YKL-40 and LAMPs as possible markers in neuroinflammation and autophagy during central nervous system infections

Abstract

Central nervous system infections continue to be a public health concern owing to the high mortality and the significant neurological sequelae among survivors. The diverse etiology with overlapping clinical and laboratory abnormalities makes the management of such patients challenging. Neuroinflammation plays an essential role in triggering oxidative stress and autophagy dysregulation. Impaired autophagy may lead to abnormal protein aggregation resulting in neurodegeneration. YKL-40 is a secreted glycoprotein, involved in several diseases accompanied by inflammation. The lysosome-associated membrane proteins (LAMPs) 1 and 2 exhibit diverse expression levels in a range of cell processes (including autophagy) and clinical conditions but the complete picture of their biological function is still unknown. This review highlights the role of YKL-40 and LAMPs in central nervous system infections. We suggest that these biomolecules might have a promising value as biomarkers or targets for therapy and could provide additional evidence for inflammatory activity in different neurological diseases.

Keywords:

• ykl-40
• LAMP-1
• LAMP-2
• central nervous system infections

Introduction

Central nervous system (CNS) infections are potentially life-threatening conditions that can be caused by various pathogens, including bacteria, viruses, fungi and parasites [1,2]. The wide variety of pathogens associated with CNS infections greatly complicates the detection of the specific causative agent and the selection of appropriate antimicrobial therapy. The rapid and precise identification of the etiology is essential for the early targeted therapy thus reducing mortality, improving the clinical course, and reducing the most possible subsequent neurological complications [1,3].

In routine clinical practice, diagnosis of CNS infections is initially made by identifying clinical symptoms, evaluation of cerebrospinal fluid (CSF) and neuroimaging. Clinical data and clinical-laboratory examination of CSF alone do not have sufficiently high specificity and sensitivity to differentiate between etiologies [1–5]. CSF samples are analyzed to search for the protein content, glucose levels, and cellular numbers and types. The microbiological methods based on culture or antigen detection have several limitations related to the identification of diverse potential pathogens [1–5]. Over the past decade, new innovative technologies have been developed including molecular assays able to detect the most common pathogens related to CNS in a single clinical specimen – multiplex PCR assays [6]. Although the utilization of these revolutionary systems in the diagnosis of CNS infections is indeed associated with an increased positive yield, a large proportion of patients remain etiologically unverified. In the developed countries, as high as 68% of meningitis cases and up to 50% of encephalitis cases are etiologically unconfirmed [5,7,8]. In clinical practice, it is essential to have an early differentiation of bacterial from viral neuroinfections, owing to the different prognosis, clinical course, and complications.

To overcome the limitations of conventional approaches to the identification of CNS infections, other strategies emerged aiming to utilize specific biomarkers in the management of acute neuroinfections. The discovery and validation of biomarkers would help clinicians with the choice of therapy, as well as the differentiation of patients at increased risk of developing complications and death. Recently, there has been a growing interest in research focused on biomarkers in various diseases including CNS infection.

The aim of this review is to summarize the available literature on the diagnostic and prognostic potential of YKL-40 and LAMPs proteins during CNS infections.

YKL-40

YKL-40, also known as chitinase 3-like 1 (CHI3L1), is a glycoprotein that has been associated with inflammation, tissue remodeling, and cancer [9]. YKL-40 is an extensively investigated glycoprotein, which has been suggested to have prognostic value in patients with several inflammatory and tumor diseases [10,11]. It is an extracellular matrix glycoprotein with still largely unknown biological functions. IL-13Rα2 is considered the main receptor for YKL-40, which determines its dominant role in inflammatory processes [12]. The conserved amino acid sequence of the glycoprotein suggests involvement and significant function (structural or regulatory) in a signaling pathway activating the MAP-kinase and PI-3K signaling cascade controlling mitogenesis. YKL-40 is expressed and secreted by activated macrophages and neutrophils, chondrocytes, and synovial cells [13,14]. In several neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, etc., YKL-40 is discussed as a potential biomarker, serving to differentiate the individual stages in the development of these diseases, as well as to monitor their progression [9,15]. YKL-40 is proposed to be a non-specific pro-inflammatory protein. Its concentration correlates with established factors such as C-reactive protein and ESR. The results of studies on patients with rheumatoid and psoriatic arthritis, osteoarthritis, and systemic sclerosis prove not only a definite relationship of YKL-40 with proinflammatory cytokines (IL-1β, IL-6, TNF-α) but also with clinical and ultra sonographic parameters [16,17]. Our studies established the local expression and secretion of YKL-40 in CSF from patients with severe brain injury, as well as a correlation with cytokines, which determined the involvement of the glycoprotein in the inflammatory process occurring after the traumatic injury [18].

