The research in our laboratory interrogates fundamental molecular, cellular and developmental biological processes. In particular, we are interested in the roles played by small ribonucleoproteins (RNPs) in a genetic disease, called Spinal Muscular Atrophy. We are also focused on the functions of histone post-translational modifications (PTMs) in the regulation of eukaryotic gene expression, important for understanding disease mechanisms in many different types of cancer.
Most plant development occurs post-embryonically, and is tied closely to environmental signals. We use techniques of genetics, molecular biology, microscopy, physiology, and biochemistry to study how environmental and endogenous signals regulate plant development. For these studies we use the model plant Arabidopsis thaliana, which has numerous technical advantages that facilitate experimental progress. We hope in the long run to reconstruct how endogenous developmental programs and exogenous signals cooperate to determine plant form and the partitioning of growth among different organs.
The hormone auxin regulates multiple developmental processes in plants, including embryo and meristem patterning, organ growth, and flower maturation. Auxin induces gene expression through a family of transcription factors called ARFs (Auxin Response Factors), whose activity is regulated by Aux/IAA proteins. Auxin switches ARF proteins between gene repressing and gene activating states by promoting Aux/IAA protein turnover. We are currently studying how this regulatory system controls seedling growth, ovule development, and flower opening and fertilization.
Among genes regulated by auxin are some (including those encoding Aux/IAA proteins) that feed back negatively on auxin response; and others encoding proteins that affect intercellular auxin transport. We are interested in how feedback controls affect the dynamics of auxin response, and in how regulated intercellular auxin movement coordinates growth and differentiation of different cells during development.
Epigenetic control of genome structure and function
Cell cycle-regulated gene expression
Developmental genetics
Synopsis
Our research focuses on understanding the molecular mechanisms that regulate DNA replication and cell proliferation during animal development. An orderly process of events called the cell cycle, which in its most familiar form consists of four phases (G1-S-G2-M) controls cell proliferation. The genome is replicated during the “S” or synthesis phase and duplicated chromosomes are segregated to daughter cells during the “M” or mitotic phase when cell division occurs. G1 and G2 are “gap” phases during cells regulate the entry into S phase and M phase, respectively. We study gene expression events that control how cells make the decision to enter S phase and proliferate, or to exit the cell cycle and differentiate. Without such control there would be no coordination between cell proliferation and the development and function of the many different types of tissues that make up an organism. In addition, the breakdown of cell cycle regulation is one of the events that contribute to the generation of cancer.
The E2F transcription factor is active (green) during S phase (red) in the asynchronous endocycles of the Drosophila salivary gland.
We study this problem using the fruit fly Drosophila melanogaster, in part because the genes controlling cell proliferation in fruit flies have been highly conserved during evolution and function the same way as in other animals, including humans. This allows us to exploit certain advantages that Drosophila has as a research tool, including the ease with which genetics (making and analyzing mutants) and cell biology (using microscopy to
In situ hybridization for histone gene expression in a Drosophila embryo.
observe cell proliferation) can be applied to the study of gene function in the context of a whole animal. Some of the cell cycle regulatory pathways that we study become defective in virtually every human cancer. Thus, one hope is that understanding how these pathways function in normal Drosophila development will give us clues to how they might malfunction in the deregulated growth typical of cancer.
Human congenital heart disease, the most common form of heart disease in childhood, occurs in about 1% of live births and up to 10% of stillbirths. Presently, the most effective therapy for cardiac diseases is heart transplantation. However, due to the shortage of organs, cost and inaccessibility of treatment for most affected individuals this remains a limited therapeutic option. Alternative treatment is the administration of drugs that improve myocardial contractility, though this treatment is only effective as a short term therapy, with the 5-year survival rate using current agents being less than 60%. An alternative therapeutic option is to treat patients with cardiac progenitor cell populations that could infiltrate and repair damaged heart tissue. Thus, the ability to isolate and propagate cell populations that can differentiate into cardiomyocytes in vivo offers the opportunity to treat a wide range of cardiac diseases. To this end, our lab is interested in understanding the relationship between cardiac progenitor proliferation and the onset of cardiac differentiation focusing on the endogenous roles of the transcription factors TBX5 and CST and the protein phosphatase SHP-2.
