1
For personal use. Only reproduce with permission The Lancet Publishing Group. THE LANCET Neurology Vol 1 December 2002 http://neurology.thelancet.com 467 Newsdesk Endocytosis in postsynaptic neuronal membranes occurs at specialised “hot spots”, according to a study published in October. “The process of endocytosis does not occur uniformly over the membrane as was previously thought: there are discrete points at which neurons take in receptors and other molecules from their surface membranes”, says author Michael Ehlers (Duke University, NC, USA). Ehlers and colleagues investigated the localisation and dynamics of clathrin coats in neuronal dendrites and spines, in particular the postsynaptic region of the neuron. They used high- resolution fluorescent imaging of hippocampal neurons expressing a fusion of clathrin light chain and GFP to show that clathrin formation, clathrin coat dispersal, and cargo transit into the cell are rapid, highly localised events. Intake of receptors tended to occur at specialised sites in the dendritic plasma membrane, notably in discrete areas at the tips of filopodia and in the lateral domains of mature spines. “These sites are stable and lie adjacent to the postsynaptic density—areas of the membrane that contain clusters of neurotransmiter receptors—and persist independently of synaptic activity”, explains Ehlers. As a consequence, he says, synaptic receptors and other membrane proteins must be translocated to endocytic sites before internalisation, which spatially and temporally separates their functional inactivation from endocytosis (Neuron 2002; 36: 434–49). The researchers looked for specific endocytic zones because previous studies indicated that synaptic strength could be controlled by removal or insertion of postsynaptic receptors from the membrane. “As well as telling us more about the biology of the neuron, the discovery of “doorways” into the postsynaptic membrane raises the possibility that drugs affecting receptor transport to and through the zones could prove useful in treating addiction, depression, stroke, epilepsy, and other neurological disorders that involve abnormal transport of receptors into the neuron”, suggests Ehlers. Moreover, Ehlers and colleagues found that these endocytic zones tended to change in character as neurons aged. “Initially, we found a lot of dynamic behaviour of the hot spots in young neurons as they were growing and forming their synapses”, said Ehlers. “But, still mysterious is that, as the neurons in culture mature and age, these hot spots seem to stabilise and specialise. They become much more well-defined in location and not to appear and disappear as often as they do in young cells”, he said. Ehlers and colleagues believe that this change with maturation provides important clues about how nerve cells differentiate and specialise during brain development. Kathryn Senior Discrete cellular entry points discovered in neuronal membrane Misfolded prion protein (PrP) in the cytosol is toxic and causes neurodegenerative disease; further- more, it can convert into the self- perpetuating PrP Sc form associated with transmissible prion diseases (Science 2002; published online October 17; DOI: 10.1126/science 1073719 & 10.1126/science 1073619). Normal PrP is transported to the membrane via the endoplasmic reticulum, during which time it adopts its final conformation. However, if misfolding occurs—a common event but perhaps more frequent if the PrP gene is mutated—the protein is moved back to the cytosol to be rapidly broken down by proteosomes. By inhibition of this proteosomal action, Jiyan Ma (Ohio State University, USA) and colleagues provoked PrP build-up in the cytosol of mouse N2A cells, as well as those transfected with a plasmid to increase PrP production. They soon discovered that even small amounts of cytosolic PrP rapidly killed the cells. “This is an entirely new view of the toxic species”, said Ma’s co-worker Lindquist (MIT, USA). “We think this toxicity wasn’t realised before simply because such small amounts of cytosolic PrP are required for cell death; other forms of the protein are present at much higher concentrations.” The researchers then produced transgenic mice that express a cytosolic form of PrP. As expected, they developed severe neurodegeneration. The team also showed that some accumlated PrP in the cytosol was spontaneously converted into a form similar to PrP Sc , a pivotal molecule in transmissible prion diseases. Further- more, new PrP was altered to assume the PrP Sc form, a hallmark of such diseases. “However, the misfolded PrP that accumulates is very toxic. Surprisingly, neurons can die even before PrP Sc clumps form”, says Ma. Prion diseases, such as kuru, scrapie, and Creutzfeldt-Jakob disease, share common features: all are neurodegenerative, all affect the CNS, and all are fatal. But they also differ: some are sporadic, others inheritable, and still others transmissible. The supposedly distinct mechanisms that cause them have remained a mystery, but these findings suggest that a common framework underlies them all: the accumulation of misfolded PrP species in the cytosol. Juan Badiola, Director of Spain’s National Spongiform Encephalopathy Laboratory, commented “this work has implications for proteosome inhibitor use both in research and in the clinic— especially if they cross the blood-brain barrier. But importantly, it suggests potential therapies could be based on stopping cytosolic PrP accumulation.” Adrian Burton Paused proteosomes add piece to prion puzzle Transgenic mice produce PrP protein directly in the cytosol (left). At seven weeks, (right), the brains of the transgenic mice are filled with holes (white space) where neurons have died Jiyan Ma, Ohio State University

Paused proteosomes add piece to prion puzzle

Embed Size (px)

Citation preview

Page 1: Paused proteosomes add piece to prion puzzle

For personal use. Only reproduce with permission The Lancet Publishing Group.

