| Research Focus HIDden targets of microRNAs for growth control Andreas Bergmann 1 and Mary Ellen Lane 2 1 Department of Biochemistry and Molecular Biology, University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard – Unit 117, Houston, TX 77030, USA 2 Department of Biochemistry and Cell Biology, Rice University, 6100 Main Street, Houston, TX 77005, USA How is the size of an animal determined? Why is it that humans grow larger than mice? Certainly, one of the most astonishing features of animal development is that every animal of a given species, and its organs and appendages, grow to approximately the same size. Sur- prisingly little is known about the biology of tissue growth and size control. Recent advances in Drosophila research have implicated a microRNA as an important regulator of animal size. These studies reveal an un- expected layer of size regulation in higher animals. It is generally believed that the size of an animal is determined by the total number and size of cells (cell mass) it contains. The appropriate cell number results from coordination of the rates of cell proliferation and apoptosis, which are regulated by both intracellular programs and extracellular signaling molecules. Of similar importance is the control of cell growth, which determines cell size. Usually, cell proliferation depends on cell growth. Cell division only occurs if a cell has reached a critical size, which is characteristic for each cell type. By contrast, however, cell growth does not depend on cell proliferation. For example, some cell types such as neurons and muscles continue to grow after they become mitotically inactive. Recently, growth control has become the focus of genetic studies in Drosophila. It has been demonstrated experi- mentally that the developmental program(s) governing animal size include mechanisms that sense – and consistently produce – the appropriate cell mass. For example, the wing imaginal disc of Drosophila is divided into anterior and posterior compartments through a clonal boundary. Cells from the anterior compartment will never migrate into the posterior compartment and vice versa [1]. It is possible experimentally to increase or decrease the cell division rate in one compartment relative to the other. However, the two compartments ultimately reach the normal size ratio and produce a wing of normal pro- portions. This is caused by faster dividing cells in one compartment exiting mitotic proliferation earlier than the slower dividing cells in the other compartment, once they have reached a critical total cell mass. Thus, one major principle of animal development appears to involve the control of total cell mass by balancing the rates of cell growth and proliferation to ensure appropriate organ and organismal growth. This example also illustrates that the mechanisms that stop cell growth are as important as those that stimulate it. How these cellular processes are coordinated during development is largely unknown. However, at least two distinct classes of growth-defective mutants have identi- fied a few individual players in Drosophila. The insulin/- phosphatidylinositol-3-kinase (PI3K) signaling pathway regulates cell growth. Overexpression of PI3K produces enlarged tissue owing to increased cell size, whereas decreased pathway activity reduces tissue growth by producing smaller cells [2–6]. Drosophila homologs of ras, myc and TOR have also been shown to promote cell growth [7–10]. In these examples, cells grow too fast and divide at abnormally large sizes. Thus, the normal balance between rates of cell growth and cell proliferation is lost. By contrast, mutations in the cdk4 and salvador/shar-pei genes affect tissue growth by alterations in cell number, but cell size is normal [11–14]. The recently discovered bantam gene in Drosophila has a similar tissue growth phenotype [15]. However, bantam does not encode a protein. bantam encodes a microRNA The bantam gene in Drosophila was first identified in an overexpression screen for genes that affect tissue growth [15]. Tissues that overexpress bantam are substantially larger than wild-type tissues. Conversely, mutations that inactivate bantam decrease tissue growth. As the name bantam indicates, mutants are smaller than normal [15]. Interestingly, although these flies are smaller, they do not display patterning defects and are proportioned normally. This suggests that the primary function of bantam is the regulation of tissue growth. Indeed, although the mutant organs contain fewer cells, the cells are of normal cell size [15]. Consistently, tissue overgrowth owing to overexpres- sion of bantam results from an increase in the number of normal-sized cells [15], indicating that bantam controls tissue growth through effects on cell number rather than cell size. However, Brennecke et al. were in for surprise when they cloned the bantam gene [16]. The strongest bantam mutation is caused by a 21-kb deletion of genomic DNA. With the exception of one expressed sequence tag (EST; which was found not to encode bantam), this region does not contain any predicted protein-coding genes. A BLAST search of the bantam region with the recently described mosquito Anopheles gambiae genome identified a sequence with 30 out of 31 identical nucleotides (nt) in a block of 90 residues with considerable similarity. This sequence is predicted to fold into a stable RNA hairpin structure (Fig. 1) and might be the precursor for Corresponding author: Andreas Bergmann (andreas@bergman.net). Update TRENDS in Biochemical Sciences Vol.28 No.9 September 2003 461 http://tibs.trends.com