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一篇很好的综述！！！ * e% p$ h2 q1 w
* Y: a, \7 p. J' E
Vertebrate Embryonic Cells Will Become Nerve Cells Unless Told Otherwise 6 A/ B: T7 U+ @/ v7 l+ k/ [ [7 O
Ali Hemmati-Brivanlou* and Douglas Melton† (Spemann and Mangold, 1924). In the absence of this
, V+ S. P" X n. q* b, |# Yinfluence, as on the ventral side, the ectoderm differenti- *Laboratory of Molecular Embryology - ?9 Q9 o9 F/ z8 p. d/ s
The Rockefeller University ates as epidermis. Thus, in the following decades, the 9 H/ I& G2 `/ @* H9 @# h# j
development of epidermis was assumed to be a fall- 1230 York Avenue . l. S7 w& f1 ]- ]2 H. l- b
New York, New York 10021–6399 back, or “default” fate for gastrula ectoderm, requiring
# q& _$ \! e1 o" c: {no cell–cell communication, whereas neural specifica- †Department of Molecular and Cellular Biology
3 {$ V6 }, A! D% AHoward Hughes Medical Institute tion was thought to be contingent upon the receipt of
' J! V7 l4 E" ~! ua positive signal (in molecular terms, a signal that acti- Harvard University
+ Q5 H! ^! W8 R7 Divinity Avenue vates a signal transduction pathway) from neighboring ; o6 q; t8 A$ L1 O: K
cells. Cambridge, Massachusetts 02138 9 d4 v( W9 b2 j$ \+ U
A considerable effort over several decades failed to 0 D4 O0 f% Z! f4 H" @- u( Z7 `
identify the gene products responsible for neural induction
" j- d2 A+ U7 F4 U' F( Q; r! |, fin the embryo. The idea of a positive signal involved The past few years have witnessed a significant change
! }' N/ H1 l" y* T! o% w% }in neural induction so dominated thinking in the field in the understanding of how the vertebrate nervous systhat ( Y1 Y G5 x6 ~' I$ P+ [8 M
the significance of results inconsistent with this tem forms during embryogenesis. More than seventy ; Z0 `3 W6 l8 J" u+ d% J
idea were not widely appreciated. For example, several years since Spemann and Mangold first demonstrated 6 F$ {# C Z: M# e, h# D: L
researchers found that when cells of ectodermal ex- the phenomenon of neural induction, the molecular ) u/ n H' V& n: `% n$ o4 u6 e
plants (also called “animal caps”), were dissociated, mechanisms underlying neural induction now appear to
) {; v: s# |' ?) I6 L% Hthey can form neural tissue whereas intact or whole be at hand. Two independent approaches, one focusing
, G2 q" p$ [8 `animal caps form epidermis (Grunz and Tacke, 1989; on a “default” or “ground-state” model for neural induc-
; t7 U( Z/ X8 m4 X# R8 @: MGodsave and Slack, 1991). In these experiments dorsal tion and the other culminating in the discovery of semesodermal
& I; H! W# L, r% x8 n9 g. Atissue (the organizer) is absent and neurali- creted neural inducing factors (noggin, follistatin, and
# U( W% [# S1 |$ }' azation occurs in a cell autonomous fashion, in contradic- chordin), have not only brought to a successful conclution ' g$ ]( w0 `& c' y
with the requirement for a positive signal derived sion the search for Spemann’s neuralizing factor, but 3 z. F% C8 ^$ u- x5 u8 q6 ~3 V* C
from the organizer. With hindsight these results using also illuminated its mechanism of action. Itnow appears
" `( ]4 n. B1 ~( x; [+ csimple cell dissociation provided a strong hint that neu- that neuralization of embryonic cells occurs when cells
- O2 [/ A9 w5 k' m7 w8 P, V: ], Qral inhibitory signal within the ectoderm, or the whole do not receive other inducing signals telling them to
' v+ L: ^9 t$ g8 f, L3 Nembryo, prevent neuralization, and in the case of animal form epidermis, mesoderm, or endoderm. This concept
( h& X2 E- p9 ^9 j% c3 r: @caps, these neural inhibitors or antagonists drive cells of neuralization allows for a reinterpretation of the classitoward / m! ?/ R) M7 S3 h
an epidermal fate. cal views on both neural and epidermal specification.
