Research

Backstory

GPR56 serves as an exemplary prototype of our "bedside to bench to bedside" paradigm. Mutation of GPR56 causes a brain malformation termed bilateral frontoparietal polymicrogyria (BFPP). More specifically, the malformation results from a global (germline) lack of GPR56 at a specific time point in brain development. Following more than a decade of subsequent research, the lab has discovered that GPR56 is dynamically regulated in several brain cell types including neural progenitor cells, oligodendrocytes, astrocytes, and microglia. In particular, GPR56 in neural progenitors governs aspects of cortical architecture. Additionally, oligodendroglial GPR56 is important for both developmental myelination and myelin repair, while microglial GPR56 mediates synapse refinement in diverse brain circuits. These observations position our studies of GPR56, a single molecular entity, to uncover how brain wiring is coordinated across multiple cell types throughout neurodevelopment. Ultimately our intention is to translate these data back to the bedside in the form of novel treatment strategies for conditions that arise in the context of aberrant neurodevelopment. In essence, GPR56 is a formidable and complex gene that exhibits differential splicing and undergoes post-translational processing with yet-to-be discovered ligands. Its full spectrum of action in the glial lineage remains to be deciphered. Therefore, continued research is necessary. Motivated by the distinct phenotypes resulting from the selective loss of GPR56 function in individual brain-cell types, we have developed GPR56- based model systems to clarify its pleiotropic and cell-type-specific functions across neurodevelopment. We look forward to deciphering aGPCR function and signaling to the point that will allow for confident therapeutic targeting to prevent or ameliorate neurodevelopmental disease. Described below are the four main ongoing lines of research in the lab.

Synapses and Circuits

Our second line of research involves studying 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, we now know that microglial physiology is dominated by their physiological roles during neurodevelopment and synapse maintenance. And so, the pathological contributions of microglia to neurological disorders relate more closely to loss of physiological function, rather than to the gain of inflammatory toxicity.

Interestingly, Gpr56 is highly expressed in microglia from embryonic to adult stages. Governed by a super-enhancer, Gpr56 is only expressed in yolk sac-derived microglia but not in microglia-like cells engrafted from fetal liver- and bone marrow-derived hematopoietic stem cells, even after long-term adaptation in the CNS in vivo. Furthermore, Gpr56 expression is promptly lost in primary cultures of microglia. Thus, Gpr56 is one of few genes that defines the microglial lineage and requires both the appropriate ontogeny and environmental cues for its expression. Motivated by the concept that cell-type-specific functions of GPR56 might coordinate multiple sequential and overlapping neurodevelopmental processes, we are investigating the role of microglial GPR56 in postnatal brain circuit formation and maintenance. Our work will shed light on the critical molecular mechanisms underlying microglia-mediated brain development and homeostasis. As noted above, GPR56 has cell-type-specific functions that support both myelination and synapse refinement, providing the first molecular entity to be implicated in both aspects of brain wiring and yielding an opportunity to understand the coordinated regulation of this process.

CNS Myelination

Our first line of research focuses on characterizing GPR56 and its binding partners in oligodendrocyte differentiation and in CNS myelination during development. The disease association here is the process of myelin disruption seen in multiple sclerosis. Oligodendrocytes are the main source of myelin in the CNS and form myelin by extending their processes and generating multiple tight wraps around each myelinated axon, allowing for both trophic support of nerve fibers as well as rapid signal transduction down the axons. We showed that GPR56 regulates developmental myelin formation through a remarkable tripartite signaling complex composed of a microglial-derived ligand, an oligodendrocyte receptor and an extracellular matrix component. Ongoing research efforts aim to further elucidate the targeting of this pathway in order to promote myelin repair. Although the obvious disease application lies in treating myelin disorders such as multiple sclerosis, we believe that the above processes may also have application in the prominent white matter degeneration characteristic of early stages of Alzheimer's Disease (AD). Our continuing efforts to investigate the underlying mechanisms by which oligodendroglial GPR56 and its ligands contribute to myelin formation and repair, show promise in developing a therapeutic approach. Finally, understanding this cellular function (of myelination) along with its associated signaling pathways, will provide useful read-out assays for addressing the effects of targeting the various GPR56 signaling domains.

Maternal Immune Activation

Epidemiological studies in humans showed that viral infection during pregnancy significantly increases the likelihood of autism spectrum disorders (ASD) in offspring. Animal studies recapitulate this finding by showing that sterile maternal immune activation (MIA) induces neurodevelopmental deficits and autism-like behaviors. However, the cellular and molecular mechanisms underlying MIA-induced circuit derangement remain largely elusive. Microglia are the tissue resident macrophages and primary innate-immune responders in the brain. Developmental functions of microglia have come vividly into view in recent years. Recently appreciated roles include providing growth factors to support layer V neuron survival, correct lamination of neocortical interneurons, and synaptic pruning. In the developing neocortex, microglia modulate neural progenitor survival by precisely timed-and-localized secretion of growth factors and they help to govern the size of neural progenitor pools by clearance of dead or stressed cells. Given the established functions of microglia and their presence at the critical time window of MIA-induced cortical malformation and behavior abnormalities, it is plausible that microglia function as the effector cells in fetal brain following MIA.

Along with collaborator Jun Huh at Harvard Medical School, the lab is investigating how maternal immune activation (MIA) deranges fetal and neonatal brain development through innate immune mechanisms. Specifically, the lab focuses on investigating the role of microglial GPR56 in development of fetal brains and how GPR56 downregulation on microglia after immune stimulation deranges 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 GPCRs play a critically important roles in brain development, synaptogenesis, myelination, blood brain barrier formation, and cancer progression. We found that collagen III is the ligand for neuron-progenitor GPR56 in the developing cerebral cortex while tissue transglutaminase is the ligand for GPR56 in oligodendrocyte precursor cells. With our collaborators, we discovered that Laminin-211 is a ligand for GPR126, which is crucial for peripheral nervous system myelination. Using in vivo and in vitro biotinylation and proteomics approaches, we plan to continue to study aGPCRs. With the progressive discoveries of ligands the biological significance and therapeutic potential of this fascinating receptor superfamily will become increasingly clear.