Harnessing neuroinflammation for neuroregeneration

Neuroinflammation’s detrimental effects

Neuroinflammation can have adverse consequences on the well-being and longevity of neurons. Activated microglia emit pro-inflammatory cytokines, including interleukin (IL)-1β and tumour necrosis factor-α (TNF-α), which cause neuronal damage and death.^[6] These cytokines stimulate excitotoxicity, apoptosis and oxidative stress, resulting in the gradual deterioration of neurons. Furthermore, the continuous activation of microglia can lead to the creation of neurotoxic substances such as nitric oxide and reactive oxygen species (ROS), which worsen the process of neurodegeneration.^[7]

Neurodegeneration and the role of inflammatory mediators

Neurodegeneration is caused not just by cytokines but also by other inflammatory mediators that are generated by activated microglia. Chemokines, such as monocyte chemoattractant protein-1, enhance the attraction of immune cells from the periphery to the CNS, resulting in increased inflammation and injury to the tissue. Matrix metalloproteinases, which are enzymes generated by activated microglia, play a role in the degradation of the blood–brain barrier (BBB), worsening neuroinflammation and enabling the entry of immune cells from outside the CNS.^[8] These immune cells continue to sustain the neuroinflammatory response, resulting in a relentless cycle of neurodegeneration.

Cellular mechanisms underlying neuroinflammation

Astrocyte reactivity

Astrocyte reactivity refers to the response of astrocytes, a kind of glial cell in the CNS, to various stimuli or injuries. Astrocytes have a reactive behaviour when exposed to inflammatory stimuli, resulting in the secretion of pro-inflammatory substances such as IL-1β and TNF-α [Figure 1]. Astrocytes also contribute to the regulation of BBB permeability^[10] which becomes altered during neuroinflammation.

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Figure 1:

Microglia and astrocytes in neuroinflammation.

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Neuroinflammation: Understanding the molecular mechanisms

Neuroinflammation and neurodegeneration

Neurodegenerative disorders, including AD, PD and multiple sclerosis (MS), are marked by the gradual deterioration of neurons’ structure and function in the CNS.^[16] Extensive research conducted over the years has demonstrated that neuroinflammation plays a vital role in the development and advancement of many debilitating illnesses.

Neuroinflammation’s contribution to AD

AD is a prevalent neurodegenerative disorder distinguished by the buildup of beta-amyloid plaques and neurofibrillary tangles in the brain. Recent research has indicated that neuroinflammation plays a direct role in the advancement of AD [Figure 2]. Upon activation, microglia release inflammatory mediators that stimulate the deposition of beta-amyloid and worsen the formation of neurofibrillary tangles. Moreover, persistent neuroinflammation results in the secretion of neurotoxic substances,^[17] which in turn leads to impaired synaptic function and the death of neurons in AD.

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Figure 2:

Neuroinflammation in Alzheimer’s disease.

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The role of neuroinflammation in PD

PD is defined by the degeneration of dopaminergic neurons in the substantia nigra, resulting in motor deficits. Accumulating data indicate that neuroinflammation plays a crucial role in the development of PD [Figure 3]. The activation of microglia and astrocytes in the substantia nigra leads to the secretion of pro-inflammatory cytokines, including TNF-α and IL-1β, which contribute to the degeneration of dopaminergic neurons.^[18] Moreover, neuroinflammation-induced oxidative stress intensifies the neuronal harm in PD.

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Figure 3:

Neuroinflammation in Parkinson’s disease.

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The role of neuroinflammation in MS

MS is an autoimmune disorder marked by the loss of myelin, the protective covering of nerve fibres, in the CNS. Neuroinflammation is a significant characteristic of MS, when immune cells invade the brain and spinal cord, resulting in the breakdown of myelin [Figure 4]. The chronic inflammation and demyelination seen in MS are caused by the release of inflammatory mediators, such as IL-17 and interferon-gamma, by immune cells that infiltrate the affected area.^[19] In addition, the activation of microglia and astrocytes in MS exacerbates the neuroinflammatory response.

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Figure 4:

Neuroinflammation in multiple sclerosis. IL: Interleukin, IFN: Interferon gamma, TNF: Tumour necrosis factor, CD4: Cluster of Differentiation-4.

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Possible regenerative functions of neuroinflammation

Although neuroinflammation is commonly linked to harmful effects, recent research indicates that it may also contribute to the promotion of brain repair and regeneration. Microglia that have been activated due to damage or disease have the ability to assume an alternate phenotype referred to as the ‘M2’ phenotype [Figure 5]. M2 microglia release anti-inflammatory cytokines, including IL-10 and transforming growth factor-β (TGF-β), which possess neuroprotective and regenerative properties.^[20] These cytokines enhance the survival of neurons, the formation of synapses and the remodelling of tissues, hence accelerating the processes of neural healing.

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Figure 5:

Neuroprotective M2 phenotype of microglia.

