Return to Hardin Lab Page Return to Hardin Lab PagePostdoctoral Researcher
2000 - Ph.D., Department of Cellular Biology, University of Georgia, Athens, GA
1993 - B.S., Department of Molecular Biology, Auburn University, Auburn, AL
Address: Rm. 329 Zoology Research Bldg.; 1117 W. Johnson St., Madison, WI 53706
Phone: (608) 265-2520
Email: tlindblom@facstaff.wisc.edu
Research Interests:
Directed cell movements are an essential component of normal development.
As cells change shape and migrate, they initiate the processes
of development that transform collections of cells into germ layers,
tissues, and organs. For example, during gastrulation, it is
the migration of cells into the interior of embryos that gives
rise to the germ layers. Neurulation in vertebrates begins when
cells in the ectoderm change shape, exit the ectoderm, and reorganize
to form the neural tube. As development proceeds, the specific
migrations of cells participate in the formation of many vertebrate
organs such as eyes and limbs. In addition to the morphogenetic
movements of cells during development, cell migrations are also
an integral element of cancer biology. To metastasize, cancer
cells must change shape, loosen their connections to neighboring
cells, and invade the circulatory system. It is the ability of
cancer cells to exit the tumor and invade the rest of the body
that requires advanced stage cancer treatments to be more invasive
than localized surgery or radiation. Therefore, information we
gather about cell migrations should impact our understanding of
both development and cancer metastasis.
The nematode, Caenorhabditis elegans, is an ideal system in which to study cell movements. The cell movements occurring during C. elegans embryogenesis are both easy to visualize and critical for development. Additionally, the wealth of genomics information and technical abilities of this system allow the rapid identification of genes of biological significance. Following gastrulation, the process of morphogenesis converts the C. elegans embryo from an ellipsoid configuration of cells into the long, thin vermiform shape of the first larval stage. Morphogenesis is dominated by the embryonic epidermis, historically called the hypodermis in C. elegans, which is born as six rows of cells oriented in the anterior/posterior axis on the dorsal surface of the embryo; two rows each of dorsal, lateral, and ventral cells. At the outset of morphogenesis, the two dorsal-most rows of cells interdigitate into a single row in a process called dorsal intercalation. Before intercalation is complete, the outer rows, or prospective ventral hypodermis, extend their lateral borders and spread over the bulk of the embryo. Thus, after enclosure, the embryo is encased in a sheet of hypodermis which is fortified by interconnected belts of adherens junctions around the apical border of each cell. The enclosure of the hypodermis is critical to morphogenesis because its architecture provides a foundation for the subsequent circumferential actin contractions that squeeze the embryo into its larval form.
While the process of enclosure is the subject of active investigation, dorsal intercalation has yet to be probed genetically. However, the process has been well described in a variety of microscopic and pharmacological investigations. Intercalation begins when the dorsal cells change shape by forming wedge-like extensions that extend towards the dorsal midline. The wedge-shaped cells extend basolateral filopodia underneath and well beyond the adherens junctions that connect the contralateral pairs of cells. As the filopodia continue to extend towards the opposite lateral border, they thicken in both the apical-basal and anterior-posterior axes. Close examination of the opposing cells reveals they are forced apart and display broken adherens junctions. Thus, by both lateral migration of filopodia and the change from a wedge to an elongated shape, an individual dorsal cell appears to forcibly separate its future anterior-posterior neighbors. With the exception of two pairs of "pointer" cells, which remain unintercalated until later in development, intercalated cells continue to change shape until they are uniformly elongated across the dorsal midline. Accompanying the shape changes of dorsal cells are concurrent changes in the cytoskeleton. As the intercalating cells elongate laterally, microtubules and microfilaments reorganize into bundles parallel to the lengthening cells. Not surprisingly, pharmacological agents that perturb either of these cytoskeletal components cause a complete cessation of intercalation.
Several features of dorsal intercalation remain unexplained and make it an intriguing biological phenomenon. First is the left-right polarity of the dorsal cells. The establishment of the left-right polarity is not obviously generated by either cell lineage or extracellular signals emanating from outside or within the hypodermis. Understanding the generation of this asymmetry could shed additional light on how cells within a seemingly uniform cellular sheet adopt different behaviors. Further, the establishment of polarity is required for any directed cell movement, and insights into polarizing signals and/or events could provide information about how metastasizing cells initiate an exit from the primary tumor. Another unexplained feature of dorsal intercalation is the apparent cell autonomous nature of individual cell movements. Laser ablations of dorsal cells indicate that the intercalation behavior of a single dorsal cell is independent of the surrounding hypodermis. The discovery of mutations blocking dorsal intercalation could reveal the origin of the necessary temporal and spatial cues for migration.
In addition to the detailed description intercalation and powerful genetics of the system, C. elegans offers several advantages to the study of cell movements. The process of dorsal intercalation is essentially invariant from animal to animal and involves only 20 cells. These cells are on the dorsal surface of the embryo and the whole process is readily viewed by DIC (differential interference contrast) microscopy in living organisms. C. elegans is particularly amenable to the identification and cloning of genes required for intercalation. Transgenesis allows the use of fluorescent reporter constructs and transgenes for the further dissection of genetic pathways. With the completion of the C. elegans genomic sequencing project, as well as public availability of cDNA and genomic clones, the molecular identification and manipulation of mutant loci is greatly facilitated.
I have taken a forward genetic approach to study dorsal intercalation.
Using a C. elegans strain specifically designed to "capture"
the temporally short intercalation process, I have screened mutagenized
populations for mutants that fail to initiate or complete intercalation.
Specifically, the strain contains a bioluminescent marker that
faithfully decorates the cellular junctions of hypodermal cells
and allows both rapid and accurate identification of dorsal cells
and also a reliable assessment of their shape and movements.
The strain also contains a mutation that blocks dorsal cell fusions
that normally mask the intercalation events but does not otherwise
deleteriously affect embryogenesis. Using this specialized strain,
I have discovered mutants that arrest with un-intercalated dorsal
cells. I am in the process of outcrossing and analyzing the mutants
to discover those with a single mutated locus and those with defects
specifically in dorsal intercalation. Ultimately, I intend to
uncover the identity of a few, select mutated loci and thus molecules
that are required for dorsal cell shape changes and migrations.
Hopefully, these molecules will be required for the fundamental
cell biology of cell migration behavior and can be investigated
in the movements of normal development and/or metastasis.
Publications:
Lindblom, T., Pierce, G. J., Sluder, A. E. (2001) A C. elegans orphan nuclear receptor contributes to xenobiotic resistance. Current Biology 11(11): 864-868.
Zhang, Y. Z., Lindblom, T., Chang, A., Sudol, M., Sluder, A. E., Golemis, A. E. (2000) Evidence that Dim1 associates with a network of proteins involved in pre-mRNA splicing, and delineation of residues essential for Dim1 interactions. Gene 257(1): 33-43.
Sluder, A. E., Lindblom, T., Ruvkun, G. (1997) The Caenorhabditis
elegans orphan nuclear hormone receptor gene nhr-2
functions in early embryonic development. Developmental Biology
184:303-319.
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