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一篇很好的综述！！！
) d3 q# k& X- }4 p' @
# N) q& i4 C% C# G; P1 pVertebrate Embryonic Cells Will Become Nerve Cells Unless Told Otherwise 7 t7 O- _( l( ?# n
Ali Hemmati-Brivanlou* and Douglas Melton† (Spemann and Mangold, 1924). In the absence of this & A& ]+ V/ ^3 p% U
influence, as on the ventral side, the ectoderm differenti- *Laboratory of Molecular Embryology & |% L: J3 z3 G: X2 Z
The Rockefeller University ates as epidermis. Thus, in the following decades, the
* v% R" P" y% n: @7 ]- ^- ~3 {2 W8 |development of epidermis was assumed to be a fall- 1230 York Avenue
. X" R8 m ` ]1 E1 W0 m/ R$ RNew York, New York 10021–6399 back, or “default” fate for gastrula ectoderm, requiring ( W0 E% _+ f* ^1 T6 _
no cell–cell communication, whereas neural specifica- †Department of Molecular and Cellular Biology
1 u3 U1 G, k/ h* ~Howard Hughes Medical Institute tion was thought to be contingent upon the receipt of
2 ^" J$ m2 u aa positive signal (in molecular terms, a signal that acti- Harvard University : p" g, Z6 _( z# f( M6 v6 x
7 Divinity Avenue vates a signal transduction pathway) from neighboring & @$ B, P4 ^6 a8 ]
cells. Cambridge, Massachusetts 02138 % @9 e: k7 L0 E% N$ h
A considerable effort over several decades failed to 5 m( X+ S: e& N! q( ?# { W
identify the gene products responsible for neural induction 5 [1 T$ w2 j6 ?* {8 ]- C) i
in the embryo. The idea of a positive signal involved The past few years have witnessed a significant change ( C0 h' w% h* p( }/ B
in neural induction so dominated thinking in the field in the understanding of how the vertebrate nervous systhat
5 }) Y9 [9 E4 Y6 w) C5 H P) Athe significance of results inconsistent with this tem forms during embryogenesis. More than seventy ; \! n: P9 P8 ~: H2 F, p! I8 Z
idea were not widely appreciated. For example, several years since Spemann and Mangold first demonstrated % }' W4 B9 z: h" j( h8 F
researchers found that when cells of ectodermal ex- the phenomenon of neural induction, the molecular
" `. L8 _( K# Tplants (also called “animal caps”), were dissociated, mechanisms underlying neural induction now appear to
" g4 F9 R' f+ g, p: Sthey can form neural tissue whereas intact or whole be at hand. Two independent approaches, one focusing
k5 F; q# q/ v' s/ uanimal caps form epidermis (Grunz and Tacke, 1989; on a “default” or “ground-state” model for neural induc- * d, t7 y3 L& N" L8 Y+ N
Godsave and Slack, 1991). In these experiments dorsal tion and the other culminating in the discovery of semesodermal & ?& G. x1 n9 G" \9 E) _
tissue (the organizer) is absent and neurali- creted neural inducing factors (noggin, follistatin, and . @: `, c0 B# w: c
zation occurs in a cell autonomous fashion, in contradic- chordin), have not only brought to a successful conclution ' r: `0 M4 d b4 m. x; }
with the requirement for a positive signal derived sion the search for Spemann’s neuralizing factor, but
. K5 f1 P4 m/ b' S) H9 F* dfrom the organizer. With hindsight these results using also illuminated its mechanism of action. Itnow appears
* _8 J1 s M" D+ xsimple cell dissociation provided a strong hint that neu- that neuralization of embryonic cells occurs when cells 3 j0 ?& M3 h2 R; {& |
ral inhibitory signal within the ectoderm, or the whole do not receive other inducing signals telling them to $ k6 ^% }: f5 k+ |* P/ R8 q
embryo, prevent neuralization, and in the case of animal form epidermis, mesoderm, or endoderm. This concept
6 {* Z8 ?4 v+ D2 \) b' m' xcaps, these neural inhibitors or antagonists drive cells of neuralization allows for a reinterpretation of the classitoward # C+ H" R# {. n6 ?. S$ ]5 h
an epidermal fate. cal views on both neural and epidermal specification.