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Targeting neuroinflammation for therapeutic purposes

Due to the simultaneous functions of neuroinflammation, there has been increased interest in treatment approaches that target the inflammatory response. Non-steroidal anti-inflammatory drugs and glucocorticoids, which are anti-inflammatory medications, have been investigated for their ability to alleviate neuroinflammation and decrease neurodegeneration. Furthermore, the focused approach of targeting particular inflammatory mediators, such as cytokines or chemokines, has demonstrated potential in preclinical investigations.

Regulating neuroinflammation using lipopolysaccharide (LPS)

LPS, which is found in the outer membrane of Gram-negative bacteria, is frequently employed to provoke neuroinflammation in experimental animals. LPS triggers the activation of TLR-4 on microglia, resulting in the synthesis of pro-inflammatory cytokines, including TNF-α and IL-1β.^[21] Research has demonstrated that administering small amounts of LPS can promote the growth of new neurons and improve the brain’s ability to adapt and change [Figure 6]. For instance, the treatment of LPS in the dentate gyrus of the hippocampus has been observed to enhance the proliferation of neural stem cells and facilitate their transformation into fully developed neurons. In addition, neuroinflammation generated by LPS can stimulate the secretion of growth factors, such as brain-derived neurotrophic factor (BDNF),^[22] which promotes additional neuroregeneration.

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Figure 6:

Neuroprotective pathway of lipopolysaccharide. LPS: Lipopolysaccharide, TLR4:Toll-like receptor 4, NFK: Nuclear factor kappa, BDNF: Brain-derived neurotrophic factor.

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Regulating neuroinflammation using IL-4/IL-13

IL-4 and IL-13 are cytokines with anti-inflammatory properties that can regulate neuroinflammation and facilitate neuroregeneration.^[23] The actions of these cytokines are mediated via the IL-4 receptor alpha and IL-13 receptor alpha 1 complexes, which are present on microglia and astrocytes.

The signalling pathway of IL-4/IL-13 has been demonstrated to inhibit the synthesis of pro-inflammatory cytokines and enhance the release of anti-inflammatory cytokines. The alteration in the cytokine profile diminishes neuroinflammation and establishes a conducive environment for neuroregeneration [Figure 7]. In addition, the signalling of IL-4/IL-13 can augment the expression of neurotrophic factors, leading to the promotion of neuronal survival and development.

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Figure 7:

Neuroprotective actions of interleukin-4 and interleukin-13. IL: Interleukin, RA: Receptor antagonist, TGF-β1: Transforming growth factor-Beta 1, TNF: Tumour necrosis factor.

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The role of adaptive immunity in neuroinflammation

Recent studies have emphasised the role of adaptive immune responses, specifically T cells, in promoting neuroinflammatory processes. T lymphocytes, namely CD4+ effector T cells and CD8+ cytotoxic T cells, invade the CNS during neuroinflammation and participate in the secretion of pro-inflammatory cytokines and chemokines.^[24]

The topic of interest is the relationship between regulatory T cells and neuroinflammation. Regulatory T cells, also known as regulatory T cells (T-regs), are a distinct subgroup of CD4+ T cells that have a vital function in maintaining immunological balance and tolerance. T-regs are distinguished by the presence of the transcription factor Foxp3.^[25] These cells carry out their inhibitory role by releasing anti-inflammatory cytokines, including IL-10 and TGF-β. Within the realm of neuroinflammation, it has been demonstrated that T-regs possess immunomodulatory characteristics that effectively suppress exaggerated immune reactions and mitigate tissue harm.

Increasing evidence suggests that T-regs have a role in the process of neuroregeneration.

T-regs have been found to not only regulate neuroinflammation but also potentially aid in neuroregeneration.

Neuroprotective benefits of T-regs

T-regs possess both immunomodulatory and neuroregenerative capabilities, as well as direct neuroprotective actions [Figure 8]. These cells have the ability to directly engage with neurons and glial cells, so enhancing the survival of neurons and safeguarding against harmful substances that might damage the nervous system. T-regs have demonstrated the ability to release neuroprotective substances, such as BDNF and nerve growth factor, which promote the survival and adaptability of neurons.^[26]

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Figure 8:

Neuroprotective actions of regulatory T cells (T-regs). BDNF: Brain-derived neurotrophic factor, GDNF: Glial cell line-derived neurotrophic factor.

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Variability in the neuroinflammatory response

A fundamental obstacle in utilising neuroinflammation for therapeutic aims is the notable variability reported in the neuroinflammatory response amongst different persons and illness circumstances.^[27] The complex interaction of immune cells, glial cells and neurons is responsible for this diversity, posing challenges in the development of universally effective therapeutic approaches. It may be imperative to employ individualised strategies that consider the distinct genetic and cellular characteristics of individuals. Advanced methodologies like single-cell RNA sequencing offer useful insights into the various cell populations implicated in neuroinflammation, facilitating the creation of precise therapies.^[28]

Duality of neuroinflammation

Neuroinflammation demonstrates a dichotomous character, manifesting both beneficial and harmful consequences depending on the circumstances.^[29] Although acute neuroinflammation plays a vital role in tissue healing and elimination of pathogens, persistent neuroinflammation can result in neuronal damage and neurodegeneration. Effectively targeting the advantageous features of neuroinflammation while mitigating the harmful components poses a substantial obstacle. Accurate control of immune responses and precise adjustment of the timing and duration of therapies may be required to achieve therapeutic outcomes.

