Hippocampal demyelination and memory dysfunction are associated with increased levels of the neuronal microRNA miR-124 and reduced AMPA receptors.

OBJECTIVE: Hippocampal demyelination, a common feature of postmortem multiple sclerosis (MS) brains, reduces neuronal gene expression and is a likely contributor to the memory impairment that is found in >40% of individuals with MS. How demyelination alters neuronal gene expression is unknown. METHODS: To explore whether loss of hippocampal myelin alters expression of neuronal microRNAs (miRNAs), we compared miRNA profiles from myelinated and demyelinated hippocampi from postmortem MS brains and performed validation studies. RESULTS: A network-based interaction analysis depicts a correlation between increased neuronal miRNAs and decreased neuronal genes identified in our previous study. The neuronal miRNA miR-124 was increased in demyelinated MS hippocampi and targets mRNAs encoding 26 neuronal proteins that were decreased in demyelinated hippocampus, including the ionotrophic glutamate receptors AMPA2 and AMPA3. Hippocampal demyelination in mice also increased miR-124, reduced expression of AMPA receptors, and decreased memory performance in water maze tests. Remyelination of the mouse hippocampus reversed these changes. INTERPRETATION: We establish here that myelin alters neuronal gene expression and function by modulating the levels of the neuronal miRNA miR-124. Inhibition of miR-124 in hippocampal neurons may provide a therapeutic approach to improve memory performance in MS patients.

Balanced Shh signaling is required for proper formation and maintenance of dorsal telencephalic midline structures.

BACKGROUND: The rostral telencephalic dorsal midline is an organizing center critical for the formation of the future cortex and hippocampus. While the intersection of WNTs, BMPs, and FGFs establishes boundaries within this critical center, a direct role of Shh signaling in this region remains controversial. In this paper we show that both increased and decreased Shh signaling directly affects boundary formation within the telencephalic dorsal midline. RESULTS: Viral over-expression of Shh in the embryonic telencephalon prevents formation of the cortical hem and choroid plexus, while expanding the roof plate. In a transgenic model where cholesterol-lacking ShhN is expressed from one allele (ShhN/+), genes expressed in all three domains, cortical hem, choroid plexus and roof plate expand. In Gli1/2 -/- mutant brains, where Shh signaling is reduced, the roof plate expands, again at the expense of cortical hem and plexus. Cell autonomous activation of Shh signaling in the dorsal midline through Gdf7-driven activated Smoothened expression results in expansion of the Wnt3a-expressing cortical hem into the plexus domain. In addition, developmental stage determines dorsal midline responsiveness to Shh. CONCLUSIONS: Together, these data demonstrate that balanced Shh signaling is critical for maintaining regional boundaries within the dorsal midline telencephalic organizing center.