6 m) F( N5 d+ b, |( r, rThe secreted growth factor BMP4 (Bone Morphogenetic
$ u7 X- X O5 aNeural, Not Epidermal, as the Default State Protein) plays a pivotal role wherein BMP signaling inof / o4 I! U# Y) a- D1 ~/ ?9 V
Embryonic Ectoderm duces epidermal differentiation. It is the absence of BMP
3 W# i5 k" f- [4 bIt is important to distinguish between direct and indirect signaling, accomplished by BMP antagonists including 9 ]; D- X8 O8 F6 R" }. F
effects in understanding assays for neural induction. noggin, follistatin, and chordin, that leads to the formation : O% n4 T6 A8 q; ~) H
of neural tissue.
/ v- u" \+ t' Q! M" Q; O1 BLessons from History
+ g9 J! d, |1 T% |+ ZThe concept of neural induction was established in 1924 # ~2 X' b) ?4 V9 I. h: p3 ]- ~+ y) r
by Spemann and Mangold’s grafting experiments using
& P) ?7 d) E n% c `/ [salamander gastrula (Spemann and Mangold, 1924). 3 g$ |. z6 ]/ p) d
During gastrulation, prospective ectodermal cells, located
9 T! b- ~4 y' z5 U! w2 h. H7 Aon top of the embryo (animal pole) make a choice ! F+ m9 t" q; H+ m$ z
between two fates: epidermal and neural. The prospective
% `) G, ~) ^& c2 z. eneural plate is defined by two boundaries: the epidermal 0 q0 j$ I6 s/ t" @4 ?. H4 ]& r
–neural boundary in the animal pole and the 8 Z" u: y: \2 e: t% l. f7 J) [) S+ @
neural–mesodermal boundary in the equatorial region ^0 Q* M$ u Y" {1 l @0 `% I' t
(Figure 1). The blastopore lip, where cells first invaginate ' ~+ j! I1 Q$ x' `* q
during gastrulation, marks the prospective dorsal side
1 k- Y4 v# [# @7 N: jwhere the neural plate forms. Transplantation of a dorsal 3 l; x: b0 k6 y$ p8 [
blastopore lip, which consists of mesoderm and endoderm, 2 V$ R* K: N9 d* x) p1 J5 K# D7 d8 m5 i
from an early salamander gastrula to the ventral ) g6 Y( L& o# `
side of another early gastrula causes formation of a
! Z @# y6 ^$ h5 psecond nervous system (Figure 2). The second nervous # z6 t4 l, x7 }! U8 L/ p
system develops not from the transplanted tissue, but
' K. B7 A2 o$ i& T" z$ d* t1 |from ventral ectoderm, which in an undisturbed embryo Figure 1. Fate Map of the Xenopus Gastrula
4 B2 X6 ~" N/ P2 U3 J- k; Sforms epidermis.Spemann namedthe dorsal blastopore
+ Z7 Q K# X7 O(Left) Lateral view.
3 q" W; o' E+ z+ x: b" Jlip the “organizer,”and proposed that innormal develop- (Right) Dorsal view. ( k h* T" _# L5 Y* \1 T
ment this region induces and organizes a correctly pat- The presumptive neural plate is delineated by presumptive epidermis
/ ~ Z; _% X9 D( y {9 Q) Qand mesoderm. Animal pole is at the top. terned nervous system in neighboring dorsal ectoderm ( a8 p4 |! v# a1 K
Cell 9 \" u# g6 C( A9 J1 j! `: ]
14
! e/ L; S" I$ h% cFigure 2. Classical Transplantation Experiment
9 w$ i% N2 K4 }- Nby Spemann and Mangold ' e; ]$ P1 W& a5 R0 B
(Top) The fate of a normal embryo.
