Nuclear Structure and Function

All eukaryotic cells enclose their genome within a dedicated, membrane-bound organelle termed the nucleus, which is the site of major cellular events such as messenger RNA synthesis and processing, ribosome subunit biogenesis and DNA replication.

 

Like the cytoplasm, the nucleus is compartmentalized to facilitate efficient coordination of these pathways, It has a highly organized three-dimensional architecture that starts with the packaging of chromatin and culminates in the self-ordered assembly of several membrane-free organelles or “nuclear bodies” that concentrate factors involved in specific molecular pathways.   Numerous disease states have been linked to dysfunction of these nuclear bodies.    

Our lab studies the contribution of specific nuclear PP1 complexes to these processes. Key nuclear targeting subunits include PNUTS, NIPP1, RepoMan, Taperin and RRP1B.

Nucleolus: the consummate nuclear body

Of all the nuclear bodies, the nucleolus has been studied in the greatest detail, due both to its prominence and ease of isolation and to increased appreciation of its essential contribution to the maintenance of cellular homeostasis.  Although best known for its originally described role as the site of ribosome production, this plurifunctional organelle has since been shown to play additional roles in the regulation of growth and cell cycle progression, DNA damage sensing and repair, telomere metabolism, RNA processing, biogenesis of ribonucleoprotein particles and coordination of cellular stress response. 

Ultrastructural analysis of nucleoli at the nanometer scale by electron microscopy confirmed that this organelle lacks a membrane and revealed the existence of a perinucleolar shell of condensed chromatin.

EM analysis also revealed a tripartite substructure comprising a concentric arrangement of: 1. an innermost lightly stained fine fibrillar structure (Fibrillar Center; FC) mostly surrounded by 2. densely packed fibrils (Dense Fibrillar Component; DFC) and embedded in 3. a grainy peripheral region (Granular Component; GC) comprising RNP particles of 15-20 nm in size

Markers are available for all 3 subregions. Shown here is a GFP-tagged GC protein (blue), an immunostained DFC protein (red) and the biotinylation pattern of a biotin ligase-tagged FC protein (green)

Ribosome Biogenesis

It is estimated that over half of the cell’s capacity is dedicated to making pre-rRNA. A large excess of ribosomal proteins, with constant turnover mediated by the proteasome, is also maintained.

 

Although this provides cells with the flexibility to rapidly adjust to changes in metabolic demand, it is an energetically expensive process, and its coordinated shutdown is thus a key strategy used by the cell to maintain energy homeostasis under conditions of cellular stress, such as nutrient deprivation and heat shock.

PP1 complexes play regulatory roles at all stages of ribosome biogenesis, from rDNA transcription through rRNA processing and ribosomal subunit assembly and export.

For more information about nuclear structures and dynamics, see:

Trinkle-Mulcahy L. Nucleolus: The Consummate Nuclear Body. Nuclear Architecture and Dynamics. Ed. Lavelle, C and Victo, J-M. Academic.

Trinkle-Mulcahy L and Sleeman, JE..The Cajal Body and the Nucleolus: "In a Relationship" or "It's Complicated"?.RNA Biol.14:739-51, 2016.

Lam, YW, Trinkle-Mulcahy, L. New insights into nucleolar structure and function. F1000Prime Rep 2015, 7:48.

Sleeman, JE and Trinkle-Mulcahy, L. Nuclear bodies: new insights into assembly/dynamics and disease relevance. Curr Op Cell Biol.28:76-83, 2014.

Trinkle-Mulcahy, L. and Lamond, A.I. Nuclear functions in space and time: Gene expression in a dynamic, constrained environment. FEBS Lett. 582:1960-70, 2008.

Trinkle-Mulcahy, L. and Lamond, A.I. Toward a high-resolution view of nuclear dynamics. Science. 318:1402-07, 2007.

 Lam, Y.W., Trinkle-Mulcahy, L. and Lamond, A.I. The Nucleolus. J. Cell Sci., 118:1335-1337, 2005.

Department of Cellular & Molecular Medicine

Ottawa Institute of Systems Biology

University of Ottawa

Ottawa, Canada

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