Ultrastructural localization of brain 'vitamin D-dependent' calcium binding proteins

The molecular identity of the mitochondrial Ca2+ sequestration system

There is ample evidence to suggest that a dramatic decrease in mitochondrial Ca(2+) retention may contribute to the cell death associated with stroke, excitotoxicity, ischemia and reperfusion, and neurodegenerative diseases. Mitochondria from all studied tissues can accumulate and store Ca(2+) , but the maximum Ca(2+) storage capacity varies widely and exhibits striking tissue specificity. There is currently no explanation for this fact. Precipitation of Ca(2+) and phosphate in the mitochondrial matrix has been suggested to be the major form of storage of accumulated Ca(2+) in mitochondria. How this precipitate is formed is not known. The molecular identity of almost all proteins involved in Ca(2+) transport, storage and formation of the permeability transition pore is also unknown. This review summarizes studies aimed at identifying these proteins, and describes the properties of a known mitochondrial protein that may be involved in Ca(2+) transport and the structure of the permeability transition pore.

Calbindin-D28k in nerve cell nuclei

Calbindin-D28k is a member of the large EF-hand family of calcium-binding proteins, that is believed to function, in part as a cytosolic calcium buffer. Recent studies have demonstrated that cells containing Calbindin-D28k are protected from degeneration caused by conditions that elevate intracellular calcium concentrations. Since its initial discovery in 1966, Calbindin-D28k has been localized in the cytoplasm of many neuronal populations, but its nuclear localization has been uncertain. Using light and electron microscopic immunohistochemistry, and nuclear fractionation methods, we demonstrate localization of Calbindin-D28k not only in the cytoplasm, but also in the nucleus of rodent midbrain dopaminergic neurons and cerebellar Purkinje cells. The Calbindin-D28k immunoreactive staining intensity in the nucleus was routinely equal or greater than that in the cytoplasm. Since calcium signals are propagated to the nucleus, where they can regulate gene expression, the existence of nuclear Calbindin-D28k has important implications for cellular function.

Calbindin D28k-containing neurons are restricted to the medial substantia nigra in humans

A controversy exists in the literature as to whether neurons containing the calcium binding protein calbindin-D28k are located within the human substantia nigra. The point of variance between reports, however, is not the anatomical distribution of these neurons, but rather the delineation of the dorsal border of the substantia nigra. It has been suggested that the dense substance P striatonigral innervation delimits the substantia nigra in the human. The aim of the present study is to re-examine the distribution of calbindin-D28k-positive neurons throughout the substantia nigra using substance P to delimit its borders. Although a few calbindin-D28k-positive neurons were found in the medial cell group of the substantia nigra, the vast majority of positive neurons were located in the adjacent A8 and A10 dopaminergic cell groups. This anatomical location of calbindin-D28k-positive neurons is consistent with previous reports, though our results indicate that when the striatonigral projection is used to define the substantia nigra, calbindin-D28k is not a notable feature of these neurons. This questions the neuroprotective role of this protein in Parkinson's disease.

Calmodulin, calbindin-D28K and calretinin in rat and chicken pineal glands: immunocytochemical and immunoblotting analysis

In pineal gland, melatonin is synthesized in pinealocytes. Pharmacological studies using calmodulin antagonists suggested that melatonin synthesis was regulated through calmodulin. However, immunohistochemical studies showed that calmodulin could only be detected in pineal glial cells, and not in pinealocytes. To further investigate this discrepancy, we have tried to detect calmodulin not seen by immunohistochemical methods. We have used rat and chicken pineal homogenate supernatants and Triton X-100-treated pellets denatured by sodium dodecyl sulfate, subjected to electrophoresis and immunoblotting using anti-calmodulin antibodies. Two different IgG (#465 and #860) purified from anti-calmodulin sera were used. In rat pineal homogenate supernatants, calmodulin could be detected by immunoblotting using both antibodies. Some calmodulin could also be detected in the Triton-treated pellet fractions, but no additional cross-reacting bands were detected. However, in both chicken pineal homogenate supernatants and Triton-extracted pellets, in addition to a calmodulin immunoreactive band, two other proteins with approximate molecular masses (M(r)) of 56 kDa and 60 kDa were detected using anti-calmodulin #465. For comparison, similar immunoblot experiments were performed for detection of calbindin-D28K and calretinin, two other calcium binding proteins expressed in different pineal cell populations. Interestingly, Triton extraction of chicken pineal pellets revealed additional bands cross-reacting with each antibody. Anti-calbindin-D28K cross-reacted strongly with a M(r) = 68 kDa protein and weakly with a M(r) = 56 kDa protein. Anti-calretinin cross-reacted strongly with a M(r) = 93 kDa protein and weakly with a M(r) = 56 kDa protein.

