An Isoform of Kalirin, a Brain-specific GDP/GTP Exchange Factor, Is Enriched in the Postsynaptic Density Fraction

Abstract

Communication between membranes and the actin cytoskeleton is an important aspect of neuronal function. Regulators of actin cytoskeletal dynamics include the Rho-like small GTP-binding proteins and their exchange factors. Kalirin is a brain-specific protein, first identified through its interaction with peptidylglycine-␣-amidating monooxygenase. In this study, we cloned rat Kalirin-7, a 7-kilobase mRNA form of Kalirin. Kalirin-7 contains nine spectrin-like repeats, a Dbl homology domain, and a pleckstrin homology domain. We found that the majority of Kalirin-7 protein is associated with synaptosomal membranes, but a fraction is cytosolic. We also detected higher molecular weight Kalirin proteins. In rat cerebral cortex, Kalirin-7 is highly enriched in the postsynaptic density fraction. In primary cultures of neurons, Kalirin-7 is detected in spine-like structures, while other forms of Kalirin are visualized in the cell soma and throughout the neurites. Kalirin-7 and its Dbl homology-pleckstrin homology domain induce formation of lamellipodia and membrane ruffling, when transiently expressed in fibroblasts, indicative of Rac1 activation. Using Rac1, the Dbl homology-pleckstrin homology domain catalyzed the in vitro exchange of bound GDP with GTP. Kalirin-7 is the first guanine-nucleotide exchange factor identified in the postsynaptic density, where it is positioned optimally to regulate signal transduction pathways connecting membrane proteins and the actin cytoskeleton.

Reversing Synapse Loss in Alzheimer's Disease: Rho-Guanosine Triphosphatases and Insights from Other Brain Disorders

“…Kalirin has since emerged as a major Rac1-GEF and as a key regulator of structural and functional plasticity at dendritic spines [64][65][66][67][68]. Several isoforms of the kalirin gene (KALRN), stemming from various promoters and transcriptional starts, have been discovered over the years, including the major isoforms kalirin-5 kalirin-7, kalirin-9, kalirin-12, and Duet [69][70][71].…”

“…Interestingly, in rodents, kalirin-7 protein levels are almost undetectable at birth, only increasing at postnatal day 14, which coincides with the onset of synaptogenesis [73,74]. Kalirin-7 plays a key role in the regulation of the size and density of dendritic spines, where it interacts with several PDZ domain-containing proteins [64][65][66][67][68].…”

“…Kalirin-7 knockout (KO) mice show a significant decrease in hippocampal dendritic spine density, deficits in LTP, and impaired cognitive function [64]. These synaptic deficits are thought to be due to abnormally attenuated signaling by Rac1, the main target of Kalirin-7 [65][66][67].…”

Plasticity in Hippocampal Peptidergic Systems Induced by Repeated Electroconvulsive Shock

“…Chronic ECS induces sprouting of mossy fibers in the hippocampus (Gombos et al 1999). Kalirin is known to alter the morphology of spine-like structures in cultured hippocampal neurons, to affect cytoskeletal organization and peptide secretion and to affect process outgrowth and branching in sympathetic neurons (Penzes et al 2000;Mains et al 1999;May et al, In press). The changes in NPY, PAM and kalirin expression that are unique to repeated ECS provide a target for further investigation into the mechanisms through which ECT ameliorates depression.…”

“…Rho proteins mediate cytoskeletal reorganization and play a central role in the specification of dendritic morphology (Threadgill et al 1997;Hall 1998). An isoform of kalirin with a PDZbinding motif is concentrated in the post-synaptic density fraction, and its over-expression altered spine morphology in mature hippocampal neurons (Penzes et al 2000(Penzes et al , 2001). Since we previously identified kalirin in several cell types in the hippocampus (Ma et al 2001), we monitored kalirin expression as a potential indicator of structural changes accompanying alterations in peptidergic systems.…”