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Vol. 18, Issue 6, 2296-2304, June 2007

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The Nucleolar Channel System of Human Endometrium Is Related to Endoplasmic Reticulum and R-Rings

Nupur Kittur^*, Gregory Zapantis, Mira Aubuchon, Nanette Santoro, David P. Bazett-Jones, and U. Thomas Meier^*

Departments of *Anatomy and Structural Biology and Obstetrics, Gynecology, and Women's Health, Albert Einstein College of Medicine, Bronx, NY 10461; and The Hospital for Sick Children, Toronto, ON M5G 1L7, Canada

Submitted February 20, 2007; Accepted April 2, 2007
Monitoring Editor: A. Gregory Matera

The nucleolar channel system (NCS) is a well-established ultrastructural hallmark of the postovulation endometrium. Its transient presence has been associated with human fertility. Nevertheless, the biogenesis, composition, and function of these intranuclear membrane cisternae are unknown. Membrane systems with a striking ultrastructural resemblance to the NCS, termed R-rings, are induced in nuclei of tissue culture cells by overexpression of the central repeat domain of the nucleolar protein Nopp140. Here we provide a first molecular characterization of the NCS and compare the biogenesis of these two enigmatic organelles. Like the R-rings, the NCS consists of endoplasmic reticulum harboring the marker glucose-6-phosphatase. R-ring formation initiates at the nuclear envelope, apparently by a calcium-mediated Nopp140-membrane interaction, as supported by the calcium-binding ability of Nopp140, the inhibition of R-ring formation by calcium chelators, and the concentration of Nopp140 and complexed calcium in R-rings. Although biogenesis of the NCS may initiate similarly, the reduced presence of complexed calcium and Nopp140 suggests the involvement of additional factors.

Address correspondence to: Thomas Meier (meier{at}aecom.yu.edu)

Abbreviations used: NCS, nucleolar channel system; ER, endoplasmic reticulum.