Biological function of MAN2B1

Based on biochemical properties, catalytic mechanisms and conserved amino acid sequences within characteristic regions, α-mannosidases are classified into three categories: class I, class II and unclassified α-mannosidases. Class I α-mannosidases exhibit a molecular mass ranging from 63 to 73 kDa and possess a conserved sequence belonging to the glycoside hydrolase 47 family. Predominantly located in the endoplasmic reticulum, Golgi apparatus and certain cell membrane regions, they play a role in N-glycan secretion and protein surveillance [Reference Zhou, Lin, Zheng, Lin, Sang, Wang, Wang, Ebbole and Lu25, Reference Mora-Montes, Robledo-Ortiz, Gonzalez-Sanchez, Lopez-Esparza, Lopez-Romero and Flores-Carreon26]. Class II α-mannosidases have a molecular mass between 107 and 136 kDa and feature conserved sequences aligned with the glycoside hydrolase 38 family. These enzymes are primarily distributed across the endoplasmic reticulum, Golgi, lysosomes and cytoplasm. They are involved in the synthesis and degradation of glycoproteins. Key members of this family include MAN2A1, MAN2A2, MAN2B1, MAN2B2 and MAN2C1 [Reference Shashidhara and Gaikwad27]. Recent research has revealed that α-mannosidase is present in not only the glycoside hydrolase 38 and 47 families but also in the glycoside hydrolase 92, 99 and 125 families. These newly discovered α-mannosidases function similarly to class I or class II α-mannosidases, primarily catalysing oligosaccharide reactions [Reference Gregg, Zandberg, Hehemann, Whitworth, Deng, Vocadlo and Boraston28, Reference Matsuda, Kurakata, Miyazaki, Matsuo, Ito, Nishikawa and Tonozuka29].

The human lysosomal gene MAN2B1, also referred to as the LAMAN gene or MANB gene, resides on chromosome 19, encompassing a genomic DNA span of 21.5 kb across 24 exons, housing a 2964 bp reading frame [Reference Malm and Nilssen3]. Notably, three primary transcription initiation sites have been identified: −309, −196 and − 191 bp upstream of the ATG initiation sites, respectively [Reference Riise, Berg, Nilssen, Romeo, Tollersrud and Ceccherini30]. The 134 bp sequence, devoid of the characteristic CAAT or TATA sequence upstream of the transcription start site, contains multiple GC-rich sites within its 5′-end region, facilitating binding to transcription factors such as SP-1, AP-2 and ETF. Upon introduction of the MAN2B1 gene’s 5′ end region into the bacterial CAT gene, it was discovered that the 150 bp sequence at the 5′ end efficiently promotes the expression of the MAN2B1 gene in COS-7 cells (African green monkey SV 40 transformed kidney cells). Furthermore, Gotoda et al. conducted an analysis of the mRNA’s 5′ end region of the human MAN2B1 gene, pinpointing the transcription site at −28 and − 20 bp distant from the start codon ‘ATG’ [Reference Kuokkanen, Riise Stensland, Smith, Kjeldsen Buvang, Van Nguyen, Nilssen and Heikinheimo31, Reference Gotoda, Wakamatsu, Kawai, Nishida and Matsumoto32].

The initial transcription product of MAN2B1 is approximately 3.5 kb and encodes a precursor consisting of 988 or 1011 amino acids, mainly due to the start site where translation can be altered. Post-translational modifications of MAN2B1 polypeptides occur within the endoplasmic reticulum. Initially synthesised as a single-stranded precursor, it undergoes processing and cleavage into fragments of 70.42 kDa (abc), 42 kDa (d) and 15 kDa (e) during maturation and subsequent endosomal transport to the lysosome [Reference Kuokkanen, Riise Stensland, Smith, Kjeldsen Buvang, Van Nguyen, Nilssen and Heikinheimo31, Reference Hansen, Berg, Riise Stensland, Heikinheimo, Klenow, Evjen, Nilssen and Tollersrud33]. The 70.42-kDa polypeptides undergo further protease-mediated hydrolysis, forming three peptides interconnected by disulfide bonds, culminating in the assembly of a protein composed of five polypeptides. Ectopic expression of MAN2B1 in COS and CHO cells revealed the secretion of an unprocessed 120-kDa precursor. In its native state, MAN2B1 exists as a homodimer, its enzymatic activity contingent upon zinc ions while being susceptible to inhibition by copper ions [Reference Heikinheimo, Helland, Leiros, Leiros, Karlsen, Evjen, Ravelli, Schoehn, Ruigrok, Tollersrud, McSweeney and Hough34, Reference Numao, He, Evjen, Howard, Tollersrud and Withers35].

