마이크로 RNA

[솔롱고]

기억을 담당하는 해마와 시각의 입구인 망막에서 새로운 메커니즘을 발견 [Japan BIO]

(특정의 "작은 RNA"의 기능을 생체뇌에서 처음으로 해명)

JST 과제달성형기초연구의 일환으로서 오사카 Bioscience 연구소의  Hurukawa Takahisa 연구부장과 Sanuki

Rikako 연구원 등은 마이크로 RNA라고 불리는 짧은 RNA의 일종, miR-124a가 뇌와 망막이라고 하는 신경회로의

형성과 신경세포의 생존에 중요한 것을 밝혀냈다.

마이크로 RNA는 18-25개의 핵산으로 이뤄진 작은 RNA로, 아미노산(단백질)으로 번역되지 않고, 기능이 밝혀지지

않았다.

그러나 최근의 연구에 따라 마이크로 RNA에는 다양한 종류가 존재하고, 그 중에는 유전자에 결합해 그 발현을 억제

하는 등 생체에 있어서 중요한 기능을 하는 것을 알게 되었다. 

그 마이크로 RNA의 한가지인 miR-124a는 중추신경계에서 가장 많이 존재하고 있으며, 뇌에서 발현하는 전마이크로

RNA량의 25-48%에나 달한다.

그 때문에, miR-124a의 중추신경계에서의 중요한 기능이 상정(想定)되어져 있었지만, 지금까지 복수의 그룹으로

부터 여러번의 상반되는 결과가 보고 되어 그 기능은 수수께끼인 체였다.

본 연구그룹은 miR-124a의 기능을 생체에서 밝히기 위해서, miR-124a가 기능하지 않는 마우스(miR-124a마우스)를

세계에서 처음으로 만드는 데에 성공했다.

이 miR-124a 결손마우스는 뇌 전체가 작게 뇌의 발달 장해를 일으키고 있었다.

그리고 나서 조사해 보니, "기억"에 중요한 해마의 신경회로 형성의 이상(異常)이 인지되었다.

또 망막에서는 "시력과 색각(色覺)"을 담당하는 신경세포가 죽어 있는 것도 발견했다.

이 이상(異常)은 miR-124a가 없으며, 그 표적이 되고 있는 유전자 Lhx2의 발현이 억제되지 않고, Lhx2가 과승하게

계속해서 기능하는 것이 원인인 것을 규명했다.

이것은 뇌와 망막이라고 하는 중추신경계의 신경회로의 형성과 신경세포의 생존에 마이크로 RNA가 중요한 역할을

담당하는 것을 생체 레벨에서 처음으로 증명한 것이다.

사람에서는 miR-124a의 유전정보를 포함한 염색체 영역의 결손 혹은 중복에 따라 간질과 자폐증 등의 정신 신경

질환이 일어난다고 알려져 있다.

이 연구성과로부터 정신 신경질환의 원인 규명과 신경계의 재생의료에 공헌할 것으로 기대된다.

본 연구성과는 2011년 8월 21일(영국시간)에 영국과학지 "Nature Neuroscience"의 온라인 속보판으로 공개된다.

원문: http://www.jst.go.jp/pr/announce/20110822/index.html
참조: http://www.nature.com/neuro/journal/vaop/ncurrent/abs/nn.2897.html

The TNA world that came before the RNA one  

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The TNA world that came before the RNA one

08 January 2012

The question of how non-living chemicals emerged from the so-called primordial soup to give rise to life on Earth is

one of the most intriguing in science. Finding an answer might not only help explain our origins but could give us

important clues as to that other intriguing question of whether or not we are alone in the universe.

Once it was recognised that DNA is key to the molecular self-replication that underpins life, chemists have sought to

understand the origins of this double-helical molecule in that primordial age. It was quickly assumed that RNA,

a single-stranded nucleic acid, may have been the precursor genetic material to DNA, and the RNA world hypothesis

was born. But what gave rise to RNA? Chemists in the US are starting to home in on another nucleic acid, TNA:

threose nucleic acid.

