Hypoxia-conditioned BMSC exosomes improve short-term spinal cord injury outcomes via the miR-615-3p/PDE4C-mediated cAMP/PKA pathway

Abstract

Spinal cord injury (SCI) remains a significant global health challenge with limited effective therapeutic options. Exosomes derived from mesenchymal stem cells (MSCs) have emerged as promising neuroprotective agents due to their biocompatibility and immunomodulatory properties. This study investigated the therapeutic potential of hypoxia-conditioned bone marrow MSC (BMSC)-derived exosomes in both in vitro and in vivo SCI models. Hypoxic preconditioning significantly enriched miR-615-3p in BMSC-derived exosomes. In spinal neuron injury models, hypoxic exosomes enhanced cell viability, reduced apoptosis, and ameliorated dysfunction of the mitochondria-associated endoplasmic reticulum membranes (MAMs). Mechanistically, miR-615-3p directly targeted and suppressed phosphodiesterase 4C (PDE4C), activating the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway. This in turn modulated calcium signaling, attenuated mitochondrial calcium overload, and reduced endoplasmic reticulum stress (ERS). In a mouse model of SCI, short-term treatment with hypoxic exosomes promoted functional recovery within a 14-day post-injury period, as evidenced by improved locomotor performance, reduced lesion volume, attenuated tissue edema, and decreased inflammatory cell infiltration. Furthermore, in vivo administration of hypoxic exosomes upregulated miR-615-3p and downregulated PDE4C expression in injured spinal cord tissues. These results demonstrate that hypoxia-conditioned BMSC-derived exosomes exert neuroprotective effects via the miR-615-3p/PDE4C axis, highlighting their potential as a novel therapeutic strategy for SCI by targeting calcium homeostasis and mitochondrial-ER dysfunction. These findings demonstrate the short-term therapeutic potential of hypoxia-conditioned exosomes in SCI. However, further preclinical studies, including long-term follow-up to assess the durability of recovery and potential late-onset effects, alongside clinical validation, are warranted before clinical translation.

1. Introduction

Spinal cord injury (SCI) is a devastating neurological condition with an annual global incidence of approximately 900,000 cases, leading to severe motor, sensory, and autonomic deficits that impose a profound burden on individuals and society [1–3]. The pathophysiology of SCI encompasses both primary mechanical damage and a complex secondary injury cascade. This secondary phase is characterized by a self-perpetuating cycle of neuroinflammation, oxidative stress, and apoptotic signaling, which collectively exacerbate tissue destruction and impede axonal regeneration [4, 5]. Key cellular events include microglial and astrocytic activation, driven partly by immunogenic myelin debris from white matter injury, which amplifies the inflammatory response [6, 7]. Recent evidence further underscores the critical role of mitochondrial-associated endoplasmic reticulum membranes (MAMs), which serve as hubs for calcium signaling and redox homeostasis. Dysfunctional mitochondria-associated endoplasmic reticulum membranes (MAMs) contribute to endoplasmic reticulum stress (ERS), NLRP3 inflammasome activation, and neuronal apoptosis [8–10]. Microglial NLRP3 signaling has been identified as a key mediator of remote neuronal damage post-SCI [11], while non-coding RNAs fine-tune glial activity, adding layers of regulatory complexity [12, 13].

Current therapeutic strategies, including pharmacotherapy, surgery, and rehabilitation, offer limited neurological recovery, particularly in elderly populations, and are often associated with significant adverse effects [14, 15]. This underscores the urgent need for innovative and more effective treatments. In recent years, extracellular vesicles (EVs), especially exosomes derived from mesenchymal stem cells (MSCs), have emerged as promising therapeutic agents due to their biocompatibility, low immunogenicity, and ability to shuttle bioactive molecules, including proteins and nucleic acids, to recipient cells [16]. MSC-derived exosomes can modulate neuroregeneration, suppress inflammation, and promote angiogenesis, demonstrating significant neuroprotective potential [17, 18]. However, challenges related to sourcing, purity, and therapeutic consistency hinder their clinical translation.