Cleavage of Hyaluronan and CD44 Adhesion Molecule Regulate Astrocyte Morphology via Rac1 Signalling

Abstract

Communication of cells with their extracellular environment is crucial to fulfill their function in physiological and pathophysiological conditions. The literature data provide evidence that such a communication is also important in case of astrocytes. Mechanisms that contribute to the interaction between astrocytes and extracellular matrix (ECM) proteins are still poorly understood. Hyaluronan is the main component of ECM in the brain, where its major receptor protein CD44 is expressed by a subset of astrocytes. Considering the fact that functions of astrocytes are tightly coupled with changes in their morphology (e.g.: glutamate clearance in the synaptic cleft, migration, astrogliosis), we investigated the influence of hyaluronan cleavage by hyaluronidase, knockdown of CD44 by specific shRNA and CD44 overexpression on astrocyte morphology. Our results show that hyaluronidase treatment, as well as knockdown of CD44, in astrocytes result in a “stellate”-like morphology, whereas overexpression of CD44 causes an increase in cell body size and changes the shape of astrocytes into flattened cells. Moreover, as a dynamic reorganization of the actin cytoskeleton is supposed to be responsible for morphological changes of cells, and this reorganization is controlled by small GTPases of the Rho family, we hypothesized that GTPase Rac1 acts as a downstream effector for hyaluronan and CD44 in astrocytes. We used FRET-based biosensor and a dominant negative mutant of Rac1 to investigate the involvement of Rac1 activity in hyaluronidase- and CD44-dependent morphological changes of astrocytes. Both, hyaluronidase treatment and knockdown of CD44, enhances Rac1 activity while overexpression of CD44 reduces the activity state in astrocytes. Furthermore, morphological changes were blocked by specific inhibition of Rac1 activity. These findings indicate for the first time that regulation of Rac1 activity is responsible for hyaluronidase and CD44-driven morphological changes of astrocytes.

CD44: a novel synaptic cell adhesion molecule regulating structural and functional plasticity of dendritic spines

“…We recently demonstrated the high knockdown efficiency of CD44 shRNA in young primary hippocampal neurons (Skupien et al. , 2014) and astrocytes (Konopka et al. , 2016).…”

Astrocyte Structural and Molecular Response to Elevated Intraocular Pressure Occurs Rapidly and Precedes Axonal Tubulin Rearrangement within the Optic Nerve Head in a Rat Model

“…The highly ordered and unique arrangement of ONH astrocyte extensions perpendicular to the axonal axis [7, 8], as well as their intimate contact with the extracellular matrix (ECM) [9], make them prime candidates for sensing and responding to mechanical strain from IOP fluctuations. Astrocytes are positioned along the connective tissues of the ONH, including the laminar beams in the primate ONH [10].…”

“…The astrocytic actin cytoskeleton is important for astrocyte morphology [35], extension formation [35], migration [36, 37], communication and interaction with the ECM [9], and axonal neuroprotection [38]. In addition, nearly 100 ONH actin cytoskeletal and integrin-related gene expression patterns are altered with early glaucomatous injury in rodent models of glaucoma [39, 40].…”

CD44 inhibits α-SMA gene expression via a novel G-actin/MRTF mediated pathway that requires TGFβR/p38MAPK activity in murine skin fibroblasts

“…For example, CD44 has a wellestablished role as an organizer of the cortical actin cytoskeleton through direct interactions with ERM (Ezrin/Radixin/Moesin) (19,53) or other membraneassociated cytoskeletal proteins such as ankyrin (54). In addition, CD44 may also regulate actin cytoskeleton by modulating the activity of Rho GTPases (55)(56)(57)(58)(59)(60)(61)(62)(63). For example, CD44 can mediate cell adhesion (64)(65)(66).…”