Regulation of neurite outgrowth and SNAP-25 gene expression by the Brn-3a transcription factor

SNAP-25 is a presynaptic nerve terminal protein which is also essential for the process of neurite outgrowth in vivo and in vitro. However the processes regulating its expression have not been characterized previously. We show that the gene encoding this protein, SNAP, is strongly activated by the Brn-3a POU (Pit-Oct-Unc) family transcription factor. Expression of both Brn-3a and SNAP-25 increases when ND7 neuronal cells are induced to extend neurite processes by serum removal. Inhibition of Brn-3a expression in these cells inhibits SNAP-25 expression and abolishes the neurite outgrowth that normally occurs in response to serum removal. These results identify Brn-3a as the first transcription factor having a role in process outgrowth in neuronal cells acting, at least in part, via the activation of SNAP-25 gene expression.

Exposure of neonatal rats to alcohol by vapor inhalation demonstrates specificity of microcephaly and Purkinje cell loss but not astrogliosis

The artificial rearing model (AR) of fetal alcohol syndrome (FAS) has been shown to induce several major pathologies in the early postnatal rat brain development: microcephaly, selective neuronal cell loss, and activation of astroglia in the neocortex. The purpose of this study was to determine whether these pathologies were specific to the action of alcohol or, in contrast, could result from confounds attributed to this model of FAS. For this purpose, the pathological effects of AR were compared with those of a vapor inhalation (VI) model of FAS. Our studies showed that the microcephaly that developed after exposure to periodic blood alcohol levels (BALs) of 300-350 mg% during postnatal days 4-9 could be achieved by both AR and IV models of FAS, and thus is independent of the method of alcohol administration. In contrast, the gliosis measured by glial fibrillary acidic protein (GFAP) mRNA levels in cortex, as well as by immunohistochemical staining for GFAP, was found only in the AR-FAS model, but not in the VI model. However, the lack of gliosis in VI was apparently not due to a less intrusive intervention of alcohol, because VI exposure resulted in a reduction in Purkinje cell number comparable with that found after AR or intragastric intubation of alcohol. Based on these observations, we conclude that the activation of gliosis observed after AR is not a specific effect of alcohol, but rather is caused by an interaction of alcohol with as yet unidentified factors present in AR.

Patient-controlled analgesia: a method for the controlled self-administration of opioid pain medications

The purposes of this article are to introduce the reader to patient-controlled analgesia (PCA) and to summarize its use in several selected pain-related conditions. patient-controlled analgesia is a relatively new technique for managing pain in which patients are able to self-administer small doses of opioid analgesic medications when needed. The authors briefly review some of the problems associated with current and previous opioid delivery strategies and highlight the advantages of PCA over these other methods. They then discuss the components of the PCA system and briefly describe how the system is operated and controlled. In this discussion, the authors indicate an appropriate therapeutic goal and suggest knowledge requirements for the effective use of PCA. The authors close with a brief summary of several reports describing the use of PCA in the management of postoperative pain, cancer pain, and pain associated with labor and delivery. Indications and contraindications for use in these conditions are presented. Because physical therapists often play a major role in pain management, it is important for them to be well informed with regard to recent developments in this rapidly developing area of clinical practice.

Differential expression of SNAP-25 protein isoforms during divergent vesicle fusion events of neural development

The presynaptic plasma membrane protein SNAP-25 (synaptosome-associated protein of 25 kDa) has been implicated as one of several neural-specific components that direct constitutive fusion mechanisms to the regulated vesicle trafficking and exocytosis of neurotransmitter release. There exist two alternatively spliced isoforms of SNAP-25, a and b, which differ in a putative membrane-interacting domain. We show that these two isoforms have distinct quantitative and anatomical patterns of expression during brain development, in neurons, and in neuroendocrine cells and that the proteins localize differently in neurites of transfected PC12 pheochromocytoma cells. These findings indicate that alternative isoforms of SNAP-25 may play distinct roles in vesicular fusion events required for membrane addition during axonal outgrowth and for release of neuromodulatory peptides and neurotransmitters.

