Multifaceted Changes in Synaptic Composition and Astrocytic Involvement in a Mouse Model of Fragile X Syndrome

Abstract

Fragile X Syndrome (FXS), a common inheritable form of intellectual disability, is known to alter neocortical circuits. However, its impact on the diverse synapse types comprising these circuits, or on the involvement of astrocytes, is not well known. We used immunofluorescent array tomography to quantify different synaptic populations and their association with astrocytes in layers 1 through 4 of the adult somatosensory cortex of a FXS mouse model, the FMR1 knockout mouse. The collected multi-channel data contained approximately 1.6 million synapses which were analyzed using a probabilistic synapse detector. Our study reveals complex, synapse-type and layer specific changes in the neocortical circuitry of FMR1 knockout mice. In layers 1 and 2/3, there is a significant decrease in the density of excitatory glutamatergic synapses, both VGluT1 and VGluT2 type, and their association with astrocytes, while the changes in inhibitory GABAergic synapses are less pronounced. Meanwhile in layer 4, there is a significant increase in the density of small VGluT1 synapses, with no changes in the astrocytic association of synapses. The ability to dissect the circuit deficits by synapse type and astrocytic involvement, will be crucial for understanding how these changes affect circuit function, and ultimately define targets for therapeutic intervention. AnimalsFMR1 KO mice were obtained from Dr. Stephen T. Warren, Emory University. Thy1-YFP-H mice were purchased from JAX. All mice were backcrossed with C57BL/6 mice more than 10 generations to produce congenic strains. For the current experiments, YFP+ WT males were crossed with YFP-FMR1+/-females, and only male offspring littermates were used for the experiments. WT mice refer to FMR1+/y, and KO mice are FMR1-/y. Because the YFP expression was highly variable between animals, we did not use it in the analysis. The mice were four months old when they were sacrificed. Further details about the mice are in Supplemental Table S1. Array tomographyThe tissue was prepared using standard array tomography protocols [42]. Mice were group-housed in the UCSC animal facility, with 12 hour light-dark cycles and access to food and water ad libitum. All procedures were performed in accordance with 2/28 4/28 5/28

Astrocytes in rare neurological conditions: Morphological and functional considerations

“…This protein is a translational repressor that is expressed in both neurons and astrocytes and is vital for normal synaptic transmission between neurons. Many studies on FXS have been conducted to look at the functional role of astrocytes in various processes including synaptic development, synaptic transmission, and dendritic spine morphology (Ariza et al, 2017; Cheng et al, 2012; Simhal et al, 2019).…”

“…Simhal et al (2019) recently conducted a study in a mice model of FXS to identify the role of astrocytes in synaptic composition. Via immunofluorescent array tomography, they found that in FMRP knockout (KO) mice, there was a significant reduction in the density and amount of excitatory glutamatergic synapses that were associated with the processes of an astrocyte, suggesting that astrocytes have a diminished functional role in the modulation of excitatory synaptic transmission in FXS, consistent with the findings of other similar studies (Jawaid et al, 2018; Simhal et al, 2019).…”

“…Simhal et al (2019) recently conducted a study in a mice model of FXS to identify the role of astrocytes in synaptic composition. Via immunofluorescent array tomography, they found that in FMRP knockout (KO) mice, there was a significant reduction in the density and amount of excitatory glutamatergic synapses that were associated with the processes of an astrocyte, suggesting that astrocytes have a diminished functional role in the modulation of excitatory synaptic transmission in FXS, consistent with the findings of other similar studies (Jawaid et al, 2018; Simhal et al, 2019). In addition to changes in excitatory synapse density, astrocytes have been shown in FMRP KO FXS models to have aberrant secretion of various proteins and gliotransmitters, including interleukin‐6 (IL‐6), hevin, SPARC proteins, GABA, glutamate, and thrombospondin‐1 which have all been shown to regulate excitatory synapse development and transmission (Cheng et al, 2016; Krasovska & Doering, 2018; Risher et al, 2014; S. K. Singh et al, 2016; Wallingford et al, 2017; Wang et al, 2016).…”