Chromogranin A is a cumbersome protein with its biochemical properties, its expression profile in a plethora of cells, its multiple fragments and molecular forms and their presumed biological activities.¹ As the name refers to, the protein belongs to a class of granins, which were first defined as proteins involved in the formation and function of secretory granules.^2-4 The other part of the name (chromo‐) relates to the fact that chromaffin granules were first to be shown to contain granin proteins. Today, the class consists of at least seven proteins, where three proteins have received the most attention: Chromogranin A, chromogranin B and secretogranin II.^{5, 6} Granins in general are acidic proteins; which is a biochemical feature that facilitates an intragranular environment with a low pH (usually around 4‐5).

From biochemistry and cellular structure/function, the proteins have also gained considerable interest in physiology and medicine. While granins at present do not represent a therapeutic target for treatment and intervention in disease processes (vide supra), measurement of the proteins in plasma has been successfully implemented in clinical laboratories supporting cancer diagnostics. Specifically, measurement of chromogranin A in neuroendocrine tumour disease has become a mainstay in the early screening for neuroendocrine neoplasia, and plasma measurement is also being used to monitor treatment effect and, ultimately, surgery.⁷ As with many other cancer biomarkers, the current data for diagnostic specificity and sensitivity are still not satisfactory. For chromogranin A, a particular issue remains as to how to uniformly define the analyte. Is chromogranin A measurement clearly defined, or are different laboratories just reporting measurement of one or another fragment as “chromogranin A” measurement? This issue is by no means trivial, where different methods for measuring the protein and its fragments cannot readily be compared just by using the same name. We recently reported on the diagnostic accuracy of different methods for “chromogranin A” measurement⁸: The results raises considerable concern and alarm. Some methods perform well in diagnosis and disease assessment, whereas others simply do not work. Thus: Some have studied the clinical value of chromogranin A measurement using methods that either are unspecific – or are too specific for a certain fragment of the protein, which are not released by tumour cells. Using a method that can quantitate the total protein product released to circulation, eg a method that reports on the number of translated protein secreted to plasma, seems to be superior to all other methods, which underscores chromogranin A as a highly meaningful protein to be measured in tumour diagnostics.^{9, 10}

The other aspect of chromogranin A in physiology or medicine is the possible biological activity of the protein and/or its processing products.¹¹ Formerly, many of the different fragments stemming from intact chromogranin A have been suggested to possess independent “hormonal” activity mainly involving the cardiovascular system. Most of the effects are proposed to be inhibitory in nature, eg operating as statins. A major issue in terms of classic physiology, however, has been a dire lack of membrane‐bound receptor targets. Thus, while some fragments may infer biological responses in experimental research involving supraphysiological doses of the various fragments, a true hormonal role with both a well‐defined ligand and a specific receptor remains to be established. In this issue of Acta Physiologica, Dr Rocca and colleagues present exciting new data to this end.¹ A particular fragment of chromogranin A, chromogranin A fragment 1‐373, was experimentally tested for its haemodynamic effects using an ex vivo Langendorff rat heart model, while further studies were performed using perfused hearts, H9c2 cardiomyocytes in culture and in silico technology. As control substance, the authors used a C‐terminally truncated form of the fragment (chromogranin A fragment 1‐372 – which, therefore, in its C‐terminus resembles the catestatin fragment – chromogranin A fragment 354‐372). From these well‐established models, the authors are able to report that the chromogranin A fragment 1‐373 (a) induces negative inotropism and vasodilation, and (b) that this effect is neutralized by antibodies specific to the C‐terminus of the fragment. Perhaps more interesting, the authors also show that (c) the fragment binds to an endothelial receptor, neuropilin‐1 (NRP1) receptor; a receptor that is known for binding vascular endothelial growth factor.¹ The authors conclude that their results suggest that chromogranin A fragment 1‐373 is a new cardioregulatory hormone and that removal of the C‐terminal arginyl residue represents a molecular switch for turning “off” its cardioregulatory activity. Thus, the study gives us more than another proposal for a chromogranin A fragment bioactivity and more specifically a potential receptor‐mediated mechanism.

