Controlling Neural Stem Cell Differentiation with MOFs

Neural stem cells need special conditions to grow and differentiate, which are not easy to mimic in culture. Kyung Min Choi, Sookmyung Women’s University, Seoul, Republic of Korea and
LabInCube Co. Ltd., Cheongju-si, Republic of Korea, Tae-Hyung Kim, Chung-Ang University, Seoul, Republic of Korea, and colleagues have developed a platform that accelerates differentiation by providing a safe and stable supply of chemicals using metal–organic frameworks (MOFs).

This platform combines a physical support for cell growth and the MOF-based reservoirs. It can be adapted to different requirements.

Cell Differentiation Factors

The brain is the natural environment for neural stem cells. Supported by proteins and supplied with the right growth factors at the right time, these stem cells differentiate into neuronal cells. In the lab, growth and differentiation are much more difficult to control. Current methodology involves placing the cells on a flat surface in a suitable medium to make them adhere and grow, and feeding them nutrients and biomolecules to trigger differentiation. However, this procedure is far from perfect.

First, the nutrients and factors in the growth medium are at a set concentration that might not be optimal for every stage of cell development. Second, some nutrients are sensitive to air and degrade over time. Third, the necessary exchange of spent medium for fresh medium can lead to abrupt concentration changes, potentially damaging the growing cells. To achieve better control over the supply of biomolecules, the team devised a platform with the molecular reservoirs already included.

MOF-Based Reservoirs in Nanopits

This platform consisted of a centimeter-sized glass plate with a large-scale array of nanowells (or “pits”), created using a technique called laser interference lithography. Less than half a micron wide and even deeper, the nanopits were designed to contain a single MOF particle each. The researchers reported that almost every nanopit could be filled with a single MOF nanocrystal; scanning electron micrographs revealed they fitted the width of the pits exactly.

With this setup, the MOF reservoirs would not touch the growing, differentiating cells but would still be close enough to supply them with the chemicals they needed. According to the researchers, maintaining physical distance was crucial because otherwise cell development would be disturbed, and the MOF reservoirs would be destroyed.

Protection from Degradation

The researchers chose zirconium-based UiO-67 as the MOF, which was synthesized from zirconium chloride and biphenyl-4,4′-dicarboxylic acid. Having a roughly octagonal shape and a diameter of less than 200 nm, the MOF nanocrystals provided a large enough pore space to take up plenty of retinoic acid molecules. Retinoic acid triggers cell differentiation; in this case, the transformation of neural stem cells into neuronal cells.

The MOF not only takes up retinoic acid and releases it slowly over several days or weeks, it also protects it from degradation. As a vitamin A (retinol) derivative, retinoic acid has alternating double bonds between the carbon atoms in its hydrophobic tail, which is attached to a six-membered ring. This all-trans electron structure reacts to sunlight and oxygen and, as a result, vanishes quickly from cell media, meaning it must be refilled at regular intervals.

However, confined within the MOF structure, there was no such degradation. The researchers reported a steady release of retinoic acid into the medium for days or even weeks. They also observed that multipotent cells fed with MOF-delivered retinoic acid differentiated more quickly to neuronal cells than control cells growing under “normal” conditions.

New MOF Usage

UiO-67, the MOF used in the experiments, is known as a robust crystalline substance with a high surface area and high stability against a range of chemicals. Suppliers mainly recommend it for gas absorption, but chemists also use it as a catalyst, for example, to destroy dangerous substances such as pesticides and nerve agents.

This reported usage as a chemical reservoir for cell growth may open a new chapter in biomedical applications. The team also highlights the versatility of the MOF crystals. Altering the organic biphenyl spacer would make it possible to adapt the pores to other biomolecules or to change the reservoir size. This could further expand its range of cell culture applications.