The X-ray crystallographic structure of bovine MAN2B1 is known at a 2.7-A resolution [Reference Heikinheimo, Helland, Leiros, Leiros, Karlsen, Evjen, Ravelli, Schoehn, Ruigrok, Tollersrud, McSweeney and Hough34]. Within the bovine MAN2B1 structure, four disulfide bridges are established between cysteines C55-C358, C268-C273, C412-C472 and C493-C501, all of which remain conserved in the human MAN2B1 sequence. Utilising the bovine crystal structure as a template, a structural model of the homologous human MAN2B1 was developed in 2011, exhibiting a sequence similarity of up to 84% to the template structure [Reference Kuokkanen, Riise Stensland, Smith, Kjeldsen Buvang, Van Nguyen, Nilssen and Heikinheimo31, Reference Heikinheimo, Helland, Leiros, Leiros, Karlsen, Evjen, Ravelli, Schoehn, Ruigrok, Tollersrud, McSweeney and Hough34, Reference Sbaragli, Bibi, Pittis, Balducci, Heikinheimo, Ricci, Antuzzi, Parini, Spaccini, Bembi and Beccari36]. Human MAN2B1 is characterised by 11 glycosylation sites, where eight of these sites align with the conserved positions identified in the bovine MAN2B1. The MAN2B1 structure comprises a seven-stranded active site a/b domain interconnected by a three-helix bundle to three b-domains. Its optimal pH range spans from 4.5 to 5, displaying only half of the maximum activity at pH levels of 7 and above, under physiological cytosolic and extracellular conditions. Additionally, in vitro assessments have determined the temperature range of 35–40 °C as optimal for maximum enzyme activity, aligning with the physiological temperature range of the human body [Reference Ajith Kumar and Siva Kumar37].

MAN2B1 plays a pivotal role in the breakdown of N-linked glycan chains released during the intracellular metabolism of glycoproteins. Primarily operating within acidic lysosomes, MAN2B1 contributes significantly to the orderly degradation of lysosomal N-linked oligosaccharides. It exhibits specificity for terminal non-reducing α-1,2, α-1,3 and α-1,6 mannosidic linkages, particularly in high-mannose and heterozygous N-linked glycans [Reference Kuokkanen, Riise Stensland, Smith, Kjeldsen Buvang, Van Nguyen, Nilssen and Heikinheimo31, Reference Venkatesan, Kuntz and Rose38]. Northern blotting revealed heightened expression of MAN2B1 in several tissues, notably in the lung, kidney, pancreas and peripheral blood leukocytes. In the central nervous system (CNS), its expression appears notably elevated in the corpus callosum and spinal cord. However, in comparison, expression levels are comparatively lower in regions such as the cerebellum, cerebral cortex, frontal and temporal lobes [Reference Nilssen, Berg, Riise, Ramachandran, Evjen, Hansen, Malm, Tranebjaerg and Tollersrud39, Reference Liao, Lal and Moremen40].

Reported mutations within the MAN2B1 gene encompass a spectrum of alterations, including deletions, insertions, duplications, missense, nonsense and splice mutations. Among these, missense mutations stand out as the most prevalent variants [Reference Borgwardt, Stensland, Olsen, Wibrand, Klenow, Beck, Amraoui, Arash, Fogh, Nilssen, Dali and Lund13, Reference Kuokkanen, Riise Stensland, Smith, Kjeldsen Buvang, Van Nguyen, Nilssen and Heikinheimo31, Reference Hansen, Berg, Riise Stensland, Heikinheimo, Klenow, Evjen, Nilssen and Tollersrud33]. Missense mutations in MAN2B1 can be classified into two primary categories: Active site-disrupting mutations: These genetic alterations result in the production of translationally defective proteins. Although these proteins undergo synthesis, processing and lysosomal translocation, they fail to execute their typical enzymatic functions due to disruptions within their active sites. Misfolded mutations: These mutations lead to the synthesis of proteins with structural misfolding, preventing them from attaining the correct folded configuration. Consequently, these proteins are retained within the endoplasmic reticulum (ER). According to the Human Gene Mutation Database (http://www.hgmd.cf.ac.uk/ac/all.php), a total of 151 mutations of MAN2B1 have been reported, comprising 57 missense mutations, 28 nonsense mutations, 22 splice-site mutations, 19 small deletions, 20 small insertions and 5 gross insertions/duplications. Among these mutations, 143 are associated with α-mannosidosis, 2 with abnormalities of the cardiovascular system, and a few others are related to autism spectrum disorder, developmental disorder and intellectual disability. The missense mutation c.2248C>T (p.Arg750Trp) stands out as a common occurrence among α-mannosidosis patients, having been reported in the majority of European populations studied, comprising over 30% of all detected disease alleles [Reference Borgwardt, Stensland, Olsen, Wibrand, Klenow, Beck, Amraoui, Arash, Fogh, Nilssen, Dali and Lund13].