Alcohol selectively attenuates stress-induced c-fos expression in rat hippocampus

The ability of ethyl alcohol to modify responses to stress has been well documented (cf. Pohorecky, 1990). However, the structural substrate mediating these effects of alcohol remains undefined. Using immediate early gene (IEG) expression in the brain as a marker of altered neuronal response, we investigated the effect of acute alcohol exposure on the activity of brain regions of rats exposed to 15 min of restraint stress. Immunocytochemical localization c-Fos protein demonstrated that restraint stress led to an induction of c-Fos expression in several brain structures including cingulate and piriform cortex, cortico-amygdaloid and hippocampo-amygdaloid transition zones, hippocampus, hypothalamus, supramammillary nucleus, and centromedial nucleus of thalamus. An intraperitoneal injection of 2 g/kg alcohol prior to stress decreased c-Fos expression in several but not all of these structures. In particular, alcohol strongly attenuated the stress-induced expression of c-Fos in hippocampus and cingulate cortex. Using slot-blot hybridization, significant induction of c-fos mRNA after restraint stress was demonstrated both in hippocampus and cortex, but prior alcohol exposure specifically attenuated c-fos induction only in the hippocampus. The response of c-fos mRNA expression to stress and alcohol differed from the effects on jun-B, c-jun and jun-D mRNA levels. Perhaps surprisingly, acute exposure to alcohol in otherwise unstressed rats did not induce significant changes in expression of IEGs in comparison to control (saline-injected) animals even with doses sufficient to elevate plasma corticosterone. In summary, these studies demonstrate a selective sensitivity of stress-induced activity of neurons of hippocampus and cingulate cortex to acute alcohol exposure.

Social problems in geriatric dentistry: an overview

Many studies have reported on the dental status of elderly patients. The results demonstrate consistent differences between normative and perceived needs in addition to varying levels of awareness of dental education among carers. This review article attempts to integrate a range of social factors which must be considered if holistic and comprehensive care is to be delivered to elderly patients.

Induction and habituation of immediate early gene expression in rat brain by acute and repeated restraint stress

Acute exposure to stress leads to activation of the pituitary-adrenal axis (PA-axis) while repeated exposure to a homotypic stressor generally results in habituation of this response. Previous studies suggested that such habituation is largely due to changes in afferents of the PA-axis. To examine where within these afferents habituation occurs, we studied the effect of acute and repeated exposure to 2 hr restraint stress on expression of c-fos mRNA, as a marker of altered neuronal activity, in brain regions previously shown to influence the activity of the PA-axis. Acute restraint stress increased expression of c-fos mRNA in cortex, hippocampus, hypothalamus, septum, and brainstem. In contrast, the effect of restraint stress on c-fos expression in the aforementioned brain regions was much smaller in animals restrained once daily for 4 d, and nonexistent in animals restrained once daily for 9 d. A similar pattern of induction and habituation of jun-B, but not zif-268, c-jun, or jun-D mRNA expression, was observed in the cortex of animals exposed to acute versus repeated restraint stress. The habituation of c-fos responses was stressor specific: exposure of restraint-adapted animals to a novel (20 min swim) stress produced an increase in levels of c-fos mRNA in every examined brain region comparable to that seen in animals exposed to this stressor for the first time. Adrenalectomy did not alter the pattern of c-fos expression induced by acute and repeated restraint stress. Therefore, activation and habituation of these c-fos responses are independent of changes in circulating levels of corticosterone.

Identification of A1 adenosine receptors in rat cochlea coupled to inhibition of adenylyl cyclase

A1 adenosine receptors (A1ARs) are found in a number of tissues in the body where their physiological roles have been identified. In the cochlea, neither the existence of these receptors nor a physiological role of adenosine has been described previously. Membranes prepared from rat cochlea demonstrated high affinity and saturable binding to N6-2-(4-amino-3-[125I]iodophenyl)ethyladenosine ([125I]APNEA), an A1AR agonist, with maximum binding capacity and dissociation constant values being 40.5 +/- 0.5 fmol/mg protein and 1.28 +/- 0.03 nM, respectively. Adenosine analogues competed for [125I]APNEA binding sites with a rank order of potency characteristic of these sites being the A1AR. The [125I]APNEA binding was significantly reduced by pertussis toxin, indicating coupling of these receptors with the Gi and/or Go proteins in cochlear membranes. Photoaffinity labeling of the receptor protein with the A1AR agonist N6-2-(4-azido-3[125I]iodophenyl)ethyladenosine showed specific labeling of a 36-kDa receptor protein. Activation of the A1AR with R-phenylisopropyladenosine (R-PIA) led to inhibition of forskolin-stimulated adenylyl cyclase activity. Amplification of reverse-transcribed RNA derived from cochlear tissue by polymerase chain reaction (using primers for the bovine A1AR) yielded a 770-bp product that hybridized to an A1AR cDNA probe on Southern blots. These data indicate the presence of an inhibitory receptor in the peripheral auditory system, which may play an important role in modulating auditory functions.

