Thread by @Biotech2k1

1/ Looking at Fusogen:

This is right from the 10K so Its not my original work, but I don't think I could say it any better.

2/ Background on Fusogens:

Fusogens are a well-studied class of naturally occurring proteins that mediate the trillions of cell-to-cell and intracellular fusion events occurring
in the human body every second.

3/ In 2013, the Nobel Prize in Physiology or Medicine was awarded for the elucidation of the roles of fusogens in
mediating intracellular trafficking in nature. First, fusogens enable recognition of a specific target membrane.

4/ Second, they promote membrane fusion by
acting as thermodynamic engines for opposing membranes, pulling them together and thereby promoting fusion.

5/ Fusogens are widely used by enveloped viruses to confer target specificity and to drive the process of introducing material in target cells. A wellknown current example of a viral fusogen is the SARS-CoV-2 coronavirus that causes COVID-19.

6/ This virus uses its spike glycoprotein to target cells
expressing the ACE2 receptor and to fuse with the cell membrane of host cells and release the viral genome into the cell.

7/ Many other biological
processes utilizing fusogens for the delivery of complex, diverse, and large payloads to specific cell types have also been found.

8/ For example, the
process of fertilization occurs as a result of a sperm fusing specifically with the egg and the transfer of the paternal genetic material to the oocyte.

9/ Similarly, the fusion of myoblasts with other myoblasts is essential for the formation, growth, and regeneration of skeletal muscle.

10/ The myoblast
delivers an entire novel nucleus to the muscle cell, highlighting the utility of this system to deliver quite large and complex payloads. These and a
myriad of other processes rely on this vast class of protein machines.

11/ Applying fusogens to in vivo cell engineering
Building on both our team’s deep understanding of fusogen biology and extensive research in protein engineering,

12/ we are developing a technology
designed to allow us to engineer the biological properties of these naturally occurring proteins.

13/ In doing so, we are developing a highly modular system that can specifically target numerous cell surface receptors and thereby deliver diverse
therapeutic payloads to a variety of cell types.

14/ Our current programs use fusogens derived from a virus from the paramyxoviridae family. The fusogen protein complex is comprised of two
proteins: the receptor recognition G protein and membrane fusion F protein.

15/ The combination of a fusogen with a delivery vehicle such as a gene
therapy vector or lipid vesicle is referred to as a fusosome. The diagram below depicts the mechanism of fusogen-mediated membrane fusion. This
protein complex is found on the outer membrane of the fusosome

16/ (1). As the fusosome interacts with cells, only those with the target receptor will
engage the G protein of the fusogen complex

17/ (2). The binding of the G protein to the receptor stimulates the F protein to initiate its membrane fusion
activity. The F protein first partially unfolds to bind to the target membrane

18/ (3) and then refolds to bring the target and fusosome membranes in
proximity

19/ (4), to ultimately promote membrane fusion (5), and subsequent payload delivery.

20/ The G protein has the potential to be engineered for a high degree of cell selectivity. To accomplish this, we first engineer the G protein so that its
natural binding domain is no longer functional.

21/ We then add a targeting scaffold to the G protein that re-directs the fusogen to a cell-specific receptor.
The targeting scaffold can be any one of naturally occurring or synthetic single chain affinity binders, such as single chain variable fragment (scFvs),

22/ camelid single-domain antibodies (VHHs), or designed ankyrin repeat proteins (DARPins). Finally, we iteratively rebuild our fusogen using insights
from protein engineering to improve titers, or potency.

23/ By serially swapping different targeting scaffolds we believe we can target multiple different cell
surface receptors, giving us the ability to target many different cell types.