PODS® – Frequently Asked Questions

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How big are PODS® nanoparticles?

PODS® crystals are cubic and typically 0.2-5 microns in size with a modal size of 1-2 microns. 

What types of molecules can you put in PODS® nanoparticles?

PODS® can be used with protein cargos. Large proteins can be loaded. Nucleic acids may be possible. Small molecules are not suitable.

What is a PODS® nanoparticle's buoyant density?

PODS® co-crystals are slightly heavier than water and will settle on the surface of a culture vessel. Care should be taken when aliquoting since PODS® crystals will sink to the bottom of a tube within a few minutes. The majority of PODS® crystals will remain in suspension for up to 60 min in a 20% glucose solution (or a solution of similar density).

How stable are PODS® nanoparticles?

Cell GS have tested a variety of conditions to assess their impact on the integrity of PODS® crystals. PODS® crystals are stable at high temperatures for short periods. They are highly stable when stored in solutions between pH 6-8 and can withstand pH 3-10. Above pH 10, PODS® crystals lose their stability and dissolve rapidly (in a few hours). PODS® crystals are stable in standard cell culturing temperatures for extended periods of time (>6 months).

How is the active protein released?

PODS® protein cargo is released by proteases. Proteases may be derived from components of the solution (e.g. serum) or secreted by nearby cells. Therefore the culture system affects the amount of growth factor available in solution. In contrast to gel-encapsulated proteins (using hydrogels such as PLGA) PODS® crystals do not produce an initial burst release of cargo proteins. For naked PODS® crystals in cell culture, peak release in culture systems typically occur at day 2, then gradually diminishes.  Peak-availability is not the same as peak-release-rate and depends on a number of factors including the rate of protease released by cells and the inherent stability (half-life) of the cargo protein.

Is it possible to modify the release profile?

The release of active proteins into cell culture occurs over a period of 1-3 weeks. This release period can be extended if the PODS® crystals are combined with a scaffold, which acts as a barrier to protease activity. Release rates will also be influenced by the presence of proteases in the scaffold. Bovine collagens, for example, will contain proteases.

Will PODS® nanoparticles adhere to plastics?

PODS® crystals will adhere to untreated plastics and are therefore ideal for micro-patterning and scaffold functionalization. For sub-aliquoting, we recommend using low adherence plastics, such as low protein binding centrifuge tubes. 

Why are PODS® sold by numbers of particles?

PODS® active proteins are incorporated into the polyhedrin crystal within the insect cell. Consequently, it is not possible to determine the amount of active protein that is present in a sample by measuring optical density. When PODS® crystals are expressed, both polyhedrin proteins and active proteins are expressed from within a single vector and both utilize the same promoter sequence. Therefore, the ratio of active to polyhedrin protein will be constant. The size distribution of PODS® crystals is also constant. Consequently, the number of crystals has been adopted as the unit of quantification for PODS®. It has been estimated that 50x106 PODS® crystals typically generates a peak concentration in the medium equivalent to 3.3 µg of standard recombinant protein.

How many PODS® nanoparticles should be used?  

PODS® crystals provide a depot of active proteins which are steadily secreted. As stated above, in one experiment it has been estimated that the biological activity of 50x106 PODS® crystals generates the same peak dose as 3.3 µg of standard recombinant protein in a LIF-dependent mouse ESC culture system. However, at 5 days following the start of seeding the PODS® crystals, more than 50% of these peak levels are still present in the culture system. Ultimately, the amount of PODS® crystals that are optimal for a particular experiment should be determined empirically, using 50x 106 PODS® crystals equivalence to 3.3 µg of standard growth factor as a good starting point.

How often should media and PODS® nanoparticles be replaced in the culture system?

The frequency of media change depends on (1) the speed with which nutrients are exhausted or degraded and (2) the speed with which toxic metabolites accumulate. In most cell culture systems, the stability of growth factors is an over-riding issue which drives media replenishment. However, particularly in cells with high levels of metabolic activity, other factors will eventually become important once the stability issue has been addressed. We have maintained growth-factor-dependent cultures for several weeks without needing to add new PODS® crystals.

Are PODS® nanoparticles transparent?

PODS® without any cargo are isotropic crystals that do not refract light. However, any cargo protein impacts on this property such that they will refract light. PODS® crystals generally do not interfere with the generation of images using imaging techniques such as phase-contrast and may also interfere with an assay that utilizes the measurement of optical density. 

Can PODS® crystals have a physical impact on cell behaviour?

Physical features on a culture surface may impact the behavior of certain cells, particularly at high densities. If PODS® crystal topology is a concern, PODS® crystals may simply be incorporated into a hydrogel surface coating. Phagocytic cells (such as macrophages) will ingest PODS®. We have not observed any effect on the health of these cells. However, if this is a concern, the PODS crystals may be incorporated into a gel. Custom-made Matrigen™ Softwell plates (flat hydrogels of defined elasticity) containing PODS® crystals are available from Cell Guidance Systems. 

Do PODS® cargo proteins have post-translational modifications? 

PODS® crystals are made in insect cells. PODS® active proteins, therefore, contain most of the post-translational modifications that are also found in mammalian cells. 

Are PODS® nanoparticles immunogenic?

The polyhedrin protein has been tested many times in-vivo and, although antibodies are generated against polyhedrin, an obvious inflammatory response has not been observed.  Lack of an inflammatory response for foreign proteins is not without precedent: silk fibroin protein from the silkworm B. mori is commonly used for surgical stitching and Botulinum toxin (Botox) is widely injected in cosmetic procedures.

Are PODS® proteins equivalent to standard growth factors?

The stability of standard recombinant growth factors varies significantly. The most labile, such as FGF-1, have a sera half-life measured in minutes limiting their utility. The stability of PODS® growth factors that are encased in PODS® crystals is much longer but once a molecule is released will have the same half-life as their standard recombinant counterparts. This may be advantageous for 3D applications as it reduces the path length

The amount of available growth factor in a culture system is a function of the speed of release from the crystal and the subsequent stability. In the first few hours of a culture containing only newly added PODS® crystals, there is little growth factor protein available for the cells. Significant amounts of growth factor protein are available after one day. The initial lag of protein availability may be corrected if necessary, by adding a small amount of standard recombinant growth factor.