Unique electrophysiological and impedance signatures between encapsulation types: An analysis of biological Utah array failure and benefit of a biomimetic coating in a rat model

Intracortical microelectrode arrays, especially the Utah array, remain the most common choice for obtaining high dimensional recordings of spiking neural activity for brain computer interface and basic neuroscience research. Despite the widespread use and established design, mechanical, material and biological challenges persist that contribute to a steady decline in recording performance (as evidenced by both diminished signal amplitude and recorded cell population over time) or outright array failure. Device implantation injury causes acute cell death and activation of inflammatory microglia and astrocytes that leads to a chronic neurodegeneration and inflammatory glial aggregation around the electrode shanks and often times fibrous tissue growth above the pia along the bed of the array within the meninges. This multifaceted deleterious cascade can result in substantial variability in performance even under the same experimental conditions. We track both impedance signatures and electrophysiological performance of 4 × 4 floating microelectrode Utah arrays implanted in the primary monocular visual cortex (V1m) of Long-Evans rats over a 12-week period. We employ a repeatable visual stimulation method to compare signal-to-noise ratio as well as single- and multi-unit yield from weekly recordings. To explain signal variability with biological response, we compare arrays categorized as either Type 1, partial fibrous encapsulation, or Type 2, complete fibrous encapsulation and demonstrate performance and impedance signatures unique to encapsulation type. We additionally assess benefits of a biomolecule coating intended to minimize distance to recordable units and observe a temporary improvement on multi-unit recording yield and single-unit amplitude.

Elastomeric and soft conducting microwires for implantable neural interfaces

Current designs for microelectrodes used for interfacing with the nervous system elicit a characteristic inflammatory response that leads to scar tissue encapsulation, electrical insulation of the electrode from the tissue and ultimately failure. Traditionally, relatively stiff materials like tungsten and silicon are employed which have mechanical properties several orders of magnitude different from neural tissue. This mechanical mismatch is thought to be a major cause of chronic inflammation and degeneration around the device. In an effort to minimize the disparity between neural interface devices and the brain, novel soft electrodes consisting of elastomers and intrinsically conducting polymers were fabricated. The physical, mechanical and electrochemical properties of these materials were extensively characterized to identify the formulations with the optimal combination of parameters including Young's modulus, elongation at break, ultimate tensile strength, conductivity, impedance and surface charge injection. Our final electrode has a Young's modulus of 974 kPa which is five orders of magnitude lower than tungsten and significantly lower than other polymer-based neural electrode materials. In vitro cell culture experiments demonstrated the favorable interaction between these soft materials and neurons, astrocytes and microglia, with higher neuronal attachment and a two-fold reduction in inflammatory microglia attachment on soft devices compared to stiff controls. Surface immobilization of neuronal adhesion proteins on these microwires further improved the cellular response. Finally, in vivo electrophysiology demonstrated the functionality of the elastomeric electrodes in recording single unit activity in the rodent visual cortex. The results presented provide initial evidence in support of the use of soft materials in neural interface applications.

In vivo effects of L1 coating on inflammation and neuronal health at the electrode-tissue interface in rat spinal cord and dorsal root ganglion

The spinal cord (SC) and dorsal root ganglion (DRG) are target implantation regions for neural prosthetics, but the tissue-electrode interface in these regions is not well-studied. To improve understanding of these locations, the tissue reactions around implanted electrodes were characterized. L1, an adhesion molecule shown to maintain neuronal density and reduce gliosis in brain tissue, was then evaluated in SC and DRG implants. Following L1 immobilization onto neural electrodes, the bioactivities of the coatings were verified in vitro using neuron, astrocyte and microglia cultures. Non-modified and L1-coated electrodes were implanted into adult rats for 1 or 4 weeks. Hematoxylin and eosin staining along with cell-type specific antibodies were used to characterize the tissue response. In the SC and DRG, cells aggregated at the electrode-tissue interface. Microglia staining was more intense around the implant site and decreased with distance from the interface. Neurofilament staining in both locations decreased or was absent around the implant, compared with surrounding tissue. With L1, neurofilament staining was significantly increased while neuronal cell death decreased. These results indicate that L1-modified electrodes may result in an improved chronic neural interface and will be evaluated in recording and stimulation studies.

The surface immobilization of the neural adhesion molecule L1 on neural probes and its effect on neuronal density and gliosis at the probe/tissue interface

Brain tissue inflammatory responses, including neuronal loss and gliosis at the neural electrode/tissue interface, limit the recording stability and longevity of neural probes. The neural adhesion molecule L1 specifically promotes neurite outgrowth and neuronal survival. In this study, we covalently immobilized L1 on the surface of silicon-based neural probes and compared the tissue response between L1 modified and non-modified probes implanted in the rat cortex after 1, 4, and 8 weeks. The effect of L1 on neuronal health and survival, and glial cell reactions were evaluated with immunohistochemistry and quantitative image analysis. Similar to previous findings, persistent glial activation and significant decreases of neuronal and axonal densities were found at the vicinity of the non-modified probes. In contrast, the immediate area (100 μm) around the L1 modified probe showed no loss of neuronal bodies and a significantly increased axonal density relative to background. In this same region, immunohistochemistry analyses show a significantly lower activation of microglia and reaction of astrocytes around the L1 modified probes when compared to the control probes. These improvements in tissue reaction induced by the L1 coating are likely to lead to improved functionality of the implanted neural electrodes during chronic recordings.