' h2 u$ n' R+ _3 f+ zThe secreted growth factor BMP4 (Bone Morphogenetic * @. M' h( ]- \1 m, f
Neural, Not Epidermal, as the Default State Protein) plays a pivotal role wherein BMP signaling inof
. S6 H3 ~5 u7 \4 O5 \1 J5 _% OEmbryonic Ectoderm duces epidermal differentiation. It is the absence of BMP 9 R* F& j' k& \
It is important to distinguish between direct and indirect signaling, accomplished by BMP antagonists including ; d! K4 k2 {. g, q. t
effects in understanding assays for neural induction. noggin, follistatin, and chordin, that leads to the formation 8 d+ V9 ^" L! f1 @; t
of neural tissue.
/ h- z J B# ]2 O* s4 GLessons from History 1 g5 j( p# U& u
The concept of neural induction was established in 1924 7 l1 l- w# u( { _3 i: V$ f) G
by Spemann and Mangold’s grafting experiments using * G$ B( B. w* A* M% p) W7 V6 _0 J9 H$ h
salamander gastrula (Spemann and Mangold, 1924). 3 m' q% j" N& X* D5 z
During gastrulation, prospective ectodermal cells, located / u; m: g5 i! R. u( B' D. r9 ]8 `
on top of the embryo (animal pole) make a choice # e' J* `- O& _1 s
between two fates: epidermal and neural. The prospective
3 @/ M' n7 N H8 g% p) Oneural plate is defined by two boundaries: the epidermal
2 q/ G. W. o& Q–neural boundary in the animal pole and the ( L- I: _& L0 w# x. P4 l7 J
neural–mesodermal boundary in the equatorial region ! Y3 {1 R" {% ?$ O) ^% Z, J! }
(Figure 1). The blastopore lip, where cells first invaginate 1 p4 W. N6 g, c. B$ K
during gastrulation, marks the prospective dorsal side 9 T6 L& R& k; S' i5 B
where the neural plate forms. Transplantation of a dorsal
% @+ y7 ?$ } c4 N# E! M( Y/ e. Iblastopore lip, which consists of mesoderm and endoderm,
2 ]& @6 a3 ]7 o$ nfrom an early salamander gastrula to the ventral
$ J$ W/ D3 f, ~1 V: q5 Tside of another early gastrula causes formation of a
" Y8 N* T( Q7 J3 zsecond nervous system (Figure 2). The second nervous
$ y! E& L( v9 K6 e6 L& }- s! Z$ msystem develops not from the transplanted tissue, but ; O- M- c& U: m4 }; w" z8 t/ q
from ventral ectoderm, which in an undisturbed embryo Figure 1. Fate Map of the Xenopus Gastrula
. a& r: n% T. _- Uforms epidermis.Spemann namedthe dorsal blastopore
2 ^3 H9 S" ^+ c6 [(Left) Lateral view.
; q9 \/ u* L4 L) l; vlip the “organizer,”and proposed that innormal develop- (Right) Dorsal view.
) {9 D$ L. Y8 g2 \2 s0 V7 r% Tment this region induces and organizes a correctly pat- The presumptive neural plate is delineated by presumptive epidermis
9 s+ Q& x. N" F6 T# E4 ?and mesoderm. Animal pole is at the top. terned nervous system in neighboring dorsal ectoderm & e R6 v/ | }* d( M# b0 F
Cell ! A% Q a/ j* w
14 5 z: C9 I m2 ~- o
Figure 2. Classical Transplantation Experiment T2 k% l# u- J0 d
by Spemann and Mangold
- D; W9 B" `$ G2 [4 b(Top) The fate of a normal embryo. , Z) S- M( A F& q; T: N
(Bottom) The transplantation experiment. 9 M2 g. ]3 {2 s, ?/ O( R$ B8 g
The transplantation of an organizer from a 7 A# l7 Z9 O$ C1 w( S/ p9 B
donor to the ventral side of a host embryo
& ]* P; x# j+ j2 ]6 L3 y( }induces a complete secondary axis and giving / }$ @$ _& t2 Z. }- L: z" R
rise to an embryo with two main body
6 j, Y u7 p6 o+ W4 v& m( W' Raxes. This experiment demonstrates that all
* p3 L( k! @2 Rthe information necessary and sufficient to
x/ O3 p8 X1 t) [( o, winduce a dorsal axis, including the entire nervous
& r0 v2 {# j; ^( f Bsystem, is contained within the cells of $ J0 H- W$ O z
the organizer (boxed here in green).
