Heat shock induces mini-Cajal bodies

Cajal bodies (CBs, formerly coiled bodies) and nucleoli share several biological features, including close physical proximity, overlapping protein profiles, disassembly during mitosis, and association with specific chromosomal loci (reviewed by Gall, 2000). Despite these similarities, biogenesis of the two organelles appears to be quite different. Nucleoli normally form on chromosomes at the nucleolus organizers (NOs), which contain actively transcribing genes coding for ribosomal RNA (Hadjiolov, 1985; Shaw and Jordan, 1995; Scheer and Hock, 1999; Olson et al., 2000). The crucial role of these genes in nucleolar biogenesis is illustrated by formation of a small nucleolus at a single transposed rDNA unit in Drosophila (Karpen et al., 1988) and by production of multiple nucleoli in oocytes of many species, including Xenopus, on amplified extrachromosomal copies of the rDNA (Brown and Dawid, 1968; Gall, 1968; Miller and Beatty, 1969). CBs also preferentially associate with specific chromosomal sites, at least in mammalian cultured cells and amphibian oocytes. These sites are the genes coding for histones (Gall et al., 1981; Callan et al., 1991; Frey and Matera, 1995) and for several small nuclear (sn) RNAs (Frey and Matera, 1995; Smith et al., 1995; Gao et al., 1997; Frey et al., 1999; Jacobs et al., 1999). However, most CBs exist free in the nucleoplasm, and there is no evidence that they are shed there after production at the chromosomal sites. Moreover, observations on nuclei assembled in Xenopus egg extract suggest that CBs can arise in nuclei that contain only bacteriophage DNA (Bell et al., 1992). Thus, formation of nucleoli depends on rDNA, whereas CBs can arise in the complete absence of eukaryotic DNA. Without compelling evidence that CBs originate at specific genetic loci, emphasis has focused on the possibility that some unique molecule or organelle might act as the seed or scaffold for their formation. In addition to factors found elsewhere in the nucleus, CBs are enriched for the marker protein coilin and the rare U7 snRNA involved in histone pre-mRNA processing (reviewed by Gall, 2000). Coilin and U7 snRNA have thus been considered as strong candidates for nucleators of CB formation. Evidence against such a role for coilin is now strong. CBs form in nuclei assembled in Xenopus egg extract, even when the extract has been immunodepleted of coilin (Bauer and Gall, 1997). Likewise, a mouse knockout for the coilin gene displays CB-like structures in its nuclei (Tucker et al., 2001). Evidence that U7 snRNA can play a role in CB biogenesis has been presented in a previous study (Tuma and Roth, 1999). The study showed that injection of U7 snRNA into the Xenopus oocyte nucleus (germinal vesicle, GV) was sufficient to induce formation of small ‘coiled body-like structures’. We describe the appearance of mini-CBs (<2 μm) in the GVs of Xenopus oocytes that have recovered from heat shock. These mini-CBs are similar in size and number to those described previously (Tuma and Roth, 1999). However, an antisense depletion experiment shows that U7 snRNA is not necessary for their induction by heat shock. Neither transcription of RNA nor import of newly translated proteins into the GV is required, suggesting that mini-CBs assemble from pre-existing components in the GV. 2011