The ability to sense and adapt to a hostile host environment is a crucial element for virulence of pathogenic fungi, including Cryptococcus neoformans. These cellular responses are evoked by diverse signaling cascades, including the stress-activated HOG pathway. Despite previous analysis of central components of the HOG pathway, its downstream signaling network is poorly characterized in C. neoformans. Here we performed comparative transcriptome analysis with HOG signaling mutants to explore stress-regulated genes and their correlation with the HOG pathway in C. neoformans. In this study, we not only provide important insights into remodeling patterns of global gene expression for counteracting external stresses but also elucidate novel characteristics of the HOG pathway in C. neoformans. First, inhibition of the HOG pathway increases expression of ergosterol biosynthesis genes and cellular ergosterol content, conferring a striking synergistic antifungal activity with amphotericin B and providing an excellent opportunity to develop a novel therapeutic method for treatment of cryptococcosis. Second, a number of cadmium-sensitive genes are differentially regulated by the HOG pathway, and their mutation causes resistance to cadmium. Finally, we have discovered novel stress defense and HOG-dependent genes, which encode a sodium/potassium efflux pump, protein kinase, multidrug transporter system, and elements of the ubiquitin-dependent system.

The cyclic AMP (cAMP) pathway plays a central role in the growth, differentiation, and virulence of pathogenic fungi, including Cryptococcus neoformans. Three upstream signaling regulators of adenylyl cyclase (Cac1), Ras, Aca1, and Gpa1, have been demonstrated to control the cAMP pathway in C. neoformans, but their functional relationship remains elusive. We performed a genome-wide transcriptome analysis with a DNA microarray using the ras1⌬, gpa1⌬, cac1⌬, aca1⌬, and pka1⌬ pka2⌬ mutants. The aca1⌬, gpa1⌬, cac1⌬, and pka1⌬ pka2⌬ mutants displayed similar transcriptome patterns, whereas the ras1⌬ mutant exhibited transcriptome patterns distinct from those of the wild type and the cAMP mutants. Interestingly, a number of environmental stress response genes are modulated differentially in the ras1⌬ and cAMP mutants. In fact, the Ras signaling pathway was found to be involved in osmotic and genotoxic stress responses and the maintenance of cell wall integrity via the Cdc24-dependent signaling pathway. Notably, the Ras and cAMP mutants exhibited hypersensitivity to a polyene drug, amphotericin B, without showing effects on ergosterol biosynthesis, which suggested a novel method of antifungal combination therapy. Among the cAMP-dependent gene products that we characterized, two small heat shock proteins, Hsp12 and Hsp122, were found to be involved in the polyene antifungal drug susceptibility of C. neoformans.

The temperature on Earth has been increased by 0.74 °C due to human activities over the past century. When the body temperature rises by 1 or 2 °C, a human feels the strangeness of health. Likewise, Earth temperature changes have resulted in various environmental abnormal phenomena, such as sea ice melting and desertification. Furthermore, temperature changes caused by global warming are causing extreme weather events, such as heat waves, large hurricanes, droughts, typhoons, heavy rains, and heavy snow in areas inhabited by humans. The main cause of global warming is carbon dioxide. Carbon dioxide is primarily emitted from fossil fuels used by humans. It is accumulated in the atmosphere and causes the greenhouse effect, thereby negatively affecting living organisms and humans' survival. The international community has recognized this problem, and members of the United Nations (UN) have agreed to the obligation of greenhouse gas (GHG) reduction at the 21st General Assembly of the Parties to the UN Framework Convention on Climate Change held in Paris in 2015. This UN obligation of GHG has been applied to all 195 UN member countries from January 2021. The convention presents the long-term goal of maintaining the global average temperature increase below 2 °C. Among anthropogenic GHG emissions, the shipping sector's contribution increased from 800 million tons in 2012 to 1 billion tons in 2018, constituting approximately 3% of the total global emissions. Hence, GHGs generated from the shipping industry must be reduced urgently. In April 2018, the International Maritime Organization (IMO) announced an early strategy to reduce GHGs emitted from ships by 50% compared with those emitted in 2008 and has continuously expressed the importance of reducing GHG emissions (Kim et al., 2018). To reduce GHGs in the shipping industry, the existing ships must be transformed into eco-friendly systems. To comply with environmental regulations, the global shipping industry changes ship fuels from heavy fuel oil (HFO) to liquefied natural gas (LNG) or marine diesel oil. And the additional emission reduction systems such as scrubber systems for SOx reduction and selective catalytic reduction systems for NOx reduction are installed on the ships (Lee, 2018). However, the solutions for GHG reduction in the shipping sector are still insufficient to achieve the IMO's GHG reduction goal. One of the well-known practical alternative solutions