The mir-279/996 cluster represses receptor tyrosine kinase signaling to determine cell fates in the Drosophila eye.

Photoreceptors in the crystalline Drosophila eye are recruited by receptor tyrosine kinase (RTK)/Ras signaling mediated by Epidermal growth factor receptor (EGFR) and the Sevenless (Sev) receptor. Analyses of an allelic deletion series of the mir-279/996 locus, along with a panel of modified genomic rescue transgenes, show that Drosophila eye patterning depends on both miRNAs. Transcriptional reporter and activity sensor transgenes reveal expression and function of miR-279/996 in non-neural cells of the developing eye. Moreover, mir-279/996 mutants exhibit substantial numbers of ectopic photoreceptors, particularly of R7, and cone cell loss. These miRNAs restrict RTK signaling in the eye, since mir-279/996 nulls are dominantly suppressed by positive components of the EGFR pathway and enhanced by heterozygosity for an EGFR repressor. miR-279/996 limit photoreceptor recruitment by targeting multiple positive RTK/Ras signaling components that promote photoreceptor/R7 specification. Strikingly, deletion of mir-279/996 sufficiently derepresses RTK/Ras signaling so as to rescue a population of R7 cells in R7-specific RTK null mutants boss and sev, which otherwise completely lack this cell fate. Altogether, we reveal a rare setting of developmental cell specification that involves substantial miRNA control.

Foxa1 and Foxa2 positively and negatively regulate Shh signalling to specify ventral midbrain progenitor identity.

Foxa2, a member of the Foxa family of forkhead/winged helix family of transcription factors, has previously been shown to be an upstream positive regulator of Shh expression in many different tissues. Recent studies also strongly suggest that Foxa2 specify cell fate by inhibiting the expression of cell fate determinants such as Helt1 and Nkx2.2. In this paper, phenotypic analyses of Wnt1cre; Foxa2flox/flox embryos in the midbrain have demonstrated a novel role for Foxa2 and its related family member, Foxa1, to attenuate Shh signalling by inhibiting the expression of its intracellular transducer, Gli2, at the transcriptional level. Chromatin immunoprecipitation experiments indicate that Foxa2 binds to genomic regions of Gli2 and likely regulates its expression in a direct manner. Our studies, involving loss and gain of function studies in mice, also provided further insights into the gene regulatory interactions among Foxa1, Foxa2 and Shh in ventral midbrain progenitors that contribute to midbrain patterning. Altogether, these data indicate that Foxa1 and Foxa2 contribute to the specification of ventral midbrain progenitor identity by regulating Shh signalling in a positive and negative manner.

Foxa1 and Foxa2 function both upstream of and cooperatively with Lmx1a and Lmx1b in a feedforward loop promoting mesodiencephalic dopaminergic neuron development.

Mesodiencephalic dopaminergic neurons control voluntary movement and reward based behaviours. Their dysfunction can lead to neurological disorders, including Parkinson's disease. These neurons are thought to arise from progenitors in the floor plate of the caudal diencephalon and midbrain. Members of the Foxa family of forkhead/winged helix transcription factor, Foxa1 and Foxa2, have previously been shown to regulate neuronal specification and differentiation of mesodiencephalic progenitors. However, Foxa1 and Foxa2 are also expressed earlier during regional specification of the rostral brain. In this paper, we have examined the early function of Foxa1 and Foxa2 using conditional mutant mice. Our studies show that Foxa1 and Foxa2 positively regulate Lmx1a and Lmx1b expression and inhibit Nkx2.2 expression in mesodiencephalic dopaminergic progenitors. Subsequently, Foxa1 and Foxa2 function cooperatively with Lmx1a and Lmx1b to regulate differentiation of mesodiencephalic dopaminergic neurons. Chromatin immunoprecipitation experiments indicate that Nkx2.2 and TH genes are likely direct targets of Foxa1 and Foxa2 in mesodiencephalic dopaminergic cells in vivo. Foxa1 and Foxa2 also inhibit GABAergic neuron differentiation by repressing the Helt gene in the ventral midbrain. Our data therefore provide new insights into the specification and differentiation of mesodiencephalic dopaminergic neurons and identifies Foxa1 and Foxa2 as essential regulators in these processes.

Foxa1 and Foxa2 regulate multiple phases of midbrain dopaminergic neuron development in a dosage-dependent manner.

The role of transcription factors in regulating the development of midbrain dopaminergic (mDA) neurons is intensively studied owing to the involvement of these neurons in diverse neurological disorders. Here we demonstrate novel roles for the forkhead/winged helix transcription factors Foxa1 and Foxa2 in the specification and differentiation of mDA neurons by analysing the phenotype of Foxa1 and Foxa2 single- and double-mutant mouse embryos. During specification, Foxa1 and Foxa2 regulate the extent of neurogenesis in mDA progenitors by positively regulating Ngn2 (Neurog2) expression. Subsequently, Foxa1 and Foxa2 regulate the expression of Nurr1 (Nr4a2) and engrailed 1 in immature neurons and the expression of aromatic l-amino acid decarboxylase and tyrosine hydroxylase in mature neurons during early and late differentiation of midbrain dopaminergic neurons. Interestingly, genetic evidence indicates that these functions require different gene dosages of Foxa1 and Foxa2. Altogether, our results demonstrate that Foxa1 and Foxa2 regulate multiple phases of midbrain dopaminergic neuron development in a dosage-dependent manner.