Ceramide and mitochondria in ischemic brain injury

Abstract

Sphingolipids are essential structural components of cellular membranes, playing prominent roles in signal transduction that governs cell proliferation, differentiation and apoptosis. Ceramides, a family of distinct molecular species characterized by various acyl chains, are synthesized de novo at the cytosolic side of the endoplasmic reticulum serving as precursors for the biosynthesis of sphingolipids in the Golgi. Recently, mitochondria emerged as an important intracellular compartment of sphingolipid metabolism. Thus, several sphingolipid-metabolizing enzymes were found to be associated with mitochondria, including neutral ceramidase, novel neutral sphingomyelinase, and (dihydro) ceramide synthase, an important ceramide-generating enzyme in de novo ceramide synthesis and recycling pathway. Mitochondrial dysfunction appears to be essential in tissue damage after brain ischemia/reperfusion (IR). Mitochondria are known to be involved in both the necrosis and apoptosis detected in animal models of ischemic stroke, and treatments that ameliorate tissue infarction were associated with better recovery of mitochondrial function. Although mitochondrial injury in stroke has been extensively studied and key mitochondrial functions affected by IR are mainly characterized, the nature of the molecule that causes loss of mitochondrial integrity and function remains obscure. Emerging data indicate a deregulation of ceramide metabolism in mitochondria damaged by IR suggesting that ceramides could play critical roles in cerebral IR-induced mitochondrial damage. This review will examine the experimental evidence supporting the key role of ceramides in mitochondrial dysfunction in cerebral IR and highlight potential targets for development of novel therapeutic approaches for stroke treatment.

Keywords: Sphingolipid; ceramide; ceramide synthase; mitochondria; neutral ceramidase; stroke.

Figure 1.

Biosynthesis of ceramide and its conversion into other bioactive sphingolipids. De novo ceramide synthesis begins with the conversion of serine and fatty acyl CoA into 3-ketosphinganine by serine palmitoyl transferase (SPT), then 3-ketosphinganine is converted into dihydrosphingosine. Myriocin is a potent inhibitor of SPT activity. (Dihydro) ceramide synthase (LASS/CerS) acylates dihydrosphingosine to form dihydroceramide, which is then reduced to ceramide by dihydroceramide desaturase. Ceramide is also produced by SMases through SM degradation in SMase pathway. Ceramidase converts ceramide into sphingosine, which is phosphorylated by sphingosine kinase (SK) to generate sphingosine-1-phosphate. Ceramide is phosphorylated by ceramide kinase (CK) yielding ceramide-1-phosphate (C1P). In the salvage or recycling pathway, complex sphingolipids are broken down to ceramide by glucosylceramidase (GCase) and then by ceramidase to sphingosine, which is re-acylated to ceramide by LASS/CerS. Fumonisin B1 inhibits LASS/CerS activity.

Figure 2.

Ceramide formation from Acyl-CoA and sphingosine (Sph) mediated by coupled activities of mitochondrial thioesterase (MTE) and nCDase (NCD).

Figure 3.

Hypothetical role of CerS6/ceramide in mitochondria after cerebral IR. IR triggers glutamate-induced cytosolic Ca²⁺ influx into the mitochondria and an activation of mitochondrial CerS6 that elevates ceramide. Ceramide blocks the MPTP opening at a low conductance state, leading to increased Ca²⁺ in the mitochondrial matrix. This MPTP inactivation would allow mitochondria to support adequate ATP production for formation of the apoptosome, and might be responsible for the initial raise in ATP production (and hence Δψ) during apoptosis [135], an observation that corresponds well with the reported transient mitochondrial hyper-polarization in the apoptosis induced by IL-3 withdrawal [136]. Rising mitochondrial Ca²⁺ activates calpain 10, which could cleave protein components of the MPTP [137] resulting in the MPTP opening at a high conductance state, swelling, and rupture of the outer mitochondrial membrane leading to necrotic cell death.

Figure 4.

Mitochondrial respiratory chain complexes. Mitochondrial respiratory chain consists of four multi-protein complexes Complex I-IV. The respiratory chain function is determined using substrates such as glutamate, succinate or ascorbate, which are oxidized via different complexes of respiratory chain. An inhibition of the electron transport through the Complex I or III could result in generation of ROS.