Surface immobilization of neural adhesion molecule L1 for improving the biocompatibility of chronic neural probes: In vitro characterization

Silicon-based implantable neural electrode arrays are known to experience failure during long-term recording, partially due to host tissue responses. Surface modification and immobilization of biomolecules may provide a means to improve their biocompatibility and integration within the host brain tissue. Previously, the laminin biomolecule or laminin fragments have been used to modify the neural probe's silicon surface to promote neuronal attachment and growth. Here we report the successful immobilization of the L1 biomolecule on a silicon surface. L1 is a neuronal adhesion molecule that can specifically promote neurite outgrowth and neuronal survival. Silane chemistry and the heterobifunctional coupling agent 4-maleimidobutyric acid N-hydroxysuccinimide ester (GMBS) were used to covalently bind these two biomolecules onto the surface of silicon dioxide wafers, which mimic the surface of silicon-based implantable neural probes. After covalent binding of the biomolecules, polyethylene glycol (PEG)-NH(2) was used to cap the unreacted GMBS groups. Surface immobilization was verified by goniometry, dual polarization interferometry, and immunostaining techniques. Primary murine neurons or astrocytes were used to evaluate the modified silicon surfaces. Both L1- and laminin-modified surfaces promoted neuronal attachment, while the L1-modified surface demonstrated significantly enhanced levels of neurite outgrowth (p<0.05). In addition, the laminin-modified surface promoted astrocyte attachment, while the L1-modified surface showed significantly reduced levels of astrocyte attachment relative to the laminin-modified surface and other controls (p<0.05). These results demonstrate the ability of the L1-immobilized surface to specifically promote neuronal growth and neurite extension, while inhibiting the attachment of astrocytes, one of the main cellular components of the glial sheath. Such unique properties present vast potentials to improve the biocompatibility and chronic recording performance of neural probes.

Self-assembled monolayers of polythiophene conductive polymers improve biocompatibility and electrical impedance of neural electrodes

There is continued interest in the development of conductive polymer coatings to improve the electrical properties and biocompatibility of electrodes for neural prostheses. We present here a new type of coating, based on mixed self-assembled monolayers (SAMs) of thiolated poly(alkylthiophene)s and functionalized alkanethiols. When assembled as a SAM on electrodes designed for in vitro electrophysiology, these polymers are able to significantly lower electrode impedance at 1 kHz. The same mixed formulation is able to promote the outgrowth of neurites from primary mouse cortical neurons when the alkanethiol component is functionalized with a neural cell adhesion molecule (NCAM) binding antibody. Atomic force microscopy of the SAMs shows that they exert their effect through the well-known mechanism of increasing electrode surface area. These new covalently bound films have the potential to be more robust and are more controllable in their composition than existing electrodeposited conductive polymer coatings.

Methylisothiazolinone, a neurotoxic biocide, disrupts the association of SRC family tyrosine kinases with focal adhesion kinase in developing cortical neurons

Methylisothiazolinone (MIT) is a biocide widely used in industrial and cosmetic products with potential as a neurotoxicant. We previously reported that short acute exposures to relatively high concentrations of MIT (100 microM) lead to widespread and selective neuronal death in vitro. To evaluate the biological properties of chronic exposures to MIT, freshly dissociated rat cortical neurons were continuously exposed to low concentrations (0.1-3 microM) of the biocide in serum-containing media. Although we observed minimal effects on cell viability, MIT induced a dramatic inhibition of neurite outgrowth. Immunoblotting and immunoprecipitation experiments revealed that focal adhesion kinase (FAK) phosphorylation was primarily affected by the MIT treatment. The phosphorylation level at tyrosines 576 and 861 of FAK was significantly decreased and likely contributed to the overall reduction of tyrosine phosphorylation of this protein. MIT inhibited Src family kinases (SFKs) in cell-free assays and led to the physical dissociation of FAK from the signaling complexes that it normally forms with c-Src and Fyn in developing neurons. High-density neuronal cultures were then employed to increase cell-to-cell contact. This approach resulted in an overall enhancement of SFKs and FAK phosphorylation and could overcome the deficits induced by MIT. This study suggests that a disruption of FAK-SFK complexes due to SFK inhibition leads to FAK dysfunction, with detrimental effects to immature neurons. Prolonged exposure to low levels of MIT and related compounds may have damaging consequences to the developing nervous system.

Electrochemically controlled release of dexamethasone from conducting polymer polypyrrole coated electrode

Chronic recordings from micromachined neural electrode arrays often fail a few weeks after implantation primarily due to the formation of an astro-glial sheath around the implant. We propose a drug delivery system, from conducting polymer (CP) coatings on the electrode sites, to modulate the inflammatory implant-host tissue reaction. In this study, polypyrrole (PPy) based coatings for electrically controlled and local delivery of the ionic form of an anti-inflammatory drug, dexamethasone (Dex), was investigated. The drug was incorporated in PPy via electropolymerization of pyrrole and released in PBS using cyclic voltammetry (CV). FTIR analysis of the surface showed the presence of Dex and polypyrrole on the coated electrode. The thickness of the coated film was estimated to be approximately 50 nm by ellipsometry. We are able to release 0.5 mug/cm(2) Dex in 1 CV cycle and a total of almost 16 mug/cm(2) Dex after 30 CV cycles. In vitro studies and immunocytochemistry on murine glial cells suggest that the released drug lowers the count of reactive astrocytes to the same extent as the added drug. In addition, the released drug is not toxic to neurons as seen by healthy neuronal viability in the released drug treated cells.

