Aging induced structural alterations in SR-Mitochondria interaction in skeletal muscle: Emerging insights

Reviewer #1 (Public review):

Summary:

This manuscript is a narrative review addressing age-related alterations in SR-mitochondria interactions in skeletal muscle and their contribution to sarcopenia. It synthesizes existing literature on calcium signaling, mitochondrial dynamics, redox balance, and structural remodeling, and discusses potential interventions including exercise and pharmacological strategies. While the topic is timely and relevant, the manuscript largely reiterates established concepts without providing sufficient conceptual novelty, critical synthesis, or mechanistic insight beyond the current literature.

Strengths:

(1) Timely topic: The focus on SR-mitochondria communication in aging muscle is relevant and of growing interest.

(2) Broad coverage: The review compiles a wide range of literature spanning calcium handling, mitochondrial biology, ROS signaling, and exercise physiology.

(3) Clear organization: The manuscript is structured logically with thematic sections (SR, mitochondria, MAMs, aging, interventions).

(4) Didactic value: Could serve as a general overview for non-specialists entering the field.

Weaknesses:

(1) Lack of novelty and conceptual advance: The manuscript does not offer new hypotheses, frameworks, or critical reinterpretation of the field. Most statements summarize already well-established knowledge, and no unifying model or novel perspective is developed to justify publication in a high-impact journal like eLife.

(2) Limited critical analysis: The review is predominantly descriptive rather than analytical. Conflicting findings (e.g., MFN2 roles, MAM density changes, Ca²⁺ overload vs deficiency) are mentioned but not critically evaluated or reconciled. There is little discussion of limitations in the cited studies or gaps in the field.

(3) Overgeneralization and speculative claims: Several assertions are presented with insufficient nuance (e.g., causal links between MAM disruption and sarcopenia, or therapeutic efficacy of interventions). The distinction between correlation and causation is often unclear, reducing scientific rigor.

(4) Insufficient depth for a specialist audience: Despite its length, the manuscript lacks mechanistic depth in key areas (e.g., precise molecular regulation of MAMs in vivo, tissue-specific differences, quantitative aspects of Ca²⁺ flux). It reads more like a textbook summary than a high-level scholarly review.

(5) Redundancy and verbosity: Many sections repeat similar concepts (Ca²⁺ dysregulation, ROS effects, mitochondrial dysfunction) without adding new insight, leading to an unnecessarily long manuscript with limited added value.

(6) Weak integration of recent literature into a coherent narrative: Although many references are cited, they are not effectively synthesized into a cohesive argument. The manuscript lacks a strong central thesis or clearly defined take-home messages.

(7) Limited translational or experimental perspective: The section on therapeutic targeting is largely speculative and does not critically assess feasibility, limitations, or current clinical evidence.

Reviewer #2 (Public review):

This review addresses a highly relevant and timely topic, namely the role of sarcoplasmic reticulum-mitochondria communication and mitochondria-associated membranes (MAMs) in skeletal muscle aging. The manuscript successfully brings together literature from several interconnected fields, including calcium signaling, mitochondrial biology, excitation-contraction coupling, muscle metabolism, and sarcopenia. Given the growing interest in organelle crosstalk as a determinant of muscle health and disease, the topic is undoubtedly of considerable interest to the readership and has the potential to make a valuable contribution to the field.

However, in its current form, the manuscript devotes a substantial proportion of its content to the description of well-established concepts that are already extensively covered in the literature. Large sections are dedicated to general skeletal muscle physiology, excitation-contraction coupling, calcium handling, mitochondrial biology, and MAM structure and composition. While this background information is useful, the level of detail is often excessive for a review that aims to focus on aging-induced alterations in SR-mitochondria interactions. As a consequence, the central theme of the manuscript becomes diluted, and the review reads more like a broad overview of skeletal muscle physiology than a focused analysis of aging-related MAM remodeling.

