The role of mast cells and their proteases in traumatic spinal cord injury

Abstract

Spinal cord injury (SCI) is a chronic condition that results in functional impairment in locomotion, loss of sensation, and it can cause neuropathic pain, spasticity, and incontinence. SCI has a significant impact on the quality of life and life expectancy; and it has also several disadvantages from an economical point of view. It is a devastating condition and patients with SCI are lifelong disabled because tissue in the central nervous system (CNS) is unable to regenerate after injury. Last years, there is increasing interest in the development of new therapies that can improve functional recovery after SCI. The pathology of SCI is characterized by a primary traumatic impact that rapidly destroys neuronal cells and axons, which is followed by secondary injury processes that induce further tissue damage and thereby worsen the neurological deficits. These secondary injury mechanisms consist amongst many other factors out of an inflammatory reaction, which is characterized by the activation and infiltration of various inflammatory cells and release of inflammatory mediators. Later, also scar tissue is formed at the lesion by reactive astrocytes and other cells (such as ependymal cells, pericytes and fibroblasts), which blocks regenerative processes. Mast cells are specialized cells of the innate immune response that are characterized by the presence of electron dense cytoplasmic granules. In these granules many preformed mediators are stored such as histamine, cytokines and several proteases; and activation of mast cells results in the release of these mediators in the extracellular space. Mast cells are mainly known for their role in allergic reactions such as asthma and hay fever, but they also play an important role in wound healing and they form a ‘first-line’ defence against pathogens. Mast cells are present in various body tissues, including the CNS. In this dissertation, we investigated the role of mast cells and their proteases on wound healing processes and repair after traumatic SCI. Our results indicate that mast cells improve functional recovery after SCI by suppressing detrimental inflammatory processes. These actions are mainly mediated by mouse mast cell protease 4 (mMCP4), a mast cell-specific chymase that cleaves inflammatory components after SCI and thereby improving the functional outcome. In addition, we showed that less scar tissue is formed at the lesion site in mice that have no mast cells. Scar tissue that is formed at the lesion after SCI has a negative impact on axon regeneration and other repair processes. More importantly, degradation or modulation of the scar is a potential therapy for SCI. In our study, we demonstrate that mast cells suppress the formation of the scar via mMCP6, a mast cell-specific tryptase that on one hand directly cleaves matrix components of the scar but on the other hand also can suppress the expression of these factors after injury on the gene level. In contrast to mMCP4, mMCP6 has no strong effect on the inflammatory response after SCI. We also demonstrated that mMCP4 limits scar formation at the lesion after SCI, but it is not clear yet whether mMCP4 directly cleaves components of the lesion scar after injury. It is plausible that the immunomodulatory effects of mMCP4 play a role in this process since the immune system and the extracellular matrix have a strong influence on each other. As a final step we explored whether mast cell proteases could be applied therapeutically to improve recovery after SCI. Therefore, we produced recombinant mMCP6 and we applied it in mice, several days after they received a spinal cord lesion. Our data showed that the functional outcome was improved in animals that received recombinant mMCP6 compared to the vehicle control group. This is a first indication that mast cell proteases have a therapeutic effect after SCI. The next step is the application of recombinant mMCP4, either alone or in combination with mMCP6. In conclusion, the results of this study indicate that mast cells play a beneficial role in repair processes and recovery after SCI. Moreover, we demonstrated that these mast cell effects were executed, to a great extent, by proteases that are specifically produced by mast cells such as mMCP4 and mMCP6. These proteases suppress the so called ‘bad’ inflammatory reactions and alternatively they also prevent the formation of scar tissue at the lesion after SCI. Our findings suggest that mast cell proteases are promising therapeutic candidates to improve repair after CNS injury. However, further research is essential to define the optimal therapeutic time window and also dosage of these proteases has to be determined. Also the possibility to apply these proteases together or in combination with other therapeutic compounds has to be investigated.