: P; R5 x1 ~9 r(Bottom) The transplantation experiment. 7 A/ X. I. P) @8 y- Z& I
The transplantation of an organizer from a 9 n6 m3 B7 D4 ]3 r5 y
donor to the ventral side of a host embryo * G0 l# l2 K' ~" Z
induces a complete secondary axis and giving
- v# q5 D" \0 Wrise to an embryo with two main body
1 B$ t1 x/ S% [2 t' daxes. This experiment demonstrates that all
+ i8 J7 @$ Z; R* Sthe information necessary and sufficient to
; q! g* ^' N$ V/ n3 ~8 W' K; n5 \induce a dorsal axis, including the entire nervous
$ j" L4 v! }4 M. b! ^system, is contained within the cells of
% p$ E# q1 ?8 n, n/ X9 y4 W5 Qthe organizer (boxed here in green).
2 p4 n) N2 E. L3 x! g: E& JSeveral growth factors with mesoderm inducing activity, these results suggested that individual cells of the early 7 M" U) }7 D8 H+ z) A4 l5 }
gastrula animal cap are predisposed to form neural tis- including activins and Vg1, can cause formation of neural $ m5 _8 @2 d+ g2 `
tissue when added to ectodermal explants, but this sue in the absence of further signals. In this view, epidermal 0 G0 P: A$ N- Z' x3 G9 z
(but not neural) specification requires a positive cell neuralization is indirect because some of the treated
; {, Z' E( @' U& K8 ycells first form dorsal mesoderm. The mesodermal cells signaling within the prospective ectoderm. When this ( ^( ?7 ]+ i& o1 L7 R5 O
signaling is interrupted experimentally by cell dissocia- then mimic the action of the organizer and induce neural ( V. v, H& f! L. L A+ {
tissue in the surrounding cells. The first direct molecular tion or molecular antagonists, neural tissue forms. Neural : u7 L/ o- p w- w. P
induction by the organizer in vivo could work in the neuralizing treatment to be described was a truncated L. E8 ?) Q6 d+ q) L
type II activin receptor (D1XAR1, here referred to as same way, that is, by blocking epidermal induction $ @% w8 J& X4 I6 L, W% D
within the animal cap (Hemmati-Brivanlou and Melton, tAR), designed to test for a requirement of activin, a
4 D$ U# |' @$ x+ Fmember of the TGFb growth factor superfamily, in meso- 1994; Figure 3). This view contrasts with the commonly 3 t {1 M* A( a7 @3 d
held textbook model wherein neural induction requires derm induction (Hemmati-Brivanlou and Melton, 1992).
5 t; t# a5 \" S4 ^7 v! z0 N1 DAnalysis of the expression of several tissue specific a positive signal. The term “neuralization” more aptly
* y' l. R- [8 ]* Q* cdescribes the situation than “neural induction”; indeed, markers following injection of tAR yielded the surprising 0 I" t T1 h, S9 V8 z
observation that a general neural marker, neural cell it is the epidermis that is induced.
. D$ b" y+ w8 Z! }& o9 eadhesion molecule (NCAM), was turned on in ectodermal 7 {6 m, P: x" s' g' m6 p
explants following inhibition of activin type II recep- Predictions of the Default or Ground State
( ~% a0 K8 _3 ?- ]4 |- SModel for Neuralization tor signaling. These explants express the activins and " s6 v) b) _$ Q+ f7 E3 H
their receptors and, as mentioned above, would make The model for neural specification described above + o* N0 `, G) n: _) `
makes two important predictions. First, the signal from epidermis when cultured alone. In addition, it was found
) T- s) N) @1 U3 U% U, L& _that the dominant negative activin receptor could also the organizer is an antagonistic secreted signal that
. t* H9 m' G7 ?3 |$ Y9 q' qinhibits the activity of a neural inhibitor/epidermal in- neuralize cells located at the bottom of the embryo or ( N; H* Z, M# Q. l9 `
vegetal pole, cells normally fated to become endoderm. ducer. This antagonism occurs specifically in the dorsal 9 B# a- A, x; q7 E
ectoderm during gastrulation. Second, the hypothesis This result suggested that neuralization by inhibition of
$ q \: `! P; x9 ]the type II receptor signaling is not confined to cells of that epidermal rather than neural specification requires
% Y# \% J. B6 g0 ~. V( c" spositive cell signaling among ectodermal cells, predicts the ectoderm but can be generalized to other germ layers. 7 G: D: ?- i3 U$ e" J- I2 m0 J
In terms of the specificity of the effect, it was noted that epidermal fate can be induced in ectodermal cells.
