Deficiency of phosphatidylethanolamine synthesis: consequences for skeletal muscle

hosphatidylethanolamine (PE) is a major gl ycer ophospholipid PL) in cellular and organellar membranes, and dysregulation of E synthesis has been associated with ener gy o v er-stora ge and nsulin resistance. 1 Due to the small size of its ethanolamine eadgr oup r elati v e to the v olume occupied by its long and, in articular, unsaturated fatty acyl side chains (which impart a kink” to the molecule), PE forms a conical shape. 2 This moleclar structure is key to sculpting the curv atur e of the inner layer f cellular and organellar membranes, where PE is quantitati v el y he most abundant type of PL. 3 In organelles where a high degree of membrane curv atur e s critical for the translation of structure to function, such s in the cristae that form the inner mitochondrial memranes, increases in PE content can impr ov e performance, hile losses can result in dysfunction that is so critical as o be incompatible with cellular and organismal life. 4 While his provides a direct connection between PE and the conrol of energy metabolism, recent studies demonstrate addiional roles for this gl ycer olipid in the regulation of wholeody metabolic homeostasis that are both complex and overlaping. For example, PE is primarily synthesized in the Kennedy athw ay thr ough the cytidine diphosphate (CDP)-ethanolamine athw ay, wher e CTP:phosphoethanolamine c ytid ylyltr ansfer ase Pcyt2) catalyzes the second and rate-limiting step. Because iacylgl ycer ol (DAG) is a substrate in the third and final step f the PE-K ennedy pathw ay, limitations in Pcyt2 activity are ssociated with reduced utilization, and therefore increased t

Phosphatidylethanolamine (PE) is a major gl ycer ophospholipid (PL) in cellular and organellar membranes, and dysregulation of PE synthesis has been associated with ener gy o v er-stora ge and insulin resistance. 1 Due to the small size of its ethanolamine headgr oup r elati v e to the v olume occupied by its long and, in particular, unsaturated fatty acyl side chains (which impart a "kink" to the molecule), PE forms a conical shape. 2 This molecular structure is key to sculpting the curv atur e of the inner layer of cellular and organellar membranes, where PE is quantitati v el y the most abundant type of PL. 3 In organelles where a high degree of membrane curv atur e is critical for the translation of structure to function, such as in the cristae that form the inner mitochondrial membranes, increases in PE content can impr ov e performance, while losses can result in dysfunction that is so critical as to be incompatible with cellular and organismal life. 4While this provides a direct connection between PE and the control of energy metabolism, recent studies demonstrate additional roles for this gl ycer olipid in the regulation of wholebody metabolic homeostasis that are both complex and overlapping.For example, PE is primarily synthesized in the Kennedy Pathw ay thr ough the cytidine diphosphate (CDP)-ethanolamine pathw ay, wher e CTP:phosphoethanolamine c ytid ylyltr ansfer ase (Pcyt2) catalyzes the second and rate-limiting step.Because diacylgl ycer ol (DAG) is a substrate in the third and final step of the PE-K ennedy pathw ay, limitations in Pcyt2 activity are associated with reduced utilization, and therefore increased accumulation of this bioactive lipid, with subsequent consequences for the risk of developing insulin resistance and obesity. 5At the same time, others have found that hepatic insulin signaling can be impr ov ed by reducing the ratio of PC:PE through a mechanism that is unrelated to DAG levels, 6 but could inv olv e other factors such as modulation of the efficiency of function of the sarco-endoplasmic reticulum calcium ATP ase pump , 7 which is r esponsib le for mor e than 20% of energy expenditur e ov erall, and up to half of resting energy use in skeletal muscle specifically. 8s the largest sink for insulin-stimulated glucose disposal, a central role for skeletal muscle in the regulation of whole body energy and glucose metabolism is well esta b lished.Questions r emain, howev er, r egarding the r ole of skeletal m uscle PE levels and Pcyt2 activity in the health and metabolic function of this tissue.These questions are particularly perplexing, given that mice heterozygous for Pcyt2 global ablation develop nonalcoholic steatohepatitis, 5 while liver-specific deletion of this enzyme causes hepatosteatosis without liver injury or impaired hepatic insulin signaling, 9 necessarily implicating extra-hepatic organ systems in the pathology.

