The Brock Grill Laboratory at The Scripps Research Institute Florida

Research Projects

The Grill lab is interested in the molecular mechanisms that govern nerve cell development and function. Two primary areas of research are now under investigation.

1) Axon termination and synapse formation in the developing nervous system. We are actively exploring the molecular and cellular mechanisms that govern development of the nervous system. To this end, we are interested in two key questions:  How does an axon terminate growth? How are synapses formed during development?

To address these questions, we rely upon a non-traditional combination of in vivo proteomics, biochemistry and genetics using the microscopic nematode C. elegans, as well as biochemistry with cultured cells. Our work has begun to unravel how a massive intracellular signaling hub called RPM-1 regulates axon termination and synapse formation. Our interest in RPM-1 signaling in the nervous system is heightened because of the emerging function of RPM-1 homologs (Drosophila Highwire, mouse Phr1 and human Pam/MYCBP2) in axon degeneration, as well as genetic links between RPM-1 signaling and neurodevelopmental disorders, such as intellectual disability.

Our recent efforts combining in vivo proteomics to identify RPM-1 binding proteins with functional genetics to understand RPM-1 signaling has shown RPM-1 utilizes several mechanisms to regulate axon termination and synapse formation. These include: 1) Ubiquitin ligase activity via the F-box protein FSN-1 which inhibits the DLK-1 and MLK-1 MAP3Ks. 2) Recruitment of the PP2C family phosphatase PPM-2, which inhibits the DLK-1 MAP3K. 3) GLO-4 which regulates the Rab GTPase GLO-1 to affect late endosome biogenesis. 4) The microtubule binding protein RAE-1. 5) The Nesprin ANC-1 which regulates ß-catenin.

When dissecting nerve cell development, we rely upon the mechanosensory neurons as an in vivo developmental readout. Because the mechanosensory neurons sense gentle touch and mediate short-term learning, these cells also allow us to couple changes in development with impacts on whole animal behavior. A critical advantage of the mechanosensory neurons is that they allow us to molecularly and anatomically distinguish multiple important developmental events in a single cell in vivo including: growth cone development, axon termination, electrical synapse formation and chemical synapse formation. In the future, we will press forward on several important and intriguing questions: Does RPM-1 signaling impact axon termination by influencing the developing growth cone? Does RPM-1 affect synapse formation by regulating initial synapse assembly or synapse stability? Last, but perhaps most interesting. Is it possible to decipher the complete RPM-1 interactome using the most advanced in vivo protein labeling and mass spectrometry techniques?

2) Balancing excitatory and inhibitory neurotransmission in a simple model circuit. Cognition requires a precise balance of excitatory “on” and inhibitory “off” transmission in neuronal circuits. At present, we know very little about the molecular and genetic mechanisms that allow a circuit to obtain excitatory/inhibitory (E/I) balance. Importantly, impaired E/I balance is a characteristic of many neurodevelopmental disorders.

Our approach to understanding E/I balance is reductionist: We use a simple model circuit, the C. elegans motor circuit, to identify molecular mechanisms required to obtain E/I balance. Using this approach, we recently showed that EEL-1, a gigantic HECT family ubiquitin ligase, is required for GABAergic presynaptic transmission and E/I balance. This finding addresses an important basic neuroscience question, and has important implications because genetic changes in the human homolog of EEL-1, called HUWE1, are linked to intellectual disability, including Juberg-Marsidi-Brooks syndrome.

Having established a fundamental function for EEL-1/HUWE1 in a simple model circuit, we now aim to crack the molecular mechanism of how EEL-1 regulates GABAergic transmission and E/I balance. To do so, we will use a combination of in vivo proteomic and genetic methods with the C. elegans. Successfully understanding how EEL-1 affects GABAergic transmission and E/I balance in our simple model circuit could provide important molecular insight into why genetic changes that increase or impair HUWE1 function in humans leads to intellectual disability.