, F! i' |: J0 n8 P) ~Several growth factors with mesoderm inducing activity, these results suggested that individual cells of the early ( i; _* A5 d7 p6 ~0 s
gastrula animal cap are predisposed to form neural tis- including activins and Vg1, can cause formation of neural
( v) L) }7 j( b O, m" r) ~tissue when added to ectodermal explants, but this sue in the absence of further signals. In this view, epidermal 0 Y6 g K9 n. O+ U" w$ L8 C
(but not neural) specification requires a positive cell neuralization is indirect because some of the treated : P8 f6 r( U6 O5 \
cells first form dorsal mesoderm. The mesodermal cells signaling within the prospective ectoderm. When this 0 r0 L3 X/ ?* `. C- U
signaling is interrupted experimentally by cell dissocia- then mimic the action of the organizer and induce neural
- p9 N5 M/ Q3 U) Ptissue in the surrounding cells. The first direct molecular tion or molecular antagonists, neural tissue forms. Neural 1 a$ w7 B" W" G. H( c/ M
induction by the organizer in vivo could work in the neuralizing treatment to be described was a truncated ' h7 E7 ~2 r1 i, @$ A
type II activin receptor (D1XAR1, here referred to as same way, that is, by blocking epidermal induction
; }; o* h. R% Z7 `, D2 @within the animal cap (Hemmati-Brivanlou and Melton, tAR), designed to test for a requirement of activin, a
. H o) M8 P/ Tmember of the TGFb growth factor superfamily, in meso- 1994; Figure 3). This view contrasts with the commonly
p+ `- D- t: q9 i( oheld textbook model wherein neural induction requires derm induction (Hemmati-Brivanlou and Melton, 1992).
. D$ A2 k- w+ b6 Q9 b/ n2 VAnalysis of the expression of several tissue specific a positive signal. The term “neuralization” more aptly 5 y" ^% g9 s) R, ?" C' X
describes the situation than “neural induction”; indeed, markers following injection of tAR yielded the surprising
1 H* j0 j |0 Wobservation that a general neural marker, neural cell it is the epidermis that is induced. ; [( [' E* S; v
adhesion molecule (NCAM), was turned on in ectodermal
& Y, s3 F9 o* W4 u- kexplants following inhibition of activin type II recep- Predictions of the Default or Ground State
, h% j& i+ a( n) `; D( ?Model for Neuralization tor signaling. These explants express the activins and
. t/ B7 l8 Y- W# ?) P9 etheir receptors and, as mentioned above, would make The model for neural specification described above
- g* M4 w& w- p6 R4 V: Umakes two important predictions. First, the signal from epidermis when cultured alone. In addition, it was found
1 |/ S3 t( N1 H j0 [1 c6 ]that the dominant negative activin receptor could also the organizer is an antagonistic secreted signal that
, L4 t5 ^/ q# Z4 X8 B- H0 z/ D. tinhibits the activity of a neural inhibitor/epidermal in- neuralize cells located at the bottom of the embryo or
, K' B) P5 Z' Q! i. [vegetal pole, cells normally fated to become endoderm. ducer. This antagonism occurs specifically in the dorsal i% M/ K7 I8 \5 N, {
ectoderm during gastrulation. Second, the hypothesis This result suggested that neuralization by inhibition of 0 v2 Y9 C7 R. N( P. d: T
the type II receptor signaling is not confined to cells of that epidermal rather than neural specification requires - q: ~ D6 _( t
positive cell signaling among ectodermal cells, predicts the ectoderm but can be generalized to other germ layers.
