Kathryn L. McCabe

Essential role for PDGF signaling in ophthalmic trigeminal placode induction

Much of the peripheral nervous system of the head is derived from ectodermal thickenings, called placodes, that delaminate or invaginate to form cranial ganglia and sense organs. The trigeminal ganglion, which arises lateral to the midbrain, forms via interactions between the neural tube and adjacent ectoderm. This induction triggers expression of Pax3, ingression of placode cells and their differentiation into neurons. However, the molecular nature of the underlying signals remains unknown. Here, we investigate the role of PDGF signaling in ophthalmic trigeminal placode induction. By in situ hybridization, PDGF receptor β is expressed in the cranial ectoderm at the time of trigeminal placode formation, with the ligand PDGFD expressed in the midbrain neural folds. Blocking PDGF signaling in vitro results in a dose-dependent abrogation of Pax3 expression in recombinants of quail ectoderm with chick neural tube that recapitulate placode induction. In ovo microinjection of PDGF inhibitor causes a similar loss of Pax3 as well as the later placodal marker, CD151, and failure of neuronal differentiation. Conversely, microinjection of exogenous PDGFD increases the number of Pax3+ cells in the trigeminal placode and neurons in the condensing ganglia. Our results provide the first evidence for a signaling pathway involved in ophthalmic (opV) trigeminal placode induction.

Pea3 Expression Is Regulated by FGF Signaling In Developing Retina

FGF signaling has been implicated as an important regulator of retinal development. As a first step in characterizing potential downstream targets of FGF signaling in the retina, we have analyzed expression of Pea3, a member of the Pea3 class of Ets‐domain transcription factors, in the developing eye. We find that Pea3 is expressed in the developing retina, and its transcription is regulated by FGF receptor activation. In addition, FGF signaling activates Cath5, a gene necessary for retinal ganglion cell differentiation. These results suggest that FGF signaling via MAPK up‐regulates transcription factors that in turn control retinal ganglion cell differentiation. Developmental Dynamics 235:327–335, 2006.

Identification of candidate secreted factors involved in trigeminal placode induction

Cranial ectodermal placodes are critical for normal development of the peripheral nervous system of the head. However, many aspects of the molecular and tissue interactions involved in their induction have yet to be elucidated. The trigeminal placode is induced by an unidentified secreted factor(s) from the dorsal neural tube. To determine candidates that may be involved in this induction process, we have performed reverse transcriptase‐polymerase chain reaction (RT‐PCR) and whole‐mount in situ hybridization to screen for receptors expressed by uninduced presumptive trigeminal level ectoderm. We have found that receptors for fibroblast growth factors, insulin‐like growth factors, platelet‐derived growth factors, Sonic hedgehog, the transforming growth factor‐beta superfamily, and Wnts all are expressed in patterns consistent with a role in trigeminal placode formation. This RT‐PCR screen for candidate receptors expressed in presumptive trigeminal ectoderm is the first systematic screen to identify potential interactions underlying induction of the trigeminal placode and represents a critical step for understanding this complex process. Developmental Dynamics 236:2925–2935, 2007.