close

Вход

Забыли?

вход по аккаунту

?

Induction and Morphogenesis.

код для вставкиСкачать
Induction and Morphogenesis
This year the 13th. Mosbacher Colloquium (May 3rd-5th)
was held under the theme “Induction and Morphogenesis”.
In his introduction, E. Klenck, Cologne (Germany), recalled
that this borderline field between biology and chemistry has
hitherto been regarded, above all, from a morphological
stand-point, but that recently progress has come about
through a more chemical approach.
This was clearly shown in the first paper: F. Lehmann, Bern
(Switzerland), spoke on “Biological and Biochemical Problems in Morphogenesis”. First structure-forming and segregation processes can take place even in the undivided ovum
(Ascidia). With other species (vertebrates, echinoderms),
morphogenesis begins after the division of the ovum. Until
the present time, however, only few relationships were
recognized between morphogenetic and biochemical segregation. Gustafson, Lenicque and Horstadius have found that
i n the case of young sea-urchin embryos the region of a given
redox potential (detected by staining with a redox dyestuff)
lies at different distances from the vegetative pole depending
upon the stage of vegetative development. Here, a region of
marked biochemical activity appears to be displaced in the
same sense as a region of particular morphological activity.
Regenerating tissues can also give information concerning the
relationship between the formation of morphological and
biochemical patterns. Substances have been discovered which
inhibit the regeneration of the tails of Xenopus larvae without
essentially impairing the vitality of the larvae themselves. Such
compounds (morphostatics) comprise, among others; colchicine, p-mercaptoethanol, 1-amino-3-methylbutyl ethyl
(11), 2,6-diketone (I), 2-methyl-3-oxo-6-ethoxyquinoline
ethyleneimino-3,5-ethoxy-p-benzoquinone
(111) as well as the
heterocycle (IV). 5-(I ,2-Dithiolan-3-yl)valeric acid (thioctic
acid) (V) and nicotinamide are also capable of morphostatic
action.
I
CH3
(1)
s
With normal regeneration of the tail, the cathepsin activity
increases and later falls of to a normal level. Cathepsins are
proteolytic enzymes and the raising of their activity during
regeneration might be related to the formation of peptides
necessary for the regeneration processes. The aminoketone (I)
raises the cathepsin activity quite considerably, which perhaps leads to such an extensive decrease of the tail proteins
that regeneration of the tail ceases. On the other hand a
colchicine
combination of quinoxaline (11) (1 :250000)
(1 : 106) inhibits the regeneration by 40 % without changing
the cathepsin activity. Evidently, therefore, several enzymes
of which the cathepsin activity is only a part, are responsible
for morphogenesis. It is to be assumed that different morphostatics influence different parts of this enzyme system.
+
J. Brachet, Brussels (Belgium), dealt with “The Influence of
Free -SH Groups on Morphogenesis”. P-Mercaptoethanol in
low concentrations (10-3 to 3x10-3 M) retards the development of Xenopus embryos; in its presence, the heads are too
small and the tails too short.Melanine formation is completely
inhibited. Dithiodiglycol, the oxidation product of mercaptoethanol, has the inverse effect (it does not influence pigment
Angew. Chem. internat. Edit. / Vol. 1 (1962) / No. 7
formation), so that it is to be assumed that the abovementioned developments depend upon the state of the
sulphydryl/disulphide equilibrium in the cells.
In higher concentrations (3x10-3 to 10-2 M), P-mercaptoethanol also inhibits the development of the nervous system;
the cavity containing the neural plate fails to close. This
effect can be reversed with adenosine triphosphate (0.1
mg/ml), although ATP has no influence on the inhibition of
tail development and melanine formation.
In the fungiform algae Acetabularia mediterranea, p-mercaptoethanol (3x10-3 to 5x10-3 M) interferes with the
formation of the pileus. In contrast, sterile whorls, from
which the pileus would normally grow, are formed without
inhibition. Here, too, dithiodiglycol(l0-4 M) has an opposite
effect; it promotes pileus formation and inhibits whorl formation. By use of reagents which react with -SH groups (pchloromercuribenzoate, 10-7 M, and p-iodosobenzoate, 10-5
M) it can be confirmed that free -SH groups are detrimental
to pileus formation.
In the case of the embryos of amphibia, thioctic acid in
minute concentrations (5 pg/ml) inhibits only the tail growth