Obstacles of the BBB

The BBB, which acts as a highly discerning barrier controlling the passage of chemicals between the blood and the brain, presents a significant obstacle in utilising neuroinflammation for therapeutic intentions. The tight connections amongst endothelial cells that compose the BBB impede the passage of medicinal substances into the brain tissue, hence constraining their effectiveness. To overcome the obstacles posed by the BBB, it is necessary to create advanced drug delivery systems such as nanoparticles, exosomes and cell-based methods.^[30] In addition, employing techniques to temporarily breach the BBB, specifically at the location of inflammation, can improve the administration of therapeutic substances.

Complexity that varies with time

The time-dependent changes in neuroinflammation introduce an additional level of intricacy to therapeutic approaches.^[31]

The immune response within the CNS is characterised by a highly dynamic process that involves distinct stages of start, resolution and chronicity. The effectiveness of therapeutic approaches may differ based on the stage of neuroinflammation being addressed. Timely therapies targeting the suppression of harmful features of neuroinflammation have the potential to avert lasting harm. Conversely, interventions implemented during the resolution period can facilitate the process of tissue repair and regeneration. Therefore, a comprehensive comprehension of the time-related changes in neuroinflammation is crucial for effective treatment approaches.

Adverse reactions

Finally, the possibility of adverse consequences and unintended outcomes presents a substantial obstacle to utilising neuroinflammation for therapeutic intentions. Manipulating the immune response in the CNS carries the potential for eliciting undesired immunological reactions or disrupting regular physiological processes. Thorough evaluation of possible adverse reactions and unintended impacts is essential throughout the development of therapeutic approaches. Pre-clinical and clinical investigations should prioritise the evaluation of the safety and effectiveness of medicines that specifically target neuroinflammation to mitigate the potential for unanticipated adverse effects.^[32]

Focusing on the activation and polarisation of microglia

Microglia, the immune cells that dwell in the CNS, have a vital function in neuroinflammation. Microglia can transition between two distinct phenotypes, namely the pro-inflammatory M1 phenotype and the anti-inflammatory M2 phenotype, as a reaction to injury or disease. The objective of innovative methods is to regulate the activation and polarisation of microglia towards the M2 phenotype, which facilitates tissue regeneration and the resolution of inflammation.^[33]

Applying nanotechnology for precise administration of medications

Nanotechnology provides novel strategies for addressing neuroinflammation by precise delivery of drugs to the CNS. Nanoparticles can be engineered to encapsulate anti-inflammatory medications and successfully traverse the BBB, guaranteeing targeted administration to specific parts of the brain that is afflicted. Furthermore, the surface changes of nanoparticles can improve their capacity to selectively target activated immune cells, such as microglia, hence reducing the occurrence of unintended consequences.^[34]

Strengthening the body’s natural healing processes

By utilising the capabilities of neuroinflammation, it is feasible to amplify the innate mechanisms of self-repair within the CNS. Facilitating tissue remodelling involves promoting a regulated inflammatory response, which in turn activates local stem cells and stimulates neural plasticity.^[35]

This method shows potential for treating neurodegenerative conditions such as AD and PD.

Neuroinflammation and neurotrophic factors

Neurotrophic factors play a crucial role in promoting the development, survival and differentiation of neurons. Neuroinflammation could impact the manifestation and discharge of these variables. Cytokines and chemokines, which are inflammatory mediators, have the ability to either stimulate or hinder the production of neurotrophic factors.^[36] Acquiring an understanding of the relationship between neuroinflammation and neurotrophic factors is crucial for the development of effective neuroregenerative therapies.

Harnessing neuroinflammation for precise medication delivery

Neuroinflammation presents a distinct chance to deliver drugs precisely to the location of injury or disease within the CNS. Enhancing the effectiveness of therapeutic treatments can be achieved by developing drug delivery systems based on nanotechnology that can specifically target activated microglia or invading immune cells. Future research should prioritise the development and refinement of targeted drug delivery systems, with a specific emphasis on verifying their safety and effectiveness, as well as assessing their potential to enhance neuroregeneration.

Enhancing stem cell transplantation

Neuroinflammation can be utilised to enhance the results of stem cell transplantation.^[37] By modulating the inflammatory response, it is possible to create a beneficial environment for transplanted stem cells, which improves their chances of surviving, integrating and differentiating. This method could potentially be utilised in the management of spinal cord injuries, stroke and other CNS illnesses.

Combination therapies

The synergistic benefits can be achieved by combining neuroregenerative medicines with techniques that specifically target neuroinflammation.^[38] Optimal outcomes can be achieved by concurrently encouraging tissue healing and regulating the inflammatory response. This method creates new opportunities for the advancement of individualised and focused neuroregenerative therapies.