Barriers to the provision of complex dental treatment for dentate older people: a comparison of dentists' and patients' views

The majority of older people live active lives and more are retaining their teeth. These people rely for their dental care on general dental practitioners. However, many find that the service they are offered falls short of their desires, and this study examined this problem from the points of view of the public and the dentists. Qualitative group discussions were conducted among 61 middle-aged and elderly people and 20 dentists, followed by a quantitative study involving 20 dentists and 57 of their patients. The aim was to discover what, if any, differences there may have been between their expectations of dental treatment. There were no significant differences between middle-aged and older peoples' views on barriers to the receipt of dental care. However, dentists assumed that their older patients would have significantly greater negative dental attitudes than their middle-aged patients. Dentists considered that ability to pay would be a significantly greater barrier for their working-class rather than their middle-class patients. However, among the patients themselves there was no significant difference. The comparison of dentists' and patients' views on barriers to treatment showed that dentists significantly overestimated their older patients' reluctance to receive dental treatment.

Internists' attitudes about clinical practice guidelines

OBJECTIVE

To assess internists' familiarity with, confidence in, and attitudes about practice guidelines issued by various organizations.

DESIGN

Cross-sectional, self-administered survey.

PARTICIPANTS

Questionnaires were mailed to a stratified random sample of 2600 members of the American College of Physicians (ACP) in 1992. Of the 2513 internists who met our eligibility criteria, 1513 responded (60%).

MEASUREMENTS AND RESULTS

Familiarity with guidelines varied from 11% of responders for the ACP guideline on exercise treadmill testing to 59% of responders for the National Cholesterol Education Program guideline. Confidence was reported in ACP guidelines by 82% of responders but by only 6% for Blue Cross and Blue Shield guidelines. Subspecialists had greatest confidence in guidelines developed by their own subspecialty organizations. It was thought that guidelines would improve the quality of health care by 70% of responders, increase health care costs by 43%, be used to discipline physicians by 68%, and make practice less satisfying by 34%. More favorable attitudes were held by internists who were paid a fixed salary, saw patients for less than 20 hours per week, had recently graduated from medical school, or were not in private practice.

CONCLUSIONS

Although most ACP members studied recognized the potential benefits of practice guidelines, many were concerned about possible effects on clinical autonomy, health care costs, and satisfaction with clinical practice.

Regulated vesicular fusion in neurons: snapping together the details

In the past year major strides have been made toward our understanding of the molecular mechanisms involved in regulated vesicle fusion and exocytosis in neurons and neuroendocrine cells. Much of this advance has come from the identification of proteins participating in these events and of their potential roles mediated by interactions with each other, the constituent membranes, and, in some cases, Ca2+ signaling. The involvement of vesicle fusion in elongation of neuronal processes during development and release of transmitters and neuromodulatory peptides in the mature nervous system indicates, however, that refinements in the fusion machinery may be required for each of these acts. For many of the participants in synaptic membrane fusion, variant isoforms have been identified that exhibit modifications that might alter interactive properties of these proteins. We discuss the idea that diversification of isoforms, as illustrated by the expression of alternatively spliced variants of SNAP-25, is likely to be an important component in providing the detail necessary to differentiate the physiology of regulated fusion of different classes of vesicles employed in development, neurotransmission, and secretion.

Deletion map of the coloboma (Cm) locus on mouse chromosome 2

The extent of the semidominant coloboma (Cm) mutation on mouse Chromosome 2 was determined by deletion mapping using interspecific hybrid mice. The Cm deletion mutation results in ophthalmic dysmorphology and behavioral deficits, including profound hyperactivity, and has been shown to encompass the gene Snap. In addition to Snap, the gene encoding phospholipase C beta-1 (Plcb-1), which maps 0.60 +/- 0.60 cM proximal to Snap, and simple sequence repeat (SSR) loci D2Mit19, D2Mit46, D2Mit28, and D2Mit136 were shown to be deleted at the Cm locus. In contrast, analysis of other closely linked SSRs and genes either proximal (Bmp-2a) or distal (Nec-1) to Snap, as well as a complementation test with the closely linked mutation lethal milk (lm), indicates that these gene sequences are unaffected by the Cm mutation. These data demonstrate that the Cm deletion represents a contiguous gene defect encompassing 1.1 to 2.2 cM that may be probed for genes, both in the mouse and in the syntenic region of human Chr 20, that independently affect elements of neurological behavior and eye development.