(to an extent of ca. 30 %). In higher concentrations (15-30
pg/ml) it exhibits the same effect as mercaptoethanol at
higher concentration levels but does not influence pigment
formation. Here also, the development of the nervous system
can be normalised with ATP. This compound has no influence on tail growth. Furthermore, oxaloacetate or succinate
reverse the under-development of tails in embryos which have
been treated with thioctic acid This is not so with embryos
which have been treated with mercaptoethanol. The formation
of the nervous system and the differentiation of the tail tissue
are therefore based on different biochemical mechanisms.
Experiments with 35s-mercaptoethanol showed that 25 % of
the radio-activity entering the cell becomes protein-bound,
presumably by formation of mixed disulphides. It seems,
therefore, that proteins are a point of attack for mercaptoethanol.
The topic discussed by H . 0. Halvarson, Madison, Wis.
(USA), was “The Control of Enzyme Synthesis in Microorganisms”.Three possibilities for the regulation of enzymatic
activity in the cells of Escherichia coli were referred to:
a) An enzyme is inhibited through the end-product of a
synthetic chain. Example: E. coli contains two P-aspartokinases of which one is inhibited by threonine, and the other
by lysine.
b) The structure of an enzyme is so altered that its specificity
changes. Example : the hormone-influenced conversion of
glutamate dehydrogenase into alanine dehydrogenase.
c) The inhibition of enzyme synthesis by means of a repressor.
A regulator gene (RG) governs the synthesis of an aporepressor (Apo-R, presumably ribonucleic acid). This combines with a cytoplasmic co-repressor (Co-R) to give the
complete repressor, which reversibly blocks the operator
RG
Cytoplasma
ApoR
CoR
:4‘
R
-+
[Rl
VI
17-
~! O I S G I
I
m-RNA
+enzyme
gene (0).As a consequence of this blockage the structure
genes (SG), which lie adjacent to the operator genes, are not
able to promote the synthesis of messenger-RNA which is
indispensable for the synthesis of protein. Inductors (I) in-
409
activate the repressor, e.g. by forming an inactive complex
with the co-repressor [I].
In this diagram, constitutive enzyme synthesis is the result
either of a mutation of the regulator gene (synthesis of an
inactive apo-repressor) or of the operator gene (no pairing
of the gene with repressor).
Hitherto, the hypothesis given under c) has only been proved
for bacteria. It has now been shown on yeast cells that other
organisms can contain several identical and independent
structure genes for the same enzyme. Presumably, then, every
structural gene has its own operator gene. A structure gene
can be controlled through a number of regulator genes.
In the case of yeast there is, evidently, a fourth possibility for
regulating enzymatic activity; in this case the removal of the
ready synthesized enzyme from the ribosome is affected.
W. Berrman, Tubingen, reported on “Cytological Aspects
of the Transfer of Information from the Chromosomes to the
Cytoplasm”. Giant chromosomes, which are formed by a
combination of many thousands of single chromosomes, are
found in the cell nuclei of Diptera larvae. These giant chromosomes display a disc-like pattern (chromomere pattern) that
arises through the alternating sequence of DNA-rich and
DNA-poor layers. At several points the giant chromosomes
are swelled out to double normal thickness (“puffs”). The
previously compact oblique discs become spongy at these
positions and DNA can usually only be detected by autoradiography and no longer by staining. Staining with lightgreen is positive, i. e. the “puffs” contain protein.
Whereas the chromomere pattern is, to a considerable extent, similar for different organs, the puff pattern changes in
a specific manner from organ to organ and also alters during
metamorphosis. Using radio-actively labelled uridine it can
be shown that RNA is synthesized in these puffs. Possibly
this RNA is related to the messenger-RNA which serves, in a
later phase, as a matrix for the synthesis of protein. It appears
that the size of a “puff’ is proportional to the amount of
RNA synthesized in it. Evidently the puff pattern changing
from organ to organ and from metamorphological phase to
metamorphological phase is an expression of di&rent genetic
activity leading to the synthesis of enzymes typical for an
organ or for a stage in the metamorphosis. This relationship
was demonstrated genetically for a “puff” in the salivary
gland chromosome of Chironomus.
