Neurodegenerative proteinopathies like amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer’s disease (AD), are incurable, adult-onset, progressive, and fatal diseases that share pathological hallmarks of aberrant protein hyperphosphorylation and aggregation into insoluble, degradation resistant inclusions. Despite heterogeneity in familial versus sporadic ALS, over 90% of all patients exhibit the key pathological hallmark of mislocalized endogenous nuclear TAR DNA-binding protein 43 (TDP-43), an RNA binding protein. In health, endogenous TDP-43 resides in the nucleus of motor neurons where it regulates essential functions for RNA metabolism, mRNA stability, alternative splicing, and transcriptional repression. However, in disease, nuclear TDP-43 is mislocalized to the cytoplasm where it forms hyperphosphorylated TDP-43 (pTDP-43) aggregates has been suggested to have a gain-of-toxicity function in neurons. Our lab identified high correlation between casein kinase 1 isoform epsilon (CK1ε) expression and pTDP-43 pathology in human spinal cord tissue of ALS patients using an unbiased transcriptomic approach. CK1ε is a serine/threonine specific phosphorylation kinase localized in the cytoplasm of cells and has been implicated in neurodegenerative diseases. In ongoing research, we are studying the regulatory role of kinases and phosphatases in phosphorylating TDP-43 and their contribution to pathologic TDP-43 aggregation and cellular toxicity in both cellular and rodent models of TDP-43 proteinopathy.

Our lab is interested in understanding early events that lead to neuronal loss in ALS patients. Recently, we identified in our human ALS tissue samples an RNA binding protein ELAVL3 that is exclusively expressed in neurons and pursued this to find that it is significantly downregulated in ALS. Little is known about this RBP in ALS, but our results after a detailed neuropathological characterization reveal ELAVL3 as a very exciting candidate of study due to it being altered in a broader number of neurons than TDP-43 (the pathological hallmark of ALS) and not only in sporadic cases, but also familial cases including SOD1 which does not present with TDP-43 abnormalities. We believe this study can make an impact on the field and are excited to inquire in this direction, being able to contribute a comprehensive understanding of the causal inferences in ALS to undercover new therapies or treatments that stop neurodegeneration in ALS and other dementias.

Nucleolar stress has been implicated in the pathology and disease pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) from repeat expansions of GGGGCC in C9orf72 (C9-ALS/FTLD) but not in sporadic ALS (SALS). Previously we reported that antisense RNA transcripts are unique in C9-ALS because of their nucleolar localization in spinal motor neurons and correlation with TDP-43 mislocalization, the hallmark proteinopathy of ALS and FTLD. We have found that nucleolar stress manifests specifically as shrinkage in nucleoli of C9-ALS spinal motor neurons. Nucleolar size reduction is greatest in similarly sized alpha motor neurons from C9-ALS cases and results are not skewed by the number of surviving neurons from each ALS spinal cord. Surprisingly, nucleolar shrinkage occurs before main pathological hallmarks-TDP-43 mislocalization or antisense RNA foci-appear and this suggest that nucleolar stress can precede pathology in C9-ALS, findings previously identified in C9-FTLD using sense RNA foci and dipeptide repeat proteins as pathological markers. Importantly, these observations are also seen in SALS motor neurons and thus nucleolar stress appears to be a significant and probably upstream problem in sporadic disease.

Sigma-1 receptors (S1Rs) are chaperone proteins usually located on the mitochondria-associated membrane (MAM) of the endoplasmic reticulum (ER) and are thought to play a neuro-protective role by modulating ER stress proteins to promote cell survival. In cases of high ER stress or high concentration of agonists, S1Rs translocate from the MAM ER to the plasma membrane (PM). The roles of S1Rs have been studied in mice and other animal models, but they have yet to be well-evaluated in humans. Recently, Nuedexta®, a sigma-1 receptor agonist, has been proven to enhance bulbar function in amyotrophic lateral sclerosis (ALS). The basis for the palliative effect of Nuedexta is speculative. To study this further, we aim to visualize S1Rs in motor neurons, which innervate both the bulbar and skeletal musculature, the former subserving speech and swallowing and the latter providing the motive force for the arms and legs. Immunohistochemistry and immunofluorescence are being used to study S1R expression in healthy and various disease phenotypes to better understand the role of this drug and the neuro-protective role S1R may play in ALS pathogenesis.

Axonal transport of TDP-43 has been studied in ALS patient-derived iPSC motor neurons, transgenic mouse models, mouse and human peripheral nerves, and the human motor cortex and medulla. However, it has not yet been described in the human spinal cord. Lower motor neurons in the spinal cord receive direct input from upper motor neurons through the corticospinal tracts. Neuropathology of corticospinal tracts reflects upper motor neuron axons, while that of roots and nerves reflect lower motor neurons. Direct evidence of TDP-43’s behavior in corticospinal tract axons is important for understanding the role of extranuclear TDP-43 and how it may impact or correlate with the pathogenesis of ALS. Immunofluorescence staining of human thoracic spinal cords revealed decreased axon densities in the lateral and ventral corticospinal tracts in sporadic ALS, suggesting axon degeneration. A significant extrasomatic presence of TDP-43 inclusions was also observed in distal neurites throughout the corticospinal tracts. In the ventral horn gray matter, there was heterogeneity in the types of inclusions that were observed. These pathological differences between corticospinal tracts and the ventral horn gray matter thus emphasize the need to distinguish upper motor neurons and lower motor neurons in further analyses.

TDP-43 pathology in oligodendrocytes is perhaps the most frequently observed sporadic ALS disease hallmark, even more so than motor neuron pathology, and is speculated to occur prior to motor neuron pathology. We are studying phosphorylation of TDP-43 in oligodendrocytes in hopes of better understanding how it may play a role in ALS disease pathology and its spread.