-
红细胞生成过程关键步骤确定
一个健康的成年人每天必须生成
1
千亿个新红血细胞，才能维持其血液循环中的红细胞数量。来自洛桑联邦理工学院（
EPFL
）的
一个研究人员小组确定了红细
胞生成过程中一个关键的步骤。这一研究发现可能不仅有助于阐明如贫血等血液疾病的病因，还使得医生
们的梦想离现实更近了一步：在实验室能够制造出红血细胞，由此提供一个潜在的取之不竭的血液
主要成分资源，用于输血。
红细
胞，其本质就是一袋将氧气输送到全身的血红蛋白。其生命起始于骨髓中的造血干细胞，经历一个高度受控的增殖
和分化过
程后，获得其最终的身份。
在这一分化过程中的一个关键步骤就是线粒体自噬（
mitop
hagy
）。随着线粒体耗尽，细胞血红蛋白负载能力达到最大。然而直
到现在，都还没有清楚了解控制线粒体自噬的机制。
在发表在本周《科学》（
Science
）杂志上的一篇论文中，洛桑联邦理工学院的
Isabelle
Barde
及其同事通过试验证实，
KRAB
型
锌指蛋白与
KAP1
辅因
子协同作用，以精细且复杂的方式调节了线粒体自噬。
论文的资深作者、病毒学家
Didier Trono
多年来一直对
KRAB/KAP1
系统感兴趣。
众所周知，其在“沉默”哺乳动物基因组反转
录因子元件中发挥作用，已有
3.5
亿年历史。它们最初是可以整合到感染生物体遗传密码中的逆转录病毒
。“它做着如此好的一份工作，
以致在进化过程中它被指派完成了很多其他的事情，”<
/p>
Trono
说。
KRAB/KAP1
系统承担的职
责之一就是调控线粒体自噬。
研究人员发现，
遗传改造缺失
KAP1
的小鼠迅速变得贫血，因为它们无法生
成红血细胞。更特别的是，他们发现，干细胞分化过程在成红血细胞（
erythr
oblast
，红细胞前体）中线粒体降解的阶段停止。且在人
类血细胞中敲除
KAP1
也会产生相似效应，表明其调控线粒体
自噬的作用在从小鼠到人类的整个进化中是保守的。
研究人员进一步证明，
KRAB/KAP1
系统是通过抑制线粒体自噬阻遏物来发挥功能。换句话说，就像负负得正，它激活了这一靶过
程。这表明，这一调控系统中的各种元件突变有可能导致了如贫血和某些类型白血病等血液疾病，从而反过来 指出了这些疾病的未来治
疗靶点。它还指出了有可能在实验室中模拟红血细胞合成的途径
。
但这些研究发现还具有更广泛
的意义。虽然线粒体对于许多细胞正常功能至关重要，但如果它们生成破坏性自由基（某些情况下
细胞呼吸作用的副产物）对于细胞也会是致命的。这些自由基引起的氧化性应激与肝脏疾病、心脏病和肥
胖有关联。因此，了解线粒体
自噬受控机制，有可能促成更好地了解以及治疗这些疾病。
Trono
认为这一多层次组合调控法则或许适应于广泛的生理系统。“它为自然完成生理活动赋予了极高水平的模块性 。”他将之
比喻为管风琴的运行方式。
每个风琴师都有一个键盘，以及受他掌控的脚踏板。他通过
各种组合应用它们来调整乐器产生的声音。相似的，微调一个或几个
控制元件可以在许多
生物过程中产生显著的影响。尽管其中任何一个元件发生突变都可能导致故障，但由于每个的贡献很小，损害往往
是有限的。反过来，这赋予了系统稳固性。
Trono
相信，这种稳固性是数亿年来进化一直在选择和改进的。（来源：生物通
何嫱）
更多阅读
《科学》发表论文摘要（英文）
A
KRAB/KAP1-miRNA Cascade Regulates Erythropoiesis
Through Stage-Specific Control of
Mitophagy
1.
Isabelle Barde
1
,
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Benjamin
Rauwel
1
,
Ray
Marcel Marin-Florez
2
,
Andrea
Corsinotti
3
,
Elisa Laurenti
1
,<
/p>
*
,
Sonia
Verp
1
,
Sandra
Offner
1
,
Julien
Marquis
1
,
?
,
Adamandia
Kapopoulou
1
,
Jiri Vanicek
4
,
Didier Trono
1
,
?
Science
DOI:
10.1126/science.1232398
?
REPORT
A KRAB/KAP1-miRNA Cascade Regulates
Erythropoiesis Through Stage-Specific Control of
Mitophagy
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Isabelle
Barde
1
,
Benjamin
Rauwel
1
,
Ray
Marcel Marin-Florez
2
,
Andrea
Corsinotti
3
,
Elisa Laurenti
1
,<
/p>
*
,
Sonia
Verp
1
,
Sandra
Offner
1
,
Julien
Marquis
1
,
?
,
Adamandia
Kapopoulou
1
,
Jiri Vanicek
4
,
Didier Trono
1
,
?
+
Author
Affiliations
1.
2.
3.
4.
1
School of Life Sciences and
Frontiers in Genetics Program, Ecole Polytechnique
Fé
dé
rale de Lausanne (EPFL),
1015 Lausanne, Switzerland.
Center for
Integrative Genomics, University of Lausanne, 1015
Lausanne, Switzerland.
Centre for
Genomic Regulation, 08003 Barcelona, Spain.
School of Basic Sciences, EPFL, 1015
Lausanne, Switzerland.
2
3
4
+
Author Notes
?
?
1.
?