Human cDNA clones encoding two different isoforms of the nerve terminal protein SNAP-25

Two distinct cDNA sequences, corresponding to alternative isoforms of the human nerve terminal protein SNAP-25 (synaptosomal associated protein of 25 kDa), were cloned and characterized. Sequence analysis demonstrated that the two isoforms are generated by alternative splicing between two distinct but homologous exons 5, a and b each encoding 39 amino acids (aa). Although the two isoforms, SNAP-25a and SNAP-25b, differ by only 9 aa, this domain encodes the portion of the protein that is a substrate for post-translational fatty acylation, and therefore might be important for regulating subcellular localization and membrane targeting.

Botulinum neurotoxins serotypes A and E cleave SNAP-25 at distinct COOH-terminal peptide bonds

SNAP-25, a membrane-associated protein of the nerve terminal, is specifically cleaved by botulinum neurotoxins serotypes A and E, which cause human and animal botulism by blocking neurotransmitter release at the neuromuscular junction. Here we show that these two metallo-endopeptidase toxins cleave SNAP-25 at two distinct carboxyl-terminal sites. Serotype A catalyses the hydrolysis of the Gln197-Arg198 peptide bond, while serotype E cleaves the Arg180-Ile181 peptide lineage. These results indicate that the carboxyl-terminal region of SNAP-25 plays a crucial role in the multi-protein complex that mediates vesicle docking and fusion at the nerve terminal.

Ontogeny, sex dimorphism, and neonatal sex hormone determination of synapse-associated messenger RNAs in rat brain

Sex hormones influence neurite outgrowth and synaptogenesis in certain hormone-dependent areas of the rat brain during neonatal development. These alterations are thought to mediate changes in brain structure and function between the sexes. Growth-associated protein 43 kDa (GAP-43) gene expression is estrogen-regulated in the adult ventromedial hypothalamus (VMH) and sexually dimorphic (M:F = 1.8:1) in adult cortex (CTX). Such effects intimate hormonal regulation of synaptic plasticity. To investigate the nature of these dimorphisms, the present study examined the ontogeny of expression of mRNAs encoding 3 neural-specific proteins: GAP-43, SCG10, and synaptosomal-associated protein 25 kDa (SNAP-25); and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in the VMH and CTX; and also the effects of altering the neonatal sex hormonal milieu on the development of these adult dimorphisms. Levels of specific mRNAs in VMH and CTX were quantitated by slot-blot hybridization in rats of both sexes at different postnatal ages. To determine the involvement of neonatal sex hormones on the levels of these mRNAs, male neonatal rat pups were treated with an estrogen receptor antagonist or an aromatase inhibitor, and neonatal female pups were treated with testosterone or estrogen prior to slot-blot evaluations in adulthood. In VMH, GAP-43 mRNA levels were high on days P1 and P4 with a 3-fold decrease by day P23; in CTX, GAP-43 mRNA first increased by day P11, then fell to baseline by day P23. In VMH, SCG10 mRNA showed only small increases with time; but in CTX, there was a 5-fold drop from days P4 to P23. In VMH, SNAP-25 mRNA was low and changed only slightly; but in CTX there was a 5-fold increase between days P4 and P60. At birth, there was no sex dimorphism in either VMH or CTX, but the levels of all 3 neural-specific mRNAs were sexually dimorphic in adult CTX (M:F = 1.76 for GAP-43, 1.46 for SCG10, 1.44 for SNAP-25). GAPDH mRNA levels were regulated developmentally in VMH and CTX, but there was no sex dimorphism in either area. In male rats who received either an estrogen antagonist or aromatase inhibitor at birth, the CTX GAP-43 and SNAP-25 mRNA levels fell by 30%, to levels similar to untreated females. Conversely, in female rats, neonatal treatment with either testosterone or estrogen increased GAP-43 and SNAP-25 mRNA levels by about 30%, to levels similar to the untreated adult male. SCG10 levels did not demonstrate neonatal hormonal dependence.(ABSTRACT TRUNCATED AT 400 WORDS)

More informative abstracts of articles describing clinical practice guidelines

Recommendations are proposed for preparing more informative abstracts of articles describing clinical practice guidelines. Information about the development and content of guidelines should be summarized with the following structure.

