Gene circuits that control morphology in Histoplasma

University Of California, San Francisco, San Francisco CA

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Abstract

Histoplasma capsulatum (Hc) is a thermally dimorphic fungus that is a primary pathogen of humans. Hc grows in the soil in a multicellular mold form. Once inhaled, in response to mammalian body temperature, Hc converts to a unicellular yeast form that colonizes macrophages. Temperature is a key signal that is sufficient to trigger the switch from the soil to host form (and vice versa); in the laboratory, room temperature promotes hyphal (mold-form) growth whereas 37ºC promotes yeast-phase growth. The long-term goal of this research is to determine the molecular basis of how temperature regulates morphology and virulence in thermally dimorphic fungi. By elucidating how Hc cells sense and respond to host temperature, we will define critical molecular landmarks that promote changes in morphology as well as the expression of virulence traits. Over the past 18 years, we have made key discoveries that illuminate the gene circuits that regulate Hc cell fate in response to temperature. We identified four transcription factors, Ryp1, 2, 3, and 4, that promote yeast-form growth in response to host temperature. The Ryp proteins associate with the upstream regions of both morphology and virulence genes, indicating that they directly regulate and link essential components of the morphology and virulence programs. We published fundamental contributions to the understanding of regulation of cell shape and virulence in response to temperature, using RNAseq and ribosomal profiling to comprehensively identify phase-specific transcripts and proteins. During the current funding period, we have made critical discoveries. First, we have discovered a naturally occurring, reversible chromosome duplication that influences filamentation in response to temperature. Cells containing the chromosomal duplication undergo robust filamentation in liquid culture at 25ºC, whereas wild-type (euploid) cells lacking the duplication remain as yeast for longer and take more time to initiate filamentation in response to temperature. We have defined a critical region within the chromosomal duplication that is responsible for enhanced filamentation, thereby providing insight into which genes on the duplication drive filamentation. Second, we have discovered a link between Ryp2 and direct control of expression of components of the heat shock pathway. Coupled with the observation that the highly conserved chaperone Hsp90 is required for yeast-phase growth at 37ºC, these data strongly suggest that the Ryp factors and the heat shock pathway intersect to drive temperature-dependent yeast-phase growth. Finally, we have made technological advances that allow us to probe critical elements of regulation such as temperature-dependent chromatin accessibility RELEVANCE (See instructions): Histoplasma is a primary pathogen that infects approximately 500,000 individuals per year in the U.S. and is a significant source of morbidity and mortality. The identification and characterization of regulatory pathways that influence cell shape and pathogenesis will significantly advance our understanding of how this organism responds to host signals such as temperature to cause disease.