MAN2B1 and immune system-related diseases

Recurrent infections represent a predominant clinical feature observed in α-mannosidosis cases, as documented in prior literature [Reference Malm, Nilssen, Adam, Feldman, Mirzaa, Pagon, Wallace, Bean, Gripp and Amemiya10]. Patients afflicted with α-mannosidosis typically exhibit immunocompromised states, characterised by impaired leukocyte chemotaxis, diminished phagocytic capabilities and disruptions within the IL-2 signalling pathway [Reference Zanetta, Bonaly, Maschke, Strecker and Michalski41]. This raises a pivotal query regarding the potential involvement of MAN2B1 in modulating immune system functionality. With the burgeoning availability of sequencing data for inflammatory disorders, facilitated by advancements in RNA sequencing technologies and database proliferation, our investigation revealed instances of aberrant MAN2B1 expression or enzymatic activity across various inflammatory diseases, animal models and cellular models (Table 2).

Table 2. Implication of MAN2B1 in diverse diseases

AD, alzheimer′s disease; BLCA, bladder urothelial carcinoma; BRCA, breast invasive carcinoma; COAD, colon adenocarcinoma; DLB, dementia with Lewy body; FTD, frontotemporal dementia; Mut, mutation; NA, not attention; SLE, systemic lupus erythematosus.

Infectious diseases and inflammatory activation

The extracellular secretion of lysosomal enzymes holds significant relevance in initiating and sustaining inflammatory responses [Reference Brade42, Reference Gros and Muller43]. Elevated α-mannosidase activity was detected in cell-free cerebrospinal fluid (CSF) obtained from patients diagnosed with bacterial meningitis, but not from those with aseptic meningitis [Reference Beratis, Mavrommatis, Hatiris, Kavaliotis, Tsagaropoulou-Stiga and Syrogiannopoulos44]. This observation strongly suggests that inflammation could potentially induce heightened extracellular secretion of MAN2B1.

In a separate clinical investigation conducted by Nicholas G et al. [Reference Beratis, Georgiou and Eliopoulou21], an assessment of lysosomal enzyme activity in the peritoneal fluid of patients with bacterial peritonitis, acute mesenteric lymphadenitis and control subjects devoid of peritoneal inflammation was conducted. The study revealed a substantial elevation in the median activity of α-mannosidase among patients with bacterial peritonitis, reaching a level 20 times higher than that observed in control subjects. Conversely, the enzyme activity in patients with acute mesenteric lymphadenitis did not significantly differ from that of the control group. Notably, no significant variance in α-mannosidase activity was observed between patients with peritonitis exhibiting negative bacterial cultures (under antibiotic treatment) and those with positive cultures. The detection of lysosomal mannosidase activity in peritoneal fluid stands as a valuable diagnostic tool for identifying patients afflicted with bacterial peritonitis. These results suggest the probable involvement of MAN2B1 specifically in bacterial-induced inflammation rather than aseptic inflammation. The observed escalation in mannosidase activity potentially correlates with bacterial recognition and phagocytic processes.

Several investigations have explored the association between MAN2B1 and bacterial infections. Notably, in studies pertaining to Mycobacterium tuberculosis infection, the alterations in MAN2B1 expression have led to conflicting findings. A study by Stephanie Widdison et al. [Reference Widdison, Watson, Piercy, Howard and Coffey45] found heightened MAN2B1 expression in Mycobacterium bovis-infected bovine alveolar macrophages compared to those infected with M. tuberculosis. Interestingly, M. bovis consistently caused disease, whereas M. tuberculosis infection was effectively controlled. Conversely, Nathan J. Hare et al. [Reference Hare, Lee, Loke, Britton, Saunders and Thaysen-Andersen46] observed a divergent outcome in human macrophages infected with M. tuberculosis. Utilising LC–MS/MS and QPCR assays, they identified several N-glycosylation-modified enzymes, noting a four-fold decrease in MAN2B1 protein levels and a significant reduction in transcript levels 72 hours post-infection. Discrepancies between these outcomes could potentially be attributed to inter-species differences.

