Alternative Splicing
Alternative splicing is a gene regulatory mechanism which is ubiquitous, vital and impressive: it produces many finely modulated protein variants from a single gene. These variants of the gene product give the system the opportunity to respond with great speed and sensitivity to changes either within or without the cell.
Alternative splicing has direct and multiple connections to cellular development and differentiation. It is particularly prevalent in terminally differentiated cells which are no longer capable of division -- for example, in the nervous, muscle and immune systems. Almost all the genes whose products either regulate or comprise the neurons undergo alternative splicing. Gross errors in alternative splicing invariably result either in hereditary disease or perinatal death. Even more interestingly, errors in alternative splicing regulation directly cause a large number of diseases, including several types of cancer, dementia, genetic defects (both morphological and cognitive) and lethal infant degenerative conditions such as spinal atrophy.
The tau gene
Tau protein, which binds to and organizes microtubules, is instrumental in establishing and maintaining the morphology of the neuronal axon. Organization of cytoskeletal elements is critical for axonal formation and neuronal migration. Furthermore, disarrangement of the axonal scaffolding is the first event in neuronal degeneration, the distal cause for most dementias. In addition to its structural role, the tau protein interacts with the plasma membrane and thus is a crucial link in the chain of communication which connects the neuronal cytoskeleton with the external environment.
The neuron-specific tau transcript gives rise to multiple isoforms via developmental stage- and tissue-specific alternative splicing. Disturbances in tau splicing result in disruption of the neuronal cytoskeleton and formation of pathological tau structures (neurofibrillary tangles) found in brains of dementia sufferers (Alzheimer's and Pick's disease, progressive supranuclear palsy, cortico-basal degeneration). Mutations which affect tau alternative splicing cause inherited frontotemporal dementia, the second most common dementia after Alzheimer's disease. In contrast to more common "frameshift" disease mechanisms which in effect produce dominant negative effectors, in the case of tau the mutant alleles produce wild-type protein, but in the wrong ratios. Thus, tau presents us with a novel disease paradigm, which is formally equivalent to dosage diseases such as Down syndrome.
In this lab, we cloned and extensively characterized the human tau gene, surveyed the expression and regulatory patterns of both the gene and its splicing variants, (discovering novel variants in the process), examined in depth the alternative splicing regulation of four tau exons (including the one whose missplicing causes frontotemporal dementia) and discovered a novel protein which interacts with a tau regulated domain. We are now launching a search for novel neuronal-specific splicing regulators and for novel ligands of the tau protein. We are also investigating the morphological and functional consequences of overexpressing tau variants.
Concluding remarks
To understand what leads to the malfunction of any system, we must first discover its correct function. Discoveries from this research will substantially increase our knowledge of the mechanisms and factors involved in alternative splicing of the nervous system, will shed light on the formation and organization of the axon at the molecular level and will show previously unknown structure-function correlations of proteins of the nervous system. In terms of applications, this research will contribute initially to the discovery and eventually to the cure of the causes which result in the sad phenomenon of dementia which is becoming increasingly common as the human life span lengthens. Within the global picture, this research forms part of the bottom-up approach in answering one of the major questions both within and beyond science -- how the brain creates a mind (the Binding Problem).