Tertiary lysosome

A problem with employment of ASON in a larger clinical setting is their poor uptake and inappropriate intracellular compartmentalization, e.g., sequestration in endosomal or lysosomal complexes. In addition, there is a need for a very careful selection of the ASON-mRNA pair sequences that would most efficiently hybridize. To date, several computer programs are used to predict the secondary and tertiary structures of the target mRNA and, in turn, which of the mRNA sequences are most accessible to the ASON. However, even with this sophisticated techniques, the choice of base-pairing partners still usually includes a component of empiricism. Despite these principal limitations, it has become clear that ASON can penetrate into cells and mediate their specific inhibitory effect of the protein synthesis in various circumstances. 

As an example, the low-density lipoprotein (LDL) molecule and its receptor (Chapter 25) are internalized by means of coated pits containing the LDL receptor. These endocytotic vesicles containing LDL and its receptor fuse to lysosomes in the cell. The receptor is released and recycled back to the cell surface membrane, but the apoprotein of LDL is degraded and the choles-teryl esters metabolized. Synthesis of the LDL receptor is regulated by secondary or tertiary consequences of pinocytosis, eg, by metabolic products—such as choles-... 

Secondary events result from primary events, for example, changes in membrane structure/permeability, mitochondrial damage, and lysosomal destabilization. Tertiary events are final observable manifestations, for example, fatty change and phospholipidosis, apoptosis, blebbing, and necrosis. 

Most polymer-based carriers for the delivery of nucleic acid drugs must escape the endosomes before complete acidification, which activates lysosomal digestion. After the discovery of the powerful endosomal destabilization activity of PEI [66], many polymer-based carriers have mimicked the structure of PEI for endosomal escape. As explained in Sect. 3.2, the proton-sponge effect of xmprotonated tertiary amines and direct contact of protonated polyamines with the endosomal membrane are two possible mechanisms of endosomal disruption by PEI. Because pH-dependent protonation is critical in both mechanisms, polymers with a high density of protonable amines during the early endosomal acidification firom pH 7.4 to 5.5 are one of the main kinds of polymer-based carriers with an endosomal escape function. Like tertiary amines in PEI, protonable moieties with low p Ta values have been frequently introduced into the polymer-based carriers. An imidazole moiety with pA"a of around 6.0 was one such candidate. The introduction of polyhistidine with an imidazole moiety on a PLL backbone showed significant increase in endosomal escape efficiency [169]. 

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