Mitochondrial Protein SYNJ2BP Emerges as Candidate Driver of Muscle Wasting in Experimental Models

A protein best known as a mitochondrial outer-membrane resident has moved to the center of an effort to understand why skeletal muscle shrinks when it is injured or unused. In a study published in Frontiers in Cell and Developmental Biology, researchers in Yangzhou, China report that synaptojanin 2 binding protein, or SYNJ2BP, becomes more abundant in atrophied human supraspinatus muscle, rises again in a mouse model of tendon injury, and, when experimentally manipulated in mice, changes the course of disuse-related muscle wasting. The findings are associative rather than definitive, but they tie together several threads—mitochondrial damage, calcium signaling, and muscle protein turnover—that have long been studied separately.

The human portion of the work began in the operating room. The team collected small samples of supraspinatus muscle, one of the rotator cuff muscles, from patients undergoing arthroscopic surgery at Subei People’s Hospital. Preoperative magnetic resonance imaging allowed the investigators to classify each sample as control or atrophic based on the ratio of the muscle’s cross-sectional area to the area of the supraspinatus fossa, the bony depression in which the muscle sits. Proteomic profiling of three control and three atrophic samples identified increased SYNJ2BP abundance in the atrophic tissue. Immunohistochemistry and western blotting on additional samples confirmed the higher expression, and the atrophic muscle also showed reduced levels of the myogenic proteins MYOD, MYOG, and MYHC.

Enrichment analysis of the proteomic data pointed toward mitochondria. Protein sets related to mitochondrial organization, respiration, and oxidative phosphorylation were downregulated in the atrophic samples, and gene set enrichment analysis showed positive enrichment of a calcium ion binding set. Western blotting revealed higher abundance of IP3R1, GRP75, and VDAC1—the components of the calcium-transfer machinery that spans contacts between the endoplasmic reticulum and mitochondria—in atrophic supraspinatus muscle from both humans and mice. The authors are careful to note that enrichment analysis alone does not establish mitochondrial dysfunction or altered channel activity, but the pattern suggested that calcium handling and mitochondrial biology deserve closer scrutiny in this context.

To test whether the same expression change appears in an animal model, the researchers transected the supraspinatus tendon in one shoulder of twelve male C57BL/6J mice, using the contralateral shoulder as a sham control. Transection produced a measurable reduction in muscle size, and hematoxylin and eosin staining showed smaller muscle fiber diameters in the transected group. SYNJ2BP abundance rose in the mouse supraspinatus muscle after tendon injury, mirroring the human finding, and the same drop in MYOD, MYOG, and MYHC accompanied it. The two species’ data are observational and complementary rather than proof of a shared mechanism, but they motivated the team to probe what SYNJ2BP actually does inside cells.

That probing took place in C2C12 mouse myoblasts, a standard laboratory cell line. Cells engineered to overexpress SYNJ2BP showed lower MYOD, MYOG, and MYHC abundance and higher levels of Atrogin-1 and MuRF-1, two proteins associated with muscle atrophy. Transmission electron microscopy revealed disrupted cristae, vacuolization, enlarged mitochondrial profiles, and fewer mitochondrial profiles in the overexpressing cells. ATP content fell, and abundance of NDUFAB1, PDHA1, MFN2, and DRP1—proteins tied to mitochondrial energetics and dynamics—also decreased. Total cytochrome c and cleaved caspase-3 abundance rose, although the authors emphasize that total cytochrome c measurement does not demonstrate its release into the cytosol.

Calcium-associated fluorescence, measured with Rhod-2 AM by flow cytometry and microscopy, was higher in SYNJ2BP-overexpressing cells than in controls. The authors interpret this cautiously: Rhod-2 fluorescence alone does not establish mitochondrial calcium overload, nor does it resolve the direction or rate of calcium transfer from the endoplasmic reticulum to mitochondria. A further complication is technical. The expression vector encodes a green fluorescent protein, gcGFP, which may contribute to green-channel signals in the ROS and membrane-potential assays. Without suitable fluorescence controls or independent validation, the reported changes in reactive oxygen species and mitochondrial membrane potential cannot be taken as established.

