Fragile X premutation rCGG repeats impair synaptic growth and synaptic transmission atDrosophilalarval neuromuscular junction

Abstract

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset neurodegenerative disease that develops in some premutation (PM) carriers of the FMR1 gene with alleles bearing 55–200 CGG repeats. The discovery of a broad spectrum of clinical and cell developmental abnormalities among PM carriers with or without FXTAS and in model systems suggests that neurodegeneration seen in FXTAS could be the inevitable end-result of pathophysiological processes set during early development. Hence, it is imperative to trace early PM-induced pathological abnormalities. Previous studies have shown that transgenic Drosophila carrying PM-length CGG repeats are sufficient to cause neurodegeneration. Here, we used the same transgenic model to understand the effect of CGG repeats on the structure and function of the developing nervous system. We show that presynaptic expression of CGG repeats restricts synaptic growth, reduces the number of synaptic boutons, leads to aberrant presynaptic varicosities, and impairs synaptic transmission at the larval neuromuscular junctions. The postsynaptic analysis shows that both glutamate receptors and subsynaptic reticulum proteins were normal. However, a high percentage of boutons show a reduced density of Bruchpilot protein, a key component of presynaptic active zones required for vesicle release. The electrophysiological analysis shows a significant reduction in quantal content, a measure of total synaptic vesicles released per excitation potential. Together, these findings suggest that synapse perturbation caused by rCGG repeats mediates presynaptically during larval NMJ development. We also suggest that the stress-activated c-Jun N-terminal kinase protein Basket and CIDE-N protein Drep-2 positively mediate Bruchpilot active zone defects caused by rCGG repeats.

Patronin regulates presynaptic microtubule organization and neuromuscular junction development in Drosophila

“…Previous studies have unambiguously ascertained that synaptic morphogenesis could be well defined at the phenomenological level by investigating Drosophila NMJ. 45 , 73 In the present study, we raise a series of lines of in vivo evidence to determine that Patronin is an unreported regulator of synaptic development at the Drosophila NMJ, and Patronin governs synaptic development mainly via presynaptic apparatus. Firstly, our genetic assay with heterogeneous RNAi lines and mutants, as well as the rescue results, unambiguously corroborate that Patronin is cell-autonomously required for sustaining bouton morphology.…”

Non-canonical DNA/RNA structures associated with the pathogenesis of Fragile X-associated tremor/ataxia syndrome and Fragile X syndrome

“…Hyperexpression of PM alleles is associated with specific disorders, including fragile X premature ovarian insufficiency (FXPOI; OMIM #311360), a condition associated with menopause in women aged <40 years ( Sullivan et al, 2011 ; Sherman et al, 2014 ), and fragile X-associated tremor/ataxia syndrome, a neurodegenerative disorder (FXTAS; OMIM #300623) that affects PM carriers, mostly men over the age of 50 years, with clinical manifestations such as action tremors, gait ataxia, Parkinsonism, and cognitive decline ( Hagerman and Hagerman, 2016 ). In model systems, hyperexpression of riboCGG repeats in the PM range leads to defects in cell development and cell toxicity ( Hagerman, 2012 ; Hagerman et al, 2018 ; Bhat et al, 2021 ). Unlike FM alleles, PM alleles alter RNA-processing mechanisms, which may be related to unusual secondary structures formed by DNA strands and RNA containing CGG and antisense CCG repeats ( Zhao and Usdin, 2021 ).…”