A novel epitope of N-CAM defines precursors of human adherent NK cells

Activated, adherent natural killer (A-NK) cells represent a distinct subpopulation of interleukin (IL)-2-stimulated NK cells, which are selectively endowed with the increased expression of integrins and ability to adhere to solid surfaces, migrate into, infiltrate, and destroy cancerous tissues. The present study defines the phenotype and functions of precursors of A-NK (pre-A-NK) cells in humans. Peripheral blood pre-A-NK cells, in contrast to the rest of NK cells, express a novel epitope of CD56 neuronal cell adhesion molecule, termed ANK-1, and increased cell-surface levels of integrins. Pre-A-NK cells also express low levels of CD56 and CD161, and some express CD162 receptor, do not express CD25 or activation markers, and are effective mediators of NK cytotoxicity. Thus, pre-A-NK cells are generally similar to CD56(dim) NK cells. However, pre-A-NK cells differ from the main NK cell subpopulation by having a lower expression level of CD16 and a lower ability to mediate redirected antibody-dependent, cell-mediated cytotoxicity. More importantly, pre-A-NK cells are preferentially endowed with the ability to rapidly respond to IL-2 by integrin-mediated adherence to endothelial cells, extracellular matrix, and plastic. This early, specific response of pre-A-NK cells to IL-2 is followed by their activation, vigorous proliferation, and differentiation into phenotypically and functionally similar A-NK cells. Pre-A-NK cells represent only approximately 26% of peripheral blood NK cells but encompass the majority of NK cells in normal and cancerous, solid tissues. We conclude that pre-A-NK cells represent a distinct subset of resting, mature NK cells with the characteristics indicative of their ability to migrate and reside in solid tissues.

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 regulates the migration of Langerhans cells from the epidermis to draining lymph nodes

Src homology 2 domain-containing protein tyrosine phosphatase substrate 1 (SHPS-1) is a member of the signal regulatory protein family in which the extracellular region interacts with its ligand, CD47. Recent studies have demonstrated that SHPS-1 plays an important role in cell migration and cell adhesion. We demonstrate in this study, using immunohistochemical and flow cytometric analyses, that murine Langerhans cells (LCs) express SHPS-1. Treatment of mice ears with 2,4-dinitro-1-fluorobenzene significantly reduced the number of epidermal LCs, and that reduction could be reversed by pretreatment with mAb to SHPS-1 or the CD47-Fc fusion protein. Treatment with the SHPS-1 mAb in vivo reduced the number of FITC-bearing cells in the lesional lymph nodes after the application of FITC to the skin. The SHPS-1 mAb inhibited the in vivo TNF-alpha-induced migration of LCs. The emigration of dendritic cells expressing I-A(b+) from skin explants to the medium was also reduced by the SHPS-1 mAb. We further demonstrate that the chemotaxis of a murine dendritic cell line, XS52, by macrophage inflammatory protein-3beta was significantly inhibited by treatment with the SHPS-1 mAb or CD47-Fc recombinant protein. Finally, we show that migration of LCs was attenuated in mutant mice that lack the intracellular domain of SHPS-1. These observations show that the ligation of SHPS-1 with the SHPS-1 mAb or with CD47-Fc abrogates the migration of LCs in vivo and in vitro, which suggests that the SHPS-1-CD47 interaction may negatively regulate LC migration.

Electroretinograms remain normal in mice lacking a synapse associated protein

Integrin-associated protein (IAP) is normally localized to the synapse rich plexiform layers of the mammalian retina. In other neuronal systems, IAP and its ligand, P84, have been implicated in synaptic function. Previously, an abnormal distribution of P84 was noted in the IAP-null retina. To examine the potential role of IAP in the function of the retinal outer plexiform layer, we recorded electroretinograms (ERGs) from IAP-null mice and wild-type littermates. Under a wide range of stimulus conditions, there was no difference between the responses of these two groups, including ERG components that reflect post-receptoral activity. These results indicate that IAP and/or P84 may not be critical for the development and maintenance of the photoreceptor-to-bipolar cell synapse.

Role of CD47 as a marker of self on red blood cells

The immune system recognizes invaders as foreign because they express determinants that are absent on host cells or because they lack "markers of self" that are normally present. Here we show that CD47 (integrin-associated protein) functions as a marker of self on murine red blood cells. Red blood cells that lacked CD47 were rapidly cleared from the bloodstream by splenic red pulp macrophages. CD47 on normal red blood cells prevented this elimination by binding to the inhibitory receptor signal regulatory protein alpha (SIRPalpha). Thus, macrophages may use a number of nonspecific activating receptors and rely on the presence or absence of CD47 to distinguish self from foreign. CD47-SIRPalpha may represent a potential pathway for the control of hemolytic anemia.

