Sirtuin-mediated nuclear differentiation and programmed degradation in Tetrahymena

Kristin M Slade,Kyle A Cottrell,Sydney Freggiaro,Emily A Wiley,Joshua J Smith

doi:10.1186/1471-2121-12-40

Abstract

BackgroundThe NAD+-dependent histone deacetylases, known as "sirtuins", participate in a variety of processes critical for single- and multi-cellular life. Recent studies have elucidated the importance of sirtuin activity in development, aging, and disease; yet, underlying mechanistic pathways are not well understood. Specific sirtuins influence chromatin structure and gene expression, but differences in their pathways as they relate to distinct chromatin functions are just beginning to emerge. To further define the range of global chromatin changes dependent on sirtuins, unique biological features of the ciliated protozoan Tetrahymena thermophila can be exploited. This system offers clear spatial and temporal separation of multiple whole genome restructuring events critical for the life cycle.ResultsInhibition with nicotinamide revealed that sirtuin deacetylase activity in Tetrahymena cells promotes chromatin condensation during meiotic prophase, differentiation of heterochromatin from euchromatin during development, and chromatin condensation/degradation during programmed nuclear death. We identified a class I sirtuin, called Thd14, that resides in mitochondria and nucleoli during vegetative growth, and forms a large sub-nuclear aggregate in response to prolonged cell starvation that may be peripherally associated with nucleoli. During sexual conjugation and development Thd14 selectively concentrates in the parental nucleus prior to its apoptotic-like degradation.ConclusionsSirtuin activity is important for several functionally distinct events requiring global chromatin condensation. Our findings suggest a novel role for sirtuins in promoting programmed pycnosis by acting on chromatin destined for degradation. The sirtuin Thd14, which displays physiological-dependent differential localization within the nucleus, is a candidate for a chromatin condensation enzyme that is coupled to nuclear degradation.

Highlights

The NAD+-dependent histone deacetylases, known as “sirtuins”, participate in a variety of processes critical for single- and multi-cellular life
We identified one sirtuin that resides in nucleoli, in mitochondria, and in distinct nuclear substructures, all in response to different physiological conditions and stages of the life cycle
Cells of two different mating types were mixed and immediately treated with 0, 10, 25 or 50 mM NAM. These concentrations were chosen based on those typically used for yeast and human cells (1-5 mM) and the fact that for Tetrahymena, other histone deacetylase inhibitors must be used at 5-10-fold higher concentrations for effects comparable to mammalian cells [19]

Summary

Introduction

The NAD+-dependent histone deacetylases, known as “sirtuins”, participate in a variety of processes critical for single- and multi-cellular life. To further define the range of global chromatin changes dependent on sirtuins, unique biological features of the ciliated protozoan Tetrahymena thermophila can be exploited. Class III histone deacetylases, known as sirtuins, are a large and ancient family of NAD+-dependent protein deacetylases that regulate a range of cellular processes. Many sirtuin-linked cell abnormalities may relate to their roles in chromatin dynamics To further probe these possibilities we turned to the single-celled protozoan Tetrahymena thermophila. The “micronucleus” is transcriptionally silent and contains chromatin that is highly condensed into constitutive heterochromatin-like structures throughout vegetative growth [12] This unique nuclear dimorphism facilitates study of factors that contribute to the differentiation and maintenance of euchromatin and heterochromatin states in the respective nuclei

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