Pcyt2 + / − Mice Have Dysregulated Skeletal Muscle Structure and Lipid Metabolism, and Exhibit Adipose Tissue Infiltr a tion
In their recent paper, Grapentine and collea gues hav e inv estigated the skeletal muscle phenotype in mice with global Pcyt2 heterozygous gene ablation ( Pcyt2 + / − ), and their findings provide substantial new data linking PE synthesis in this tissue not only to muscle health but also to systemic glucose dysregulation. 3 In the current work, the authors examined skeletal muscles from wildtype and Pc yt2 + / − mice , whic h w ere used since homozygous loss of this gene is embryonic lethal.Most noticea b l y, the quality of skeletal muscle c hanged dr amatically with Pcyt2 deficiency, highlighting the critical role of this enzyme in the structural integrity of this tissue.Changes included evidence of degeneration, including the appearance of hyper-eosinophilic cells and vacuolization, as well as disruptions visible under electron microscopy, which included disordering of myofilaments and sarcomer es, r esulting in irr egular or disrupted Z-lines.Not surprisingl y, skeletal m uscle tissues fr om Pcyt2 + / − mice had higher levels of fibrosis and macrophage infiltration compared to tissues from wildtype littermates.
Ele vated le vels of triacylglycerol (TAG) were also found in skeletal m uscle fibr es fr om these mice, which was not unexpected, gi v en the disruption to DAG utilization and concomitant rise in DAG content within the tissue.Increased lipogenesis, r esulting fr om elev ated Sr ebp1c acti v ation and enhanced de phosphor ylation of acetyl-coenzyme A (CoA) carboxylase, and r educed lipol ysis r esulting fr om r educed acti v ation of hormone-sensiti v e lipase and reduced Cpt1 levels, supported that both higher lipid synthesis and reduced breakdown were factors in the observed m y osteatosis.What was surprising, howev er, w as the observ ation of appar ent infiltration of skeletal muscle with adipose tissue cells, rather than just intr am yocellular lipid droplets, which appeared in clusters to give a "cloud-like" appearance characteristic of classical adipose tissue depots.

Pcyt2 + / − Mice Have Dysregulated Skeletal Muscle Glucose Metabolism
Lipotoxicity significantly impairs glucose handling in numerous models, so the authors studied glucose metabolism in skeletal muscle, and found a near doubling in of levels and activity of the DAG-acti v ated negati v e insulin signaling r egulator pr otein kinase c (Pkc) α, 10 together with reduced contents of the insulin signaling mediators IRS1, p85-PI3k, and phosphorylated and total Akt, as well as an 87% reduction in the ratio of plasma membrane:total Glut4 protein content.Not surprisingly, this reduced soleus and gastrocnemius glucose uptake under both basal and stimulated conditions in Pc yt2 + / − mice , in association with reduced glycogen synthesis, but enhanced glycogen content.

What Does This Tell Us About the Role of Skeletal Muscle Pcyt2 + / − in Liver Metabolic Dysregulation
Gi v en that m y osteatosis is closely associated with the pathogenesis of nonalcoholic fatty li v er disease, to the extent that it is used as a prognostic indicator in patients with end-stage liver disease, 3 the findings from Grapentine et al. str ongl y suggest that the a ge-de pendent non-alcoholic steatohepatitis (NASH) and insulin resistance reported in global Pcyt2 + / − mice 5 (but not in li v er-specific Pcyt2 knockout mice 9 ) originates, at least in part, from pathological changes within skeletal muscle.To explain the overall outcomes, the authors postulated a model of meta bolic r e-r outing.They suggest that r educed acti vity of Pcyt2 in skeletal muscle results in DAG accumulation that activates Pkc α to impair insulin signaling and glucose uptake, while pr ob lems with mitochondrial oxidation and muscle function ov erall decr ease fuel utilization.The authors suggest that this increased the diversion of glucose in skeletal muscle into nonoxidati v e pathw ays, suc h as lipogenesis, whic h caused inflammation that further inhibited insulin signaling, and together with elev ated gl ycogen stor es, also further limited the uptake and utilization by skeletal muscle of glucose.With this major glucose sink diminished, and the tissue inflamed and dysfunctional, the authors suggest that mice became glucose intolerant, and r er outed this excess glucose into the production of TAG in li v er.
Results from this study highlight the importance of PE synthesis in skeletal m uscle structur e, function, and metabolic health, providing a significant and novel insight into the role of this enzyme and this bioacti v e lipid in this important tissue.One limitation, which the authors ac knowledge , is the use of a global Pc yt2 + / − mouse .Gi v en the magnitude of effects observed in the current study, future work in mice deficient in Pcyt2 specifically in skeletal muscle would provide an exciting and direct insight into the nature of skeletal m uscle-li v er cr osstalk in he patosteatosis dev elopment and r elated meta bolic complications.