Astrocyte-Secreted Factors in Fragile X Mice

Abstract

Astrocytes are required for the proper development of synapses within the brain and astrocyte dysfunction has been indicated in many neurodevelopmental disorders, including Fragile X Syndrome (FXS). FXS is characterized by a deficiency in fragile X mental retardation protein (FMRP). FMRP regulates the translation of numerous mRNAs and its loss disturbs the composition of proteins for dendritic spine and synapse development. Here we investigate whether particular astrocyte-derived factors that have been implicated in governing excitatory synapse development in the brain show altered expression within the context of FXS. Using Fmr1 knock-out (KO) mice lacking FMRP, we analyzed the early postnatal protein expression of the astrocyte-secreted factors hevin, SPARC, and glypicans 4/6 in the cortex and hippocampus (regions in which dendritic spine abnormalities have been associated with FXS) via Western blotting. Hevin and SPARC showed altered expression patterns in Fmr1 KO mice, compared to WT, in a brain-region-specific manner. In the cortex, we found a transient increase in the level of hevin in P14 Fmr1 KO mice, compared to WT, as well as decreases in Fmr1 KO levels of SPARC at P7 and P14. In the hippocampus, hevin expression was lower in P7 Fmr1 KO mice than in WT mice, and then surpassed WT levels by P21. Hevin is involved in the proper establishment of thalamocortical synapses, and so, we also examined the number of thalamocortical synapses formed in co-cultures of WT thalamic and cortical neurons with either WT or KO astrocytes. Preliminary data from these experiments show an increase in the number of thalamocortical synapses formed when WT neurons were cultured with KO astrocytes, suggesting the involvement of astrocytes in the proper development of thalamocortical connectivity. Together, these findings indicate aberrant astrocyte-signalling in FXS and suggest that altered expression of hevin and SPARC may contribute to abnormal neurobiology in FXS.