Expression of huntingtin-associated protein-1 in neuronal cells implicates a role in neuritic growth

Huntingtin-associated protein-1 (HAP1) binds more tightly to mutant huntingtin, but its function and distribution in neurites are not clear. Using PC12 cells and cultured hippocampal neurons, we found that two HAP1 isoforms, HAP1-A and HAP1-B, have different subcellular localizations. While most HAP1-B is diffusely distributed within the cytoplasm, HAP1-A is enriched in the growth cones and neuritic puncta of developing neurons. In mature neurons and adult brain neurons, however, HAP1-A is concentrated in axon terminals and associated with synaptic vesicles. Transfection of HAP1-A in PC12 cells significantly promotes neurite extension, with HAP1-A distributed in the distal regions of the growing neurites. Cotransfection of mutant huntingtin with HAP1-A in PC12 cells results in the accumulation of HAP1-A on huntingtin aggregates and the inhibition of neurite promotion by HAP1-A. This study suggests that HAP1-A has a function in neuritic development and synaptic function and that mutant huntingtin may alter this function.

Ultrastructural basis for gene expression at the synapse: synapse-associated polyribosome complexes

This review summarizes what is known about the protein synthetic machinery that is selectively localized beneath postsynaptic sites on the dendrites of CNS neurons. This machinery, made up of polyribosomes and associated membranous cisterns, allows a local synthesis of key proteins at individual postsynaptic sites.

Protein synthesis within dendrites: glycosylation of newly synthesized proteins in dendrites of hippocampal neurons in culture

There is increasing evidence that certain mRNAs are present in dendrites and can be translated there. The present study uses two strategies to evaluate whether dendrites also possess the machinery for protein glycosylation. First, precursor labeling techniques were used to conjunction with autoradiography to visualize glycosyltransferase activities that are characteristic of the rough endoplasmic reticulum (RER) (mannose) or the Golgi apparatus (GA) (galactose and fucose) in dendrites that had been separated from their cell bodies and in intact neurons treated with brefeldin A or low temperature. Second, immunocytochemical techniques were used to define the subcellular distribution of proteins that are considered markers of the RER (ribophorin I) and GA (p58, alpha-mannosidase II, galactosyltransferase, and TGN38/41). Autoradiographic analysis revealed that isolated dendrites incorporated sugar precursors in a tunicamycin-sensitive and protein synthesis-dependent manner. Moreover, when intact neurons were pulse-labeled with 3H-labeled sugars at low temperature or after treatment with brefeldin A, labeling was distributed over proximal and sometimes distal dendrites. Immunolabeling for RER markers was predominantly localized in cell bodies but extended for a considerable distance into dendrites of all neurons. Immunolabeling for GA markers was confined to the cell body in approximately 70% of the neurons, but in 30% of the neurons, the staining extended into proximal and middle dendrites. These results indicate that the machinery for glycosylation extends well into dendrites in many neurons.

Glial response to neuronal activity: GFAP-mRNA and protein levels are transiently increased in the hippocampus after seizures

We have recently demonstrated that electrically induced seizures lead to dramatic increases in mRNA for GFAP in areas in which seizures occur. The present study evaluates the time course of the changes in the GFAP-mRNA levels after seizures and the relationship between these changes and GFAP protein levels to understand the role of neuronal activity in regulating glial gene expression. GFA protein and mRNA levels were measured in hippocampi from rats in which seizures were induced by: (1) 50-Hz stimulus trains delivered 12 times over the course of 1 day via indwelling electrodes implanted chronically in the CA3 region of the hippocampus; and (2) intraperitoneal injections of pentylenetetrazol. In the case of the electrically induced seizures, we also compared the glial response in animals that had never experienced a seizure with the response in animals that previously had been kindled but had not experienced a seizure for 30 days. Electrically induced seizures led to rapid transient increases in GFAP-mRNA levels in the hippocampus ipsi- and contralateral to the stimulation. GFAP-mRNA increased about five-fold 1 day after the end of seizure activity and returned to near-control levels by 4 days. There were no detectable increases in GFA protein at 1 day but by 2 days GFA protein levels had increased about two-fold. GFA protein levels remained elevated until 4 days poststimulation and then began to decrease. The responses were similar when seizures were induced in kindled animals, except that the GFAP protein levels remained elevated for somewhat longer.(ABSTRACT TRUNCATED AT 250 WORDS)

