What Makes Each Brain Unique


By Fred H. Gage /Alysson R. Muotri F. H. ゲージ /A. R. モートリ
English 日本語 日本語
Your brain is special. So is mine. Differences arise at every level od the organ’s astonishingly intricate architecture; the human brain contains 100 billion neurons, which come in thousands of types and collectively form an estimate of more than 100 trillion interconnections. These differences, in turn, lead to variances in the ways we think, learn and behave and in our propensity for mental illness.  あなたの脳は世界に1つしかない独自のもので,私の脳もそうだ。脳の個人差は,驚くほど複雑なその構造のあらゆるところで生まれている。ヒトの脳には数千種類の神経細胞が1000億個以上ある。そしてそれらが何百兆ともいわれる配線でつながって緻密なネットワークを形成している。これらの違いは,私たちの考え方や学習能力,行動パターンのほか,精神疾患のかかりやすさの差異につながっている。
How does diversity in brain wiring and function arise? Variations in the genes we inherit from our parents can play a role. Yet even identical twins raised by the same parents can differ markedly in their mental functioning, behavioral traits, and risk of mental illness or neurodegenerative disease. In fact, mice bred to be genetically identical that are then handled in exactly the same way in the laboratory display differences in learning ability, fear avoidance and responses to stress even when age, gender and care are held constant. Something more has to be going on.  脳の神経回路や機能の差異はどのように生まれるのだろうか? 両親から受け継いだ遺伝子の違いが一因であるのは確かだ。しかし,同じ両親に育てられた一卵性双生児でも,精神機能や行動特性,精神疾患や神経疾患の発症リスクがまるで違うことがある。実際,同じ遺伝系統のマウスを研究室で全く同じように育てても,性別と年齢,生育環境が同じマウスの間で,学習能力や恐怖に対する反応,ストレス応答性に違いが生じる。遺伝子のほかにも何かが働いているはずだ。
Certainly the experiences we have in life matter as well; they can, for instance, influence the strength of the connections between particular sets of neurons. But researchers are increasingly finding tantalizing indications that other factors are at work—for instance, processes that mutate genes or affect gene behavior early in an embryo’s development or later in life. Such phenomena include alternative splicing, in which a single gene can give rise to two or more different proteins. Proteins carry out most of the operations in cells, and thus which proteins are made in cells will affect the functioning of the tissues those cells compose. Many researchers are also exploring the role of epigenetic changes—DNA modifications that alter gene activity (increasing or decreasing the synthesis of specific proteins) without changing the information in genes.  もちろん,私たちの日々の経験も関係していて,例えば特定の神経細胞間の結合強度に影響しうる。しかし様々な研究から,ほかの因子の関与を示唆する興味深い証拠が増えてきた。例えば胎生期や生まれた後に遺伝子が変異したり発現の仕方が変化したりすることによる影響だ。
In the past few years the two of us and our colleagues have come on especially intriguing suspects that seem to operate more in the brain than in other tissues: jumping genes. Such genes, which have been found in virtually all species, including humans, can paste copies of themselves into other parts of the genome (the full set of DNA in the nucleus) and alter the functioning of the affected cell, making it behave differently from an otherwise identical cell right next to it. Many such insertions in many different cells would be expected to yield subtle or not so subtle differences in cognitive abilities, personality traits and susceptibility to neurological problems.  ここ数年で私たちは,実に驚くべきことが脳の中で他の組織よりも頻繁に起きているのではないかと考えるようになった。“ジャンプする遺伝子”だ。こうした遺伝子はヒトを含むほぼすべての生物種で見つかっており,自分自身を複製してゲノム(核内の全DNA)の別の場所に挿入,つまり“コピー・アンド・ペースト”する。この結果,その細胞は機能が変化し,隣の同じ細胞とは異なる振る舞いをするようになる。様々な細胞でこうした遺伝子の挿入が頻繁に起これば,認知能力や性格,神経疾患のかかりやすさに,多かれ少なかれ違いが生じてくるだろう。
Our early findings of gene jumping in the brain have led us to another question: Given that the brain’s proper functioning is essential to survival, why has evolution allowed a process that tinkers with its genetic programming to persist? Although we still do not have a definite answer, mounting evidence suggests that by inducing variability in brain cells, jumping genes may imbue organisms with the flexibility to adapt quickly to changing circumstances. Therefore, these jumping genes—or mobile elements, as they are called—may have been retained evolutionarily because, from the standpoint of promoting survival of the species, this adaptation benefit outweighs the risks.  また,脳におけるジャンピング遺伝子についての私たちの初期の発見から,別の疑問が浮かび上がってきた。脳が適切に機能することが生存には不可欠なのに,遺伝プログラムを下手にいじくるような仕組みがなぜ進化の過程で残りえたのか?