The state of synapses in fragile X syndrome

Abstract

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading genetic cause of autism. There is increasing evidence in both FXS and other forms of autism that alterations in synapse number, structure, and function are associated and contribute to these prevalent diseases. FXS is caused by loss of function of the Fmr1 gene, which encodes the RNA binding protein, fragile X mental retardation protein (FMRP). Therefore, FXS is a tractable model to understand synaptic dysfunction in cognitive disorders. FMRP is present at synapses where it associates with mRNA and polyribosomes. Accumulating evidence finds roles for FMRP in synapse development, elimination, and plasticity. Here, the authors review the synaptic changes observed in FXS and try to relate these changes to what is known about the molecular function of FMRP. Recent advances in the understanding of the molecular and synaptic function of FMRP, as well as the consequences of its loss, have led to the development of novel therapeutic strategies for FXS.

Figure 1. FMRP regulation of mRNA transport and local translation impacts synaptic structure and function

FMRP is shuttles to and from the nucleus where it may play a role in nuclear export of mRNAs. FMRP is found both in growth cones, immature axons and mature dendrites, as well as dendritic spines. In these compartments, FMRP is associated with mRNPs and larger RNA granule structures which also contain FMRP-interacting proteins such as FXRs and CYFIP. RNA granules and FMRP travel into dendrites via kinesin motors on microtubules. During transport, it is thought that FMRP functions to translationally suppress cargo mRNAs. Inset: Once in the spine FMRP phosphorylation and ubiquitination are regulated by mGluR activity which is thought to play a role is activation of translation initiation and elongation. Proteins whose translation is regulated by FMRP include Arc and MAP1b, all of which are known to regulate AMPA receptor endocytosis and thereby synaptic function.

Figure 2. Evidence for a direct, postsynaptic role of FMRP in synapse elimination

A Cultured Fmr1 KO hippocampal neurons (equivalent postnatal day 12–16) have more structural synapses. Top Left, Mixed dissociated culture prepared from both wild-type and Fmr1-KO mice labeled with an antibody against FMRP (green fluorescence) and the synaptic marker GluR1 (red fluorescence) (scale 10 μm). Bottom Left, High resolution image of highlighted sections in top left panel (scale = 5 μm). Right, Quantification of the number of synaptic puncta on Fmr1-KO neurons and neighboring WT neurons for three synaptic markers: surface GluR1, PSD-95, and synapsin. B. Acute postsynaptic expression of FMRP results in fewer functional synapses. Top, Depiction of experimental paradigm in which organotypic hippocampal slice cultures are prepared from Fmr1-KO mice and biolistically transfected with GFP-tagged FMRP. Simultaneous whole cell recordings are then obtained from untransfected Fmr1 KO (FMRP lacking) and FMRP-expressing neurons to quantify electrophysiological changes. Middle, Representative traces for evoked NMDA receptor-mediated synaptic responses (upper left, scale 20 pA, 20 ms), AMPA receptor-mediated synaptic responses (lower left, scale 20 pA, 20 ms), and miniature EPSCs (right, scale 10 pA, 500 ms). Traces from FMRP-expressing (transfected) neurons are shown in grey and those from FMRP-lacking (untransfected) neurons are shown in black. Bottom, Quantification of synaptic function following FMRP expression in Fmr1-KO neurons for evoked AMPA receptor-mediated responses, evoked NMDAR-mediated responses, miniature EPSC frequency and amplitude, and synaptic failure rate. *p-value < 0.05, **p-value < 0.01, ***p-value < 0.001. Modified with permission from (Pfeiffer and Huber, 2007).

Figure 3. Alterations in synaptic structure, function and plasticity in sensory neocortex of Fmr1 KO mice

Schematic summarizing major synaptic alterations in sensory neocortex: Size of arrows indicate the magnitude and direction of change. 1) A reduction in the synaptic connectivity of layer (L) 4 excitatory (Ex) neurons with neighboring L4 excitatory neurons, fast spiking (FS) inhibitory interneurons and L2/3 pyramidal neurons. 2) Increase in dendritic spine density and length of layer 2/3 and 5 neurons, although to date no functional change in synapse number has been demonstrated. 3) A reduced threshold for LTP in L2/3 and 5 synapses. 4) Increased L4 axonal length and spread into L2/3. In addition to synaptic changes, increases in intrinsic excitability are observed in L4 excitatory neurons. Overall, there is a circuit hyperexcitability in response to thalamic stimulation in sensory neocortex which may be mediated by the reduction in excitatory drive onto L4 FS inhibitory neurons, the increase in intrinsic excitability of L4 excitatory neurons and the increase in spine density on pyramidal neurons in L2/3 and 5.