The team also explored whether SYNJ2BP physically associates with the calcium-transfer complex. Protein docking predicted a candidate SYNJ2BP–VDAC1 interface with an interface solvation free-energy change of −7.9 kcal/mol, a computational value that describes a structural hypothesis rather than a measured binding affinity. Fluorescence microscopy showed overlap between SYNJ2BP or VDAC1 signals and mitochondrial staining. In VDAC1 immunoprecipitates, fold recoveries relative to control were 1.54 for VDAC1, 2.17 for IP3R1, 1.51 for GRP75, and 2.42 for SYNJ2BP, yielding partner-to-VDAC1 recovery ratios of approximately 1.41, 0.98, and 1.57. Notably, GRP75 recovery relative to VDAC1 was close to the control value, which limits evidence for enhanced assembly of the entire complex. The authors stress that these descriptive ratios do not account for input protein abundance and do not establish stronger relative association, binding affinity, or complex stoichiometry.

The most striking results came from a separate mouse model of disuse. Twenty-four four-week-old male mice were divided into four groups of six: unrestricted controls, mice whose left hindlimb was immobilized in a plaster cast for six weeks, and two immobilized groups that also received local adeno-associated viral delivery of either a SYNJ2BP-overexpression or a SYNJ2BP-knockdown vector into the tibialis anterior and gastrocnemius muscles. Overexpression worsened the picture: greater mitochondrial swelling and cristae disruption, lower NDUFAB1, PDHA1, MFN2, and DRP1 abundance, higher total cytochrome c and cleaved caspase-3, smaller muscles, and reduced grip force, maximum treadmill running time, and maximum running distance. Knockdown moved every one of these measures in the opposite direction, with better-preserved cristae, more mitochondrial profiles, larger gastrocnemius and tibialis anterior muscles, and improved physical performance relative to immobilization alone.

The authors themselves catalog the caveats that keep these results firmly in the category of hypothesis-generating. The human proteomic cohort was small and observational, with three samples per group. The AAV experiments used saline as the comparator, so without matched empty-vector and non-targeting shRNA controls, vector-related and sequence-specific off-target effects cannot be fully separated from SYNJ2BP manipulation. Supraphysiological overexpression may induce nonspecific cellular stress. VBIT-4, a compound originally developed to interact with VDAC1 and inhibit oligomerization-associated apoptosis, attenuated several overexpression-induced changes in the C2C12 experiments, but it also reduced SYNJ2BP abundance itself, so the rescue cannot be assigned exclusively to VDAC1 inhibition or selective disruption of the IP3R1/GRP75/VDAC1 complex. The tendon-transection and immobilization models differ in anatomical site, age at entry, inflammation, and mechanical loading, and directionally consistent findings across them support broader relevance without proving a common mechanism.

What the study delivers, then, is a well-documented association and a concrete set of next questions. SYNJ2BP abundance is higher in atrophic supraspinatus muscle in two species, and reducing it locally in immobilized mouse hindlimbs was associated with preserved mitochondrial morphology, less muscle wasting, and better physical performance. Prior work cited by the authors shows that SYNJ2BP’s role is context-dependent: overexpression increases contacts between mitochondria and rough endoplasmic reticulum in some systems, reducing it improves mitochondrial oxidative function in disease-associated motor neurons, and it has reported protective effects in nucleus pulposus cells. Whether the IP3R1/GRP75/VDAC1-associated calcium pathway mediates the phenotypes observed here remains untested. The authors call for independent genetic perturbation of complex components, direct measurement of compartment-specific calcium dynamics, and establishment of physiological expression relevance before SYNJ2BP can be considered a validated target for preserving muscle structure and function in human disease.

Subject of Research: The role of the mitochondrial protein SYNJ2BP in skeletal muscle atrophy and mitochondrial dysfunction in human tissue, cultured cells, and mouse models.

Article Title: SYNJ2BP is associated with mitochondrial abnormalities and skeletal muscle atrophy in experimental models

Article References: Zhang, J., Yang, Y., Meng, X., Wang, X., Dai, J., Zhang, P., Wang, J., & Fei, W. (2026). SYNJ2BP is associated with mitochondrial abnormalities and skeletal muscle atrophy in experimental models. Frontiers in Cell and Developmental Biology, 14, 1996512. https://doi.org/10.3389/fcell.2026.1996512

Image Credits: AI Generated

DOI: 10.3389/fcell.2026.1996512

Keywords: SYNJ2BP, skeletal muscle atrophy, mitochondrial dysfunction, VDAC1, IP3R1, GRP75, calcium signaling, supraspinatus, muscle wasting, C2C12 cells, hindlimb immobilization, rotator cuff injury