Huey Hing , Ph.D
- Associate Professor
Office: Lennon Hall 203
From a young age, I have asked “Where do I come from?” and “Where does life come from?” I received my B.S. from the National University of Singapore and my Ph.D. from Yale University, where I trained in the lab of Dr. Spyros Artavanis in genetics and development. After graduation, I moved to the lab of Dr. Larry Zipursky at the University of California, Los Angeles, where I fell in love with the study of brain development. I established my lab studying the neurobiology of brain development first at the University of Illinois at Urbana-Champaign and now at SUNY Brockport.
Given the central role of dendritic branching and targeting in neural circuit assembly and function, disruptions in dendritic development are linked to a number of psychiatric and neurodevelopmental disorders (Forrest, Penzes, 18). Despite the importance of dendritic morphogenesis, its underlying molecular mechanisms have remained incompletely understood. The long-term goal of my lab is to elucidate the molecular mechanisms of dendritic development and understand how its dysfunction leads to neurological diseases.
Our Model System
We take advantage of the stereotyped neural circuit of the Drosophila olfactory system to unravel the mechanisms of neural circuit development. In the antennal lobe (AL), dendrites of 50 classes of uniglomerular projection neurons (PNs) form synapses with the axons of 50 classes of olfactory receptor neurons (ORNs) in stereotyped glomeruli (Couto et al., 2005; Fishilevich and Vosshall, 2005). We previously reported that the DA1/VA1d dendritic pair, located within a cluster of dorsolateral PN dendrites, undergoes a novel rotational migration of ~45˚ around each other to attain their final adult pattern (Wu et al., 2014). This rearrangement (in the lateral à dorsal à medial à ventral direction) occurs between 16 and 30 hour After Puparium Formation (hAPF), a period of major ORN axon invasion of the AL (Jefferis et al., 2004; Jhaveri et al., 2000).
The Wnt5 Guidance Signal
We showed that the Wnt5 protein drives the rotation of the DA1/VA1d dendritic pair by acting as a chemorepellent (Wu et al., 2014). Wnt5 is expressed by a set of AL-extrinsic cells and forms a dorsolateral-high to ventromedial-low (DL>VM) gradient in the AL neuropil, which provides positional information to align the dendritic pattern relative to the axes of the brain. We also showed that the Derailed (Drl)/Ryk atypical receptor tyrosine kinase, a putative Wnt5 receptor (Fradkin et al., 2004; Harris and Beckendorf, 2007; Yasunaga et al., 2015; Yoshikawa et al., 2003), is differentially expressed by the PN dendrites, providing intrinsic information for their targeting in the Wnt5 gradient. Interestingly, drl opposes wnt5 repulsive signaling so that dendrites expressing high levels of drl terminate in regions of high wnt5 concentration and vice versa. To further unravel the mechanisms of PN dendritic targeting, we have screened for mutations that disrupted the rotation of the DA1/VA1d dendritic pair.
Wnt5 acts through the Planar Cell Polarity Pathway
We recently observed that mutations in the Van Gogh (Vang) gene disrupted the rotation of the DA1/VA1d dendritic pair, mimicking the wnt5 mutant phenotype. Vang encodes a four-pass transmembrane protein (Taylor et al., 1998; Wolff and Rubin, 1998) of the core Planar Cell Polarity (PCP) group, an evolutionarily conserved signaling module that imparts polarity to cells (Goodrich and Strutt, 2011; Yang and Mlodzik, 2015). The loss of Vang suppressed the repulsion of the VA1d dendrites by wnt5, indicating that Vang is a downstream component of wnt5 signaling. Surprisingly, Vang acts in the ORNs, indicating an obligatory codependence of ORN axon and PN dendritic migration. We also show that the drl gene is selectively expressed in the DA1 dendrites where it antagonizes Vang and converts wnt5 repulsion of the DA1 glomerulus into attraction. The bipolar responses of the DA1/VA1d glomeruli could create the torsion by which wnt5 directs the rotation of the glomeruli. Our work shows that converging pre- and postsynaptic processes contribute key signaling components of the PCP pathway, allowing the processes to be co-guided by the wnt5 signal.
My future research will be divided into the two following parts:
(1) Elucidating the Biochemical Mechanisms of Wnt5-Drl/Ryk-Vang Signaling
Our genetic evidence indicated that Drl/Ryk acts in the PN dendrites to antagonize the function of Vang in the ORN axons. Drl/Ryk encodes a transmembrane receptor belonging to the atypical receptor tyrosine kinase family, while Vang encodes a four-pass transmembrane protein. It is possible that the Drl and Vang proteins directly interact across the synaptic cleft. We will test this hypothesis by asking (a) if Drl physically interact with Vang and (b) if Vang activity is down regulated upon interaction. We will test the hypothesis by assessing the behaviors of the proteins in vitro and in vivo. For the in vitro experiments we have made constructs that express the Drl/Ryk and Vang proteins in the Drosophila S2 cell line. For the in vivo experiments we have used the Crispr/Cas9 technique to edit the two genes, which will allow us to visualize the trafficking of the two proteins in the developing neuronal processes.
Showing that Drl/Ryk could act in trans to down regulate Vang would be an important finding for two reasons. First, the role of the Ryk protein in PCP signaling is controversial. Our finding would show that the Drl/Ryk protein is a legitimate component of PCP signaling. Second, the mammalian Ryk protein has previously been shown to modulate the stability of the Vangl2 protein in cis. Our finding would show for the first time that the two proteins could also interact in trans, possibly using a very different signaling mechanism.
(2) Identifying other Components of Wnt5-Drl/Ryk-Vang Signaling
The observation that the Drl/Ryk transmembrane protein, which is expressed by the PN dendrites, antagonizes Wnt5 signaling prompts the question of what receptor transduces the Wnt5 signal in the dendrites. We hypothesize that the Frizzled seven-pass transmembrane protein (Fz) could act as the Wnt5 receptor in the PN dendrites. To test this hypothesis we will use the mosaic technique to selectively delete the Fz function from either the ORN axons or PN dendrites to determine if Fz is required by either of the pre- or postsynaptic processes for formation of the olfactory neural circuit.
Identifying the Wnt5 receptor in the PN dendrites will be a crucial advancement in understanding the mechanism by which Wnt5 regulate the co-migration of the ORN axons and PN dendrites. Linking the Fz transmembrane protein to the signaling pathway will allow us to (a) visualize the extracellular synaptic complex that forms between the ORN axon and PN dendrites and (b) elucidate the intracellular signaling cascade that transduces the Wnt5 signal into dendritic growth or collapse.