Introduction
Adenine nucleotides are interconverted by phosphotransferase enzymes called adenylate kinases (AKs). The AKs play a crucial role in maintaining cellular energy homeostasis and high levels of ATP through a combination of AK activity and mitochondrial oxidative phosphorylation [Reference Dzeja, Vitkevicius and Redfield1]. Adenylate kinase 5 (AK5) gene expression was initially found in brain tissue [Reference Van Rompay, Johansson and Karlsson2]. Research accumulated over time indicates that in addition to the brain, AK5 also appears in endometrial tissues, cardiac tissue, Wharton’s jelly cells, pancreatic beta cells, etc [Reference Angelucci, Marchisio and Di Giuseppe3–Reference Stanojevic, Habener and Holz6]. Furthermore, according to data from The Human Protein Atlas (www.proteinatlas.org), which employs stringent evaluation methods including immunohistochemical staining patterns and ribonucleic acid sequencing (RNA-seq) data analysis, AK5 is also expressed prominently in kidney, testis, breast, stomach, duodenum, and small intestine, with particularly high expression observed in the neurons of the hippocampus and cerebral cortex [Reference Uhlén, Fagerberg and Hallström7].
Van Rompay et al. [Reference Van Rompay, Johansson and Karlsson2] found a novel human AK complementary deoxyribonucleic acid (cDNA), designated AK5, by searching the Expressed Sequence Tag (EST) database. This protein has a glycine-rich region that can bind nucleoside triphosphates and a distinct site that can bind nucleoside monophosphates. Additionally, it possesses a lid domain that covers the substrate when it binds [Reference Van Rompay, Johansson and Karlsson2] The AK5 gene is 278 kb long and has 14 exons. It has three variants that are different in how start codons are used, and the full-length protein has 562 amino acids. The two catalytic domains are known as AK5p1 and AK5p2, and they both contain a glycine-rich p-loop, a nucleoside monophosphate (NMP)-binding domain, and a LID domain. One of the variants encodes a 536 amino acid protein with both AK domains, but it has a shorter N-terminus than full-length AK5. A third transcript encodes a deduced 198-amino acid protein that only contains AK5p2 and is identical to the transcript described earlier [Reference Solaroli, Panayiotou and Johansson8].
Along with the other AKs, AK5’s main job is to maintain the homeostasis of nucleotide metabolism and energy metabolism. Recent research has uncovered its involvement in various cellular processes, including the differentiation of cardiac cells, the stemness of Wharton’s jelly cells, and the inhibition of proteasome activity in mantle cell lymphoma [Reference Angelucci, Marchisio and Di Giuseppe3,Reference Dzeja, Chung and Faustino4,Reference Li, Li and Jiang9,Reference Weinkauf, Zimmermann and Hartmann10]. However, the significance of AK5 extends far beyond these functions encompassing a wide range of pathophysiological conditions, including autoimmune encephalitis, epilepsy, neurodegenerative diseases, diabetes, lower extremity arterial disease, celiac disease (CD), and several cancers (Table 1). Given this broad spectrum of involvement, understanding the role of AK5 in various diseases has emerged as an area of intense research interest. In this review, we aim to provide a comprehensive summary of studies investigating the role of the AK5 since its discovery. Furthermore, we highlight the significance of AK5 in disease pathogenesis and its implications for developing novel diagnostic and therapeutic strategies. Through this review, we seek to elucidate the potential of AK5 as a therapeutic target and prognostic indicator and we aim to underscore the importance of further research into AK5 and its multifaceted roles in health and disease.
Table 1. The role of AK5 in different pathophysiological conditions
Role in cancers
Breast cancer
In most nations around the world, breast cancer is the most frequently diagnosed cancer in women, accounting for one-fourth of all cancer diagnoses in females and it also ranks first among factors that lead to the death of women from cancer [Reference Ferlay, Soerjomataram and Dikshit29,Reference Servick30]. The landscape of cancer genome methylation has been examined in a variety of tumor types, with microarrays serving as the primary analytical tool to date. DNA methylation has been the most thoroughly characterized [Reference Ciriello, Cerami and Sander31,Reference Saghafinia, Mina and Riggi32].
