A New Generation of IMiDs as Treatments for Neuroinflammatory and Neurodegenerative Disorders.

The immunomodulatory imide drug (IMiD) class, which includes the founding drug member thalidomide and later generation drugs, lenalidomide and pomalidomide, has dramatically improved the clinical treatment of specific cancers, such as multiple myeloma, and it combines potent anticancer and anti-inflammatory actions. These actions, in large part, are mediated by IMiD binding to the human protein cereblon that forms a critical component of the E3 ubiquitin ligase complex. This complex ubiquitinates and thereby regulates the levels of multiple endogenous proteins. However, IMiD-cereblon binding modifies cereblon's normal targeted protein degradation towards a new set of neosubstrates that underlies the favorable pharmacological action of classical IMiDs, but also their adverse actions-in particular, their teratogenicity. The ability of classical IMiDs to reduce the synthesis of key proinflammatory cytokines, especially TNF-α levels, makes them potentially valuable to reposition as drugs to mitigate inflammatory-associated conditions and, particularly, neurological disorders driven by an excessive neuroinflammatory element, as occurs in traumatic brain injury, Alzheimer's and Parkinson's diseases, and ischemic stroke. The teratogenic and anticancer actions of classical IMiDs are substantial liabilities for effective drugs in these disorders and can theoretically be dialed out of the drug class. We review a select series of novel IMiDs designed to avoid binding with human cereblon and/or evade degradation of downstream neosubstrates considered to underpin the adverse actions of thalidomide-like drugs. These novel non-classical IMiDs hold potential as new medications for erythema nodosum leprosum (ENL), a painful inflammatory skin condition associated with Hansen's disease for which thalidomide remains widely used, and, in particular, as a new treatment strategy for neurodegenerative disorders in which neuroinflammation is a key component.

Recent advances in the molecular mechanism of thalidomide teratogenicity.

Thalidomide was first marketed in 1957 but soon withdrawn because of its notorious teratogenicity. Studies on the mechanism of action of thalidomide revealed the pleiotropic properties of this class of drugs, including their anti-inflammatory, antiangiogenic and immunomodulatory activities. Based on their notable activities, thalidomide and its analogues, lenalidomide and pomalidomide, have been repurposed to treat erythema nodosum leprosum, multiple myeloma and other haematological malignancies. Thalidomide analogues were recently found to hijack CRL4CRBN ubiquitin ligase to target a number of cellular proteins for ubiquitination and proteasomal degradation. Thalidomide-mediated degradation of SALL4 and p63, transcription factors essential for embryonic development, very likely plays a critical role in thalidomide embryopathy. In this review, we provide a brief retrospective summary of thalidomide-induced teratogenesis, the mechanism of thalidomide activity, and the latest advances in the molecular mechanism of thalidomide-induced birth malformations.

Post-translational modification of Parkin and its research progress in cancer.

Clinical practice has shown that Parkin is the major causative gene found in an autosomal recessive juvenile parkinsonism (AR-JP) via Parkin mutations and that the Parkin protein is the core expression product of the Parkin gene, which itself belongs to an E3 ubiquitin ligase. Since the discovery of the Parkin gene in the late 1990s, researchers in many countries have begun extensive research on this gene and found that in addition to AR-JP, the Parkin gene is associated with many diseases, including type 2 diabetes, leprosy, Alzheimer's, autism, and cancer. Recent studies have found that the loss or dysfunction of Parkin has a certain relationship with tumorigenesis. In general, the Parkin gene, a well-established tumor suppressor, is deficient and mutated in a variety of malignancies. Parkin overexpression inhibits tumor cell growth and promotes apoptosis. However, the functions of Parkin in tumorigenesis and its regulatory mechanisms are still not fully understood. This article describes the structure, functions, and post-translational modifications of Parkin, and summarizes the recent advances in the tumor suppressive function of Parkin and its underlying mechanisms.

Pleiotropic effects for Parkin and LRRK2 in leprosy type-1 reactions and Parkinson's disease.

Type-1 reactions (T1R) are pathological inflammatory episodes and main contributors to nerve damage in leprosy. Here, we evaluate the genewise enrichment of rare protein-altering variants in 7 genes where common variants were previously associated with T1R. We selected 474 Vietnamese leprosy patients of which 237 were T1R-affected and 237 were T1R-free matched controls. Genewise enrichment of nonsynonymous variants was tested with both kernel-based (sequence kernel association test [SKAT]) and burden methods. Of the 7 genes tested 2 showed statistical evidence of association with T1R. For the LRRK2 gene an enrichment of nonsynonymous variants was observed in T1R-free controls (PSKAT-O = 1.6 × 10-4). This genewise association was driven almost entirely by the gain-of-function variant R1628P (P = 0.004; odds ratio = 0.29). The second genewise association was found for the Parkin coding gene PRKN (formerly PARK2) where 7 rare variants were enriched in T1R-affected cases (PSKAT-O = 7.4 × 10-5). Mutations in both PRKN and LRRK2 are known causes of Parkinson's disease (PD). Hence, we evaluated to what extent such rare amino acid changes observed in T1R are shared with PD. We observed that amino acids in Parkin targeted by nonsynonymous T1R-risk mutations were also enriched for mutations implicated in PD (P = 1.5 × 10-4). Hence, neuroinflammation in PD and peripheral nerve damage due to inflammation in T1R share overlapping genetic control of pathogenicity.

