CHARACTERIZATION OF SPIN90 IN THE HIPPOCAMPUS AND IMPLICATIONS FOR ALZHEIMER'S DISEASE

Excitatory synapses are regulated through diverse protein complexes assembled at the postsynaptic compartment of dendritic spines. Upon NMDA receptor stimulation, such proteins induce dynamic remodeling of actin-rich postsynaptic density, causing alteration in spine structure and function. This anatomical plasticity is crucial for information storage and processing in the brain. SPIN90, an actin-binding protein, was found to modulate dendritic spine morphology depending on its phosphorylation state. Sequestration of ADF/cofilin, an actin depolymerizing factor, by phosphorylated SPIN90 suppressed activity-dependent spine shrinkage. To further characterize SPIN90 in the brain, SPIN90 knockout mice were generated and changes in synaptic transmission, plasticity and memory consolidation were examined. Brain slice recording. Coronal brain sections (300um) of wildtype (wt) and SPIN90 knockout mice brain were incubated in aCSF and subjected to extracellular whole-cell patch clamp recordings. HFS-LTP and LFS-LTD were induced on the SC-CA1 circuit of the hippocampus. Immunohistochemistry. 4-week old wt and SPIN90 knockout mice brain were fixed in 10% neutral formaldehyde solution then embedded in paraffin. Sagittal sections of 5 um were made, followed by H&E staining. Contextual Fear Conditioning. 4-week old wt and SPIN90 knockout mice were subjected to an intial handling/habituation stage followed by contextual training with electrical foot shock (US) of 0.8mA for 2sec. Then mice were stimulated with context (CS) and later put through retention test for 3min. SPIN90 KO mice showed reduced neuronal contacts at the CA1 region of hippocampus, impaired NMDAR-LTD but normal NMDAR-LTP compared with wt controls. SPIN90 KO mice showed dysfunction in contextual fear memory and learning. SPIN90 KO hippocampal neurons showed enhanced expression of phosphorylated GSK3beta, Akt, and acetylated tubulin but decreased phosphorylated HDAC6 (active) levels in normal conditions. Reduced spinal contacts due to absence of SPIN90 show its importance in the maintenance of spine structure. In physiological conditions, SPIN90 might have a role in initiating or maintaining LTD (upregulated in memory related neurological disorders). Consequently, SPIN90 KO mice showed reduced memory consolidation. Akt-GSK3beta pathway and acetylation on alpha-tubulin are enhanced in SPIN90 KO hippocampal neurons. This implies possible role of SPIN90 in fine-tuning of memory although a comprehensive study is needed.