The effect of L-DOPA on Cryptococcus neoformans growth and gene expression

Cryptococcus neoformans is unusual among melanotic fungi in that it requires an exogenous supply of precursor to synthesize melanin. C. neoformans melanizes during mammalian infection in a process that presumably uses host-supplied compounds such as catecholamines. L-3,4-dihydroxyphenylalanine (L-DOPA) is a natural catecholamine that is frequently used to induce melanization in C. neoformans and L-DOPA-melanized cryptococci manifest resistance to radiation, phagocytosis, detergents and heavy metals. Given that C. neoformans needs exogenous substrate for melanization one question in the field is the extent to which melanin-associated phenotypes reflect the presence of melanin or metabolic changes in response to substrates. In this study we analyze the response of C. neoformans to L-DOPA with respect to melanization, gene expression and metabolic incorporation. Increasing the concentration of L-DOPA promotes melanin formation up to concentrations > 1 mM, after which toxicity is apparent as manifested by reduced growth. The timing of C. neoformans cells to melanization is affected by growth phase and cell density. Remarkably, growth of C. neoformans in the presence of L-DOPA results in the induction of relatively few genes, most of which could be related to stress metabolism. We interpret these results to suggest that the biological effects associated with melanization after growth in L-DOPA are largely due to the presence of the pigment. This in turn provides strong support for the view that melanin contributes to virulence directly through its presence in the cell wall.