Negative regulation of phagocytosis in murine macrophages by the Src kinase family member, Fgr

Ingestion of opsonized pathogens by professional phagocytes results in the generation and release of microbicidal products that are essential for normal host defense. Because these products can result in significant tissue injury, phagocytosis must be regulated to limit damage to the host while allowing for optimal clearance and destruction of opsonized pathogens. To pursue negative regulation of phagocytosis, we assessed the effect of the Src kinase family member, Fgr, on opsonin-dependent phagocytosis by mouse macrophages. We chose Fgr because it is present in high concentrations in circulating phagocytes but is not essential for Fcgamma receptor-mediated ingestion by mouse macrophages. Although expression of Fgr both in a macrophage cell line and in primary macrophages significantly attenuates ingestion mediated by Fcgamma receptors and CR3, it does not affect macropinocytosis or receptor-mediated endocytosis. This selective effect of Fgr is independent of its tyrosine kinase function. After Fcgamma receptor cross-linking, Fgr becomes associated with the immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptor, SIRPalpha (a member of the signal-regulatory protein family, also known as Src homology 2 domain-containing protein tyrosine phosphatase [SHP] substrate 1 [SHPS-1], brain immunoglobulin-like molecule with tyrosine-based activation motifs [BIT], and P84) and potentiates the association of the phosphatase SHP-1 with SIRPalpha. This association is responsible, at least in part, for decreasing positive signaling essential for optimal phagocytosis. These data demonstrate an important negative regulatory role for this Src kinase family member and suggest that this homeostatic function must be overcome for optimal uptake and clearance of opsonized pathogens.

Expression of a synapse-associated membrane protein, P84/SHPS-1, and its ligand, IAP/CD47, in mouse retina

P84 and integrin associated protein (IAP) are heterophilic binding partners that are expressed in the central nervous system in addition to a variety of other tissues. Both molecules are known to be involved in cell signaling in nonneural tissues. In the retina, both molecules are expressed prominently in plexiform layers, suggesting a possible association with synapses. Here, we examined the cellular expression and ultrastructural localization of the two molecules in the developing mouse retina. Both appeared to be expressed at one or both sides of synaptic sites, although the expression of IAP in the retina precedes that of P84. Examination of transgenic IAP-null retinae revealed a failure of P84 to become associated with synaptic sites, suggesting the interaction of P84 with IAP was necessary for P84's synaptic localization. These findings suggest that the signaling activities of P84 and IAP are localized to sites of synaptic contact in the retina. Thus this pair of synapse-associated molecules represents a bidirectional signaling system that could function to modify synaptic activity or possibly trophic interactions between central neurons.

Integrin-associated protein is a ligand for the P84 neural adhesion molecule

P84 (also known as SHPS-1, BIT, and SIRP) is a heterophilic adhesive membrane protein involved in receptor tyrosine kinase signaling that is found at synapses in the mammalian central nervous system and in non-neural tissues. We have identified a binding partner for P84 using an expression cloning strategy. Here we report that integrin-associated protein (IAP/CD47) is a predominant binding partner of P84. Immunohistochemistry reveals a virtually identical distribution of P84 and IAP in a variety of adult brain regions. Because IAP has been implicated in cell signaling in cells of the immune system, P84 and IAP represent a heterophilic binding pair that is likely to be involved in bi-directional signaling at the synapse and in other tissues.

Maturational changes in cell surface antigen expression in the mouse retina and optic pathway

The distribution of the cell surface molecules M6 and L1 was studied using the immunohistochemistry and in situ hybridization in the developing and adult mouse retina and optic nerve. L1 is a cell adhesion molecule while M6 is a cell surface molecule homologous to the myelin protein proteolipid protein (PLP/DM20). Although both molecules were expressed in retina and optic nerves of embryonic and neonatal mice, our studies show that their patterns of postnatal expression are quite different. While L1 continues to be expressed in optic axons throughout adulthood, expression of M6 on optic axons declines after birth and instead becomes strongly expressed on Müller glial endfeet and in the inner plexiform layer. The modulation of these molecules after birth could provide clues to changing cell-cell interactions occurring in the proximal portion of the optic pathway.

The murine P84 neural adhesion molecule is SHPS-1, a member of the phosphatase-binding protein family

P84 is a neuronal membrane glycoprotein that promotes the attachment and neurite outgrowth of cultured murine cerebellar cells. The heterophilic adhesive properties of P84 and its localization at sites of synaptogenesis suggest that it may be involved in regulation of synapse formation or maintenance. P84 is expressed in subsets of neurons throughout the CNS. By cloning the cDNA encoding murine P84, we have discovered that this molecule is a member of a family of phosphatase-binding proteins and is identical to the murine SHPS-1 cDNA. Here we report the cloning of two alternatively spliced forms of P84 and describe its localization within the CNS by in situ hybridization.

Alternate strategies in lesion-induced reactive synaptogenesis: differential expression of L1 in two populations of sprouting axons

In the CNS the cell adhesion molecule L1 plays a role in axonal growth and fasciculation. Since its roles in synapse formation and CNS regeneration are unknown, we followed the staining of L1 through the sequence of degeneration and reactive axon sprouting in the denervated outer molecular layer (ML) of the hippocampal dentate gyrus following ipsilateral entorhinal cortex (ERC) lesion. We compared immunohistological and ultrastructural localization of L1 and employed image analysis to evaluate lamina-specific changes over time. L1 staining was uniformly distributed over the ML in unlesioned animals. Following ERC lesion, L1 staining markedly declined in the outer ML; L1 staining in the inner ML remained constant. Over 30 days postlesion, commissural and associational (C/A) afferents from inner ML sprouted partway into the denervated zone, and L1 was expressed on these sprouting afferents. L1 staining exactly corresponded to fiber outgrowth as assessed by Holmes fiber stain. As the L1-bearing axons of the C/A projection expanded, staining for embryonic N-CAM (reexpressed on the dendrites of the denervated zone) appeared to recede. There was never overlap of L1 and embryonic N-CAM staining; the difference always marked the boundary between inner and outer ML. Ultrastructural analysis confirmed localization of L1 staining to axonal profiles, indicating that the new pattern of L1 staining reflected distinct types of axonal growth. These changes in cell adhesion molecule expression closely paralleled the known sequence of reactive synaptogenesis and axonal sprouting and demonstrate a link between cell adhesion molecule expression and axonal sprouting during self-repair by the CNS.

