E.V. Kiseleva
K.N. Morozova
M.W. Goldberg |
ALZHEIMER DISEASE, RETICULON NOGOA AND NUCLEAR ENVELOPE
ASSEMBLY IN GROWING OOCYTES |
Institute of Cytology and Genetics, Novosibirsk, Russia
Durham University, Durham, UK; e-mail elka@bionet.nsc.ru
|
Alzheimer’s disease is a progressive neurodegenerative disorder, characterized
by deposition of amyloid protein, formation of neurofibrillary tangles,
and neuronal death in brain regions. There is much interest in recent years
in the possible role of reticulons (Rtns) that are a family of evolutionary
conserved proteins in vertebrates which includes: Rtn1, Rtn2, Rtn3, and Rtn4a.
While the exact functions of Rtn1 to Rtn3 are unknown, mammalian Rtn4a/Nogo-A
was shown to inhibit the regeneration of severed axons in the mammalian central
nervous system. This inhibitory function is exerted via two distinct regions,
one within the Rtn4 specific N-terminus and the other in the conserved
reticulon homology domain. Rtn4a has been extensively studied with regards to
its neurite outgrowth inhibitory function, both in limiting plasticity in the
healthy adult brain and regeneration during central nervous system injury. Rtn4a
activities are presumably associated with Nogo splice isoforms expressed on
the cell surface. At the same time Rtn4a and other reticulon paralogues and
orthologues, are mainly localized to the endoplasmic reticulum (ER), and are
likely to have cell autonomous functions that are not yet clear. Emerging evidence
suggests that Rtn4a may have a role in modulating the morphology and functions
of the ER. Recently it was demonstrated that Rtn4a is a membrane protein that
shapes tubules of the ER (Voeltz et al., 2006). Because ER is attached to the
nuclear envelope (NE) during interphase and has a role in post mitotic/meiotic
NE reassembly, we speculated that Rtn4a could play a role in NE dynamics. Using
high resolution scanning electron microscopy together with immuno-electron microscopy
we found that Rtn4a is located at tubular-like junctions between fusing membrane
vesicles in the cytoplasm, and between cytoplasmic membranes and the outer nuclear
membrane in growing Xenopus oocyte nuclei. Previously it was shown that incubation
of egg extracts with antibodies against Rtn4a caused ER to form into large vesicles
instead of tubules (Voeltz et al., 2006). To test whether Rtn4a contributes
to the NE assembly, we added the same Rtn4a antibody to nuclear assembly reactions
in vitro. During NE assembly in Xenopus egg extracts, Rtn4a localises to the
region of high membrane curvature over the edge of membranes that are flattening
onto the chromatin. Chromatin was enclosed by membranes containing nuclear pore
complexes, but after that nuclei did not grow. Instead large sacs of ER membranes
attached to, but did not integrate into the NE. Both in vivo and in vitro experiments
have shown that Rtn4a is preferentially concentrated in the region of highest
curvature, supporting the model that this protein has a function related to
stabilization of membrane curvature. Additionally our findings indicate
for the first time that Rtn4a may have a role in the NE assembly in growing
cells (Kiseleva et al. 2007). New function of Rtn4a which is involved in development
of neurodegenerative disorders opens a new view on its participation in the
regulation of intracellular events and dynamics of cell membrane compartments.
The study was supported by grant no 07-04-00416-a of the Russian Fund for Basic
Research and the Program of the Russian Academy of Sciences: “ Molecular
and Cellular Biology”.