OBJECTIVE

a succinct statement of the objective of the guideline, including the targeted health problem, the targeted patients and providers, and the main reason for developing recommendations concerning this problem for this population.

OPTIONS

principal practice options that were considered in formulating the guideline.

OUTCOMES

significant health and economic outcomes identified as potential consequences of the practice options.

EVIDENCE

Methods used to gather, select, and synthesize evidence, and the date of the most recent evidence obtained.

VALUES

persons and methods used to assign values (relative importance) to potential outcomes of alternative practice options.

BENEFITS, HARMS, AND COSTS

the type and magnitude of the main benefits, harms, and costs that are expected to result from guideline implementation.

RECOMMENDATIONS

a brief and specific list of key recommendations.

VALIDATION

the results of any external review, comparison with guidelines developed by other groups, or clinical testing of guideline use.

SPONSORS

key persons or groups that developed, funded, or endorsed the guideline. Abstracts adhering to these recommendations could enhance readers' ability to appraise the applicability, importance, and validity of guidelines for specific providers, patients, and settings. More informative abstracts could also promote the use of more explicit methods of guideline development, more consistent reporting of guideline documents, and the more appropriate use of guidelines by clinicians.

In a drug discrimination procedure isolation-reared rats generalize to lower doses of cocaine and amphetamine than rats reared in an enriched environment

Rats with different behavioral histories, defined by rearing and housing in either an enriched condition (EC) or an isolation condition (IC), were trained in a two-lever operant procedure to discriminate 5.0 mg/kg cocaine from saline. In cocaine dose-generalization tests, the IC rats exhibited an ED50 (1.01 mg/kg) significantly lower than the EC rats (ED50: 1.55 mg/kg). The cocaine-appropriate responding was emitted when the rats were treated with d-amphetamine, and for the d-amphetamine test doses the ED50 (0.19 mg/kg) was again significantly lower for the IC rats compared to the ECs (ED50: 0.33 mg/kg). These data suggest that IC rats are more sensitive to the stimulus properties of indirect dopaminergic agonists than EC rats and highlight the importance of environmental variables in governing an organism's response to the stimulus properties of abused drugs.

The 25 kDa synaptosomal-associated protein SNAP-25 is the major methionine-rich polypeptide in rapid axonal transport and a major substrate for palmitoylation in adult CNS

A conspicuous correlate of the developmental transformation of axonal growth cones to synaptic terminals is a marked increase in synthesis and axonal transport of a methionine-rich, acidic polypeptide of approximately 25 kDa. This polypeptide, designated "super protein" (SuP), is the most prominent species among methionine-labeled proteins conveyed by rapid axonal transport in mature CNS and PNS neurons of warm- and cold-blooded vertebrates. We show here that SuP is identical to SNAP-25, a highly conserved synaptic protein of known primary structure, by immunoprecipitation with anti-SNAP-25 antiserum of SuP labeled with 35S-methionine and transported by retinal ganglion cells of rat and cat. In addition, we show that SNAP-25/SuP is the most prominent species among retinal polypeptides that incorporate 3H-palmitate in vivo, that it is fatty acylated through a hydroxylamine-labile, thioester bond, and that palmitoylated SNAP-25/SuP is axonally transported. Thus, SNAP-25/SuP is a rapidly transported constituent of the presynaptic apparatus and a major neuronal substrate for long-chain fatty acylation.