Calmodulin immunoreactivity in the chicken pineal gland: comparison with calbindin-D28k, calretinin, and S100

Calmodulin distribution in the chicken pineal organ was investigated by immunohistochemistry. Calmodulin immunoreactivity was detected in ependymocytes in the follicular zone and in interstitial cells in the parafollicular zone. No calmodulin immunoreactivity was detected in pinealocytes. Lack of calmodulin immunoreactivity in pinealocytes raises questions about its proposed function in melatonin synthesis as suggested by pharmacological studies using calmodulin antagonists. The calmodulin distribution was comparable to that of S100, a glial cell marker. Two other markers, calbindin-D28k and calretinin, which in neuroanatomical studies give excellent cytoarchitectonic staining, in the chick pineal permitted the detection of two subclasses of pinealocytes. One was darkly stained by calbindin-D28k and rare. The other was very abundant and calretinin positive. In the parafollicular zone, calbindin-D28k and/or calretinin antibodies allowed us to visualize cells presenting a neuron-like morphology. Calretinin immunoreactivity was detected in nearly all pinealocytes in which hydroxy-indol-O-methyl transferase was also located. Comparison between the lack of calmodulin and the presence of calretinin, belonging to the same calcium-binding protein family, in chick pinealocytes raises the hypothesis about a possible role of calretinin in melatonin synthesis.

Reduction of vitamin D hormone receptor mRNA levels in Alzheimer as compared to Huntington hippocampus: correlation with calbindin-28k mRNA levels

Receptors for vitamin D hormone (VDR) and the calcium binding protein, calbindin-28k, have been localized in many tissues, including brain. In brain, VDR and calbindin-28k were reported to colocalize in hippocampal CA1 cells. We have shown that mRNA pool size for calbindin-28k was reduced, on average, by 35% in Alzheimer hippocampal CA1 cells, as compared to Huntington control (manuscript in preparation). In the present study, in situ hybridization with tritiated antisense RNA probes was used to examine VDR expression in paired Alzheimer and Huntington brain tissue. Message levels for VDR were reduced, on average, by 34% and 31%, respectively, in Alzheimer hippocampal CA1 and CA2 pyramidal cells, as compared to Huntington control. However, VDR message levels were not significantly different from control in Alzheimer temporal cortex or cerebellum. There was no correlation between VDR message levels and brain weight, autopsy interval, patient age or the extent of neurofibrillary degeneration. Instead, VDR mRNA pool size in hippocampal CA1 cells correlated significantly with calbindin-28k message levels (r = 0.52, P less than 0.001). Decreased message levels for VDR and calbindin-28k in these cells were due to an increased percentage of cells expressing lower message levels for these proteins. These results show that in Alzheimer hippocampal CA1 cells, VDR mRNA pool size is downregulated and that this downregulation may play a role in the reduction of calbindin-28k expression.