In the context of meningitis, a disease characterized by a significant inflammatory reaction within the subarachnoidal space and/or the brain parenchyma, YKL-40 has gained interest in several aspects including diagnosis, disease progression, and prognosis. However, in the international scientific literature, the data on the expression and secretion of YKL-40 in neuroinfections are scarce, and in Bulgaria, they are missing. In the CNS, YKL-40 is mostly expressed by microglia, especially when responding to acute and chronic inflammation [19–21]. One study reported that patients with purulent meningitis/encephalitis had significantly higher CSF YKL-40 levels than patients with lymphocytic meningitis or patients without meningitis [19]. Another research group found that changes in CSF levels of the glycoprotein correlated with improvement in meningo-radicular syndrome, suggesting that YKL-40 may be involved in the pathogenesis of the anti-N-methyl-D-aspartate receptor (NMDA-R) encephalitis [21]. Recently, it was reported that CSF YKL-40 levels in autoimmune encephalitis (anti-LGI1) correlated with the severity and prognosis [22]. Interestingly, in tick-borne encephalitis, it has been demonstrated that CSF YKL-40 has the potential to differentiate between meningitis and meningoencephalitis with a sensitivity of 62.5% but provides better specificity of 87.5% [23]. It was shown that YKL-40 can serve as a prognostic biomarker. Kronborg et al. [24] demonstrated that YKL-40 can be elevated in S. pneumoniae bacteremia and the high levels correlate strongly to the poor prognosis.

LAMPs

Neuronal autophagy is crucial for neuronal development and signaling, as well as for their growth and function. It has been shown that the main factors that regulate autophagy also control neuroinflammation aiming to improve the immune system’s ability to respond to changing microenvironment. Many authors suggested that autophagy was an adaptive, pro-survival process during CNS viral infection rather than a response triggering cell death [25]. It was proven that autophagy inhibited the replication of several viruses in neurons [26]. Autophagy can support viral development even it is a cellular reaction involved in the antiviral response [27].

Lysosomes play a major role in autophagy. LAMP-1 and LAMP-2 are conservative proteins of the lysosomal membrane, but little is known about their role. Involvement in processes such as autophagy, cell death, and neoangiogenesis has been proposed [28]. Basic information about YKL-40 and LAMP-1, LAMP-2 regarding gene and protein structure and biological functions is presented in Table 1.

Table 1. Basic characteristics of YKL-40, LAMP-1 and LAMP-2.

Name	Gene symbol, chromosome localization	Protein structure, interaction, amino acids	Biological function	Cellular source in healthy conditions	Subcellular location	Signaling pathways
YKL-40	CHI3L1, chromosome 1; 10 exons	383 AA, monomer	Tissue remodeling and angiogenesis; induced Th2 inflammatory response and M2 macrophage differentiation; modulates antibacterial responses [10,12,29]	Expressed by monocytes/macrophages, chondrocytes, synovial cells, osteoclasts, and microglia [30,31].	Cytoplasmic granular [32]	Activates the Erk, Akt, and Wnt-linked signaling pathways [33,34]
LAMP-1	LAMP1, chromosome 13	417 AA, Homodimer	Lysosome biogenesis, autophagy, and cholesterol homeostasis; NK-cells cytotoxicity [35]; Cytotoxic granule movement to the cell surface and perforin trafficking to the lytic granule [36]. NK-cells are protected by degranulation [37]. Presents carbohydrate ligands to selectins. Implicated in tumor cell metastasis.	Distributed among autophagic and endolysosomal organelles [28]	Endosome-lysosomal membrane of cells and plasma membrane [38]	Participates in endocytic and autophagic pathways and lysosomal biogenesis [39]
LAMP-2	LAMP2, chromosome X	410 AA, monomer, homodimer	Chaperone-mediated autophagy, supports the FURIN-mediated cleavage of mumps virus fusion protein F (P13473) [40]	Distributed among autophagic and endolysosomal organelles [41].	Endosome-lysosomal membrane of cells and plasma membrane [38]	Mediates response to elevated platelet cytosolic Ca2+ and innate immune response