In an in vitro study conducted by Anna Torri et al. [Reference Torri, Beretta, Ranghetti, Granucci, Ricciardi-Castagnoli and Foti22], dendritic cells (DCs) were subjected to various compounds, revealing intriguing insights. Stimulation of DC cells with bacterial strains such as Listeria monocytogenes, Lactobacillus paracasei and Toll-like receptor (TLR) ligands (LPS, poly I:C, ZymA) prompted inflammatory activation, concomitant with a noteworthy reduction in MAN2B1 mRNA expression. Interestingly, treatments involving vitD, IL-10, Nismesulide and IFNa failed to elicit changes in MAN2B1 expression. Similarly, Mohd M. Khan et al. [Reference Khan, Koppenol-Raab, Kuriakose, Manes, Goodlett and Nita-Lazar47] observed a reduction in MAN2B1 protein levels in macrophages 24 hours post-infection by Escherichia coli and Burkholderia cenocepacia bacteria. These collective findings suggest a nuanced relationship between MAN2B1 expression and the inflammatory response to specific bacterial stimuli.

Furthermore, studies have implicated MAN2B1 in the immune response against viral infections. In a genome-wide RNAi screen conducted by Alexander Karlas et al. [Reference Karlas, Machuy, Shin, Pleissner, Artarini, Heuer, Becker, Khalil, Ogilvie, Hess, Maurer, Muller, Wolff, Rudel and Meyer48], knockdown of MAN2B1 using siRNA inhibited the replication of influenza A/WSN/33 or A/Hamburg/04/2009 viruses in A549 cells, without affecting cell viability. Additionally, in Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection of porcine alveolar macrophages, a cell type primarily targeted by the virus, MAN2B1 was among the genes showing significant downregulation [Reference Jiang, Zhou, Michal, Wu, Zhang, Zhang, Ding, Liu, Manoranjan, Neill, Harhay, Kehrli and Miller49].

While existing research primarily concentrates on innate immune cells, there is a scarcity of reports on the atypical expression of MAN2B1 in adaptive immune cells. This paucity could potentially be linked to varying MAN2B1 expression levels across different cell types. To explore this, we analysed MAN2B1 mRNA levels across diverse immune cell types using data from the Human Protein Atlas (HPA) database (https://www.proteinatlas.org). Intriguingly, our analysis unveiled high expression levels of MAN2B1 in monocytes and DCs, whereas it exhibited lower expression in granulocytes, T cells, B cells and NK cells (Figure 1). These findings indicate a distinctive pattern of MAN2B1 expression across various immune cell types, possibly highlighting its varied role within the immune system.

Figure 1. MAN2B1 expression across various immune cell types. The transcript expression values, computed as nTPM (normalised transcripts per million), were derived from an internal normalisation pipeline encompassing 18 distinct immune cell types and total peripheral blood mononuclear cells (PBMCs). Colour-coding is assigned according to the lineage of blood cell types. This depiction of blood cell-type expression provides an overview of RNA-seq data sourced from multiple references, including internally generated Human Protein Atlas (HPA) data, as well as datasets from studies by Monaco et al. [Reference Monaco, Lee, Xu, Mustafah, Hwang, Carre, Burdin, Visan, Ceccarelli, Poidinger, Zippelius, Pedro de Magalhaes and Larbi97] and Schmiedel et al. [Reference Schmiedel, Singh, Madrigal, Valdovino-Gonzalez, White, Zapardiel-Gonzalo, Ha, Altay, Greenbaum, McVicker, Seumois, Rao, Kronenberg, Peters and Vijayanand98].

MAN2B1 and neurodegenerative disorders

Lysosomal dysfunction and perturbation of lysosomal protein function are recognised features in various neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Lewy body dementia (DLB) and anterior temporal dementia (FTD) [Reference Parnetti, Chiasserini, Persichetti, Eusebi, Varghese, Qureshi, Dardis, Deganuto, De Carlo, Castrioto, Balducci, Paciotti, Tambasco, Bembi, Bonanni, Onofrj, Rossi, Beccari, El-Agnaf and Calabresi57-Reference Udayar, Chen, Sidransky and Jagasia60]. It is well-documented that lysosomal dysfunction contributes to the pathogenesis of neurodegenerative diseases [Reference Root, Merino, Nuckols, Johnson and Kukar61]. In a comprehensive study involving 536 patients with sporadic early-onset Parkinson’s disease (SEOPD) and 600 patients with familial PD across Asian populations, researchers conducted genetic assessments of 67 candidate lysosome-related genes using whole exome sequencing. Their findings indicated a significant enrichment of rare damaging variants in MAN2B1 associated with PD [Reference Chen, Gu, Song, Hou, Ou, Zhang, Liu, Su, Cao, Wei, Zhao, Wu and Shang62].

