Division of Clinical Chemistry, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden.
Bone. 2013 Apr;53(2):399-408. doi: 10.1016/j.bone.2012.12.048. Epub 2013 Jan 10.
Mouse serum alkaline phosphatase (ALP) is frequently measured and interpreted in mammalian bone research. However, little is known about the circulating ALPs in mice and their relation to human ALP isozymes and isoforms. Mouse ALP was extracted from liver, kidney, intestine, and bone from vertebra, femur and calvaria tissues. Serum from mixed strains of wild-type (WT) mice and from individual ALP knockout strains were investigated, i.e., Alpl(-/-) (a.k.a. Akp2 encoding tissue-nonspecific ALP or TNALP), Akp3(-/-) (encoding duodenum-specific intestinal ALP or dIALP), and Alpi(-/-) (a.k.a. Akp6 encoding global intestinal ALP or gIALP). The ALP isozymes and isoforms were identified by various techniques and quantified by high-performance liquid chromatography. Results from the WT and knockout mouse models revealed identical bone-specific ALP isoforms (B/I, B1, and B2) as found in human serum, but in addition mouse serum contains the B1x isoform only detected earlier in patients with chronic kidney disease and in human bone tissue. The two murine intestinal isozymes, dIALP and gIALP, were also identified in mouse serum. All four bone-specific ALP isoforms (B/I, B1x, B1, and B2) were identified in mouse bones, in good correspondence with those found in human bones. All mouse tissues, except liver and colon, contained significant ALP activities. This is a notable difference as human liver contains vast amounts of ALP. Histochemical staining, Northern and Western blot analyses confirmed undetectable ALP expression in liver tissue. ALP activity staining showed some positive staining in the bile canaliculi for BALB/c and FVB/N WT mice, but not in C57Bl/6 and ICR mice. Taken together, while the main source of ALP in human serum originates from bone and liver, and a small fraction from intestine (<5%), mouse serum consists mostly of bone ALP, including all four isoforms, B/I, B1x, B1, and B2, and two intestinal ALP isozymes dIALP and gIALP. We suggest that the genetic nomenclature for the Alpl gene in mice (i.e., ALP liver) should be reconsidered since murine liver has undetectable amounts of ALP activity. These findings should pave the way for the development of user-friendly assays measuring circulating bone-specific ALP in mouse models used in bone and mineral research.
鼠血清碱性磷酸酶（ALP）在哺乳动物骨研究中经常被测量和解释。然而，人们对小鼠循环中的 ALP 知之甚少，并且它们与人类 ALP 同工酶和同工型的关系也知之甚少。从肝脏、肾脏、肠道和椎骨、股骨和颅骨组织中提取了鼠 ALP。研究了来自混合野生型（WT）小鼠血清和单个 ALP 敲除株的血清，即 Alpl（-/-）（也称为编码组织非特异性 ALP 或 TNALP 的 Akp2）、Akp3（-/-）（编码十二指肠特异性肠 ALP 或 dIALP）和 Alpi（-/-）（也称为编码全球肠 ALP 或 gIALP 的 Akp6）。通过各种技术鉴定 ALP 同工酶和同工型，并通过高效液相色谱法进行定量。WT 和敲除小鼠模型的结果显示与人类血清中发现的相同的骨特异性 ALP 同工型（B/I、B1 和 B2），但除了 B1x 同工型外，鼠血清还含有仅在慢性肾病患者和人类骨组织中更早检测到的 B1x 同工型。还在鼠血清中鉴定出两种鼠肠同工酶，dIALP 和 gIALP。在鼠骨中鉴定出所有四种骨特异性 ALP 同工型（B/I、B1x、B1 和 B2），与在人骨中发现的相一致。除了肝脏和结肠外，所有鼠组织均含有大量的 ALP 活性。这是一个显著的差异，因为人类肝脏含有大量的 ALP。组织化学染色、Northern 和 Western blot 分析证实肝组织中无法检测到 ALP 表达。ALP 活性染色显示 BALB/c 和 FVB/N WT 小鼠的胆小管有一些阳性染色，但 C57Bl/6 和 ICR 小鼠没有。总之，虽然人类血清中 ALP 的主要来源是骨和肝，一小部分来自肠（<5%），但鼠血清主要由骨 ALP 组成，包括所有四种同工型 B/I、B1x、B1 和 B2，以及两种肠 ALP 同工型 dIALP 和 gIALP。我们建议重新考虑小鼠中 Alpl 基因（即 ALP 肝）的遗传命名，因为鼠肝中无法检测到 ALP 活性。这些发现将为在用于骨和矿物质研究的小鼠模型中开发用户友好的循环骨特异性 ALP 测定方法铺平道路。
