Forebrain and Eye: The forebrain originates from the anterior neural plate and gives rise to the anterior brain structures and the eyes during embryonic development. We use a critical regulator of the development of these structures, the transcription factor Six3, as a tool to help us dissect the molecular and cellular processes controlling forebrain and eye morphogenesis (Fig. 1). For these studies, we use mouse models as well as 3-D organ culture of stem cells and iPSCs (Fig. 2 and 4). Our laboratory demonstrated that functional inactivation of Six3 in the anterior neural plate (prospective forebrain) leads to the ectopic anterior expansion of Wnt1 and the loss of telencephalic tissue. This work demonstrated that Six3 is a direct in vivo repressor of Wnt1 during vertebrate forebrain formation and that this repression is required for normal telencephalic development. We also unraveled how mutations in Six3 lead to holoproscencephaly (HPE): the dominant mutation results in various degrees of midline fusion and cyclopia in humans. We demonstrated that haploinsufficiency of Six3 fails to activate Shh expression inthe ventral forebrain, leading to HPE.
In addition to the brain, the developing eye (lens and retina) expresses Six3. Taking advantage of the Six3–conditional mutant strain, we demonstrated that Six3 is crucial for lens formation. Conditional deletion of Six3 in the presumptive lens ectoderm (PLE) disrupted lens induction and specification; therefore, the lens placode and lens were absent. Using the conditional inactivation approach to remove Six3 from the prospective eye field territory, we also demonstrated that NR specification in mouse embryos requires Six3-mediated suppression of Wnt8b in the anterior neural plate. In these conditional-mutant embryos, eye field induction was not affected, and RPE specification appeared normal; however, NR specification did not occur (Fig. 3)
Congenital retina abnormalities such as anophthalmia (absence of eye) and microphthalmia (small eye), and retinal degenerative diseases such as retinitis pigmentosa and macular degeneration cause varying degrees of irreversible vision loss in millions of people worldwide. Recent advances using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) suggest that one day cell-replacement therapy will be used to treat these ocular diseases. Our lab is currently using the 3D organotopic method of growing eyes on a petri dish initially reported by the Sasai lab at the Riken Institute in Japan, to better understand the process controlling eye morphogenesis (Fig. 4).
Lymphatic Vasculature in Health and Disease: A better understanding of normal lymphatic development should allow us to address pathological lymphatic conditions that lead to inflammation, autoimmunity and cancer, and to improve the clinical treatment of primary and secondary forms of lymphoedema. We identified the transcription factor Prox1 as the first specific marker of lymphatic endothelial cells, a finding that allowed us to confirm a century-old theory proposing a venous origin of the mammalian lymphatic vasculature (Wigle and Oliver, 1999, Fig. 5). We also demonstrated that Prox1 is necessary for lymphatic cell specification in vivo, and is sufficient for lymphatic differentiation in culture (Wigle and Oliver, 1999; Wigle et al., 2002; Oliver and Detmar, 2002; Hong et al., 2002). Moreover, we showed that the lymphatic endothelial cell fate is plastic and reprogrammable, and requires constant Prox1 expression (Johnson et al., 2008, Fig. 6).
A few years ago, we published the first report of adult onset obesity resulting of lymphatic vasculature defects (Fig. 7). Our current efforts focus on characterizing the early steps leading to the formation of the lymphatic vasculature and to better understand the role of lymphatics in obesity and other metabolic diseases. Using loss and gain of function approaches we continue dissecting the early steps of lymphatic endothelial cell specification in mice. We are also using different approachs to restore lymphatic function and ameliorate the obesity phenotype in mouse models of this disease. We are also trying to identify reliable biomarkers that could be used to identify human patients in which obesity could also be a consequence of subtle lymphatic defects.