Carla Emiliani and colleagues reported an upregulation of α-D-mannosidase activity and transcript levels in skin fibroblasts of individuals with AD [Reference Emiliani, Urbanelli, Racanicchi, Orlacchio, Pelicci, Sorbi, Bernardi and Orlacchio63]. Conversely, Parnetti’s team investigated lysosomal α-D-mannosidase activity in the CSF of patients diagnosed with DLB, AD and FTD, along with healthy subjects. Their findings revealed significantly lower enzyme activity in the CSF of patients compared to healthy individuals [Reference Parnetti, Balducci, Pierguidi, De Carlo, Peducci, D’Amore, Padiglioni, Mastrocola, Persichetti, Paciotti, Bellomo, Tambasco, Rossi, Beccari and Calabresi23]. In a subsequent study, researchers analysed α-mannosidase in venous blood and CSF from patients diagnosed with mild neurological disorders, utilising DEAE cellulose chromatography [Reference Tasegian, Paciotti, Ceccarini, Codini, Moors, Chiasserini, Albi, Winchester, van de Berg, Parnetti and Beccari64]. The results highlighted that α-mannosidase from venous blood did not penetrate the blood–brain barrier, while the enzyme activity detected in CSF originated from cerebral lysosomes. These differing outcomes could be attributed to the diversity in sample types examined. CSF, being more directly correlated with pathological changes in the CNS, contrasts with fibroblasts in the skin. The observed enzyme activity in CSF is believed to potentially reflect pathological cerebral changes associated with neurodegenerative diseases [Reference Tasegian, Paciotti, Ceccarini, Codini, Moors, Chiasserini, Albi, Winchester, van de Berg, Parnetti and Beccari64].

Progressive intellectual decline, motor dysfunction and cerebellar atrophy stand out as prominent neurological symptoms in individuals diagnosed with mannosidosis. Line Borgwardt’s research [Reference Borgwardt, Stensland, Olsen, Wibrand, Klenow, Beck, Amraoui, Arash, Fogh, Nilssen, Dali and Lund13] explored genotype–phenotype associations in α-mannosidosis, revealing a robust correlation between MAN2B1 genotype, intracellular localisation and CNS symptoms. Specifically, this association extended to pulmonary function, upper limb coordination, balance, FVC% and the accumulation of oligosaccharides in CSF.

Extensive evidence from diverse animal models further underlines the link between α-mannosidosis and CNS abnormalities. Guinea pigs affected by α-mannosidosis exhibited significant neurological abnormalities as early as 2 months of age, marked by neuronal lysosomal vacuolation and secondary GM3 ganglioside accumulation. They also demonstrated extensive axonal globus pallidus and diminished white matter myelin sheaths [Reference Crawley and Walkley65]. In mice with α-mannosidosis, observable traits included activated Bergman glial populations, macrophage infiltration and the buildup of free cholesterol and gangliosides [Reference Damme, Stroobants, Walkley, Lullmann-Rauch, D’Hooge, Fogh, Saftig, Lubke and Blanz66]. Notably, ERT exhibited partial reversal of cerebellar pathological changes, reducing activated macrophages and astrocytes while enhancing neurocognitive function [Reference Damme, Stroobants, Walkley, Lullmann-Rauch, D’Hooge, Fogh, Saftig, Lubke and Blanz66, Reference Stroobants, Damme, Van der Jeugd, Vermaercke, Andersson, Fogh, Saftig, Blanz and D’Hooge67]. Recently, a seven-month-old dog displaying progressive neurological symptoms and brain atrophy was euthanised, and microscopic examination confirmed a lysosomal storage disease. Whole-genome sequencing unveiled a MAN2B1 missense mutation (Asp104Gly) with undetectable α-mannosidase activity [Reference Bullock, Johnson, Pattridge, Mhlanga-Mutangadura, Guo, Cook, Campbell, Vite and Katz68].