The process of reinnervation in the dentate gyrus of adult rats: temporal relationship between changes in the levels of glial fibrillary acidic protein (GFAP) and GFAP mRNA in reactive astrocytes

The present study evaluates the temporal relationships between increases in glial fibrillary acidic protein (GFAP) mRNA, GFA protein levels, and GFAP immunostaining in the hippocampus of adult rats following unilateral lesions of the entorhinal cortex (EC). GFAP mRNA levels were assessed at 12 h, 1, 2, 4, 6, 8, 10, 12, 14, and 30 days postlesion by dot blot assays using 35S-labeled cRNA probes against the mRNA. Animals were also prepared for in situ hybridization during the peak of GFAP mRNA expression (2 days postlesion) to explore the nature of individual differences in the spatial extent of the increases. GFA protein levels were assessed by Western blot and dot immunoblot techniques in a separate group of animals prepared at 1, 2, 4, 6, 8, and 10 days postlesion and by immunostaining at 1, 2, 4, 6, and 8 days postlesion. The dot blot analyses of GFAP mRNA levels confirmed previous studies, in that we observed dramatic increases in the levels of GFAP mRNA in the hippocampus ipsilateral to the EC lesions. The increases were biphasic, with a large peak in mRNA levels at 1-2 days postlesion (about 10-fold greater than control) and a second peak at 6-8 days. In most animals, the increases were predominantly ipsilateral to the lesion. However, in some animals, there were also large increases on the contralateral side. In situ hybridization experiments revealed two different spatial patterns of increased gene expression, one in which the increases in GFAP mRNA occurred bilaterally and one in which increases were restricted primarily to the hippocampus ipsilateral to the lesion. Immunochemical measures revealed that GFA protein levels increased gradually in the hippocampus ipsilateral to the lesion, reaching a peak at about 2-fold higher than control at 4 days postlesion, and then remained near this level until at least 10 days postlesion. In the contralateral hippocampus, GFA protein levels were increased to about the same extent as on the ipsilateral side at 1, 2, and 4 days postlesion, but then began to decline, returning to near control levels by 8 days. Increases in immunostaining occurred with about the same time course as the increases in GFA protein levels as measured immunochemically. These results define the temporal relationship between increases in GFAP mRNA and increases in GFA protein, providing new insights into the regulation of gene expression in reactive astrocytes.

Demonstration of local protein synthesis within dendrites using a new cell culture system that permits the isolation of living axons and dendrites from their cell bodies

The presence of polyribosomes within dendrites suggests a capability for local dendritic protein synthesis. However, local synthesis is difficult to evaluate because of rapid somatodendritic protein transport. The present study describes a two-surfaced culture system that allowed the separation of living axons and dendrites from their cell bodies of origin. Because this system eliminates the transport of proteins produced in the cell body, it was possible to study the extent of dendritic protein synthesis directly. Hippocampal neurons were plated on a Nucleopore polycarbonate membrane that was mounted on a thick matrix of proteins (Matrigel) fixed on a coverslip. As the neurons grew, axons and dendrites grew through the membrane into the Matrigel. To evaluate local protein synthesis within dendrites, the membrane with the cell bodies was removed, leaving a dense array of transected dendrites and axons on the coverslip with few contaminant cell bodies. Absence of cell bodies was confirmed by staining with the nuclear stain Hoechst 33258. Coverslips with isolated neurites were pulse labeled with 3H-leucine for 30 min, and fixed for autoradiography to identify sites of protein synthesis. Autoradiographic analyses revealed that isolated dendrites (immunochemically identified using antibodies against MAP2) became heavily labeled, whereas axons exhibited little if any labeling. The labeling was essentially eliminated when the neurites were pulse labeled with 3H-leucine in the presence of puromycin, whereas labeling was affected only minimally by chloramphenicol. The puromycin-sensitive incorporation of 3H-leucine in dendrites demonstrates that the polyribosomes previously described are active in protein synthesis. This system will allow a characterization of synthetic activity within isolated neurites and provide a new approach to identifying proteins that are produced within dendrites.

