16 August 2007. Dysfunction of the ubiquitin-proteasome system (UPS), by which cells dispose of defective proteins, is gaining attention as a mechanism leading to neurodegeneration. Suppression of the UPS led to Parkinson-like behaviors and neuropathology in rats (see ARF related news story). In Alzheimer disease (AD), postmortem analyses reveal impaired proteasome activity in AD brains (Keller et al., 2000).
The UPS has also been implicated in synaptic plasticity (see ARF related news story), but scientists have not known precisely how. Now Peter Kalivas and colleagues at the Medical University of South Carolina have demonstrated that a protein called NAC1 enables proteasome migration to dendritic spines.
NAC1 was originally described as an immediate early gene product in a 1997 report by Kalivas’s lab. The protein has been observed to move from the nucleus to the cytoplasm in response to cellular stimulation (Korutla et al., 2005). Now the Kalivas team reports in the August 15 Journal of Neuroscience that NAC1 is involved in shuttling proteasomes from the nucleus to dendrites following activation.
They first confirmed that NAC1 interacted with proteasomes. In a series of protein binding experiments, first author Haowei Shen and coauthors found that NAC1 binds to two proteasome complex proteins Cul3 and Mov34. Confocal microscopy revealed nuclear colocalization of NAC1 with each of these two proteins.
Because the researchers had previously found that NAC1 can move out of the nucleus in response to activity (Korutla et al., 2005), they wondered whether NAC1 could pull the proteasome complex along with it. They used various ways to elicit cellular activity in their cultured cortical neurons in order to coax NAC1 out of the nucleus. Reasoning that inhibition of proteasomes would cause compensatory translocation of nuclear proteasomes, the researchers treated cells with the proteasome inhibitor MG132 and observed NAC1 and proteasome protein 20S move from the nucleus to the cytoplasm. A similar migration pattern occurred following treatment with bicuculline, which induces synaptic activity. Further, by knocking out NAC1 and also by mutating its binding site, the researchers found that treatments of MG132 and bicuculline no longer budged proteasomes from the nucleus.
But is NAC1 targeting a particular destination when it leaves the nucleus? Upon observing that following bicuculline treatment NAC1 colocalized with the proteasome protein Cul3 near dendrites, the researchers suspected that NAC1 was heading toward sites of activity. Indeed, the researchers went on to demonstrate that NAC1 colocalized with the synaptic protein synapsin and the spine structural protein actin. Time-course experiments showed that it took NAC1 as long as 6 hours after bicuculline treatment to reach the dendrites and that levels of the translocated proteins remained elevated for 48 hours after treatment.
“NAC1 was not just moving proteasomes out of the cytoplasm, but NAC1 is involved in taking them all the way out to sites of synapses,” Kalivas said. “It’s the first protein that I and the reviewers are aware of that is involved in activity-mediated movement of the proteasome.”
What’s more, the authors write, “The fact that NAC1 was required for mobilizing the UPS into the cytoplasm in response to pharmacologically inhibiting the proteasome indicates that NAC1 could be an important cellular response to overproduction of these proteins associated with classic neurodegenerative diseases.”—Molly McElroy.
Molly McElroy is a freelance writer based in Melbourne, Florida.
Shen H, Korutla L, Champtiaux N, Toda S, LaLumiere R, Vallone J, Klugmann M, Blendy JA, Mackler SA, Kalivas PW. NAC1 regulates the recruitment of the proteasome complex into dendritic spines. Journal of Neuroscience. 2007 Aug 15;27:8903-8913. Abstract