It has long been understood that CpG islands, which are typically unmethylated, can become methylated in cancer, which causes gene repression [Reference Herman, Graff and Myöhänen33]. CpG islands in the 5’ upstream promoter region of the AK5 gene were found to be aberrantly methylated in primary breast cancer samples, and in MCF-7, and MDA-MB-231 breast cancer cell lines (Fig. 1a). It was also found to be expressed in normal human mammary epithelial cells and adjacent non-cancerous tissues from a breast cancer patient in stage 1 and stage 2. Twenty genes were identified to be aberrantly methylated in human breast cancer cells [Reference Miyamoto, Fukutomi and Akashi-Tanaka20]. Based on this, Miyamoto et al. proposed that AK5 might serve as a promising biomarker for breast cancer. However, data from The Human Protein Atlas (www.proteinatlas.org), suggest AK5 does not act as a prognostic indicator in breast cancer. This data is based on the antibody staining of more than 1000 human breast cancer samples [Reference Pontén, Jirström and Uhlen34,Reference Uhlen, Zhang and Lee35]. Interestingly, AK2 was found to be highly expressed in breast cancer tissues, with its overexpression associated with increased malignancy [Reference Wang, Zhao and Wang36]. The Human Protein Atlas data also support AK2 as a prognostic factor in breast cancer, where high expression is linked to a favorable outcome [Reference Pontén, Jirström and Uhlen34,Reference Uhlen, Zhang and Lee35]. Therefore, AK2 may be more relevant as a therapeutic target for breast cancer than AK5.
Figure 1. ( a ) in breast cancer patients, the promoter region AK5 gene was found aberrantly methylated. ( b ) AK5 knockdown using small interfering ribonucleic acid (siRNA) in human gastric cancer cell lines drastically inhibits cell proliferation and autophagy and promotes apoptosis at the molecular level.
Gastric cancer
In terms of cancer-related mortality and incidence globally, gastric cancer (GC) comes in third and fifth positions, respectively [Reference Bray, Ferlay and Soerjomataram37]. The disease is frequently diagnosed at an advanced stage due to the late onset of clinical symptoms, which limits the available therapeutic approaches in more than 50% of cases [Reference Ferlay, Soerjomataram and Dikshit29].
In GC patients, a high level of AK5 protein was suggested as an indicator of a significantly unfavorable prognostic factor and is associated with T-stage and N-stage cancer rather than with metastasis and differentiation. AK5 knockdown using AK5 siRNA in human GC cell lines AZ521 and MKN74 drastically inhibited proliferation and autophagy and promoted apoptosis. AK5 is, therefore, a potential oncogene, prognostic marker, and therapeutic target for GC (Fig. 1b) [Reference Zhang, Wang and Fan24].
Colorectal carcinoma (CRC)
CRC is a disorder that only affects the colon or rectum and is brought on by the colon’s abnormally high rate of glandular epithelial cell proliferation [Reference Servick30]. In 2023, CRC accounted for 10% of all cancer-related cases, being the second leading cause of cancer-related deaths worldwide [38]. For patients with metastatic lesions, the prognosis for CRC has never been satisfactory [Reference Xie, Chen and Fang39].
Comparing the AK5 gene in CRC patients’ samples to nearby normal tissue and controls revealed significant hypermethylation in tumor DNA. Real-time PCR analysis further indicates that the relationship between AK5 DNA methylation and expression in CRC samples is inverse. In CRC cell lines, the AK5 promoter commonly undergoes hypermethylation, which causes methylation-mediated gene silencing and AK5 gene expression inhibits cell migration and invasion via modulating the adenosine monophosphate-activated protein kinase/mammalian target of rapamycin (AMPK/mTOR) signaling pathway, which strongly suggests its possible association with metastasis (Fig. 2) [Reference Ahn, Chae and Lee23].
Figure 2. AK5 gene promoter region in colorectal carcinoma cell lines is hypermethylated which inhibits the expression of the AK5 gene. AK5 gene expression inhibits cell migration and cell invasion.
Prostate cancer
Prostate cancer (PCa) affects millions of men worldwide and is a complex heterogeneous disease. The progression of PCa is strongly associated with the accumulation of mutations leading to genome instability over a patient’s lifetime [Reference Hawley, Zhou and Arlidge40,Reference Rebello, Oing and Knudsen41]. The multidisciplinary field of research into prostate cancer is very active and now includes computational biology in addition to laboratory and clinical science [Reference Rebello, Oing and Knudsen41].
