Mechanisms of Amine Accumulation in, and Egress from, Lysosomes

Abstract

The human body is continuously exposed to small organic molecules containing one or more basic nitrogen atoms. Many of these are endogenous (i.e., neurotransmitters, polyamines and biogenic amines), while others are exogenously supplied in the form of drugs, foods and pollutants. It is well-known that many amines have a strong propensity to specifically and substantially accumulate in highly acidic intracellular compartments, such as lysosomes, through a mechanism referred to as ion trapping. It is also known that cells have acquired the unique ability to sense and respond to amine accumulation in lysosomes in an effort to prevent potential negative consequences associated with hyperaccumulation. We describe here methods that are used to evaluate the dynamics of amine accumulation in, and egress from, lysosomes. Moreover, we highlight specific proteins that are thought to play important roles in these pathways. A theoretical model describing lysosomal amine dynamics is described and shown to adequately fit experimental kinetic data. The implications of this research in understanding and treating disease are discussed.For over a century, scientists have studied the interactions between low-molecular-weight dyes and cells and tissues. Initially, it was serendipitously discovered that certain dye molecules preferentially associated with specific subcellular structures/compartments. These so-called 'vital stains', together with advances in cell imaging, provided a basis for diagnosing and understanding disease and later became crucial in basic scientific research, providing investigators tools with which to study basic aspects of cell structure and function.In this article, we will focus our attention on the accumulation and egress pathways for small-molecular-weight amine-containing molecules that specifically localize within lysosomes. Lysosomes are found in virtually all mammalian cells and contain various hydrolytic enzymes that function in the digestion and recycling of cellular materials (i.e., proteins, lipids and nucleic acids) [1][2][3][4][5]. These hydrolytic enzymes have optimal activity at low pH (4-5), which is maintained through the activity of the vacuolar H + ATPase (VATPase), which is located on the limiting membrane and functions in pumping protons from the cell cytosol to the luminal space [6,7]. The steep intracellular pH gradient that exists †

Lysosomal Sequestration (Trapping) of Lipophilic Amine (Cationic Amphiphilic) Drugs in Immortalized Human Hepatocytes (Fa2N-4 Cells)

“…The rapid reversibility of lysosomal trapping of a lipophilic amine is shown by the effects of asphyxiating rats with carbon dioxide, which slightly acidifies the blood and causes a slight decrease in tissue levels and an increase in plasma drug levels (Angus et al, 2008). Although passive diffusion is one mechanism by which lipophilic amines can exit lysosomes, there are other mechanisms by which this can occur, namely mechanisms more relevant for the recycling of phospholipidbound drugs (Goldman et al, 2009).…”

Atypical antipsychotics alter cholesterol and fatty acid metabolism in vitro

“…Antipsychotics can be classifi ed as hydrophobic amine or cationic amphiphile/ amphipath drugs, as they are positively charged by virtue of an amine group that can be protonated, and they display both hydrophilic and hydrophobic properties. It is well recognized that many amines have a strong propensity to specifi cally and substantially accumulate in highly acidic intracellular compartments, such as lysosomes, through a mechanism referred to as ion trapping ( 48 ). In their unionized form, weak basic amines are relatively membrane permeable, but they are membrane impermeable in their ionized conjugate acidic state ( 48 ).…”

“…It is well recognized that many amines have a strong propensity to specifi cally and substantially accumulate in highly acidic intracellular compartments, such as lysosomes, through a mechanism referred to as ion trapping ( 48 ). In their unionized form, weak basic amines are relatively membrane permeable, but they are membrane impermeable in their ionized conjugate acidic state ( 48 ). Some antipsychotic and antidepressant drugs have been shown to extensively accumulate in lysosomes ( 49,50 ).…”

Long Receptor Residence Time of C26 Contributes to Super Agonist Activity at the Human β2 Adrenoceptor

“…As such, the prolonged receptor internalisation caused by C26 may provide an additional mechanism of long duration of action at the β 2 adrenoceptor, which may be further exploited by targeting drugs to intracellular compartments e.g. by incorporating physiochemical properties that facilitate ion trapping (Goldman et al, 2009). …”