Neurocalcin-immunoreactive cells in the rat hippocampus are GABAergic interneurons

Classes and continua of hippocampal CA1 inhibitory neurons revealed by single-cell transcriptomics

Understanding any brain circuit will require a categorization of its constituent neurons. In hippocampal area CA1, at least 23 classes of GABAergic neuron have been proposed to date. However, this list may be incomplete; additionally, it is unclear whether discrete classes are sufficient to describe the diversity of cortical inhibitory neurons or whether continuous modes of variability are also required. We studied the transcriptomes of 3,663 CA1 inhibitory cells, revealing 10 major GABAergic groups that divided into 49 fine-scale clusters. All previously described and several novel cell classes were identified, with three previously described classes unexpectedly found to be identical. A division into discrete classes, however, was not sufficient to describe the diversity of these cells, as continuous variation also occurred between and within classes. Latent factor analysis revealed that a single continuous variable could predict the expression levels of several genes, which correlated similarly with it across multiple cell types. Analysis of the genes correlating with this variable suggested it reflects a range from metabolically highly active faster-spiking cells that proximally target pyramidal cells to slower-spiking cells targeting distal dendrites or interneurons. These results elucidate the complexity of inhibitory neurons in one of the simplest cortical structures and show that characterizing these cells requires continuous modes of variation as well as discrete cell classes.

Visinin-like proteins (VSNLs): interaction partners and emerging functions in signal transduction of a subfamily of neuronal Ca2+ -sensor proteins

The visinin-like protein (VSNL) subfamily, including VILIP-1 (the founder protein), VILIP-2, VILIP-3, hippocalcin, and neurocalcin delta, constitute a highly homologous subfamily of neuronal calcium sensor (NCS) proteins. Comparative studies have shown that VSNLs are expressed predominantly in the brain with restricted expression patterns in various subsets of neurons but are also found in peripheral organs. In addition, the proteins display differences in their calcium affinities, in their membrane-binding kinetics, and in the intracellular targets to which they associate after calcium binding. Even though the proteins use a similar calcium-myristoyl switch mechanism to translocate to cellular membranes, they show calcium-dependent localization to various subcellular compartments when expressed in the same neuron. These distinct calcium-myristoyl switch properties might be explained by specificity for defined phospholipids and membrane-bound targets; this enables VSNLs to modulate various cellular signal transduction pathways, including cyclic nucleotide and MAPK signaling. An emerging theme is the direct or indirect effect of VSNLs on gene expression and their interaction with components of membrane trafficking complexes, with a possible role in membrane trafficking of different receptors and ion channels, such as glutamate receptors of the kainate and AMPA subtype, nicotinic acetylcholine receptors, and Ca(2+)-channels. One hypothesis is that the highly homologous VSNLs have evolved to fulfil specialized functions in membrane trafficking and thereby affect neuronal signaling and differentiation in defined subsets of neurons. VSNLs are involved in differentiation processes showing a tumor-invasion-suppressor function in peripheral organs. Finally, VSNLs play neuroprotective and neurotoxic roles and have been implicated in neurodegenerative diseases.

Gene expression in cortical interneuron precursors is prescient of their mature function

At present little is known about the developmental mechanisms that give rise to inhibitory gamma-aminobutyric acidergic interneurons of the neocortex or the timing of their subtype specification. As such, we performed a gene expression microarray analysis on cortical interneuron precursors isolated through their expression of a Dlx5/6(Cre-IRES-EGFP) transgene. We purified these precursors from the embryonic mouse neocortex at E13.5 and E15.5 by sorting of enhanced green fluorescent protein-expressing cells. We identified novel transcription factors, neuropeptides, and cell surface genes whose expression is highly enriched in embryonic cortical interneuron precursors. Our identification of many of the genes known to be selectively enriched within cortical interneurons validated the efficacy of our approach. Surprisingly, we find that subpopulations of migrating cortical interneurons express genes encoding for proteins characteristic of mature interneuron subtypes as early as E13.5. These results provide support for the idea that many of the genes characteristic of specific cortical interneuron subtypes are evident prior to their functional integration into cortical microcircuitry. They suggest interneurons are already relegated to specific genetic subtypes shortly after they become postmitotic. Moreover, our work has revealed that many of the genes expressed in cortical interneuron precursors have been independently linked to neurological disorders in both mice and humans.

Expression of the neuronal calcium sensor visinin-like protein-1 in the rat hippocampus

Visinin-like protein-1 (VILIP-1) belongs to the neuronal calcium sensor (NCS) family of EF-hand Ca(2+)-binding proteins which are involved in a variety of Ca(2+)-dependent signal transduction processes in neurons. VILIP-1 has been implicated in the pathology of CNS disorders including Alzheimer's disease and schizophrenia, but its expression has also been found to be regulated following induction of hippocampal synaptic plasticity underlying learning and memory processes. VILIP-1 is strongly expressed in different populations of principal and non-principal neurons in the rat hippocampus. VILIP-1-containing interneurons are morphologically and neurochemically heterogeneous. On the basis of co-localizing markers, VILIP-1 is rarely present in perisomatic inhibitory parvalbumin containing cells. However, VILIP-1 is frequently expressed in mid-proximal dendritic inhibitory cells characterized by calbindin immunoreactivity, and most strongly co-expressed in calretinin-positive disinhibitory interneurons. Partial co-localization of the metabotropic glutamate receptor mGluR1alpha with VILIP-1 was often found in interneurons located in the stratum oriens of the hippocampal CA1 region and in hilar interneurons. Partial co-localization of alpha4beta2 nicotinic acetylcholine receptor with VILIP-1 was seen in stratum oriens interneurons and particularly at the border of the hilus in the dentate gyrus, where VILIP-1 also strongly co-localized with calretinin. We speculate that depending on the regulation of the expression of VILIP-1 in hippocampal pyramidal cells or defined types of interneurons, it may have different effects on hippocampal synaptic plasticity and network activity in health and disease.

