Stephan Kindler, PhD, Co-Principal Investigator (2010)
Hans-Jurgen Kreienkamp, PhD, Co-Principal Investigator (2010-2011)

FRAXA Awards:

by Hans-Jürgen Kreienkamp (picture left) and Stefan Kindler, 6/8/2011

Postsynaptic signaling events are believed to be the starting point of long-term changes in synaptic transmission, which accompany all forms of learning and memory. In the fragile X syndrome, the lack of FMRP leads to excessive synthesis of proteins in neuronal dendrites, affecting postsynaptic protein composition and ultimately synaptic function. Comparing the postsynaptic protein structure of wildtype and FMRP deficient mice, we identified a set of molecules, which are enriched in postsynaptic regions after loss of FMRP.

One protein that is significantly increased in abundance is Shank1, which is a rather large scaffold protein connecting various types of glutamate receptors, signaling proteins and cytoskeletal filaments. Biochemical experiments indicate that FMRP physically associates with Shank1 mRNAs, is co-transported with these transcripts into dendrites and represses their translation. Increased levels of Shank1 are likely to be relevant for the pathogenesis of the fragile X syndrome as Shank1 contributes to the formation of dendritic spines during neuronal development and stabilizes synapses on nascent dendritic spines. Thus, an overabundance of Shank1 could prevent the physiological elimination of synapses, which is compromised in fragile X patients and FMRP deficient mice.

Therefore, our hypothesis is that FMRP regulates Shank1 synthesis close to postsynaptic sites, and that loss of this regulatory role in fragile X patients interferes with the pruning of synapses, a developmental process that is essential for proper wiring of the nervous system.

To put this theory to the test, we have eliminated one (of two) Shank1 allels in FMRP deficient mice, thereby reducing Shank1 mRNA and protein levels. By analyzing these mice with respect to dendrite morphology, synaptic function and behavior, we hope to establish whether Shank1 is a promising target for a potential treatment of the FXS.

by Hans-Jürgen Kreienkamp and Stefan Kindler, 4/1/2010

One major assumption for the pathogenesis of the fragile X syndrome is that due to the lack of FMRP, excessive proteins synthesis occurs in neuronal dendrites, leading to disproportionate levels of postsynaptic proteins. We have started to compare the protein composition of the postsynaptic density (PSD) isolated from wildtype and FMRP deficient mice.

The PSD is a large protein complex containing glutamate receptors and their associated intracellular proteins. Signaling events occurring in the PSD are believed to be the starting point of long-term structural changes, which accompany all forms of learning and memory. We observed that the concentrations of several glutamate receptor subunits, as well as a select set of postsynaptic scaffold proteins are increased in PSDs derived from FMRP deficient mice.

One protein that is significantly increased in abundance is Shank1, which is a rather large scaffold protein connecting various types of glutamate receptors, signaling proteins and cytoskeletal filaments. Increased levels of Shank1 are likely to be relevant for the pathogenesis of the fragile X syndrome as Shank1 contributes to the formation of dendritic spines during neuronal development and stabilizes synapses on nascent dendritic spines. Thus, an overabundance of Shank1 could prevent the physiological elimination of synapses, which is compromised in fragile X patients and FMRP deficient mice. Interestingly, the Shank1 mRNA is one of the few neuronal transcripts, which are highly abundant in dendrites, making it likely that FMRP, which is also present in dendrites, represses the translation of the Shank1 mRNA close to postsynaptic sites. Biochemical experiments demonstrate that FMRP physically associates with Shank1 mRNAs and that FMRP is actually present in dendritic mRNA transport particles.

Thus, our current hypothesis is that FMRP regulates Shank1 synthesis close to postsynaptic sites, and that loss of this regulatory role in fragile X patients interferes with the pruning of synapses, which is essential for proper functioning of the nervous system.