metabolic status and PT remodeling. Consistently, the activation of skeletal muscle metabolism through long-chain fatty acid supplementation partially restored PT formation in vitro. Together, these findings highlight the utility of human skeletal muscle tissue models for investigating NMJ dynamics in pathological contexts and for identifying therapeutic strategies aimed at delaying or preventing neuromuscular decline.