This finding has several interesting perspectives. First and foremost, the prospect of a novel ligand for NPR1 could have an impact on how we are to stimulate new growth of vasculature; that is, a proangiogenic substance. Vascular endothelial growth factor has been tried in several studies for enhancing angiogenesis in vivo in ischemic heart disease: So far without convincing results. Perhaps this chromogranin A fragment could be a new way of stimulating the vasculature in the coronary artery tree and, speculatively, perhaps without stimulating growth of unwanted vasculature in other organs (in particular, in dormant tumour tissue). One important study will be to directly compare the effect of vascular endothelial growth factor to the chromogranin A fragment in terms of cardiac angiogenesis in human disease. While the two molecules share the receptor‐binding site (PGPQLR), the remaining sequence of the chromogranin A fragment may clearly affect receptor binding and the degradation profile in plasma. Whether this fragment is released to circulation also remains to be determined. In extension, the plasma concentrations need to be scrutinized, if we are to truly consider the fragment as a “cardioregulatory hormone” in a classic endocrine context. In the present study, the used doses (optimal dose for effect was 4000 pmol/L) may deviate grossly from normal physiology and thus not represent in vivo conditions. Another important issue to address will be the maturation and degradation of chromogranin A to harbour the 1‐373 fragment. Thrombin cleavage is not likely to be the true biological maturation mechanism, where intracellular cleavage by a protease/convertase specific for dibasic cleavage sites seems more plausible. If thrombin indeed is the responsible protease for the maturation of chromogranin A fragment 1‐373, it would then be a local and extracellular process tightly coupled to ongoing thrombosis.¹² C‐terminal trimming of the fragment in vivo should also be considered, as the fragment may be trimmed immediately upon release or shortly thereafter, thus inactivating the fragment from a hormonal point of view.

With these considerations in mind, it is highly interesting with a chromogranin A fragment involved in angiogenesis. The author group has previously reported chromogranin A production in the human heart,¹³ which, in combination with the present findings, suggests an organ‐specific role of chromogranin A in human heart disease. Whether this fragment is released from the heart to circulation would be interesting to examine, where cardiac chromogranin A release so far has not been possible to establish.¹⁴ Finally, to truly consider the fragment as a novel hormone, we need to develop specific and accurate methods for measuring the fragment in plasma. What are the concentrations in basal state and in stimulated scenarios? This will simply be instrumental for future research on this fascinating fragment.

嗜铬粒蛋白A是一种繁琐的蛋白质，具有生化特性，在大量细胞中的表达特征，其多个片段和分子形式以及其推测的生物学活性。¹顾名思义，该蛋白质属于一类颗粒蛋白，首先被定义为与分泌颗粒的形成和功能有关的蛋白。^2-4名称（chromo-）的另一部分与以下事实有关：首先显示出嗜铬素颗粒含有格兰宁蛋白。如今，该类别至少包含7种蛋白质，其中三种蛋白质受到了最广泛的关注：嗜铬粒蛋白A，嗜铬粒蛋白B和促胰泌素II。^5、6格兰宁通常是酸性蛋白；这是一种生化特征，可促进pH值较低（通常约为4-5）的颗粒内环境。

从生物化学和细胞结构/功能来看，蛋白质还对生理学和医学引起了相当大的兴趣。尽管目前颗粒蛋白尚不代表疾病过程中的治疗和干预治疗目标（参见上文），但血浆蛋白的测量已在支持癌症诊断的临床实验室中成功实施。具体而言，在神经内分泌肿瘤的早期筛查中，嗜铬粒蛋白A的测定已成为早期筛查神经内分泌肿瘤的主要手段，血浆测定也被用于监测治疗效果，并最终监测手术效果。⁷与许多其他癌症生物标志物一样，当前的诊断特异性和敏感性数据仍不令人满意。对于嗜铬粒蛋白A，如何均匀地定义分析物仍然是一个特殊的问题。嗜铬粒蛋白A的测定是否明确定义，或者不同的实验室只是将一个或另一个片段的测定报告为“嗜铬粒蛋白A”的测定？这个问题绝非易事，其中仅通过使用相同的名称就无法轻易比较用于测量蛋白质及其片段的不同方法。我们最近报道了“嗜铬粒蛋白A”测量方法的诊断准确性⁸：结果引起了相当大的关注和警觉。有些方法在诊断和疾病评估中效果很好，而另一些方法根本行不通。因此：有些人研究了嗜铬粒蛋白A测定的临床价值，该方法使用的是非特异性–或对于某些特定的蛋白质片段而言过于特异性，而肿瘤细胞不会释放这种方法。使用一种可以定量释放到循环中的总蛋白产物的方法，例如报告血浆分泌的翻译蛋白数量的方法，似乎优于所有其他方法，这些方法都强调了嗜铬粒蛋白A是一种非常有意义的待测蛋白在肿瘤诊断中。^9、10

