Yale > Molecular Cellular and Developmental Biology

Associate Professor of Molecular, Cellular & Developmental Biology
Yale University, KBT 716
PO Box 208103, 266 Whitney Ave
New Haven, CT 06520.
Phone (203) 432 3492; Fax (203) 432 6161
Email: frank.slack@yale.edu
Web site

B.Sc.(Hons.) University of Cape Town 1987; Ph.D. Tufts University School of Medicine 1993



			Micrograph of C. elegans with a mutation in the microRNA gene let-7. Adult animals herniate and burst through the vulva..



			Wild-type animal expressing two green fluorescence protein (GFP) fusions that highlight the seam cell nuclei and junctions.



			Top. A C. elegans embryo expressing a green fluorescence protein (GFP) fusion in seam cells. Bottom. A DIC image of the same embryo



			Black arrows show in vitro binding of the let-7 microRNA to the lin-41 3'UTR RNA containing let-7 complementary sites (lane 2). let-7 fails to bind to the same 3'UTR RNA missing those sites (lane 3). Grey arrows show let-7 binding to RNA containing just the let-7 complementary sites (lane 4). Free let-7 probe is in lane 1

Development is a four dimensional process. Just as pattern formation in the anterior-posterior and dorsal-ventral axis is controlled by the explicit action of genes, so too is pattern formation in the 4th axis, time, under explicit genetic control. However, we know significantly less about patterning this 4th axis. Genes that pattern the temporal axis, by controlling developmental timing, are known as heterochronic genes and are best understood in the nematode, C. elegans. Our lab focuses on using the advantages of C. elegans to find the important genes and molecules that control developmental timing and to understand the underlying mechanisms utilized. A pathway of heterochronic genes has been identified through the genetic identification of mutants that express cell fates either too early or too late relative to wild-type animals. This pathway controls the temporal progression of C. elegans development by regulating the abundance or activities of a succession of these heterochronic genes over time. The pathway that is emerging gives us an idea of the mechanisms that are employed for temporal patterning - microRNA control, key temporal morphogens, translational control, transcriptional control - but it is not complete. Our goal is to make this a fully mechanistic pathway. In addition, since many of the C. elegans heterochronic genes control timing of cell differentiation and are related to human cancer genes, we are examining the role of their human homologues in cancer. We are also extrapolating our work in C. elegans to provide an understanding of how tissues and organs are specified at the correct time during development of higher animals.

We are using molecular, genetic, bioinformatic and genomic approaches to understand:

the role of microRNAs like lin-4 and let-7 in control of gene expression in development and cancer.lin-4 and let-7 are founding member of a large family of recently discovered microRNAs. These two ~20 nucleotide RNAs regulate gene expression in the heterochronic pathway. These RNAs bind to complementary sequences in the 3’UTR of their target gene mRNAs and through a mechanism that we are trying to understand, down-regulate their translation. We have isolated protein factors that bind to miRNAs and hope that their identity will shed light on the miRNA mechanism. We have described the minimal sequences necessary for miRNA control of target sequences and have used this information to design bioinformatics screens to identify novel miRNA targets, in an effort to understand how these miRNAs control differentiation. Most of the dozen or so targets we have identified encode transcription factors, leading to our assertion that these miRNAs are master temporal control genes. Another target is the C. elegans homologue of the human proto-oncogene RAS (see below). Both the lin-4 and let-7 miRNAs are transcriptionally regulated and begin to be expressed at critical times in development, just prior to the down-regulation of their targets. Developmental timing can therefore be distilled down to the timing of expression of these miRNAs - we are interested in what controls their transcription, as well as how they control the expression of their targets. We are also investigating the role of additional temporally regulated microRNAs during C. elegans development. Mis-regulation of genes that control cell proliferation and cell fate determination often contributes to cancer development. In C. elegans, let-7 controls the timing of proliferation versus differentiation decisionsby epidermal cells. In let-7 mutants, cells frequently fail to terminally differentiate, and instead elect to divide again, a hallmark of cancer. In C. elegans, let-7 directly regulates RAS, and another gene, lin-41, which is homologous to cancer genes, including PML, mutated in almost all cases of promylocytic leukemia. let-7 is conserved in humans, where it has been linked to cancer. Specifically, human let-7 is poorly expressed or deleted in lung cancer, and over-expression of let-7 in lung cancer cells inhibits their growth, demonstrating a role for let-7 as a tumor suppressor in lung tissue. We have also shown that human let-7 is expressed in the lung and regulates the expression of important oncogenes implicated in lung cancer, including RAS. We are focusing on the role of let-7 in regulating proto-oncogene expression during lung development and cancer. We are also testing whether over-expression of let-7 suppresses activating oncogenic mutations in RAS, as it does in C. elegans.
the temporal patterning role of the HBL-1 transcription factor and the LIN-41 RING finger protein.By genetic arguments, hbl-1 and lin-41 are major targets of the let-7 RNA, and there are let-7 complementary sequences in the 3’UTR of hbl-1 and lin-41 that are responsive to let-7. While let-7 mutations lead to a reiteration of larval fates in the adult animal (in this case cells divide instead of terminally differentiate), hbl-1 and lin-41 mutations display the opposite phenotype and precociously express adult fates in the larval animals (cells terminally differentiate instead of proliferate). hbl-1 and lin-41 encode switches that must be tripped to progress from early fates to later fates. LIN-41 encodes a protein that belongs to a large super family that includes many human oncogenes and tumor suppressor genes. LIN-41 provides temporal cues that allow cells to decide whether to divide or terminally differentiate. We are examining the mechanism of action of LIN-41 and HBL-1. An emerging theme is of universal patterning mechanisms acting throughout the animal kingdom. We are testing this idea by knocking out lin-41 in the mouse.
the role of the novel, nuclear LIN-14 gene in temporal patterning and aging. The lin-14 gene produces a temporal gradient of LIN-14 protein, such that early in the L1 stage LIN-14 protein levels are high and at the end of the L1 stage LIN-14 protein levels are low. The drop in LIN-14 concentration trips a switch that allows cells to adopt post-L1 cell fates. Inappropriate expression of LIN-14 at post-L1 times results in a reiteration of L1 fates, while elimination of LIN-14 during the L1 results in L2 fates being expressed precociously. We have little idea how LIN-14 functions to specify the correct timing of cell fate but we have candidate downstream genes and genes whose products interact with LIN-14 to provide us with clues. lin-4 and lin-14 mutants display defect in lifespan, that depend on genes in the insulin signaling pathway. We are currently investigating how these developmental timing genes influence timing of aging.