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We examine the effects of imperfect phase estimation of a reference signal on the bit error rate and mutual information over a communication channel influenced by fading and thermal noise. The Two-Wave Diffuse-Power (TWDP) model is utilized for statistical characterization of propagation environment where there are two dominant line-of-sight components together with diffuse ones. We derive novel analytical expression of the Fourier series for probability density function arising from the composite received signal phase. Further, the expression for the bit error rate is presented and numerically evaluated. We develop efficient analytical, numerical and simulation methods for estimating the value of the error floor and identifying the range of acceptable signal-to-noise ratio (SNR) values in cases when the floor is present during the detection of multilevel phase-shift keying (PSK) signals. In addition, we use Monte Carlo simulations in order to evaluate the mutual information for modulation orders two, four and eight, and identify its dependence on receiver hardware imperfections under the given channel conditions. Our results expose direct correspondence between bit error rate and mutual information value on one side, and the parameters of TWDP channel, SNR and phase noise standard deviation on the other side. The results illustrate that the error floor values are strongly influenced by the phase noise when signals propagate over a TWDP channel. In addition, the phase noise considerably affects the mutual information.

Cryptococcus neoformans (Cn) is a pathogenic yeast that is the leading cause of fungal meningitis in immunocompromised patients. Various Cn virulence factors, such as the enzyme laccase and its product melanin, phospholipase, and capsular polysaccharide have been identified. During a screen of knockout mutants, the gene resistance to aminocholesterol 1 (RTA1) was identified, the function of which is currently unknown in Cn. Rta1 homologs in S. cerevisiae belong to a lipid-translocating exporter family of fungal proteins with transmembrane regions and confer resistance to the antimicrobial agent 7-aminocholesterol when overexpressed. To determine the role of RTA1 in Cn, the knock-out (rta1Δ) and reconstituted (rta1Δ+RTA1) strains were created and phenotypically tested. RTA1 was involved in resistance to 7-aminocholesterol, and also in exocyst complex component 3 (Sec6)-mediated secretion of urease, laccase, and the major capsule component, glucuronoxylomannan (GXM), which coincided with significantly smaller capsules in the rta1Δ and rta1Δ+RTA1 strains compared to the wild-type H99 strain. Furthermore, RTA1 expression was reduced in a secretory 14 mutant (sec14Δ) and increased in an RNAi Sec6 mutant. Transmission electron microscopy demonstrated vesicle accumulation inside the rta1Δ strain, predominantly near the cell membrane. Given that Rta1 is likely to be a transmembrane protein located at the plasma membrane, these data suggest that Rta1 may be involved in both secretion of various fungal virulence factors and resistance to 7-aminocholesterol in Cn.

Cryptococcus neoformans is a human fungal pathogen that adapts its metabolism to cope with limited oxygen availability, nutrient deprivation and host phagocytes. To gain insight into cryptococcal metabolism, we optimized a protocol for the Seahorse Analyzer, which measures extracellular acidification rate (ECAR) and oxygen consumption rate (OCR) as indications of glycolytic and respiratory activities. In doing so we achieved effective immobilization of encapsulated cryptococci, established Rotenone/Antimycin A and 2-deoxyglucose as effective inhibitors of mitochondrial respiration and glycolysis, respectively, and optimized a microscopy-based method of data normalization. We applied the protocol to monitor metabolic changes in the pathogen alone and in co-culture with human blood-derived monocytes. We also compared metabolic flux in wild-type C. neoformans, its isogenic 5-PP-IP5/IP7-deficient metabolic mutant kcs1∆, the sister species of C. neoformans, Cryptococcus deuterogattii/VGII, and two other yeasts, Saccharomyces cerevisiae and Candida albicans. Our findings show that in contrast to monocytes and C. albicans, glycolysis and respiration are tightly coupled in C. neoformans and C. deuterogattii, as no compensatory increase in glycolysis occurred following inhibition of respiration. We also demonstrate that kcs1∆ has reduced metabolic activity that correlates with reduced mitochondrial function. Metabolic inflexibility in C. neoformans is therefore consistent with its obligate aerobe status and coincides with phagocyte tolerance of ingested cryptococcal cells.

Opportunistic fungi are a major cause of morbidity and mortality world-wide, particularly in immunocompromised individuals. Developing new treatments to combat invasive fungal disease is challenging given that fungal and mammalian host cells are eukaryotic, with similar organization and physiology. Even therapies targeting unique fungal cell features have limitations and drug resistance is emerging. New approaches to the development of antifungal drugs are therefore needed urgently. Cryptococcus neoformans, the commonest cause of fungal meningitis worldwide, is an accepted model for studying fungal pathogenicity and driving drug discovery. We recently characterized a phospholipase C (Plc1)-dependent pathway in C. neoformans comprising of sequentially-acting inositol polyphosphate kinases (IPK), which are involved in synthesizing inositol polyphosphates (IP). We also showed that the pathway is essential for fungal cellular function and pathogenicity. The IP products of the pathway are structurally diverse, each consisting of an inositol ring, with phosphate (P) and pyrophosphate (PP) groups covalently attached at different positions. This review focuses on (1) the characterization of the Plc1/IPK pathway in C. neoformans; (2) the identification of PP-IP5 (IP7) as the most crucial IP species for fungal fitness and virulence in a mouse model of fungal infection; and (3) why IPK enzymes represent suitable candidates for drug development.