A recent study on prostate cancer (PCa) using bioinformatics analysis based on high-throughput next-generation sequencing technology proposed a novel risk model: “a three-gene signature model (KCNK3, AK5, and ARHGEF38)” strongly associated with cancer-related pathways, overall survival (OS), and tumor microenvironment-related immune cells in PCa. A reduced chance of survival was linked to high expression of ARHGEF38, whereas a higher chance of survival was linked to high expression of KCNK3 and AK5 (Fig. 3). Gene expression alterations in the model have clinical importance since they are strikingly related to PCa patients’ OS [Reference Liang, Wang and Pei27].
Figure 3. High AK5 gene expression is associated with better survival probability of prostate cancer patients.
Colon adenocarcinoma
The most frequent malignant tumor of the digestive tract is colon adenocarcinoma (COAD), whose etiology is currently unclear [Reference Chang, Gen and Yeo42,Reference Lian, Xia and Chen43].
In recent research that used both patient samples and cell lines, the role of AK5 in the onset and progression of COAD was examined. It was found that low AK5 expression can serve as a standalone prognostic biomarker in clinical diagnosis and that low AK5 protein also promoted proliferation and metastasis by controlling the tumor-suppressor proteins p16 and p21, which are involved in the cell-cycle pathway (Fig. 4). These findings open new perspectives on COAD and could have a significant impact on the creation of novel COAD therapeutics and clinical diagnostic approaches [Reference Zhang, Li and Liu22].
Figure 4. In colon adenocarcinoma, higher AK5 protein expression is found in adjacent non-cancerous tissue compared to cancerous tissue. Low AK5 expression promotes proliferation and metastasis.
Role in brain disorders
Alzheimer
Alzheimer’s disease (AD) is the most prevalent form of dementia and is characterized by neuritic plaques and neurofibrillary tangles because of amyloid-beta peptide (Aβ) buildup in the medial temporal lobe and neocortical structures, the area of the brain most affected [Reference De-Paula, Radanovic and Diniz44]. All eukaryotic cells express the 14-3-3 protein family of regulatory molecules, but they are particularly abundant in the cytoplasm of neurons in cerebral tissue [Reference Huang and Marie45]. There are numerous brain functions and various brain disorders that are closely associated with 14-3-3 proteins, most of which are expressed in the brain [Reference Cho and Park46]. Evidence suggests that these proteins are involved in brain development, memory formation, and several neurological disorders [Reference Mackintosh47]. These proteins are associated with tau deposition in neurofibrillary tangles [Reference Qureshi, Li and MacDonald48]. In AD, there is an abnormal increase in the phosphorylation of the tau protein [Reference Gong and Iqbal49]. In a study, it was discovered that a specific isoform of 14.3.3 proteins, namely 14-3-3 ζ, actively facilitated the glycogen synthase kinase-3 beta-dependent phosphorylation of tau [Reference Agarwal-Mawal, Qureshi and Cafferty50].
In vivo proteomic analysis of brain tissue from transgenic mice using tandem affinity purification and LC-MS method identified almost 40 novel 14-3-3ζ binding proteins including AK5, which had consensus sequences for 14-3-3 binding. Further, co-immunoprecipitation studies confirmed the association of 14-3-3ζ with AK5 (Fig. 5b). AK5 interacts directly in the brain with the 14.3.3 ζ complex [Reference Angrand, Segura and Völkel11]. In humans, whole-transcript expression assays found deregulated expression of genes involved in purine metabolism in AD samples compared to controls. Further, transcriptome analysis of these genes found differential expression of messenger ribonucleic acid (mRNAs) with regional variations. AD stages V-VI, AK5 mRNA was found to be differentially downregulated in the frontal cortex (Fig. 5a) [Reference Ansoleaga, Jové and Schlüter12].
Figure 5. ( a and b ) AK5 association with Alzheimer: ( a ) AK5 gene showing downregulation in the frontal cortex; ( b ) AK5 protein co-immunoprecipitated with 14-3-3ζ complex. ( c ) AK5 association with Parkinson: AK5 gene showing upregulation at transcriptional level in the frontal cortex area eight of the brain.
