Alexander F. Schier

Our research focuses on three areas:

(i) vertebrate embryogenesis - how do signals influence the fate and movement of cells?

(ii) sensory neuron development and function - how does an organism sense potentially harmful stimuli?

(iii) sleep and wakefulness - what are the genes and circuits that regulate sleep?

We mainly use zebrafish as a model system, because genetic and imaging approaches can be combined to study complex behaviors and developmental processes in a vertebrate.

1. Vertebrate embryogenesis

The vertebrate body plan is set up during gastrulation, when a ball of undifferentiated, totipotent cells is transformed into an embryo. This process results in the formation of the three germ layers (ectoderm, mesoderm, and endoderm) and the three axes (anterior-posterior, dorsal-ventral and left-right; reviewed in Schier & Talbot 2005). We wish to understand how signaling between cells regulates this process.

Nodal signaling: We have focused on the dissection of the Nodal signaling pathway, because of its key role during gastrulation. Our studies have revealed that the interplay of extracellular factors of the Nodal, EGF-CFC and Lefty families controls gastrulation (reviewed in Schier 2003). Nodal signals induce mesoderm and endoderm via EGF-CFC coreceptors. In contrast, Lefty signals block mesoderm and endoderm formation by inhibiting EGF-CFC coreceptors (Cheng et al. 2004).

We have found that the Nodal signal Squint functions as a morphogen i.e. it is produced locally, acts over several cell diameters and specifies cell fates in a concentration-dependent manner (Chen et al. 2001). In contrast, Lefty proteins act as long-range feedback inhibitors of Nodal signaling (Chen & Schier 2002). These finding led us to propose that Nodal and Lefty are a molecular correlate of the reaction-diffusion system introduced by Alan Turing in 1952 to explain pattern formation during development. This model describes how the interaction between an agonist (e.g. Nodal) and an antagonist (e.g. Lefty) determines the size and fate of a field of cells. We are now studying how these signals move through the embryo, elicit concentration-dependent effects and induce the movement of cells.

MicroRNAs: In addition to signaling molecules and transcription factors, microRNAs have recently emerged as potentially important regulators of biological processes. These short RNA molecules regulate gene expression by binding to mRNAs, but their role during development is largely unknown. We have therefore generated zebrafish embryos that lack an enzyme required for microRNA processing. These mutants lack all microRNAs and have defects in gastrulation and brain morphogenesis (Giraldez et al. 2005). We have isolated the miR-430 microRNA family as a regulator of these processes. Analysis of its target mRNAs revealed that miR-430 promotes the maternal-to-zygotic transition. This transition is a universal process in animal development, when the embryo activates zygotic gene expression and thus no longer solely relies on maternally provided transcripts. The activation of zygotic transcription coincides with the elimination of many maternal mRNAs. We found that that miR-430 accelerates the deadenylation and clearance of several hundred maternal transcripts during zygotic stages (Giraldez et al. 2006).

2. Sensory neuron development and function

Animals protect themselves by sensing potentially harmful thermal, mechanical or chemical stimuli. This process of nociception is mediated by specific sensory receptors and circuits. We wish to understand how this circuit develops and functions.

We have focused on the development of trigeminal sensory neurons, the primary nociceptors in the vertebrate head. Our studies have revealed that chemokine signals assemble trigeminal sensory neurons into a nerve center (ganglion). In the absence of chemokine signals, neurons are mispositioned (Knaut et al. 2005). We have also found that mutual repulsion limits the size of sensory arbors. For example, when a neuron is part of the ganglion it will only innervate a small area of the head; in contrast, a single isolated neuron innervates the entire head (Sagasti et al. 2005). These results provide a model for how sensory neurons assemble and generate complex innervation patterns. We are now studying which molecules mediate these interactions.

Our ultimate goal is to link sensory neuron development to behavior. We have thus started to isolate receptors implicated in nociception and found zebrafish orthologues of the major classes of genes implicated in mammalian nociception. We have started to interfere with the activity of nociceptive genes and test their roles in behavioral assays.

3. Sleep and wakefulness

As many as 10% of Americans suffer chronic sleep disturbances, but the genetic and cellular mechanisms that control sleep and wake states remain largely elusive. We wish to find genes and circuits that regulate this behavior.
We are establishing zebrafish as a model system for sleep research. Zebrafish have the basic hallmarks of sleep-like behaviors. Sleeping fish require stronger stimuli than awake fish to initiate movement (increased arousal threshold), and sleep deprivation is followed by increased sleep (sleep rebound). In addition, the zebrafish brain expresses peptides that have been implicated in human sleep disorders. We are developing assays to measure sleep and wakefulness in zebrafish with the long-term goal of using genetic screens to isolate novel sleep regulators.