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Collagen XVIII (COL18A1) is an abundant heparan sulfate proteoglycan in vascular basement membranes. Here, we asked (i) if the loss of collagen XVIII would result in blood-brain barrier (BBB) breakdown, pathological alterations of small arteries and capillaries and neuroinflammation as found in cerebral small vessel disease (CSVD) and (ii) if such changes may be associated with remodeling of synapses and neural extracellular matrix (ECM). We found that 5-month-old Col18a1-/- mice had elevated BBB permeability for mouse IgG in the deep gray matter, and intravascular erythrocyte accumulations were observed in capillaries and arterioles in the cortex, hippocampus, and deep gray matter, with a lesser accumulation in the white matter. The permeability of the BBB increased with age in Col18a1-/- mice and affected cortical regions and the hippocampus in mice aged 12 months. However, none of the Col18a1-/- mice displayed hallmarks typical of more advanced stages of CSVD, such as perivascular space enlargement or large bleeds or infarcts. Collagen XVIII deficiency-induced BBB leakage was accompanied by activation of microglia and astrocytes, a loss of aggrecan in the ECM of perineuronal nets associated with fast-spiking inhibitory interneurons and accumulation of the perisynaptic ECM proteoglycan brevican and the microglial complement protein C1q at excitatory synapses. As the pathway underlying these regulations, we found increased signaling through the TGF-ß1/Smad3/TIMP-3 cascade. Thus, collagen XVIII proved to be crucial for the structural integrity of small vessels and its absence leads to pathological changes typical of early stages of CSVD. Furthermore, this study highlights an association between the alterations of perivascular ECM, extracellular proteolysis, and perineuronal/perisynaptic ECM, as a possible substrate of synaptic and cognitive alterations in CSVD.

BackgroundThe pathological alterations of small arteries and capillaries defined as cerebral small vessel disease (CSVD) are characterized by age-dependent blood-brain barrier (BBB) breakdown and vessel wall remodeling manifesting in small vessel occlusions and perivascular bleeds. As collagen XVIII (Col18a1) is an abundant heparan sulfate proteoglycan in vascular basement membranes, we focused on studying the role of collagen XVIII in vascular integrity and neurovascular unit maintenance in the context of CSVD.MethodsIn this study, we examined BBB breakdown, neuroinflammation and (peri)synaptic protein alterations using immunohistochemistry in 5- and 12-month-old Col18a1-/- and Col18a1+/+ mice. To further characterize molecular alterations, we used qPCR and compared the expression levels of major genes encoding tight junction and basement membrane proteins, inflammatory markers, neural extracellular matrix (ECM) proteins and proteinases.ResultsOur immunohistochemical analysis revealed progression of small vessel leakage in Col18a1-/- mice between 5 and 12 months. A large fraction (60%) of the small vessel area became mouse IgG-immunopositive in the hippocampus and retrosplenial cortex of 12-month-old Col18a1-/- mice. However, none of the Col18a1-/- mice displayed hallmarks typical of more advanced stages of CSVD, such as perivascular or large bleeds or infarcts. Collagen XVIII deficiency-induced BBB leakage was accompanied by activation of microglia and astrocytes, leading to perivascular ECM remodeling, upregulation of Timp3 and accumulation of perisynaptic ECM proteoglycan brevican and complement protein C1q, which may underlie impaired synaptic plasticity and loss of synapses.ConclusionOur findings highlight that Col18a1-/- mice represent a valuable model of early CSVD and call for more mechanistic analysis of underlying mechanisms and cognitive dysfunction in this model.