THE LANCET Neurology Vol 1 December 2002 http://neurology.thelancet.com 467

Newsdesk

Endocytosis in postsynaptic neuronalmembranes occurs at specialised “hotspots”, according to a study publishedin October. “The process of endocytosisdoes not occur uniformly over themembrane as was previously thought:there are discrete points at whichneurons take in receptors and othermolecules from their surfacemembranes”, says author MichaelEhlers (Duke University, NC, USA).

Ehlers and colleagues investigatedthe localisation and dynamics ofclathrin coats in neuronal dendrites andspines, in particular the postsynapticregion of the neuron. They used high-resolution fluorescent imaging ofhippocampal neurons expressing afusion of clathrin light chain and GFPto show that clathrin formation,clathrin coat dispersal, and cargo transitinto the cell are rapid, highly localisedevents. Intake of receptors tended tooccur at specialised sites in the dendriticplasma membrane, notably in discrete

areas at the tips of filopodia and in thelateral domains of mature spines.“These sites are stable and lie adjacentto the postsynaptic density—areas ofthe membrane that contain clusters ofneurotransmiter receptors—and persistindependently of synaptic activity”,explains Ehlers. As a consequence, hesays, synaptic receptors and othermembrane proteins must betranslocated to endocytic sites beforeinternalisation, which spatially andtemporally separates their functionalinactivation from endocytosis (Neuron2002; 36: 434–49).

The researchers looked for specificendocytic zones because previousstudies indicated that synaptic strengthcould be controlled by removal orinsertion of postsynaptic receptorsfrom the membrane. “As well as tellingus more about the biology of theneuron, the discovery of “doorways”into the postsynaptic membrane raisesthe possibility that drugs affecting

receptor transport to and through thezones could prove useful in treatingaddiction, depression, stroke, epilepsy,and other neurological disorders thatinvolve abnormal transport of receptorsinto the neuron”, suggests Ehlers.

Moreover, Ehlers and colleaguesfound that these endocytic zonestended to change in character asneurons aged. “Initially, we found a lotof dynamic behaviour of the hot spotsin young neurons as they were growingand forming their synapses”, saidEhlers. “But, still mysterious is that, asthe neurons in culture mature and age,these hot spots seem to stabilise andspecialise. They become much morewell-defined in location and not toappear and disappear as often as theydo in young cells”, he said. Ehlers andcolleagues believe that this change withmaturation provides important cluesabout how nerve cells differentiate andspecialise during brain development. Kathryn Senior

Discrete cellular entry points discovered in neuronal membrane

Misfolded prion protein (PrP) in thecytosol is toxic and causesneurodegenerative disease; further-more, it can convert into the self-perpetuating PrPSc form associatedwith transmissible prion diseases(Science 2002; published onlineOctober 17; DOI: 10.1126/science1073719 & 10.1126/science 1073619).

Normal PrP is transported to themembrane via the endoplasmicreticulum, during which time it adoptsits final conformation. However, ifmisfolding occurs—a common eventbut perhaps more frequent if the PrPgene is mutated—the protein is movedback to the cytosol to be rapidly brokendown by proteosomes. By inhibition ofthis proteosomal action, Jiyan Ma(Ohio State University, USA) andcolleagues provoked PrP build-up inthe cytosol of mouse N2A cells, as wellas those transfected with a plasmid toincrease PrP production. They soondiscovered that even small amounts ofcytosolic PrP rapidly killed the cells.“This is an entirely new view of thetoxic species”, said Ma’s co-worker

Lindquist (MIT, USA). “We think thistoxicity wasn’t realised before simplybecause such small amounts ofcytosolic PrP are required for celldeath; other forms of the protein arepresent at much higherconcentrations.”

The researchers then producedtransgenic mice that express a cytosolicform of PrP. As expected, theydeveloped severe neurodegeneration.

The team also showed that someaccumlated PrP in the cytosol wasspontaneously converted into a formsimilar to PrPSc, a pivotal molecule intransmissible prion diseases. Further-more, new PrP was altered to assume

the PrPSc form, a hallmark of suchdiseases. “However, the misfolded PrPthat accumulates is very toxic.Surprisingly, neurons can die evenbefore PrPSc clumps form”, says Ma.

Prion diseases, such as kuru,scrapie, and Creutzfeldt-Jakob disease,share common features: all areneurodegenerative, all affect the CNS,and all are fatal. But they also differ:some are sporadic, others inheritable,and still others transmissible. Thesupposedly distinct mechanisms thatcause them have remained a mystery,but these findings suggest that acommon framework underlies themall: the accumulation of misfolded PrPspecies in the cytosol.

Juan Badiola, Director of Spain’sNational Spongiform EncephalopathyLaboratory, commented “this work hasimplications for proteosome inhibitoruse both in research and in the clinic—especially if they cross the blood-brainbarrier. But importantly, it suggestspotential therapies could be based onstopping cytosolic PrP accumulation.”Adrian Burton

Paused proteosomes add piece to prion puzzle

Transgenic mice produce PrP protein directlyin the cytosol (left). At seven weeks, (right), thebrains of the transgenic mice are filled withholes (white space) where neurons have died

Jiya

n M

a, O

hio

Sta

te U

nive

rsity