Cholesterol synthesis and nerve regeneration

ERAD components Derlin-1 and Derlin-2 are essential for postnatal brain development and motor function

Derlin family members (Derlins) are primarily known as components of the endoplasmic reticulum-associated degradation pathway that eliminates misfolded proteins. Here we report a function of Derlins in the brain development. Deletion of Derlin-1 or Derlin-2 in the central nervous system of mice impaired postnatal brain development, particularly of the cerebellum and striatum, and induced motor control deficits. Derlin-1 or Derlin-2 deficiency reduced neurite outgrowth in vitro and in vivo and surprisingly also inhibited sterol regulatory element binding protein 2 (SREBP-2)-mediated brain cholesterol biosynthesis. In addition, reduced neurite outgrowth due to Derlin-1 deficiency was rescued by SREBP-2 pathway activation. Overall, our findings demonstrate that Derlins sustain brain cholesterol biosynthesis, which is essential for appropriate postnatal brain development and function.

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Derlin-1 and Derlin-2 are essential for postnatal brain development and function

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Chemical chaperon does not ameliorate the phenotype of Derlin-deficient neuron

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Derlin regulates SREBP-2 activation and promotes brain cholesterol biosynthesis

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Derlin-mediated cholesterol biosynthesis is essential for neurite outgrowth

CYP46A1 gene therapy deciphers the role of brain cholesterol metabolism in Huntington's disease

Dysfunctions in brain cholesterol homeostasis have been extensively related to brain disorders. The main pathway for brain cholesterol elimination is its hydroxylation into 24S-hydroxycholesterol by the cholesterol 24-hydrolase, CYP46A1. Increasing evidence suggests that CYP46A1 has a role in the pathogenesis and progression of neurodegenerative disorders, and that increasing its levels in the brain is neuroprotective. However, the mechanisms underlying this neuroprotection remain to be fully understood. Huntington's disease is a fatal autosomal dominant neurodegenerative disease caused by an abnormal CAG expansion in huntingtin's gene. Among the multiple cellular and molecular dysfunctions caused by this mutation, altered brain cholesterol homeostasis has been described in patients and animal models as a critical event in Huntington's disease. Here, we demonstrate that a gene therapy approach based on the delivery of CYP46A1, the rate-limiting enzyme for cholesterol degradation in the brain, has a long-lasting neuroprotective effect in Huntington's disease and counteracts multiple detrimental effects of the mutated huntingtin. In zQ175 Huntington's disease knock-in mice, CYP46A1 prevented neuronal dysfunctions and restored cholesterol homeostasis. These events were associated to a specific striatal transcriptomic signature that compensates for multiple mHTT-induced dysfunctions. We thus explored the mechanisms for these compensations and showed an improvement of synaptic activity and connectivity along with the stimulation of the proteasome and autophagy machineries, which participate to the clearance of mutant huntingtin (mHTT) aggregates. Furthermore, BDNF vesicle axonal transport and TrkB endosome trafficking were restored in a cellular model of Huntington's disease. These results highlight the large-scale beneficial effect of restoring cholesterol homeostasis in neurodegenerative diseases and give new opportunities for developing innovative disease-modifying strategies in Huntington's disease.

Dysregulated Brain Cholesterol Metabolism Is Linked to Neuroinflammation in Huntington's Disease

Huntington's disease is an autosomal-dominant, neurodegenerative disorder caused by a CAG repeat expansion in exon-1 of the huntingtin gene. Alterations in cholesterol metabolism and distribution have been reported in Huntington's disease, including abnormal interactions between mutant huntingtin and sterol regulatory element-binding proteins, decreased levels of apolipoprotein E/cholesterol/low-density lipoprotein receptor complexes, and alterations in the synthesis of ATP-binding cassette transporter A1. Plasma levels of 24S-hydroxycholestrol, a key intermediary in cholesterol metabolism and a possible marker in neurodegenerative diseases, decreased proportionally to the degree of caudate nucleus atrophy. The interaction of mutant huntingtin with sterol regulatory element-binding proteins is of particular interest given that sterol regulatory element-binding proteins play a dual role: They take part in lipid and cholesterol metabolism, but also in the inflammatory response that induces immune cell migration as well as toxic effects, particularly in astrocytes. This work summarizes current evidence on the metabolic and immune implications of sterol regulatory element-binding protein dysregulation in Huntington's disease, highlighting the potential use of drugs that modulate these alterations. © 2020 International Parkinson and Movement Disorder Society.