- U2 V) [4 F% U' s) p, @' u/ AIn terms of the specificity of the effect, it was noted that epidermal fate can be induced in ectodermal cells. + N \. l# g% _. x2 r- q
These predictions have very recently received experi- that while tAR does not interfere with receptor tyrosine ! e2 v9 d, E( R4 d
kinase signaling (such as FGF), tAR could inhibit signal- mental support from several fronts.
. j+ ^9 ]. h# jing of other TGFb factors. It is now appreciated that
/ J4 L6 q5 R' w& OTGFb receptors are heterodimers and thus interfering BMP4 Inhibits Neuralization and Induces Epidermis " c% l7 k! R! B5 {$ q
The neuralizing activity of the truncated activin receptor, with a particular pathwaymay affect signalingfrom other " {; ]2 p3 Y! l t! K
members of the family. Indeed, it was subsequently de- and the observation that an activin antagonist, follistatin, & t" X9 u* _& _( Z+ I9 B
has direct neural inducing activity (see below) pointed to termined that tAR blocks more than just activin signaling,
* g- r) i4 I* R6 Q( z' nand appears to inhibit other TGFbs including Vg1 activin as an endogenous neural inhibitor. Nonetheless,
# r' ]1 Y e" ?: Vthese data provided no direct evidence that activin could and BMPs (Schulte-Merker et al., 1994; Hemmati-Brivanlou
( s5 ^, h! P; z: n* L' Qand Thomsen, 1995). specify or induce epidermis. To address this prediction,
2 v0 [( s- _1 k4 E' U( t/ j. |. ca complementation assay was used where cells of the The fact that tAR expression directly initiated nerve 9 o: R7 m; F1 F2 ~3 _2 J
cell formation was significant because not only did it animal cap were dissociated and incubated in the presence & Z9 Z: b1 R$ X o+ w. \" p9 y
or absence of activin or BMP4, which are both occur in the complete absence of dorsal mesoderm, but
" K- i) U1 H' ^' ymore importantly, it demonstrated that neuralization can TGFb ligands inhibited by the truncated activin receptor.
( X+ O. p) Q0 y. h/ w4 K' X8 UWhile activin did inhibit neuralization of dissociated ec- occur by inhibition of signaling. Moreover, injection of 1 U/ i5 p: c( {% J
tAR showed that cells in any germ layer would become todermal cells by inducing mesoderm, activin did not ) m: t& k9 @/ E( n
induce expression of epidermal markers. In contrast, neural if TGFb signaling was blocked (Hemmati-Brivanlou 2 ?0 v T/ K3 [3 C! _0 u
and Melton, 1994). Since both cell dissociation and BMP4 not only inhibited neuralization but induced epidermal
5 \# u2 I* A5 {. G" W$ o8 V# f* lfate. The two activities of BMP4, neural suppres- expression of a dominant negative activin receptor in ) _2 l% `" c+ q/ p9 y+ |
intact ectodermal explants can be interpreted as an sion and epidermal induction, always occur together, & f( a z1 v. d9 b* ~; ]+ W0 L* K0 q
leading to the conclusion that they represent a single interference with the communication between cells, # e# n- V% J, W5 ?
Review " d5 \' B. x4 q7 p* E! q
15
, c9 m; q$ q" v; s2 s, S7 e+ lFigure 3. Schematic of the “Default Model”
4 q! J g E' k& iof Vertebrate Neuralization " G! g$ I# c5 y4 \6 b; \; g' b
(Top) In normal intact ectodermal explants
, u: t: D# X0 F8 _(animal caps), BMP4 (the blue arrows) induces 0 i- C, p8 W7 G- o
and maintains the epidermal fate. ; t' M* }5 v; c- j3 u3 E3 L" E
Upon dissociation this secreted neural inhibitor/
- E+ I( f* }: h! Mepidermal inducer is diluted and ineffective, . Z$ P- S( H3 V3 j; Q2 U
and thus the neural fate in unveiled by & S4 Q# I% r \# Y( V+ L
derepression.