Parkinson
Parkinson’s disease (PD) is a progressive neurodegenerative condition marked by tremors, rigidity, postural instability, and bradykinesia in motor function. The fastest-growing neurological disease in terms of deaths, years lived with a disability, and prevalence is the second most prevalent neurodegenerative condition after AD [Reference Bandres-Ciga, Diez-Fairen and Kim51]. A similar study like Alzheimer’s was performed on human cases of PD brains, which explored alterations in gene expression at mRNA level, and encoding enzymes involved in purine metabolism, finding upregulation of AK5 mRNA in the frontal cortex area 8 at stages 5–6 (Fig. 5c) [Reference Garcia-Esparcia, Hernández-Ortega and Ansoleaga26].
Autoimmune limbic encephalitis
Autoimmune limbic encephalitis (ALE) is a syndrome of rapidly progressive neurological disease characterized by psychiatric disturbances, memory deficits, and seizure activity caused by antibodies that target antigens of the central nervous system either intracellularly or on the cellular surface [Reference Ng, Kramer and Centurion18,Reference Graus, Saiz and Dalmau52]. Initially considered paraneoplastic, the condition was later found, with the discovery of disease-causing antibodies, to be non-paraneoplastic in many cases [Reference Budhram, Leung and Nicolle53].
In severe non-paraneoplastic ALE refractory to immunotherapy, adenylate kinase 5 (AK5) immunoglobulin G (IgG) antibodies were found either in the serum or cerebrospinal fluid (CSF) targeting neuronal cytosolic AK5 protein. IgG is considered a biomarker of the disease (Fig. 6) [Reference Bataller, Kleopa and Wu14–Reference Vicino, Loser and Salvioni Chiabotti19]. As of 2023 more than 30 cases of anti-AK5 encephalitis have been recognized and they were administrated with immunotherapeutic agents [Reference Guillaume, Saguin and Peroux54–Reference McKeon-Makki, McKeon and Yang56]. The specific etiology of anti-AK5 encephalitis is yet to be determined, and the precise pathogenic mechanism leading to its development remains ambiguous. Presently, there are no available AK5-blocking antibodies or compounds for therapeutic intervention.
Figure 6. Anti-AK5 IgG antibodies present in cerebrospinal fluid targeting the neuronal cytosolic target, AK5 protein, in the brain limbic area of autoimmune limbic encephalitis.
Temporal lobe epilepsy
Among epilepsy, TLE is the most prevalent type, which is frequently resistant to anti-epileptic drugs (AEDs) [Reference Moshé, Perucca and Ryvlin57]. Chronic spontaneous recurrent seizures are the most common hallmarks of TLE, and epileptic patients show frequent symptoms of memory loss, psychiatric disorders, and behavioral problems [Reference Moshé, Perucca and Ryvlin57,Reference Berg58].
AK5 protein considerable downregulation was reported in temporal lobe epilepsy patients and in epileptic rat models (Fig. 7a). Co-immunoprecipitation analysis confirmed the interaction of AK5 protein with a brain-specific protein, calcium-dependent phospholipid-binding protein (CPNE6), and this AK5-CPNE6 complex could play an important role in controlling epileptic seizures and could be involved in the development of refractory epilepsy (Fig. 7b) [Reference Lai, Hu and Chen28]. Studies on CPNE6 provide evidence of its link to brain development, memory formation, long-term potentiation and its upregulation is closely related to axonal growth and epilepsy [Reference Nakayama, Yaoi and Kuwajima59–Reference Zhu, Zha and Long62].
Figure 7. Association of AK5 in temporal lobe epilepsy. ( a ) AK5 protein is downregulated in the temporal lobe area of temporal lobe epilepsy patients and epileptic rat model. ( b ) AK5 protein co-immunoprecipitated with brain specific protein, calcium-dependent phospholipid-binding protein (CPNE6) complex.
Possible association with type 2 diabetes
In many tissues, ATP-sensitive potassium (K-ATP) channels – so named because intracellular ATP inhibits them – play important physiological roles. These channels control glucose-dependent insulin secretion in pancreatic cells and are the target of sulfonylurea medications used to treat type 2 diabetes [Reference Ashcroft63]. K-ATP channels are membranous proteins found in many organs and play important roles in linking metabolism with the electrical activity of the cell [Reference Ashcroft and Rorsman64].
