Aging as an emergent factor that contributes to phenotypic variation in Cryptococcus neoformans

Abstract

Cryptococcus neoformans, similar to other eukaryotes, undergoes replicative aging. Replicative life spans have been determined for clinical C. neoformans strains, and although they are a reproducible trait, life spans vary considerably among strains. C. neoformans has been proposed as an ideal model organism to investigate the contribution of replicative aging in a fungal pathogen population to emerging phenotypic variation during chronic cryptococcal infections. C. neoformans cells of advanced generational age manifest a distinct phenotype; specifically, a larger cell size, a thicker cell wall, drug resistance, as well as resistance to hydrogen peroxide-mediated killing. Consequently, old cells are selected in the host environment during chronic infection and aging could be an unanticipated mechanism of pathogen adaptation that contributes to persistent disease. Aging as a natural process of phenotypic variation should be further studied as it likely is also relevant for other eukaryotic pathogen populations that undergo asymmetric replicative aging.

Keywords: Aging; C. neoformans; Pathogen; Phenotypic variation.

Figure 1. Birth size of clinical C. neoformans cells does not correlate with the strain’s replicative life span (RLS)

Birth cell body size of C. neoformans cells (n = 360) did not correlate with the median RLS of the strain (Spearman r = −0.006).

Figure 2. The cell walls of old C. neoformans cells are not weakened with age unlike those of old S. cerevisiae cells

(A) Old S. cerevisiae cells can be stained with calcofluor for bud scars, which weaken the cell wall with each division. (B) Old C. neoformans cells cannot be accurately stained for bud scars, and the cell wall appears to thicken with each division.

Figure 3. Schemata demonstrating the rare probability of finding old cells

The probability of finding a mutant phenotype (red) in a clonally expanding population is 1/1 regardless of the number of replications because the phenotype is inherited by all progeny. However, the probability of finding a cell with a specific age (20-generation-old cell in red) is rare, and is 1/10⁶ in a population that has replicated 20 times.

Figure 4. A model that depicts the relationship between replicative age and intracellular proliferation rate of C. neoformans

The pathogen is most likely to benefit during middle age, when it has acquired the old age phenotype and still proliferates at a rate comparable to that of young cells.