Changes in cortical and striatal neurons predict behavioral and electrophysiological abnormalities in a transgenic murine model of Huntington's disease.

Neurons in Huntington's disease exhibit selective morphological and subcellular alterations in the striatum and cortex. The link between these neuronal changes and behavioral abnormalities is unclear. We investigated relationships between essential neuronal changes that predict motor impairment and possible involvement of the corticostriatal pathway in developing behavioral phenotypes. We therefore generated heterozygote mice expressing the N-terminal one-third of huntingtin with normal (CT18) or expanded (HD46, HD100) glutamine repeats. The HD mice exhibited motor deficits between 3 and 10 months. The age of onset depended on an expanded polyglutamine length; phenotype severity correlated with increasing age. Neuronal changes in the striatum (nuclear inclusions) preceded the onset of phenotype, whereas cortical changes, especially the accumulation of huntingtin in the nucleus and cytoplasm and the appearance of dysmorphic dendrites, predicted the onset and severity of behavioral deficits. Striatal neurons in the HD mice displayed altered responses to cortical stimulation and to activation by the excitotoxic agent NMDA. Application of NMDA increased intracellular Ca(2+) levels in HD100 neurons compared with wild-type neurons. Results suggest that motor deficits in Huntington's disease arise from cumulative morphological and physiological changes in neurons that impair corticostriatal circuitry.

Functional limits of conformation, hydrophobicity, and steric constraints in prokaryotic signal peptide cleavage regions. Wild type transport by a simple polymeric signal sequence.

These experiments examine the role of conformation, hydrophobicity, and steric constraints in the function of the prokaryotic signal peptide cleavage region. The experimental strategy involves replacement of the wild type Escherichia coli alkaline phosphatase signal peptide cleavage region with a series of idealized model sequences designed to epitomize the particular structural and physical variables under study. By analyzing model sequences whose conformations have been determined by physical studies, we have demonstrated that efficient transport does not depend on the structural preference of the cleavage region. Although previous studies based on Chou-Fasman analysis have suggested that the cleavage region forms a beta-turn which is required for transport, our results demonstrate that either a beta-turn- or alpha-helix-fostering sequence in the cleavage region functions indistinguishably from wild type. Furthermore, the presence of a proline residue between the core and cleavage region, although common in natural sequences, is not essential for export. Cleavage regions of varying hydrophobicities can support translocation across the inner membrane, but the placement of bulky residues at positions -1 and -3 upstream of the cleavage site abolishes processing and transport to the periplasm. By reducing the signal peptide to simplified, idealized segments, this study has identified a largely polymeric sequence, MKQST(L10)-(A6), that functions equivalently to the wild type alkaline phosphatase signal peptide. This work starts to provide a basis for the design of a universal prokaryotic signal peptide that incorporates all the critical physical and structural characteristics required for transport function.

Signal peptide subsegments are not always functionally interchangeable. M13 procoat hydrophobic core fails to transport alkaline phosphatase in Escherichia coli.

Bacterial signal peptides display little amino acid sequence homology despite their shared role in mediating protein transport. This heterogeneity may exist to permit the establishment of signal peptide conformations that are appropriate for transport of particular proteins. In this paper we explore how signal peptides are composed of structural units that may interact with each other and with the mature protein to effect transport. Using a new application of cassette mutagenesis, we have replaced the hydrophobic core of the Escherichia coli alkaline phosphatase signal peptide with cores from the signals of maltose-binding protein, OmpA, and M13 major coat protein. The core regions from maltose-binding protein and OmpA effectively replaced the alkaline phosphatase core; the resultant hybrid signals performed as well as wild type in periplasmic transport and processing of alkaline phosphatase. However, the core region from M13 major coat protein generated a transport-incompetent hybrid signal peptide. Elimination of a proline-containing portion of the M13 major coat protein core did not improve transport effectiveness. However, restoration of the procoat cleavage region and the negatively charged amino terminus of the mature protein did ameliorate the transport defect. These results suggest that at least in the case of these procoat-derived signal peptide mutants, there is a requirement for complementarity among the hydrophobic core, cleavage region, and part of the mature protein in order for efficient protein transport to occur.

Peptide and protein synthesis by segment synthesis-condensation.

The chemical synthesis of biologically active peptides and polypeptides can be achieved by using a convergent strategy of condensing protected peptide segments to form the desired molecule. An oxime support increases the ease with which intermediate protected peptides can be synthesized and makes this approach useful for the synthesis of peptides in which secondary structural elements have been redesigned. The extension of these methods to large peptides and proteins, for which folding of secondary structures into functional tertiary structures is critical, is discussed. Models of apolipoproteins, the homeo domain from the developmental protein encoded by the Antennapedia gene of Drosophila, a part of the Cro repressor, and the enzyme ribonuclease T1 and a structural analog have been synthesized with this method.