Differential expression of calbindin-D 28 kDa in rat incisor ameloblasts throughout enamel development

Calbindin-D 28 kDa (CaBP 28 kDa), a vitamin D-dependent calcium-binding protein, has been associated with calcium handling by cells. We have investigated the expression of this protein in the rat incisor enamel organ, an epithelium interposed between a mineralizing matrix and connective tissue rich in blood vessels, by radioimmunoassay (RIA), Western blotting, and quantitative protein A-gold immunocytochemistry with antibodies to rat kidney CaBP 28 kDa. RIA of cytosolic extracts showed that enamel organs contained relatively high concentrations of CaBP 28 kDa (compared to kidney; see review by Christakos S., C. Gabrielides, and W.B. Rhoten 1989 Endocr. Rev., 10:3-25). Immunoblotting of proteins extracted from enamel organ strips revealed an intensely-stained band near 28 kDa throughout amelogenesis following ameloblast differentiation. Immunocytochemically, CaBP 28 kDa was localized exclusively within ameloblasts. The density of labelling increased from the presecretory stage to the secretory stage and fluctuated across the maturation stage in relation to ameloblast modulation. Ruffle-ended ameloblasts consistently showed the most intense immunoreaction. Gold particles were present throughout the cytoplasm and nuclei of ameloblasts but regions rich in rough endoplasmic reticulum or cell webs showed a higher immunolabelling. Some gold particles were also associated with the external face of the rough endoplasmic reticulum. Multivesicular bodies in maturation stage ameloblasts were occasionally immunoreactive. These data suggest that the intracellular concentration of CaBP 28 kDa is regulated throughout amelogenesis reflecting a stage-specific control of calcium homeostasis in ameloblasts.

Molecular identification of the Lugaro cell in the cat cerebellar cortex

The cerebellar cortex contains five major classes of neurons that can be differentiated from one another on the basis of their location, size, shape, and, in some cases, molecular characteristics. The cerebellar cortex also contains other, less numerous neuronal types, including the Lugaro cell, which has been described on only a few occasions. The Lugaro cell is a relative rare cell type and is characterized by a fusiform cell body with thick, horizontally oriented dendrites. It is located in or slightly below the Purkinje cell layer. Because the Lugaro cell shares some morphological characteristics with the other large granular layer neurons, it often has been classified as a Golgi cell. In the present study we have taken advantage of differences in the molecular properties of neurons and have used monoclonal antibodies to identify and classify the Lugaro cell. Three large neuronal types in the cerebellar cortex were examined with cell-type-specific antibodies: Cat-301 and Cat-304 for Lugaro cells; Rat-303 for Golgi cells; and anti-calbindin for Purkinje cells. Double label immunocytochemistry on sections of the cat cerebellum was performed with subclass- or species-specific secondary antibodies. Each of the three antibodies was selective for one of the three large neuron classes. Cat-301 and Cat-304 recognized Lugaro cells but not Golgi or Purkinje cells. Our results demonstrate that the Lugaro cells are molecularly, as well as morphologically, distinct from Purkinje and Golgi cells and thus constitute a distinct cell type in the cerebellar cortex.

Calbindin D28k-containing splanchnic and cutaneous dorsal root ganglion neurons of the rat

Calbindin D28k (CaBP)-containing splanchnic and cutaneous sensory neurons in the rat dorsal root ganglia (DRGs) were investigated immunocytochemically in combination with a fluorescent dye tracer (Fluoro gold). About 15% of the DRG neurons at Th9-10 levels showed CaBP-like immunoreactivity. Eighty-four % of the splanchnic sensory neurons were immunoreactive to CaBP, while only 3% of the cutaneous sensory neurons were. The diameters of the splanchnic and cutaneous sensory neurons containing CaBP were 23.4 +/- 6.3 microns and 38.4 +/- 8.8 microns, respectively. Splanchnic sensory neurons containing CaBP were sensitive to capsaicin while cutaneous ones were not. These findings suggest that CaBP-containing splanchnic and cutaneous sensory neurons constitute different subgroups among the DRG neurons at the lower thoracic level.

