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PODS™ FAQs

PODS™ FAQs

1)  How big are PODS™ crystals?

PODS crystals are cubic and typically 1-15 microns in size with a modal size of 3-4 microns. During the manufacturing process, some PODS crystals may fracture resulting in smaller PODS crystal fragments.

2)  What is a PODS crystal's buoyant density?

PODS crystals are 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).

3)  How stable are PODS crystals?

A variety of conditions have been tested to assess impact PODS crystal integrity. PODS crystals are stable in high temperatures and remain intact after boiling for 1 hour. They are highly stable when stored in solutions between pH 6-8. 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 (> 10 days).

4)  How is the cargo protein released?

In addition to pH, PODS protein crystals are broken down in solutions containing proteases. Proteases may be derived from components of the solution (e.g. serum) or secreted by 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 has been shown to occur at day 2, then gradually diminishes.

5)  Is it possible to modify the release profile?

The release of cargo proteins into cell culture occurs over a period of 2-3 weeks. This release period can be extended to several months, if the PODS crystals are combined with a scaffold, such as a collagen.

6)  Will PODS crystals adhere to plastics?

PODS crystals will adhere to untreated plastics. For sub-aliquoting, we recommend using low adherence plastics, such as low protein binding centrifuge tubes.

7)  Why are PODS proteins sold by numbers of crystals?

PODS cargo proteins are incorporated into the polyhedrin crystal within the insect cell. Consequently, it is not possible to obtain a direct measurement of the amount of cargo protein that is present in a sample. When PODS crystals are expressed, both polyhedrin proteins and cargo proteins are expressed from within a single vector and both utilize the same promoter sequence. Therefore, the ratio of cargo 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 50x10^6 PODS crystals typically generates a peak concentration in the medium equivalent to 3.3 µg of standard recombinant protein.

8)  How many PODS crystals should be used?

PODS crystals provide a depot of proteins which are steadily secreted. As stated above, it has been estimated that the biological activity of 50x10^6 PODS crystals generates the same peak dose as 3.3 µg of standard recombinant protein. 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 is optimal for a particular experiment should be determined empirically, using 50x 10^6 PODS crystals equivalence to 3.3 µg of standard growth factor as a good starting point.

9)  How often should media and PODS crystals 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.

10)  Are PODS crystals transparent?

PODS are crystalline and will refract light. A large number of PODS crystals may interfere with the generation of images using imaging techniques such as phase-contrast.

11)  Can PODS crystals have a physical impact on cell behavior?

Physical features on a culture surface may impact the behavior of certain cells although we have not observed any noticeable effects in the cultures we have tested. If PODS crystal topology is a concern, PODS crystals may be incorporated into a hydrogel surface coating.

12)  Do PODS proteins have important post-translational modifications?

PODS crystals are made in insect cells. PODS cargo proteins, therefore, contain most of the post-translational modifications that are found in mammalian cells. For example, PODS Wnt-3a, which requires palmitoylation and glycosylation for biological activity, is functional.

13)  Are PODS crystals immunogenic?

The polyhedrin protein has been tested in vivo and an inflammatory response has not been observed.  Lack of immunogenicity for foreign proteins is not without precedent: silk fibroin protein from the silkworm B. mori is commonly used for surgical stitching and Botulinum toxin is widely injected in cosmetic procedures to reduce the appearance of wrinkles.

14)  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 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 released will be the same as their standard recombinant counterparts.

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.

15)  Why are PODS proteins 100% bioactive?

PODS cargo proteins such as growth factors are captured and purified without denaturation. Unlike standard recombinant proteins, they do not require refolding or successive column purification steps. Consequently, biological activity is inherent.

16)  Do you offer custom services?

Yes, we can produce your protein in PODS crystals for both research and therapeutic applications. For further information, please contact: sales@mayflowerbio.com