Hyaluronic Acid Receptor CD44 Deficiency Is Associated with Decreased Cryptococcus neoformans Brain Infection (original) (raw)

2012, Journal of Biological Chemistry

Background: Previous studies suggest that the C. neoformans hyaluronic acid and host CD44 interaction could be the initial step of brain invasion. Results: CD44 knockout mouse model verified the pathophysiology of C. neoformans brain invasion. Conclusion: The CD44 receptor on membrane lipid rafts is involved. Significance: The mechanism of invasion and a potential clinical intervention strategy were identified. Cryptococcus neoformans is a pathogenic yeast that can invade the brain and cause meningoencephalitis. Our previous in vitro studies suggested that the interaction between C. neoformans hyaluronic acid and human brain endothelial CD44 could be the initial step of brain invasion. In this report, we used a CD44 knockout (KO or CD44 ؊/؊) mouse model to explore the importance of CD44 in C. neoformans brain invasion. Our results showed that C. neoformans-infected CD44 KO mice survived longer than the infected wild-type mice. Consistent with our in vitro results, the brain and cerebrospinal fluid fungal burden was reduced in CD44-deficient mice. Histopathological studies showed smaller and fewer cystic lesions in the brains of CD44 KO mice. Interestingly, the cystic lesions contained C. neoformans cells embedded within their polysaccharide capsule and were surrounded by host glial cells. We also found that a secondary hyaluronic acid receptor, RHAMM (receptor of hyaluronanmediated motility), was present in the CD44 KO mice. Importantly, our studies demonstrated an in vivo blocking effect of simvastatin. These results suggest that the CD44 and RHAMM receptors function on membrane lipid rafts during invasion and that simvastatin may have a potential therapeutic role in C. neoformans infections of the brain. Cryptococcus neoformans invades the brain and causes meningoencephalitis, primarily in immunocompromised patients and sporadically in normal hosts. It is the most common fungal infection of the central nervous system and is one of the major causes of death in AIDS patients (1, 2). C. neoformans is encompassed by a polysaccharide capsule composed mainly of glucuronoxylomannan (GXM) 3 (3); this capsule is known to be the major virulence factor of this yeast (4-6). As such, the role and biogenesis of the capsule has become an area of focus for research in this pathogen (7). Previously, we characterized the C. neoformans gene CPS1 (capsule polysaccharide synthase 1) (8). Deletion of CPS1 from C. neoformans cells causes alterations in the ultrastructure between the cell wall and capsule, and the deletant shows a reduced ability to associate with human brain microvascular endothelial cells (HBMEC). Polysaccharide hyaluronic acid (HA; also known as hyaluronan) can be detected in the wild-type C. neoformans strain but not in cps1⌬ cells. Testing of C. neoformans strains with different concentrations of HA demonstrated that the ability of yeast to bind to HBMEC is proportional to their HA content. Subsequent analysis indicated that the CPS1 gene encodes an HA synthase (9). Taken together, these findings indicate that the C. neoformans CPS1 gene product, HA, plays a role as an adhesion molecule during interaction with endothelial cells. Several HA-binding proteins (or HA receptors) localized in the membranes, such as CD44, RHAMM, Ivd4, LEC receptor, and others, have been identified in different cell types (10, 11). The most common HA receptor is CD44, which plays different roles in different cell types. As CD44 is a major membrane HA receptor, it is conceivable that C. neoformans HA directly engages with the HBMEC CD44 as a part of its invasion mechanism. Indeed, we have demonstrated that C. neoformans HA interacts with CD44 on HBMEC, a primary receptor in C. neoformans infection (12). For example, an anti-CD44 neutralizing antibody treatment significantly reduces C. neoformans association with HBMEC. Association of C. neoformans with HBMEC is also found to be considerably impaired either in CD44 knockdown HBMEC or HA-deficient C. neoformans