Fate of grafted embryonic Purkinje cells in the cerebellum of the adult "Purkinje cell degeneration" mutant mouse. II. Development of synaptic responses: an in vitro study

Solid pieces of cerebellar primordia from 12-day-old C57Bl embryos were implanted in the cerebellar vermis of 3-4-month-old "Purkinje cell degeneration" mutant mice. Ten to 22 days after grafting, mutant mice were sacrificed, and synaptic responses of grafted Purkinje cells were studied by intracellular recordings performed in 400 microns thick sagittal slices in vitro. As early as 10 days after transplantation, grafted Purkinje cells have already completed their migration from the implant into the host molecular layer. Accordingly, inhibitory as well as excitatory responses were already elicited in these cells by electrical stimulation of the host subcortical white matter. Furthermore, a transient stage of multiple innervation of Purkinje cells by climbing fibers exists between 10 and 15 days after grafting, as revealed by the stepwise variation in amplitude of the climbing fiber-mediated excitatory postsynaptic potentials recorded before 15 days after grafting. Thirteen days after transplantation, typical all-or-none climbing fiber-mediated responses, parallel fiber-mediated excitatory postsynaptic potentials, and inhibitory postsynaptic potentials were also already present. Finally, normal adult-type synaptic responses were observed in all tested cells 15 to 17 days after grafting. Together with the companion paper (Sotelo et al., 1990), these results demonstrate that grafted Purkinje cells are able to impose on host afferents a pattern of synaptogenesis which closely follows that occurring during normal development, in particular, the transient stage of multiple innervation of Purkinje cells by climbing fibers.

Immunocytochemical demonstration of the calcium-binding proteins calbindin-D 28k and parvalbumin in the subiculum, hippocampus and dentate area of the domestic pig

The distribution of the calcium-binding proteins calbindin-D 28k (CaBP) and parvalbumin (PV) in the hippocampal region of the domestic pig was demonstrated by immunocytochemistry. Scattered CaBP-immunoreactive cell bodies were present in the subiculum, stratum oriens, pyramidal cell layer and stratum radiatum of the hippocampal regio superior and inferior, and the outer plexiform layer and outer hilar cell layer of the dentate hilus. Other cell bodies and bundles of stained fibers were present in stratum moleculare of regio superior and inferior, and in the outer third of the molecular layer of the fascia dentata. Terminal-like CaBP-immunoreactivity was seen in the subiculum and around cell bodies in the pyramidal cell layer of regio superior and inferior and the dentate granular cell layer. Scattered PV-immunoreactive cell bodies were present in stratum oriens and the pyramidal cell layer of regio superior and inferior, and in the outer plexiform layer and outer hilar cell layer of the dentate hilus. Terminal-like PV-immunoreactivity surrounded the cell bodies in the pyramidal cell layer of regio superior and inferior and in the dentate granular cell layer. The distribution of CaBP and PV in the pig hippocampus is compared to that of other more commonly used experimental animals. Whereas the distribution of PV-immunoreactivity in the pig hippocampus appears identical to that of the rat hippocampus, the distribution of CaBP-immunoreactivity in the pig hippocampus differs markedly from that of the rat hippocampus, the most prominent feature being a lack of CaBP-immunoreactivity in the granule cells, mossy fibers and pyramidal cells in the pig. The functional implications of calcium-binding proteins in the brain are discussed.

Ultrastructural localization of immunoreactive calbindin-D28k in the rat and monkey basal ganglia, including subcellular distribution with colloidal gold labeling

