The transcription factor Lmx1b maintains Wnt1 expression within the isthmic organizer

Abstract

Cells in the caudal mesencephalon and rostral metencephalon become organized by signals emanating from the isthmus organizer (IsO). The IsO is associated with the isthmus, a morphological constriction of the neural tube which eventually defines the mesencephalic/ metencephalic boundary (MMB). Here we report that the transcription factor Lmx1b is expressed and functions in a distinct region of the IsO. Lmx1b expression is maintained by the glycoprotein Fgf8, a signal capable of mediating IsO signaling. Lmx1b, in turn, maintains the expression of the secreted factor Wnt1. Our conclusions are substantiated by the following: (i) Lmx1b mRNA becomes localized to the isthmus immediately after Fgf8 initiation, (ii) Wnt1 expression is localized to the Lmx1b expression domain, but with slightly later kinetics, (iii) Fgf8-soaked beads generate similar domains of expression for Lmx1b and Wnt1 and (iv) retroviral-mediated expression of Lmx1b (Lmx1b/RCAS) maintains Wnt1 expression in the mesencephalon. Ectopic Lmx1b is insufficient to alter the expression of a number of other genes expressed at the IsO, suggesting that it does not generate a new signaling center. Instead, if we allow Lmx1b/RCAS-infected brains to develop longer, we detect changes in mesencephalic morphology. Since both ectopic and endogenous Lmx1b expression occurs in regions of the isthmus undergoing morphological changes, it could normally play a role in this process. Furthermore, a similar phenotype is not observed in Wnt1/RCAS-infected brains, demonstrating that ectopic Wnt1 is insufficient to mediate the effect of ectopic Lmx1b in our assay. Since Wnt1 function has been linked to the proper segregation of mesencephalic and metencephalic cells, we suggest that Lmx1b and Wnt1 normally function in concert to affect IsO morphogenesis.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

“…The Lmx1b gene encodes an LIM homeodomain protein with a dynamic expression pattern in the developing central nervous system, including the MH domain (Adams et al, 2000 ; Liu and Joyner, 2001 ; Matsunaga et al, 2002 ; O’Hara et al, 2005 ; Guo et al, 2007 ; Kim et al, 2016 ; Pose-Méndez et al, 2016 ). In chicks, Lmx1b starts to be expressed immediately after Fgf8 initiation (Adams et al, 2000 ).…”

“…The Lmx1b gene encodes an LIM homeodomain protein with a dynamic expression pattern in the developing central nervous system, including the MH domain (Adams et al, 2000 ; Liu and Joyner, 2001 ; Matsunaga et al, 2002 ; O’Hara et al, 2005 ; Guo et al, 2007 ; Kim et al, 2016 ; Pose-Méndez et al, 2016 ). In chicks, Lmx1b starts to be expressed immediately after Fgf8 initiation (Adams et al, 2000 ). At stage HH15–20, Lmxb1 expression is observed in a band located just rostral to the MH boundary, as defined by the caudal Otx2 expression, and in the dorsal and ventral midlines of the developing midbrain.…”

“…At stage HH15–20, Lmxb1 expression is observed in a band located just rostral to the MH boundary, as defined by the caudal Otx2 expression, and in the dorsal and ventral midlines of the developing midbrain. The caudal borders of the Wnt1 - and Lmx1b -expressing areas are coincident and abut the Fgf8 -positive area (Adams et al, 2000 ; Matsunaga et al, 2002 ). Using Fgf8 -soaked beads and retroviral-mediated Lmx1b expression ( Lmx1b/RCAS ), it has been reported that Lmx1b seems to act as an effector of Fgf8 in the regulation of Wnt1 expression (Adams et al, 2000 ).…”

“…The caudal borders of the Wnt1 - and Lmx1b -expressing areas are coincident and abut the Fgf8 -positive area (Adams et al, 2000 ; Matsunaga et al, 2002 ). Using Fgf8 -soaked beads and retroviral-mediated Lmx1b expression ( Lmx1b/RCAS ), it has been reported that Lmx1b seems to act as an effector of Fgf8 in the regulation of Wnt1 expression (Adams et al, 2000 ). Thus, Lmxb1 could be both necessary and sufficient to maintain Wnt1 expression in the MH domain, regulating the midbrain morphogenesis (Adams et al, 2000 ).…”

“…Using Fgf8 -soaked beads and retroviral-mediated Lmx1b expression ( Lmx1b/RCAS ), it has been reported that Lmx1b seems to act as an effector of Fgf8 in the regulation of Wnt1 expression (Adams et al, 2000 ). Thus, Lmxb1 could be both necessary and sufficient to maintain Wnt1 expression in the MH domain, regulating the midbrain morphogenesis (Adams et al, 2000 ). In another line of evidence, Lmx1b misexpression by in ovo electroporation induced Wnt1 and Otx2 expressions, while Fgf8 expression was repressed (Matsunaga et al, 2002 ).…”