Homeostasis Of Energy Conduction, Neurotransmitters, Cytotoxic Compounds And Metal Ions

Neurotransmission is very dynamic process that superficially appears to contradict homeostasis. However, close examination of this process indicates the necessity of neurotransmitter homeostasis for brain function. The fate of monoamines and glutamic acid within and outside nerve cells is a good example of the dynamics of such homeostasis. Although glutamic acid can be accumulated in the cytoplasm of neurones to relatively high concentrations (millimolar) without apparent damage, similar concentrations outside the cell are cytotoxic and frequently lead to cell death (Attwell et al. 1993). Fig. 1 depicts some elements involved in neurotransmitter and metal-ion homeostasis in nerve cells. ATP is usually maintained at a high concentration in the cytoplasm and at a low concentration outside the cell. Mitochondria are the main providers of ATP except under oxygen stress. Most animals cannot survive prolonged anoxia without brain damage. Those animals that can survive prolonged anoxia utilize two main strategies to maintain high levels of ATP under oxygen deprivation: a drastic decrease in metabolic processes or an increase in their glycolytic activity (Perez-Pinzon et al. 1992; Lutz et al. 1995). Neuromodulators and neurotransmitters play a key role in the processes that are accompanied by changes in metal-ion homeostasis. ATP drives several primary pumps that function in the maintenance of the appropriate concentrations of neurotransmitters and metal ions in the various cell compartments. However, most of the energy for the transport systems is provided by two primary pumps: the Na⁺/K⁺-ATPase on the plasma membrane and the vacuolar H⁺-ATPase (V-ATPase) in the vacuolar system (Nelson, 1992). Consequently, most of the transport processes in the vacuolar system are driven by an electrochemical gradient of protons and most of the transport systems in the plasma membrane are driven by an electrochemical gradient of Na⁺.

Monoamines function as hormones and neurotransmitters in a variety of systems both within and outside the CNS. They are very reactive compounds and are cytotoxic at quite moderate concentrations (Schuldiner, 1994). Therefore, their homeostasis inside and outside the cell as well as their accumulation at high concentrations in specialized granules is one of the consequences of their cytotoxicity. Two principal transporters function in their homeostasis. One is drive by a Na⁺ gradient and is located in the plasma membrane, and the other (vesicular monoamine transporter) is located in the vacuolar system and is driven by an H⁺ gradient. Transporters similar to the mammalian vesicular monoamine transporters are present in bacteria. Remarkably, these transporters function in detoxification of the bacteria and can render them drug-resistant (Schuldiner, 1994). Evolution has apparently led to the utilization of pre-existing bacterial multidrug transporters for monoamine neurotransmission in the vertebrate brain.

The environment of unicellular and higher organism is laced with poisonous compounds, some of which are self-produced by their own metabolic pathways. To sustain life, the cells must get rid of these cytotoxic compounds and, in essence, achieve a homeostasis of the poison. The process of waste and poison removal from the cells involves special transporters that recognize the damaging compounds and transport them out of the cell. These transporters are divided into two main families, one utilizing the energy of ATP and the other utilizing the energy of ion gradients (Kanner, 1989; Nelson, 1992). Most transporters of both families have a very broad specificity for substrates and most of them are hydrophobic compounds, but they have no apparent structural similarity. Recent studies on the genes and cDNAs encoding these transporters shed some light on their general structure and function but have revealed no clues to the molecular mechanism mediating substrate recognition and translocation.

In contrast to monoamines, the homeostasis of glutamate in the brain is not influenced by the reactivity of glutamate but by the interaction between glutamate and specific receptors on the cell surface. Because glutamate is not a reactive compound, it can be accumulated to quite high concentrations in the cytoplasm (see Fig. 1). Therefore, there is no need for a very active vesicular glutamate transporter, and the concentration of glutamate inside the synaptic vesicles may be only 10 times that in the cytoplasm (monoamines are present at a concentration more than four orders of magnitude higher than that in the cytoplasm). However, effective glutamate transport by the cytoplasmic membrane of nervous cells is crucial for the function and viability of these cells (Attwell et al. 1993). During hypoxia or ischaemia, glutamate transporters can run backwards, and the glutamate released reacts with its receptors, resulting in increased Ca²⁺ entry into the cells. The high Ca²⁺ concentrations inside the cells may trigger the death of neurones and thus cause brain damage.

Metal-ion homeostasis encompasses all the factors mentioned for neurotransmitters and poisonous compounds. Although metal-ion homeostasis is vital for every eukaryotic cell, it may serve a special function in certain organs and cell types. The presence of unusual concentrations of metal ions can cause impaired brain function or cell death. The different ions may be distinct as redox-active ions such as Fe²⁺, Cu²⁺, Co²⁺ and to a lesser extent Mn²⁺ or non-redox-active ions such as Ca²⁺ and Zn²⁺. Zn²⁺ and Ca²⁺ may be targeted to transcription factors and other enzymes involved in DNA metabolism. Targeting redox-active metal ions to these places can lead to the promotion of radical reactions that result in nucleic acid damage. The redox-active ions normally function in enzymes that participate in redox reactions and in the conversion of active oxygen-containing components. All of these processes require defined amounts of specific metal ions at the right position and at the right time. In brain cells, Zn²⁺ is accumulated in presynaptic vesicles of excitatory neurones and is released during synaptic activity (Assaf and Chung, 1984; Howell et al. 1984). Zn²⁺ interacts with some ionotropic receptors in the brain. For example, the ionotropic ATP receptor (P2x3) is potentiated by Zn²⁺ (Seguela et al. 1996) and it blocks currents mediated by N-methyl-D-aspartate (NMDA) or γ-aminobutyric acid (GABA) as well as voltage-gated Ca²⁺ channels (Westbrook and Mayer, 1987; Peters et al. 1987). Since Zn²⁺ is secreted from synaptic vesicles, interacts with certain receptors and should have a specific transport system, it may be considered as a neurotransmitter. All of these considerations call attention to the importance of metal-ion homeostasis in brain cells. Through a mutation in the yeast gene CDC1, a gene encoding a manganese transporter (SMF1) has been identified (Supek et al. 1996). It was observed that the yeast SMF1 gene shares homology with the mouse Nramp gene. Nramp (Bcg) was cloned as a gene responsible for mouse resistance to infection with mycobacteria and is identical with the Ity and the Lsh genes conferring resistance to infection by Salmonella typhimurium and Leishmania donovani, respectively. We propose that the mammalian protein, like the yeast transporter, is a Mn²⁺ and/or Zn²⁺ transporter. This result led to the proposal that Nramp functions in the induction of resistance or sensitivity to mycobacteria (Supek et al. 1996). The hypothesis is based on manganese homeostasis in macrophage phagosomes that are maintained at low concentration to reduce the mycobacteria infectivity.