Honors & Awards


  • Samuel Eiduson Student Lecture Award, University of California, Los Angeles (2017)
  • Dissertation Year Fellowship, University of California, Los Angeles (2016-2017)
  • Phi Beta Kappa International Scholarship Award, Phi Beta Kappa Alumni in Southern California (2016)
  • Provost Fellowship, New Jersey Institute of Technology (2006-2007)

Professional Education


  • Bachelor of Engineering, CMR Institute of Technology (2006)
  • Master of Science, New Jersey Institute of Technology, Biomedical Engineering (2008)
  • Doctor of Philosophy, University of California Los Angeles (2017)

All Publications


  • Netrin1 Produced by Neural Progenitors, Not Floor Plate Cells, Is Required for Axon Guidance in the Spinal Cord NEURON Varadarajan, S. G., Kong, J. H., Phan, K. D., Kao, T., Panaitof, S. C., Cardin, J., Eltzschig, H., Kania, A., Novitch, B. G., Butler, S. J. 2017; 94 (4): 790-?

    Abstract

    Netrin1 has been proposed to act from the floor plate (FP) as a long-range diffusible chemoattractant for commissural axons in the embryonic spinal cord. However, netrin1 mRNA and protein are also present in neural progenitors within the ventricular zone (VZ), raising the question of which source of netrin1 promotes ventrally directed axon growth. Here, we use genetic approaches in mice to selectively remove netrin from different regions of the spinal cord. Our analyses show that the FP is not the source of netrin1 directing axons to the ventral midline, while local VZ-supplied netrin1 is required for this step. Furthermore, rather than being present in a gradient, netrin1 protein accumulates on the pial surface adjacent to the path of commissural axon extension. Thus, netrin1 does not act as a long-range secreted chemoattractant for commissural spinal axons but instead promotes ventrally directed axon outgrowth by haptotaxis, i.e., directed growth along an adhesive surface.

    View details for DOI 10.1016/j.neuron.2017.03.007

    View details for Web of Science ID 000401415100016

    View details for PubMedID 28434801

  • Netrin1 establishes multiple boundaries for axon growth in the developing spinal cord. Developmental biology Varadarajan, S. G., Butler, S. J. 2017

    Abstract

    The canonical model for netrin1 function proposed that it acted as a long-range chemotropic axon guidance cue. In the developing spinal cord, floor-plate (FP)-derived netrin1 was thought to act as a diffusible attractant to draw commissural axons to the ventral midline. However, our recent studies have shown that netrin1 is dispensable in the FP for axon guidance. We have rather found that netrin1 acts locally: netrin1 is produced by neural progenitor cells (NPCs) in the ventricular zone (VZ), and deposited on the pial surface as a haptotactic adhesive substrate that guides Dcc(+) axon growth. Here, we further demonstrate that this netrin1 pial-substrate has an early role orienting pioneering spinal axons, directing them to extend ventrally. However, as development proceeds, commissural axons choose to grow around a boundary of netrin1 expressing cells in VZ, instead of continuing to extend alongside the netrin1 pial-substrate in the ventral spinal cord. This observation suggests netrin1 may supply a more complex activity than pure adhesion, with netrin1-expressing cells also supplying a growth boundary for axons. Supporting this possibility, we have observed that additional domains of netrin1 expression arise adjacent to the dorsal root entry zone (DREZ) in E12.5 mice that are also required to sculpt axonal growth. Together, our studies suggest that netrin1 provides "hederal" boundaries: a local growth substrate that promotes axon extension, while also preventing local innervation of netrin1-expressing domains.

    View details for DOI 10.1016/j.ydbio.2017.08.001

    View details for PubMedID 28780049

  • Type Ib BMP receptors mediate the rate of commissural axon extension through inhibition of cofilin activity DEVELOPMENT Yamauchi, K., Varadarajan, S. G., Li, J. E., Butler, S. J. 2013; 140 (2): 333-342

    Abstract

    Bone morphogenetic proteins (BMPs) have unexpectedly diverse activities establishing different aspects of dorsal neural circuitry in the developing spinal cord. Our recent studies have shown that, in addition to spatially orienting dorsal commissural (dI1) axons, BMPs supply 'temporal' information to commissural axons to specify their rate of growth. This information ensures that commissural axons reach subsequent signals at particular times during development. However, it remains unresolved how commissural neurons specifically decode this activity of BMPs to result in their extending axons at a specific speed through the dorsal spinal cord. We have addressed this question by examining whether either of the type I BMP receptors (Bmpr), BmprIa and BmprIb, have a role controlling the rate of commissural axon growth. BmprIa and BmprIb exhibit a common function specifying the identity of dorsal cell fate in the spinal cord, whereas BmprIb alone mediates the ability of BMPs to orient axons. Here, we show that BmprIb, and not BmprIa, is additionally required to control the rate of commissural axon extension. We have also determined the intracellular effector by which BmprIb regulates commissural axon growth. We show that BmprIb has a novel role modulating the activity of the actin-severing protein cofilin. These studies reveal the mechanistic differences used by distinct components of the canonical Bmpr complex to mediate the diverse activities of the BMPs.

    View details for DOI 10.1242/dev.089524

    View details for Web of Science ID 000312741400009

    View details for PubMedID 23250207

    View details for PubMedCentralID PMC3597210