Normal cellular function depends on the controlled flux of Ca++ within intracellular compartments and across the plasma membrane. Proteins that bind Ca++ are thought to contribute to the regulation of intracellular Ca++ and, perhaps more importantly, signal functional changes in cell activity. In the brain, calbindin-D28k is among a class of calcium-binding proteins that are widely and heterogeneously distributed in select populations of neurons, among them neostriatal cells, but whose function is largely unknown. In this study of the monkey and rat neostriatum and globus pallidus, calbindin-D28k was localized with immunoperoxidase and immunogold methods in order to identify striatal cell populations that contain this protein and the subcellular compartments in which it is likely to function. Light and electron microscopy showed intense and extensive labeling of immunoreactive calbindin-D28k in the cell bodies, dendrites, and spines of medium-sized neostriatal spiny neurons and in their axon terminals which end in the globus pallidus. More discrete labeling with a gold-conjugated second antibody showed that the predominant site of calbindin-D28k was the matrix of the cytoplasm. Gold label was also associated with the karyoplasm of spiny cells and with the neurofilaments and axoplasmic matrix of striatopallidal axons and terminals, respectively. Membranes were either sparsely labeled (endoplasmic reticulum, mitochondria) or devoid of gold particles (nuclear envelope and plasmalemma). Radioimmunoassays of striatal subcellular fractions supported the anatomical findings by indicating that the soluble fractions of neostriatal tissue homogenates contained most of the calbindin-D28k immunoreactivity and that washes from forebrain synaptosomes treated with Triton X-100 yielded high levels of immunoreactive calbindin-D28k. These findings show that immunoreactive calbindin-D28k is localized to spiny neurons of the striatopallidal pathway and are consistent with previous observations on subcellular localization in nonneuronal tissues. If, as recently speculated, calbindin-D28k regulates calcium concentrations in neostriatal spiny neurons, this feature may be particularly involved with the high density of glutamatergic inputs to these cells. More work is needed to determine whether calbindin-D28k, when complexed to Ca++ in neostriatal spiny cells, signals the activation of protein kinases, phosphorylation, and/or neurotransmitter release, as has been shown for other Ca++-binding proteins in mammalian tissues.

On the molecular mechanism of intestinal calcium transport

The intestinal absorption of calcium is certainly a complex process, dependent on several factors of which vitamin D, via 1,25(OH)2D3, is the major controlling hormone. The efficiency of calcium absorption is a function of calcium status and calcium need. As the body's demand for calcium increases, the process commonly termed, adaptation, is activated in which the synthesis of 1,25(OH)2D3 from precursor is increased, resulting in the stimulation of the rate of calcium absorption. The increased demand for calcium might result from the ingestion of a diet deficient in calcium, from growth, pregnancy, lactation and egg shell formation in the laying hen. Accomapanying the change in calcium absorptive efficiency are molecular modifications of the transporting enterocytes, some mentioned herein and elsewhere (Wasserman & Chandler, 1985; Wasserman, 1980; Wasserman et al., 1984). Highly correlated with the rate of calcium absorption under a wide variety of conditions is the concentration of the vitamin D-induced calcium-binding protein, calbindin-D28K (avian type) and calbindin-D9K (mammalian intestinal type). The role of calbindin-D in this transport process is not precisely known but is considered to act at the present time as a cytosolic facilitator of Ca2+ diffusion from the brush border membrane to the basolateral membrane. In addition to the induction of calbindin-D synthesis, 1,25(OH)2D3 exerts other effects on the intestinal epithelium that can have consequences on the calcium absorptive process. Some of these effects are summarized in Figure 14. Vitamin D-dependent reactions might be either direct effects of 1,25(OH)2D3 or indirect effects due to elevated intracellular Ca2+ concentrations. These include changes in the fluidity of the brush border membrane, an increase in microvillar alkaline phosphatase-low affinity Ca-activated ATPase activity, an association of calmodulin with the 105 kD brush border cytoskeletal protein and, following calbindin D synthesis, the binding of calbindin D to a 60 kD brush border protein and to microtubules. The latter has been suggested to be related to the proposed transfer of Ca2+ by an endocytotic-exocytotic mechanism. In addition, a vitamin D-dependent intestinal membrane calcium-binding protein has been identified (Kowarski & Schachter, 1980). Playing into this multi-component system is a stimulation of cyclic nucleotide synthesis by 1,25(OH)2D3 which, through activation of cyclic nucleotide-dependent protein kinases, might modify membrane Ca2+ "channels" by phosphorylation reactions.4+ Intracellular organelles, i.e., the endoplasmic reticulum, mitochondria, the Golgi apparatus, are potent sequesters of Ca2+ and could contribute to the protection of the cell from excessively high Ca2+ concentrations by transiently storing absorbed Ca2+.

