Biological Sciences
Bone Morphogenetic Protein-2 expression is sexually dimorphic in the adult zebra finch forebrain: an immunocytological study.
Songbirds are excellent models towards understanding the estrogenic modulation of constitutive and induced neurogenesis, synaptic plasticity and learning. Bone morphogenetic proteins (BMPs), which are critical for the establishment of the vertebrate CNS, also affect neurogenesis and synaptic plasticity later in development and adulthood. Several BMPs are expressed in the adult zebra finch brain, and telencephalic BMP2 transcription is increased following brain injury and aromatization. However, whether (or not) BMP2 is translated constitutively or in association with injury is unclear. Adult finches (4 per sex) received unilateral, penetrating brain injuries and were perfused 3 or 7 days later. The brains were stained using an antibody specific to a recombinant BMP2 peptide. BMP2 expression was prominent in cells clustered around the damaged meso- and nidopallia. Immunopositive cells were also seen in many other areas including the neurogenic subventricular zone (SVZ), and song nuclei Area X and RA. A comparison of cell densities across these areas and the entopallium (an undamaged nucleus ventral to the needle tract) revealed significant differences due to sex, brain area and the interaction of these variables. All these effects appear to be driven by immunoreactive cells that define Area X in males but fail to distinguish it from the surrounding medial striatum in females. Thus, BMP2 is expressed in a sexually dimorphic manner in areas involved in song learning and immediately surrounding areas of brain damage in the zebra finch. Future studies will focus on the role(s) of BMP2 in synaptic plasticity, learning and injury-induced neurogenesis.
Supported by NS042767.
The MPNmag of Syrian Hamsters is Sexually Dimorphic in Neuronal Subtype
The medial preoptic area (MPOA) plays a critical role in male sex behavior. A subdivision of the MPOA, the magnocellular division of the medial preoptic nucleus (MPNmag), regulates male mating behavior in the Syrian hamster. Lesions of the MPNmag eliminate male mating behavior. Several lines of evidence suggest that the MPNmag integrates pheromonal and hormonal signals to regulate behavior. For example, the MPNmag is stimulated in males exposed to female pheromones with circulating testosterone; the MPNmag of the female is not stimulated even if testosterone is present. These results suggest that there are fundamental sex differences in MPN mag. Previous studies in our lab show that males have significantly more neurons in the MPNmag, in cresyl violet stained tissue. In this study neurons were identified as large round cells with single nucleoli. More recently we found that immunolabeling with NeuN, a neuronal marker, uncovered an additional set of neurons with multiple nucleoli that were not assessed in the previous study. The goal of this study is to determine if there are sexual dimorphisms in the number of single versus multiple nucleoli neurons within the MPNmag.
Neurons with single and multiple nucleoli were counted in tissue immunolabeled for NeuN and counterstained with cresyl violet. Our results indicate that adult males have more neurons with a single nucleolus, yet females have more neurons with multiple nucleoli making total neuron number the same between the sexes. As neurons with multiple nucleoli have been shown to be more active our results support the hypothesis that the MPN mag of male is fundamentally different from that of females.
Caribbean damselfish with varying territory quality: correlated behaviors but not a syndrome
The behavioral syndrome hypothesis suggests that individuals within a population behave differently due to specific behavioral types and these should be consistent in a variety of contexts. In contrast, for territorial animals that live in stochastic environments, natural selection should favor animals that show behavioral flexibility and can modulate behavior in relation to current territory quality. We examined the territorial behavior of a natural population of male beaugregory damselfish by enhancing territory quality using artificial breeding sites and comparing their behavior to males on lower quality natural sites. When male fish were defending high quality artificial territories, they had higher levels of aggression toward male conspecifics and courtship toward females than when on low quality natural territories. We also found that aggression and courtship behaviors were correlated on natural sites, but not artificial. Behaviors were not correlated when males switched from natural to artificial territories or from artificial to natural territories. These results indicate that males assess their current territories and adjust behaviors accordingly and that courtship and aggression behaviors are not linked within a permanent behavioral syndrome.
Novel Splicing Generates Unique mRNA Isoforms from the Ribosomal Protein L22-like Gene in Drosophila melanogaster
Alternative splicing contributes to protein diversity by creating multiple mRNAs from a single gene that may encode functionally-distinct protein variants. Little is known about alternative splicing of ribosomal protein (rp) genes as a possible mechanism to generate ribosome diversity itself. As complex organelles, eukaryotic ribosomes contain several RNAs and ~80 rp components, each represented as a single copy. Paralogous proteins exist in several rp families and their presence may indicate functional redundancy or specialized functions. Incorporation of specialized paralogs into ribosomes could generate functionally-distinct classes of ribosomes, possibly dedicated to translation of specific mRNAs or to differential regulation of protein synthesis within cells/tissues/developmental stages. Drosophila rp L23a and L22 contain a unique N-terminal extension (of unknown function) with homology to histone H1 (Koyama et al., 1999). Gene expression profiling revealed a novel transcript called “L22-like” that displays an embryonic gonad-specific expression pattern by in situ hybridization compared to a constitutive pattern for L22 mRNA (Shigenobu et al., 2006; Kai et al., 2005). Noticeably, this paralog contains a similar histone H1- like domain, and if expressed at the protein level and assembled into ribosomes, could define a unique class of ribosomes within the fly gonad. Whether or not L22-like and L22 are interchangeable within the ribosome is unknown. Further insight into L22-like expression was determined by RT-PCR analysis using RNA from embryo, larval, and adult stages, as well as from S2 cells. Full-length L22-like mRNAs were detected at all stages. Surprisingly, lower molecular weight (MW) amplicons were also present. Cloning and sequencing revealed that the lower MW amplicons (designated “L22-like short”) are mRNA variants, in which non-canonical splice sites within an exon are used for RNA splicing to remove a previously unidentified intron. Northern blot analysis confirmed the presence of multiple L22-like mRNAs. Preliminary Western blot analyses show protein products of the predicted size for both L22-like and L22-like short isoforms in S2 cells. Whether or not the L22-like isoforms are incorporated into ribosomes is yet to be determined. Collectively, these experiments provide a foundation for understanding developmental regulation of L22-like rp gene expression and how rp variants may contribute to ribosome diversity within cells.
Response characteristics of the avian Superior Olivary Nucleus
The processing of acoustic cues relies on the integration of both ascending and descending neural activity. One of the most well understood auditory circuits is the pathway devoted to processing interaural time disparities, the primary cue for low frequency sound localization. While the ascending ITD circuitry is well characterized both anatomically and physiologically, descending components of the circuit have received much less attention. The superior olivary nucleus in birds (SON) provides the major descending and inhibitory input to all major nuclei in the avian ITD pathway. Despite its central role in avian ITD processing, little is known about the SON’s physiology. We have begun to investigate the cellular and acoustic properties of SON neurons. Our in vivo studies have confirmed previous reports that identified two general classes of SON neuron, phasic and sustained, that are distinguishable on the basis of responses to tones. In addition, we have identified two classes of SON neuron in response to current injection using whole cell current clamp. The first type fires one or two spikes at the beginning of depolarizing current steps, and shows a rectified membrane response for the remainder of the pulse. These properties are common among auditory neurons specialized to process timing information and may correspond to the phasic cells observed in vivo. The second and more common type fires multiple action potentials with depolarizing current steps. This type may correspond to the sustained neurons observed in vivo. These data represent the initial studies toward our goal to build a comprehensive understanding of the SON, and its role in ITD processing.
Supported by the Deafness Research Foundation.