" p8 ~) N" n9 J* p0 V0 p7 n- [(Middle) Expression of tAR or tBR interferes ; v( t: U9 F) ]' l5 {* s6 Q
with the cells ability to receive the BMP4 signal.
4 Y/ n; d* R/ j# t- MThe epidermal fate can no longer be _9 o: S. Q' y+ m9 T# l, l, C5 Y
maintained and the neural fate is unveiled. . _) @# Y$ A5 K& H
(Bottom) In the embryo, the ectoderm has a
0 p5 @5 v8 g { v& j, D5 S2 {dorsal-ventral polarity. Secreted factors,
* ~8 H8 Y) e& P' \1 T2 P Bsuch as noggin, chordin, and follistatin, interfere
. U- ?' M. O* x& W& I! gdirectly with the BMP4 signal (in the case $ X: q! p+ K* i( P# f6 L) f/ z
of noggin and chordin, by direct binding to
% I" @: _3 I; y2 E& L- z% ABMP4). The consequence of this interference
}6 D& ~. _$ B7 g, b8 {is that BMP4 can no longer have access to ) Z% v& I Z7 }8 | C
its receptor and therefore can no longer induce # z6 b% F! d/ D1 \% _/ G
or maintain the epidermal fate and thus
" r5 v' A* `# E" T6 Y+ {" {9 c; d8 h9 Wneural tissue forms on the dorsal side.
* O7 x! Z s) v" {; i* R) \action, as expected from the neuralization model (Wil- Noggin, Follistatin, Chordin, and Others 2 k8 t3 E! |2 @1 |0 H; w5 R; R6 y
Isolation of the first endogenous direct neural inducing son and Hemmati-Brivanlou, 1995). Induction of epidermis ( y- N3 A2 y! G2 }
is inhibited if the dissociated cells express the trun- factors was reported shortly after the characterization 0 E0 o+ ^! T4 g1 w) r* m
of tAR activity. A functional screening strategy using cated activin receptor. These findings demonstrated
& N3 |- v6 w _6 m. K+ ^that epidermis is an induced fate rather than the default ventralized (UV irradiated) embryos allowed for the identification 3 B x! q. |6 _2 l8 d
of noggin. Because of its localized expression state of the ectoderm. ) a/ ?$ e: H& }' F) b3 H) b
BMP4 is expressed at the appropriate time and place in the organizer and its neural inducing effect, noggin 3 E$ ?* z2 l" K9 T
was proposed to be the instructive positive signal de- to be the endogenous neural inhibitor/epidermal inducer.
. ~/ b, b7 P U% fIn situ hybridization shows that BMP4 RNA is fined by Spemann’s experiments, and thus presented a
5 U/ a3 [0 f d# ~5 k" U: Fserious challenge to the double inhibition mechanism present in the entire animal cap at the start of gastrulation,
/ h& g2 m6 R v$ e" gas well as in ventral and lateral marginal zone (Fain- which is the trademark of the default model. In fact the & a3 w/ ^3 X# Y; S4 V3 M
cloning of the noggin receptor was much anticipated as sod et al., 1994; Hemmati-Brivanlou and Thomsen, " ?" `+ f+ J% g
1995). At later stages, transcripts disappear from the a way into the signal transduction involved in neural ; R6 u9 _8 ?" o2 u+ w0 n; h1 H7 ]
induction (Lamb et al., 1993). portion of the ectoderm that becomes the neural plate,
% z% U( G/ z U8 J3 z' Rsuggesting that repression of BMP4 transcription is one On another front, an obvious extension of the demonstration
& a& z L$ v$ o" d+ r' T/ Gof neuralizing activity by tAR was to examine of the mechanisms by whichBMP4 activity can be inhibited " k3 Z0 K/ B% a& w
in the prospective neuroectoderm.ABMP4 receptor the embryonic distribution and activities of other activin
9 T. @9 R. p& e: h/ v _antagonists in embryos. Follistatin, an inhibitor which is also expressed in the animal cap. Thus the pattern
$ n) j, l# U* L2 w7 W" Dof BMP4 transcription is consistent with its proposed binds activin, also expressed in the organizer, was " }3 C, T, g! U" [! N I
shown to turn on neural markers directly (Hemmati-Bri- functions in epidermal induction and the suppression ' I* s7 F4 w2 o% i" \; G
of neural development. vanlou et al., 1994). However, as it was the case for tAR,
