# Single-Cell Transcriptomic Profiling of Longissimus Dorsi and Biceps Femoris Muscles in Kazakh Horses Reveals Cellular Heterogeneity and Myogenic Regulation

## TL;DR

This study uses single-cell RNA sequencing to compare the muscle cells of Kazakh horses, revealing how different muscles are specialized for endurance or explosive movements.

## Contribution

The study provides the first single-cell transcriptomic profiling of equine LD and BF muscles, identifying cell-type-specific metabolic and functional adaptations.

## Key findings

- LD muscle cells show lipid metabolism and energy production adaptations for endurance.
- BF muscle cells exhibit glycolytic energy pathways and contraction-related gene expression for explosive movements.
- Candidate markers like MYL1 and LDHA were identified for potential use in breeding and training.

## Abstract

Kazakh horses are famous for thriving in harsh climates and for outstanding endurance. They rely on different muscles for different jobs: the longissimus dorsi (LD) along the back supports posture and stamina, while the biceps femoris (BF) in the hindlimb powers fast, strong movements. Using single-cell RNA sequencing, we mapped the cell types in LD and BF and asked how they work at the molecular level. We found that LD is tuned for long-lasting activity—shifting toward lipid (fat) metabolism and energy production. By contrast, BF appears primed for quick bursts—showing higher expression of genes for muscle contraction and glycolytic (sugar-based) energy, together with signals of oxidative stress response. These results reveal clear, cell-level specializations of two key equine muscles and suggest candidate markers (e.g., MYL1, LDHA) that may inform training and selective breeding for performance and health in Kazakh horses. As this study analyzed three male horses, future work with larger, diverse cohorts and targeted validation (e.g., qPCR) will further strengthen these insights.

Kazakh horses are renowned for their endurance and adaptability, with distinct muscle groups such as the longissimus dorsi (LD) and biceps femoris (BF) muscles serving specialized functions. However, the molecular mechanisms underlying the functional specialization of these muscles in Kazakh horses remain poorly understood. This study aims to address this gap by utilizing single-cell RNA sequencing (scRNA-seq) to investigate the transcriptomic differences between these muscle groups, with a focus on understanding their molecular adaptations. Our analysis revealed that the BF muscle, specialized for explosive movements, exhibited upregulation of genes associated with anaerobic metabolism, muscle contraction, and oxidative stress response, reflecting its reliance on glycolysis for sustained energy production. In contrast, the LD muscle, primarily responsible for postural support and endurance, showed a metabolic shift toward lipid utilization and energy production. Differential gene expression analysis also revealed distinct enrichment in biological pathways, with LD cells being enriched in pathways related to muscle contraction and calcium signaling, while BF cells were enriched in energy metabolism pathways. These findings provide valuable insights into the molecular adaptations of Kazakh horses’ muscle tissues, highlighting the functional specialization of LD and BF muscles and offering a foundation for future research on improving muscle performance and breeding programs in equines.