' \( Y6 F) ]8 A2 i; {: XThese predictions have very recently received experi- that while tAR does not interfere with receptor tyrosine % z9 l0 d4 Y/ U n" C" r, }) y
kinase signaling (such as FGF), tAR could inhibit signal- mental support from several fronts. & k0 V! d1 ?; p. W9 y
ing of other TGFb factors. It is now appreciated that
9 b2 |. [1 f) G1 A1 GTGFb receptors are heterodimers and thus interfering BMP4 Inhibits Neuralization and Induces Epidermis
% m$ c) X$ B$ \% z* MThe neuralizing activity of the truncated activin receptor, with a particular pathwaymay affect signalingfrom other
* G9 L3 Q3 a& ^; rmembers of the family. Indeed, it was subsequently de- and the observation that an activin antagonist, follistatin,
, R% ^+ W* C! u* r# z$ f; w5 nhas direct neural inducing activity (see below) pointed to termined that tAR blocks more than just activin signaling, 8 y# J' h, u" ^' _$ j8 i
and appears to inhibit other TGFbs including Vg1 activin as an endogenous neural inhibitor. Nonetheless,
7 ?+ W: r+ D" o; x$ N" nthese data provided no direct evidence that activin could and BMPs (Schulte-Merker et al., 1994; Hemmati-Brivanlou
0 e: \ H$ i! @; u( ]and Thomsen, 1995). specify or induce epidermis. To address this prediction,
# a: f: Y( j7 j6 u# ]7 Q: oa complementation assay was used where cells of the The fact that tAR expression directly initiated nerve
6 o% R0 S+ ?0 jcell formation was significant because not only did it animal cap were dissociated and incubated in the presence
4 W& {- h! n+ F% C, l3 K8 hor absence of activin or BMP4, which are both occur in the complete absence of dorsal mesoderm, but
1 `8 R, j1 _' b6 qmore importantly, it demonstrated that neuralization can TGFb ligands inhibited by the truncated activin receptor. 6 {; C. e! m4 G" ]$ s7 J
While activin did inhibit neuralization of dissociated ec- occur by inhibition of signaling. Moreover, injection of . t6 q( V" {. i# C/ @9 ?& V# h; I
tAR showed that cells in any germ layer would become todermal cells by inducing mesoderm, activin did not , N- S" a1 _& H: P
induce expression of epidermal markers. In contrast, neural if TGFb signaling was blocked (Hemmati-Brivanlou
' P) k6 t' V% A6 gand Melton, 1994). Since both cell dissociation and BMP4 not only inhibited neuralization but induced epidermal 1 H. `7 a% Z6 H
fate. The two activities of BMP4, neural suppres- expression of a dominant negative activin receptor in
1 p- @! R# e; x5 ointact ectodermal explants can be interpreted as an sion and epidermal induction, always occur together, ( n: j" j5 E# }$ K% c4 i' Q
leading to the conclusion that they represent a single interference with the communication between cells, / g+ C6 n, q! h. y6 R0 d
Review
1 N Q( `5 h5 r15 8 Q. U5 F N3 l) `
Figure 3. Schematic of the “Default Model” : Q. Y8 ]6 K2 ^
of Vertebrate Neuralization
. h; V3 F+ }' `. I4 P' |(Top) In normal intact ectodermal explants
6 P0 I$ d4 K# E7 ~" e- [/ a(animal caps), BMP4 (the blue arrows) induces * P" |1 p4 y$ X8 }4 c( g( T
and maintains the epidermal fate. ; e* k8 Y+ A# L& e; \3 H
Upon dissociation this secreted neural inhibitor/
. }3 H4 M3 {0 n& \3 E! A( depidermal inducer is diluted and ineffective,
: p# i5 }; ?& B% Hand thus the neural fate in unveiled by 1 ^$ R$ ]) Y2 q3 U" H- H i5 H
derepression. ' H, P+ K0 q h( O7 g# D+ z$ j
(Middle) Expression of tAR or tBR interferes 1 D" t9 d3 v5 h. {8 O, z c/ N+ e( {
with the cells ability to receive the BMP4 signal.