Calbindin 28 kDa in endocrine cells of known or putative calcium-regulating function. Thyro-parathyroid C cells, gastric ECL cells, intestinal secretin and enteroglucagon cells, pancreatic glucagon, insulin and PP cells, adrenal medullary NA cells and some pituitary (TSH?) cells

The distribution of calbindin in some endocrine glands (thyroid, parathyroid, ultimobranchial body, pituitary and adrenals) and in the diffuse endocrine cells of the gut and pancreas has been investigated immunohistochemically using an antiserum raised against the 28 kDa calbindin from chicken duodenum. The identity of calbindin-immunoreactive cells in a number of avian and mammalian species was ascertained by comparison with hormone-reactive cells in consecutive sections or by double immunostaining of the same section with both calbindin and hormone antibodies. Calcitonin-producing C cells of the mammalian and avian thyroid, parathyroid or ultimobranchial body, PP, glucagon and insulin cells of the mammalian and avian pancreas, enteroglucagon cells of the avian intestine, secretin cells of the mammalian duodenum, histamine-producing ECL cells of the mammalian stomach, as well as noradrenaline-producing cells of the adrenal medulla and some (TSH?) cells of the adenohypophysis were among the calbindin-immunoreactive cells. Although some species variability has been observed in the intensity and distribution of the immunoreactivity, especially in the pancreas and the gut, a role for calbindin in the mechanisms of calcium-mediated endocrine cell stimulation or of intracellular and extracellular calcium homeostasis is suggested.

Immunoreactivity for calretinin and other calcium-binding proteins in cerebellum

Two calcium-binding proteins, calbindin and parvalbumin, have been reported to be abundant in Purkinje cells and other cell types in the cerebellum. Immunoreactivity for a related protein, calretinin, is now reported in cerebellum of chick and rat. In the chick, antibodies against calretinin stain mossy fibres throughout, and climbing fibres in a distinct group of folia. They also stain several cell types in the molecular layer. As there is no detectable calretinin mRNA in the cerebellar cortex, this cellular staining may be due to cross-reaction with an unknown antigen. In the rat, antibodies against calretinin stain the Lugaro cells, and some granule cells in lobe X; they also give weak staining of all the granule cells in the other lobes. Thus almost all the neuronal cell types in the cerebellum show immunoreactivity for at least one of the calcium-binding proteins in one or both species.

Postnatal development of parvalbumin-, calbindin- and adult GABA-immunoreactivity in two visual nuclei of zebra finches

The characterization of neuron populations by their immunoreactivity against parvalbumin- and calbindin (28-kDa)-antisera has been used to study the postnatal development of the visual diencephalic nucleus rotundus and the mesencephalic nucleus isthmi complex in zebra finches. In nucleus rotundus, parvalbumin-immunoreactivity was restricted to the neuropil during the first 10 days and appears additionally in somata around day 12 where it remains until adulthood. Calbindin-immunoreactivity of the very scarce neuropil and the few somata, which can be observed during the first two weeks, disappears until adulthood. Thus, the adult nucleus rotundus shows an almost complementary distribution of calbindin- and parvalbumin-immunoreactive structures: the numerous, heavily parvalbumin-positive somata, which are surrounded by dense immunoreactive neuropil are in sharp contrast to the complete absence of calbindin-immunoreactive somata. Only a thin rim surrounding this nucleus contains punctate calbindin-positive neuropil. In the nucleus isthmi complex, parvalbumin and calbindin staining patterns show markedly different developmental profiles. While the density of parvalbumin-immunoreactive neuropil in the parvocellular part of the nucleus isthmi continuously increases and the somata remain unstained, the initially heavily calbindin-positive somata gradually lose their immunoreactivity during the first two weeks. In the adult nucleus isthmi complex, parvalbumin- and calbindin show nearly identical staining patterns. A comparison between the two calcium-binding proteins and GABA-immunoreactivity in adult brains revealed different relationships in the two nuclei: while in nucleus rotundus GABA-staining pattern neither resembles that of parvalbumin nor of calbindin, in the nucleus isthmi complex all three staining patterns coincide.

