Mechanisms Underlying Axonopathy in Hereditary Spastic Paraplegia

Project Summary
Axonal degeneration within the corticospinal tract leads to several neurological diseases, including
hereditary spastic paraplegias (HSPs), which are a clinically and genetically heterogeneous group of gait
disorders characterized by poor balance, spasticity, and progressive muscle weakness that can ultimately result
in paralysis. Leveraging parallel animal (rat) and induced pluripotent stem cell (iPSC)-based models, our goal is
to develop a better understanding of the pathomechanisms that underlie neurodegeneration resulting from
mutations in genes that cause HSP, with a longer term goal of using these models as platforms to identify new
therapeutics to combat disease. Using CRISPR-mediated genome editing, we have developed physiologically
relevant models that recapitulate phenotypes exhibited by patients suffering from HSP. Specifically, CRISPR-
modified rats expressing pathological variants of SPG4 (spastin) and SPG57 (TFG) demonstrate early onset
hind limb spasticity and ataxia, which rapidly progresses to hind limb paralysis. Other rat models, including those
harboring a truncation of SPG80 (UBAP1) identified previously in patients, exhibit later onset disease phenotypes,
enabling us to examine disease progression in multiple, unique contexts. We now have an unprecedented
opportunity to determine the mechanistic basis of the axonopathies observed. In particular, we plan to use high-
resolution, live cell confocal imaging and electron tomography to test the hypothesis that changes in the
trafficking of specific factors, including neurofilament proteins implicated previously in neurodegenerative
disease, contribute to impaired neuronal function in HSP. We will also determine how neurofilament trafficking
defects observed relate to disease onset based on a combination of electromyography studies, histopathology,
and comprehensive gait and kinematic analysis of rodent movement as spasticity and muscle weakness ensues.
Furthermore, we will determine mechanisms by which mutations that underlie HSP impact neuronal excitability,
again using live cell imaging approaches, but also in vitro biochemistry and genetic studies. Collectively, this
work will help to uncover several of the mechanisms that contribute to neuronal dysfunction observed in patients
with HSP and lay the foundation for the future development of drug screening approaches.

Grant Number: 5R01NS124165-05
NIH Institute/Center: NIH
Principal Investigator: Anjon Audhya