Although autophagy is intensively investigated in CNS infection, studies on LAMP-1 and LAMP-2 proteins are insufficient in the literature. Our previous experience shows that they change their cellular localization in tumor cell lines and membrane expression promotes binding to extracellular matrix molecules and thus facilitates tumor progression [42]. It was proven that LAMP-2 is involved in the viral cycle of HSV-1. Researchers used a LAMP-2 deficient neuroblastoma cell line in which a lower production of infectious was detected. It was indicated that LAMP2 deficiency significantly attenuates the neurodegenerative causes, increase of phosphorylated tau and inhibition of Aβ secretion induced by HSV-1 in chronic conditions [43].

Autophagy-inflammation interplay during infection

Autophagy is a highly conserved cellular degradation process performed in the autophagosome. It plays a major role during cellular development and differentiation, and tumor suppression. Inflammation is triggered by recognition of the pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) from pattern recognition receptors (PRRs) of immune cells. The increased rate of inflammation resulted in cytokine secretion and higher YKL-40 levels. On the other hand, autophagy inhibits the function of pathogen PAMPs [44].

Both autophagy and inflammation could be induced by pathogens, and their interaction persists during the host response. Many participants in the inflammation as receptors (TLRs, NOD-like receptors), proinflammatory cytokines activate autophagy. The autophagic process is triggered also by starvation, cellular damage, and the production of dysfunctional proteins [45]. Removal of pathogens by autophagy eliminates the causative agent and suppresses inflammation, whereas dysfunction in autophagy leads to increased inflam­mation. Furthermore, it was proven that autophagy modulated inflammation through suppressing the inflammasome or down-regulating proinflammatory cytokines [46]. We attempted to summarize the crosstalk between autophagy and inflammation in Figure 1.

Figure 1. Interplay between autophagy and inflammation.

Inflammation and autophagy are triggered by pathogens (bacteria, viruses), external or internal factors (starvation, stress, hypoxia). Autophagy negatively regulates inflammation by degrading its stimuli, including bacteria, viruses, DAMPs, PAMPs, and inflammasome components. On the other hand, activation of DAMPs, NF-κβ induces immune response leading to Th1- and Th2-related cytokines secretion. The Th2 immune response inhibits LAMPs proteins and autophagy, while Th1-related cytokines trigger autophagy [61].

Autophagy and inflammation are the two sides of the same coin, and their controlled cooperation is required for cellular homeostasis. Dysbalanced autophagy often leads to inflammatory responses, resulting in severe host damage [44]. It is still unclear under which circumstances enhanced autophagy plays a role in cell death or represents a mechanism with protective effects [40].

Late endosomes and lysosomes are rich in LAMPs. Notably, S. pneumoniae has been confirmed to target LAMP-enriched compartments so it can traffic to late phagosomes. On the other hand, Mycobacterium tuberculosis has developed strategies to interfere with phagosome maturation by preventing LAMP integration. Similarly, Neisseria disrupts macrophage-killing mechanisms by slowing LAMP’s integration into phagosomes. LAMP-1 is susceptible to degradation of IgA proteases, produced by Neisseria, S. pneumoniae, and H. influenzae, promoting their survival. These findings demonstrate the complex interactions between pathogens and host cellular defense mechanisms involving phagosome functions [47–50].

It was recently identified that another lysosomal-associated protein (LIMP-2) is a receptor for enterovirus 71 (EV71), a virus known to have the potential to infect the CNS [51–53]. In addition, it was shown that the intracellular development of L. monocytogenes, can involve specific vacuoles, both expressing LAMP-1 [54–56]. These findings suggest the role of this group of proteins in the pathogenesis of CNS infections making them valuable candidates for biomarker research. Table 2 summarizes studies on YKL-40 and LAMPs in CNS infections.

Table 2. Summary of YKL-40, LAMPs studies in CNS infections.

