Our group uses the model organism C. elegans, a soil nematode, to study genetic mechanisms that control the development and function of the nervous system. We have a particular interest in understanding the mechanisms that superimpose sex-specific characteristics onto neural development and function. Its relative simplicity, exceptional experimental tractability, and use of conserved developmental mechanisms makes C. elegans an ideal model for exploring basic biological processes that are relevant to health and disease in the human nervous system.

We have three related areas of active interest in the lab. In the first, we aim to understand the means by which genetic networks regulate the patterning of male-specific sensory neurons. In the second, we are studying a sensory transduction pathway mediated by the C. elegans polycystins, orthologs of human genes associated with Polycystic Kidney Disease. In the third, we are exploring sexual dimorphisms in the nominally non-sex-specific "core" nervous system of C. elegans, with an emphasis on the chemosensory system.

Cell fate specification in the nervous system

The function of the nervous system relies on its extraordinary diversity of cell type. A central question in developmental neuroscience has been to understand how neurogenesis is coupled to the establishment of specific neural subtypes, and how this is regulated according to developmental time and position.

The C. elegans male tail contains nine pairs of sensilla called rays; these represent an ideal model in which to address this problem. Each ray comprises two distinct sensory neurons (A-type and B-type) and an associated glial-like ray structural cell. These three cells, along with a cell that undergoes programmed cell death, are generated from a single ray precursor through two rounds of asymmetric division. We are working to define the genetic regulatory network that controls the precise establishment of these four distinct cell types from a single precursor. This project has particular relevance for human health in two ways: first, the disruption of mechanisms that specify neural subtype in humans is likely to contribute both to neural birth defects and to complex diseases such as schizophrenia. Second, a robust understanding of the mechanisms that control neural subtype will be critical for the use of stem-cell therapies, as the success of this approach will depend on our ability to guide neural precursors down the appropriate path of subtype-specific differentiation.

Our previous work has shown that the conserved atonal-class bHLH transcription factor LIN-32 is necessary for multiple regulatory steps in ray development. Early in ray development, lin-32 implements neural competence in the ray precursor cell. Later, lin-32 has multiple independent roles that are required for the differentiation of each terminal ray cell type. We are currently working to understand how lin-32 function is coupled to the specification of each neuroglial cell type.

More recently, we have used DNA microarrays to identify a number of genes, including the DM-domain factor DMD-3 and the LIM-homeodomain factor LIM-7, that have important roles in patterning individual branches of the ray lineage. We have also found that the regulated asymmetric expression of lin-32 itself in the ray lineage may have important roles in this process. Finally, we are also taking a forward genetic approach to this question and have recently isolated several novel mutants that show specific defects in the development of one class of ray neurons. We expect these mutants to identify new genes that act downstream of or in parallel with lin-32 to implement neural subtype diversity among the progeny of the ray precursor cell. Vertebrate homologs of these genes will be excellent candidates for factors that act in similar subtype-specification pathways.

This project is supported by an R01 grant from NINDS.

(A) The wild-type male tail (two of the 18 rays are indicated by arrows).

(B) Each ray contains the sensory endings of two neurons (RnA and RnB) surrouned by the process of the ray structural cell Rnst.

(C) The three cell types of each ray descend clonally from a single ray precursor cell, Rn.

Sensory transduction in the C. elegans male tail

The C. elegans male tail also provides an outstanding opportunity to dissect the genetic contributions to the sensory transduction pathways that operate in ray neurons. Surprisingly, recent genetic studies of male behavior in C. elegans have found that two genes critical for ray sensory function have human homologs with important functions in the kidney. These genes, called PKD1 and PKD2 in humans, are associated with Autosomal Dominant Polycystic Kidney Disease (ADPKD), a leading cause of renal failure in the US. Current treatment of ADPKD focuses only on the amelioration of symptoms, as no means for halting or reversing the decline of renal function have been found.

In C. elegans, the polycystin proteins are present in the ciliated tips of ray sensory neurons, where they are thought to be necessary for transducing sensory cues. In renal tubules, the polycystins are present in the sensory cilia of epithelial cells. In this context, the polycystins act to transduce a calcium signal in response to the bending of the cilium by fluid flow. Disruption of this function leads to cystogenesis and the fibrosis of the kidney seen in PKD. Because very little is known about the mechanisms that mediate the sensory functions of the polycystins, C. elegans provides an excellent genetic system in which to address this question.

The polycystin genes PKD1 and PKD2 encode transmembrane proteins that are thought to form a receptor-channel complex. Our recent microarray studies led unexpectedly to the identification of five novel extracellular proteins whose expression is restricted to the precise set of 21 neurons that express the polycystins in C. elegans. We believe that these factors, which we call CWP-1 through CWP-5, are likely to have important roles in the polycystin signaling pathway. Consistent with this, we have recently found that mutations in at least one of these genes can suppress the sensory defects associated with polycystin mutations. We expect the characterization of these factors to lead to significant insight into the mechanisms of polycystin signaling in the context of a sensory cilium.

In addition, we are using the calcium indicator cameleon to monitor polycystin-mediated signaling in vivo. This integrative physiological approach will provide a unique and powerful means to define the specific stimuli that trigger a polycystin-mediated calcium response and will allow us to genetically define the signaling components acting upstream and downstream of the polycystins. A more thorough characterization of normal polycystin function is important both for defining therapeutic targets in PKD and for identifying the genetic modifiers that are thought to play a critical role in the progression of this disease.

This project is supported by an R21 grant from NIDDK. Pilot studies were funded by a Research Grant from the PKD Foundation.

Sexual dimorphism in the C. elegans nervous system

A number of recent findings in vertebrate systems have challenged the established idea that gonadal hormones are the sole determinant of sexually-dimorphic characteristics in the CNS. According to this hypothesis, cell-autonomous mechanisms that depend on differences in chromosome content (XX or XY) act in parallel with hormonal pathways to direct male- or female-specific development in the nervous system. Because a variety of neurological and psychiatric disorders (including autism and mood disorders) have significantly different incidences and courses between the sexes, a better understanding of the mechanisms that generate sexual dimorphisms in the CNS is critical. However, virtually nothing is known about the genetic mechanisms that might translate chromosomal cues into sex-specific neural characteristics. As with so many other basic biological processes, significant insight is likely to emerge from the analysis of simple genetic model systems. The well-defined nervous system of C. elegans, coupled with its reliance on a cell-autonomous sex-determination mechanism, ideally suits it to the study of these pathways.

We have recently made two important observations that indicate that the sex-determination pathway of C. elegans directly modifies its "core" non-sex-specific nervous system. C. elegans has two sexes, hermaphrodites (essentially females that are able to self-fertilize) and males. First, we unexpectedly identified a gene, srj-54, that is expressed in a single neuron pair (the RIG interneurons) only in males, despite the fact that this neuron pair is present in both sexes. We believe this is indicative of a hidden but large-scale molecular dimorphism in the worm nervous system that is not apparent at the cell-lineage or anatomical level. We aim to identify additional components of this dimorphism, and to define the genetic pathway that connects differential chromosome content to the sexually-dimorphic expression of these genes. Second, our recent behavioral studies have demonstrated striking and robust differences in olfactory behavior between the sexes. Because the olfactory system is well characterized and is anatomically and lineally equivalent between hermaphrodites and males, this indicates the presence of subtle differences in neural circuitry and/or gene expression. Again, these are amenable to genetic analysis. We expect these studies to lend novel insight into the genetic pathways that modulate sex-specific neural properties as a function of the chromosomal sex of an individual neuron.

This project is supported by a Research Grant from the National Alliance for Autism Research.