In the substantia nigra, cerebral cortex, hippocampus, basal ganglia, and thalamic nucleus, K-ATP channels are highly expressed. These channels can be found in presynaptic membranes, axons, and cell bodies [Reference Moriguchi, Ishizuka and Yabuki65]. The K-ATP channels are hetero-octamer protein complexes having four Kir6.2 subunits, which make pores and four regulatory sulphonylurea receptor subunits in the islet cells [Reference Aguilar-Bryan, Nichols and Wechsler66,Reference Nichols67].
Food intake leads to an increase in glucose metabolism which inhibits β-cell K-ATP channel activity due to rise in intracellular adenosine triphosphate/adenosine diphosphate [ATP]/[ADP] ratio, leading to insulin secretion by affecting the membrane electrical activity [Reference Nichols67]. Patch clamp study on AK1 gene knockout mice found reduction in β-cells K-ATP channel activity though the activity was not completely suppressed. Increase in the concentration of ADP in pancreatic beta cells favors insulin secretion and decreases K-ATP channel activity. When AMP concentration increases, AK1 catalyzes the formation of ADP in presence of ATP and thereby reduces K-ATP channel activity [Reference Schulze, Düfer and Wieringa68]. Two cytosolic isoforms of AK, AK1, and AK5 are expressed in human islets and INS-1 cells (mouse islet), and AK1 was immunoprecipitated with the Kir6.2 subunit of K-ATP channel [Reference Stanojevic, Habener and Holz6]. We propose that these studies raised the possibility that the other cytosolic isoform, AK5, might be involved in compensating the role of cytosolic AK1 in AK1 gene knockout mice by regulating ATP/ADP ratio and K-ATP channel activity (Fig. 8).
Figure 8. K-ATP channel activity is reduced in pancreatic β-cell cells of AK1 gene knockout mice. ADP production is decreased in the absence of the AK1 enzyme, and ADP is crucial for improving K-ATP channel performance. A similar experiment using mice lacking the AK5 gene to study its role in K-ATP channel activity remains to be done.
Role in other disorders
Lower extremity arterial disease (LEAD)
In atherosclerosis, the wall of the artery builds lesions which often lead to narrowing due to the accumulation of atheromatous plaque [69,Reference Tsukahara, Tsukahara and Haniu70]. The most common appearance of atherosclerosis is peripheral arterial disease (PAD) which is a chronic inflammatory process in which formation of atheromatous plaques in arteries is accelerated [Reference Hamburg and Creager71,Reference Norgren, Hiatt and Dormandy72].
LEAD is one of the characteristics of PAD, in which chronic degeneration occurs due to the check-in blood flow caused by stenosis or occlusion of lower limb vessels [Reference Aboyans, Ricco and Bartelink73]. In patients with LEAD, high-throughput sequencing was used to study microRNAome and transcriptome of peripheral blood mononuclear cells (PBMCs), finding dysregulation of 26 microRNA and 14 genes (AK5 gene was downregulated), that can be used as novel biomarkers of LEAD in new diagnostic and therapeutic approaches [Reference Bogucka-Kocka, Zalewski and Ruszel25].
Celiac disease
CD is a multi-factorial long-term auto-inflammatory disorder, mainly affecting the small intestine where individuals have permanent intolerance to gluten, present in foods such as wheat, rye, and barley and it occurs in both sporadic and familial forms [Reference Al-Aama, Shaik and Banaganapalli21,Reference Lindfors, Ciacci and Kurppa74,Reference Di Sabatino and Corazza75]. Next-generation sequencing of exome of a Saudi Arabia consanguineous family with CD found a rare 3 bp ATT insertional mutation in AK5 gene and showed the autosomal recessive mode of inheritance, which suggests AK5 gene probably playing a role in this disease [Reference Al-Aama, Shaik and Banaganapalli21].
Asthma
Asthma (AS) is a heterogeneous clinical syndrome that is defined as a chronic inflammatory disease of the respiratory airways. Airway hyper-responsiveness, or an exaggerated airway-narrowing response to certain triggers like viruses, allergens, and exercise, is linked to chronic inflammation and causes recurrent episodes of wheezing, breathlessness, chest tightness, and/or coughing that can change over time and in intensity [Reference Quirt, Hildebrand and Mazza76]. The most prevalent leukocytes among inflammatory cells in alveoli, distal airspaces, and conducting airways are macrophages [Reference Arjomandi, Witten and Abbritti77–Reference Leung, Wong and Ko79]. Macrophages play a variety of roles in AS inflammation, such as changing the production of cytokines or chemokines that are anti-inflammatory and activating inflammasomes to control cellular functions [Reference van der Veen, de Groot and Melgert80].