Ultrastructure and synaptic relationships of calbindin-reactive, Dogiel type II neurons, in myenteric ganglia of guinea-pig small intestine

Immunoreactivity for calbindin D 28K was localized ultrastructurally in nerve cell bodies and nerve fibres in myenteric ganglia of the guinea-pig small intestine. Reactive cell bodies had a characteristic ultrastructure: the cytoplasm contained many elongate, electron-dense mitochondria, numerous secondary lysosomes that were peripherally located, peripheral stacks of rough endoplasmic reticulum and dispersed Golgi apparatus. The cells were generally larger than other myenteric neurons and had mainly smooth outlines. The cytoplasmic features of these neurons were shared by a small group of immunonegative cells, but the majority of negative cells had clearly different ultrastructural appearances. Of 310 cells from 16 ganglia that were systematically examined, 38% were immunoreactive for calbindin, 10% were unreactive but similar in ultrastructure to the calbindin-reactive neurons and 51% were unreactive and dissimilar in the appearance of their cytoplasmic organelles. Immunoreactive varicosities with synaptic specializations were found on most unreactive neurons, but were markedly less frequent on the calbindin-immunoreactive cell bodies. Non-reactive presynaptic fibres were also more common on non-reactive neurons than on the calbindin-positive cell bodies. Numerous reactive varicosities, some showing synaptic specializations, were found adjacent to other fibres in the neuropil. Light microscopic studies show calbindin immunoreactive neurons to have Dogiel type-II morphology. Thus the present work links distinguishing ultrastructural features to a specific nerve cell type recognized by light microscopy in the enteric ganglia for the first time.

Epithelial and neuronal calbindin in avian intestine. An immunohistochemical study

It is well known that calbindin immunoreactivity is highly concentrated in the duodenal absorptive cells of young birds. We have shown that in the adult intestine of three avian species, calbindin content is much more variable. In addition to absorptive cells, we have detected throughout the gut of both sexes of the domestic fowl and in the large intestine of the Japanese quail a second type of calbindin-positive epithelial cell which has the shape of a typical endocrine cell. These cells were particularly abundant in the large intestine, in contrast to the usual distribution of endocrine cells along the gut. Calbindin was also detected in the nervous system of the intestine. Calbindin-positive nerve fibres were rare in the duodenum and ileum, numerous in plexuses and nerve processes in both muscular layers and lamina propria of the large intestine in domestic fowl and Japanese quail. In the mallard, nerve fibres were rarely calbindin positive while definitively positive for VIP. Calbindin of the peripheral nervous system of the domestic fowl and Japanese quail comigrates with the duodenal calbindin (27,000 dalton) in SDS gel electrophoresis.

Electrophysiological demonstration of a synaptic integration of transplanted Purkinje cells into the cerebellum of the adult Purkinje cell degeneration mutant mouse

