Objective  To investigate the influence of the protamine sulfate on endocytosis and intracellular stability of tetrahedral framework nucleic acid (tFNA).

  Methods  Articular cartilage cells were collected from 3-day-old C57BL mice. Cells at passage 1-2 were used in the experiments. 4 single-strand DNAs (S1 was marked by Cy5) were utilized to synthesize tFNAs via annealing process and ultrafiltration for purification. High-performance capillary electrophoresis (HPCE) was used to verify synthesis of tFNAs and transmission electron microscope was used to photo morphological characteristics. The 1 mg/mL protamine sulfate solution was slowly dropped into newly synthesized tFNAs (N/P=5/1). Then, Zeta potential was detected. Cells were treated with 100 nmol/L tFNAs with protamine sulfate in Dulbecco’s Modified Eagle’s medium (DMEM) (Exp.1), 100 nmol/L tFNAs in DMEM (Exp.2), and DMEM (Control), respectively. Flow cytometry was used to quantitatively detect intracellular Cy5 fluorescence after 6 h and 12 h treatments. Immunofluorescence staining was used to qualitatively observe internalized Cy5 fluorescence after 12 h treatment by laser confocal microscope. Lysosome of living cells were stained with lysosome probe. Colocalization between lysosome and tFNAs was observed by laser confocal microscope.

  Results  After incubating protamine sulfate, negative potential was transformed into positive one ( (−1.567±0.163) mV to (4.700±0.484) mV). The fluorescence intensity of tFNAs in the Exp.1 group was higher than that of the Exp.2 group in 6 h and 12 h (P<0.05). This was consistent with the results of immunofluorescence staining after 12 h. Colocalization of Cy5 fluorescence and lysosome in the Exp.1 group was more rare than that in the Exp.2 group at 6 h and 12 h. Furthermore, a large amount of Cy5 fluorescence was still seen in the Exp.1 group at 12 h, while Cy5 fluorescence of the Exp.2 group was less.

  Conclusion  Protamine sulfate can effectively enhance endocytosis, and to some extent it can achieve lysosome escape of tFNAs.