Combined DiI and Antibody Labeling Reveals Complex Dysgenesis of Hippocampal Dendritic Spines in a Mouse Model of Fragile X Syndrome

Abstract

Structural, functional, and molecular alterations in excitatory spines are a common hallmark of many neurodevelopmental disorders including intellectual disability and autism. Here, we describe an optimized methodology, based on combined use of DiI and immunofluorescence, for rapid and sensitive characterization of the structure and composition of spines in native brain tissue. We successfully demonstrate the applicability of this approach by examining the properties of hippocampal spines in juvenile Fmr1 KO mice, a mouse model of Fragile X Syndrome. We find that mutant mice display pervasive dysgenesis of spines evidenced by an overabundance of both abnormally elongated thin spines and cup-shaped spines, in combination with reduced density of mushroom spines. We further find that mushroom spines expressing the actin-binding protein Synaptopodin—a marker for spine apparatus—are more prevalent in mutant mice. Previous work identified spines with Synaptopodin/spine apparatus as the locus of mGluR-LTD, which is abnormally elevated in Fmr1 KO mice. Altogether, our data suggest this enhancement may be linked to the preponderance of this subset of spines in the mutant. Overall, these findings demonstrate the sensitivity and versatility of the optimized methodology by uncovering a novel facet of spine dysgenesis in Fmr1 KO mice.

Loss of synaptopodin impairs mGluR5 and protein synthesis–dependent mGluR-LTD at CA3-CA1 synapses

“…In FXS, a neurodevelopmental disorder characterized by enhanced mGluR-LTD, basal protein synthesis is increased and SP is found more abundantly in dendritic spines ( 70 , 71 ). The fragile X messenger ribonucleoprotein (FMRP)—silenced in FXS—regulates RNA translation and was found associated with the SA ( 72 ).…”

Mouse models of fragile X-related disorders

“…FXS patients have been reported to have a higher density of cortical dendritic spines that have an immature morphology ( Irwin et al, 2002 ). FXS KO mice also have submicroscopic neuroanatomical abnormalities, which are brain region and age dependent, including dendritic spines that are immature in their appearance, evidenced by the overrepresentation of both elongated, thin spines and short, cup-shaped ones ( Speranza et al, 2022 ; Wijetunge et al, 2014 ).…”