Identification of pathways of degeneration and protection in motor neuron diseases

Abstract

Motor neuron diseases preferentially affect specific neuronal populations with distinct clinical features even if disease-causing genes are expressed in many cell types. In spinal muscular atrophy (SMA), somatic motor neurons are selectively vulnerable to a deficiency in the broadly expressed survival of motor neuron 1 (SMN1) gene. In amyotrophic lateral sclerosis (ALS), mutations in multiple ubiquitously expressed genes have been identified that result in the same selective vulnerability. However, certain somatic motor neuron groups, including oculomotor and trochlear (CN3/4) neurons, are for unknown reasons relatively resistant to degeneration. We hypothesized that we could use CN3/4 motor neuron resistance as a tool to dissect mechanisms of vulnerability and protection, which would aid in identifying drug targets for the treatment of so far incurable motor neuron diseases.

Within this thesis work, we developed a robust method for spatial transcriptomic profiling of closely related neuronal populations that is sensitive down to single cells and can be applied to partly degraded human post-mortem tissues. We called this method LCM-seq (laser capture microscopy coupled with RNA sequencing). We applied LCM-seq to reveal longitudinal changes in gene expression in a mouse model of SMA in order to elucidate distinct adaptation mechanisms of several motor neuron populations that could account for their differential susceptibility. We revealed a common activation of DNA damage response and apoptosis pathways in somatic motor neurons independent of their susceptibility. We furthermore found gene expression changes that were preferential to the resistant CN3/4 motor neurons. Of particular interest were genes that function in regeneration, synaptic vesicle release and those that protect cells from oxidative stress and apoptosis. We speculate that these genes could play a role in the resistance of CN3/4 motor neurons and their manipulation in vulnerable motor neurons could be used to protect these from degeneration. As proof of concept, we further investigated candidates with implications for differential vulnerability that we had previously identified in a transcriptome analysis in the normal rat. We demonstrated a relative conservation across species by confirming the expression patterns of multiple proteins in mouse and human and in health and disease. Moreover, we provided functional evidence that the oculomotor restricted insulin-like growth factor 2 (IGF-2) can rescue vulnerable spinal motor neurons in in vitro and in vivo models of ALS. This indicates that IGF-2 could in part play a role in the preservation of CN3/4 motor neurons in ALS. By combining comprehensive studies in mouse models and the use of human ALS patient tissues as well as patient-derived induced pluripotent stem cell based in vitro assays we could maximize the chance of identifying mechanisms with relevance in human disease.

In conclusion, we provide a tool box for transcriptional profiling of neuronal populations with differential vulnerability followed by functional studies in mouse and human aiding in elucidating pathological mechanisms in neurodegenerative diseases, which could lead to the identification of drug targets for the treatment of motor neuron diseases.