Dev Bio SuperGroup Meeting

Lhx9 regulates hypocretin neuron specification in zebrafish and mice
Justin Liu - Prober Lab

Loss of neurons that express the neuropeptide hypocretin (Hcrt) has been implicated in
narcolepsy, a debilitating disorder characterized by excessive daytime sleepiness and cataplexy.
Cell replacement therapy using Hcrt-expressing neurons is a potentially useful therapeutic
approach, but factors sufficient to specify Hcrt neurons are unknown. Using zebrafish as a highthroughput
system to screen for factors that can specify Hcrt neurons in vivo, we identified the LIM
homeobox transcription factor Lhx9 as necessary and sufficient to specify Hcrt neurons. We found
that Lhx9 can directly induce hcrt expression and identified two essential Lhx9 binding sites in the
zebrafish hcrt promoter. Akin to its function in zebrafish, we found that Lhx9 is sufficient to
specify hcrt-expressing neurons in the developing mouse hypothalamus. Our results elucidate an
evolutionarily conserved role for Lhx9 in Hcrt neuron specification that improves our understanding
of Hcrt neuron development and may enable a novel therapeutic approach for narcolepsy.

piRNA pathway guides establishment of repressive H3K9me3 mark on active LINE1 elements in germ cells
Dubravka Pezic - Aravin lab

Transposable elements (TEs) occupy large fraction of metazoan genomes and play important role
in large-scale chromatin structure, expression of adjacent protein-coding genes, and evolution of
genomes. TEs represent large threat to genomic integrity due to their capacity to induce doublestranded
DNA breaks, insertional mutations and chromosome rearrangements. Many species have
evolved efficient strategies to control TE activity using two often linked mechanisms that operate at
the levels of chromatin and small RNA-mediated silencing. In both mammals and plants, many
families of TEs are silenced by DNA methylation. Studies in mammals also suggest that TEs are
repressed through histone modifications, particularly H3K9me3 mark, however, except for a few
cases, it is not clear how TE sequences are discriminated from normal genes and targeted for
chromatin-based repression. In mammalian germ cells, a specific class of small RNAs, the
piRNAs, recognizes and silences a diverse set of TEs through post-transcriptional repression and
establishment of CpG DNA methylation mark on their sequences. Here, we employed a ChIP-Seq
approach to analyze the H3K9me3 mark on TEs in different tissues and explored the link between
this mark and the piRNA pathway. Our results show that all classes of retrotransposons are targets
of this repressive histone mark in mouse tissues. In germ cells, H3K9me3 levels on LINE elements
are further increased. Upon impairment of the piRNA pathway, young, potentially active families of
LINE elements lose H3K9me3, and their transcripts are overexpressed. Our data therefore