Precursors for cholesterol synthesis (7-dehydrocholesterol, 7-dehydrodesmosterol, and desmosterol): cholesterol/7-dehydrocholesterol ratio as an index of development and aging in PNS but not in CNS

In rat sciatic nerve, the 7-dehydrocholesterol content decreased dramatically during the postnatal period and slowly during adulthood and aging. In contrast, the 7-dehydrodesmosterol content peaked at 14 days and was nearly undetectable after 60 days. The desmosterol content peaked at 21 days and was nearly undetectable after 1 year. The cholesterol content increased up to 21 days and remained nearly constant thereafter. In brain (in contrast to sciatic nerve), 7-dehydrodesmosterol and desmosterol contents decreased dramatically during development and slightly during adulthood and aging; the 7-dehydrocholesterol content peaked at 21 days and remained constant during aging. Only 7-dehydrocholesterol was dramatically more concentrated in PNS than in CNS. In brain, the cholesterol/7-dehydrocholesterol ratio increased during development and remained stable after 6 months. In contrast, in sciatic nerve, this ratio continuously increased during development and aging (950-fold between 5 days and 18 months). Thus, the cholesterol/7-dehydrocholesterol ratio is a useful biochemical index of development and aging in the PNS.

Alterations of cholesterol synthesis precursors (7-dehydrocholesterol, 7-dehydrodesmosterol, desmosterol) in dysmyelinating neurological mutant mouse (quaking, shiverer and trembler) in the PNS and the CNS

In brain, levels of cholesterol, desmosterol and 7-dehydrodesmosterol are reduced in shiverer and quaking, but not in trembler 60-day-old dysmyelinating mutant mice. Very interestingly, 7-dehydrocholesterol is not altered in any mutant. The amount of cholesterol is similar in the different normal control mouse strains and in rat. In contrast, levels of precursors are not the same. In sciatic nerve, cholesterol is slightly reduced in shiverer, reduced 2-fold in quaking, and dramatically reduced in trembler (10-fold). 7-Dehydrocholesterol is affected in all mutants.

Biosynthesis of membrane cholesterol during peripheral nerve development, degeneration and regeneration

Biosynthesis of peripheral nerve cholesterol was investigated by the in vivo and in vitro incorporation of [1-14C]-acetate into sciatic endoneurium of normal rats during development, degeneration and regeneration. Labeled sterols were rapidly formed (less than 10 min) within the endoneurial portion of sciatic nerve after [1-14C]acetate administration by intraneural injection. The majority of labeled sterols were initially found in lanosterol and desmosterol. After six hr, the 14C-labeling in both precursors was decreased to minimum, whereas cholesterol became the major labeled product of sterol. As myelination proceeded, the incorporation of [1-14C]acetate into endoneurial cholesterol decreased rapidly and reached a minimum after six mo. In mature adult nerve, an increased proportion of biosynthesis of lanosterol and desmosterol also was demonstrated. The in vitro incorporation of [1-14C]acetate into cholesterol was inhibited during Wallerian degeneration. Instead, cholesteryl esters were labeled as the major sterol product. Such inhibition, however, was not observed in the adult Trembler nerve (Brain Res. 325, 21-27, 1985), which is presumed to be due to a primary metabolic disorder of Schwann cells. The cholesterol biosynthesis was gradually resumed in degenerated nerve by either regeneration of crush-injured nerve or reattachment of the transected nerve. These results suggest that cholesterol biosynthesis in peripheral nerve relies on the axon to provide necessary substrates. De novo synthesis appears to be one of the major sources of endoneurial cholesterol that forms and maintains peripheral nerve myelin.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.