LSDs and age-related neurodegenerative disorders such as PDs and ADs are characterised by a common pathological feature: the accumulation of undigested material within the organelles of the endolysosomal system [Reference Filippini, Gennarelli and Russo69, Reference Tancini, Buratta, Delo, Sagini, Chiaradia, Pellegrino, Emiliani and Urbanelli70]. Moreover, members of the class II α-mannosidase family, namely MAN2A1 [Reference Vedin, Cederholm, Freund-Levi, Basun, Garlind, Irving, Eriksdotter-Jonhagen, Wahlund, Dahlman and Palmblad71] and MAN2C1 [Reference Gialluisi, Reccia, Modugno, Nutile, Lombardi, Di Giovannantonio, Pietracupa, Ruggiero, Scala, Gambardella, Iacoviello, Gianfrancesco, Acampora, D’Esposito, Simeone, Ciullo and Esposito72], have also been reported to be associated with neurodegenerative conditions. This implies that the pathologic alterations in lysosomes related to neurodegenerative diseases may involve glycosylation beyond the sole influence of MAN2B1.

MAN2B1 and cancers

Glycosylation stands as a hallmark of cancer, intricately involved in various aspects of cancer cell biology encompassing cell signalling, tumour cell dissociation, invasion, cell–matrix interactions, angiogenesis and immune modulation [Reference Caval, Alisson-Silva and Schwarz73]. Eliminating N-glycan expression on tumour cells has shown promise in enhancing the efficacy of immunotherapy [Reference Greco, Malacarne, De Girardi, Scotti, Manfredi, Angelino, Sirini, Camisa, Falcone, Moresco, Paolella, Di Bono, Norata, Sanvito, Arcangeli, Doglioni, Ciceri, Bonini, Graziani, Bondanza and Casucci74, Reference Lee, Wang, Xia, Chen, Rau, Ye, Wei, Chou, Wang, Yan, Tu, Hsia, Chiang, Chao, Wistuba, Hsu, Hortobagyi and Hung75]. Disruptions in glycosylation patterns can arise from irregularities in the expression, localisation and functioning of glycosyltransferases and glycosyl hydrolases [Reference Hu, Zhang, Yang, Zhao, Liu, Huang, Lyu, Xiao, Guo, Zhou and Tang76].

Notably, abnormal upregulation of α-D-mannosidase has been linked to Ras activation in Alzheimer’s disease [Reference Emiliani, Urbanelli, Racanicchi, Orlacchio, Pelicci, Sorbi, Bernardi and Orlacchio63]. Ras signalling, pivotal in normal cell proliferation, frequently undergoes activation across a spectrum of malignancies. While the correlation between α-D-mannosidase and Ras activation remains unreported in cancers, upregulation of MAN2B1 and heightened α-mannosidase activity have been documented in various cancers, including gynaecologic cancer, malignant glial tumours and leukaemia [Reference Urbanelli, Magini, Ercolani, Trivelli, Polchi, Tancini and Emiliani19, Reference Wielgat, Walczuk, Szajda, Bien, Zimnoch, Mariak and Zwierz77, Reference Beratis, Kaperonis, Eliopoulou, Kourounis and Tzingounis78]. Analysis of The Cancer Genome Atlas (TCGA) data unveiled elevated mRNA levels of MAN2B1 across numerous human cancers, encompassing bladder urothelial carcinoma (BLCA), breast invasive carcinoma (BRCA) and colon adenocarcinoma (COAD) [Reference Lin, Liu, Zhao, Xia, Li, Wang, Huang, Wanggou and Li20].

N.G. Beratis [Reference Beratis, Kaperonis, Eliopoulou, Kourounis and Tzingounis78] reported a notable elevation in α-mannosidase activity in the peritoneal fluid of patients with gynaecologic cancer compared to women with infertility, serving as control subjects, in a study conducted in 2004. However, no notable variance in α-mannosidase activity was detected in the fluid extracted from ovarian cysts, indicating its potential diagnostic significance in distinguishing malignant gynaecologic cancers. α-D-mannosidase activity surged by nearly 70-fold in the HL60 cell line and 35-fold in the NB4 promyelocytic leukaemia cell line compared to skin fibroblasts [Reference Urbanelli, Magini, Ercolani, Trivelli, Polchi, Tancini and Emiliani19]. Gene expression analysis unveiled a close relationship between heightened enzyme activity and transcriptional upregulation. Notably, MAN2B1 transcriptional regulation differs between normal and tumour cell lines. In HEK-293 cells, Sp1 binds to the promoter region −101/−71 of MAN2B1, encompassing two overlapping GC boxes. Conversely, NF-κB regulates MAN2B1 transcription in the HL60 cell line by binding to the segment −373/−269.

