In vivo function of the ER-Golgi transport protein LMAN1 in photoreceptor homeostasis

Mammalian cargo receptors for endoplasmic reticulum-to-Golgi transport: mechanisms and interactions

Proteins that are destined to enter the secretory pathway are synthesized on the rough endoplasmic reticulum (ER) and then translocated into the ER lumen, where they undergo posttranslational modifications, folding, and assembly. After passing a quality control system, the cargo proteins are packaged into coat protein complex II (COPII) vesicles to exit the ER. In metazoans, most COPII subunits have multiple paralogs, enabling COPII vesicles the flexibility to transport a diverse range of cargo. The cytoplasmic domains of transmembrane proteins can interact with SEC24 subunits of COPII to enter the ER exit sites. Some transmembrane proteins may also act as cargo receptors that bind soluble secretory proteins within the ER lumen, enabling them to enter COPII vesicles. The cytoplasmic domains of cargo receptors also contain coat protein complex I binding motifs that allow for their cycling back to the ER after unloading their cargo in the ER-Golgi intermediate compartment and cis-Golgi. Once unloaded, the soluble cargo proteins continue maturation through the Golgi before reaching their final destinations. This review provides an overview of receptor-mediated transport of secretory proteins from the ER to the Golgi, with a focus on the current understanding of two mammalian cargo receptors: the LMAN1-MCFD2 complex and SURF4, and their roles in human health and disease.

Altered phenotype in LMAN1-deficient mice with low levels of residual LMAN1 expression

Combined deficiency of coagulation factors V and VIII (F5F8D) is an autosomal recessive bleeding disorder caused by loss-of-function mutations in either LMAN1 or MCFD2. The latter genes encode 2 components of a mammalian cargo receptor that facilitates secretion of coagulation factor V (FV) and factor VIII (FVIII) from the endoplasmic reticulum (ER) to the Golgi via coat protein complex II vesicles. F5F8D patients exhibit FV and FVIII levels that are ∼10% to 15% of normal. We report herein a comparative analysis for a series of murine Lman1 alleles. Consistent with previous reports, mice completely deficient in LMAN1 (Lman1-/-) exhibit ∼50% FV and FVIII levels. In contrast, mice carrying a hypomorphic Lman1 allele (Lman1cgt/cgt) that expresses ∼6% to 8% of wild-type Lman1 mRNA levels exhibit intermediate plasma FV and FVIII reductions (∼70% of wild-type levels). Lman1-/- mice exhibit ER accumulation of another LMAN1 cargo, alpha-1 antitrypsin (A1AT), with an intermediate level of A1AT ER retention observed in Lman1cgt/cgt mice. Finally, the previously reported strain-specific, partially penetrant, perinatal lethality of LMAN1-deficient mice (Lman1gt1/gt1) was confirmed in Lman1-/- mice, although it was not observed in Lman1cgt/cgt mice. Taken together, these results show a dose-dependent effect of residual LMAN1 on the secretion of its cargo proteins. The results also suggest that human subjects with hypomorphic LMAN1 mutations might present with mild bleeding phenotypes resulting from more modest reductions in FV and FVIII, which could be missed by routine clinical evaluation. Finally, these findings suggest that therapeutic targeting of LMAN1 to reduce FV and FVIII as an anticoagulant strategy may only require partial inhibition of LMAN1 function.

Brg1 coordinates multiple processes during retinogenesis and is a tumor suppressor in retinoblastoma

Retinal development requires precise temporal and spatial coordination of cell cycle exit, cell fate specification, cell migration and differentiation. When this process is disrupted, retinoblastoma, a developmental tumor of the retina, can form. Epigenetic modulators are central to precisely coordinating developmental events, and many epigenetic processes have been implicated in cancer. Studying epigenetic mechanisms in development is challenging because they often regulate multiple cellular processes; therefore, elucidating the primary molecular mechanisms involved can be difficult. Here we explore the role of Brg1 (Smarca4) in retinal development and retinoblastoma in mice using molecular and cellular approaches. Brg1 was found to regulate retinal size by controlling cell cycle length, cell cycle exit and cell survival during development. Brg1 was not required for cell fate specification but was required for photoreceptor differentiation and cell adhesion/polarity programs that contribute to proper retinal lamination during development. The combination of defective cell differentiation and lamination led to retinal degeneration in Brg1-deficient retinae. Despite the hypocellularity, premature cell cycle exit, increased cell death and extended cell cycle length, retinal progenitor cells persisted in Brg1-deficient retinae, making them more susceptible to retinoblastoma. ChIP-Seq analysis suggests that Brg1 might regulate gene expression through multiple mechanisms.

Summary: The SWI/SNF protein Brg1 controls cell cycle length, cell cycle exit and cell survival, and is required for cell differentiation and retinal lamination, in the developing mouse retina.