# Cmpk2 Gene and Protein Expression in Saliva or Salivary Glands of Dyslipidemic Mice
## 1 Introduction  
With global dietary shifts, obesity has become a widespread problem [1, 2]. Obesity-induced dyslipidemia is collectively characterized by low-density lipoprotein cholestericolemia, hypertriglyceridemia, and high-density lipoprotein cholesterol [3]. Elevated total cholesterol (T-CHO) is also commonly associated with these dyslipidemias. Dyslipidemia increases the risk of age-related chronic inflammatory diseases, such as cardiovascular disease [4, 5], diabetes [6], and stroke [7], necessitating early detection. Consequently, there is a need to develop biomarkers for early detection. To date, biomarkers have been developed using blood [8, 9], urine [10, 11], cerebrospinal fluid [12], and saliva [13]. Among these, saliva testing offers the advantages of being easily collected and noninvasive.  
Saliva is mainly secreted by the major and minor salivary glands, containing exfoliated epithelium and gingival crevicicular fluid with antibacterial and buffering properties [14]. Saliva also includes biomarkers valuable for disease identification and screening, facilitating its development for systemic and dental diseases [15]. In dental applications, saliva biomarkers are used to screen for oral cancer [16, 17], dental caries [18, 19], periodontal disease [20], and Sogren's syndrome [21, 22]. Saliva has been used diagnostically for systemic diseases in Europe and the U.S., such as heart failure [23], cardiovascular disease [24], diabetes [25], Parkinson's disease [26], and Alzheimer's disease [27]. During the COVID-19 pandemic, saliva\(-\)based PCR testing became widely recognized [28]. Salivaconsists of 99.5% water and 0.5% inorganic and organic components [29]. The proteins, lipids, mRNAs, miRNAs, and hormones that make up this 0.5% component are already being used and investigated as potential biomarkers [30]. On the other hand, mRNA levels are studied in diseases such as vascular aging [31], oral cancer [16], and asthma [32], presenting potential as a lipid abnormality biomarker.  
Cytidine/uridine monophosphate kinase 2 (_CMPK2_), a mitochondrial DNA (mtDNA) synthase, is a molecule whose function is rapidly being elucidated [33]. mtDNA synthesis activates the Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome [34]. _CMPK2_ activates mtDNA synthesis in atherogenic macrophages from mice with abnormal lipid metabolism, thereby exacerbating inflammation [35]. Moreover, elevated _CMPK2_ expression has been demonstrated to activate mitochondrial synthesis in the upstream pathway of inflammatory cytokine release [36]. _CMPK2_ mRNA has been implicated in diseases such as atherosclerosis [37], rheumatoid arthritis [38], systemic lupus erythematosus [39], ischemic stroke [40], sepsis therapy [41], osteoarthritis [42], and primary Sjogren's syndrome [43]. The mechanism of _CMPK2_ expression activation via Toll-like receptors (TLRs) has recently been elucidated [44]. Typically, TLRs are known to accept lipopolysaccharide [45], damage-associated molecular patterns (DAMPS) [40], and lipids [46], among others. Dyslipidemia, which increases blood lipid levels, may increase lipid binding to TLRs, further upregulating _CMPK2_ expression in cells. The increased inflammation and mitochondrial dysfunction in the salivary glands of mice fed a high-fat diet [47] suggests that dyslipidemia may also affect the salivary glands. Given that our previous study identified _CMPK2_ mRNA in human saliva, we hypothesized that CMPK2 in salivary glands and saliva may be altered by dyslipidemia. However, the direct relationship between dyslipidemia and elevated _CMPK2_ mRNA expression in saliva has not yet been fully characterized.  
This study aimed to examine the association between serum lipid levels and _Cmpk2_ mRNA in saliva, as well as CMPK2 protein localization in salivary glands, utilizing two types of dyslipidemia mice (high-fat and high-cholesterol [HFHC] diet mice and ApoE-deficient mice) for the prospective development of an early detection method for dyslipidemic states.