## 1 Introduction  
Nicotinamide mononucleotide (NMN), a naturally occurring biologically active nucleotide, can be naturally synthesized in cells and is also found in a variety of plants and animals [1; 2; 3], such as hairy beans, broccoli, shrimp, etc., with hairy beans containing the highest amount of NMN, up to 1.88 mg/100 g [4]. Studies have shown that NMN has a variety of physiological activities and has therapeutic effects on brain diseases such as Alzheimer's disease, Parkinson's disease, and cerebral hemorrhage [5; 6; 7]. The prerequisite for a drug to act on the brain and have therapeutic effects is that the drug reaches the site of action. As a highly specialized system of endothelial cells (mainly including brain endothelial cells, astrocytes, pericytes, tight junctions, etc.) that exists between circulating blood and central extracellular fluid, the blood-brain barrier (BBB) is a key barrier separating the blood from the underlying brain cells and is limiting for the delivery of drugs for the treatment of brain diseases [8; 9; 10; 11]. However, to our best knowledge, the ability of NMN to cross the blood-brain barrier (BBB) and the mechanism remains unclear.  
To date, the immortalized hCMEC/D3 cell line has become a useful model for screening drug candidates in the central nervous system [12; 13]. This cell line has the major advantages of a high proliferation rate and phenotypic invariance during passaging and retains most of the morphological and functional characteristics of brain endothelial cells when cultured alone [14; 15], which is more suitable for drug uptake and pathological modeling studies. The main pathways of drugs across the blood-brain barrier are passive transport, active transport, and cytosolization and cytotranspiration [16]. Although some drugs can penetrate the blood-brain barrier, drug-metabolizing enzymes and drug transporters on the blood-brain barrier inhibit the diffusion of the drug to the therapeutic site, resulting in poor bioavailability of the drug in the central nervous system [17; 18]. Researchindicates that liposomes modified with particular peptides have demonstrated remarkable effectiveness in promoting the passage of molecules across the blood-brain barrier [19; 20; 21]. This strategy shows promise for delivering drugs to the central nervous system, and our study intends to advance this field of NMN-focused research.  
In this study, the immortalized hCMEC/D3 cell line was selected to construct an in vitro cell monolayer BBB model to investigate the transmembrane capacity of NMN across the BBB and its transport mechanism. Moreover, we constructed an NMN-loaded liposome and modified the liposome surface with two short peptides, ANG and RVG, to enhance blood-brain barrier penetration and target nanovesicles to gliomas [22; 23; 24]. ANG peptides have a high affinity for low density lipoprotein receptor-related protein (LRP), which is overexpressed in both the blood-brain barrier and gliomas [22], and studies have shown that ANG peptides can enhance blood-brain barrier penetration and target nanovesicles to gliomas [22; 23; 24]. By evaluating their ability to penetrate the BBB, this study could lay a basis for the study of NMN in the development of new drugs in the central nervous system and provide new ideas for research related to NMN-targeted delivery.