The screening of differentially expressed genes (DEGs) related to macrophages in AS through samples obtained from a public database to build a risk prediction model and explore its predictive abilities in AS diagnosis suggests a new diagnostic model based on 10 macrophages-related DEG signatures to predict AS risk. Among these novel biomarkers associated with macrophages in AS, AK5 gene was found downregulated when compared to controls [Reference Ai, Shen and Wang13].
Conclusion
As a member of the AK family, the function of AK5 is not only restricted to preserving the homeostasis of nucleotide and energy metabolism in living cells, but studies suggest it also plays a significant role in cardiac cell differentiation, WJC stemness, and proteasome inhibition in mantle cell lymphoma. A growing body of research has shown that, in addition to the brain, AK5 is expressed at the transcriptional and translational levels in a variety of other tissues, plays a critical role in a wide range of pathophysiological disorders, and has the potential to serve as a diagnostic biomarker and a target for treating the disease.
AK5 is also associated with cell death and cell cycle signaling pathways. It has varying molecular effects on tumorigenesis in various cancers. In a three-gene signature model high AK5 gene expression is related to better OS of prostate cancer patients. In gastric cancer patients, high AK5 protein is considered an unfavorable prognostic marker whereas in COAD low AK5 protein promotes metastasis and proliferation. In breast cancer and colorectal carcinoma tissues, AK5 gene promoter region was found methylated when compared to normal tissue and methylation was inversely proportional to gene expression. By regulating AMPK/mTOR signaling and the tumor suppressor proteins p16 and p21 in colorectal carcinoma and COAD, respectively, the AK5 protein is crucial in preventing metastasis.
Mutated AK5 gene with 3 bp ATT insertional mutation is a potential candidate for CD in Saudi patients. AK5 protein might be involved in regulating ATP/ADP ratio and K-ATP channel activity in type 2 diabetes mellitus. Transcriptional dysregulation of the AK5 gene has been reported in asthma, LEAD, and neurodegenerative diseases like Alzheimer’s and Parkinson’s, though translational downregulation of the AK5 gene has been reported in Temporal Lobe Epilepsy patients. IgG antibodies of CSF or serum target neuronal cytosolic antigen-AK5 in severe ALE, a rapidly progressing neurological syndrome with unclear etiology. Anti-AK5 encephalitis recognized in over 30 cases, lacks specific treatment, emphasizing the need for further research into its causes and therapeutic options.
It is crucial to conduct additional research studies on AK5 because of its potential role as a prognostic indicator and therapeutic target for several different types of disorders, as well as its relationships with numerous cellular processes. There are many questions that remain to be explored. In AD, the evidence, such as the involvement of the 14.3.3 ζ protein in tau phosphorylation, deregulation of the AK5 gene, and the direct interaction between AK5 and 14.3.3 ζ in the brain, suggests that further exploration of the AK5-14.3.3 ζ complex through expanded in vivo studies and detailed investigation of intricate molecular mechanisms could yield valuable insights into its role, potentially paving the way for therapeutic advancements. A similar approach could be applied to explore the role of the AK5-CPNE6 complex role in temporal lobe epilepsy for a comprehensive understanding and potential therapeutic applications. AK5 gene expression is strongly associated with different stages of cancer by regulating signaling pathways, but its role in many other aggressive cancers like glioblastoma, pancreatic adenocarcinoma, lung cancer, etc remains to be explored.
In conclusion, while AK5 demonstrates promise as a potential diagnostic biomarker and therapeutic target for various diseases, it is essential to recognize that most studies discussed are observational or preclinical, limiting the ability to draw definitive conclusions regarding its clinical utility. Further research is warranted to validate AK5’s clinical applications and elucidate its precise mechanisms of action.
Author contributions
M.S.S. wrote the first draft, J.S.C. edited the drafts and submitted the manuscript to the journal, and M.H.S. edited the drafts and directed the work.
Funding statement
Open access funding is provided by the University of Navarra.
Competing interests
None.
Open access