% R3 B7 \( B* b+ a( rthe specificity of follistatin for activin was uncertain. 4 s2 z$ R3 z1 x. _ C" q$ W' G
Recently it was shown that follistatin can interfere with Additional Evidence for the Default Model & ^' @$ Z H4 B6 i( p7 m
of Neuralization in Vertebrates the function ofBMP7 (Yama *** a et al., 1995; see below),
8 y' \3 w8 O6 [3 ]5 Gand can dorsalize ventral mesoderm (Sasai et al., 1995). Dominant Negative BMP Receptors and Ligands
6 k4 n3 i# i6 m8 |: L% GAs would be predicted from the default model, antago- Another important gene expressed in the organizer, / w0 C9 D0 {3 t$ p6 w
chordin, was originally isolated in a differential screen nists of BMP4 signaling lead to neuralization. For example, + t( x! ]9 {; g: _& n$ |" {% M
a truncated type I BMP4/2 receptor, tBR, induces for dorsal specific genes (Sasai et al., 1994). Chordin, 5 m7 b2 o3 n" `* `2 p
a secreted factor and the vertebrate homolog of the neural tissue directly in intact animal cap explants as ) n F0 P& p1 L! a
does the truncated activin receptor, tAR (Suzuki et al., Drosophila gene short gastrulation (sog) has direct neural
# l% x- E" j4 b9 l1 O+ u3 Rinducing ability. Though the possibility of an antago- 1994; Xu et al., 1995). However, while tAR blocks all $ f9 u5 G$ s2 h- O8 h+ k7 f; u
TGFbs tested so far, tBR seems to be more specific in nism between chordin and BMP4 was noted, chordin
' M& A9 e# y& t% x+ iwas also suggested to be a positive neural inducing that it does not inhibit activin or Vg1 signaling. In addition,
1 L8 ] e: e# h( _dominant negative forms of ligands such as BMP4 signal derived from the organizer with a possible receptor 6 _4 T/ `+ V# g9 O+ B+ l6 A& `
and a signal transduction pathway (Sasai et al., 1995). and BMP7, but not activin, induce neural tissue directly @$ ~5 j2 }/ r5 ~ x& D
in ectodermal explants (Hawley et al., 1995). The fact Just like noggin and follistatin, chordin can also recue
3 {6 A7 n. G2 A6 `( ]ventralized UV embryos and dorsalize mesoderm. that BMP7 dominant negative ligand can also induce
7 I. }" _* t) {neural markers suggests that either other BMPs can ' m! \7 |5 o$ H
fulfill the same neural inhibitory activity or that dominant Biochemical Mechanisms for Neuralization
2 v) p8 v7 g5 _$ \4 l4 q7 z; UTwo significant papers recently published in Cell shed negative BMPligands have a pleiotropic inhibitory effect
6 T/ K$ B: P% u+ X9 Non all BMPs. light on the mechanism of neural tissue formation by
0 g8 y7 F- S7 ]& xCell $ ~5 \/ V& F1 x
16
% H9 B9 I0 V2 n! \: p% Q0 O: T8 lnoggin and chordin. Biochemical studies demonstrated Onthe otherend of the spectrum, homozygous knockthat ! S& I8 i( O& }0 F
both chordin and noggin directly bind BMP4 (Pic- out mice for BMP4 or the BMP receptor (BMPR1) die 6 }$ X: } U+ K/ C7 ]( ~
colo et al., 1996; Zimmerman et al., 1996). The binding mostly at gastrulation stage at the time when these types
& S9 |1 |# A# i, }# b# \affinity is higher for noggin–BMP4 (20 pM) than for of cell fate decisions are being made (for review, see
0 Q& m2 A4 \, Ychordin–BMP4 (300 pM), but chordin protein seems to Hogan, 1996). Even though this is a negative result, it / T2 {% ^, O2 O0 K
be more abundantly expressed in the organizer. Interest- highlights the pivotal role that BMP4 seems to play. It
Y$ u: a2 R4 b Bingly, in both cases, this binding can be competed effi- is also important to remember that since more than one
% Q: w7 o9 r# N3 \0 h+ k" Cciently with BMP2 and to a lesser degree with BMP7. BMP4 inhibitor is present in vertebrates, it is likely that
1 V" a* @# _8 C! `, d. Z9 cThe consequence of this binding for both noggin and the knock out of single BMP4 antagonist will have no ) n* c. U7 _! |" f3 o
chordin is that the neural inhibitor/epidermal inducer obvious neural phenotype.