Exploitation of the host cell ubiquitin machinery by microbial effector proteins.

Pathogenic bacteria are in a constant battle for survival with their host. In order to gain a competitive edge, they employ a variety of sophisticated strategies that allow them to modify conserved host cell processes in ways that favor bacterial survival and growth. Ubiquitylation, the covalent attachment of the small modifier ubiquitin to target proteins, is such a pathway. Ubiquitylation profoundly alters the fate of a myriad of cellular proteins by inducing changes in their stability or function, subcellular localization or interaction with other proteins. Given the importance of ubiquitylation in cell development, protein homeostasis and innate immunity, it is not surprising that this post-translational modification is exploited by a variety of effector proteins from microbial pathogens. Here, we highlight recent advances in our understanding of the many ways microbes take advantage of host ubiquitylation, along with some surprising deviations from the canonical theme. The lessons learned from the in-depth analyses of these host-pathogen interactions provide a fresh perspective on an ancient post-translational modification that we thought was well understood.This article is part of a Minifocus on Ubiquitin Regulation and Function. For further reading, please see related articles: 'Mechanisms of regulation and diversification of deubiquitylating enzyme function' by Pawel Leznicki and Yogesh Kulathu (J. Cell Sci.130, 1997-2006). 'Cell scientist to watch - Mads Gyrd-Hansen' (J. Cell Sci.130, 1981-1983).

Cereblon and its downstream substrates as molecular targets of immunomodulatory drugs.

Thalidomide was first developed as a sedative around 60 years ago, but exhibited teratogenicity, leading to serious defects such as limb deformities. Nevertheless, thalidomide is now recognized as a therapeutic drug for the treatment of Hansen's disease and myeloma. Immunomodulatory drugs (IMiDs), a new class of anti-cancer drug derived from thalidomide, have also been developed and exert potent anti-cancer effects. Although the molecular mechanism of thalidomide and IMiDs remained unclear for a long time, cereblon, a substrate receptor of the CRL4 E3 ubiquitin ligase was identified as a primary direct target by a new affinity technique. A growing body of evidence suggests that the effect of IMiDs on myeloma and other cancer cells is mediated by CRBN. Each IMiD binds to CRBN and alters the substrate specificity of the CRBN E3 ubiquitin ligase complex, resulting in breakdown of intrinsic downstream proteins such as Ikaros and Aiolos. Here we give an overview of the current understanding of mechanism of action of IMiDs via CRBN and prospects for the development of new drugs that degrade protein of interest.

Discovery of the target for immunomodulatory drugs (IMiDs).

Half a century ago, the sedative thalidomide caused a serious drug disaster because of its teratogenicity and was withdrawn from the market. However, thalidomide, which has returned to the market, is now used for the treatment of leprosy and multiple myeloma (MM) under strict control. The mechanism of thalidomide action had been a long-standing question. We developed a new affinity bead technology and identified cereblon (CRBN) as a thalidomide-binding protein. We found that CRBN functions as a substrate receptor of an E3 cullin-Ring ligase complex 4 (CRL4) and is a primary target of thalidomide teratogenicity. Recently, new thalidomide derivatives, called immunomodulatory drugs (IMiDs), have been developed by Celgene. Among them, lenalidomide (Len) and pomalidomide (Pom) were shown to exert strong therapeutic effects against MM. It was found that Len and Pom both bind CRBN-CRL4 and recruit neomorphic substrates (Ikaros and Aiolos). More recently it was reported that casein kinase 1a (Ck1a) was identified as a substrate for CRBN-CRL4 in the presence of Len, but not Pom. Ck1a breakdown explains why Len is specifically effective for myelodysplastic syndrome with 5q deletion. It is now proposed that binding of IMiDs to CRBN appears to alter the substrate specificity of CRBN-CRL4. In this review, we introduce recent findings on IMiDs.

The ubiquitin ligase parkin mediates resistance to intracellular pathogens.

