RESUMO
Transmembrane prolyl 4-hydroxylase (P4H-TM) is an enigmatic enzyme whose cellular function and primary substrate remain to be identified. Its loss-of-function mutations cause a severe neurological HIDEA syndrome with hypotonia, intellectual disability, dysautonomia and hypoventilation. Previously, P4H-TM deficiency in mice was associated with reduced atherogenesis and lower serum triglyceride levels. Here, we characterized the glucose and lipid metabolism of P4h-tm-/- mice in physiological and tissue analyses. P4h-tm-/- mice showed variations in 24-h oscillations of energy expenditure, VO2 and VCO2 and locomotor activity compared to wild-type (WT) mice. Their rearing activity was reduced, and they showed significant muscle weakness and compromised coordination. Sedated P4h-tm-/- mice had better glucose tolerance, lower fasting insulin levels, higher fasting lactate levels and lower fasting free fatty acid levels compared to WT. These alterations were not present in conscious P4h-tm-/- mice. Fasted P4h-tm-/- mice presented with faster hepatic glycogenolysis. The respiratory rate of conscious P4h-tm-/- mice was significantly lower compared to the WT, the decrease being further exacerbated by sedation and associated with acidosis and a reduced ventilatory response to both hypoxia and hypercapnia. P4H-TM deficiency in mice is associated with alterations in whole-body energy metabolism, day-night rhythm of activity, glucose homeostasis and neuromuscular and respiratory functions. Although the underlying mechanism(s) are not yet fully understood, the phenotype appears to have neurological origins, controlled by brain and central nervous system circuits. The phenotype of P4h-tm-/- mice recapitulates some of the symptoms of HIDEA patients, making this mouse model a valuable tool to study and develop tailored therapies.
RESUMO
HIF prolyl 4-hydroxylases (HIF-P4Hs, also known as PHDs and EGLNs) are crucial enzymes that modulate the hypoxia inducible factor (HIF) response and help to maintain cellular oxygen homeostasis. This function is especially well-known for cytoplasmic or nuclear enzymes HIF-P4H-1-3 (PHDs 1-3, EGLNs 2, 1 and 3, respectively), but the physiological role is still obscure for a fourth suggested HIF-P4H, P4H-TM that is a transmembrane protein and resides in the endoplasmic reticulum. Recently however, both experimental and clinical evidence of the P4H-TM involvement in CNS physiology has emerged. In this study, we first investigated the expression pattern of P4H-TM in the mouse brain and found a remarkably selective abundance in brains areas that are involved in social behaviors and anxiety including amygdala, lateral septum and bed nucleus of stria terminalis. Next, we performed behavioral assays in P4h-tm-/- mice to investigate a possible phenotype associated to these brain areas. In locomotor activity tests, we found that P4h-tm-/- mice were significantly more active than their wild-type (WT) littermate mice, and habituation to test environment did not abolish this effect. Instead, spatial learning and memory seemed normal in P4h-tm-/- mice as assessed by Morris swim task. In several tests assessing anxiety and fear responses, P4h-tm-/- mice showed distinct courageousness, and they presented increased interaction towards fellow mice in social behavior tests. Most strikingly, P4h-tm-/- mice practically lacked behavioral despair response, a surrogate marker of depression, in forced swim and tail suspension tests. Instead, mutant mice of all other Hif-p4h isoforms lacked such a behavioral phenotype. In summary, this study presents a remarkable anatomy-physiology association between the brain expression of P4H-TM and the behavioral phenotype in P4h-tm-/- mice. Future studies will reveal whether P4H-TM may serve as a novel target for anti-depressant and anti-anxiety pharmacotherapy.
