Summary Control of body size by SMA-5 , a homolog of MAP kinase BMK 1 / ERK 5 , in C . elegans

There are many studies on the size of an animal or a plant in the literature. For example, mice that overexpressed growth hormone became larger (Palmiter et al., 1982), and in Dictyostelium, smlA mutant cells oversecrete a factor to form very small fruiting bodies (Brock and Gomer, 1999). In Drosophila, mutants in the components of the insulin/insulinlike growth factor 1 (IGF1) signaling pathway have been reported to be smaller (Leevers et al., 1996; Böhni et al., 1999). Many factors known to control body size or growth in various animals are involved in insulin/insulin-like growth factor signaling as described above, or TGFβ signaling (Patterson and Padgett, 2000; Massague, 2000), suggesting that they are major and conserved signal pathways controlling body size. However, the mechanisms of body size determination are largely unknown (Conlon and Raff, 1999). C. elegans is an excellent model animal for studies on body size control. The number of somatic nuclei is fixed at 959 in an adult hermaphorodite, and their entire cell lineages were elucidated (Sulston and Horvitz, 1977). There are many mutants with an abnormal body size or shape. For example, several mutants in TGFβ signaling factors such as DBL1/CET-1 (ligand), DAF-4 and SMA-6 (receptor), SMA-2, SMA-3 and SMA-4 (Smad transcriptional factors) are small (Estevez et al., 1993; Savage et al., 1996; Krishna et al., 1999; Suzuki et al., 1999; Morita et al., 1999). We and others have shown that egl-4 mutants have a larger body size (Daniels et al., 2000; Fujiwara et al., 2002; Hirose et al., 2003), and that the egl-4 gene encodes cyclic GMP-dependent protein kinases (L’Etoile et al., 2002; Fujiwara et al., 2002; Hirose et al., 2003). We have developed methods to measure body volume, and to analyze morphology and volume of major organs using a confocal microscope: cell size in the major organs is increased in the egl-4 mutants, while cell numbers are not. Genetic interaction studies strongly suggest that the DBL-1/TGFβ pathway functions downstream of EGL-4 for body size control as the body size of a double mutant carrying egl-4 and sma-6 or dbl-1 mutations is close to that of the single small mutant (Hirose et al., 2003). In contrast to the egl-4 mutants, three small mutants in the DBL-1 pathway have much smaller cell size and indistinguishable cell numbers in major organs (Nagamatsu and Ohshima, 2004). We have also shown that cGMP downregulates body size through EGL-4 (Nakano et al., 2004). Here, we present studies on the sma-5 gene. Its mutant is very small, and has additional phenotypes that are not seen in the mutants of the DBL-1/TGFβ signaling factors. Our findings, based on the identification of the sma-5 gene encoding a MAP kinase homolog, provide novel and interesting insights into the mechanisms that control body size. We have analyzed the sma-5(n678) mutant in C. elegans to elucidate mechanisms controlling body size. The sma-5 mutant is very small, grows slowly and its intestinal granules look abnormal. We found a 15 kb deletion in the mutant that includes a 226 bp deletion of the 3′ end of the W06B3.2-coding sequence. Based on this result, rescue experiments, RNAi experiments and a newly isolated deletion mutant of W06B3.2, we conclude that W06B3.2 is the sma-5 gene. The sma-5 mutant has much smaller intestine, body wall muscles and hypodermis than those of the wild type. However, the number of intestinal cells or body wall muscle cells is not changed, indicating that the sma-5 mutant has much smaller cells. In relation to the smaller cell size, the amount of total protein is drastically decreased; however, the DNA content of the intestinal nuclei is unchanged in the sma-5 mutant. The sma-5 gene is expressed in intestine, excretory cell and hypodermis, and encodes homologs of a mammalian MAP kinase BMK1/ERK5/MAPK7, which was reported to control cell cycle and cell proliferation. Expression of the sma-5 gene in hypodermis is important for body size control, and it can function both organ-autonomously and non-autonomously. We propose that the sma-5 gene functions in a MAP kinase pathway to regulate body size mainly through control of cell growth.