Soria et al. has published this year an interesting study about the modification leading by synucleopathy on the nanoscale organization and diffusivity of the extracellular space, through hyaluronan remodeling.
Indeed the extracellular space (ECS), composed by the interstitial fluid and the extracellular matrix (ECM), plays several roles in health and disease, including biochemical signal diffusion and intercellular communication, and also in the large-scale distribution of molecules. Contrary to extracellular matrix of other tissues, the brain extracellular matrix contains a very low level of collagen and is mainly composed of long chain of hyaluronic acid (HA). In interaction with other ECM components, HA forms a scaffold that allows cell adhesion and maintenance of ECS permeability. Moreover, hyaluronan displays multiple properties depending on its size: high molecular weight HA (HMWHA) is involved in anti-inflammatory response, and low molecular weight HA (LMWHA) actives pro-inflammatory pathways in specific cells like microglia. However, the implication of this complex system is largely unexplored in neurodegenerative diseases, like Parkinson’s disease or other proteinopathies.
In this study, Soria et al. described the alterations of ECS organization and diffusivity properties, in an in vivo model of alpha-synuclein-induced neurodegeneration, then they investigated the HA modification impact in the context of neurodegeneration. As a model, they injected Lewy Bodies (LB) of patient in the substancia nigra (SN) of mice to induce neurodegeneration: after 4 months, they observed about 50% of dopaminergic neuron loss, so they studied the nanoscale impact on neurodegeneration on the ECS.
First, they studied the ECS volume fraction and organization by imaging of high-pressure cryofixation sample, which preserves the characteristic of ECS by avoiding dehydration. They found that alpha-synclein-induced neurodegeneration enlarges the ECS and modifies the organization of ECS, with for example larger polls in LB-treated mice compared to controls. Then they quantified the ECS diffusivity by single-molecule traking of fluorescent SWCNT. They observed an increase in nanoscale diffusion in the ECS after LB-induced neurodegeneration. However, they measured a weak correlation between ECM dimensions and diffusivity, suggesting that the modification of diffusivity is not completely linked to the spatial structure.
Then they studied the impact of synucleinopathy on HA of the ECM, because the HA is a major component of the brain ECM and thus its modification can impact ECS parameters. They analyzed the HA in the SN of mice by HA Binding Protein (HABP) staining. They showed the HA organization was altered, with a more diffuse ECM and HA-cable like structure were limited to disperse dots. Moreover, the analysis of the cross-linking of HA showed a decrease in network complexity in degenerated SN. Conversely, the chondroitin sulfate of the ECM remained unaffected. The ARNm analysis showed no difference in HA synthases and hyaluronidases expression, but an increase in expression of CD44, a HA receptor. Furthermore, they found an increase in ARNm level of microglia activation marker CD68 and pro-inflammatory cytokine TNF-alpha, suggesting an inflammatory state in the SN after LB-induced degeneration.
Therefore they characterized the microglia phenotype, and they found more microglia with HA inclusion in degenerative SN, which suggest an involvement of reactive microglia in HA degradation during neurodegeneration.
Interestingly, they showed that the co-inoculation of LB with hyaluronidase to induce neurodegeneration lead to 20% of dopaminergic cell loss, versus about 50% without hyaluronidase. They also found a similar decrease in neurodegeneration with injection of LMWHA. Moreover, mice treated with LB and hyaluronidases displayed a higher microglial activation. They concluded that the HA fragmentation increased the early activation of microglia after LB inoculation, and provided a long-term neuroprotection. In addition, they showed by HA synthase inhibitor treatment in mice that chronic HA depletion increased ECS width, especially in LB-treated mice, and increased its diffusivity.
This study provides interesting insights in the interaction between the ECM and neurodegenerative or neuroprotective mechanisms. With this same objective, HCS Pharma works on cerebral in vitro model that include both dopaminergic neurons and glial cells, and also the extracellular environment thank its technology BIOMIMESYS® Brain, a hyaluronan-based matrix. This model aims at better mimicking cerebral pathophysiological conditions, et will allow to select relevant drug candidates against brain-related disorders.
Feel free to contact us if you want more information about our cerebral in vitro model in 3D which that include the extracellular microenvironment!
Soria, F. N. et al. (2020) ‘Synucleinopathy alters nanoscale organization and diffusion in the brain extracellular space through hyaluronan remodeling’, Nature Communications, 11(1), p. 3440. doi: 10.1038/s41467-020-17328-9.