Background Upper motor neurons (UMNs) are a key component of motor neuron circuitry. Their degeneration is a hallmark for diseases, such as hereditary spastic paraplegia (HSP), primary lateral sclerosis (PLS), and amyotrophic lateral sclerosis (ALS). Currently there are no preclinical assays investigating cellular responses of UMNs to compound treatment, even for diseases of the UMNs. The basis of UMN vulnerability is not fully understood, and no compound has yet been identified to improve the health of diseased UMNs: two major roadblocks for building effective treatment strategies. Methods Novel UMN reporter models, in which UMNs that are diseased because of misfolded superoxide dismutase protein (mSOD1) toxicity and TDP‐43 pathology are labeled with eGFP expression, allow direct assessment of UMN response to compound treatment. Electron microscopy reveals very precise aspects of endoplasmic reticulum (ER) and mitochondrial damage. Administration of NU‐9, a compound initially identified based on its ability to reduce mSOD1 toxicity, has profound impact on improving the health and stability of UMNs, as identified by detailed cellular and ultrastructural analyses. Results Problems with mitochondria and ER are conserved in diseased UMNs among different species. NU‐9 has drug‐like pharmacokinetic properties. It lacks toxicity and crosses the blood brain barrier. NU‐9 improves the structural integrity of mitochondria and ER, reduces levels of mSOD1, stabilizes degenerating UMN apical dendrites, improves motor behavior measured by the hanging wire test, and eliminates ongoing degeneration of UMNs that become diseased both because of mSOD1 toxicity and TDP‐43 pathology, two distinct and important overarching causes of motor neuron degeneration. Conclusions Mechanism‐focused and cell‐based drug discovery approaches not only addressed key cellular defects responsible for UMN loss, but also identified NU‐9, the first compound to improve the health of diseased UMNs, neurons that degenerate in ALS, HSP, PLS, and ALS/FTLD patients.

There are no effective cures for upper motor neuron (UMN) diseases, such as amyotrophic lateral sclerosis (ALS), primary lateral sclerosis, and hereditary spastic paraplegia. Here, we show UMN loss occurs independent of spinal motor neuron degeneration and that UMNs are indeed effective cellular targets for gene therapy, which offers a potential solution especially for UMN disease patients. UCHL1 (ubiquitin C-terminal hydrolase-L1) is a deubiquitinating enzyme crucial for maintaining free ubiquitin levels. Corticospinal motor neurons (CSMN, a.k.a UMNs in mice) show early, selective, and profound degeneration in Uchl1 nm3419 (UCHL1 −/− ) mice, which lack all UCHL1 function. When UCHL1 activity is ablated only from spinal motor neurons, CSMN remained intact. However, restoring UCHL1 specifically in CSMN of UCHL1 −/− mice via directed gene delivery, was sufficient to improve CSMN integrity to the healthy control levels. In addition, when UCHL1 gene was delivered selectively to CSMN that are diseased due to misfolded SOD1 toxicity and TDP-43 pathology via AAV-mediated retrograde transduction, the disease causing misfolded SOD1 and mutant human TDP-43 were reduced in hSOD1 G93A and prpTDP-43 A315T models, respectively. Diseased CSMN retained their neuronal integrity and cytoarchitectural stability in two different mouse models that represent two distinct causes of neurodegeneration in ALS.