Rapid expression and transport of embryonic N-CAM in dentate gyrus following entorhinal cortex lesion: ultrastructural analysis

Neural cell adhesion molecules are known to be important in axon guidance and synapse formation in the developing brain. The embryonic form of neural cell adhesion molecule (eN-CAM) is reexpressed in the outer molecular layer (OML) of the dentate gyrus following entorhinal cortex (ERC) lesion. Ultrastructural analysis revealed localization of eN-CAM to the membrane of granule-cell dendritic membranes and occasionally axons within the denervated zone. Because eN-CAM is expressed rapidly (within 2 days) after ERC lesion, we were interested in the temporal sequence of expression. Denervated hippocampi (12, 15, 24, and 48 hours post-ERC lesion) were stained with anti-eN-CAM and processed for immunoelectron microscopy. At 12 hours, there was no evidence of staining for eN-CAM. By 15 hours after lesion, membranes of both dendrites and axons throughout the molecular layer exhibited moderate eN-CAM staining, and dendritic cytoplasm was heavily labeled. Twenty-four hours following lesion, plasma membrane staining of eN-CAM on both axons and dendrites had increased in intensity within the OML, whereas membrane eN-CAM staining was diminished in the inner molecular layer (IML), and the intradendritic cytoplasmic staining disappeared. By 48 hours after lesion, eN-CAM staining had disappeared from the IML but remained intense and widely distributed in the OML. These findings suggest a rapid transport of de novo synthesized protein. A generalized reaction appears to occur immediately following denervation, and eN-CAM is up-regulated in the complete expanse of the dendritic membrane, despite the fact that only the OML is denervated.(ABSTRACT TRUNCATED AT 250 WORDS)

Embryonic neural cell adhesion molecule (N-CAM) is elevated in the denervated rat dentate gyrus

We evaluated the immunohistological changes in neural cell adhesion molecule (N-CAM) expression in the adult rat dentate gyrus during the period of synaptic degeneration, axonal sprouting, and synaptogenesis following ipsilateral entorhinal cortex (ERC) lesion. This lesion denervates the outer two-thirds of the dentate granule cells dendrites and induces compensatory sprouting from the subjacent inner one-third into the denervated zone, as well as reactive synaptogenesis in the denervated outer molecular layer. In unlesioned adult hippocampus, antibodies to total N-CAM stained the inner molecular layer intensely, and the outer molecular layer (ML) more lightly. After ERC lesion the intense staining of the inner layer widened, the expansion following the known temporal sequence of commissural and associational (C/A) axon sprouting into the denervated zone. In normal unlesioned controls there was very light, uniform staining of the ML with antibodies directed against embryonic N-CAM (eN-CAM). By 2 d post-ERC lesion, the outer two-thirds of the ML stained robustly with antibody to eN-CAM. This area of intense staining receded as the C/A axon collaterals from the inner one-third entered the denervated zone, so that by 30 d the intense eN-CAM staining only occupied the outer half of the ML. The increased expression of eN-CAM remained present at 60 d post-ERC lesion, past the point that synaptic volume density has returned to normal levels in the denervated zone. Ultrastructural studies showed that the newly expressed eN-CAM was located on the surface of dendrites in the denervated zone, but was not found at the synaptic contacts.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell adhesion molecules in the early developing mouse retina: retinal neurons show preferential outgrowth in vitro on L1 but not N-CAM

Both L1 and N-CAM are present on optic axons early in the developing mouse retina and optic nerve. In in vitro assays on substrates of purified cell adhesion molecules cells derived from E13 mouse retinae showed vigorous neurite extension on L1 but not on N-CAM. Although retinal neurons on N-CAM showed only limited attachment to the substrate, they were able to form lamellipodia immediately around the cell perimeter. In contrast, similarly derived cortical cells showed extensive neurite outgrowth on both substrates. Under these culture conditions, nearly all of the L1 and N-CAM present in the cell membrane appeared to be sequestered on the lower surface of the growth cones and neurites, indicating that most of these cell adhesion molecules were involved in homophilic interactions. Our results suggest differential roles for L1 and N-CAM in initiation and establishment of the optic pathway.