. K* k! K: E) P. Z; k9 W. lBMP4 can no longer access its receptor; thus, BMP4
/ y) |" t( Z/ w7 Z7 O5 D/ g4 P Isignaling, which would otherwise occur throughout the Is the Inhibition of BMP Signaling Sufficient
) A$ l; k6 j: n) B+ t+ y& Kanimal cap, is inhibited on the dorsal side and neural for Neuralization? # l# C( [6 [* O! ~$ h0 A% P
fate is unveiled. Finally, the interaction of both noggin ) \& m8 k$ a- ^* w: B- |, X
Although noggin, follistatin, and chordin can neuralize
9 j6 c z5 X! oand chordin seems to be specific to BMP2 and BMP4
7 o9 k1 E: }+ _7 z5 e( Q T$ M* oby antagonizing BMP4 epidermalizing activity, is it possince
8 I/ g" j' o1 d9 v: X) F) Mthey both fail to bind activin or TGFb1. l/ r5 z: T* }& Q
sible that, in addition, they transduce a signal via a
# f9 x* R6 b1 kWhile the biochemical mechanism of neuralization by
( ~8 r8 M: Q J$ t" O+ {0 Ureceptor, as was originally postulated for noggin and * w8 a p4 o: f" ]
noggin and chordin seems to be solved, the case for
# @( `! E5 j! P) p- hchordin? The default model would predict not. There ; B. h( Z& A7 X8 m/ F0 A
follistatin is unresolved. First, it is clear that follistatin % y6 w$ Y U/ @9 q
are indeed three lines of evidence that strongly argue directly binds activin with very high affinity (Nakamura et
* {' v! ?( j- i6 q' W1 H# ~5 d gagainst the existence of receptors for follistatin, noggin, al., 1990). Activin, however, does not have an epidermal 0 m' F* o/ ?/ Q/ c* _
or chordin, at least in the pathway mediating neuraliza- inducing activity; instead, it inhibits neural formation by
. M' c& D- u7 N3 Z6 ktion, and that inhibition of BMP signaling is sufficient to pushing the cells toward a mesodermal fate. It is thus + c- ]) a* O8 v' m
unveil the neural fate. First, there is the evidence from possible that activin mediates cell fate choices at the
7 D1 z* S6 L" p* c) Y9 |cell dissociation experiments discussed above: when ectodermal-marginal zone boundary. Also, there is eviembryonic
" \. ~. d$ j2 X; R+ V! }- Gcells are dissociated for several hours they dence that follistatin can inhibit BMP7 activity (Yamawillmakeneural
% M; M% j6 i! c. ^$ S* ~% Y2 q$ xtissue (Grunz and Tacke, 1989; Godsave *** a et al., 1995). Because there is evidence that heteroand ! A, V6 Y: w+ D! V9 r9 N; @
Slack, 1991), and the addition of BMP4 will inhibit dimers of BMP4/7 have a muchhigher activty thanBMP4
# m! b$ [0 T I. V/ ~4 Xthis effect and induce epidermis (Wilson and Hemmati- or BMP7 homodimer, and that there is overlap of expres- # y! a# f2 w0 |; s% U4 f' E& Z) j
Brivanlou, 1995). The second evidence comes from ex- sion for BMP4 and BMP7 in the ventral side of the emperiments
7 {: f0 x, u/ n! {% m. N/ ~1 frecently performed in Drosophila. Holley et bryo (reviewed by Hogan, 1996), it is tempting to specual.