Ubiquitin-mediated targeting of intracellular bacteria to the autophagy pathway is a key innate defence mechanism against invading microbes, including the important human pathogen Mycobacterium tuberculosis. However, the ubiquitin ligases responsible for catalysing ubiquitin chains that surround intracellular bacteria are poorly understood. The parkin protein is a ubiquitin ligase with a well-established role in mitophagy, and mutations in the parkin gene (PARK2) lead to increased susceptibility to Parkinson's disease. Surprisingly, genetic polymorphisms in the PARK2 regulatory region are also associated with increased susceptibility to intracellular bacterial pathogens in humans, including Mycobacterium leprae and Salmonella enterica serovar Typhi, but the function of parkin in immunity has remained unexplored. Here we show that parkin has a role in ubiquitin-mediated autophagy of M. tuberculosis. Both parkin-deficient mice and flies are sensitive to various intracellular bacterial infections, indicating parkin has a conserved role in metazoan innate defence. Moreover, our work reveals an unexpected functional link between mitophagy and infectious disease.

PARK2 mediates interleukin 6 and monocyte chemoattractant protein 1 production by human macrophages.

Leprosy is a persistent infectious disease caused by Mycobacterium leprae that still affects over 200,000 new patients annually. The host genetic background is an important risk factor for leprosy susceptibility and the PARK2 gene is a replicated leprosy susceptibility candidate gene. The protein product of PARK2, Parkin, is an E3 ubiquitin ligase that is involved in the development of various forms of Parkinsonism. The human macrophage is both a natural host cell of M. leprae as well as a primary mediator of natural immune defenses, in part by secreting important pro-inflammatory cytokines and chemokines. Here, we report that down-regulation of Parkin in THP-1 macrophages, human monocyte-derived macrophages and human Schwann cells resulted in a consistent and specific decrease in interleukin-6 (IL-6) and monocyte chemoattractant protein 1 (MCP-1/CCL2) production in response to mycobacteria or LPS. Interestingly, production of IL-6 at 6 hours by THP-1 cells stimulated with live M. leprae and M. bovis BCG was dependent on pretreatment with 1,25-dihydroxyvitamin D(3) (VD). Parkin knockdown in VD-treated cells blocked IL-6 induction by mycobacteria. However, IκB-α phosphorylation and levels of IκB-ξ, a nuclear protein required for IL-6 expression, were not affected by Parkin silencing. Phosphorylation of MAPK ERK1/2 and p38 was unaffected by Parkin silencing while JNK activation was promoted but did not explain the altered cytokine production. In a final set of experiments we found that genetic risk factors of leprosy located in the PARK2 promoter region were significantly correlated with M. leprae sonicate triggered CCL2 and IL6 transcript levels in whole blood assays. These results associated genetically controlled changes in the production of MCP-1/CCL2 and IL-6 with known leprosy susceptibility factors.

Deciphering the mystery of thalidomide teratogenicity.

Thalidomide was originally developed in 1954 as a sedative that was commonly used to ameliorate morning sickness. However, thalidomide exposure during the first trimester of pregnancy caused multiple birth defects (e.g. phocomelia and amelia), affecting ≈ 10,000 children worldwide in the late 1950s and early 1960s. Thalidomide is now recognized as a clinically effective, albeit strictly restricted, drug for the treatment of leprosy and multiple myeloma. Investigators have studied thalidomide teratogenicity for half a century, proposing over 30 hypotheses to account for its actions. Among these, the anti-angiogenesis and oxidative stress models have gained widespread support. Nonetheless, the precise molecular mechanisms and direct targets of thalidomide have not heretofore been elucidated. We developed ferrite-glycidyl methacrylate beads that enable magnetic separation and efficient purification of ligand-binding molecules; the beads were recently employed to identify cereblon as a primary target of thalidomide. Cereblon forms an E3 ubiquitin ligase complex with DDB1, Cul4A, and Roc1, which is important for the expression of fibroblast growth factor 8, an essential regulator of limb development. Expression of a drug binding-deficient mutant of cereblon suppressed thalidomide-induced effects in zebrafish and chicks. This suggests that thalidomide downregulates fibroblast growth factor 8 expression and induces limb malformation by binding to wild-type cereblon, inhibiting the function of the associated E3 ubiquitin ligase. The present review summarizes the teratogenicity of thalidomide, including existing models for its mode of action, and discusses the identification of cereblon as a key molecule for deciphering the longstanding mystery of thalidomide teratogenicity.

Identification of a primary target of thalidomide teratogenicity.

Half a century ago, thalidomide was widely prescribed to pregnant women as a sedative but was found to be teratogenic, causing multiple birth defects. Today, thalidomide is still used in the treatment of leprosy and multiple myeloma, although how it causes limb malformation and other developmental defects is unknown. Here, we identified cereblon (CRBN) as a thalidomide-binding protein. CRBN forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1) and Cul4A that is important for limb outgrowth and expression of the fibroblast growth factor Fgf8 in zebrafish and chicks. Thalidomide initiates its teratogenic effects by binding to CRBN and inhibiting the associated ubiquitin ligase activity. This study reveals a basis for thalidomide teratogenicity and may contribute to the development of new thalidomide derivatives without teratogenic activity.