Postnatal expression of polysialic acid-neural cell adhesion molecule in the hypothalamus of the male rhesus monkey (Macaca mulatta)

Puberty in primates is triggered by a gonad-independent reinitiation of a pulsatile mode of GnRH release. The purpose of the present study was to begin to examine the hypothesis that this neuroendocrine event is the result of structural or plastic changes within the neural network governing the activity of GnRH neurons. Specifically, we sought to determine whether polysialic acid neural cell adhesion molecule (PSA-NCAM), a plasma membrane-associated glycoprotein that has previously been proposed to be a marker for postnatal neuronal plasticity, was expressed within GnRH neuron containing areas of the rhesus monkey hypothalamus. The study employed male monkeys that were castrated prepubertally. Immunocytochemistry of hypothalamic tissue from four animals of pubertal age employing a monoclonal antibody (12F8) specific for PSA-NCAM revealed the presence of PSA-NCAM immunoreactivity within the region of the arcuate nucleus and median eminence of the medial basal hypothalamus (MBH) and in the region of the organum vasculosum of the lamina terminalis of the rostral hypothalamus, two areas in the monkey brain where GnRH neurons are concentrated. As expected, immunostaining for total NCAM using a polyclonal rabbit antibody to mouse total NCAM was uniformly distributed throughout hypothalamic sections containing the MBH. Double staining showed that some, though not all, GnRH cell bodies of the MBH were located within the PSA-NCAM-immunopositive region of the arcuate nucleus and the median eminence. The pattern of PSA-NCAM immunoreactivity in the MBH of three prepubertal monkeys was similar to that seen for the older animals. Western analysis of a membrane extract from the MBH of a monkey of pubertal age, employing antibody 12F8, identified a broad band of staining at the expected molecular weight for this adhesion molecule. A similar, but less intense, immunoreactive band was observed for the preoptic area. In contrast, an immunoblot of a membrane extract of cerebral cortex was only faintly positive for PSA-NCAM. Taken together, the foregoing findings are consistent with the notion that structural changes within the MBH may underlie the pubertal reinitiation of pulsatile GnRH release. Moreover, the presence of PSA-NCAM in the MBH of prepubertal monkeys suggests that the role, if any, of this molecule in the onset of sexual maturation in primates is permissive in nature.

Expression of rat renal gamma-glutamyltranspeptidase in LLC-PK1 cells as a model for apical targeting

In the rat, gamma-glutamyltranspeptidase (gamma GT) is transcribed into four unique mRNAs from a single gene by use of at least three different promoters and alternative splicing. For the first time, two distinct full-length cDNAs encoding the protein for rat renal gamma-glutamyltranspeptidase have now been isolated. Characterization by restriction enzyme mapping and nucleotide sequencing indicates that the two cDNAs, corresponding to transcripts I and II, differ only in the 5' noncoding region. However, transcription from promoter I, most proximal to the coding sequence, apparently began 20 bases upstream from the major transcription start previously reported. Since in vitro transcription and translation of these two new gamma GT cDNAs were found to produce a full-length peptide (M(r) approximately 62,000), both cDNAs were used to transfect LLC-PK1 (porcine) cells, a polarized cell line most representative of the renal proximal tubule. Rat gamma GT was expressed in transfected cells as judged by immunofluorescence analysis, direct immunoprecipitation after metabolic labeling with [35S]methionine, and an increase in gamma GT specific enzymatic activity (up to 5-fold). When clonal cell lines (I or II) were grown on Falcon filter inserts, the increased gamma GT activity was found only at the apical surface, consistent with polarized expression of the rat gamma GT. In contrast, transfection of the same cells with cDNA of human growth hormone resulted in both apical (70%) and basal lateral (30%) secretion of the expressed hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of neural cell adhesion molecule (N-CAM) and L1 in human and rodent hippocampus

Cell surface adhesion molecules N-CAM and L1 are implicated in central nervous system (CNS) cell migration and axon outgrowth in in vitro and in vivo developmental studies. These molecules show a differential distribution during CNS development, thus suggesting that they subserve different roles in process outgrowth and tissue organization. A variety of N-CAM isoforms are known, and individual N-CAMs undergo posttranslational modification. Such changes and the potential for generating numerous molecules may mediate development of specific neural cell contacts and circuitry. We evaluated immunohistochemical staining of polyclonal antibodies to L1 and N-CAM, as well as monoclonal antibodies directed against embryonic N-CAM and the 140 and 180 kDa species of N-CAM in human, rat, and mouse hippocampus. Staining patterns in the three species were qualitatively similar, but staining in the mouse hippocampus was quantitatively greater for some epitopes. A distinctive pattern of staining was found, corresponding to the known anatomy of the structure. Total N-CAM staining was intense in the hilus and inner molecular layer (ML) of the dentate gyrus with lighter staining in the dentate outer ML. The mossy fiber tract (MFT), comprising axons traveling from the dentate granule cells to CA3 pyramidal cells, was strongly stained by polyclonal antibody to N-CAM. There was abundant staining of the stratum radiatum (SR) and stratum oriens (SO) of CA1, but stratum lacunosum moleculare (LM) showed very little staining. The monoclonal antibody 12F11, which recognizes the 140 and 180 kDa forms of N-CAM, intensely stained the MFT, hilus, and inner ML.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental expression of neural cell adhesion molecules in the mouse neocortex and olfactory bulb

