One fundamental role of mitochondria is ATP production via oxidative phosphorylation, a process critically dependent on the integrity of the electron transport chain (ETC) [19]. When oxidative phosphorylation becomes compromised, decreased ATP production occurs, which is particularly detrimental to neurons due to their high energy demands. For instance, in AD, there is a marked decline in oxidative phosphorylation efficiency, leading to energy deficits that contribute to synaptic dysfunction and neuronal death [20]. Mitochondrial dysfunction can manifest as various mitochondrial abnormalities, such as loss of mitochondrial membrane potential, reduced activity of key ETC complexes (like complex IV), and elevated production of ROS, which further exacerbate mitochondrial damage and impair ATP synthesis [21].

Studies have shown that an increase in oxidative stress can inhibit the mitochondrial ETC, leading to bioenergetic failure in neurons. For example, oxidative modifications to proteins within the ETC can hinder their function, creating a vicious cycle where energy deficiency perpetuates further mitochondrial dysfunction [22]. This deficient ATP production impacts neurons’ ability to maintain membrane potential and perform essential physiological functions, ultimately resulting in neuronal dysfunction and activation of apoptotic pathways [23].

Mitochondria are crucial for calcium homeostasis, which is fundamental for neuronal signaling and excitability. Calcium imbalance occurs due to disrupted mitochondrial calcium uptake and buffering capacity, often leading to excitotoxicity, a pathological process characterized by excessive stimulation of neurons by excitatory neurotransmitters like glutamate [24]. The regulation of intracellular calcium by mitochondria is vital as it affects neurotransmitter release and synaptic plasticity. Damaged mitochondria can result in elevated cytosolic calcium levels, overwhelming buffering capacity and triggering excessive neuronal activation. This process leads to neuronal injury through activation of calcium-dependent apoptotic pathways [24,25].

Moreover, oxidative stress can exacerbate calcium dysregulation by promoting the opening of the mitochondrial permeability transition pore, which facilitates the unnecessary release of calcium into the cytoplasm and promotes the release of pro-apoptotic factors such as cytochrome c. The importance of calcium homeostasis underscores that mitochondrial failure entails not only energy deficits but also disruptions in key signaling mechanisms leading to neuronal excitotoxicity, culminating in neurodegeneration.

mtDNA is particularly susceptible to mutations due to its proximity to the ETC, where ROS are generated as byproducts. The accumulation of mtDNA mutations contributes to mitochondrial dysfunction, implicated in various neurodegenerative diseases [26]. Impaired mtDNA repair mechanisms can exacerbate this issue, compromising mitochondrial function, altering proteostasis, and reducing mitochondrial biogenesis [27].

The failure of mitophagy, the selective removal of damaged mitochondria, is critical in compounding mitochondrial dysfunction in neurons. Proteins like PINK1 and Parkin play pivotal roles in recognizing and targeting dysfunctional mitochondria for degradation [28]. Impairments in these pathways can lead to the accumulation of damaged mitochondria, further contributing to oxidative stress and calcium dysregulation, ultimately resulting in neuronal death. Research indicates that neurons exhibit slower mitophagy rates compared to other cell types, making them particularly vulnerable to mitochondrial dysfunction over time [29,30]. Thus, the interplay between mtDNA mutations, proteostasis, and ineffective mitophagy exemplifies a multifaceted mechanism through which mitochondrial integrity is compromised, promoting neurodegenerative processes.

Mitochondrial transport along axons is crucial for maintaining energy homeostasis and meeting the local energy demands of synapses. Disruption in mitochondrial transport can lead to an inadequate supply of ATP to distal neuronal compartments, contributing to synaptic dysfunction. Mutations affecting proteins necessary for mitochondrial transport, such as dynein and kinesin, can impede mitochondrial movement, leading to energy deficits in neurotransmission areas [25].

Starvation or pathological conditions can exacerbate mitochondrial transport issues, resulting in stationary mitochondria that fail to support synaptic function and ultimately leading to neuron death. In rodent models of AD, impairments in axonal transport were linked to reduced mitochondrial motility along axonal microtubules, demonstrating how such disruptions can affect synaptic plasticity and overall neuronal function [31]. The interplay between mitochondrial transport and neuronal health underscores the critical need for effective mitochondrial dynamics to maintain neuronal integrity and function.

The first approach to mitochondrial rejuvenation involves the use of small molecules that target mitochondrial pathways. Four main strategies have emerged: nicotinamide adenine dinucleotide (NAD+) boosters, antioxidants, sirtuin activators, and mitochondrial biogenesis activators.

Another innovative approach to restore mitochondrial function is through mitochondrial augmentation therapies, including stem cell-mediated transfer, isolated mitochondrial transplantation, and extracellular vesicle (EV)-based delivery.

Recent advancements in neuromodulatory and photonic approaches provide novel means to modulate mitochondrial function and improve neuronal health through non-invasive techniques.

Delivering therapeutic agents to the distal parts of long-range neuronal networks poses considerable challenges. Neurons, especially those with extensive axonal projections, such as motor neurons and cortical pyramidal neurons, have unique architectures that require tailored approaches for effective drug delivery [45]. Traditional systemic administration of therapeutic molecules often struggles to achieve sufficient concentrations at synaptic terminals, which depend on mitochondrial dynamics for energy production and health.

Studies have shown that mitochondrial dysfunction can propagate from localized areas along the axonal length, where ineffective mitochondrial transport exacerbates the problem [46]. Thus, strategies for localized delivery of mitochondrial therapeutics via intranasal or intrathecal routes are currently under investigation. Novel methods, such as the formulation of drug carriers targeting mitochondrial pathways directly, hold potential but require further refinement to ensure efficient uptake and transport across neuronal membranes [47]. Overcoming the complexities of neuronal architecture will be crucial for the successful translation of mitochondrial therapies.

