Abstract

   Astragalus polysaccharides (APS) have a wide range of biological
   activities. Most researchers discuss total APS as the main research
   object. However, because the relative molecular weight of APS has a
   wide distribution, in-depth studies on the mechanisms of the biological
   activity of notable molecules are limited. For example, the
   relationship between the immunomodulatory effect of APS and its
   relative molecular weight has not been clearly defined. Therefore, in
   this paper, we separated and obtained APS of different molecular
   weights by ultrafiltration technology and then constructed a mouse
   cyclophosphamide-induced immunosuppression model to investigate the
   immune activity of APS of different molecular weights. The immune
   enhancement mechanism of APS was explored by examining changes in
   routine blood indicators, body weight, immune organs, and differential
   metabolites in mouse serum. Results showed that APS-I (molecular
   weight, >2,000 kDa), APS-II (molecular weight, 1.02 × 10^4 Da) and
   APS-III (molecular weight, 286 Da) could increase the number of immune
   cells in mouse serum and improve immune organ damage to varying
   degrees. Among the samples obtained, APS-II showed the best effects.
   Compared with those in the blank group, 29 metabolites determined by
   UHPLC Q-Exactive MS in the serum of the model group changed remarkably,
   and APS-I, APS-II, and APS-III respectively restored 13, 25, and 19 of
   these metabolites to normal levels. Metabolomics analysis revealed that
   APS-II is mainly responsible for the immunomodulatory activity of APS.
   Metabolomics analysis revealed that the mechanisms of this specific
   molecule may involve the regulation of phenylalanine metabolism,
   cysteine and methionine metabolism, tricarboxylic acid cycle (TCA
   cycle) and arginine and proline metabolism.

   Keywords: Astragalus polysaccharides, structural analysis,
   metabolomics, serum, immunomodulation, differential metabolites

Introduction

   Astragali Radix (AR) refers to the dried root of the perennial legume
   Astragalus mongholicus Bunge, which is a traditional Chinese medicine.
   It is listed as an upper-grade material in Shen Nong’s Herbal Classic
   and has been used as an immunomodulator in traditional Chinese medicine
   prescriptions for over 2,000 years. AR has a wide range of uses; for
   example, it can invigorate the qi, solidify surfaces, reduce water
   retention and swelling, and support sore muscles. The Ministry of
   Commerce of China has listed AR as among the country’s 60 strategic key
   Chinese medicinal materials and 18 main Chinese medicinal materials. In
   addition, in 2018, AR was included in the Chinese medicine and food
   homology list.

   Modern phytochemical and pharmacological experiments have shown that
   polysaccharides have very strong immunomodulatory activity and are
   highly abundant in AR ([38]Wu X. et al., 2011). Complex polysaccharide
   mixtures may be obtained by water extraction and alcohol precipitation
   from AR. Astragalus polysaccharides (APS) are composed of
   polysaccharides with diverse structures and different molecular weights
   distributed over a wide range (5.6 × 10^3–4.8 × 10^6 Da) ([39]Jin et
   al., 2014). The molecules of APS are mainly composed of different types
   of functional groups, including dextrans and arabinogalactans.

   Research on the activity of APS often describes their ability to
   regulate non-specific immunomodulation, enhancement of specific and
   non-specific immunity. APS are known to have a variety of biological
   activities, such as antitumor, antidiabetic, antiviral, cardiovascular,
   and neuroprotective effects. Studies have shown that these
   pharmacological activities generally involve immunomodulation
   ([40]Morten et al., 2008; [41]Jiang et al., 2013; [42]Li et al., 2017).
   Unfortunately, most available studies focus on total APS or a certain
   component of APS ([43]Table 1). At present, reports on the separation
   of total APS according to molecular weight and the structural analysis
   and immunological screening of the resulting components are scarce.
   Therefore, clarifying the relationship between the molecular weight,
   structure and activity of different APS components is necessary to
   elucidate the medicinal material basis of AR.

TABLE 1.

   Structure characterization of Astragalus polysaccharides (APS).
   No. Name Molecular weight Main structure Bioactivity References