*
Present address: Campbell Family Institute for
Cancer Research, Ontario Cancer Institute,
Princess Margaret Cancer Centre, University Health
Network
and Department of Molecular
Genetics, University of Toronto, Toronto, Ontario
M5S 1A8, Canada.
?
?
Present
address: Functional Genomics Core,
Nestlé
Institute of Health Sciences,
EPFL Campus, 1015 Lausanne, Switzerland.
?
?
Corresponding
author. E-mail:
@
ABSTRACT
?
During
hematopoiesis, lineage- and stage-specific
transcription factors work in concert with
chromatin modifiers to direct the differentiation
of all blood cells.
Here, we explored
the role of KRAB-containing zinc finger proteins
(KRAB-ZFPs) and their cofactor KAP1 in this
process. Hematopoietic-restricted deletion of
Kap1 in the mouse resulted in severe
hypoproliferative anemia. Kap1-deleted
erythroblasts failed to induce mitophagy-
associated genes and retained
mitochondria. This was due to
persistent expression of microRNAs targeting
mitophagy transcripts, itself secondary to a lack
of repression by stage-specific
KRAB-
ZFPs. The KRAB/KAP1-miRNA regulatory cascade is
evolutionary conserved, as it also controls
mitophagy during human erythropoiesis. Thus, a
multilayered transcription regulatory
system is present, where protein- and RNA-based
repressors are superimposed in combinatorial
fashion to govern the
timely triggering
of an important differentiation event.
Through the process of erythropoiesis,
about one hundred billion new red cells are
generated every day in the human adult bone
marrow. This process is
initiated by
the differentiation of hematopoietic stem cells
(HSC) into the earliest erythroid progenitor,
which was identified ex vivo as a slowly growing
burst-forming unit-erythroid (BFU-E).
This erythroid progenitor morphs into the rapidly
dividing CFU-E (colony-forming unit-erythroid),
the proliferation of
which is
stimulated by the hypoxia-induced hormone
erythropoietin. Further differentiation occurs
through a highly sophisticated program
orchestrated by
lineage- and stage-
specific combinations of protein- and RNA-based
transcription regulators
(
1
–
3
).
It culminates in the elimination of intracellular
organelles
including mitochondria and
the nucleus to yield the fully mature erythrocyte,
containing on the order of 250 million molecules
of hemoglobin as almost sole
cargo.
Much is still to be learned about the molecular
mechanisms of these events, not only to understand
the cause of red cell disorders, but also to aid
the in
vitro manufacturing of the large
supplies of oxygen-carrying cells for transfusion.
Higher vertebrate genomes encode
hundreds of KRAB-ZFPs that can bind DNA in a
sequence-specific fashion through a C-terminal
array of C2H2 zinc
fingers and recruit
the corepressor KAP1 via their N-terminal KRAB
domain
(
4
–
7
).
KAP1, also known as TRIM28 (tripartite motif
protein 28), TIF1β
(transcription
intermediary factor 1 beta) or KRIP-1 (KRAB-
interacting protein 1), acts as a scaffold for a
multi-molecular complex that silences
transcription
through the formation of
heterochromatin (
8
–
11
). The KRAB/KAP1 system probably
evolved initially to minimize retroelement-induced
genome perturbations
(
12
–
14
), but recent data
indicate that it also regulates multiple aspects
of mammalian physiology (
15
–
24
). The present study was
undertaken to explore its role
in
hematopoiesis.
The hemato-specific
knockout of Kap1 in the mouse, whereby the
hematopoietic system of otherwise wild type
animals is reconstituted from Kap1-deleted
hematopoietic stem cells and
progenitors (fig. S1), resulted in a series of
hematological abnormalities (table S1). Mutant
mice displayed fatal
hypo-regenerative
anemia, characterized by the accumulation of
transferrin receptor/CD71+ glycophorin-A-
associated/Ter119- early erythroblasts and an
almost complete absence of mature
CD71-Ter119+ cells in the bone marrow
(
Fig. 1A
). Electron
microscopy and Mitotracker staining revealed that
KO
erythroblasts contain more
mitochondria than their wild type counterparts
(
Fig. 1B
), correlating with
decreased expression of mitophagy genes such as
Nix/Bnip3L, Ulk1, GABARAPl2, Sh3glb1,
Atg12, Becn1 and Bcl2l1 (
Fig.
2A
). Since the KRAB/KAP1 pathway is
mostly known to induce transcriptional
repression (
10
,
11
), it seemed likely that
this effect was indirect. An examination of the
miRNA expression profile of control and Kap1 KO
CD71+Ter119+
cells revealed that, among
455 miRNAs tested, 5 were downregulated and 11
upregulated more than two-fold in KO cells (data
are presented in the Gene
Expression
Omnibus dataset GSE44061). A recently described in
silico approach (
25
,
< br>26
) suggested that six of these
upregulated miRNAs had
mitophagy-
associated deregulated transcripts as their
targets, notably miR-351, predicted to act on
Bnip3L (
Fig. 2A
). Consistent
with this hypothesis, levels of
miR-351
abruptly dropped in CD71+Ter119+ cells, compared
to their CD71+Ter119- precursors, mirroring Bnip3L
induction (
Fig. 2B
).
Furthermore,
transduction of mouse
erythroleukemia (MEL) cells with a
GFP-
expressing lentiviral vector
harboring, 3′ of GFP, the Bnip3L 3′UTR sequence
predicted to be
targeted by miR-351
resulted in miR-351-dependent downregulation of
the reporter (
Fig. 2C
).
Finally, similar to their KAP1-depleted
counterparts,
miR-351-overexpressing
MEL cells were blocked in differentiation and
accumulated mitochondria, and this phenotype was
reversed by expression of a
Bnip3L
transcript devoid of this 3′UTR sequence (fig.
S2).