The formation and retrogression of the “puffs” can be
brought about by several means (incubation medium, temperature change, transplantation). The observation that the
injection of the hormone ecdysone into Chironomus larvae
induces the formation of two puffs is of particular interest.
The earliest signs of metamorphosis are visible only several
hours after the puff formation.
One of the effects brought about by ecdysone is sclerotisation
(hardening) of the skin during the transformation of the
larvae into pupae. According to P. Karlson, Munich (Germany), this process is comparable to an o-quinone tannage:
o-quinone arising from tyrosine cross-links the skin protein
by reaction with free amino-groups. Ecdysone so changes the
Tyrosine
decarboxylase
+ Dopa -
j
Dopa-amine
+ N-Acetyldopa-amine
phenoloxydase
--+
o-quinone
protein
-+
“sclerotin”
[ I ] F. Jacob and J. Monod, Cold Spring Harbor Sympos. quantitat. Biol. 26, 193 (1962).
410
tyrosine metabolism of the larvae (through formation of a
decarboxylase and a phenoloxidase) that an o-quinone is
finally produced via dihydroxyphenylalanine (Dopa) and
Dopa-amine.
The phenoloxidase could be crystallised. It is formed (under
the influence of ecdysone) from inactive intermediates contained in the larvae. This activation appears to consist of a
limited hydrolysis and it was shown that ecdysone causes the
formation of the necessary proteolytic enzyme. Together
with the observation that puffs form on the chromosomes
under the influence of ecdysone (cf. paper delivered by Beerman) the following scheme can be given for the action of
ecdysone in sclerotisation.
ecdyson
DNAinactive
-4
”%active
3.
RNA
-+
protein (enzyme)
H . Holtzer, Philadelphia (USA), has found that the spinal
chord of chick embryo induces undifferentiated mesenchyme cells (somites) of the same embryo to develop into
cartilage cells. This applies in vivo (transplantation of spinal
chord into the mesenchyme tissue) as well as in vitro, i. e. in a
tissue culture. If the mesenchyme tissue and spinal chord are
separated in vitro through a fine filter (pore size ca. 40 p),
induction still takes place. Three days pass between the
commencement of the mutual incubation, and the first
appearance of cartilage cells. Presumably this period is required for the synthesis of the enzymes and co-factors necessary for cartilage formation.
A substance which induces the development of somites
(from chick embryo) into cartilage cells was isolated from
the spinal chords of 4 day-old chick embryos by F. Zilliken,
Nijmegen (Holland). The spinal chords were homogenised
with 0.25 M HC104 at 0 “ C ,neutralised and centrifuged. The
supernatant was treated with charcoal which was then eluted
with 10 % pyridine. Further purification was carried out on
Dowex 1-X8 by gradient elution with formic acid and
ammonium formate. The substance purified in this manner is
most stable at pH 5.5-6.0, and is neither found in yeast nor
in tissues which are not able to promote the development of
cartilage cells. It consists of guanosine monophosphate
(GMP), glucose, galactose, mannose, ribose and an unidentified sugar which migrates rapidly on chromatography
(idose?). Furthermore it contains two uronic acids, glucosamine, N-acetylneuraminic acid and a peptide residue
comprised of 15 types of amino-acids, (asp., thr., ser., glu.,
pro., hydroxy-pro., glyc., ala., val., meth., iieu., leu., tyr.,
phe.). Up to the present time, only hypothetical arguments can
be put forward concerning the mutual linkages of these
building units.
Materials which are able to cause induction of fore-heads
(fore-brain, middle-brain and eyes), hind-heads (hind-brain,
auditory capsules and head muscles), or trunks and tails of
salamander embryos were obtained from 9 day-old chick
embryos by H. Tiedemann, Heiligenberg (Germany). The
trunk-and tail-inducing “mesodermal factor” is a protein of
mol. wt. 50000 to 100000. Fore-heads are induced by the
“neural factor” which is a nucleoproteide. Both factors are
inactivated by trypsin or pepsin. In the presence of thioglycollic acid or performic acid, or when the temperature is
raised, both factors behave differently. The mesodermal
factor is quickly inactivated in all cases. Evidently a combination of both factors is mecessary for the induction of the
[VB 583/30 IE]
hind-heads.
Angew. Chem. internat. Edit.I Vol. I (I962) 1 No. 7
Документ
Категория
Без категории
Просмотров
0
Размер файла
264 Кб
Теги
induction, morphogenesis
1/--страниц
Пожаловаться на содержимое документа