The hereditary spastic paraplegias (HSP) are a group of over 80 neurodegenerative conditions and the most
common cause of inherited spasticity and associated disability. This K08 proposal focuses on prototypical
forms of HSP in children caused by biallelic loss-of-function variants in four genes that encode subunits of the
adaptor protein complex 4 (AP-4): AP4B1, AP4M1, AP4E1, and AP4S1. Progressive degeneration of the
cortico-spinal tracts renders most children with AP-4-associated HSP wheelchair-dependent by the age of 10
years. Currently, there are no therapies that halt disease progression, and few patients are known to have
survived into adulthood, highlighting the urgency for research into the fundamental biology of HSP.
The AP-4 complex is crucial for the intracellular trafficking of transmembrane proteins, including the
autophagy-related protein ATG9A. How altered trafficking of ATG9A leads to impaired neurodevelopment and
axonal degeneration and how ATG9A distribution can be restored is currently unknown. In this proposal, I will
address this unmet question by developing neuronal models of AP-4 deficiency and testing novel modulators
of AP-4-dependent protein trafficking. In preliminary experiments, we have systematically screened small
molecule modulators of ATG9A trafficking using a cell-based phenotypic assay that measures ATG9A
distribution as a surrogate of AP-4 function. We identified several modulators of ATG9A distribution. I will test
the hypothesis that these restore trafficking and function of ATG9A in vitro in neurons derived from AP-4-HSP
patients and in vivo in an ap4b1-/- zebrafish model.
This proposal presents a five-year research career development program focused on the study of AP-4 in HSP
to expand the breadth and depth of understanding the role of protein trafficking and autophagy in this group of
diseases. The goal is the establishment of a cross-organismal screening platform to identify and develop novel
modulators of protein trafficking for the treatment of HSP.
The candidate is currently a resident in Child Neurology at the Department of Neurology at Boston Children's
Hospital and Harvard Medical School. The outlined proposal builds on the candidate's previous research on
protein trafficking, autophagy and neurodegeneration and integrates new domains of expertise in cell biology,
advanced microscopy, and iPSC-derived neurons and genetically-engineered zebrafish to model human
diseases. These skills are reflected in his mentoring team consisting of primary mentor, Dr. Mustafa Sahin, and
a scientific advisory committee consisting of Dr. Craig Blackstone, Dr. Thomas Schwarz, Dr. Annapurna Poduri
and Dr. Leonard Zon. The proposed experiments and didactic work will position the candidate with a unique
set of cross-disciplinary skills that will enable his transition to independence as a physician-scientist in the field
of translational neuroscience in childhood-onset neurological diseases.
