Cellular and Molecular Biology Training Program

Sandy is a student in the Cell and Developmental Biology home area of the MBIDP, where she works in the laboratory of Dr. Samantha Butler.  She received a B.S. in Biological Sciences from UC Irvine in 2014 and a M.S. in Biology, with an emphasis in mucosal immunity, from CSULA in 2016.  She joined the CMB training program in 2018. 

Mentor: Dr. Samantha Butler

     The formation of neuronal circuits is critical for normal nervous system function.  During development, neurons project axons toward their synaptic targets.  Failure to make appropriate connections can result in errors processing sensory stimuli, movement disorders, and intellectual disabilities.  Neural circuits are generated when growth cones at the tips of axons use molecular cues in the environment to guide axon extension.  Netrin1 is an example of an axon guidance cue and was first characterized in the developing spinal cord.

     Early studies suggested that floor plate (FP)-derived netrin1 is a diffusible attractant required for commissural axons expressing the netrin1 receptor, Dcc, to grow towards the ventral midline.  However, recent work from the Butler laboratory has shown that this chemotactic model is incorrect.  In the mouse spinal cord, netrin1 is expressed by both FP cells and neural progenitor cells (NPCs) in the ventricular zone (VZ).  Furthermore, netrin1 protein is found on both axons and the outer edges of the spinal cord, in a manner that does not fit with a model of a simple continuous gradient emanating from the FP.  My laboratory has shown that in the absence of either netrin1 or Dcc, spinal axons aberrantly innervate the VZ and commissural axons either stall or are dramatically defasciculated.  Surprisingly, these phenotypes are only observed when netrin1 is removed from NPCs and not the FP cells, demonstrating that NPCs, not the FP, supply the netrin1 that guides commissural axons to extend ventrally.

A new model for the role of netrin1 in the spinal cord: 

     Our findings suggest that NPCs transport and deposit netrin along the outer edges of the spinal cord.  The deposited netrin forms an adhesive substrate that positions and promotes commissural axon outgrowth.  Axons are directed to extend in a fasciculated manner around the VZ and towards the ventral midline through a Dcc-dependent mechanism.  The netrin1+ substrate promotes bundled axonal growth by transferring on to Dcc+ axons as they travel along the substrate.  Thus, netrin1 promotes directed axon growth through haptotaxis, the directed growth of cells along an adhesive substrate, and not by chemotaxis.  The molecular mechanism(s) underlying haptotaxis-mediated axon guidance by netrins are not yet understood, therefore I propose to:

1. Elucidate the mechanisms by which spinal NPCs establish a netrin1+ substrate that promotes directed, fasciculated axonal growth towards the FP at the ventral midlineusing real-time, near super high-resolution microscopy to follow the path and trafficking dynamics of netrin1 made by NPCs both in ex vivo and in vivo systems
2. Assess the mechanism by which a netrin1+ substrate promotes fasciculated axonal growth using in vitro co culture systems composed of segments of spinal cord in the presence of a netrin substrate
3. Define the role of FP-derived netrin1 on the pre and post-crossing behavior of spinal axons upon reaching the FP by labeling and imagining of axonal trajectories