Jennifer Bruce

Loss of brain tau defines novel sporadic and familial tauopathies with frontotemporal dementia

Dementia lacking distinctive histopathology (DLDH) or frontotemporal lobe degeneration (FTLD) is the most common neuropathological diagnosis for sporadic frontotemporal dementias (FTDs). The hallmarks of DLDH are neuron loss and gliosis in the absence of any disease‐specific brain lesion. Similar brain pathology is also seen in a familial FTD pedigree known as hereditary dysphasic disinhibition dementia 2 (HDDD2). Abnormality in the function or isoform composition of the microtubule binding protein tau is a prominent feature in the brains of many patients with sporadic and hereditary FTDs. Therefore, we studied the tau protein in different brain regions from DLDH and HDDD2 patients. Our results indicate that a selective loss of all six tau isoforms, but not tau mRNA, occurs in these brains compared to normal control and Alzheimers disease brains. Loss of tau protein was identified by Western blot analysis of protein extracts from DLDH and HDDD2 brains in regions both with and without neuronal degeneration. Functionally, this loss of tau protein may be equivalent to pathogenic mutations in the tau gene identified in familial FTD with parkinsonism linked to chromosome 17 (FTDP‐17). Thus, DLDH and HDDD2 are novel tauopathies with a unique mechanism of pathogenesis. The presence of tau mRNA in these brains suggests that the level of tau protein may be controlled posttranscriptionally, at the level of either translation or mRNA stability. Ann Neurol 2001:49:165–175

Stabilization of neurofilament transcripts during postnatal development

Neurofilament (NF) mRNAs in primary sensory neurons are long-lived transcripts that undergo transcription-dependent destabilization when placed in primary culture [32]. Destabilization of NF transcripts implies that the transcripts are stabilized in high-expressing neurons and that stabilization may coordinate and increase levels of NF expression. The present study examines the stabilities of the three NF subunit mRNAs in postnatal cultures of dorsal root ganglia (DRG) to determine whether increased stability of NF mRNAs could be responsible for the coordinate postnatal upregulation of the three NF subunits [29]. The studies show that the light (NF-L), mid-sized (NF-M) and heavy (NF-H) NF mRNAs are lost at 8 and 16 h in primary cultures from postnatal day 2 (P2) rats, but much less so in cultures from postnatal day 16 (P16) and day 30 (P30) rats. Losses of each NF mRNAs in P2 cultures occurs simultaneously in the presence or absence of actinomycin. The findings support the view that stabilization of NF transcripts contribute to the high and coordinate level NF expression and that components of the stabilizing process are acquired during postnatal development.

Deletion of 3′-Untranslated Region Alters the Level of mRNA Expression of a Neurofilament Light Subunit Transgene

High levels of neurofilament (NF) mRNA expression are attained during early postnatal development and are a major determinant of axonal size. High level NF expression is also dependent upon axonal continuity since NF mRNA levels are down-regulated after nerve transection. This study shows that both postnatal up-regulation and axotomy-induced down-regulation are altered by deletion of 3′-UTR from the mouse light NF subunit (NF-L). Transgenes with (NF-L+) or without (NF-L−) 3′-UTR display similar patterns of neuron-specific expression but differ in their respective levels of expression. Whereas changes in the level of NF-L+ mRNA parallel those of the endogenous mouse NF-L mRNA, changes in the level of NF-L− mRNA differ from the pattern of endogenous NF-L expression during postnatal up-regulation and axotomy-induced down-regulation. Specifically, the NF-L− transgene undergoes a 3-fold aberrant up-regulation between embryonic days 15 (E15) and 18 (E18) and has lost its susceptibility to axotomy-induced down-regulation. Studies of transfected P19 cells show that 3′-UTR deletion leads to a severalfold stabilization of NF-L mRNA and an increase in steady-state mRNA level. The findings support the working hypothesis that the 3′-UTR contains determinants that alter stability and that stabilization of NF-L mRNA regulates the levels of NF-L mRNA in neuronal tissues and cells.

Neurofilament Proteins Are Distributed in a Diminishing Proximodistal Gradient Along Rat Sciatic Nerve

Neurofilament (NF) proteins are distributed in a diminishing proximodistal gradient along rat sciatic nerve when compared with total noncollagen or other proteins in nerve. About a twofold decline of NF proteins can be detected by quantitating nerve proteins that have been separated by gel electrophoresis. A similar decrease of immunoreactivity to each NF subunit is seen in distal nerve segments when noncollagen nerve proteins are immunoblot ed. Parallel decreases occur in all three NF proteins, thereby maintaining neurofilament subunit stoichiometry along the neuraxis. The same NF gradient can be detected when the NF contents in nerve branches to the gluteus and gastrocnemius muscles are compared with each other and with those in nerve segments taken from the same proximodistal levels of the parent sciatic nerve. The gradient of NF proteins increases during postnatal development and is readily detected by postnatal day 16. During the same period of development, the heavy NF subunit appears for the first time and is rapidly incorporated throughout the sciatic nerve. Hence, the NF gradient becomes manifest during the development and maturation of the adult form of the axonal cytoskeleton. The basis for the proximodistal gradient of NF proteins in peripheral nerve is presently unknown. The extent of the gradient cannot be accounted for on the basis of diminishing numbers of nerve fibers or increasing amounts of other nerve proteins, e.g., collagen, in distal nerve. An alternative interpretation is that the gradient reflects a low level of NF protein turnover during axonal transport.

Studies of neurofilaments that accumulate in proximal axons of rats intoxicated with β,β'-iminodipropionitrile (IDPN)

The paradigm of IDPN neuropathy was produced in rats in order to examine the neurofilaments (NFs) that accumulate in the proximal motor and sensory axons of intoxicated animals, and to compare the aggregated NFs with control NFs and with the depleted populations of NFs in the distal portions of the same experimental nerves. NFs were probed biochemically and histochemically, using a large and well-characterized library of monoclonal antibodies that included antibodies that are monospecific for each of the rat NF protein subunits (NF-H, NF-M, and NF-L) as well as antibodies that recognized differential phosphorylated states of rat NF-H and NF-M. All antibodies tested showed enhanced immunostaining of enlarged axons and of large spheroids in the spinal cord and dorsal root ganglia of experimental animals. Biochemical analyses of IDPN-treated animals revealed enrichment of NF-H, NF-M, and NF-L in homogenates of dorsal root ganglia and of proximal motor and sensory nerve roots as well as depletion of the three subunits in distal nerve roots and in sciatic nerves. Immunoblots revealed a uniform enrichment of NF-H, NF-M, and NF-L in NF aggregates as well as the same admixture of phosphorylated and dephosphorylated epitopes of NF-H and NF-M in experimental and in control tissues. The global increase of immunoreactivity in axonal swellings to antibodies that react with phosphorylated, nonphosphorylated,and phosphorylation-independent NF epitopes suggests that IDPN induces an accumulation of NFs in proximal axons without necessarily altering the state of NF phosphorylation.