The effects of dihydrotestosterone on amino acid transport in ageing mammalian skeletal muscle fibres

衰老会导致骨骼肌质量和力量逐渐下降，通常被称为骨质疏松症。骨质疏松症主要发生在80岁的人(和2岁的啮齿动物)，其后果包括缺乏运动，更容易摔倒，最终失去独立性。由于英国80岁以上的人口预计将在未来20年内翻一番，从2006年的270万人增加到2031年的540万人，患有骨质疏松症的人口比例以及由此对NHS和整个社会施加的要求预计将相应上升。尽管它在物理和社会经济上具有重要意义，但人们对石棺减少的原因知之甚少。在这项研究中，我们认为它的发生是因为骨骼肌细胞将氨基酸(蛋白质的组成成分)运入和运出细胞的能力下降。我们还认为，这种下降是由于将氨基酸运入和运出细胞的转运体/泵的数量减少，或者是由于激素浓度的减少，例如通常刺激这些泵的男性荷尔蒙。为了检验这些假设，我们计划调查：(1)年龄对氨基酸转运体数量及其将氨基酸运入和运出小鼠骨骼肌细胞的能力的影响。(2)用睾酮的活性代谢物双氢睾酮(DHT)处理肌细胞是否能逆转这些年龄依赖性的改变。(3)最后，我们将测试用DHT治疗560日龄的小鼠4周是否逆转/减缓骨质疏松症。我们还将研究DHT处理是否增加了转运氨基酸和新蛋白质的合成。第一种类型将使用在正常实验室条件下饲养的不同年龄(60-730天)的小鼠。从每个年龄组的EDL和比目鱼肌分离出的小肌纤维束将被用来研究年龄对各种氨基酸转运体表达和生理功能的影响以及这些变化背后的细胞和分子机制。大多数先前的研究已经调查了睾酮处理对骨骼肌质量和力量的影响，还没有研究DHT处理的那些。因此，第二个实验将调查DHT治疗年轻、老年和衰老小鼠至少4周的效果。在第三个实验中，我们将研究DHT对衰老小鼠骨骼肌质量、强度、各种氨基酸转运体的表达和功能、异亮氨酸(Ile)和MeAIB的摄取以及Ile在蛋白质中的掺入的急性影响。这些效应背后的细胞和分子机制也将被研究。大多数实验将在CD1小鼠身上进行，因为它们价格低廉，具有与正常小鼠和人类相似的基因变异，并且从24个月起患有严重的骨质疏松症。这些实验的结果将提供重要的信息，说明氨基酸运输的下降和合成类固醇生物利用度的降低在骨质疏松症中所起的作用。它们还将增加我们对骨骼肌随年龄增长而发生的变化以及这些变化背后的机制的理解。他们还将提供关于DHT治疗是否可以延迟/减缓石棺减少的发展以及其潜在作用机制的信息。这些发现将对未来开发新的治疗石棺减少症的方法具有重要意义。

Ageing leads to a gradual decline in skeletal muscle mass and strength, commonly referred to as sarcopenia. Sarcopenia occurs mostly in people >80yrs (and rodents >2 years old) and its consequences include inactivity, increased susceptibility to falls and eventually to loss of independence. As the number of people over 80yrs in the United Kingdom is expected to double in the next 20 years, from 2.7 million in 2006 to 5.4 million by 2031, the percentage of the population suffering from sarcopenia and the demands this imposes on the NHS and society in general is expected to rise accordingly. Despite its physical and socioeconomic importance, little is known about the causes of sarcopenia. In this study we propose that it occurs because of a decrease in the ability of skeletal muscle cells to transport amino acids (the building blocks of proteins ) into and out of the cells. We also suggest that this decline is due to a reduction in the number of the transporters/pumps that move amino acids into and out of the cells or is due to a decrease in the concentration of hormones such as the male sex hormones that normally stimulate these pumps.To test these suggestions we plan to investigate; (1) the effects of age on the number of amino acid transporters and their ability to move amino acids into and out of mouse skeletal muscle cells. (2) Whether treating muscle cells with dihydrotestosterone (DHT), the active metabolite of testosterone, can reverse these age-dependent changes. (3) Finally we will test whether treating mice >560 days old with DHT for 4 weeks reverses/slows down sarcopenia. We will also investigate whether DHT treatment increases the transport amino acids and the synthesis of new proteins.Three types of experiments will be used in the study. The first type will use mice of different ages (range 60-730 days) kept under normal laboratory conditions. Small muscle fibre bundles isolated from the edl and soleus muscles of each age group will then be used to investigate the effects of age on the expression and physiological function of various amino acid transporters and the cellular and molecular mechanisms underlying these changes.Most previous studies have investigated the effects of testosterone treatment on skeletal muscle mass and strength and no studies have investigated those of DHT treatment. Therefore, the second experiment will investigate the effects of treating young, old and senescent mice with DHT for at least 4 weeks. Small muscle fibre bundles isolated from the edl and soleus of treated and untreated mice will then be used to investigate the effects of DHT treatment on skeletal muscle mass, strength, the expression and function of various amino acid transporters, the uptake of isoleucine (Ile) and MeAIB and the incorporation of Ile into proteins.In the third experiment, the acute effects of DHT treatment on force production, amino acid transport and incorporation into proteins in muscle fibre bundles isolated from the edl and soleus of senescent mice will be investigated. The cellular and molecular mechanisms underlying these effects will also be investigated.Most of these experiments will be performed using CD1 mice because they are inexpensive, have similar genetic variations as normal mouse and human populations and suffer from severe sarcopenia from 24 months onwards.The results from these experiments will provide important information on the role a decline in amino acid transport and reduced bioavailability of anabolic steroids, play in sarcopenia. They will also increase our understanding of the changes that occur in skeletal muscle with ageing and the mechanisms underlying these changes. They will also provide information on whether DHT treatment can delay/slow the development of sarcopenia as well as the mechanisms underlying its effects. These findings will be important in the development of new ways of managing sarcopenia in the future.