A Developmental Switch Exists in the Effects of FMRP

$90,000 FRAXA Research Grant for 2010-2011

With a $90,00 grant from FRAXA Research Foundation, Dr. Kimberly Huber and her team at the University of Texas at Southwestern found that there is a developmental switch of postsynaptic FMRP on synaptic function. This switch is controlled by MEF2 transcriptional activity.

Results Published: Postsynaptic FMRP bidirectionally regulates excitatory synapses as a function of developmental age and MEF2 activity
Dr. Kimberly Huber

Kimberly Huber, PhD
Principal Investigator

Tong Zang, PhD
FRAXA Postdoctoral Fellow

University of Texas at Southwestern

by Tong Zang, PhD

Proper synapse maturation and elimination is crucial for the establishment of appropriate neural circuits that underlie sensory processing and cognition. Neuron of Fragile X patients as well as in the mouse model of Fragile X, Fmr1 KO mice, display more dendritic spines, the point of contact for excitatory synapses, as well as long and thin filopodia resembling immature spines. This suggests Fragile X mental retardation protein (FMRP) has a role in promoting synapse maturation and elimination. Altered regulation of these processes in Fragile X syndrome likely underlies many of the cognitive deficits associated with Fragile X syndrome.

There is considerable evidence that synaptic plasticity and structure are altered in Fmr1 KO mice, but the mechanisms by which this occurs are unknown. Previously we demonstrated that acute postsynaptic expression of FMRP in CA1 pyramidal neurons in slice cultures elicits synapse elimination providing a direct cell autonomous role for FMRP in synapse elimination (Pfeiffer et al., 2007). We have gained important mechanistic insight into FMRP induced synapse elimination. The activity-dependent transcription factor, myocyte enhancer factor 2 (MEF2) also causes synapse elimination in neurons. We find that FMRP is required for MEF2 to elicit synapse elimination. Importantly, FMRP functions downstream of MEF2 activation and active MEF2 is required for FMRP to eliminate synapses (Pfeiffer, Zang et al., Neuron, 2010).

Most recently, we have observed developmental differences in the effects of acute FMRP expression on synaptic function. The developmental difference will give us a hint of how the disease occurs and proceeds. We hypothesize that these differences are due to developmental regulation of candidate transcription factors. We anticipate that our results will gain a better understanding of how FMRP regulates synapse development, why synapse density, maturation and connectivity are altered in Fragile X syndrome and may lead to novel therapeutic strategies for the disease.

Our project has these aims:

Aim 1: Determine if there is a developmental switch in the effects of FMRP on dendritic spine structure and number.
Aim 2: Determine whether developmental regulation of MEF2 accounts for the switch of FMRP on synapse number.
Aim 3: Test MeCP2 as a transcription factor which regulates FMRP function in young neurons.

Results: The team finds that the developmental switch of postsynaptic FMRP on synaptic function is controlled by MEF2 transcriptional activity.

Dr. Kimberly Huber

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