We are interested in understanding how cells develop into organisms. We love the nematode C. elegans, because it allows us to readily combine a great number of useful techniques, including techniques of cell biology, direct manipulation of cells, forward and reverse genetics, biochemistry, molecular biology, biophysics, and live imaging of cells and their dynamic, cytoskeletal components. Current work in the lab addresses several fundamental questions in cell and developmental biology — how cells move to specific positions during development, how cells change shape, how developmental patterning mechanisms tell cell biological mechanisms what to do where and when, how intercellular signals act to polarize cells, and how the mitotic spindle is positioned in cells.
We have also been developing a relative of C. elegans and Drosophila, a water bear (tardigrade), to study how developmental mechanisms can evolve to produce organisms with different forms and how biological materials can survive unusual extremes.
Albert Harris is an embryologist interested in the “amoeboid” locomotion of the body’s component cells, and the ways in which cell movements generate anatomical patterns.
He invented and developed the elastic substratum method, by which he (followed by later researchers) used silicone rubber and various gels embedded with particles to measure and map locations, strengths and directions of cell traction forces at the micrometer level. He also co-discovered focal adhesions, retrograde surface transport, that ruffling is upfolding, that sponges crawl, that sponge cells continually rearrange even without dissociation, and has collaborated on mathematical and computer simulation studies of cell migration and cell division. Videos from this research are posted under “research” above.
His graduate students have worked on diverse topics ranging from the special adhesive properties of macrophages, the generation of tendons by cellular forces, the locomotion of sponges, cellular responses to electric fields, and the effects of tumor promoters on cell contractility.
He was raised in Durham and Wrightsville Beach NC., and in Norfolk, VA., and for a long time was the one and only graduate of The Norfolk Academy ever to attend Swarthmore College. He did his PhD. with J. P. Trinkaus at Yale, and was a Post-Doctoral Fellow of the Damon Runyon Cancer Research Foundation at Cambridge (England) working under Michael Abercrombie, FRS. He is the son of Kenneth Harris, the prominent artist, author and spearhead of the Civil Rights Movement in Tidewater, Virginia. His family were plaintiffs in the (1958-9) Federal and State court cases that forced the racial integration of the Virginia Public Schools. His wife Elizabeth Holder Harris has been a leader in molecular genetics research on chloroplasts and the alga Chlamydomonas. They have three children, one of whom also has a PhD in Biological Research, and another of whom is a teacher. His special interests are whitewater canoeing, turtles and computer programming.
Formation and differentiation of Drosophila neurons and glia
Regulation of nervous system transcription
Synopsis
Our laboratory is concerned with the molecular mechanisms that govern the development of the Drosophila central nervous system, including: (1) how nervous system precursor cells are generated, and (2) how neurons and glia acquire their differentiated properties. The primary focus of the lab is using genomic technologies to study the regulation of transcription during neuronal and glial development.
We study the growth and interactions of cells in their natural environment – the animal – and how these interactions are modified in disease. We focus on the mechanisms that control the process of blood vessel formation, which is crucial to successful development and required in cancer and other diseases. There are currently several areas of investigation that utilize genetically altered mice and cells derived from those mice.
We developed a cell culture model of developmental blood vessel formation to study cross-talk between cellular processes such as cell division and sprouting migration to expand vessel networks. Mouse embryonic stem cells are induced to differentiate in dishes to form structures that contain some embryonic tissues, including primitive blood vessels. We have incorporated GFP reporter genes to visualize the dynamic processes of blood vessel formation via time-lapse imaging, and we have used both genetic manipulation and inhibitors to dissect the role of an important signaling pathway (VEGF) in these processes.
