Mitochondrial biogenesis for the treatment of spinal cord injury

Introduction

Neuronal integrity is dependent on mitochondrial homeostasis and function, resulting in the central nervous system (CNS) being particularly sensitive to mitochondrial dysfunction ( ). Spinal cord injury (SCI) results in an intricate pathology involving heterogeneous cell types with unique roles in injury and recovery. Following SCI, mitochondria are dysfunctional, resulting in an array of consequences including decreased mitochondrial respiration and adenosine triphosphate (ATP) production, depolarization of the mitochondrial membrane, mitochondrial DNA (mtDNA) fragmentation, oxidative stress, compromised calcium homeostasis, altered mitochondrial dynamics, and cell death ( ). These cellular dysfunctions contribute to the secondary injury cascade of SCI, exacerbating injury, and hindering recovery.

Mitochondrial biogenesis (MB) is an intricate process involving the generation of new, functional mitochondria ( ). In recent years, there has been an increase in published data documenting that pharmacological induction of MB restores mitochondrial and organ function following various pathological events in vivo, including SCI ( ). These findings, in conjunction with the plethora of evidence indicating that restoring mitochondrial homeostasis promotes recovery following SCI, speak to the potential for this treatment strategy. Additionally, in vitro reports have elucidated cell type-specific consequences of mitochondrial dysfunction and MB in SCI-relevant cell types, which could aid in the future development of targeted therapies.

Importantly, there exists an increasing number of U.S. Food and Drug Administration (FDA)-approved pharmaceuticals capable of MB induction, demonstrating the clinical applicability of this approach ( ). This perspective will briefly review and explore CNS cell type-specific mitochondrial dysfunction and MB, as well as describe current therapeutic strategies employing inducers of MB post-SCI.

Mitochondrial dysfunction

Mitochondrial dysfunction results from alterations to homeostatic mitochondrial processes, leading to deficient energy metabolism, primarily through decreased oxidative phosphorylation (OXPHOS) and ATP synthesis. Examples of such alterations include inadequate mitochondrial number and/or mass, altered membrane potential, mitochondrial DNA (mtDNA) mutation/fragmentation, defective electron transport chain (ETC) activity, increased production of reactive oxygen species (ROS), intracellular calcium dysregulation, and impaired mitochondrial dynamics and mitophagy ( ; ).

Due to high energy demand within the CNS, mitochondrial dysfunction and ensuing loss of ATP can prevent the function of various ATPases (H ⁺ , Ca ^{2 +} , Na ⁺ /K ⁺ -ATPase) required for effective neurotransmission, thereby deregulating cellular ion gradients. Mitochondrial dysfunction can also disrupt calcium buffering and signaling, which is crucial for neuronal synapses, leading to calcium overload and excitotoxicity ( ). In SCI pathology, activated astrocytes and glial cells release pro-inflammatory cytokines, resulting in mitochondrial dysfunction and ultimately apoptosis ( ). Additionally, SCI is characterized by vasculature disruption, leading to loss of blood flow and local ischemia, which contributes to oxidative stress, mitochondrial dysfunction and the propagation of cell death observed during secondary injury ( Fig. 1 ) ( ). Therefore, evidence suggests that restoring mitochondrial function could be an effective strategy to mediate multiple facets of injury progression and aid in preventing further cell death.

Mitochondrial biogenesis

Although many studies exist assessing mitochondria-targeted strategies for treatment of SCI, the majority address singular aspects of downstream mitochondrial dysfunction, such as increasing antioxidant defenses or inhibiting opening of the mitochondrial permeability transition pore (mPTP) ( ). In contrast, MB is the production of new, functional mitochondria via the growth and division of pre-existing mitochondria, which could therefore address multiple, if not all, facets of mitochondrial dysfunction ( Fig. 2 ). This complex process involves the cooperation of multiple cellular pathways, requiring the synthesis of mtDNA, transcription and translation of mitochondrial- and nuclear-encoded proteins and ultimately assembly of ETC complexes ( ).

Regulation of MB

Coordination of the nuclear and mitochondrial genomes is modulated by transcriptional coactivators, with the most relevant to MB being the “master regulator” peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α) ( ). Activation of PGC-1α can be initiated via multiple post-translational modifications, including deacetylation by sirtuin-1 (SIRT1) and phosphorylation by adenosine monophosphate-activated kinase (AMPK) and p38-mitogen-activated protein kinase (MAPK) ( ). Additionally, agonism of G-protein-coupled receptors (GPCRs), such as 5-hydroxytryptamine (5-HTRs) and β-adrenergic receptors (β-ARs), can activate the protein kinase B (Akt)/endothelial nitric oxide synthase (eNOS)/cyclic guanosine monophosphate (cGMP) pathway leading to activation of PGC-1α, translocation to the nucleus and, subsequently, induction of MB ( Fig. 3 ) ( ; ; ). Importantly, studies have shown that PGC-1α is decreased in the spinal cord after SCI, indicative of disrupted MB ( ; ).

PGC-1α regulates MB through interactions with transcription factors such as peroxisome proliferator-activated receptors (PPARs) and nuclear respiratory factors (Nrf1/2), among others ( ; ). PPARs are involved in the expression of fatty acid oxidation and Krebs cycle enzymes and OXPHOS components ( ; ). Activation of Nrf1/2 induces transcription of mitochondrial transcription factor A (TFAM) ( ), which translocates to the mitochondria, where it activates mitochondrial gene expression and mtDNA replication ( ). Coordination of these transcription factors via PGC-1α culminates in the induction of MB.

Intimately related to MB is mitochondrial dynamics, namely fusion and fission. Fusion is the joining of two mitochondria mediated by mitofusins (Mfn1/2) and optic atrophy 1 (OPA1), whereas fission initiates cleavage and division of mitochondria, and is mediated, in part, by dynamin-related protein 1 (DRP1) and outer membrane receptor fission 1 (FIS1). Dysfunctional mitochondria contain impaired proteins, damaged membranes and fragmented mtDNA, increasing fission mechanisms and mitophagy, which is the selective degradation of damaged mitochondria by autophagosomes mediated, in part, by PTEN-induced kinase 1 (PINK1) and E3-ubiquitin ligase (Parkin) interaction ( Fig. 4 ) ( ). Conversely, promotion of fusion mechanisms has been implicated in MB and recovery ( ). Proper balance of MB, dynamics and mitophagy is critical for mitochondrial function and response to various stressors, including SCI.

Cell type-specific mitochondrial dysfunction and MB

SCI is a complex pathology involving heterogeneous cell types with distinct roles in the progression of injury and recovery. Many in vivo studies exist evaluating the global effects of mitochondrial-based therapies for SCI, while few reports exist detailing cell type-specific effects. Understanding the role of mitochondrial dysfunction and MB in relevant cell types can uncover not only underexplored mechanisms, but also potential therapeutic strategies following SCI.