Shank protein

Chromosome 22q13 Deletion Syndrome

     Chromosome 22q13 deletion syndrome (22q13DS) is a neurobehavioral syndrome marked by global developmental delay, and autism spectrum disorder (ASD) features.^[3] The Shank-3 gene is located within this region of chromosome 22. Studies have revealed that point mutations in Shank-3 can cause the neurodevelopmental symptoms associated with 22q13DS, accounting for 1% of all autism cases.^[5] At the molecular level, disruption of the full length Shank-3 protein reduces AMPA receptor signaling and spine remodeling.^[4]Mice who were haploinsufficient for Shank-3, emitted fewer ultrasonic vocalizations during interactions with estrus female mice, a behavior reminiscent of that seen in Autism patients. Further, Shank knockout mice have less dendritic spine development, a diminished PSD size, decreased levels of proteins GKAP and Homer, and greatly impaired synaptic signaling. Interestingly, overexpression of Shank-3 may also result in an ASD, supporting the hypothesis that Autism is caused by improper Excitatory/Inhibitory neuronal ratios in the brain.^[4] Measurements of broad miRNA expression levels in Autism patients uncovered aberrant levels of miRNAs for genes involved in ASDs like MeCP2, the cause of Rett Syndrome, NRXN-1, a gene implicated in ASDs, and Shank-3, adding support to Shank-3’s role in autism.^[6] Due to the marked reduction in AMPA receptor signalling in Shank-3 mutants, compounds that enhance AMPA transmission (AMPAkinses) serve as potential therapeutic approaches to treating some ASDs.^[4]

βPIX Structure

     βPIX is a protein belonging to a group of guanine nucleotide exchange factors used by Rho GTPase family members, like Rac1 and Cdc42. Rac1 and Cdc42 regulate the actin cytoskeleton of synapses.^[7] PIX has an N-terminal Src homology 3 (SH3) domain which associates with PAK, a coiled-coil (CC) domain, which is critical for multimerization, and a C-terminal PDZ binding domain which interacts with the PDZ domain of Shank.^[7] The interaction of Shank with βPIX promotes the synaptic localization of βPIX and βPIX associated p21 Associated Kinase (PAK). Since PAK regulates actin cytoskeletons, and dendritic spines are actin-rich structures, it is believed that Shank recruits βPIX to dendritic spines to regulate the PSD.^[1]

Shank Family Protein Structure

     The contains 90 amino acids and folds into a compact consisting of a six-stranded β-sandwich flanked by two alpha helices.^[7] βPIX possess a via within its CC domain, a , and a at the C-terminus. Interestingly, only 1 Shank molecule is bound to the CC domain trimer of βPIX in an . The of βPIX forms a number of with the Shank PDZ domain. Shank-3-Arg 679 forms the with βPIX, tightly H-Bonding Glutamate 643, forming 2 weak bonds with Phe 696, and Van der Waals interactions with ring of Phe 696. Abolishing this interaction through mutagenesis completely eliminates the assembly. Upon binding of βPIX, the PDZ domain undergoes a significant . Lys 682 undergoes a nearly to make room for the βPIX PDZ binding domain.^[7]

Shank Oligomerization

     Shank proteins are positioned between scaffolding proteins that are bound to either neurotransmitter receptors or the actin cytoskeleton. This puts Shank proteins in a perfect position to create the underlying structure of the PSD.^[2] The of Shank-3 can () to form large sheets composed of helical fibers stacked side by side. The proposed sheet structure with radially projecting protein interaction domains, is ideal architecture for a protein that must contact both membrane and cytoplasmic components at a synaptic surface.^[2] It resembles the structure of a peg board, with Shank oligomers forming the board and PIX proteins forming the pegs to which things attach. Models of this sort validate the importance of Shank-3 as master scaffolding proteins and illustrate how slight mutations can disrupt an entire PSD and synaptic function.