The timing of therapeutic interventions in the context of mitochondrial dysfunction is another critical factor influencing therapeutic efficacy. Interventions targeted at mitochondria must be strategically timed to coincide with critical windows of neurodegeneration or compensatory capacity. For instance, therapeutic strategies directed at enhancing mitophagy or mitochondrial biogenesis may be particularly effective when initiated early during neurodegeneration when compensatory mechanisms can still be activated.

In models of AD, evidence suggests that the restoration of mitochondrial function is more effective in earlier disease states compared to advanced stages characterized by extensive neuronal loss and irreversible damage [48]. Preclinical findings indicate that initiating therapies at pre-symptomatic stages or at early symptom appearances can significantly boost the therapeutic window for functional recovery [49]. Thus, a better understanding of disease progression and timely interventions may yield improved outcomes in clinical settings.

The integration of transplanted or transferred mitochondria from donor cells into host neurons raises significant questions regarding compatibility. Mitochondrial transplant therapies must ensure that donor mitochondria can efficiently integrate, complement the host cellular machinery, and become functionally operative within the recipient neuron [50]. Various intrinsic and extrinsic factors may influence the fate of transplanted mitochondria, including the degenerative state of host neurons and the compatibility of mitochondrial membranes and signaling pathways between donor and recipient cells.

Current studies utilize induced pluripotent stem cells and other cellular models to explore compatibility issues at a molecular level. Mitochondrial transfer experiments have indicated that differences in membrane potentials and mitochondrial dynamics can dictate the success of integration into host systems [51]. Optimization of mitochondrial donor sources to match recipient cell types or preconditioning recipient cells to enhance compatibility may represent viable strategies for improving therapeutic outcomes and ensuring efficacy [52].

Another major barrier to the successful translation of mitochondrial therapies remains the lack of standardized functional assessments. While improved survival rates among treated populations indicate therapeutic success, a comprehensive understanding of mitochondrial functionality and overall neuronal health is pivotal. Current assessments often focus on cell viability rather than robust mitochondrial measures like ATP production efficiency, mitochondrial bioenergetics, or the restoration of metabolic pathways.

To facilitate a rigorous evaluation of therapeutic interventions, standardized methods to assess mitochondrial function post-treatment must be developed. Functional assessments could include measuring ATP levels, evaluating mitochondrial dynamics, and quantifying ROS production alongside morphometric analyses of mitochondrial morphology. These assessments will provide critical data to correlate changes in mitochondrial performance with clinical outcomes in neurodegenerative diseases [53].

Research focused on targeted mitochondrial therapeutics aims to mitigate age-related mitochondrial dysfunction, which is a critical factor in several neurodegenerative diseases, including AD, PD, and ALS. Literature indicates that dysfunctional mitochondria are implicated in the pathogenesis of these diseases, often leading to increased oxidative stress, mtDNA mutations, and diminished neuroprotection [45,54].

Aging is associated with a gradual decline in mitochondrial function; hence, interventions aimed at enhancing mitochondrial biogenesis, improving mitophagy, and boosting mitochondrial efficiency are particularly vital in older populations. For instance, studies have explored mitochondrial-targeting antioxidants that may relieve oxidative stress in neurodegeneration by protecting mitochondrial integrity and enhancing neuronal survival [55,56]. Additionally, activation of mitophagy has shown potential in alleviating the neurotoxic burden associated with the accumulation of damaged mitochondria, potentially improving cognitive outcomes in Alzheimer’s models [50].

To maximize therapeutic benefit, there is growing interest in combining mitochondrial repair strategies with regenerative therapies. This integrative approach can offer a dual mechanism—promoting recovery and resilience in neurons while also addressing mitochondrial dysfunction. Stem cell-based therapies can provide a viable means to restore functionality by replenishing damaged cells or enhancing the activity of resident neuronal stem cells.

Combining stem cell therapy with mitochondrial transfer protocols has demonstrated promise in preclinical studies, with evidence suggesting that healthy mitochondria can be delivered to distressed neuronal populations via stem cell exosomes or as isolated organelles [29]. Such strategies could facilitate more effective mitochondrial function restoration in diseases characterized by severe energy deficits. Engineering approaches that merge mitochondrial augmentation with regenerative techniques may further enhance neuronal repair and synaptic plasticity, amplifying the regenerative potential of the aging nervous system [57].

Advancements in engineering, including nanotechnology, gene delivery systems, and biomaterials, present exciting opportunities to enhance the targeted delivery of mitochondrial therapies. The development of nanoformulations capable of carrying mitochondrial-targeted therapeutics directly to neuronal tissues can minimize off-target effects and increase local concentrations at sites of mitochondrial dysfunction [58]. Moreover, innovative biomaterials could serve as scaffolds promoting neural growth while simultaneously integrating mitochondrial repair capabilities responsive to cellular needs [59].

Safety is paramount when developing these interventions; thus, ongoing efforts aim to ensure that mitochondrial augmentation therapies do not trigger excessive ROS production or other detrimental effects that could exacerbate existing neurodegenerative conditions. Engineering strategies that allow for precise temporal and spatial control over mitochondrial delivery could revolutionize clinical practices in neurodegeneration, enhancing therapy effectiveness while reducing potential side effects [60].

The landscape of mitochondrial therapeutic strategies is rich with opportunity, particularly in connection to age-related disorders and neurodegenerative diseases. By focusing on combinations of mitochondrial repair with regenerative approaches and leveraging engineering advancements, the potential for substantial clinical applications becomes increasingly tangible. Future studies will need to validate these strategies in clinical trials to confirm their safety, efficacy, and long-term benefits for patients facing neurodegeneration challenges [61].