Developmental regulation and localization of carnitine palmitoyltransferases (CPTs) in rat brain

Abstract

While the brain’s high energy demands are largely met by glucose, brain is also equipped with the ability to oxidize fatty acids for energy and metabolism. The brain expresses the carnitine palmitoyltransferases (CPTs) that mediate carnitine-dependent entry of long-chain acyl-CoAs into the mitochondrial matrix for β-oxidation – CPT1a and CPT2 located on the outer and inner mitochondrial membranes, respectively. Their developmental profile, regional distribution and activity as well as cell type expression remain unknown. We determined that brain CPT1a RNA and total protein expression were unchanged throughout post-natal development (PND0, PND7, PND14, PND21 and PND50); however, CPT2 RNA peaked at PND 21 and remained unchanged through PND50 in all regions studied (cortex, hippocampus, midbrain, and cerebellum). Both long-chain acyl CoA dehydrogenase (ACADL) and medium acyl-CoA dehydrogenase (ACADM) showed a similar developmental profile to CPT2. Acylcarnitines, generated as result of CPT1a activity, significantly increased with age and peaked at PND21 in all brain regions, concurrent with the increased expression of enzymes involved in mitochondrial β-oxidation. The CPT system is highly enriched in vivo in hippocampus and cerebellum, relative to cortex and midbrain, and is exclusively present in astrocytes and neural progenitor cells, while absent in neurons, microglia, and oligodendrocytes. Using radiolabeled oleate, we demonstrate regional differences in brain fatty acid oxidation that may be blocked by the irreversible CPT1a inhibitor etomoxir. This study contributes to the field of knowledge of brain cell-specific metabolic pathways, which are important for understanding normal brain development and aging, as well as pathophysiology of neurological diseases.

GSEA of mouse and human mitochondriomes reveals fatty acid oxidation in astrocytes

“…In Cellular energy homeostasis, 19 out of the 36 over‐expressed genes are related to canonical FAO. Notably, they encompass all the enzymes associated with this pathway (Figure b), including long‐chain fatty acid transport to mitochondria by Ctp1a and Ctp2 , in agreement with recent findings (Jernberg, Bowman, Wolfgang, & Scafidi, ). There is also over‐expression of enzymes related to the oxidation of unsaturated fatty acids, namely Decr1 , Eci1 and Eci2 (Table ).…”

“…Our experiments in vitro confirm the co‐existence of aerobic glycolysis and FAO in astrocytes, as forecast in a review (Panov et al, ). They also agree with recent findings that suggest FAO occurs in the presence of glucose in mouse hippocampus and cortex (Jernberg et al, ). We show that FAO in astrocytes is versatile, because it is inhibited by high concentrations of glutamate, resembling states of high excitatory neuronal activity.…”

“…For example, when neurogenesis is induced by PGC1alfa‐mediated mitochondrial biogenesis, there is no upregulation or downregulation of FAO enzymes (Stoll et al, ). However, according to our data, astrocyte maturation implies an increase in FAO and amino acid catabolism, in agreement with the developmental upregulation of carnitine palmitoyl transferases and acyl‐CoA dehydrogenases in young mice (Jernberg et al, ). We thus argue that the key point in deriving astrocytes from iPSCs, or neurons from astrocytes, is mitochondrial specialization, and not the switch from a non‐oxidative to an oxidative phenotype.…”

“…Also there are a further five genes related to Redox homeostasis and aldehyde dehydrogenases over-expressed in astrocytes compared with neurons. Moreover, we should highlight that fatty acid availability is not a limiting step for astrocyte FAO as fatty acids are transported into the brain by either rapid passive diffusion and/or facilitated transport across the Brain Blood Barrier (Ebert, Haller, & Walton, 2003;Jernberg et al, 2017;Schonfeld & Reiser, 2013). Further, astrocytes could oxidize fatty acids from intracellular lipid droplets (Cabodevilla et al, 2013;Gubern et al, 2008).…”

PPARβ/δ‐agonist GW0742 ameliorates dysfunction in fatty acid oxidation in PSEN1ΔE9 astrocytes

“…In the intermembrane space of mitochondria, the fatty acyl‐CoA is converted to a fatty acylcarnitine by CPT1 and subsequently transported across the mitochondrial inner membrane space. An increase in the expression level of CPT1a, the main isoform of CPT1 expressed in astrocytes, is indicative of increased FAO since it is the rate‐limiting transport enzyme of FAs (Blazquez et al, ; Jernberg et al, ). Etomoxir, which blocks the activity of CPT1, is widely used to inhibit β‐oxidation.…”