	Markers
Disease	YKL-40	LAMP-1 and LAMP-2
Anti-leucine-rich glioma inactivated 1(LGI1) encephalitis	CHI3L1 levels in CSF and serum are highly elevated compared to controls. CSF CHI3L1 levels are correlated with the severity and prognosis of LGI1 [22]	N/D
Tick-borne encephalitis: meningoencephalitis and meningitis [23]	The concentration is significantly elevated in the group than the controls the after 7 days from the last dose of treatment.	Autophagy restricts the replication of several viruses in neurons [21]
N-methyl-D-aspartate receptor encephalitis	CSF levels of YKL-40 and inflammatory cytokines (TNF-a, IL-6, IL-10) are all higher in patients with anti-NMDAR encephalitis at the acute stage of disease compared with the controls [21]	N/D
Creutzfeldt–Jakob (CJD) disease	Concentrations are elevated in patients with CJD vs patients with Alzheimer’s disease, frontotemporal dementia and controls (CSF) [57] Levels are elevated in patients with CJD vs patients with neurodegenerative disorders, neurologic controls, and healthy individuals [58]	N/D
HIV-1 infections, Herpes simplex virus type 1 (HSV1)	CSF YKL-40 is a candidate biomarker for understanding the pathogenesis of HIV in the CNS. Strong correlation between CSF YKL-40 and NFL indicates a pathogenic association between astroglial activation and axonal injury [59]	HIV infection induces damage-regulated autophagy modulator (DRAM) expression in a p53-dependent manner [60] LAMP-2 deficiency affects more severely the lysosomal dysfunction induced by HSV 1 infection [43]

Note: N/D – not determined.

There are several limitations of using YKL-40 and LAMPs as diagnostic and prognostic biomarkers in patients with CNS infections that should be addressed. There is a lack of specificity since these biomolecules are overexpressed also in a wide range of conditions, including cancer, non-infectious inflammatory diseases, and cardiovascular diseases. The broad expression could limit their use in patients with CNS infection as stand-alone biomarkers. Furthermore, the levels of YKL-40 and LAMPs may show variability among populations and be influenced by age, gender, and other demographic differences. In the context of measuring YKL-40 and LAMPs levels, it is worth mentioning that different methods may yield fluctuating results due to variations in sensitivity, specificity, and calibration. This variability can make it challenging to compare results across studies or clinical settings. In addition, discrepancies could also be attributed to a lack of standardized protocols for such assays, including the time of sample collection, specimen handling, as well as storage conditions.

These limitations could complicate the interpretation of results in patients with CNS infections making it difficult to determine cut-off values for diagnostic and prognostic purposes. However, integrating these proteins in combination with other biomarkers could possibly contribute to a more accurate diagnosis, better disease stratification, and improved monitoring and response to treatment in patients with CNS infections. To address such limitations additional studies in various conditions and populations are needed to integrate more clinical data with biomarker levels. Identifying complementary biomarkers that correlate strongly with specific diseases could be particularly valuable. Moreover, the advances in artificial intelligence and machine learning models could facilitate future application of YKL-40 and LAMPs in patients with neuroinfections.

Conclusions

In order to improve the management of patients with CNS infections, there is a growing interest in studying various biomolecules. YKL-40 and LAMPs have also emerged as potential biomarkers for a number of diseases, also including infections of the CNS. YKL-40 is linked with inflammation and tissue remodeling, while LAMPs are key in autophagy, lysosomal stability, and immune regulation, making them valuable for diagnosing and managing patients with CNS infections. Research on YKL-40 and LAMPs in meningitis is still evolving, with studies aiming to better define their role and application in clinical practice. Current studies potential use in diagnostics and clinical assessment. Despite initial findings, transitioning from research to clinical practice requires further studies to validate and integrate these biomarkers in the care of patients with acute CNS infections. This could lead to improved patient outcomes, personalized treatment approaches, and the development of novel therapeutic targets, thus making significant advances in the field of neuroinfectious diseases.

Author contributions

All the authors contributed to the data research, writing, editing, and reviewing of the manuscript. All authors have read and agreed to the final version of the manuscript.

Acknowledgment

The authors thank Valentina Mihaylova for the help in the design of the figure.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data is available from the corresponding author upon request.

Additional information

Funding

This study was funded by the European Union-NextGenerationEU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project № BG-RRP-2.004-0007-C01 and by MU-Plovdiv project № 02/2022.