Fibrosis in the central nervous system : the role of perivascular cells

Abstract

Regeneration in the adult mammalian central nervous system (CNS) is very limited. One limiting factor is the formation of chronic scar tissue, which inhibits axonal regeneration and functional recovery. While scar formation has been recognized for more than a century, research on the origin and function of the fibrotic scar component has been mostly neglected. The objective of this thesis is to thoroughly characterize the origin of fibrotic scar forming stromal cells and their transformation upon spinal cord injury. Furthermore, we aimed to decipher if fibrotic scar formation and the contribution of perivascular cells to fibrosis is a general mechanism, in brain and spinal cord and in response to different kinds of lesions. And ultimately, the goal was to determine, if modulation of fibrotic scarring represents a therapeutic potential to achieve functional recovery after CNS injury.

The Göritz laboratory previously established GLAST+ perivascular cells, named type A pericytes, as the origin of fibrotic scar tissue after spinal cord injury. In paper III, we employed genetic in vivo lineage tracing to heritable label the GLAST (Glutamate aspartate transporter) expressing subpopulation of Pdgfrβ (Platelet-derived growth factor receptor beta) positive perivascular cells in combination with single-cell RNA sequencing to characterize the cells at the molecular level. Our results show that GLAST+ perivascular cells encompass pericytes and perivascular fibroblasts, based on their transcriptome. To distinguish between pericytes and perivascular fibroblasts, we genetically labeled perivascular fibroblasts, using an inducible Col1a1-CreERT2 transgenic mouse line. Our results show that GLAST+Col1a1+ fibroblasts are more readily observed along larger diameter penetrating blood vessels in the spinal cord white matter, while GLAST+Col1a1- pericytes partially cover the abluminal surface of smaller vessels in the arteriole-capillary transitional zone. Importantly, both populations contributed to stromal fibroblasts in fibrotic scar tissue. Remarkably, cells derived from perivascular fibroblasts contributed mostly to the white matter portion of the scar, while pericyte progeny mainly contributed to grey matter areas, together establishing heterogeneity of fibrotic scar composition.

The aim of paper II was to determine fibrotic tissue formation in response to different kinds of lesions in brain and spinal cord, in mice and humans. Furthermore, we asked to which extend GLAST+ perivascular cells contribute to fibrotic tissue in different parts of the CNS and in response to distinct lesions. For this, we compared fibrotic tissue formation as well as the contribution of GLAST+ perivascular cells after complete crush-, contusion- (paper III), dorsal hemisection- (paper I), and dorsal funiculus incision spinal cord injury, large cortical, cortico-striatal brain stab wound lesions, striatal-, cortical- and striatal-cortical ischemic stroke lesions, experimental autoimmune encephalomyelitis-induced lesions and the Gl261 glioblastoma model. In all lesion models and pathologies investigated, we found stromal tissue formation. However, the cellular arrangement and ECM distribution was lesion dependent. In all lesion models in which stromal fibroblasts accumulated outside the vessel wall, the vast majority was derived from GLAST+ perivascular cells, except for the Gl261 glioblastoma model. We also showed that stromal tissue is formed upon spinal cord injury, multiple sclerosis, stroke and glioblastoma in humans and that a subset of pdgfrb+ perivascular cells in the human brain and spinal cord expresses GLAST (SLC1A3). Our results show that fibrotic scarring by GLAST+ perivascular cells is conserved throughout the CNS.

In paper I, we investigated the therapeutic potential of mitigating fibrotic scarring after spinal cord injury by genetic reduction of GLAST+ perivascular cell proliferation. We demonstrate that decreased fibroblast accumulation is attended by reduced deposition of extracellular matrix in the injury core, modulated glial scar architecture and diminished inflammation, leading to increased regeneration of corticospinal- and raphespinal tract axons. Furthermore, regenerated corticospinal tract axons functionally integrate caudal to the lesion as shown by electrophysiologic recordings upon optogenetic activation. Mice with reduced perivascular cell-derived scarring and the highest number of regenerated axons showed best recovery of sensorimotor functions.

In summary: Various CNS pathologies trigger fibrosis by perivascular fibroblasts and a subset of pericytes in a region-dependent manner. Interfering with the scarring process, to moderately reduce fibrotic scarring by GLAST+ perivascular cells, may represent a strategy to improve functional recovery after several detrimental CNS maladies.