1 F5 w/ {" \1 Z7 _The epidermal fate can no longer be / l V2 ~+ K) ^* C' h, J3 ?
maintained and the neural fate is unveiled. - ^! l: d( e/ h+ s% e$ y1 o6 s
(Bottom) In the embryo, the ectoderm has a
( Q; Q) f1 L$ @% V8 @dorsal-ventral polarity. Secreted factors,
7 m# H) T* s3 j0 S6 tsuch as noggin, chordin, and follistatin, interfere ' Y8 F- r9 n% Z3 T2 I0 l$ v
directly with the BMP4 signal (in the case 3 ~4 D8 R+ S0 U$ z+ m* k. x: S
of noggin and chordin, by direct binding to ! h0 D5 p5 X/ E
BMP4). The consequence of this interference ! H3 L2 Z6 D8 y# `8 t# ?* u5 P! E
is that BMP4 can no longer have access to $ }! X% @( O2 w& O
its receptor and therefore can no longer induce
% S( S. Y! R- f/ k$ ^# W( N4 B6 Nor maintain the epidermal fate and thus 7 s1 _2 O, h* [
neural tissue forms on the dorsal side.
$ K5 D2 v) V( caction, as expected from the neuralization model (Wil- Noggin, Follistatin, Chordin, and Others 1 }! z9 j) L S9 x4 s U+ W8 u
Isolation of the first endogenous direct neural inducing son and Hemmati-Brivanlou, 1995). Induction of epidermis
, f) V. ?9 B' e5 s7 lis inhibited if the dissociated cells express the trun- factors was reported shortly after the characterization 1 C& F/ d! M- o: j5 ]+ `3 l" U3 L
of tAR activity. A functional screening strategy using cated activin receptor. These findings demonstrated & }! N1 S2 x5 h& g, Y% B/ e
that epidermis is an induced fate rather than the default ventralized (UV irradiated) embryos allowed for the identification 4 i% m% B* K3 }$ B+ r f3 u8 |
of noggin. Because of its localized expression state of the ectoderm.
. Y# {& g! J3 B- R& _BMP4 is expressed at the appropriate time and place in the organizer and its neural inducing effect, noggin 3 a$ s/ H5 J2 p" q/ |
was proposed to be the instructive positive signal de- to be the endogenous neural inhibitor/epidermal inducer. / ^: [. |( l& u* L- Y/ E; _' r2 E
In situ hybridization shows that BMP4 RNA is fined by Spemann’s experiments, and thus presented a
0 e2 G5 ~4 V- t {serious challenge to the double inhibition mechanism present in the entire animal cap at the start of gastrulation, 0 e7 t, \ d; h
as well as in ventral and lateral marginal zone (Fain- which is the trademark of the default model. In fact the
4 y# m h+ A$ U1 R! m) n. o- _cloning of the noggin receptor was much anticipated as sod et al., 1994; Hemmati-Brivanlou and Thomsen, o. n! X9 @* x+ T0 V6 v
1995). At later stages, transcripts disappear from the a way into the signal transduction involved in neural & _5 k: W7 E( L9 ]
induction (Lamb et al., 1993). portion of the ectoderm that becomes the neural plate, " Y6 r+ z3 }6 e( }' w( ?
