Jody de Jong

The central nervous system (CNS) consists of the brain and spinal cord. The CNS receives, processes, and transmits information to and from the CNS itself, as well as to other parts of the body. It regulates essential functions such as movement, sensation, smell, hearing, and thinking. The CNS consists of nerve cells (neurons), glial cells, blood vessels, and the extracellular matrix (ECM). Neurons communicate with each other via projections called axons, which act like electrical wires carrying signals produced by the neurons. Axons are encased in a fatty layer called myelin, which ensures rapid signal transmission, provides nutrients, and protects the axon from damage. The CNS is divided into grey matter, containing neuronal cell bodies, and white matter, containing mostly myelinated axons.

Glial cells, including oligodendrocytes and their precursors (OPCs), astrocytes, and microglia, are also vital. Oligodendrocytes generate and maintain the myelin sheath. Astrocytes produce and maintain the ECM, support the blood-brain barrier, and provide nutrients. Microglia are tissue-specific immune cells that clear diseased cells and debris and signal other cells to initiate repair.

The ECM is a network of molecules between cells, primarily glycoproteins and proteoglycans, regulated by enzymes like matrix metalloproteinases (MMPs) and ADAMTs. It is a dynamic structure essential for development, homeostasis, and tissue repair. Following injury, the ECM temporarily changes its composition to guide repair, but stalled or aberrant remodeling is observed in diseases like multiple sclerosis (MS).

MS is a chronic inflammatory disease characterized by demyelination and neuronal loss. While remyelination occurs in early stages, it becomes less effective over time, leading to secondary neurodegeneration. Progression also occurs through diffuse pathology in normal-appearing grey (NAGM) and white matter (NAWM), a process known as progression independent of relapse activity (PIRA). Promoting remyelination and preventing diffuse pathology are key therapeutic goals.

This thesis focuses on the role of the ECM in MS. In white matter, disrupted ECM remodeling leads to pathological changes. Differences exist between lesion types and locations (core vs. rim). Unlike in successful experimental repair models, changes in MS lesions often become permanent. These changes, including accumulations of chondroitin sulfate proteoglycans (CSPGs) and fibronectin, inhibit OPC differentiation. Additionally, ECM stiffness influences cell behavior; although the brain is generally soft, local variations in stiffness affect OVC and microglial migration and differentiation. To improve MS treatment, it is essential to bypass or neutralize the inhibitory effects of the ECM.

In Chapter 3, novel rapid decellularization protocols were developed for thin slices of rat and human brain tissue. These methods effectively remove DNA and cellular components while preserving the ECM composition and surface ultrastructure. This allows for the analysis of regional ECM composition and the study of cell behavior on a natural, cell-free scaffold.

Chapter 4 investigated the ECM composition in decellularized NAGM and subpial grey matter lesions (GSL). Analysis revealed changes in perineuronal nets and unique proteins linked to synaptic plasticity and blood-brain barrier integrity in the NAGM, even before demyelination is evident. Microglia cultured on these scaffolds lost their specific marker IBA1 and partially adopted a pro-inflammatory state (iNOS expression), suggesting that ECM changes contribute to diffuse pathology.

Chapter 5 identified eight differentially abundant proteins in white matter lesions (WSL), including reduced levels of a 55 kDa brevican fragment. Five proteins related to blood-brain barrier integrity were uniquely detected in WSL. Interestingly, OPCs differentiated well on decellularized WSL scaffolds but failed to mature on surrounding perilesional white matter (PLWM), suggesting that while the ECM in lesions is permissive, other factors or the surrounding environment might inhibit repair.

Collectively, the data show that ECM remodeling varies across MS pathology stages and between tissue types. In grey matter, changes occur before visible demyelination, potentially driving early diffuse pathology. In white matter, changes are most prominent within lesions. These findings highlight the importance of region-specific ECM structures and suggest the ECM as a target for future MS therapies.