Immobilized Pool of NCAM180 in the Postsynaptic Membrane Is Homeostatically Replenished by the Flux of NCAM180 from Extrasynaptic Regions

Abstract

Homeostatic mechanisms maintaining high levels of adhesion molecules in synapses over prolonged periods of time remain incompletely understood. We used fluorescence recovery after photobleaching experiments to analyze the steady state turnover of the immobile pool of green fluorescent protein-labeled NCAM180, the largest postsynaptically accumulating isoform of the neural cell adhesion molecule (NCAM). We show that there is a continuous flux of NCAM180 to the postsynaptic membrane from nonsynaptic regions of dendrites by diffusion. In the postsynaptic membrane, the newly delivered NCAM180 slowly intermixes with the immobilized pool of NCAM180. Preferential immobilization and accumulation of NCAM180 in the postsynaptic membrane is reduced after disruption of the association of NCAM180 with the spectrin cytoskeleton and in the absence of the homophilic interactions of NCAM180 in synapses. Our observations indicate that the homophilic interactions and binding to the cytoskeleton promote immobilization of NCAM180 and its accumulation in the postsynaptic membrane. Flux of NCAM180 from extrasynaptic regions and its slow intermixture with the immobile pool of NCAM180 in the postsynaptic membrane may be important for the continuous homeostatic replenishment of NCAM180 protein at synaptic contacts without compromising the long term synaptic contact stability.Neurons are assembled into circuits via specialized contacts, called synapses, that are essential for information processing, learning, and storage in the brain. Synapses are stable long lasting structures (1-3). This stability is at least partially rendered by a number of cell adhesion molecules accumulating in the pre-and postsynaptic membranes (4 -9). During development, cell adhesion molecules are delivered to nascent synapses in intracellular organelles or as cell surface aggregates (10 -12). A similar type of quantal delivery of adhesion molecules has been also observed during induction of long term potentiation (13). Long lasting stability of synapses (months or years), however, suggests that synaptic pools of cell adhesion molecules (protein life time hours or days) have to be also homeostatically and continuously replenished over prolonged periods of time without compromising synapse stability and composition. Mechanisms of such replenishment remain poorly understood.Among synaptic adhesion molecules, the neural cell adhesion molecule (NCAM), 2 a member of the immunoglobulin superfamily of adhesion molecules, was shown to be involved in mechanical stabilization of interneuronal contacts and their transformation into synapses (12,14). In mice, deficiency in NCAM results in impaired long term potentiation, increased aggression and anxiety, deficits in spatial learning, and exploratory behavior (15-21). In humans, genetic variations in the NCAM gene have been reported to confer risk factors associated with bipolar affective disorders and schizophrenia (22)(23)(24). The largest isoform of NCAM, NCAM180, accumulates in the postsynaptic membrane and recruits the s...

Epidermal control of axonal attachment via β-spectrin and the GTPase-activating protein TBC-10 prevents axonal degeneration

“…There are multiple examples of β-spectrin interacting with integral membrane proteins and cell adhesion molecules 35 . β-spectrin directly interacts with the neural cell adhesion molecule NCAM 36,37 , as well as with CHL1, a close homolog of the immunoglobulin superfamily cell adhesion molecule L1 38 , and is required independently of α-spectrin for the polarized localization of the Na + /K + ATPase in the epithelia of the Drosophila midgut 39 . On its own, disruption to unc-70/β-spectrin causes only a mild axonal breakage phenotype; however, in combination with mutations in the Rab-GAP tbc-10, and subsequent increases in RAB-35 activity, it causes loss of hemidesmosomes, detachment of the axon from the surrounding epidermis, and axonal degeneration.…”

Dynein Interacts with the Neural Cell Adhesion Molecule (NCAM180) to Tether Dynamic Microtubules and Maintain Synaptic Density in Cortical Neurons

“…Polo-Parada et al (71) have previously described a conserved C-terminal motif in NCAM that may signal via myosin light chain kinase to reg-ulate myosin-driven synaptic vesicle trafficking at the presynaptic terminal. At the postsynaptic terminal, an association with a spectrin-based scaffold mediates the accumulation of NCAM at synaptic contacts (23,72). Dynein has also been shown to interact indirectly with the spectrin cytoskeleton via dynactin (35) and ankyrin (73), so multiple interactions probably contribute to dynamically link the cytoskeleton to the cell cortex at active trafficking sites, such as synapses.…”

Reciprocal Interactions between Cell Adhesion Molecules of the Immunoglobulin Superfamily and the Cytoskeleton in Neurons

“…In mammalian cells, interactions with the βI spectrin meshwork reduce the lateral diffusion in the cell surface plasma membrane of NCAM, and particularly its largest isoform NCAM180 with the longest intracellular domain (Pollerberg et al, 1986 ). This reduction in the mobility is required for the accumulation of NCAM180 at the post-synaptic sites of excitatory synapses in cultured mouse hippocampal neurons (Leshchyns'ka et al, 2011 ). Similarly, depolymerization of the actin cytoskeleton using latrunculin A induces dispersion of Nectin-1, an IgSF cell adhesion molecule which interacts with actin via afadin (Takahashi et al, 1999 ), from synapses of developing neurons accompanied by the shedding of Nectin-1 from the cell surface (Lim et al, 2008 ).…”