The present disclosure relates to polypeptides, such as fibronectin type III (FN3) domains that can bind CD71, their conjugates, isolated nucleotides encoding the molecules, vectors, host-cells, as well as methods of making and using the same.

Carriers such as lipid nanoparticles are useful for delivering payloads (such as RNA, DNA, proteins, or small molecules) to cells such as erythroid cells. This disclosure provides improved carriers that comprise a targeting agent that binds a cell surface receptor. In some embodiments, the cell surface receptor is one that undergoes a high rate of endocytosis.

The present disclosure relates to polypeptides, such as fibronectin type III (FN3) domains that can bind CD71, their conjugates, isolated nucleotides encoding the molecules, vectors, host-cells, as well as methods of making and using the same.

Provided herein are polypeptides that bind to a transferrin receptor, methods of generating such polypeptides, and methods of using the polypeptides to target a composition to a transferrin receptor-expressing cell.

Provided herein are polypeptides that bind to a transferrin receptor, methods of generating such polypeptides, and methods of using the polypeptides to target a composition to a transferrin receptor-expressing cell.

Provided are genetically modified non-human animals that express a human or chimeric (e.g., humanized) TFR1, and methods of use thereof.

In some aspects, the present invention provides chimeric transferrin receptor (TfR) polynucleotides and polypeptides. In other aspects, this invention provides chimeric TfR transgenic animal models and methods of using the animal models to identify therapeutics that can cross the blood-brain barrier.

Methods for the in vitro production of enucleated red blood cells and the enucleated red blood cells thus prepared are provided. Such enucleated red blood cells may express a sortaggable surface protein, which allows for surface modification in the presence of a sortase. Also described herein are surface modified enucleated red blood cells, e.g., conjugated with an agent of interest such as a peptide, a detectable label, or a chemotherapeutic agent, and uses thereof in delivering the agent to a subject.

In some aspects, the present invention provides chimeric transferrin receptor (TfR) polynucleotides and polypeptides. In other aspects, this invention provides chimeric TfR transgenic animal models and methods of using the animal models to identify therapeutics that can cross the blood-brain barrier.

Provided herein are polypeptides that bind to a blood-brain barrier (BBB) receptor, methods of generating such polypeptides, and methods of using the polypeptides to target a composition to a BBB receptor-expressing cell, e.g., for transport across the BBB. Also provided herein are transferrin receptor (TfR) constructs that comprise a monomeric TfR apical domain or one or more portions of the TfR apical domain which have been circularly permuted relative to the full-length TfR sequence.