Axon pathfinding
During nervous system development (including brain development), neurons extend thin cellular processes called axons that make contacts with other neurons. The axon scaffold is fundamental to the function of the nervous system. The powerful genetic and molecular tools of C. elegans are used to discover and analyze genes and molecules that control the outgrowth and guidance of axonal growth cones to their targets in the developing nervous system. Experiments in the lab have identified a network of cytoskeletal signaling molecules that have similar and overlapping roles in the process. Many axon pathfinding molecules have been identified, but it is unclear how these molecules act together to control axon pathfinding. Experiments in the Lundquist lab have shown that three Rac GTPases redundantly control axon pathfinding in parallel to the cytoskeletal signaling molecule UNC-34 Enabled, and that the novel actin-binding protein UNC-115 abLIM acts as a downstream cytoskeletal effector of Rac signaling during growth cone migration.
Future directions include the identification of new genes involved in axon pathfinding and understanding how these new genes and previously-identified genes work together to control cytoskeletal signaling during axon pathfinding.
Neuronal Migration
Nascent neurons must often migrate long distances in the developing nervous system. This problem is exemplified in the cortex of the brain, where neurons must migrate radially to form distinct cortical layers of neurons that mediate brain function. The simpler C. elegans system is used to study the migrations of neuroblasts in the developing worm in an attempt to understand the molecular and genetic mechanisms of neuronal migration.
Two distinct molecular mechanisms have been identified in the migration of the Q neuroblasts. First, the Rac GTPases and UNC-34 Enabled are required for the ability of the Q cells to migrate, likely due to the fact that they are required for membrane protrusion of lamellipodia and filopodia that underlie migration. A distinct mechanism involving the MIG-2 Rac GTPase and the NIK kinase MIG-15 control the direction of Q cell migration (polarity of migration). These data indicate that cell migration is a two-step process: the cells must know which direction to migrate (polarity); and the cells must be able to execute migration (lamellipodia and filopodia formation).
Future directions include understanding how MIG-2 Rac and MIG-15 Nik control cell polarity and how they act in realtion to other known mechanisms that control Q cell polarity, including Wnt signaling.
UNC-115 abLIM and lamellipodia and filiopodia formation
Migrating growth cones and neurons dynamically extend and retract plasma membrane protrusions called lamellipodia and filopodia whichare required for cell and growth cone motility. Lamellipodia and filopdia are formed by the differential dynamics and form of the actin cytoskeleton. It will be important to understand how the actin cytoskeleton is regulated to form lamellipodia and filopodia during cell and growth cone migration.
The UNC-115 abLIM protein is an actin-binding protein composed of three N-terminal LIM domains and a C-terminal actin-binding villin headpiece domain. UNC-115 is required for axon pathfinding and acts downstream of Rac GTPases to control lamellipodial and filopodial formation. UNC-115 can also mediate the formation of lamellipodia and filopodia in worm neurons and in cultured mammalian fibroblasts, and causes actin rearrangements in cultured fibroblasts away from a stress fiber morphology to a lamellipodial and filopodial morhpology. UNC-115 abLIM is a key regulator of lamellipodia and filopodia formation during cell and growth cone migration.
Future directions include understanding how UNC-115 regulates the cytoskeleton and what other molecules act with UNC-115 in this process.
Key to understanding organogenesis will be to understand the molecular mechanisms that coordinate cell fate and tissue morphogenesis during development. The C. elegans germline represents such a system. The mitotic germline stem cells at the distal tip of the gonad enter meiosis as they move away from the distal tip. As they do so, the syncytial meiotic nuclei associate with the cortex of the germ cell plasma membrane leaving a central, nucleus-free region called the rachis. The Ras GTPase controls the switch of germ nuclei from mitotic to meiotic fate.
Experiments in the lab are aimed at understanding the role of Ras in the morphogenetic switch to rachis formation and the role of the FLI-1 Flightless-1 protein in this system.