Polyclonal antibodies to N-CAM and L1 and monoclonal antibodies to epitopes of N-CAM (designated 12F11, 8A2, and 12F8) were used to investigate the spatial and temporal distribution of these neural cell adhesion molecules during the development of mouse cortex and olfactory bulb. The aim of the study was to correlate developmental events such as cell migration, dendritic and axonal outgrowth, and synaptogenesis with the appearance and disappearance of specific molecules involved in cell-cell interactions. Western transfer studies indicated that 12F8 antibody recognized polysialic acid found on embryonic N-CAM; 8A2 antibody primarily recognized the 140 kD component of N-CAM while the 12F11 antibody recognized the 180 and the 140 kD forms. The study demonstrates a high degree of cell surface molecular specialization of different compartments in developing neocortex and olfactory bulb. L1 is found on a variety of unmyelinated fiber tracts including thalamocortical fibers, olfactory nerve, and inner plexiform layer of the olfactory bulb. In contrast, N-CAM epitope recognized by 12F11 antibody is present on olfactory nerve fibers but appears later and is much weaker than L1 on thalamocortical fibers and is absent from the olfactory lobe inner plexiform layer. Dendritic regions are best labeled by 12F8 antibody; the epitope becomes faint in adult cortex but remains strongly expressed in olfactory bulb. This study reveals that widespread N-CAM expression in the central nervous system is constituted by a diversity of local expression of different molecular forms of N-CAM; their different anatomical distributions suggest they may each have unique roles.

Cellular events associated with peripherally induced rejection of mature neural xenografts placed into neonatal rat brains

Various circumstances have brought about a dispute concerning the immunologically priviledged status of the central nervous system (CNS). Using a transplantation paradigm, we have examined the cellular events associated with an experimentally induced focal assault on the CNS by the immune system. Chunks of embryonic mouse cortex were transplanted into neonatal rat brains and allowed to survive for 4 weeks. The adult rats then received a skin graft of donor origin to induce rejection of the transplanted tissue. Animals were sacrificed at various time points and examined histologically and immunocytochemically. Under these circumstances, the transplant is rejected via a first-set rejection response, and astrocytes of donor origin appear to be the primary target of the host immune system. Expression of class I and class II major histocompatibility antigens is noted to correlate with lymphocytic invasion of the transplant.

Central nervous system antigen P84 can serve as a substrate for neurite outgrowth

Neurite outgrowth promoting properties of neural cell surface proteins can be assessed by immobilizing isolated membrane proteins on nitrocellulose-coated petri dishes. Using this method, we have identified a unique cell surface antigen, designated P84, as a new neural cell adhesion molecule. Immunoaffinity purified P84 contains three polypeptides with molecular weights of 167, 85, and 66 kDa. When spotted onto nitrocellulose-coated plates, P84 supports adhesion of mouse cerebellar neurons and neurite outgrowth. Glial cell attachment was also observed. Intact monoclonal antibodies directed against P84 inhibit adhesion and outgrowth on a P84 substrate. This antigen is found on the surfaces of neurons in cultures of cerebellar cells. It is also found on a subclass of unidentified flat cells. P84 is not found on oligodendrocytes or GFAP-positive astrocytes. As early as E9, P84 could be detected in the floor plate region of the spinal cord. This pattern persists throughout embryonic development. Postnatally, widespread expression of P84 is observed in a variety of CNS tissues.

Disulfide linkage of biotin identifies a 106-kDa Ca2+ release channel in sarcoplasmic reticulum

Reactive disulfide reagents (RDSs) with a biotin moiety have been synthesized and found to cause Ca2+ release from sarcoplasmic reticulum (SR) vesicles. The RDSs oxidize SH sites on SR proteins via a thiol-disulfide exchange, with the formation of mixed disulfide bonds between SR proteins and biotin. Biotinylated RDSs identified a 106-kDa protein which was purified by biotin-avidin chromatography. Disulfide reducing agents, like dithiothreitol, reverse the effect of RDSs and thus promoted active re-uptake of Ca2+ and dissociated biotin from the labeled protein indicating that biotin was covalently linked to the 106-kDa protein via a disulfide bond. Several lines of evidence indicate that this protein is not Ca2+, Mg2+-ATPase and is not a proteolytic fragment or a subunit of the 400-kDa Ca2+-ryanodine receptor complex (RRC). Monoclonal antibodies against the ATPase did not cross-react with the 106-kDa protein, and polyclonal antibodies against the 106-kDa did not cross-react with either the ATPase or the 400-kDa RRC. RDSs did not label the 400-kDa RRC with biotin. Linear sucrose gradients used to purify the RRC show that the 106-kDa protein migrated throughout 5-20% linear sucrose gradients, including the high sucrose density protein fractions containing 400-kDa RRC. Protease inhibitors diisopropylfluorophosphate used to prevent proteolysis of 400-kDa proteins did not alter the migration of 106-kDa in sucrose gradients nor the patterns of biotin labeling of the 106-kDa protein. Incorporation of highly purified 106-kDa protein (free of RRC) in planar bilayers revealed cationic channels with large Na+ (gNa+ = 375 +/- 15 pS) and Ca2+ (gCa2+ = 107.7 +/- 12 pS) conductances which were activated by micromolar [Ca2+]free or millimolar [ATP] and blocked by micromolar ruthenium red or millimolar [Mg2+]. Thus, the SR contains a sulfhydryl-activated 106-kDa Ca2+ channel with apparently similar characteristics to the 400-kDa "feet" proteins.

