Crystals in humans

13 March 2023

At high enough concentrations and under the right conditions, most molecules, including complex proteins and nucleic acids, will crystallize. Crystals of molecules have been important in allowing their structure to be determined using X-ray crystallography. Although the conditions for most molecules to crystalize do not exist in nature, there are a few exceptions, and some of these occur in humans. So if you look hard enough, microscopic crystals are very common in animals, including humans. Naturally occurring crystals play important roles in health as well as disease. Recombinant protein crystals are also being used, or are in development, as drug delivery systems providing therapies to treat diseases including diabetes, cancer, osteoarthritis and macular degeneration.

Insulin

Insulin is initially synthesised as proinsulin. This self-assembles into a soluble hexamer in the presence of zinc irons. The beta cells in which insulin forms have higher levels of zinc than the blood. This zinc reduces insulin solubility and crystallization occurs rapidly. Consequently, insulin is stored as crystalline granules in the insulin-producing beta cells located in the islets of Langerhans in the pancreas.

The function of the crystalline form of insulin is not clear, but it may serve to protect the insulin from further proteolytic processing in the production cell. When glucose levels rise in the blood, these insulin nano-scale crystals, which are 200–300 nm across, are secreted into blood by exocytosis. Upon secretion the granules experience increased pH and decreased concentrations of zinc which causes the crystals to dissolve rapidly to give the functionally active monomeric form of insulin which can be adsorbed by other cells.

This particular propensity of insulin to form crystals has been exploited to create crystalline depot drug formulations. For example, adding zinc ions to recombinant insulin in solution promotes the formation of hexagonal insulin crystals. This formulation, known as Ultralente insulin, generates microcrystals about 25 x 25 x 5 microns in size. When injected into the blood, the crystals slowly dissolve over 36 hours providing sustained availability of insulin.

Glargine insulin (Lantus®) is an insulin molecule with additional disulphide bridges. Rather than being crystalline before deployment, Lantus is soluble in its storage buffer. It becomes crystalline upon injection into the blood which has a different pH to the buffer. Glargine insulin has replaced Ultralente and is able to provide stable baseline levels of insulin over a 24-hour period.

White blood cells

Granulocytes (including neutrophils, eosinophils and basophils) are characterized by secretory vesicles packed with potent proteins and chemicals which can be rapidly released to combat infection. Some of the proteins in these vesicles are stored in crystal forms. For example, eosinophils contain crystals of the eosinophil basic protein (EMBP). In addition, Galectin-10 released by Eosinophils during an inflammatory response can result in hexagonal bipyramidal protein crystals known as Charcot-Leyden crystals which are subsequently taken up by macrophages. It has been suggested that these crystals might provide a quick way of removing excess protein from the microenvironment.

Pineal Gland

The pineal gland (glandula pinealis) is the neuroendocrine gland localized at the epithalamus of the brain. Light/dark (LD) conditions control pineal gland activity and secretion of melatonin. This hormone influences circadian rhythms, reproductive function in photoperiodic mammals, body temperature, immunity, the cardiovascular system and antioxidant activity. Calcium plays an important role in the pineal gland. Two types of calcified granular material can be detected in the mammalian pineal gland: 1) the meningeal calcification and 2) the intraparenchymal calcification, which origin relates to the function of pinealocytes. It has been suggested that calcification reduces the production of melatonin which may, in turn, lead to health issues such as the promotion of neuronal disease.

Inner ear

Otoliths, or otocania (ear stones, ear dust) are formed of calcium carbonate mixed with the otoconin protein. In humans, these crystals have a hexagonal shape. These structures are found suspended in the canals of the inner ear. It has been demonstrated that otocania, which are found in fish and birds as well as mammals, are involved in balance.

Bone

Bone is mostly made of collagen and apatite, a mineral. In addition, needle-shaped crystals have been identified twisted into patterns that resemble propeller blades. These patterns form natural fractals which repeat adding strength to the bone.

Articular joints

Gout is a form of inflammatory arthritis that results in painful, swollen joints. It is caused by the accumulation of monosodium urate (MSU). This forms needle-like crystals that are associated with a rich diet as well as genetic factors. Gout affects about 2% of adults in developed countries.

Inflammation is triggered when the needle-like crystals are phagocytically ingested by macrophages. Typically, particles that have undergone phagocytosis are broken down within the enzyme-laden, acidic phagolysosome. However, the integrity of this structure is not maintained with the needle-like MSU crystals which pierce the phagolysosome membrane. This releases its contents which leads to the activation of the inflammasome and subsequently inflammation

PODS® crystals

PODS is a nature-mimetic technology that uses a polyhedron protein to create co-crystals 0.2 – 7 microns that can carry a cargo protein. PODS crystals have been generated for a variety of proteins which have been used to sustainably deliver the cargo protein for a sustained period of time. PODS crystals have been tested in pre-clinical models of disease including macular degeneration, hearing loss, osteoarthritis, bone remodelling and cancer.

IMAGE: Gout-causing MSU crystals Gabrielle Caponetti