suggesting that repression of BMP4 transcription is one On another front, an obvious extension of the demonstration
3 q* o5 O5 F8 E( a8 P tof neuralizing activity by tAR was to examine of the mechanisms by whichBMP4 activity can be inhibited
+ ] e- O! h* Tin the prospective neuroectoderm.ABMP4 receptor the embryonic distribution and activities of other activin
# {4 M# p, q. v1 \antagonists in embryos. Follistatin, an inhibitor which is also expressed in the animal cap. Thus the pattern
1 |& w: i: j1 `of BMP4 transcription is consistent with its proposed binds activin, also expressed in the organizer, was
* W: T( b0 ~7 Y( sshown to turn on neural markers directly (Hemmati-Bri- functions in epidermal induction and the suppression " O7 V# e0 |# p8 w0 i& r8 W
of neural development. vanlou et al., 1994). However, as it was the case for tAR,
i7 W r; s8 _$ Jthe specificity of follistatin for activin was uncertain. 7 K/ t) _3 \- S7 K' V9 p
Recently it was shown that follistatin can interfere with Additional Evidence for the Default Model {, F5 K& t# Z1 g$ t
of Neuralization in Vertebrates the function ofBMP7 (Yama *** a et al., 1995; see below),
W, Q; t& P5 L- mand can dorsalize ventral mesoderm (Sasai et al., 1995). Dominant Negative BMP Receptors and Ligands
/ c m- \9 v4 }5 a! AAs would be predicted from the default model, antago- Another important gene expressed in the organizer,
! |1 [0 }" x' ~9 t6 A/ E9 ychordin, was originally isolated in a differential screen nists of BMP4 signaling lead to neuralization. For example, + \! U) J B: U% a: ]; d
a truncated type I BMP4/2 receptor, tBR, induces for dorsal specific genes (Sasai et al., 1994). Chordin,
7 ?' b( ]. l t2 M9 V3 ta secreted factor and the vertebrate homolog of the neural tissue directly in intact animal cap explants as
2 \- t. ^2 k" c/ v) \1 ^does the truncated activin receptor, tAR (Suzuki et al., Drosophila gene short gastrulation (sog) has direct neural # K" a' X- r( ^% s/ j* ~
inducing ability. Though the possibility of an antago- 1994; Xu et al., 1995). However, while tAR blocks all # P7 f( o! B/ ]/ `4 x. g
TGFbs tested so far, tBR seems to be more specific in nism between chordin and BMP4 was noted, chordin
, ^4 J" z! D$ h2 {6 j7 zwas also suggested to be a positive neural inducing that it does not inhibit activin or Vg1 signaling. In addition,
% q" L' k* f& l; @dominant negative forms of ligands such as BMP4 signal derived from the organizer with a possible receptor
2 \7 M, n* a' T) _ U( Yand a signal transduction pathway (Sasai et al., 1995). and BMP7, but not activin, induce neural tissue directly
& E) q& d/ ]! i. }' g Nin ectodermal explants (Hawley et al., 1995). The fact Just like noggin and follistatin, chordin can also recue 2 m; Z4 }( E# H- P4 Y6 |
ventralized UV embryos and dorsalize mesoderm. that BMP7 dominant negative ligand can also induce 8 P0 h {: Q O5 D6 z
neural markers suggests that either other BMPs can 6 e/ y$ |7 i0 Z1 v# t
fulfill the same neural inhibitory activity or that dominant Biochemical Mechanisms for Neuralization & ~ S) b- E3 S# d
Two significant papers recently published in Cell shed negative BMPligands have a pleiotropic inhibitory effect 5 D8 C. t: `* Q/ [. u- s
on all BMPs. light on the mechanism of neural tissue formation by
6 ~ a1 T6 n9 k3 PCell 1 j) _$ ^. p" F5 O' _# D
16
/ r. w& O6 K) G$ J+ D3 f ?noggin and chordin. Biochemical studies demonstrated Onthe otherend of the spectrum, homozygous knockthat 6 y. e w, c& U- `
both chordin and noggin directly bind BMP4 (Pic- out mice for BMP4 or the BMP receptor (BMPR1) die . B) O7 \# Q+ R* ~: g3 z ]
colo et al., 1996; Zimmerman et al., 1996). The binding mostly at gastrulation stage at the time when these types 4 @6 Q3 Q" w8 n2 G( D: h: N; x
affinity is higher for noggin–BMP4 (20 pM) than for of cell fate decisions are being made (for review, see : W" q% a% T; D4 w$ X: L
chordin–BMP4 (300 pM), but chordin protein seems to Hogan, 1996). Even though this is a negative result, it
7 B; o5 a6 N3 E, Xbe more abundantly expressed in the organizer. Interest- highlights the pivotal role that BMP4 seems to play. It
) w# U6 X9 }& c% S# Q( [1 D: xingly, in both cases, this binding can be competed effi- is also important to remember that since more than one 7 Z/ H5 c4 X+ T* C' ?6 c
ciently with BMP2 and to a lesser degree with BMP7. BMP4 inhibitor is present in vertebrates, it is likely that
0 r3 ?) u" @% W9 M. X% C& D1 {6 zThe consequence of this binding for both noggin and the knock out of single BMP4 antagonist will have no
8 m( b) d/ A, k$ b/ ychordin is that the neural inhibitor/epidermal inducer obvious neural phenotype. ( B9 N$ ~$ U, T }+ s' u8 f* f+ u. t
BMP4 can no longer access its receptor; thus, BMP4 + o- e2 P- k. r6 n" V$ F
signaling, which would otherwise occur throughout the Is the Inhibition of BMP Signaling Sufficient
- m) }+ a. F0 E% u# Nanimal cap, is inhibited on the dorsal side and neural for Neuralization?
