RTL1/PEG11 imprinted in human and mouse brain mediates anxiety-like and social behaviors and regulates neuronal excitability in the locus coeruleus.

RTL1/PEG11, which has been associated with anxiety disorders, is a retrotransposon-derived imprinted gene in the placenta. However, imprinting patterns and functions of RTL1 in the brain have not been well-investigated. We found Rtl1 was paternally, but not maternally, expressed in brain stem, thalamus, and hypothalamus of mice, and imprinting status of RTL1 was maintained in human brain. Paternal Rtl1 knockout (Rtl1m+/p-) mice had higher neonatal death rates due to impaired suckling, and low body weights beginning on embryonic day 16.5. High paternal expression of Rtl1 was detected in the locus coeruleus (LC) and Rtl1m+/p- mice showed an increased delay in time of onset for action potentials and inward currents with decreased neuronal excitability of LC neurons. Importantly, Rtl1m+/p- mice exhibited behaviors associated with anxiety, depression, fear-related learning and memory, social dominance, and low locomotor activity. Taken together, our findings demonstrate RTL1 is imprinted in brain, mediates emotional and social behaviors, and regulates excitability in LC neurons.

LINC complex regulation of genome organization and function.

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

The mammalian LINC complex component SUN1 regulates muscle regeneration by modulating drosha activity.

Here we show that a major muscle specific isoform of the murine LINC complex protein SUN1 is required for efficient muscle regeneration. The nucleoplasmic domain of the isoform specifically binds to and inhibits Drosha, a key component of the microprocessor complex required for miRNA synthesis. Comparison of the miRNA profiles between wildtype and SUN1 null myotubes identified a cluster of miRNAs encoded by a non-translated retrotransposon-like one antisense (Rtl1as) transcript that are decreased in the WT myoblasts due to SUN1 inhibition of Drosha. One of these miRNAs miR-127 inhibits the translation of the Rtl1 sense transcript, that encodes the retrotransposon-like one protein (RTL1), which is also required for muscle regeneration and is expressed in regenerating/dystrophic muscle. The LINC complex may therefore regulate gene expression during muscle regeneration by controlling miRNA processing. This provides new insights into the molecular pathology underlying muscular dystrophies and how the LINC complex may regulate mechanosignaling.

SUN4 is essential for nuclear remodeling during mammalian spermiogenesis.

One of the more dramatic examples of cellular reorganization occurs during spermiogenesis in which a roughly spherical spermatid is transformed into a mature sperm cell. A highlight of this process involves nuclear remodeling whereby the round spermatid nucleus is sculpted into an elongated and polar structure. This transformation in nuclear architecture features chromatin condensation, changes in the composition and organization of the nuclear lamina and redistribution and elimination of nuclear pore complexes. The manchette, a cytoplasmic microtubule-based structure is thought to play a crucial role in the remodeling process. Here we show that SUN4, a spermatid nuclear membrane protein has an essential function in coupling the manchette to the nuclear periphery. In the absence of SUN4, manchette microtubules appear highly disorganized and the nucleus itself fails to elongate. Consequently, mice deficient in SUN4 display globozoospermia with associated infertility.

A trans-homologue interaction between reciprocally imprinted miR-127 and Rtl1 regulates placenta development.

The paternally expressed imprinted retrotransposon-like 1 (Rtl1) is a retrotransposon-derived gene that has evolved a function in eutherian placentation. Seven miRNAs, including miR-127, are processed from a maternally expressed antisense Rtl1 transcript (Rtl1as) and regulate Rtl1 levels through RNAi-mediated post-transcriptional degradation. To determine the relative functional role of Rtl1as miRNAs in Rtl1 dosage, we generated a mouse specifically deleted for miR-127. The miR-127 knockout mice exhibit placentomegaly with specific defects within the labyrinthine zone involved in maternal-fetal nutrient transfer. Although fetal weight is unaltered, specific Rtl1 transcripts and protein levels are increased in both the fetus and placenta. Phenotypic analysis of single (ΔmiR-127/Rtl1 or miR-127/ΔRtl1) and double (ΔmiR-127/ΔRtl1) heterozygous miR-127- and Rtl1-deficient mice indicate that Rtl1 is the main target gene of miR-127 in placental development. Our results demonstrate that miR-127 is an essential regulator of Rtl1, mediated by a trans-homologue interaction between reciprocally imprinted genes on the maternally and paternally inherited chromosomes.

Schizosaccharomyces pombe Pak-related protein, Pak1p/Orb2p, phosphorylates myosin regulatory light chain to inhibit cytokinesis.

p21-activated kinases (Paks) have been identified in a variety of eukaryotic cells as key effectors of the Cdc42 family of guanosine triphosphatases. Pak kinases play important roles in regulating the filamentous actin cytoskeleton. In this study, we describe a function for the Schizosaccharomyces pombe Pak-related protein Pak1p/Orb2p in cytokinesis. Pak1p localizes to the actomyosin ring during mitosis and cytokinesis. Loss of Pak1p function leads to accelerated cytokinesis. Pak1p mediates phosphorylation of myosin II regulatory light chain Rlc1p at serine residues 35 and 36 in vivo. Interestingly, loss of Pak1p function or substitution of serine 35 and serine 36 of Rlc1p with alanines, thereby mimicking a dephosphorylated state of Rlc1p, leads to defective coordination of mitosis and cytokinesis. This study reveals a new mechanism involving Pak1p kinase that helps ensure the fidelity of cytokinesis.

GIT1 activates p21-activated kinase through a mechanism independent of p21 binding.

p21-activated kinases (PAKs) associate with a guanine nucleotide exchange factor, Pak-interacting exchange factor (PIX), which in turn binds the paxillin-associated adaptor GIT1 that targets the complex to focal adhesions. Here, a detailed structure-function analysis of GIT1 reveals how this multidomain adaptor also participates in activation of PAK. Kinase activation does not occur via Cdc42 or Rac1 GTPase binding to PAK. The ability of GIT1 to stimulate alphaPAK autophosphorylation requires the participation of the GIT N-terminal Arf-GAP domain but not Arf-GAP activity and involves phosphorylation of PAK at residues common to Cdc42-mediated activation. Thus, the activation of PAK at adhesion complexes involves a complex interplay between the kinase, Rho GTPases and protein partners that provide localization cues.