嗜铬粒蛋白A在生理学或医学上的另一方面是蛋白质和/或其加工产物可能的生物学活性。¹¹以前，已经提出了许多来自完整嗜铬粒蛋白A的不同片段具有独立的“激素”活性，主要涉及心血管系统。提议大多数效应本质上是抑制性的，例如作为他汀类药物。然而，就经典生理学而言，一个主要问题是极度缺乏膜结合受体靶标。因此，尽管某些片段可能会在涉及超片段的超生理剂量的实验研究中推断出生物学反应，但明确定义的配体和有待确定的特异性受体。在本期《生理学报》中，Rocca博士及其同事为此提供了令人兴奋的新数据。^1个使用离体Langendorff大鼠心脏模型对嗜铬粒蛋白A的特定片段chromogranin A片段1-373进行了血流动力学效应的实验测试，同时使用灌注心脏，H9c2心肌细胞进行了文化和计算机技术的进一步研究。作为对照物质，作者使用了片段的C末端截短形式（嗜铬粒蛋白A片段1–372 –因此，在其C末端类似于catestatin片段–嗜铬粒蛋白A片段354–372）。从这些公认的模型中，作者能够报告嗜铬粒蛋白A片段1–373（a）诱导负性肌力和血管舒张作用，以及（b）该作用被该片段C端特异的抗体所中和。 。也许更有趣，作者还表明（c）该片段与内皮受体Neuropilin-1（NRP1）受体结合；已知与血管内皮生长因子结合的受体。¹作者得出结论，他们的结果表明嗜铬粒蛋白A片段1–373是一种新的心脏调节激素，C末端精氨酰基残基的去除代表了“关闭”其心脏调节活性的分子开关。因此，这项研究为我们提供了关于嗜铬粒蛋白A片段生物活性的另一项建议，更具体地说，它为潜在的受体介导的机制提供了更多建议。

这个发现有几个有趣的观点。首先，NPR1新型配体的前景可能会影响我们如何刺激脉管系统的新生长。即促血管生成物质。血管内皮生长因子已在多项研究中尝试增强缺血性心脏病的体内血管生成：迄今为止，尚无令人信服的结果。嗜铬粒蛋白A片段可能是刺激冠状动脉树中脉管系统的一种新方法，而且可能是不刺激其他器官（尤其是在休眠的肿瘤组织中）不想要的脉管系统生长的新方法。一项重要的研究将是直接比较血管内皮生长因子与嗜铬粒蛋白A片段在人类疾病中的心脏血管生成方面的作用。虽然这两个分子共享受体结合位点（PGPQLR），但嗜铬粒蛋白A片段的其余序列可能会明显影响受体结合和血浆中的降解情况。该片段是否释放到流通中还有待确定。此外，如果我们要真正将片段视为经典内分泌环境中的“心脏调节激素”，则需要仔细检查血浆浓度。在本研究中，所使用的剂量（最有效的剂量为4000 pmol / L）可能与正常的生理状况大相径庭，因此不代表体内条件。要解决的另一个重要问题将是嗜铬粒蛋白A的成熟和降解，以保留1-373片段。凝血酶裂解不可能是真正的生物成熟机制，在这里，通过对二元裂解位点特异的蛋白酶/转化酶进行的细胞内裂解似乎更合理。如果凝血酶确实是嗜铬粒蛋白A片段1-373成熟的负责蛋白酶，那么它将是局部和细胞外过程，与正在进行的血栓形成紧密相关。还应考虑在体内对该片段进行¹² C末端修剪，因为该片段可在释放后立即修剪或在此后不久修剪，从而从激素的角度使该片段失活。

考虑到这些考虑因素，嗜铬粒蛋白A片段参与血管生成非常有趣。作者小组以前曾报道过人心脏中嗜铬粒蛋白A的产生，¹³与目前的发现相结合，表明了嗜铬粒蛋白A在人类心脏病中的器官特异性作用。该片段是否从心脏释放到循环中将是一个有趣的问题，因为迄今为止尚无法确定心脏嗜铬粒蛋白A的释放情况。¹⁴最后，要真正将该片段视为一种新型激素，我们需要开发一种特定且准确的方法来测量血浆中的片段。基础状态和刺激情景中的浓度是多少？这将仅仅是对这个引人入胜的片段进行进一步研究的工具。