+ [+ i/ U: u2 T1 o" afate is unveiled. Finally, the interaction of both noggin
9 ]; A5 ?- x2 _+ z) z' k$ R4 m& G+ e' XAlthough noggin, follistatin, and chordin can neuralize
( \6 L: X6 T8 |1 q3 kand chordin seems to be specific to BMP2 and BMP4 ) v2 G6 z! j. Z) g4 V2 t
by antagonizing BMP4 epidermalizing activity, is it possince 0 y, Q, A2 m) L8 V" r+ k5 _
they both fail to bind activin or TGFb1.
$ f3 y$ O% J! p2 j5 [sible that, in addition, they transduce a signal via a / X2 O7 X6 u! e, d) V: G/ `! [5 {
While the biochemical mechanism of neuralization by , m% b8 G* |( h; }2 H
receptor, as was originally postulated for noggin and 0 n8 |. ~1 U0 A9 m
noggin and chordin seems to be solved, the case for ! f: q! [1 E' M7 M* C: a
chordin? The default model would predict not. There
; j7 x. c$ _0 s" q" {; c- Kfollistatin is unresolved. First, it is clear that follistatin " o) f0 M, ~2 m9 Q' F \
are indeed three lines of evidence that strongly argue directly binds activin with very high affinity (Nakamura et
2 [7 B! R7 ^5 k! K' l" R3 O6 I: u2 M" gagainst the existence of receptors for follistatin, noggin, al., 1990). Activin, however, does not have an epidermal
) L8 d) ^0 M' o4 o) r3 nor chordin, at least in the pathway mediating neuraliza- inducing activity; instead, it inhibits neural formation by 8 T: h* ?+ |0 Z! V8 W/ p
tion, and that inhibition of BMP signaling is sufficient to pushing the cells toward a mesodermal fate. It is thus ( S/ r; C: ~4 q8 D$ J
unveil the neural fate. First, there is the evidence from possible that activin mediates cell fate choices at the 7 n- s* U2 c+ D2 [3 I0 p( z0 M4 _
cell dissociation experiments discussed above: when ectodermal-marginal zone boundary. Also, there is eviembryonic 9 V& z# q$ M2 s: x2 u
cells are dissociated for several hours they dence that follistatin can inhibit BMP7 activity (Yamawillmakeneural * {: v( q: U% M/ G8 o5 e" g
tissue (Grunz and Tacke, 1989; Godsave *** a et al., 1995). Because there is evidence that heteroand 9 I8 z3 J. x Z3 D7 i
Slack, 1991), and the addition of BMP4 will inhibit dimers of BMP4/7 have a muchhigher activty thanBMP4
- W, V+ Y4 q: o& `this effect and induce epidermis (Wilson and Hemmati- or BMP7 homodimer, and that there is overlap of expres- & V6 b3 r7 [0 H" D/ V
Brivanlou, 1995). The second evidence comes from ex- sion for BMP4 and BMP7 in the ventral side of the emperiments 2 u* _! Q# C; K
recently performed in Drosophila. Holley et bryo (reviewed by Hogan, 1996), it is tempting to specual. & _% _$ G% d. k( x* Y* W! E/ u9 e
(1996) demonstrate that at least for SOG/chd, and late that the inhibition of BMP4 by follistatin is mediated
+ u: s, v# G( R5 u/ D6 d. b: B5 Uperhaps for noggin, binding DPP/BMP is their only func- by its binding to BMP7. Alternatively, because RNA intion.
