Research Summary

Locomotion is an ancient behavior displayed by vertebrate and invertebrate animals. Despite anatomical differences between species, locomotion invariably relies on the function of a specialized network of neuronal and muscle cells known as the motor circuit. Motor neurons lie at the heart of this circuit and display remarkable diversity based on anatomy, electrophysiology and molecular composition. Motor neurons are also the cellular substrates of a number of human diseases. By focusing on this ancient and clinically-relevant cell type, we hope to reveal broadly applicable molecular mechanisms underlying motor neuron development and disease.

The research objectives of our lab are:
1. To understand how motor neuron diversity is generated during development.
2. To uncover the molecular mechanisms that enable motor neurons to remain functional throughout life.
3. To reveal conserved and divergent molecular mechanisms underlying motor neuron development.
4. To study the molecular mechanisms that cause motor neuron disease.

To this end, we harness the specific strengths of two genetic model organisms. We use the nematode Caenorhabditis elegans (C. elegans) as a gene discovery tool and then aim to translate our findings to the vertebrate nervous system using the mouse Mus musculus as a model. We are also employing a number of in vitro cellular systems, such as motor neurons derived from human induced pluripotent stem cells (iPSC).

​​Studying motor neuron development  in C. elegans and mice

Its simple nervous system (302 neurons that fall into 118 distinct neuron types), the ease to perform forward genetic screens, its complete connectome, and the plethora of available molecular markers for specific neuron types make C.elegans an excellent model to study neuronal development and neurodegeneration. Specifically, our lab focuses on the C. elegans ventral nerve cord, which is populated with distinct subtypes of cholinergic and GABAergic motor neurons necessary for locomotion. By performing forward genetic screens, we identified novel, evolutionarily conserved regulatory factors (e.g., transcription factors, chromatin factors) required for motor neuron diversity and function. Using state-of-the-art methodology including whole genome sequencing, CRISPR/Cas9 genome editing, single-molecule mRNA fluorescent in situ hybridization (FISH) and cell type-specific transcriptome profiling (single cell RNA-Sequencing), we aim to elucidate the gene regulatory mechanisms that enable motor neurons to mature and become functional. 

We previously showed that the phylogenetically conserved COE (Collier, Olf, EBF)-type transcription factor UNC-3 is required for terminal differentiation of the majority of C.elegans nerve cord motor neurons 
(PMID: 22119902, PMID: 25913400). To extend our findings to more complex nervous systems, we used the simple chordate Ciona intestinalis (in collaboration with the lab of Michael Levine) and found that the UNC-3 ortholog in C. intestinalis (CiCOE) is also required for motor neuron terminal differentiation 
(PMID: 22119902). This striking phylogenetic conservation throughout millions of years of evolution (from worms to simple chordates) suggests an ancient role for UNC-3. Intriguingly, UNC-3 orthologs are also expressed in the mouse spinal cord.

Our recent study (PMID: 30658714) suggests that the function of UNC-3 is conserved in mouse spinal motor neurons. Motivated by these observations, our lab aims to systematically test whether the function of other transcription factors we study in C. elegans (e.g., Hox family of TFs) is conserved across phylogeny. To this end, we employ conditional mouse mutagenesis (Cre/loxP technology) combined with cutting-edge molecular (e.g., RNA-Seq) and behavioral approaches.

Motor neurons in the mouse embryonic spinal cord

Motor neuron axonal projections to the mouse forelimb

Studying motor neuron disease in C. elegans, mice, and in vitro models

More recently, our lab became interested in the molecular mechanisms underlying motor neuron disease. In collaboration with the Roos lab (Neurology, U Chicago), we developed a new C. elegans model that recapitulates aspects of motor neuron disease (ALS). Check our collaborative paper here: https://doi.org/10.1101/2020.06.13.150029

A C. elegans model of C9ORF72 ALS/FTD implicates a new molecule in motor neuron degeneration

We focus on mutations in the C9ORF72 gene, the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) - two devastating diseases with no available cure. These mutations are unusual in nature, they are expansions of the GGGGCC nucleotide sequence in the first intron of C9ORF72. The nucleotide expansions are transcribed and subsequently translated to produce neurotoxic proteins through a poorly understood process termed “repeat-associated non-AUG (RAN) translation”. In this paper, we showed for the first time that RAN translation occurs in C. elegans by developing a C. elegans model of C9ORF72 ALS/FTD. This discovery significantly contributes to the ALS/FTD field, as it enables the use of an in vivo model system with powerful genetic tools for the study of ALS/FTD pathogenesis (Sonobe et al., Nature Communications, 2021, PMID: 34654821). We have already shared this animal model with multiple labs across the globe. Importantly, this model enabled us to identify the eukaryotic translation initiation factor eif-2D/eIF2D as a critical driver of motor neuron dysfunction and degeneration in C. elegans. 
Related to this line of research, our recent manuscript entitled “Translation of dipeptide repeat proteins in C9ORF72 ALS/FTD through unique and redundant AUG initiation codons” can be found at bioRxiv: https://www.biorxiv.org/content/10.1101/2022.08.06.503063v1

​Outlook

A detailed understanding of how motor neurons develop and function may provide novel entry points into the etiology, diagnosis or treatment of motor neuron disorders, such as spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). From the basic science perspective, our research will reveal novel regulatory factors (e.g., transcription factors, chromatin factors) and the molecular mechanisms through which these factors act to control motor neuron development.