Primordial life may have used TNA as its genetic material

© Dreamtime

John Chaput and colleagues at Arizona State University in Tempe, explain that TNA is, in some senses, chemically

simpler than RNA and, although it is considered a potentially natural derivative of that molecule, there are hints that it

may have been the primordial precursor. Despite its simplicity it can exchange information with RNA by forming

complementary strands with itself and with RNA as demonstrated by Albert Eschenmoser and colleagues at ETH

Zurich, Switzerland.¹ However, there is one characteristic of TNA that has not been observed to date but

would be essential to allow the progression from a putative TNA world to one based on RNA: the ability to

undergo tertiary folding into novel functional structures that bind ligands and catalyse reactions.

'Threose, which has one fewer carbon atoms than ribose, is simpler than RNA not because it has fewer atoms,

but rather because it can be synthesised from a single starting material,' explains Chaput. It is difficult to design

functional nucleic acid molecules from first principles, he adds, so his group used in vitro selection to evolve a

population of TNA receptors that could bind to an arbitrary target. In the present work, the team used the clotting protein

thrombin as the target. The process was made possible by the development of polymerase enzymes that can

transcribe strands of DNA into their TNA counterparts. The present study builds on a decade's worth of effort to develop

the chemical technology that allows a TNA combinatorial library to be evolved.

TNA can fold into complex shapes and is complementary to RNA

© Nat. Chem.

The researchers have now demonstrated that these selected TNA molecules can fold into complex shapes with

discrete ligand-binding properties.² Fundamentally, the work demonstrates a property of TNA that was not clear

before the team began but was known, of course, in RNA and DNA. 'This provides evidence that TNA could

have served as an ancestral genetic system during an early stage of life,' Chaput tells Chemistry World.

'This is exciting work,' says Steven Benner at the Westheimer Institute of the Foundation for Applied Molecular Evolution,

US. 'We have found that threose, the backbone of Chaput's new DNA, is one of the sugars that (like ribose) is stabilised

by borate minerals, so this makes it also a candidate for a prebiotic carbohydrate.'

David Bradley

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Simpler Times: Did an Earlier Genetic

Molecule Predate DNA and RNA?

ScienceDaily (Jan. 9, 2012) — In the chemistry of the living world, a pair of nucleic acids -- DNA and RNA -- reign

supreme. As carrier molecules of the genetic code, they provide all organisms with a mechanism for faithfully

reproducing themselves as well as generating the myriad proteins vital to living systems.

Yet according to John Chaput, a researcher at the Center for Evolutionary Medicine and Informatics, at Arizona State

University's Biodesign Institute^?, it may not always have been so.

Chaput and other researchers studying the first tentative flickering of life on earth have investigated various alternatives

to familiar genetic molecules. These chemical candidates are attractive to those seeking to unlock the still-elusive

secret of how the first life began, as primitive molecular forms may have more readily emerged during the planet's

prebiotic era.

One approach to identifying molecules that may have acted as genetic precursors to RNA and DNA is to examine other

nucleic acids that differ slightly in their chemical composition, yet still possess critical properties of self-assembly and

replication as well as the ability to fold into shapes useful for biological function.

According to Chaput, one interesting contender for the role of early genetic carrier is a molecule known as TNA, whose

arrival on the primordial scene may have predated its more familiar kin. A nucleic acid similar in form to both DNA and

RNA, TNA differs in the sugar component of its structure, using threose rather than deoxyribose (as in DNA) or ribose

(as in RNA) to compose its backbone.

In an article released online January 9 in the journal Nature Chemistry, Chaput and his group describe the Darwinian

evolution of functional TNA molecules from a large pool of random sequences. This is the first case where such

methods have been applied to molecules other than DNA and RNA, or very close structural analogues thereof.

Chaput says "the most important finding to come from this work is that TNA can fold into complex shapes that can bind

to a desired target with high affinity and specificity." This feature suggests that in the future it may be possible to evolve

TNA enzymes with functions required to sustain early life forms.

Nearly every organism on earth uses DNA to encode chunks of genetic information in genes, which are then copied

into RNA. With the aid of specialized enzymes known as polymerases, RNA assembles amino acids to form essential

proteins. Remarkably, the basic functioning of the genetic code remains the same, whether the organism is a snail or

a senator, pointing to a common ancestor in the DNA-based microbial life already flourishing some 3.5 billion years ago.

