While we investigate several aGPCRs, GPR56 serves as an exemplary prototype of our "bedside to bench to bedside" paradigm. My interest in aGPCRs stemmed from my postdoctoral work in Christopher Walsh's lab at Harvard Medical School where I used a genetic approach to study human brain development and discovered mutant GPR56 as a cause of neurodevelopmental disease. The GPR56-associated neurodevelopmental disease, bilateral frontoparietal polymicrogyria (BFPP), results from universal (germline) lack of GPR56 at a specific point in brain development, due to homozygosity of any of several function-null mutants. In more than 10 years since then, we've discovered that GPR56 is highly expressed in several brain cell types including neural progenitor cells, oligodendrocytes, astrocytes, and microglia. The GPR56 gene is formidably complex, harboring multiple transcriptional start sites, differential splicing, post-translational processing and yet-to-be discovered ligands. Motivated by finding distinct phenotypes for selective loss of GPR56 function in individual brain-cell types we've taken GPR56 into model systems to clarify its pleiotropic and cell-type-specific functions in development. We look forward eagerly to deciphering aGPCR function and signaling at a level that will enable therapeutic targeting.

Cortical Development

There are four major lines of ongoing research in the Piao lab. The first identifies cellular and molecular components of the GPR56 signaling pathways involved in cerebral cortical development. Our previous work has shown that first-born neurons, known as preplate neurons, express GPR56 most strongly in the frontal cortex, the region of the cortex most devastated in GPR56 mutations. Although the molecular mechanisms underlying the function of these preplate neurons remain largely unknown, our data indicates that GPR56 plays an important role in their function. In the next phase of research, we will continue to study how GPR56 mediates the interaction between preplate neurons and the pial basement membrane, particularly in an attempt to elucidate how it helps to define the boundary between the neocortex and pial basement membrane while providing a framework for the developing brain. At a molecular level, we discovered that collagen III is the ligand for GPR56 in the developing brain, whose binding activates the RhoA pathway by coupling to Gÿ12/13. Given that collagen III is a known constituent of the pial basement membrane, this strongly implies that GPR56-expressing migrating cells actively communicate with the basement membrane during normal brain development.

CNS Myelination

The second line of ongoing research focuses on characterizing GPR56 and its binding partners in oligodendrocyte development and CNS myelination during development. The disease association here is the process of myelin repair (remyelination), the only robust neuroregenerative mechanism that operates in the adult human brain. Oligodendrocytes are the main source of myelin in the CNS and myelinate by extending their processes and repeatedly wrapping them around the axons, allowing both for trophic support of nerve fibers and rapid signal transduction down the axons. Our research previously showed that GPR56 plays an important role in the proliferation of oligodendrocyte precursor cells. We are now working on identifying the ligands of GPR56 present during myelination. Further, we are investigating the interactions between different glial cells mediated by GPR56 and their binding partners on CNS remyelination. Importantly, remyelination may be relevant for disease beyond the evident myelin disorders such as multiple sclerosis (MS). For example, prominent white matter degeneration is also characteristic of early Alzheimer disease (AD). We pursue the mechanisms by which oligodendroglial GPR56 and its ligands contribute to myelin formation and repair, mindful of the potential of this research to contribute to therapeutic strategies.

Microglia in Brain Development

Our third line of research is to study the role of microglial GPR56 in the development and function of the brain. Originating from the primitive myeloid cells in the yolk sac, microglia are tissue-resident macrophages in the CNS and enter the brain at the start of brain development. First described in the early years of the twentieth century, microglia have long been considered innate immune cells that primarily serve to clear injured/dead cells and infectious agents from the CNS. However, what we now know is that microglial physiology is dominated by physiological roles during neurodevelopment and synapse maintenance in the adult CNS. The pathological contributions of microglia to neurological disorders relate more closely to loss of physiological function than to gain of inflammatory toxicity. Tantalizingly, GPR56 is highly expressed in the microglia of the postnatal brain with currently unknown function. Our work will shed light on the molecular mechanisms underlying microglia-mediated brain development.

De-orphan aGPCRs

The fourth major line of ongoing research is the identification of ligands for aGPCRs, the majority of which remain orphan receptors. These poorly studied GPCRs play a critically important role in brain development, synaptogenesis, myelination, blood brain barrier formation, and cancer progression. We have already found that collagen III is the ligand for GPR56 in the developing cerebral cortex, and, with our collaborators, we have discovered that Laminin-211 is a ligand for GPR126, which is crucial for peripheral nervous system myelination. Using in vitro biotinylation and proteomics approaches, we plan to continue to study aGPCRs. With the progressive discoveries of ligands, we plan to use this information in the future for drug-screening and development.