A pericellular hyaluronan matrix is required for the morphological maturation of cortical neurons

Alteration of mechanical stresses in the murine brain by age and hemorrhagic stroke

Residual mechanical stresses, also known as solid stresses, emerge during rapid differential growth or remodeling of tissues, as observed in morphogenesis and tumor growth. While residual stresses typically dissipate in most healthy adult organs, as the growth rate decreases, high residual stresses have been reported in mature, healthy brains. However, the origins and consequences of residual mechanical stresses in the brain across health, aging, and disease remain poorly understood. Here, we utilized and validated a previously developed method to map residual mechanical stresses in the brains of mice across three age groups: 5-7 days, 8-12 weeks, and 22 months. We found that residual solid stress rapidly increases from 5-7 days to 8-12 weeks and remains high in mature 22 months mice brains. Three-dimensional mapping revealed unevenly distributed residual stresses from the anterior to posterior coronal brain sections. Since the brain is rich in negatively charged hyaluronic acid, we evaluated the contribution of charged extracellular matrix (ECM) constituents in maintaining solid stress levels. We found that lower ionic strength leads to elevated solid stresses, consistent with its unshielding effect and the subsequent expansion of charged ECM components. Lastly, we demonstrated that hemorrhagic stroke, accompanied by loss of cellular density, resulted in decreased residual stress in the murine brain. Our findings contribute to a better understanding of spatiotemporal alterations of residual solid stresses in healthy and diseased brains, a crucial step toward uncovering the biological and immunological consequences of this understudied mechanical phenotype in the brain.

Assembly of neuron- and radial glial-cell-derived extracellular matrix molecules promotes radial migration of developing cortical neurons

Radial neuronal migration is a key neurodevelopmental event for proper cortical laminar organization. The multipolar-to-bipolar transition, a critical step in establishing neuronal polarity during radial migration, occurs in the subplate/intermediate zone (SP/IZ), a distinct region of the embryonic cerebral cortex. It has been known that the extracellular matrix (ECM) molecules are enriched in the SP/IZ. However, the molecular constitution and functions of the ECM formed in this region remain poorly understood. Here, we identified neurocan (NCAN) as a major chondroitin sulfate proteoglycan in the mouse SP/IZ. NCAN binds to both radial glial-cell-derived tenascin-C (TNC) and hyaluronan (HA), a large linear polysaccharide, forming a ternary complex of NCAN, TNC, and HA in the SP/IZ. Developing cortical neurons make contact with the ternary complex during migration. The enzymatic or genetic disruption of the ternary complex impairs radial migration by suppressing the multipolar-to-bipolar transition. Furthermore, both TNC and NCAN promoted the morphological maturation of cortical neurons in vitro. The present results provide evidence for the cooperative role of neuron- and radial glial-cell-derived ECM molecules in cortical development.

Impact of hyaluronan size on localization and solubility of the extracellular matrix in the mouse brain

Hyaluronan (HA) is a central component of the extracellular matrix (ECM) in the brain and plays a pivotal role in neural development and plasticity. Brain HA exists in 2 distinct forms of the ECM: the diffuse ECM, which is soluble in saline and detergents, and the condensed ECM, which forms aggregates, such as perineuronal nets (PNNs). Although the physiological functions of HA significantly differ depending on its size, size differences in HA have not yet been examined in the 2 ECM types, which is partly because of the lack of methods to rapidly and accurately measure the molecular weight (MW) of HA. In this study, we established a simple method to simultaneously assess the MW of HA in multiple crude biological samples. HA was purified through single-step precipitation from tissue extracts using biotinylated HA-binding protein and streptavidin-coupled magnetic beads, followed by separation on gel electrophoresis. By applying this method to HA in the mouse brain, we revealed that the condensed ECM contained higher MW HA than the diffuse ECM. Higher MW HA and lower MW HA exhibited different spatial distributions: the former was confined to PNNs, whereas the latter was widely present throughout the brain. Furthermore, the limited degradation of HA showed that only higher MW HA was required to form an insoluble HA-aggrecan complex. The present study demonstrated that the MW of HA in the brain strongly correlates with the localization and solubility of the ECM it forms.

Removal of perineuronal nets leads to altered neuronal excitability and synaptic transmission in the visual cortex with distinct time courses

Parvalbumin-expressing (PV) interneurons fast inhibit excitatory neurons in various brain areas. Perineuronal nets (PNNs), accumulating around PV neurons, have been shown to play critical roles in neuronal function and plasticity. The cellular mechanisms underlying their functions are still in debate, for example, do PNNs contribute significantly to the excitability of inhibitory neurons especially those containing PV? On the other hand, whether PNNs have significant contributions to synaptic transmission of PV neurons is much less unknown. In this study, we designed experiments to address these questions and found that removing PNNs in vivo using chondroitinase ABC (ChABC) led to distinct changes in neuronal excitability and synaptic transmission, depending on the duration of ChABC treatment. The results showed 7 days after ChABC treatment reduced both intrinsic excitability of PV neurons and synaptic transmission to both PV neurons and excitatory neurons in the primary visual cortex. However, 1 day after ChABC treatment digested PNNs effectively but had no effects on intrinsic excitability and synaptic transmission. These results suggest the contribution of PNNs to neuronal excitability and synaptic transmission depends on different time courses of ChABC digestion.

Matters of size: Roles of hyaluronan in CNS aging and disease

Involvement of extracellular matrix (ECM) components in aging and age-related neurodegeneration is not well understood. The role of hyaluronan (HA), a major extracellular matrix glycosaminoglycan, in malignancy and inflammation is gaining new understanding. In particular, the differential biological effects of high molecular weight (HMW-HA) and low molecular weight hyaluronan (LMW-HA), and the mechanism behind such differences are being uncovered. Tightly regulated in the brain, HA can have diverse effects on cellular development, growth and degeneration. In this review, we summarize the homeostasis and signaling of HA in healthy tissue, discuss its distribution and ontogeny in the central nervous system (CNS), summarize evidence for its involvement in age-related neurodegeneration and Alzheimer Disease (AD), and assess the potential of HA as a therapeutic target in the CNS.

Hyaluronan degradation and release of a hyaluronan-aggrecan complex from perineuronal nets in the aged mouse brain

BACKGROUND

Perineuronal nets (PNNs) are insoluble aggregates of extracellular matrix molecules in the brain that consist of hyaluronan (HA) and chondroitin sulfate proteoglycans (CSPGs). PNNs promote the acquisition and storage of memories by stabilizing the formation of synapses in the adult brain. Although the deterioration of PNNs has been suggested to contribute to the age-dependent decline in brain function, the molecular mechanisms underlying age-related changes in PNNs remain unclear.

METHODS

The amount and solubility of PNN components were investigated by sequential extraction followed by a disaccharide analysis and immunoblotting. We examined the interaction between HA and aggrecan, a major HA-binding CSPG, by combining mass spectrometry and pull-down assays.

RESULTS

The solubility and amount of HA increased in the brain with age. Among several CSPGs, the solubility of aggrecan was selectively elevated during aging. In contrast to alternations in biochemical properties, the expression of PNN components at the transcript level was not markedly changed by aging. The increased solubility of aggrecan was not due to the loss of HA-binding properties. Our results indicated that the degradation of high-molecular-mass HA induced the release of the HA-aggrecan complex from PNNs in the aged brain.

CONCLUSION

The present study revealed a novel mechanism underlying the age-related deterioration of PNNs in the brain.