Neuronal activity up-regulates astroglial gene expression

Neuronal gene expression is known to be modulated by functional activity. This modulation is thought to play a key role in determining the differentiation of developing neurons and regulating the operation of mature neurons. Here we describe a regulation of astroglial gene expression by neuronal activity. We report that intense neuronal activity (electrically induced seizures) in rat hippocampus leads to rapid and dramatic increases in mRNA for glial fibrillary acidic protein (GFAP), an astroglia-specific intermediate filament protein. GFAP mRNA levels increased at sites of stimulation as well as in areas that were synaptically activated by the resultant seizures. When seizures were induced repetitively for many days, levels of GFAP mRNA remained chronically elevated. However, GFAP mRNA returned to control levels within a few days after the cessation of stimulation. The coupling between astroglial gene expression and neuronal activity may be a mechanism through which neuronal activity modulates the function of supporting cells that are responsible for regulating the extracellular microenvironment of the brain.

The process of reinnervation in the dentate gyrus of adult rats: time course of increases in mRNA for glial fibrillary acidic protein

The present study evaluates the time course of increased expression of the mRNA for glial fibrillary acidic protein (GFAP) within the dentate gyrus and hippocampus after unilateral lesions of the entorhinal cortex. Levels of GFAP mRNA were evaluated by dot blot hybridization of mRNA isolated from the hippocampus and quantitative in situ hybridization. For dot blot hybridization, RNA was isolated from pooled hippocampi obtained from animals killed at 12 hr, 1, 2, 4, 6, 8, 10, 14, and 30 d postlesion. A separate set of animals killed at 2, 4, 6, 8, 10, 12, 14, and 32 d were prepared for in situ hybridization. The dot blot analyses of mRNA isolated from the hippocampus revealed that on the side ipsilateral to the lesion, the levels of GFAP mRNA increased rapidly, reaching a peak at 2 d postlesion. The increases were not evident by 12 hr postlesion, but by 24 hr, levels of GFAP mRNA were 5-fold higher than control, and by 48 hr, the levels were over 6-fold higher than control. The levels of GFAP mRNA decreased after 2 d postlesion. At 4 and 6 d postlesion the levels were about 2-fold higher than control. At later postlesion intervals, mRNA levels were comparable to the control. At 2 d postlesion, the levels of GFAP mRNA were also increased about 2-fold over control levels on the contralateral side. After 2 d, the levels of GFAP mRNA on the contralateral side were comparable to the control. In situ hybridization revealed a complex pattern of changes in the levels of GFAP. At 2 d postlesion, the levels of GFAP mRNA increased dramatically throughout the hippocampus bilaterally. The increases were most pronounced in the denervated portions of the neuropil; however, the levels of GFAP mRNA were also substantially elevated in laminae that do not receive direct projections from the entorhinal cortex. GFAP mRNA levels were also increased in other areas that receive projections from the entorhinal cortex, including the septum, lateral-dorsal thalamus, and entorhinal cortex contralateral to the lesion. In addition, GFAP mRNA levels were increased in regions bordering the ventricles throughout the brain, and over the surface of the tectum. After 2 d postlesion, the increases in the levels of GFAP mRNA were for the most part restricted to areas containing terminal degeneration. The generalized increases throughout the hippocampus were no longer apparent. Areas bordering the ventricles continued to exhibit higher labeling than in control animals, but this effect was not as prominent as at 2 d postlesion.(ABSTRACT TRUNCATED AT 400 WORDS)