& I- B& o2 s" r+ fThey showed that while noggin inhibits DPP and jections with follistatin are done at the two cell stage $ a; X$ I/ L, M& S1 d! ?9 A
phenocopies a dpp2 mutation, it can only operate out- and the animal cap explants are removed about 4 hours 2 @/ x6 R/ ]5 h- O
side of the cell, and in the presence of an activated DPP later, at blastula stages, it could be argued that an intact 5 @3 n5 E( ~- I) S) g! g
receptor, its effect is abolished. More compelling is the activin pathway is required for BMP4 signaling, whose ( y; L. Y, q* g* W& C# f
fact that the double sog2 dpp2 mutant has the same disruption eliminates BMP4 activity.
- d4 v& t/ _6 X* o Aphenotype as the dpp2 mutant. If SOG had any other It is also noteworthy that three other secreted factors ' N$ b0 j( z$ f3 ^0 U
function than just inhibiting DPP, the double mutant FGF, FRL1 (Harland, 1994; Kino *** a et al., 1995), and
! F: {: }9 }9 ~2 U- rphenotype should have been different than that of dpp2 Xnr3 have been reported to have direct neural inducing ' M* _; N, ]3 T+ _. H
alone. The final line of evidence comes from the fact activity. The mode of action of both FGF and FRL1,
- z8 ]$ h& E2 E& O: ?. cwhich is an FGF related factor, is unclear. Xnr3, however, that, while chordin can reverse the osteogenic induction 3 c Y8 C, T4 k) t) u
is a member of theTGFb family localized in the organizer caused by BMP4 in 10T1/2 cell lines, it cannot block
D7 l" d5 e# c# j/ b# m: Owith direct neural inducing activity also mediated the one mediated by retinoic acid (Piccolo et al., 1996). : q' `0 B7 s+ m+ B; m9 n2 j
through a BMP4 inhibition (Hansen et al., 1996). Taken together, these observations strongly suggest
# \% s. o9 ?" h# ?4 @" ythat inhibition of BMP signaling is sufficient for neurali- ( y' }! \* \+ y' U$ o
Evolutionary Considerations zation.
) V' S( E' L5 m8 y" j4 t+ B8 kIn an interesting turn of events, recent studies of ecto- Contribution from many groups working with the amdermal
$ M/ v1 D* l3 V1 D9 rpatterning in vertebrate embryos may have phibian system has provided a molecular solution to the
5 M) `1 P7 x7 L7 g; W1 b. Bhelped us understand the situation in Drosophila. The problem of vertebrate neural induction originally defined
9 S* c# i" t/ w1 x# c2 c* Pstrategy of neuralization by inhibition of an inhibitor by Spemann and Mangold. The challenge for the future " P2 f7 u- ?7 U8 l9 _8 A
seems to have been conserved from arthropods to will inevitably include the establishment of a link bemammals. : H6 i$ ~0 \1 R/ t# l% Q. a0 x
tween the early neural specification process, described $ e: E7 k8 b9 p7 j
In Drosophila, the homolog of BMP4 is decapen- above, and the function of neurogenic genes operating
9 B0 m9 O" B8 Ataplegic (dpp) and the homolog of chordin is short gas- downstream of these signaling events, ultimately leadtrulation & J4 y& w+ \2 k1 f O
(sog). DPP/BMP4 and SOG/chd can function- ing to the generation of a mature neuron.
: X8 i5 A j& o4 J) Z/ T: d# W# pally substitute for each other in both organisms despite " s4 Y N$ `! W. c+ h
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发表于 2012-5-3 20:03
谢谢各位前辈的赐教了！还想问下如果用DMEM/F12，用哪个公司的比较好哩？