Nevertheless, such ancestors were by this time quite complex, leading some scientists to speculate about still earlier

forms of self-replication. Before DNA emerged to play its dominant role as the design blueprint for life, a simpler genetic

world dominated by RNA may have prevailed. The RNA world hypothesis as it's known alleges that ribonucleic acid

(RNA) acted to store genetic information and catalyze chemical reactions much like a protein enzyme, in an epoch

before DNA, RNA and proteins formed the integrated system preval‎ent today throughout the living world.

While the iconic double helix of DNA is formed from two complimentary strands of nucleotides, attached to each other

by base pairing in a helical staircase, RNA is single-stranded. The two nucleic acids DNA and RNA are named for the

type of sugar complex that forms each molecule's sugar-phosphate backbone -- a kind of molecular thread holding

the nucleotide beads together.

Could a simpler, self-replicating molecule have existed as a precursor to RNA, perhaps providing genetic material for

earth's earliest organisms? Chaput's experiments with the nucleic acid TNA provide an attractive case. To begin with,

TNA uses tetrose sugars, named for the four-carbon ring portion of their structure.

They are simpler than the five-carbon pentose sugars found in both DNA and RNA and could assemble more easily in

a prebiotic world, from two identical two-carbon fragments.

This advantage in structural simplicity was originally thought to be an Achilles' heel for TNA, making its binding

behavior incompatible with DNA and RNA. Surprisingly, however, research has now shown that a single strand of TNA

can indeed bind with both DNA and RNA by Watson-Crick base pairing -- a fact of critical importance if TNA truly

existed as a transitional molecule capable of sharing information with more familiar nucleic acids that would eventually

come to dominate life.

In the current study, Chaput and his group use an approach known as molecular evolution to explore TNA's potential as

a genetic biomolecule. Such work draws on the startling realization that fundamental Darwinian properties -- self-

replication, mutation and selection -- can operate on non-living chemicals.

Extending this technique to TNA requires polymerase enzymes that are capable of translating a library of random DNA

sequences into TNA. Once such a pool of TNA strands has been generated, a process of selection must successfully

identify members that can perform a given function, excluding the rest. As a test case, the team hoped to produce

through molecular evolution, a TNA strand capable of acting as a high-specificity, high-affinity binding receptor for the

human protein thrombin.

They first attempted to demonstrate that TNA nucleotides could attach by complementary base pairing to a random

sequence of DNA, forming a hybrid DNA-TNA strand. A DNA polymerase enzyme assisted the process. Many of the

random sequences, however, contained repeated sections of the guanine nucleotide, which had the effect of pausing

the transcription of DNA into TNA. Once random DNA libraries were built excluding guanine, a high yield of DNA-TNA

hybrid strands was produced.

The sequences obtained were 70 nucleotides in length, long enough Chaput says, to permit them to fold into shapes

with defined binding sites. The DNA-TNA hybrids were then incubated with the target molecule thrombin. Sequences

that bound with the target were recovered and amplified through PCR. The DNA portion was removed and used as a t

emplate for further amplification, while the TNA molecules displaying high-affinity, high specificity binding properties

were retained.

Additionally, the binding affinity of the evolved and selected TNA molecules was tested against two other common

proteins, for which they displayed no affinity, strengthening the case that a highly specific binding molecule had

resulted from the group's directed evolution procedure.

Chaput suggests that issues concerning the prebiotic synthesis of ribose sugars and the non-enzymatic replication of

RNA may provide circumstantial evidence of an earlier genetic system more readily produced under primitive earth

conditions. Although solid proof that TNA acted as an RNA precursor in the prebiotic world may be tricky to obtain,

Chaput points to the allure of this molecule as a strong candidate, capable of storing information, undergoing selection

processes and folding into tertiary structures that can perform complex functions. This result provides the motivation

to explore TNA as an early genetic system.

Chaput is optimistic that major questions about the prebiotic synthesis of TNA, its role in the origin and early evolution

of life on earth, and eventual genetic takeover by RNA will, over time, be answered.

Story Source:

The above story is reprinted from materials provided by Arizona State University. The original article was written by

Richard Harth.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

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Journal Reference:

1. Hanyang Yu, Su Zhang, John C. Chaput. Darwinian evolution of an alternative genetic system provides
2. support for TNA as an RNA progenitor. Nature Chemistry, 2012; DOI:  10.1038/nchem.1241