Reactive disulfides trigger Ca2+ release from sarcoplasmic reticulum via an oxidation reaction

Reactive disulfide compounds (RDSs) with a pyridyl ring adjacent to the S-S bond such as 2,2'-dithiodipyridine (2,2'-DTDP), 4,4'-dithiodipyridine, and N-succinimidyl 3(2-pyridyldithio)propionate (SPDP) trigger Ca2+ release from sarcoplasmic reticulum (SR) vesicles. They are known to specifically oxidize free SH sites via a thiol-disulfide exchange reaction with the stoichiometric production of thiopyridone. Thus, the formation of a mixed S-S bond between an accessible SH site on an SR protein and a RDS causes large increases in SR Ca2+ permeability. Reducing agents, glutathione (GSH) or dithiothreitol reverse the effect of RDSs and permit rapid re-uptake of Ca2+ by the Ca2+, Mg2+-ATPase. The RDSs, 2,2'-DTDP, 4,4'-dithiodipyridine and SPDP displaced [3H]ryanodine binding to the Ca2+-receptor complex at IC50 values of 7.5 +/- 0.2, 1.5 +/- 0.1, and 15.4 +/- 0.1 microM, respectively. RDSs did not alter the rapid initial phase of Ca2+ uptake by the pump, stimulated ATPase activity, and induced release from passively loaded vesicles with nonactivated pumps; thus they act at a Ca2+ release channel and not at the Ca2+, Mg2+-ATPase. Efflux rates increased in 0.25-1.0 mM [Mg2+]free then decreased in 2-5 mM [Mg2+]free. Adenine nucleotides inhibited the oxidation of SHs on SR protein by RDSs and thus reduced Ca2+ efflux rates. However, once RDSs oxidized these SH sites and opened the Ca2+ release pathway, subsequent additions of nucleotides stimulated Ca2+ efflux. In skinned fibers, 2,2'-dithiodipyridine elicited rapid twitches which were blocked by ruthenium red. These results indicate that RDSs trigger Ca2+ release from SR by oxidizing a critical SH group, and thus provide a method to covalently label the protein(s) involved in causing these changes in Ca2+ permeability.

Characterization of primary cell cultures derived from rat renal proximal tubules

Proximal tubules were prepared from rat kidney cortex by collagenase digestion and purified by Percoll gradient centrifugation. Their enrichment was estimated by comparing the specific activities of various cell-specific enzymes in homogenates of renal cortex and of the isolated tubules. The tubules were cultured in a 50:50 mixture of Dulbecco's modified Eagle's and Ham's F12 media supplemented with insulin, transferrin, epidermal growth factor, hydrocortisone, and prostaglandin E1. After 2 to 3 d an extensive outgrowth of epithelial cells developed from the attached tubules. After 5 to 7 d near confluent monolayers were obtained. Hormonal responsiveness, marker enzyme activities, and transport properties were determined to further characterize the primary cultures. The cultured cells exhibited increased cyclic AMP production in response to parathyroid hormone but not calcitonin or vasopressin, consistent with the absence of cells derived from distal and collecting tubules. The cells also retained significant levels of 25-hydroxyvitamin D3-1 alpha-hydroxylase, alkaline phosphatase, and gamma-glytamyl-transpeptidase, three enzymes that are primarily associated with the proximal tubule. The cultured epithelial cells also exhibit a Na+-dependent phosphate and glucose transport systems. Therefore, the cells retain many functional properties that are characteristic of proximal tubules. Thus, the primary cultures should be suitable for the study of processes that occur specifically within this segment of the rat nephron.

A new marker for identifying quail cells in embryonic avian chimeras: a quail-specific antiserum

The characterization of cell behavior in quail chick chimeras has greatly increased our knowledge of the ontogeny of embryonic cell populations and the role of cell-cell interactions in development. We sought to extend the value of avian chimeras by producing a marker that would recognize cell surface components and that could be used instead of the traditional nuclear marker to identify quail cells within chimeras. We describe here a quail-specific antiserum produced by injecting chickens with a membrane fraction of 6-10-day quail embryos. By use of peroxidase coupling of a second antibody, serum reactivity was tested in tissue sections of normal quail and chick embryos and of somitic mesoderm and neural tube chimeras. The primary time period examined was 6-10 days of development. At these stages, the antiserum recognizes only quail cells and stains both plasma membrane-associated and cytoplasmic cell components. The latter characteristics allow the identification of quail axons in chimeras and facilitate visualization of quail cells at low magnification. We show that antiserum staining can also be used to identify quail cells in culture and can be combined with orthograde HRP labeling of neurons.

Cell surface changes in the developing optic nerve of mice

A recently defined antibody to a cell surface protein, M6, inhibits neurite outgrowth in culture (Lagenaur, Fushiki, and Schachner: Soc. Neurosci. Abstr. 10:739, '84). In the developing mouse, the antibody stains all parts of the primary optic pathway at birth. Over the next week, staining is lost from the proximal segment of the optic nerve and a week later from the more central part of the nerve. By contrast staining persists through adulthood in the optic fiber layer of the retina. This means that single axons in the mature optic nerve express the antigen over only the proximal few millimeters of their course and over their terminal region. The results are discussed in relation to the overall maturation of the optic pathway and to the processes of membrane maturation and myelination.

Use of a species-specific antibody for demonstrating mouse neurons transplanted to rat brains

A cell surface monoclonal antibody specific for mouse central nervous system neurons was used to identify mouse tissue transplanted to neonatal rat brains. Neuronal cell bodies and processes were stained in the transplants. Immature axons were stained growing out of the transplants into the host brain; and in mature brains unmyelinated axons and terminal plexuses were demonstrated. The technique allows a variety of studies to be performed on transplant-host interactions, especially in circumstances where the two are closely apposed.