BMAL1 Proteostasis, Circadian Dysfunction, and Ferroptotic Vulnerability in Osteoporosis: Current Evidence and Experimental Priorities.

Osteoporosis is increasingly recognized as a disorder of impaired skeletal adaptation in which endocrine stress, metabolic injury, altered mechanotransduction, remodeling imbalance, and redox vulnerability converge. Bone is also a circadian tissue, and ferroptosis is now directly implicated in skeletal cell dysfunction across several osteoporosis-related settings. This review examines whether proteostatic instability of brain and muscle ARNT-like 1 (BMAL1) could represent a mechanistically relevant link between these processes. To keep interpretation proportionate to the available data, bone-established evidence is distinguished from extra-skeletal bridge mechanisms and disease-contextual extrapolations that remain unproven in bone. Current evidence supports three conclusions. First, circadian regulation in bone extends beyond BMAL1 transcript abundance and must be interpreted within the broader clock network. Second, ferroptosis is directly established in osteoblasts and osteocytes across postmenopausal, glucocorticoid-related, diabetic, iron-overload, and unloading-related contexts. Third, BMAL1 is functionally important for skeletal differentiation and stress adaptation, but selective BMAL1 degradation and clockophagy have not yet been demonstrated in skeletal cells. Taken together, the literature does not justify describing clockophagy as an established skeletal pathway, but it does support a testable framework for defining when BMAL1 proteostasis may become pathogenic in osteoporosis and which experiments could most decisively validate or refute that possibility.