Rho GTPases play key roles in neuronal development and in the formation and function of dendritic spines. Mental retardation is associated with deficits in individual Rho proteins, in the guanine nucleotide exchange factors (GEFs) that activate Rho proteins and in downstream targets of activated Rho proteins. Deficits in expression of Kalirin, a large, dual Rho GEF protein, are associated with increased iNOS levels in Alzheimer's disease brain and with decreased spine density in post-mortem prefrontal cortex from schizophrenic patients. In addition, Kalirin interacts with DISC1 (a candidate schizophrenia gene) and HAP1 (a Huntingtin interactor). Single nucleotide polymorphisms predict a significant number of individuals heterozygous for Kalirin function, with only one expressed copy or a normal and a mutated copy. These observations make a compelling case for analyzing mice in which expression of Kalirin can be manipulated both during development and in the adult. Mammals also express Trio, a highly homologous, but non- redundant gene. The single Kalirin/Trio gene in Drosophila and C. elegans plays an essential role within and outside of the nervous system. Based on our studies in cultured neurons, Kalirin plays a central role in axon initiation and outgrowth and in dendritic growth. Over-expression of the major adult splice variant of Kalirin, Kalirin-7, increases the formation of dendritic spines in pyramidal neurons and in normally aspiny interneurons. Reductions in the expression of Kalirin-7 and Kalirin-7 (an N-terminally truncated variant generated from a different promoter) result in deficits in spine formation and maintenance. We generated mouse models in which expression of Kalirin-7 and Kalirin-7 can be varied. Mice lacking the single exon unique to Kalirin-7/ Kalirin-7 (Kal7KO) are born at half the expected frequency, but survive to adulthood and reproduce. Mice heterozygous for this exon (Kal7+/KO) have diminished levels of Kalirin-7 and Kalirin-7 and show deficits in synaptic transmission. At the ultrastructural level, Kal7KO mice have a reduced number of normal excitatory synapses, plus many aberrant synaptic profiles not seen in normal mice. When tested in the elevated zero maze, Kal7+/KO and Kal7KO mice show a graded decrease in anxiety-like behavior. Mice in which the Kal7 exon is surrounded by lox-p sites (Kal7CKO) allow tissue-specific, developmentally regulated elimination of Kalirin-7/ Kalirin-7. These mice will be assessed using behavioral tests, morphological assessment of pre- and post-synaptic elements, electrophysiological recordings of slices and biochemical analysis of subcellular fractions. Spine formation in hippocampal neurons prepared from Kal7KO mice can be rescued by expressing exogenous Kalirin-7, allowing detailed analysis of the role of Kalirin-7 and the isolated Sec14p, spectrin-like, DH and PH domains. The role of Kal7 in spine formation in response to proteins such as Shank3, GluR2 and Neuroligin-1 will be assessed. The ability of the six known human Kalirin-7 mutants to rescue spine formation and synaptic function will be assessed using the Kal7KO mice.
Excitatory synapses onto dendritic spines account for much of the communication that goes on between neurons. The number and shape of dendritic spines respond to developmental cues, environmental stimuli and hormonal changes. Changes in spine morphology play key roles in learning and memory and it is clear that many signaling pathways affect spine formation and function. Kalirin-7, an activator of small GTP binding proteins of the Rho family, is localized to dendritic spines and is one of a small number of factors known to be capable of increasing the number of dendritic spines. We plan to use the Kalirin-7 knockout mouse that we generated to elucidate the pathway(s) controlling spine formation and structure.
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