We study the role of a novel gene that activates cellular homologs of oncogenes (ie Ras) in the proper function of blood vessels. We showed that while this gene is not required for development, it is required for the response of vessels to phorbol esters, which are tumor promoters. This suggests that the signaling pathway using this gene is important in diseases such as diabetes and cancer, and we are testing these models.
We investigate how blood vessels know where to go as they form, by using chimeric embryos that consist of a host bird embryo with inserted mouse tissue. This technique allows us to determine the migration patterns of blood vessels over time during fetal development, and we can genetically manipulate the mouse tissue. We can also introduce exogenous DNA into the host via electroporation of the embryos. This analysis has thus far uncovered a crucial role for VEGF in the patterning of vessels around the neural tube, which will form the brain and spinal cord.
Our research group has interests in telomere biology and germline immortality. We are studying these problems using the nematode Caenorabditis elegans, a multicellular eukaryote with excellent genetics, which can be combined with biochemical and cell biology approaches.
How does the germline achieve immortality?
The germline is an immortal cell lineage that is passed from one generation to the next, indefinitely. In order to determine how germ cells achieve immortality, we have identified C. elegans mutants with Mortal Germlines: mutants that reproduce normally for several generations, but eventually became sterile (Figure 1). Gametes of these mutants transmit forms of stress that can accumulate over the generations to levels that cause strong effects on germ cell development (sterility or reproductive arrest). Stress transmitted by mortal germline mutants could also affect somatic development. One pathway that promotes germ cell immortality is the Piwi/piRNA genome silencing pathway. Deficiency for this pathway allows for growth for many generations but then results in an adult reproductive arrest phenotype that resembles a response to acute starvation. Furthermore, deficiency for the daf-2 insulin/IGF1 receptor, which results in stress resistance and adult longevity, can suppress the sterility of Piwi mutants. This creates an interesting link between germ cell immortality and somatic longevity whose basis we seek to understand.
Figure 1. mortal germline mutants eventually become sterile after several generations.
Given that the soma is only needed for a single generation, evolutionary theory posits that somatic cells may be deficient for pathways that ensure germ cell immortality. Substantial evidence from vertebrates indicates that this is likely to be the case. We utilize genetic, cell biology and biochemical approaches to study pathways that enable germ cells to achieve proliferative immortality in C. elegans. This work could shed light on epigenetic forms of macromolecular stress that can be transmitted by gametes and could be generally relevant to heredity and possibly to human disease.
Analysis of Telomere Biology
A second pathway that promotes germ cell immortality is telomerase, which is a ribonucleoprotein that adds repeats to chromosome termini. The ends of linear chromosomes, telomeres, are usually composed of simple repetitive sequences whose length is maintained by telomerase. Humans deficient for telomerase suffer from the lethal hereditary disorders Aplastic Anemia, Dyskeritosis Congenita and Pulmonary Fibrosis. C. elegans is the most highly evolved multicellular eukaryote in which an unbiased genetic approach can be used to study the problem of telomere replication. A fraction of the mortal germline mutants described above are completely defective for telomerase activity in vivo (Figure 2). Studies of these genes may help to elucidate the mechanism by which telomerase functions at chromosome termini and may provide significant insight into telomerase-related diseases. We also study proteins that bind to telomeres, some of which form nuclear foci that are dynamic during development.
Telomerase is deficient in most human somatic cells, and telomere erosion in this context may contribute to genome instability that fuels the development of many forms of cancer. C. elegans has holocentric chromosomes, so fused chromosomes that occur when telomerase is deficient can be genetically isolated and mapped. These stable end-to-end chromosome fusions allow us to address the mechanism by which dysfunctional telomeres are repaired with unparalleled clarity, and may provide insight into how large-scale DNA rearrangements occur during tumorigenesis.
Figure 2. Teomere replication defects such as (a) telomere shortening and (b) end-to-end chromosome fusions are observed in a subset of mortal germline mutants, suggesting that teomerase does not function properly in these mutants.