Vasoactive intestinal polypeptide-containing elements in the olfactory bulb of the hedgehog (Erinaceus europaeus)

The distribution of vasoactive intestinal polypeptide (VIP)-immunopositive elements was analyzed in the olfactory bulb (OB) of the Western European hedgehog (Erinaceus europaeus) under light and electron microscopy. The immunoreactivity appeared in an abundant population of periglomerular cells of the glomerular layer, in interneurons of the external plexiform layer, and in a restricted group of deep short-axon cells of the internal plexiform layer, the granule cell layer and the white matter. In the glomerular layer, VIP-containing periglomerular cells constituted a population of non-GABAergic neurons and did not receive synapses from olfactory axons. In the EPL, VIP-immunoreactivity appeared in a morphologically heterogeneous population of GABAergic interneurons, most of them identified as satellite cells and Van Gehuchten cells. These interneurons exerted an abundant and selective innervation of the somata, primary and secondary dendrites of the principal mitral and tufted cells, but did not contact granule cells. Perisomatic innervation of the principal cells followed two different patterns. The first included 'normal' basket-like arrangements of VIP-containing varicosities surrounding the somata of mitral and tufted cells. In the second, a set of satellite cells gave rise to short dendritic shafts that embraced the somata of principal cells in an 'exuberant' basket-like arrangement. These two morphological patterns of perisomatic innervation of principal cells were correlated with a neurochemical specificity of the target. In this sense, the 'exuberant' basket-like structures were always found surrounding a subpopulation of principal cells that did not contain the calcium-binding protein parvalbumin (PV). By contrast, they were never found surrounding the subpopulation of PV-containing principal cells, which only showed 'normal' basket-like structures. This study provides new data on the connectivity and neurochemical features of the hedgehog olfactory bulb and suggests that the olfactory circuits in this species are more complex than those described in other mammals.

Abnormal localization of two neuronal calcium sensor proteins, visinin-like proteins (vilips)-1 and -3, in neocortical brain areas of Alzheimer disease patients

The anatomical distribution of the neuronal calcium sensor proteins visinin-like protein-1 and -3 (VILIP-1 and -3) was investigated in various neocortical areas of Alzheimer's disease (AD) patients and controls. In AD and normal brains their cellular localization was confined to pyramidal and non-pyramidal neurons. In AD brains the intracellular immunostaining for VILIP-1 and to a lesser extent for VILIP-3 was found to be reduced in comparison to controls. Also, significantly less VILIP-1-immunoreactive neurons were found in the temporal cortex of AD patients as compared to normal brains. Accordingly, Western blot analysis revealed that immunoreactivity for VILIP-1 is less concentrated in tissue extracts of the temporal cortex of AD patients compared to controls. Extracellularly, VILIP-1 and VILIP-3 immunoreactive material was detected in close association with typical pathologic hallmarks of AD such as dystrophic nerve cell processes, amorphous and neuritic plaques, and extracellular tangles. In control brains an extraneuronal localization of VILIP-1 or VILIP-3 was never observed. Our morphological and neurochemical findings point to an involvement of these two neuronal calcium sensor proteins in pathology and possibly pathophysiology of changed calcium homeostasis in AD.

Distribution of parvalbumin immunoreactivity in the brain of the tench (Tinca tinca L., 1758)

The distribution of parvalbumin (PV) immunoreactivity in the tench brain was examined by using the avidin-biotin-peroxidase immunocytochemical method. This protein was detected in neuronal populations throughout all main divisions of the tench brain. In the telencephalic hemispheres, PV-immunopositive neurons were distributed in both the dorsal and ventral areas, being more abundant in the area ventralis telencephali, nucleus ventralis. In the diencephalon, the scarce distribution of PV-containing cells followed a rostrocaudal gradient, and the most evident staining was observed in the nucleus periventricularis tuberculi posterioris and in a few nuclei of the area praetectalis. In the mesencephalon, abundant PV-immunoreactive elements were found in the tectum opticum, torus semicircularis, and tegmentum. In the tectum opticum, PV-immunoreactivity presented a laminar distribution. Three PV-containing neuronal populations were described in the torus semicircularis, whereas in the tegmentum, the PV staining was mainly located in the nucleus tegmentalis rostralis and in the nucleus nervi oculomotorii. In the metencephalon, Purkinje cells were PV-immunopositive in the valvula cerebelli, lobus caudalis cerebelli, and in the corpus cerebelli. In the myelencephalon, PV immunoreactivity was abundant in the nucleus lateralis valvulae, in the nucleus nervi trochlearis, nucleus nervi trigemini, nucleus nervi abducentis, nucleus nervi glossopharyngei, and in the formatio reticularis. Mauthner cells were also PV immunostained. By contrast to other vertebrate groups, only a restricted population of PV-containing neurons was GABA-immunoreactive in the tench, demonstrating that this calcium-binding protein cannot be considered a marker for GABAergic elements in the teleost brain. This study demonstrates a low phylogenetic conservation of the distribution of PV comparing teleosts and tetrapods.