9 I8 @% d% j; [/ K2 C# h8 G4 [(1996) demonstrate that at least for SOG/chd, and late that the inhibition of BMP4 by follistatin is mediated
. Y6 P9 ?" n# Z3 Y9 `7 e: sperhaps for noggin, binding DPP/BMP is their only func- by its binding to BMP7. Alternatively, because RNA intion. ' G% i3 D; s' R7 E9 U0 u. v6 J+ C: K
They showed that while noggin inhibits DPP and jections with follistatin are done at the two cell stage , X1 P* S0 P. S+ @6 E! W
phenocopies a dpp2 mutation, it can only operate out- and the animal cap explants are removed about 4 hours
( h+ H q" m+ T. q/ j4 ~# a: }+ y2 bside of the cell, and in the presence of an activated DPP later, at blastula stages, it could be argued that an intact
$ e% q5 p$ c) Qreceptor, its effect is abolished. More compelling is the activin pathway is required for BMP4 signaling, whose
# J! F9 O C$ x: r5 `fact that the double sog2 dpp2 mutant has the same disruption eliminates BMP4 activity. ) H7 @& s# H# n6 f9 \( ]/ E
phenotype as the dpp2 mutant. If SOG had any other It is also noteworthy that three other secreted factors
4 V9 `) Q, I5 sfunction than just inhibiting DPP, the double mutant FGF, FRL1 (Harland, 1994; Kino *** a et al., 1995), and 1 g, I# ~8 @; z! J/ L, S
phenotype should have been different than that of dpp2 Xnr3 have been reported to have direct neural inducing
4 X. r& Z( u H0 N$ [5 ealone. The final line of evidence comes from the fact activity. The mode of action of both FGF and FRL1, 9 h8 @' `1 H. K3 K
which is an FGF related factor, is unclear. Xnr3, however, that, while chordin can reverse the osteogenic induction
& l% F$ Y0 U) Z4 n3 his a member of theTGFb family localized in the organizer caused by BMP4 in 10T1/2 cell lines, it cannot block 4 [6 M$ f9 ?, Y0 b& p; j. f
with direct neural inducing activity also mediated the one mediated by retinoic acid (Piccolo et al., 1996).
( [3 _& }/ U* r& `( Xthrough a BMP4 inhibition (Hansen et al., 1996). Taken together, these observations strongly suggest 0 G! N2 M+ x3 O$ R
that inhibition of BMP signaling is sufficient for neurali- 8 \) d; J A: d2 B& c
Evolutionary Considerations zation. $ ?8 x( a5 Z0 D0 L+ N
In an interesting turn of events, recent studies of ecto- Contribution from many groups working with the amdermal
) Q+ @9 E, B, g9 }% fpatterning in vertebrate embryos may have phibian system has provided a molecular solution to the
* g5 s% A0 Q2 g J8 Dhelped us understand the situation in Drosophila. The problem of vertebrate neural induction originally defined % u! V3 N$ z5 }) R! N
strategy of neuralization by inhibition of an inhibitor by Spemann and Mangold. The challenge for the future
) e8 j/ y I% F4 |+ d) nseems to have been conserved from arthropods to will inevitably include the establishment of a link bemammals. 9 _( l$ n% R6 l/ }5 R T
tween the early neural specification process, described $ l1 J! }" Q8 {. ~5 E. r
In Drosophila, the homolog of BMP4 is decapen- above, and the function of neurogenic genes operating
) m* L' K3 ?$ I& S% z) Xtaplegic (dpp) and the homolog of chordin is short gas- downstream of these signaling events, ultimately leadtrulation ( X0 }; K) H3 B9 J# u& T9 G4 e$ w
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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？