In a 2006 study conducted by P. Wielgat [Reference Wielgat, Walczuk, Szajda, Bien, Zimnoch, Mariak and Zwierz77], the expression of lysosomal exoglycosidases was examined in 29 specimens of human gliomas along with unchanged brain tissue typically excised alongside the tumours. The study revealed a significant increase in α-mannosidase activity within malignant glial tumours compared to both control tissue (normal brain tissue) and non-glial tumours. The most substantial rise in α-mannosidase activity was observed in anaplastic astrocytomas. Moreover, primary tumours exhibited markedly higher α-mannosidase activity than metastatic tumours. This study was the first to suggest a correlation between α-mannosidase activity and the progression of brain tumours, implying a potential role of lysosomal exoglycosidases in the advancement and dynamic development of glial tumours. Two recent RNA sequencing-based bioinformatic analytical studies have identified MAN2B1 as a novel prognostic biomarker for glioma, indicating that higher expression of MAN2B1 correlates with shorter survival times among patients [Reference Lin, Liu, Zhao, Xia, Li, Wang, Huang, Wanggou and Li20, Reference Lin, Wang, Xiong, Zhou, Yang, Chen, Xu, Zhou, Mao, Lv, Wang and Zhou79]. These studies highlighted a significant increase in MAN2B1 expression within glioma tissue, revealing associations with WHO classification, IDH1 mutation status and various histological subgroups among individuals diagnosed with glioma. Additionally, the research illustrated a positive connection between MAN2B1 and immune response pathways, coupled with infiltration of immune cells. Notably, MAN2B1 exhibited a strong positive correlation with M2 macrophage markers (TGFBI and CD163), while its relationship with M1 macrophage markers (NOS2 and TNF) appeared comparatively weaker.

These studies collectively suggest a potential link between heightened α-mannosidase activity and the advancement of malignant tumours. Furthermore, the correlation between MAN2B1 and tumour-associated macrophages indicates possible involvement of MAN2B1 in shaping the tumour microenvironment.

MAN2B1 and others disorders

α-Mannosidosis, an inherited metabolic disorder, exhibits characteristic cellular changes such as multiple membrane-bound cytoplasmic vacuoles found in various cell types—hepatocytes, pancreatic exocrine cells, renal cells, thyroid cells, smooth muscle cells, bone cells and neurons in the CNS and peripheral nervous system [Reference Stinchi, Lullmann-Rauch, Hartmann, Coenen, Beccari, Orlacchio, von Figura and Saftig80]. Reduced α-mannosidase activity affects not only the nervous and immune systems but also involves other organs, leading to liver or spleen enlargement, lung issues [Reference Beck, Olsen, Wraith, Zeman, Michalski, Saftig, Fogh and Malm81] and kidney complications [Reference Segoloni, Colla, Messina and Stratta82]. In a guinea pig model of α-mannosidosis, α-mannosidase deficiency triggers substantial morphological alterations in foetal pathology [Reference Auclair and Hopwood83]. ERT exhibits potential in reducing lysosomal vacuolation in the liver, kidneys, spleen and pancreas [Reference Crawley, King, Berg, Meikle and Hopwood84]. MAN2B1 has been associated with pathological alterations in multiple organs, in addition to its role in α-mannosidosis.

In a multi-ethnic genome-wide association study encompassing 7,914 participants, Ani Manichaikul and colleagues [Reference Manichaikul, Hoffman, Smolonska, Gao, Cho, Baumhauer, Budoff, Austin, Washko, Carr, Kaufman, Pottinger, Powell, Wijmenga, Zanen, Groen, Postma, Wanner, Rouhani, Brantly, Powell, Smith, Rabinowitz, Raffel, Hinckley Stukovsky, Crapo, Beaty, Hokanson, Silverman, Dupuis, O’Connor, Boezen, Rich and Barr85] explored the association between lung/single nucleotide polymorphism (SNP) health. They identified the MAN2B1 SNP (rs10411619) in Hispanic patients and the MAN1C1 SNP (rs12130495) in African Americans. In an independent multi-ethnic cohort, they found that gene expression of MAN2B1 in peripheral monocytes showed a statistically significant association with a reduced upper-lower lobe ratio, whereas the gene expression of MAN1C1 was associated with an increased percentage of emphysema.