Differential expression of the presynaptic protein SNAP-25 in mammalian retina

We have studied the expression of the nerve terminal protein synaptosomal associated protein 25 (SNAP-25) in the retina of adult rat, mouse, and monkey, as well as in the developing mouse retina. To evaluate SNAP-25 expression, its distribution was compared to those of the synaptic vesicle-associated proteins synapsin I and synaptophysin. In situ hybridization in adult rat retinas suggested that SNAP-25 mRNA is mainly expressed by ganglion, amacrine, and horizontal cells, but not by photoreceptors and bipolar cells. In all species, the SNAP-25 polypeptide was most abundant in the inner part of the inner and outer plexiform layers and was also found in the ganglion cell axons. In adult retina, synapsin I and synaptophysin were also mainly localized in synaptic fields and processes but all three proteins showed a distinct pattern of distribution. Finally, in mouse retina, the three proteins were first detectable at embryonic day 16 and subsequently showed developmentally regulated changes in their cellular localization. These results suggest that SNAP-25 is predominantly expressed in specific subtypes of conventional synapses, but not ribbon synapses, and that it may also be involved in the physiology of nonvesicular terminals of horizontal cells. Our study also suggests that combinatorial expression of different components of the presynaptic specialization may contribute to synaptic functional diversity.

Spontaneous locomotor hyperactivity in a mouse mutant with a deletion including the Snap gene on chromosome 2

The gene encoding the synaptosomal-associated protein--25 kDa (SNAP-25) was mapped by analysis of somatic cell hybrids and an intersubspecies backcross to mouse Chromosome 2. To identify potential mutants for SNAP-25, mice bearing mutations mapping to this region of Chromosome 2 were screened for Snap gene abnormalities. Mice heterozygous for the semidominant mutation coloboma (Cm/+) were identified that carried a deletion of Snap gene sequence. Analysis of genomic DNA revealed that the Snap gene dosage in Cm/+ mice was 50% lower than control littermates. Additionally, SNAP-25 mRNA and protein expression were 50% lower in coloboma mice than control littermates. The coloboma mouse phenotype is characterized by small eyes and head bobbing; in addition, we observed that these mice were extremely hyperactive with spontaneous locomotor activity exceeding three times control mouse activity. The localization of the genetic abnormality in coloboma mice using the Snap gene marker will provide a powerful tool for studying the biologic basis of locomotor hyperactivity.

Distribution and expression of SNAP-25 immunoreactivity in rat brain, rat PC-12 cells and human SMS-KCNR neuroblastoma cells

Immunocytochemical, immunoblotting and in situ hybridization studies were used to map the distribution of SNAP-25 protein and mRNA in the rodent nervous system. These experiments demonstrated that subsets of neurons expressed SNAP-25, and that several patterns of expression emerged: SNAP-25 expression in caudate nucleus was initially concentrated in axons, which subsequently was localized in presynaptic regions of these axons. Other regions, typified by neocortex, showed developmental increases and persistent adult neuronal immunoreactivity for SNAP-25. Finally, olfactory bulb contained neurons which initially expressed SNAP-25, but lost expression during maturation. Additional studies in cultured human and rat cell lines derived from neural crest suggested that SNAP-25 is expressed in such lines, but not in glial or fibroblast lines. Differentiation of rat PC-12 cells with nerve growth factor failed to alter steady-state levels of SNAP-25 protein; similar responses were seen in human SMS-KCNR neuroblastoma cells differentiated using retinoic acid. The presence of SNAP-25 in presynaptic regions of numerous neuronal subsets and in neural crest cell lines suggests that this protein subserves an important function in neuronal tissues.

The dispersion of defective endogenous murine retroviral elements suggests retrotransposition-mediated amplification

The dispersion of four replication-defective endogenous proviruses, originally detected in 129 strain mice and shown to have extensive deletions of gag, pol, and env gene regions, was investigated in 13 inbred strains and substrains of mice. Using probes to sequences flanking the integration sites in 129 mice, unique genomic Eco RI fragments were assigned to each of the four endogenous proviral elements. Analyses revealed that certain of these proviral elements are present both in strains closely related to strain 129 (i.e., strains 101 and LP/J) and in more distantly related strains (i.e., strains BALB/cJ, A/J, and C3H/HeJ). In mouse strains lacking proviral integration at a particular locus, the size of the corresponding Eco RI genomic fragment and absence of a characteristic Kpn I site indicated the lack of a residual solitary long terminal repeat. Hybridization of oligonucleotide probes that distinguish the specific deletions present within these elements identified additional analogous proviral integrations at many different sites in all strains investigated. These data indicate that the diversification of these proviral elements found in inbred strains is generated by integration of new copies, rather than excision through homologous recombination. Moreover, the results are consistent with other endogenous retroviruses providing the trans-acting proteins necessary to package the defective viral RNA.