The present invention relates to modular systems for vaccination against infectious agents that involves the delivery of, e.g., exosome-loaded, antigen-encoding mRNAs to and into cells and tissues of the immunized subject. The present invention also relates to compositions and methods for the design, preparation, manufacture, formulation, and/or use of vaccines, e.g., nucleic acid vaccines, loaded into extracellular vesicles, e.g., exosomes loaded with synthetic mRNAs encoding multiple surface and cytoplasmic antigens of interest, e.g., antigenic polypeptides derived from an infectious virus, e.g., SARS-CoV-2, designed to elicit strong humoral and cellular immune responses due to the simultaneous expression of antigens in their native state and as exosome-associated antigens.

The disclosure relates to nucleic acid expression cassettes for the treatment of neurodegenerative disorders. Methods of treating neurodegenerative disorders such as Alzheimer's disease, frontotemporal dementia, frontotemporal lobar degeneration, Pick's disease, Lewy body dementia, memory loss, cognitive impairment, and mild cognitive impairment are also provided.

The disclosure features compositions, systems, and uses thereof, comprising polynucleotide(s) encoding target molecule(s), and polynucleotide(s) encoding repressor(s) or timer(s). The repressor(s) or timer(s) delay expression of the target molecule(s).