Lysosomal enzymes play pivotal roles in various biological processes and cellular reactions. Reports indicate that specific stimuli can induce alterations in MAN2B1 expression. For instance, Daila S. Gridley demonstrated that low-dose photons and solar particle events notably upregulated MAN2B1 expression in mouse liver [Reference Gridley, Coutrakon, Rizvi, Bayeta, Luo-Owen, Makinde, Baqai, Koss, Slater and Pecaut86]. In another study by Paula Juricic et al. [Reference Juricic, Lu, Leech, Drews, Paulitz, Lu, Nespital, Azami, Regan, Funk, Frohlich, Gronke and Partridge87], chronic rapamycin treatment in female Drosophila and mice elicited a geroprotective response. Rapamycin induced autophagy in the mouse intestine, as observed by an increased count of lysozyme⁺/p62⁺ granules per Paneth cell. Immunofluorescence data displayed an increased presence of MAN2B1⁺ punctae in intestinal crypts, persisting even six months post-treatment. This elevation in MAN2B1 expression potentially signifies alterations within cellular lysosomes, potentially serving as a marker for Rapamycin-induced autophagic activity and implicating MAN2B1 in intestinal homeostasis regulation post-Rapamycin treatment. Additionally, an experimental study on rabbits revealed that inhalation of morpholine vapour significantly augmented α-mannosidase activity in alveolar macrophages [Reference Tombropoulos, Koo, Gibson and Hook88]. in vitro experiments further confirmed that exposing alveolar macrophages to morpholine induced a time-dependent elevation in α-mannosidase activity. These alterations in α-mannosidase activity may indicate the cellular response to the biological toxicity of certain substances.

Conclusion and prospection

Investigations extending beyond α-mannosidosis have unveiled notable insights into the potential role of MAN2B1 in inflammatory conditions, particularly in macrophage infection and activation. These studies delineate variations in MAN2B1 expression and activity across various pathological contexts, shedding light on its associations with inflammation and tumorigenesis. However, comprehensive exploration through systematic experimental studies remains pending. Intriguingly, the involvement of MAN2B1 in infections, nervous system, autoimmune disorders and organ damage shows certain correlations with symptoms observed in α-mannosidosis (Figure 2). A comprehensive investigation into MAN2B1-related research is anticipated to significantly advance our understanding of the pathogenic mechanisms underlying α-mannosidosis, potentially opening avenues for more diverse and effective therapeutic strategies. However, we have no ability to draw definitive conclusions regarding the relationship between MAN2B1 and various disease processes now. The potential cellular and molecular mechanisms remain to be confirmed through further experimental validation. Consequently, there is a pressing need for a concerted effort to enhance the basic and clinical knowledge of MN2B1 for the treatment of these diseases.

Figure 2. Clinical manifestations of α-mannosidosis and disorders associated with MAN2B1.

Lysosomal changes and dysfunction have been identified in a wide variety of diseases, including autoimmune, metabolic and kidney diseases, PD, diabetes mellitus and lysosomal storage diseases [Reference Gros and Muller43, Reference Cao, Luo, Wu and He89]. Targeting the lysosome has emerged as a promising approach in various therapeutic strategies, especially in treating LSD and some other diseases related to lysosomal dysfunction. However, challenges persist in achieving specific drug delivery, controlling lysosomal enzyme activity and ensuring effective disease impact. The complexity of lysosomal functions complicates the development of targeted therapies. Consequently, unravelling the molecular intricacies within lysosomes would offer more precise therapeutic targets for diseases associated with lysosomal dysfunction.

Aberrant glycosylation, considered a hallmark of cancer, has been implicated in its initiation and progression across various malignancies [Reference Yue, Huang, Lan, Xiao and Luo90-Reference Duca, Malagolini and Dall’Olio92]. The transmembrane protein GManII, encoded by MAN2A1, catalyses the initial step in complex N-glycan biosynthesis. Inhibition of GManII in cancer cells impedes cancer progression and enhances chemosensitivity. MAN2A1 has emerged as a therapeutic target for cancer treatment. Swainsonine, the most potent GManII inhibitor, exhibiting nanomolar affinity with the GH 38 mannosidase family, has garnered attention as a potential anticancer agent [Reference Lee, Loo, Wibowo, Mohammat and Foo93]. However, challenges arise in targeting GManII due to the high similarity in functionality and active sites between LMan encoded by the MAN2B1 gene and GManII. Swainsonine, a reversible inhibitor, has shown promising results in preclinical and phase I trials but was discontinued in phase II due to disease progression or adverse reactions [Reference Shaheen, Stadler, Elson, Knox, Winquist and Bukowski94]. The adverse effects included mild symptoms like fatigue, anorexia, nausea and diarrhoea, while severe reactions were primarily associated with neurological toxicity such as depression, anxiety and cognitive impairment. Nonetheless, these adverse effects were fully reversible upon treatment cessation. Researchers are inclined to develop swainsonine analogues with improved selectivity to overcome these side effects. Similarly, developing small molecule drugs targeting MAN2B1 might face functional interference and side effects on MAN2A1. Hence, gene therapy or targeted delivery could be pivotal strategies in treating related disorders, especially targeting macrophages.