In contrast, the sections specifically dedicated to aging and MAM dysfunction, which represent the most novel and potentially impactful aspects of the review, are comparatively brief and largely descriptive. The discussion of how aging alters MAM architecture, calcium microdomains, mitochondrial calcium signaling, and organelle communication would benefit from substantially greater depth. For example, although the manuscript highlights alterations in proteins such as MFN2, IP3R, VDAC, and MCU, the mechanistic implications of these changes for sarcopenia and age-associated muscle dysfunction are not critically developed. Similarly, the review would be strengthened by a more comprehensive discussion of the evidence linking MAM disruption to impaired muscle performance, metabolic inflexibility, denervation, and mitochondrial dysfunction during aging.

Another limitation is that much of the manuscript summarizes published findings without sufficiently evaluating the strength of the evidence or discussing existing controversies. Several statements imply causal relationships between MAM disruption and sarcopenia, whereas in many cases, the available data remain largely correlative. The authors should more clearly distinguish between established mechanisms, experimental observations, and emerging hypotheses. A more critical assessment of conflicting findings, particularly regarding the role of MFN2 and the dual consequences of altered mitochondrial calcium uptake, would considerably improve the scientific rigor of the review.

A major omission concerns the role of mitochondrial Ca²⁺ uptake in skeletal muscle physiology and aging. Throughout the manuscript, mitochondrial Ca²⁺ uptake is presented as a central determinant of muscle function and as a key mechanism linking MAM disruption to sarcopenia. However, the authors do not adequately discuss evidence that challenges this view. In particular, genetic mouse models lacking MCU exhibit surprisingly mild skeletal muscle phenotypes under basal conditions despite a near-complete abolition of rapid mitochondrial Ca²⁺ uptake. These findings have generated considerable debate regarding the physiological importance of mitochondrial Ca²⁺ uptake for muscle function and metabolic regulation. While MCU deletion clearly affects exercise adaptation and certain stress responses, the relatively modest baseline phenotype suggests the existence of compensatory pathways and raises important questions regarding the extent to which impaired mitochondrial Ca²⁺ uptake alone can explain age-associated muscle dysfunction. A balanced review should acknowledge these observations and discuss the ongoing debate regarding the relative contributions of mitochondrial Ca²⁺ deficiency versus mitochondrial Ca²⁺ overload in aging skeletal muscle.

Similarly, the discussion of MFN2 would benefit from greater nuance. The manuscript largely presents MFN2 as a structural tether linking the sarcoplasmic reticulum and mitochondria. However, MFN2 is a multifunctional protein with well-established roles in mitochondrial fusion, mitochondrial network organization, mitophagy regulation, and metabolic signaling. Consequently, many of the phenotypes associated with altered MFN2 expression cannot be unequivocally attributed to changes in MAM formation. The review does not sufficiently distinguish between the effects of MFN2 on organelle tethering and its effects on mitochondrial dynamics. This distinction is particularly important because several studies have questioned whether MFN2 acts primarily as a positive tether, a negative regulator of contacts, or whether its influence on organelle communication is secondary to its role in controlling mitochondrial morphology. As a result, attributing age-related alterations in SR-mitochondria communication solely to changes in MFN2-mediated tethering may oversimplify a considerably more complex biological scenario.

The manuscript's organization could also be improved. The sections discussing aging-related alterations, mitochondrial dysfunction, calcium dysregulation, oxidative stress, and therapeutic interventions contain significant overlap and repetition. Streamlining some background sections and reallocating space to a more detailed discussion of aging-specific mechanisms would help maintain focus and improve readability. In particular, the manuscript would benefit from expanding the sections on aging-induced MAM remodeling, age-dependent changes in MAM composition and ultrastructure, and the potential of MAM-targeted interventions as therapeutic strategies for sarcopenia.

Finally, the review would gain from a stronger future perspectives section. Several important questions remain unresolved, including whether MAM disruption is a primary driver of muscle aging or a secondary consequence of mitochondrial dysfunction, how MAM architecture differs among muscle fiber types during aging, and whether MAM-associated proteins could serve as reliable biomarkers or therapeutic targets in human sarcopenia. Highlighting these knowledge gaps would further enhance the review's impact.

Overall, the manuscript covers an important and emerging area of research and contains a valuable compilation of the relevant literature. Nevertheless, substantial revision is required to reduce the emphasis on well-established background information, deepen and critically analyze the aging-specific sections, and provide a more focused discussion of the role of MAMs in skeletal muscle aging and sarcopenia.