Highlights d Specific membrane-embedded ER proteins target and accumulate on LDs d Minimal targeting motifs use Trp and positive-charged residues for LD accumulation d Distribution of these residues within motifs correlates with LD accumulation
Lipid droplets (LDs) are intracellular
storage organelles composed
of neutral lipids, such as triacylglycerol (TG), surrounded by a phospholipid
(PL) monolayer decorated with specific proteins. Herein, we investigate
the mechanism of protein association during LD and bilayer membrane
expansion. We find that the neutral lipids play a dynamic role in
LD expansion by further intercalating with the PL monolayer to create
more surface-oriented TG molecules (SURF-TG). This interplay both
reduces high surface tension incurred during LD budding or growth
and also creates expansion-specific surface features for protein recognition.
We then show that the autoinhibitory (AI) helix of CTP:phosphocholine
cytidylyltransferase, a protein known to target expanding monolayers
and bilayers, preferentially associates with large packing defects
in a sequence-specific manner. Despite the presence of three phenylalanines,
the initial binding with bilayers is predominantly mediated by the
sole tryptophan due to its preference for membrane interfaces. Subsequent
association is dependent on the availability of large, neighboring
defects that can accommodate the phenylalanines, which are more probable
in the stressed systems. Tryptophan, once fully associated, preferentially
interacts with the glycerol moiety of SURF-TG in LDs. The calculation
of AI binding free energy, hydrogen bonding and depth analysis, and
in silico mutation experiments support the findings. Hence, SURF-TG
can both reduce surface tension and mediate protein association, facilitating
class II protein recruitment during LD expansion.
Mortality caused by age-related bone fractures or osteoporosis is steadily increasing worldwide as the population ages. The pace of the development of bone regeneration engineering to treat bone fractures has consequently increased in recent years. A range of techniques for bone regeneration, such as immunotherapy, allografts, and hydrogel therapy, have been devised. Cell-based therapies using bone marrow-derived mesenchymal stem cells and induced pluripotent stem cells derived from somatic cells are considered to be suitable approaches for bone repair. However, these cell-based therapies suffer from a number of limitations in terms of efficiency and safety. Somatic cells can also be directly differentiated into osteoblasts by several transcription factors. As osteoblasts play a central role in the process of bone formation, the direct reprogramming of fibroblasts into osteoblasts may hence be a new way to treat bone fractures in elderly individuals. Here, we review recent developments regarding the therapeutic potential of the direct reprogramming of cells for bone regeneration.
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