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

Abstract

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. INTRODUCTIONFor close to half a century, the nucleolar channel system (NCS) has been known as an ultrastructural hallmark of the postovulation human endometrium (Dubrauszky and Pohlmann, 1960;Clyman, 1963;Moricard and Moricard, 1964;Terzakis, 1965). It consists of several layers of tubular membrane cisternae in the nuclei of endometrial epithelial cells and is often associated with the nuclear envelope and nucleoli, hence the name (for a review see Spornitz, 1992). This organelle appears during a 3-to 4-d window in the midsecretory phase of the menstrual cycle when the endometrium is receptive to implantation of the fertilized egg (Clyman, 1963;Gordon, 1975). Oral contraceptives and intrauterine devices interfere with the timing and/or overall appearance of the NCS (Wynn, 1967;Feria-Velasco et al., 1972;Azadian-Boulanger et al., 1976;van Santen et al., 1988;Dockery et al., 1997). Several cases of unexplained primary infertility have been characterized by the absence or delayed development of NCSs as the only ultrastructural alteration in endometrial epithelial cells (Kohorn et al., 1972;Gore and Gordon, 1974;Dockery et al., 1996). This and other correlative evidence suggests a role for the NCS in the preparation of the uterine surface for embryo implantation and points to the NCS as a long sought after marker for human receptivity. Nevertheless, despite this tantalizing data and the detailed ultrastructural description of the NCS, it has resisted molecular characterization. Here we provide first insights as to the composition of this enigmatic organelle.On the basis of their remarkable ultrastructural resemblance to the NCS, we previously characterized membrane systems, R-rings, that are induced in nuclei of tissue culture cells by overexpression of the nucleolar protein Nopp140 (Isaac et al., 1998;Isaac et al., 2001). Nopp140 functions as a chaper...

Intranuclear membranes induced by lipidated proteins are derived from the nuclear envelope

“…ER marker proteins did not induce intranuclear membranes on their own when expressed NCS-related structures, the R-rings, induced by overexpression of the nucleolar chaperon Nopp140, 6 for ODVs produced in insect cells during baculovirus infection, 17 for membranes formed after overexpression of particular nuclear proteins [22][23][24][25] as well as for those membranes formed in nuclei of several pathological cells. 13 In the above-mentioned cases the membrane budding from the INM has been either followed directly by live cell imaging 6,15,22 or membrane continuity between the intranuclear membranes and the INM has been demonstrated by TEM analysis. 6,10,13 In addition, the presence of ER marker proteins within the intranuclear membrane structures has been taken as conclusive evidence for their NE/ER origin.…”

“…13 In the above-mentioned cases the membrane budding from the INM has been either followed directly by live cell imaging 6,15,22 or membrane continuity between the intranuclear membranes and the INM has been demonstrated by TEM analysis. 6,10,13 In addition, the presence of ER marker proteins within the intranuclear membrane structures has been taken as conclusive evidence for their NE/ER origin. 23,24 Overexpression of lamins leads to formation of multilayered stacked membrane cisternae.…”

“…The best-analyzed ones are those of the nucleolar channel system (NCS) that appears transiently in the epithelial cell nuclei of post-ovulatory human endometrium. [5][6][7][8] The NCS consists of layers of tubular membrane cisternae and is often associated with nucleoli and the NE. Overexpression of the nucleolar chaperone Nopp140 in tissue culture cells induces the formation of R-rings, which show remarkable ultrastructural resemblance to the NCS.…”

“…Budding might be either into the perinuclear space, as in the case of herpesvirus, 20 baculovirus 17 and plant nucleorhabdoviruses 50 or into the nucleoplasm. The later holds for the formation of the NCS and the R-rings, 6 the production of ODVs in the course of baculovirus infection, 17 and for membrane formation after experimental overexpression of nuclear proteins. Depending on the frequency of budding continuities between INM and the membranes of the arrays might be observed regularly or, as in case of lamininduced arrays, not at all.…”

Mechanism of the AAA+ ATPases pontin and reptin in the biogenesis of H/ACA RNPs

“…For Western blots, mouse monoclonal anti-tubulin (Sigma; 0.3 mg/mL) and antifibrillarin antibodies (D77 at 1 mg/mL; Aris and Blobel 1988), and rabbit polyclonal anti-Nopp140 (RS8; Kittur et al 2007) and anti-NHP2 antisera (CG2; Grozdanov et al 2009b) were used in addition to those employed in the S100 inhibition experiments above. Probes and procedures for Northern blots were identical to those reported previously (Grozdanov et al 2009b).…”