Bridging time and space in the brain : 14C dating and spatial transcriptomic analysis give new insight into neurological disease

Abstract

Conditions affecting the nervous system are collectively the largest contributor to ill health and disability worldwide, and the therapeutic options for many neurological diseases remain limited. For a long time, it was believed that the central nervous system was fixed, with very limited potential for repair after injury. However, studies over the recent decades have demonstrated that the brain exhibits more plasticity than previously understood, which has led to significant efforts to harness these mechanisms for regenerative therapies. In addition, our increased understanding of how the central nervous system interacts with the immune system, opens further possibilities for novel therapeutic strategies. While animal models have provided important insights to brain plasticity and neuroimmune interactions, the involvement of these processes in neurological disease remain largely unexplored in the human brain. This is primarily due to a lack of methods to study cellular dynamics, such as cell generation and immune cell infiltration in brain tissues, without harming the individual. Using two innovative techniques, we have in this thesis explored cellular processes in time and space, in the healthy as well as the pathological human brain.

In paper I, we utilized a 14C-based retrospective birth dating method to study how adult neurogenesis, the generation of new neurons in the adult brain, in the hippocampus may be affected in individuals that suffered from alcohol or cocaine addiction. We did not observe any difference in adult hippocampal neurogenesis between healthy controls and the addiction groups, indicating that reduced neurogenesis throughout life does not contribute to addiction vulnerability. Although it was not possible to exclude smaller, yet clinically relevant, changes in cell turnover during the time of abuse, we found that individuals with long-term abuse of alcohol must have similar levels of neuronal generation as many healthy controls during the time of abuse. Furthermore, we found large interindividual variability in neuronal turnover in the healthy control group, suggesting that lifestyle and genetic factors greatly influence adult neurogenesis in the healthy brain.

The retrospective birth dating method was also applied in paper II to investigate the dynamics of oligodendrocyte generation in patients with multiple sclerosis (MS). We specifically examined shadow plaque lesions, which are believed to be remyelinated through the generation of new myelinating oligodendrocytes. Contrary to this belief, we found that oligodendrocytes in shadow plaques are old, indicating that potential remyelination in humans is performed by older oligodendrocytes. However, increased oligodendrocyte generation was found in non-lesion tissue of a few patients, possibly suggesting a heterogeneous reactive cell generation response.

MS is generally considered a T cell mediated autoimmune disease. However, the clinical success of B cell-depleting therapies suggest an important role of B cells in MS pathology. The specific T and B cells involved, and their target antigens, are not yet fully characterized. However, the diverse clones can be identified by their highly specific T and B cell receptors, and studying the lymphocyte clonality in a spatial context could shed light on specific clones’ involvement in MS pathology. However, high throughput methods for simultaneous analysis of the spatial distribution B and T cell clones in human tissues were not available. Therefore, in paper III, we developed a spatial transcriptomics method for variable, diversity and joining (VDJ) sequences, called Spatial VDJ, that maps the spatial distribution of T and B cell receptors in tissue sections, while simultaneously capturing the spatial gene expression. This method allows us to study lymphocyte clonal dynamics in a spatial context, link specific lymphocyte clones to local gene expression profiles as well as to lineage trace B cell clones, which can inform us of the development of B cell immune responses.

In paper IV, we utilized the Spatial VDJ technique to characterize the spatial distribution of lymphocyte clones in MS lesions, with the goal of identifying candidate clones that may be involved in driving the lesion pathology. We found significant infiltrates of clones of the B cell lineage, possibly terminally differentiated and antibody producing plasma cells, in different lesion types. We identified locally expanded clones that had infiltrated the lesion parenchyma close to vessels and along active lesion borders This potentially suggest a local antigen-driven activity and involvement in driving lesion pathology.

Together, these studies provide novel insights into how the generation of two types of brain cells might be involved in the pathological human brain. These studies also identify candidate B cell clones that potentially could be drivers of MS lesions. These are key insights into fundamental cellular dynamics of the human brain and how they may differ from the processes described in more commonly used animal models. Such new insights may help guide the development of future therapies for devastating diseases of the human brain.