After implantation of solid pieces of cerebellar primordia from 12-day-old C57BL embryos into the cerebellar parenchyma of 3- to 4-month-old "Purkinje cell degeneration" mutant mice, Purkinje cells from the donor leave the implant and differentiate while migrating into the host molecular layer. Electrophysiological studies were performed using in vitro cerebellar slice preparations from "Purkinje cell degeneration" mutants 1-2 months after grafting, when grafted Purkinje cells have reached their final location in the host molecular layer and have completed their morphological differentiation. Intracellular recordings obtained from 45 Purkinje cells in mutant mice demonstrated that such grafted neurons have normal bioelectrical properties including sodium and calcium conductances and inward rectification. Moreover, all grafted Purkinje cells responded to electrical white matter stimulation by a typical all-or-none climbing fiber response. Responses mediated through the activation of mossy and parallel fibers, as well as inhibitory postsynaptic potentials, were also recorded in a significant number of grafted Purkinje cells. On the whole, all these excitatory and inhibitory responses in grafted "Purkinje cell degeneration" mutant mice have characteristics comparable to those in control mice. After electrophysiological studies, Purkinje cells were further characterized by their positive staining by calbindin antibody. Neurons of this class were dispersed throughout the molecular layer of the host folia in which the electrophysiological recordings had been performed. The ectopic location of their perikarya, the presence of dendritic trees spanning most of the molecular layer (without entering the granular layer), and the occasional presence of axons emerging from the ectopic neurons and forming loose bundles at the white matter axis of the folia, corroborate the grafted nature of the Purkinje cells studied. Therefore, these experiments demonstrate that embryonic Purkinje cells from the graft can complete differentiation in the adult host cerebellum, and establish specific synaptic contacts with the presynaptic elements previously impinging on the missing neurons of "Purkinje cell degeneration" mutants. This process leads to a qualitative functional synaptic restoration of the cortical cerebellar network.

Pineal-retinal molecular relationships; immunocytochemical evidence of calbindin-27 kDa in pineal transducers

Calbindin-27 kDa immunocytochemical localization was studied concurrently in the pineal organ and retina from human as well as representatives of all vertebrate classes. Calbindin immunoreactivity was demonstrated in retinal cones (but not in rods) and in pineal transducers (cone-like and modified photoreceptor cells, pinealocytes) of a majority of amniotes. In contrast, no labelling was observed in anamniotes, except in retinal cones of the toad. Labelling was distributed through all cellular compartments (outer and inner segments, perikarya, pedicles or processes) of pineal transducers and retinal cones. Intra- and interspecific variations of calbindin contents are discussed.

Calretinin: a gene for a novel calcium-binding protein expressed principally in neurons

A novel gene of the calmodulin superfamily, encoding a 29-kD neuronal protein here named "calretinin," has been isolated as a cDNA clone from chick retina. The encoded sequence includes four putative calcium-binding sites and a fusion protein binds calcium. The most similar protein known is the 28-kD intestinal calcium-binding protein, calbindin (58% homology). Both genes date from before the divergence of chicks from mammals. The distribution of calretinin and calbindin mRNAs in chick tissues has been mapped using RNA gel blots and in situ hybridization. RNAs from both genes are abundant in the retina and in many areas of the brain, but calretinin RNA is absent from intestine and other nonneural tissues. Calretinin and calbindin are expressed in different sets of neurons throughout the brain. Calretinin RNA is particularly abundant in auditory neurons with precisely timed discharges.

Embryonic and adult neurons interact to allow Purkinje cell replacement in mutant cerebellum

It has often been proposed that one way of replacing degenerating neurons in the brain is to implant embryonic neurons of the same type. However, in the case of so-called 'point-to-point' systems, as opposed to the 'paracrine' systems which mainly involve local release of neurotransmitter, functional recovery requires a precise re-establishment of the missing circuitry. We recently showed that in one point-to-point system, the cerebellum of adult mice homozygous for the mutation Purkinje cell degeneration (pcd)2, missing Purkinje cells can be replaced by grafting cerebellar primordia from normal mouse embryos. Here, we present studies of the cellular mechanisms underlying this successful replacement. Grafted Purkinje cells leave the graft to migrate along stereotyped pathways to their final position in the deficient molecular layer, where they receive synaptic contacts from adult host neurons. Both the detailed timetable and the precise cellular interactions observed are remarkably similar to those occurring during normal development. Our results suggest that the deficient molecular layer exerts a selective neurotropic effect on neurons of the missing category, and that the embryonic neurons are able to respond to this signal during a period defined by their own internal clock. We also raise the possibility that embryonic Purkinje cells can induce in adult neural cells a new type of plasticity, that of recreating a permissive microenvironment for the integration of embryonic neurons.